GABA AND GLYCINE IN THE CENTRAL AUDITORY-SYSTEM OF THE MOUSTACHE BAT - STRUCTURAL SUBSTRATES FOR INHIBITORY NEURONAL ORGANIZATION

The distribution and morphology of neurons and axonal endings (puncta) immunostained with antibodies to gamma-aminobutyric acid (GABA) and g lycine (Gly) were analyzed in auditory brainstem, thalamic, and cortic al centers in the mustache bat. The goals of the study were (1) to com pare and contrast the location of GABAergic and glycinergic neurons an d puncta, (2) to determine whether nuclei containing immunoreactive ne urons likewise have a similar concentration of puncta, (3) to assess t he uniformity of immunostaining within a nucleus and to consider regio nal differences that were related to or independent of cytoarchitectur e, and (4) to compare the patterns recognized in this bat with those i n other mammals. There are nine major conclusions. (1) Glycinergic imm unostaining is most pronounced in the hindbrain. (2) In the forebrain, GABA alone is present. (3) Some nuclei have GABAergic or glycinergic neurons exclusively; a few have neither. (4) Although there is sometim es a close relationship between the relative number of immunopositive neurons and the density of the puncta, just as often there is no parti cular correlation between them; this reflects the fact that many GABAe rgic and glycinergic neurons project beyond their nucleus of origin. ( 5) Even nuclei devoid of or with few GABAergic or glycinergic neurons contain relatively abundant numbers of puncta; some neurons receive ax osomatic terminals of each type. (6) In a few nuclei there are physiol ogical subregions with specific local patterns of immunostaining. (7) The patterns of immunostaining resemble those in other mammals; the pr incipal exceptions are in nuclei that, in the bat, are hypertrophied ( such as those of the lateral lemniscus) and in the medial geniculate b ody. (8) Cellular colocalization of GABA and Gly is specific to only a few nuclei. (9) GABA and glutamic acid decarboxylase (GAD) immunostai ning have virtually identical distributions in each nucleus. Several i mplications follow. First, the arrangements of GABA and Gly in the cen tral auditory system represent all possible patterns, ranging from mut ually exclusive to overlapping within a nucleus to convergence of both types of synaptic endings on single neurons. Second, although both tr ansmitters are present in the hindbrain, glycine appears to be dominan t, and it is often associated with circuitry in which precise temporal control of aspects of neuronal discharge is critical. Third, the audi tory system, especially at or below the level of the midbrain, contain s significant numbers of GABAergic or glycinergic projection. neurons. The latter feature distinguishes it from the central visual and somat ic sensory pathways. (C) 1995 Wiley-Liss, Inc.