PHARMACOLOGICALLY INDUCED ELEMENTS OF THE HUNTING AND FEEDING-BEHAVIOR IN THE PTEROPOD MOLLUSK CLIONE-LIMACINA .2. EFFECTS OF PHYSOSTIGMINE

1. A contact of the pteropod mollusk Clione limacina with its prey (small pteropod mollusk Limacina helicina) evokes a complex pattern of hunting and feeding behavior. protraction of tentacles to seize the prey, activation of buccal apparatus to swallow the prey, activation of locomotor system (speeding up of wing beating), reversal of reaction to tactile stimulation of the head, loss of normal (vertical) orientation in space, and swimming in circles. After injection of physostigmine (PhS), the acetylcholinesterase inhibitor, into the hemocoel of intact Clione, all these manifestations of the hunting and feeding behavior could be evoked by tactile stimulation of the head, or they arose spontaneously. 2. In the preparation of the isolated CNS, the effect of PhS on the neural networks controlling different aspects of the hunting and feeding behavior was studied by recording from neurons monitoring activity of different networks (less-than-or-equal-to 4 neurons simultaneously). Tactile stimulation of the head was mimicked by a short-term electrical stimulation of the corresponding nerve. Before PhS application, the nerve stimulation evoked elements of the defense reaction, i.e., long-lasting inhibition of all main motor control systems: the locomotor network in the pedal ganglia, the tentacle control network in the cerebral ganglia, and the network controlling radula and hook movements in the buccal ganglia. However, after PhS application, the same stimulus evoked a long-lasting bout of excitation in all the three networks accompanied by activation of the heart-exciting neuron as well as by a modification of the activity of statocyst receptor cells controlling Clione's spatial orientation (the ''fictive hunting bout''). Similar hunting bouts could arise spontaneously. 3. Injection of acetylcholine (ACh) into the hemocoel of intact Clione was less effective than injection of PhS. After ACh injection, reversal of reaction to head stimulation was observed in less-than-or-equal-to 20% of the experiments (the percentage of positive results was higher if ACh was injected into the head just over the CNS). Bath application of ACh to the isolated CNS did not produce the hunting bouts. However, a short-term local application of ACh to the cerebral ganglia in the isolated CNS resulted in activation of the main motor systems controlling locomotion, protraction of tentacles, and movements of buccal mass. 4. During spontaneous PhS-induced bouts, excitation of different networks involved in hunting behavior was sometimes not quite synchronous. Different networks could be excited in variable order over a period of up to several seconds. Sometimes the bouts did not contain all components typical of hunting behavior. These findings suggest that generation of long-lasting bouts of activity, either spontaneously or in response to a brief stimulus, is a property of each of the networks. Synchronization of the bouts arising in different networks may be due to mutual excitatory interconnections between networks. It is supposed that PhS, in addition to reversal of reaction to head/head nerve stimulation, increases interactions between different networks. This supposition is supported by the fact that after PhS application, the hunting bouts could be produced by stimulation of some individual cells from different motor systems. 5. A possible role of cholinergic mechanisms in the control of hunting and feeding behavior in Clione is compared with the role of gamma-aminobutyric acid (GABA)ergic and serotonergic mechanisms.