Distribution of GABA immunoreactivity in the retino-recipient layer of the viper optic tectum. A light and electron microscope quantitative study

The distribution of GABA-immunoreactivity was investigated in the principal retino-recipient layer of the optic tectum in the snake Vipera aspis. This layer, the stratum griseum et fibrosum superficiale, contained an important proportion (approximately 50%) of small GABA-immunoreactive interneurons, characterized by a voluminous invaginated nucleus surrounded by a thin rim of cytoplasm poor in organelles and occasionally showing pleiomorphic synaptic vesicles, which could also be observed in some of the dendrites that contained synaptic vesicles. In the neuropile, the GABA-immunoreactive profiles containing synaptic vesicles could be subdivided into dendrites containing synaptic vesicles and axon terminals with pleiomorphic synaptic vesicles. The dendrites containing synaptic vesicles (23.4% of all profiles containing synaptic vesicles) were postsynaptic either to optic terminals (39.2%), GABA-immunoreactive axon terminals with pleiomorphic synaptic vesicles (48.2%) or to immunonegative (S₁) boutons with round synaptic vesicles (12.6%). These dendrites were presynaptic to GABA-immunoreactive (18%) neurons or immunonegative (82%) neurons. The axon terminals with pleiomorphic synaptic vesicles, which represented 47.4% of all profiles, were predominantly (99%) GABA-immunoreactive and four types could be distinguished according to cytological criteria. These axon terminals made synaptic contacts for the most part (78%) with immunonegative profiles, and more rarely (22%) with immunoreactive neurons. These data are compared to those previously obtained in the homologous structure of other vertebrate species, birds and mammals in particular.