Enkephalin-like immunoreactivity in the cat superior colliculus: distribution, ultrastructure, and colocalization...

The distribution of enkephalin (ENK) immunoreactivity has been examined in the cat superior colliculus (SC) by means of light and electron microscope immunocytochemistry. The antisera were directed against leucine enkephalin but also recognized methionine enkephalin. Colocalization of ENK with gamma aminobutyric acid (GABA) was studied with a two-chromagen double-labeling technique. Enkephalin antiserum labeling was highly specific. Dense neuropil labeling was found only in a thin band 75-100 microns wide within the upper superficial gray layer of SC. Negligible neuropil labeling was seen deeper, except for patches of label within the intermediate gray layer. Intensely labeled neurons also had a specific distribution. Forty-seven percent were located within the upper 200 microns of SC, 40% within the deep superficial gray layer, 11% in the optic layer, and only 2% below that layer. Almost all ENK-labeled cells were small (mean area of 117 microns2). Some of these had horizontal fusiform cell bodies and horizontally oriented dendrites. Others had small round somata and thin, obliquely oriented dendrites. In double-labeling experiments, 18% of anti-ENK-labeled cells were also immunoreactive for GABA. Four distinct types of ENK-labeled profile were identified with the electron microscope. Presynaptic dendrites (PSD) with loose accumulations of synaptic vesicles were densely labeled with the antiserum. Conventional dendrites were also labeled. Both types of labeled profile received input from unlabeled synaptic terminals, including those from the retina that contained pale mitochondria and round synaptic vesicles and formed asymmetric synaptic contacts. Retinal terminals were never labeled with the antisera. However, some axon terminals with round synaptic vesicles, dark mitochondria, and symmetric synaptic densities were labeled by the antisera, as were some thinly myelinated axons. These results show that there is a small population of enkephalinergic neurons in the cat SC, some of which also contain GABA. Because not all cells with identical morphologies were double labeled, it appears that neurons of like morphology are chemically heterogeneous.