GABAergic axons in the ventral forebrain of the rat: an electron microscopic study

The ventral forebrain, including the ventral striatum, the ventral pallidum and the substantia innominata, is an important region involved in the functions of the basal ganglia and the limbic system, as well as the magnocellular corticopetal neurons of the nucleus basalis of Meynert. Although previous studies have shown that this region is richly innervated by GABAergic fibers, little is known with respect to the relative densities of GABAergic to non-GABAergic axon terminals in this region. To address this issue, we have developed a specific rabbit antiserum to GABA and used a postembedding immunocytochemical reaction to analyse the distribution of GABA-like immunoreactive axon terminals in the rat ventral striatum, ventral pallidum and substantia innominata. Of all axon terminals that form identifiable synapses within single ultrathin sections taken from these regions, 11.6% in the ventral striatum, 85.5% in the ventral pallidum and 64.8% in the substantia innominata were GABAergic. Differences were also found in the distribution patterns of these terminals with respect to the size of their synaptic target dendrites. These findings are consistent with previous findings that a majority of inputs to the ventral striatum are excitatory, and that a majority of inputs to the ventral pallidum are inhibitory. Our results provide a first approximation of the anatomical substrate for the physiology and pharmacology of GABA actions in the ventral forebrain region. These results also show that GABA may play an important role in the substantia innominata, where both the cholinergic and the non-cholinergic magnocellular corticopetal neurons reside within a neuropil innervated by many different non-cholinergic fibers.