This week, I was graciously invited to join the Southern Fried Science blog network. I accepted, and so will be relocating to www.cephalove.southernfriedscience.com . All of the old posts have been copied to that site, and no new posts will show up here. I'm sorry if this is a hassle for all of you generous souls who link to me, but please continue to link to me at my new and improved location!
Thanks so much to all of my readers, and those of you who put up with my rambling comments on your own blogs. If there is one reason I have enjoyed blogging, it's your feedback and encouragement.
A blog documenting my intellectual love affair with our many-armed friends.
Saturday, August 7, 2010
Thursday, August 5, 2010
Memory, observation, and consciousness in Octopus Vulgaris
A while back, I wrote a post about short and long term memory processes in cephalopods. I wrote then that there is good evidence for a dissociation of short and long term memory process in cephalopods, but that this isn't a good basis (alone) for inferring the presence of consciousness, or in the case of arguments about animal's rights, the capacity to suffer (which, I guess, usually comes along with being conscious.) I stand by this; I just want to cover a neat study that I missed while writing that post: Lesions of the vertical lobe impair visual discrimination learning by observation in Octopus vulgaris by Fiorito and Chichery (1995). This uses an observational learning task that Fiorito and Scotto used in their 1992 article on observational learning in the octopus, where the test octopus watches another octopus perform a visual discrimination, and then is tested on that discrimination. Octopuses can (apparently) learn a simple task by watching another octopus do the task pretty well, and so in their 1995 paper, Fiorito and Chichery examine the effect of brain lesions to the vertical lobe of O. vulgaris on their retention of this task, as well as a discrimination learned through the more traditional method of reward and aversion (in the case of the octopus, some fish for a correct answer and a small electric shock for an incorrect answer, usually.)
The vertical lobe is one of many lobes in the cephalopod brain. It sits above the oesophagus, and receives input from the sensory systems of the arms and visual information from the optic lobes. It is classically associated with learning, so that removal of the vertical lobe results rather reliably in deficits in the learning of a discrimination task. When asking questions about the presence of short and long term memory processes, one has to differentiate between the two. Thus, Fiorito and Chichery test their animals at two time points, 1.5 hours after training and 24 hours after training. It's important to note that this 24 hours would not nearly qualify as long-term in human memory, where memories can be stored for many years. In the octopus, tactile memories have only been shown to be retained for up to 50 days, although interestingly enough, the removal of the vertical lobe after a task has been learned appears to improve memory retention (Sanders, 1970.) I'll get back to this.
On to the procedure! Fiorito and Chichery trained one group of octopuses to disciminate between a white ball and a red ball - specifically, to attack the white ball and not the red ball. Then, another group (which had been operated on, some having their vertical lobe partially removed and others having a sham surgery) watched the first group perform the discrimination for 4 trials. They were then tested, to see if they could remember the discrimination at 1.5 hours after training and at 24 hours after training. The results are shown below:
This is a bit of an odd way of showing the data (I would have done a line graph, myself.) First of all, the bars in each graph show how many of the tested octopuses chose which ball, the red (R) or the white (W). NA is used for trials in which the octopuses did not make a valid response (ie. did not attack either ball.) The white ball can be thought of as the "correct" choice. The top row of graphs shows animals with the vertical lobe removed, and the bottom row shows animals who received a sham surgery. The first column of graphs shows the 1.5 hour test, and the second column shows the 24 hour test. The sham-operated group looks much as one might expect them to - they learn, and they retain the learning. The lesioned group is strikingly impaired. At 1.5 hours, it's clear that the removal of the vertical lobe has hurt performance, as these animals are performing at chance levels. By 24 hours, however, they seem to have improved! This is odd. If we explain this by analogy to human learning processes, we would have to say that these octopuses formed a long-term memory of the task without forming a short-term memory of it first. This indicates that "short-term" and "long-term" memory like what we talk about in mammals is not readily applicable to the description of learning in cephalopods.
Consider for a moment the results of Sanders (1970), who found that octopuses who learned a task and had their vertical lobes removed (unfortunately, I cannot find the full text of the paper at the moment and so I don't know the exact procedure) retained it better than those who had intact vertical lobes - that is, they retained it for a longer period of time. If Fiorito and Chichery had tested their octopuses at longer intervals, we might expect that they would find the same results, with vertical lobe remove leading to a greatly delayed acquisition of the memory as well as a slower decay of the memory. This strikes me as odd, as I do not believe that this can be shown to be the case with people. In general, if people cannot remember something for a short time, they cannot thereafter remember it better after a long interval - it is simply gone from the system. I may be wrong about this point (and please point out any counter-examples you know), but it seems to me that the memory of cephalopods doesn't correspond very cleanly to the "working memory-consolidation-long term memory" model that is used to describe human memory.
And why should it? Cephalopods may not have memory that looks like ours, but they have highly developed memory systems that serve them well enough. If anything, we should be excited that our theories of human memory cannot explain cephalopod memory very well; the more varieties of memory systems we have to study, the more we can learn about learning, period.
This paper is a big deal (theoretically speaking) for a reason besides its illustration of the role of the vertical lobe in the time course of memory. Did you catch it? The authors used an observational learning task. That is, the octopuses being tested did not receive fish for the correct answer and shocks for the incorrect answer in the task. They did the task (correctly, at that) without ever being rewarded or punished for it; instead, they learned how to do the task by watching another animal perform it. When Fiorito and Scotto published a paper on observational learning in the octopus in 1992, people had a hard time swallowing it. It simply did not make sense, critics contended, that octopuses, being such loners, would have the capacity for observational learning. Why would they have evolved the capacity to be cooperatively social? The fact that they can learn by observation is one of the arguments that proponents of cephalopod consciousness (that is, the idea that cephalopods have some form of conscious awareness) often cite this as evidence of their general powers of cognitive representation. The octopuses are not being social, they're just being smart. At some level, they appear to have a representation of themselves and other beings, enough that they can learn a simple task by observing another octopus do it. In any case, replicating this finding adds some weight to Fiorito and Scotto's argument that octopuses can learn by observation.
Thanks for reading!
Fiorito G, & Chichery R (1995). Lesions of the vertical lobe impair visual discrimination learning by observation in Octopus vulgaris. Neuroscience letters, 192 (2), 117-20 PMID: 7675317
Fiorito, G., & Scotto, P. (1992). Observational Learning in Octopus vulgaris Science, 256 (5056), 545-547 DOI: 10.1126/science.256.5056.545
SANDERS, G. (1970). Long-term memory of a tactile discrimination in Octopus vulgaris and the effect of vertical lobe removal Brain Research, 20 (1), 59-73 DOI: 10.1016/0006-8993(70)90154-X
The vertical lobe is one of many lobes in the cephalopod brain. It sits above the oesophagus, and receives input from the sensory systems of the arms and visual information from the optic lobes. It is classically associated with learning, so that removal of the vertical lobe results rather reliably in deficits in the learning of a discrimination task. When asking questions about the presence of short and long term memory processes, one has to differentiate between the two. Thus, Fiorito and Chichery test their animals at two time points, 1.5 hours after training and 24 hours after training. It's important to note that this 24 hours would not nearly qualify as long-term in human memory, where memories can be stored for many years. In the octopus, tactile memories have only been shown to be retained for up to 50 days, although interestingly enough, the removal of the vertical lobe after a task has been learned appears to improve memory retention (Sanders, 1970.) I'll get back to this.
On to the procedure! Fiorito and Chichery trained one group of octopuses to disciminate between a white ball and a red ball - specifically, to attack the white ball and not the red ball. Then, another group (which had been operated on, some having their vertical lobe partially removed and others having a sham surgery) watched the first group perform the discrimination for 4 trials. They were then tested, to see if they could remember the discrimination at 1.5 hours after training and at 24 hours after training. The results are shown below:
Consider for a moment the results of Sanders (1970), who found that octopuses who learned a task and had their vertical lobes removed (unfortunately, I cannot find the full text of the paper at the moment and so I don't know the exact procedure) retained it better than those who had intact vertical lobes - that is, they retained it for a longer period of time. If Fiorito and Chichery had tested their octopuses at longer intervals, we might expect that they would find the same results, with vertical lobe remove leading to a greatly delayed acquisition of the memory as well as a slower decay of the memory. This strikes me as odd, as I do not believe that this can be shown to be the case with people. In general, if people cannot remember something for a short time, they cannot thereafter remember it better after a long interval - it is simply gone from the system. I may be wrong about this point (and please point out any counter-examples you know), but it seems to me that the memory of cephalopods doesn't correspond very cleanly to the "working memory-consolidation-long term memory" model that is used to describe human memory.
And why should it? Cephalopods may not have memory that looks like ours, but they have highly developed memory systems that serve them well enough. If anything, we should be excited that our theories of human memory cannot explain cephalopod memory very well; the more varieties of memory systems we have to study, the more we can learn about learning, period.
This paper is a big deal (theoretically speaking) for a reason besides its illustration of the role of the vertical lobe in the time course of memory. Did you catch it? The authors used an observational learning task. That is, the octopuses being tested did not receive fish for the correct answer and shocks for the incorrect answer in the task. They did the task (correctly, at that) without ever being rewarded or punished for it; instead, they learned how to do the task by watching another animal perform it. When Fiorito and Scotto published a paper on observational learning in the octopus in 1992, people had a hard time swallowing it. It simply did not make sense, critics contended, that octopuses, being such loners, would have the capacity for observational learning. Why would they have evolved the capacity to be cooperatively social? The fact that they can learn by observation is one of the arguments that proponents of cephalopod consciousness (that is, the idea that cephalopods have some form of conscious awareness) often cite this as evidence of their general powers of cognitive representation. The octopuses are not being social, they're just being smart. At some level, they appear to have a representation of themselves and other beings, enough that they can learn a simple task by observing another octopus do it. In any case, replicating this finding adds some weight to Fiorito and Scotto's argument that octopuses can learn by observation.
Thanks for reading!
Fiorito G, & Chichery R (1995). Lesions of the vertical lobe impair visual discrimination learning by observation in Octopus vulgaris. Neuroscience letters, 192 (2), 117-20 PMID: 7675317
Fiorito, G., & Scotto, P. (1992). Observational Learning in Octopus vulgaris Science, 256 (5056), 545-547 DOI: 10.1126/science.256.5056.545
SANDERS, G. (1970). Long-term memory of a tactile discrimination in Octopus vulgaris and the effect of vertical lobe removal Brain Research, 20 (1), 59-73 DOI: 10.1016/0006-8993(70)90154-X
Monday, August 2, 2010
Cephalopod Links Number 4.5
At National Geographic, you can check out some footage and a story about the successful attachment of a critter cam to a Humboldt squid. I'll have to get ahold of their television show somehow, seeing as I don't have TV, and it was last Friday, anyways.
Although it's an old piece, I want to point out this article on the expansion of the Humboldt squid's range. It's a very interesting topic, and one that is still (as far as I know) ripe for investigation and theorizing.
PZ Meyers points out a video claiming (among otherthings fallacies) that the fossil record of coleoid cephalopod ancestors provides evidence against "macroevolution". Of course, the alternative, and so presumably correct, theory is "Intelligent Design". Wow.
Folks in Delaware can head down to the Delaware Seashore State Park and dissect a squid for $8, among other great activities. Now that's good use of public facilities!
Departing from cephalopod-related links for a moment, Virginia Heffernan has written this piece over at the New York Times that's gotten science bloggers in a tizzy, and which I can't help but feel a wee bit offended by. While most of the article is spent blasting ScienceBlogs in particular (which I find catty and off-topic much of the time, too,) she has a few cracks at science blogging in general, including this gem:
"...science blogging, apparently, is a form of redundant
and effortfully incendiary rhetoric that draws bad-faith
moral authority from the word “science” and from
occasional invocations of “peer-reviewed” thises and
thats."
Please, stop me the next time I start throwing around "incendiary rhetoric"on Cephalove.
Check out this cool video of giant octopus kites.
And then this one of S. latimanus in an agonistic encounter, complete with some great changes in coloration.
Circus of the Spineless #53, that virtual periodical on all things invertebrate, is up at The Birder's Lounge. Make sure to check it out for an interesting assortment of writing on all sorts of spineless wonders.
I'll be back later in the week with some more cephalopod photographers, and a few science-y posts. I'm thinking about tackling the "cephalopod consciousness" issue, although it will take a bit of work to get ahold of the literature, read it, and then work out where I stand. Nonetheless, I'll see what I can get up here.
Thanks for reading!
Although it's an old piece, I want to point out this article on the expansion of the Humboldt squid's range. It's a very interesting topic, and one that is still (as far as I know) ripe for investigation and theorizing.
PZ Meyers points out a video claiming (among other
Folks in Delaware can head down to the Delaware Seashore State Park and dissect a squid for $8, among other great activities. Now that's good use of public facilities!
Departing from cephalopod-related links for a moment, Virginia Heffernan has written this piece over at the New York Times that's gotten science bloggers in a tizzy, and which I can't help but feel a wee bit offended by. While most of the article is spent blasting ScienceBlogs in particular (which I find catty and off-topic much of the time, too,) she has a few cracks at science blogging in general, including this gem:
"...science blogging, apparently, is a form of redundant
and effortfully incendiary rhetoric that draws bad-faith
moral authority from the word “science” and from
occasional invocations of “peer-reviewed” thises and
thats."
Please, stop me the next time I start throwing around "incendiary rhetoric"on Cephalove.
Check out this cool video of giant octopus kites.
And then this one of S. latimanus in an agonistic encounter, complete with some great changes in coloration.
Circus of the Spineless #53, that virtual periodical on all things invertebrate, is up at The Birder's Lounge. Make sure to check it out for an interesting assortment of writing on all sorts of spineless wonders.
I'll be back later in the week with some more cephalopod photographers, and a few science-y posts. I'm thinking about tackling the "cephalopod consciousness" issue, although it will take a bit of work to get ahold of the literature, read it, and then work out where I stand. Nonetheless, I'll see what I can get up here.
Thanks for reading!
Cephalopod Photography: Barry Fackler
Today's cephalopod photographer of note is Barry Fackler, a physical therapist originally from Pennsylvania who lives and dives in Hawai'i. All of the photos in this post are his, and they're all click-through-able, so check out the larger sizes. It turns out that octopods (mostly O. cyanea) are the only cephalopods in his Flickr portfolio, so I'm sorry to disappoint the squid and cuttlefish lovers out there. I promise they are all wonderful photos, though!
Here is an O. cyanea showing a mostly white color pattern while jetting, a behavior often associated with defensive flight.
Here is another gorgeous octopus of the same species, giving the camera an inquisitive look. This was taken at at Keone'ele Cove in Honaunau (another place to add to my "List of incredible spots to visit when I get money" spreadsheet.)
I always love to see octopuses express dramatic papillae. In this next shot, we see an octopus who is apparently trying to look like just another chunk of coral, even if he's not doing a terribly good job.
This next one shows an octopus (O. cyanea again) in a defensive posture. Notice the high contrast color pattern, the curled arms, and the spread interbrachial web. The point of this behavior is to look big enough to make a potential predator think twice before he eats you - it's a common strategy among prey species. According to Barry, the animal adopted this pose when approached by some fish.
I like this next shot simply because you can see right into the octopus's mantle. It's somehow fascinating to me to see the inside and outside of an animal at the same time like this.
These next two photos show some behavior that I had never heard of before reading the photographer's description. Apparently, the fish (a peacock grouper) was following this octopus around to feed on small prey that the octopus stirred up as it foraged over the reef. According to Barry, they usually follow eels, but he found this one hanging around a hunting O. cyanea. I'd argue that it is probably not cooperative hunting per se, as it's unclear how the octopus would benefit from it, but it's fascinating behavior nonetheless.
I'll finish up with two gorgeous portraits of O. cyanea just sitting on the reef. I love the colors of these octopuses.
Thanks for reading! Make sure to click on over to Barry's photostream and check out his other underwater photos.
Here is an O. cyanea showing a mostly white color pattern while jetting, a behavior often associated with defensive flight.
Here is another gorgeous octopus of the same species, giving the camera an inquisitive look. This was taken at at Keone'ele Cove in Honaunau (another place to add to my "List of incredible spots to visit when I get money" spreadsheet.)
I always love to see octopuses express dramatic papillae. In this next shot, we see an octopus who is apparently trying to look like just another chunk of coral, even if he's not doing a terribly good job.
This next one shows an octopus (O. cyanea again) in a defensive posture. Notice the high contrast color pattern, the curled arms, and the spread interbrachial web. The point of this behavior is to look big enough to make a potential predator think twice before he eats you - it's a common strategy among prey species. According to Barry, the animal adopted this pose when approached by some fish.
I like this next shot simply because you can see right into the octopus's mantle. It's somehow fascinating to me to see the inside and outside of an animal at the same time like this.
These next two photos show some behavior that I had never heard of before reading the photographer's description. Apparently, the fish (a peacock grouper) was following this octopus around to feed on small prey that the octopus stirred up as it foraged over the reef. According to Barry, they usually follow eels, but he found this one hanging around a hunting O. cyanea. I'd argue that it is probably not cooperative hunting per se, as it's unclear how the octopus would benefit from it, but it's fascinating behavior nonetheless.
I'll finish up with two gorgeous portraits of O. cyanea just sitting on the reef. I love the colors of these octopuses.
Thanks for reading! Make sure to click on over to Barry's photostream and check out his other underwater photos.
Sunday, August 1, 2010
Serotonin in the octopus learning system.
(Note: I apologize if this post seems jargon-ey. I've tried to explain or reference any hard to get terms, but I do assume that readers know the very basics of neural functioning. If you need a primer on this, check out wikipedia's page on neurons or this great tutorial. Feel free to post in the comments if there's anything you want explained more thoroughly, and I'll give it a crack.)
The Octopus research group in Jerusalem is back with a paper in the August issue of Neuroscience about the function of serotonin in the octopus vertical lobe, Serotonin is a facilitatory neuromodulator of synaptic transmission and “reinforces” long-term potentiation induction in the vertical lobe of Octopus vulgaris. I'm very excited to blog about this paper - it's the very first time in my short blogging career that I've gotten to cover a study as it was coming out! You can read my other posts about their work here and here (that second one has a basic description of the technique of stimulation-induced LTP, which I'll be very brief with here.)
Basically, LTP (long-term potentiation) is one of the mechanisms by which neurons are thought to adjust how they connect to each other during the process of learning - specifically, they become stronger (or potentiated,) meaning that signals are carried across the synapse more effectively. The authors of this paper use a technique by which they induce LTP in synapses in the octopus vertical lobe (a structure thought to be involved in learning and memory) and study the effects of serotonin (also called 5-HT, which is short for 5-hydroxytryptamine, the terminology I'll be using from now on) on the properties of the induced LTP. Presumably, this can tell us something about the function of 5-HT in the normal functioning of the vertical lobe, although this point is very debatable.
Why look at 5-HT? Well, for starters, it's one of the big neurotransmitters these days (along with such illustrious nearly-lay-term chemicals as dopamine, norepinephrine, GABA and glutamate.) You hardly need to have a specific reason to study it these days because it's involved in pretty much every process that contemporary neurobiology cares about: consumptive behavior, mood and depression, social cognition, the action of addictive drugs. More than that, though, it's conserved across all bilaterians, the group of bilaterally symmetrical animals including people, the rest of the vertebrates, the insects, and, among many others, the molluscs! If there is any neurotransmitter that is interesting to study comparatively, it's 5-HT, as it's been shown to be involved in learning in animals as distantly related to each other as sea slugs, rats, humans, and (now) cephalopods. If we learn how 5-HT does its job in a wide variety of animals, it will help us understand how neurotransmitters function within nervous systems in general. This is, we will hopefully agree, a Good Thing.
The authors begin with the hypothesis that, as has been shown in Aplysia (a beautiful little sea slug who is relatively widely studied in neuroscience,) 5-HT probably has a role in the modulation of LTP rather than inducing it directly, making it a putative neuromodulator. It is not hard to imagine how this might be a good thing to have in a memory system. Let's pretend that our animals has just been injured, or that it has just found a great big source of food. All of these events call for a general upregulation in the formation of memories, since remembering what happened around these events will help the animal repeat or avoid them in the future, depending on whether they were good or bad. If a chemical can increase the amount of LTP (a process thought to be involved in learning,) it would make sense that it might be selectively secreted or expressed during times when the animal's memory system needs to pay attention to what's going on, and not when there is nothing of consequence happening. This is an extremely limited view of the role of neuromodulators in learning, but it illustrates the principal as well as I know how to. In short, neuromodulators, while not responsible for neurotransmission and plasticity themselves, have some effect on it. This sort of effect is one of the things that allows the great flexibility of neural systems, one of their key features.
In the first part of their study, the authors stained slices of the octopus vertical lobe for 5-HT, and then described what they say - this is good old fashioned neuroscience. They found that 5-HT shows up in fibers from the medial superior frontal lobe (MSF) that innervate large areas of the vertical lobe. The MSF is thought to be one of the main sources of input of sensory information to the vertical lobe, and this tract of fibers (known as the MSF-VL tract) is thought to be involved in the formation of sensory memories in the octopus, as per J. Z. Young's early lesion experiments in the octopus. The authors note that this wide spread of 5-HT is typical of neuromodulators, supporting the idea that MSF neurons use 5-HT to modulate LTP in the vertical lobe.
In the second part of the study, the authors use a technique where they induce LTP in live slices of octopus brain (cool, right?) by repeatedly stimulating the axons running from the MSF to the vertical lobe. They measure the "strength" of neurotransmission as fPSP's, or synaptic field potential, which is roughly an indicator of how much electrical activity is generated by activity in many synapses within a small area of the tissue. I'll only summarize one of their several experiments here, because it is the one that really illustrates the neuromodulatory effect.
This figure shows the results of an experiment using induced LTP in octopus brain slices. The experimenters stimulated the brain slices along the MSF-VL tract and recorded the resultant electrical activity in the VL. Let's start with the first graph. The y-axis shows the amount of activity recorded in the vertical lobe after a very small electrical stimulation (this is what each data point is.) The x-axis shows the time from the beginning of the experiment. At about 30 minutes, MSF-VL neurons were stimulated with a "triplet", which consisted of three pulses in quick succession. As we can see in the control preparation (the blue line,) this w pas not enough to induce LTP, which would be evident as an increase in the field potential. In a preparation treated with 5-HT, however, this stimulation was enough to elicit some LTP, which is apparent as a stable elevation of the recorded field potential at times 50 and 60 minutes. After 60 minutes, each preparation was subject to high-frequency stimulation, which caused maximal LTP in both cases. The bar graph next to it (B) shows the results of multiple experiments, showing that before high-frequency stimulation, the treatment with 5-HT caused an increase in the LTP resulting from the triple-pulse, indicating that the presence of 5-HT made MSF-VL synapses prone to undergo LTP. The second line graph (C) shows the results of a set of similar experiments, except that the stimulation was done once per minute. As is apparent, treatment with 5-HT (shown by the red bar) increased the rate of LTP; however, as indicated in the adjacent bar graph (D), it did not increase the maximum amplitude of LTP.
It's important to remember that in the active nervous system, it's unlikely that synapses are ever stably at a maximal strength. That increase in the rate of induction of LTP, modest though it may seem in this experiment, could be crucial in affecting the functioning of a memory system in a behaving animal. In the "real world", the stimuli involved in learning are often only present for a short time, and the state of any particular synapse in the nervous system is determined by an incredibly complex set of chemical factors. Neuromodulatory activity (like that argued for in this paper) provides a sensitive mechanism by which the functioning of a neural system could be finely coordinated, allowing the integration of a variety of information into one system that can make a timely decision about whether an action was good enough to repeat or bad enough to avoid in the future.
For convenience's sake, I skipped a variety of other interesting experiments that the authors did, and I encourage you to get the paper yourself and read it, if you can. I very much like this type of research, and I like the challenge that blogging about it presents. Anyways, I hope you've enjoyed this as much as I have!
Thanks for reading!
Shomrat T, Feinstein N, Klein M, & Hochner B (2010). Serotonin is a facilitatory neuromodulator of synaptic transmission and "reinforces" long-term potentiation induction in the vertical lobe of Octopus vulgaris. Neuroscience, 169 (1), 52-64 PMID: 20433903
The Octopus research group in Jerusalem is back with a paper in the August issue of Neuroscience about the function of serotonin in the octopus vertical lobe, Serotonin is a facilitatory neuromodulator of synaptic transmission and “reinforces” long-term potentiation induction in the vertical lobe of Octopus vulgaris. I'm very excited to blog about this paper - it's the very first time in my short blogging career that I've gotten to cover a study as it was coming out! You can read my other posts about their work here and here (that second one has a basic description of the technique of stimulation-induced LTP, which I'll be very brief with here.)
Basically, LTP (long-term potentiation) is one of the mechanisms by which neurons are thought to adjust how they connect to each other during the process of learning - specifically, they become stronger (or potentiated,) meaning that signals are carried across the synapse more effectively. The authors of this paper use a technique by which they induce LTP in synapses in the octopus vertical lobe (a structure thought to be involved in learning and memory) and study the effects of serotonin (also called 5-HT, which is short for 5-hydroxytryptamine, the terminology I'll be using from now on) on the properties of the induced LTP. Presumably, this can tell us something about the function of 5-HT in the normal functioning of the vertical lobe, although this point is very debatable.
Why look at 5-HT? Well, for starters, it's one of the big neurotransmitters these days (along with such illustrious nearly-lay-term chemicals as dopamine, norepinephrine, GABA and glutamate.) You hardly need to have a specific reason to study it these days because it's involved in pretty much every process that contemporary neurobiology cares about: consumptive behavior, mood and depression, social cognition, the action of addictive drugs. More than that, though, it's conserved across all bilaterians, the group of bilaterally symmetrical animals including people, the rest of the vertebrates, the insects, and, among many others, the molluscs! If there is any neurotransmitter that is interesting to study comparatively, it's 5-HT, as it's been shown to be involved in learning in animals as distantly related to each other as sea slugs, rats, humans, and (now) cephalopods. If we learn how 5-HT does its job in a wide variety of animals, it will help us understand how neurotransmitters function within nervous systems in general. This is, we will hopefully agree, a Good Thing.
The authors begin with the hypothesis that, as has been shown in Aplysia (a beautiful little sea slug who is relatively widely studied in neuroscience,) 5-HT probably has a role in the modulation of LTP rather than inducing it directly, making it a putative neuromodulator. It is not hard to imagine how this might be a good thing to have in a memory system. Let's pretend that our animals has just been injured, or that it has just found a great big source of food. All of these events call for a general upregulation in the formation of memories, since remembering what happened around these events will help the animal repeat or avoid them in the future, depending on whether they were good or bad. If a chemical can increase the amount of LTP (a process thought to be involved in learning,) it would make sense that it might be selectively secreted or expressed during times when the animal's memory system needs to pay attention to what's going on, and not when there is nothing of consequence happening. This is an extremely limited view of the role of neuromodulators in learning, but it illustrates the principal as well as I know how to. In short, neuromodulators, while not responsible for neurotransmission and plasticity themselves, have some effect on it. This sort of effect is one of the things that allows the great flexibility of neural systems, one of their key features.
In the first part of their study, the authors stained slices of the octopus vertical lobe for 5-HT, and then described what they say - this is good old fashioned neuroscience. They found that 5-HT shows up in fibers from the medial superior frontal lobe (MSF) that innervate large areas of the vertical lobe. The MSF is thought to be one of the main sources of input of sensory information to the vertical lobe, and this tract of fibers (known as the MSF-VL tract) is thought to be involved in the formation of sensory memories in the octopus, as per J. Z. Young's early lesion experiments in the octopus. The authors note that this wide spread of 5-HT is typical of neuromodulators, supporting the idea that MSF neurons use 5-HT to modulate LTP in the vertical lobe.
In the second part of the study, the authors use a technique where they induce LTP in live slices of octopus brain (cool, right?) by repeatedly stimulating the axons running from the MSF to the vertical lobe. They measure the "strength" of neurotransmission as fPSP's, or synaptic field potential, which is roughly an indicator of how much electrical activity is generated by activity in many synapses within a small area of the tissue. I'll only summarize one of their several experiments here, because it is the one that really illustrates the neuromodulatory effect.
This figure shows the results of an experiment using induced LTP in octopus brain slices. The experimenters stimulated the brain slices along the MSF-VL tract and recorded the resultant electrical activity in the VL. Let's start with the first graph. The y-axis shows the amount of activity recorded in the vertical lobe after a very small electrical stimulation (this is what each data point is.) The x-axis shows the time from the beginning of the experiment. At about 30 minutes, MSF-VL neurons were stimulated with a "triplet", which consisted of three pulses in quick succession. As we can see in the control preparation (the blue line,) this w pas not enough to induce LTP, which would be evident as an increase in the field potential. In a preparation treated with 5-HT, however, this stimulation was enough to elicit some LTP, which is apparent as a stable elevation of the recorded field potential at times 50 and 60 minutes. After 60 minutes, each preparation was subject to high-frequency stimulation, which caused maximal LTP in both cases. The bar graph next to it (B) shows the results of multiple experiments, showing that before high-frequency stimulation, the treatment with 5-HT caused an increase in the LTP resulting from the triple-pulse, indicating that the presence of 5-HT made MSF-VL synapses prone to undergo LTP. The second line graph (C) shows the results of a set of similar experiments, except that the stimulation was done once per minute. As is apparent, treatment with 5-HT (shown by the red bar) increased the rate of LTP; however, as indicated in the adjacent bar graph (D), it did not increase the maximum amplitude of LTP.
It's important to remember that in the active nervous system, it's unlikely that synapses are ever stably at a maximal strength. That increase in the rate of induction of LTP, modest though it may seem in this experiment, could be crucial in affecting the functioning of a memory system in a behaving animal. In the "real world", the stimuli involved in learning are often only present for a short time, and the state of any particular synapse in the nervous system is determined by an incredibly complex set of chemical factors. Neuromodulatory activity (like that argued for in this paper) provides a sensitive mechanism by which the functioning of a neural system could be finely coordinated, allowing the integration of a variety of information into one system that can make a timely decision about whether an action was good enough to repeat or bad enough to avoid in the future.
For convenience's sake, I skipped a variety of other interesting experiments that the authors did, and I encourage you to get the paper yourself and read it, if you can. I very much like this type of research, and I like the challenge that blogging about it presents. Anyways, I hope you've enjoyed this as much as I have!
Thanks for reading!
Shomrat T, Feinstein N, Klein M, & Hochner B (2010). Serotonin is a facilitatory neuromodulator of synaptic transmission and "reinforces" long-term potentiation induction in the vertical lobe of Octopus vulgaris. Neuroscience, 169 (1), 52-64 PMID: 20433903
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