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1. The intracellular mechanism of heterosynaptic facilitation (HSF) formation in identified neurons from the snail Planorbis corneus has been studied. 2. Facilitation of excitatory postsynaptic currents (EPSC) were induced by (a) stimulation of pallial nerve, and (b) addition to extracellular saline of serotonin, NaF, papaverine, theophylline, caffeine or dibutril-cAMP. 3. A depression of EPSC in solutions containing tolbutamide, a cAMP-dependent protein kinase inhibitor was observed. 4. In some cases the similar facilitation or depression of the current induced by acetylcholine application (ACh-current) was found in the same neuron. 5. The effects on ACh-current were distorted in solutions containing caffeine, a well-known activator of calcium ions release from the intracellular depot. 6. According to our findings, we suggest that adenylate cyclase activity of postsynaptic cells could underlie the formation of HSF and it is likely that this activity was modulated by intracellular concentration of calcium ions.

Changes in the EPSC and Ach-current amplitudes of Planorbis corneus LC-1 and RC-1 neurons has been comparatively investigated after the influence on their adenylate cyclase system in order to reveal postsynaptic mechanisms of the heterosynaptic facilitation. Both responses are n-cholinergic and depend on the membrane conductivity for Na+ and K+. Application of 5-HT has led to an increase of the EPSC and ACh current (in most cases) amplitudes by 100-300%. A negligible increase of the EPSC and at the same time a decrease of the Ach-current were observed in 30% of cells. It was, probably, a result of different contribution made by Na+ and K+ to the activation mechanism of the channel-receptor complex conductivity of the nonsynaptic cell membrane. Effects of 5-HT on EPSCs and Ach-current were imitated by actions of the phosphodiesterase blockers and adenylate cyclase activators. Both the blockers and activators depressed the EPSCs and Ach-current. Thus, activation of the adenylate cyclase system by serotonin has promoted development of the postsynaptic mechanisms of heterosynaptic facilitation in command neurons of Planorbis corneus.

Monoamines (NE, DA) were studied for their effect on heterosynaptic facilitation of n-cholinergic synaptic transmission in identified giant neurons of the cerebral ganglion of Planorbis corneus. It was shown that NE and DA blocked n-cholinergic EPSP and facilitated acetylcholine potential evoked by ionophoretic acetylcholine application. The monoamines blocked heterosynaptic facilitation as well. Washing of the monoamines after blocking heterosynaptic facilitation evoked recovery and long maintenance of the synaptic facilitation. It is concluded that DA and NE release may be the mechanism for conducting the intensity and duration of the heterosynaptic facilitation in the natural activity of the central nervous system of molluscs.

It is shown that heterosynaptic facilitation of the synaptic transmission in the giant neurons of the cerebral ganglion in Planorbis corneus develops due to the diffuse neurohumoral effects on the pre- and postsynaptic structures but not as a result of the local synaptic influences on the presynaptic mechanisms. Using the new biological object it has been confirmed that serotonin from the perfuse solution was a source of the facilitatory action on the n-cholinergic synaptic transmission. It was established that conditioning stimulations facilitated not only the n-cholinergic synaptic transmission but also the ionophoretic acetylcholine potentials. Amplitudes of EPSPs and acetylcholinic potentials increased 4-6 times, while the postsynaptic membrane impedance increased only by 20%. Based on such data it is summarized that sensitization of n-cholinergic receptors of the postsynaptic membrane contributed much to the heterosynaptic facilitation. The possible role of n-cholinergic receptors of the postsynapcit membrane in the processes of the heterosynaptic facilitation and of the conditional training is discussed.

Heterosynaptic facilitation evoked in the giant neuron of the Planorbis corneus cerebral ganglion by stimulation of the remote visceral nerve was more effective than facilitation achieved by paired stimulation of cerebral nerves. Its effectiveness depended not on the efficacy of the heterosynaptic input to the giant neuron, but on the strength of the connections between the chosen heterosynaptic input with neurons surrounding the giant cell. This finding as well as prolonged latency and slow rise-time of facilitation may indicate that the latter is of neurosecreory origin. The existence of a mechanism of drastic increase in the efficacy of synaptic connections may play an important role in animal habituation and during conditioning in particular, though such plasticsynaptic changes do not reflect the specificity of the conditioned reflex.