Glutamate decarboxylase-immunoreactive neurons and terminals in the periaqueductal gray of the rat

The periaqueductal gray of 5 rats was processed for immunocytochemistry using an antiserum to glutamate decarboxylase. In both colchicine-pretreated (4 rats) and untreated (1 rat) animals, glutamate decarboxylase-positive cell bodies were present in all periaqueductal gray subdivisions, especially in the dorsal and ventrolateral subdivision. The perikaryal cross-sectional area of labelled neurons was smaller than that of periaqueductal gray projecting neurons retrogradely labelled with horseradish peroxidase in separate experiments. The morphology of glutamate decarboxylase-containing neurons resembled that of small polygonal, triangular and fusiform cells described in previous Golgi studies. Glutamate decarboxylase immunoreactivity was also observed in a large number of terminal-like structures, most of which were distributed close to the somata and dendrites of both glutamate decarboxylate-positive and -negative neurons. At all rostrocaudal levels the highest concentration of these elements was observed around the aqueduct. These results suggest that two sub-populations of neurons are present in the periaqueductal gray of rats, one consisting of small-sized glutamate decarboxylase-positive neurons (intrinsic neurons) and the other of large-sized glutamate decarboxylase-negative neurons (projecting neurons). Intrinsic circuits could be present between glutamate decarboxylase-positive and -negative neurons and between glutamate decarboxylase-positive neurons.