Histaminergic synaptic transmission in the cerebral ganglion of Aplysia.

Standard intracellular stimulating and recording techniques including voltage-clamp were used to analyze the synaptic responses mediated by two identifiable histamine-containing neurons (HCNs), designated C2 neurons, located in bilaterally symmetric clusters of the isolated cerebral ganglion of Aplysia california. Activation of each C2 induced unitary chemically mediated synaptic potentials in over 15 identified ipsilateral follower neurons. Several additional followers were connected to the HCNs by nonrectifying electrical synapses. Most of the follower neurons examined received only chemical synapses from the C2s. Some of the followers were reciprocally connected with each other through nonrectifying electrical synapses. A single C2 action potential can evoke six distinctive types of chemically mediated postsynaptic potentials (PSPs) in different follower neurons. Most of the PSPs have been shown to be multicomponent, i.e., they are comprised of various combinations of individual fast (less than or equal to 150 ms), slow (1-2 s), and very slow (greater than or equal to 4 s) depolarizing and hyperpolarizing components. The combination of these components produces PSPs of varying complexity, from simple monophasic responses such as the frequently observed slow excitatory PSPs and slow inhibitory PSPs to responses consisting of two to three components such as fast excitatory, slow inhibitory PSPs or fast inhibitory, slow excitatory PSPs. All of the multicomponent PSPs appear to be mediated through monosynaptic connections from the C2, as determined by various electrophysiological criteria. The slow and very slow synaptic components of the multicomponent PSPs were markedly potentiated in amplitude and duration after repetitive C2 activation. This property of the slow components permits the slower PSPs to exert a major influence on the excitability and integrative properties of the follower neurons.