Differential distribution of GABA- and serotonin-containing afferents on an identified central neuron

Abstract

The distributions of γ-aminobutyric acid (GABA)- and serotonin (5-HT)-containing terminals impinging on the surface of the Mauthner (M-) cell were studied at the light microscopic level using double immunofluorescent labeling and were compared with that of the glycine receptor. The latter was visualized indirectly, using a monoclonal mouse antibody which recognizes its 93-kDa associated protein. This neuron has two large principal dendrites: one extending ventrorostrally (ventral dendrite) and the other dorsolaterally (lateral dendrite). There are also two other classes of smaller processes: one that projects ventrally (small ventral dendrites) and one penetrating in the axons cap (cap dendrites), a peculiar neuropil surrounding the initial segment of the M-cell axon. A cellular regionalization of these afferent systems was found: GABA boutons, labeled for glutamic acid decar☐ylase (GAD), were localized preferentially on the lateral dendrite while 5-HT-filled endings predominated on the ventral one. The density of these two classes of inputs was comparable in the other areas of the M-cell: less of their terminals were in contact with the soma outside the axon cap, and more numerous boutons, which presented either GABA or 5-HT immunoreactivities, were apposed to the small ventral dendrites. This preferential pattern of innervation differed with the ubiquitous presence of glycine receptor clusters on the M-cell membrane. Finally no evidence of a colocalization of GABA and 5-HT in afferent endings was detected at any portion of the M-cell. Since the studied networks are involved in the inhibitory control of this neuron, and in its modulation, our results are discussed in relation with possible heterosynaptic interactions between them, and between other similarly segregated excitatory inputs.