The marine snail Aplysia is perhaps best known for its Nobel-award-winning learning ability. In the classic work, a small water jet to the body of the animal was paired with electric shock to accomplish classical (or Pavlovian) conditioning. Since about the 1930s, it has been a longstanding debate among psychologists whether classical and operant conditioning can be explained by similar underlying learning mechanisms. To answer this question, new conditioning protocols have been established in Aplysia, to make them as comparable as possible.
The feeding behavior of this snail can be conditioned both operantly and classically. As you can see in this video:
Aplysia snails bite spontaneously and if every bite is followed by a reward (either a squirt of seaweed juice or an electric stimulus to the nerve mediating food reward), the animal will learn to bite more, even in the absence of food. This protocol thus establishes operant conditioning. Classical conditioning is established when you touch the lips of the animal with a brush and then feed the animal, irrespective of what it did before or after the brush stimulus. The effect of both protocols on feeding is the same: feeding is enhanced, either spontanous feeding (operant) or feeding elicited by a brush stimulus (classical).
The nice thing about Aplysia is that you can take its nervous system out, place it in a dish and still perform experiments. Using these techniques, researchers have found out that neuron B51 is critical for these kinds of learning: if B51 fires, the animal is feeding, if B51 is not firing, even if the mouthparts move, it is not a feeding movement. Thus, not surprisingly, operant learning makes B51 more likely to fire (Brembs et al. 2002). In classical conditioning, however, it’s the reverse: B51 is less likely to fire, even though there is more feeding after conditioning. Apparently, this can be explained by the sensory input from the brush to B51 being so potentiated, that it overcomes the reduction in B51 excitability.
How can two memories form in a single neuron? This question can now also be tackled, because then graduate student Fred Lorenzetti developed a single-neuron analog of both classical and operant conditioning. In operant conditioning the neuron is being depolarized until it fires, just as it would be during feeding, and then a puff of dopamine is applied, just as the esophageal nerve mediating the food-reward would do. In classical conditioning, the neurotransmitter Acetylcholine (ACh) is puffed onto the neuron (mimicking the brush-CS) and then dopamine (US).
Using this technique, Lorenzetti et al. have now found out that after such CS-US pairing in the dish, B51 responded more strongly to ACh puffs (just as intact animals responded with feeding to brush strokes), while general excitability of B51 was reduced, just like after classical conditioning of the intact animal. The D1-like dopamine receptor that mediates the changes after operant conditioning, doesn’t seem to be involved in the changes brought about by classical conditioning, as driving it with an agonist failed to bring about the changes in B51.
Thus, it seems as if dopamine acts through different receptors depending on what just happened prior to dopamine application: if the neuron fired during feeding, a general increase in excitability is observed, if the neuron is not firing, but ACh is applied, then a different dopamine receptor is activated and leads to a decrease in excitability.
Lorenzetti, F., Baxter, D., & Byrne, J. (2011). Classical Conditioning Analog Enhanced Acetylcholine Responses But Reduced Excitability of an Identified Neuron Journal of Neuroscience, 31 (41), 14789-14793 DOI: 10.1523/JNEUROSCI.1256-11.2011
Tags: Aplysia, classical, conditioning, dopamine, operant
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