Electron microscopic autoradiography of the uptake of [3H]GABA in dispersed cell cultures of rat cerebellums. I. The morphology of the GABAergic synapse.

Dispersed cell cultures of rat cerebellums 21 days in vitro were incubated in saline containing 0.5 micrometer [3H]GABA for 5 min, fixed and prepared for electron microscopic autoradiography. Quantitative methods for analyzing the resulting autoradiographs were developed to distinguish lightly labeled neurons from heavily labeled (i.e. GABAergic) neurons. After a 3 week exposure, 2% of the presynaptic elements were found to be GABAergic. A longer incubation time of 15 min resulted in a greatly increased grain density over lightly labeled neurons, making it more difficult to distinguish them from the GABAergic neurons. When the concentration of [3H]GABA was increased to 1.0 micrometer, the grain densities over both GABAergic and other neurons increased to a similar extent. The morphology of the GABAergic synapses differed from that of the total population synapses. The GABAergic synapses had larger cross-sectional areas, lower densities of vesicles, greater numbers of vesicles, and thinner postsynaptic densities. Both the GABAergic and total population had similar lengths of the postsynaptic densities, cleft widths, vesicle sizes and types of postsynaptic elements. The morphology of the GABAergic synapse reported here is similar to that described for basket and stellate presynaptic elements in vivo.