Axosomatic synapses in the visual cortex of adult rat. A comparison between GABA-accumulating and other neurons

Differences in axosomatic synapses between GABA-accumulating [G(+)N] or non-accumulating [G(-)N] neurons have been investigated in the visual cortex of adult rat. The neurons were classified and localized in light microscopic autoradiograms after [3H]GABA injections. The cells resectioned for electron microscopic identification of type 1 synapses (T1S) and type 2 synapses (T2S). A total of 167 neurons [45 G(+)N, 122 G(-)N] situated in laminae II-VI were evaluated. The two groups of neurons were not uniform populations. G(-)N included both pyramidal and non-pyramidal neurons, whereas no typical pyramidal neurons were found among G(+)N. A total of 691 synaptic contacts was evaluated for these groups of somata. The density of synapses was higher on G(+)N than on G(-)N. This was mainly due to a difference in the number of T1S. On G(-)N the frequency distributions of both types of synapses represented Poisson distributions, indicating that there were stochastic variations around mean values. In contrast, on G(+)N the distribution was exponential which suggests that G(+)N include several subpopulations with different densities of T1S. On all cortical neurons the average density of T2S was 509-60 T2S per 1000 microns2 of soma surface, which resembles the density in the neuropil. In contrast, T1S varied from zero to a mean of 12 per 1000 microns2 on G(-)N and to a mean of 51 per 1000 microns2 on G(+)N, i.e. the density of T1S, unlike T2S, is much smaller on neuronal somata than in the surrounding neuropil. It is suggested that the formation of axosomatic T1S, but not of T2S, is suppressed to a variable degree on almost all cortical neurons. Only on pyramidal neurons does the suppression of T1S seem to be complete.