Immunohistochemistry of Diverging and Converging Neurotransmitter Systems in Mollusks.

Abstract

This series of studies was undertaken to compare the distribution of several transmitter-specific neuron systems in the nervous systems of the marine mollusks, Aplysia californica and Pleurobranchaea californica. Several specimens of each of the major ganglia of both species were sectioned serially, and each series was stained immunohistochemically to reveal one of the neuron systems. The present paper reports the results of stainings for acetylcholine, histamine, serotonin, gamma-aminobutyric acid (GABA), vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), Phe-Met-Arg-Phe-NH2 (FMRFamide), and small cardioactive peptide B (SCPB). For all the transmitter-specific sets of neurons examined, relatively few neurons send diverging projections to large areas of the neuropil in one or more ganglia. Moreover, different transmitters converge onto similar areas, and several transmitters evidently project onto the same identified neuron. Many of these diverging and converging projections are sufficiently extensive and overlapping that they are unlikely to be specific for a particular motorpattern. The overall findings are consistent with our previous neurophysiological data. These indicate that activity does not necessarily arise through circuit-specific, identifiable connections. Instead, appropriate response patterns, often containing mixtures of several behaviors, emerge variably through diffuse connections. These findings are also consistent with recent reports from other laboratories indicating that even light topical stimulation generates highly distributed neural activity, and that the responses of identifiable neurons are not constant. The anatomical data presented here should be combined with physiological experiments aimed at verifying the neurotransmitter action of the substances examined and elucidating the role of these agents in controlling the dynamics of neuronal responses.