The fine structure of the caudal periaqueductal gray of the cat: morphology and synaptic organization of normal and immunoreactive enkephalin-labeled profiles.

Although the midbrain periaqueductal gray (PAG) is thought to have a major role in an endorphin-mediated analgesia system, little is known about its neuroanatomical organization. To determine the microcircuitry within the PAG through which exogenous and endogenous opiates may act, we analyzed the synaptic organization of normal and immunoreactive enkephalin (ENK)-labeled profiles in the caudal PAG, a region of particular interest because of its effectiveness in generating analgesia. Examination of the normal fine structure of this region demonstrated that there is no characteristic synaptic morphology that distinguishes individual regions of the caudal PAG (ventromedial, ventrolateral and dorsolateral) from one another. In all 3 regions of the caudal PAG, axodendritic synapses are the predominant form of synaptic interaction making up 93-97% of all synapses counted. Axosomatic synapses are much less common, as are presumed axoaxonic and dendrodendritic synapses. In the caudal ventral PAG, the largest population of ENK-labeled axonal boutons are found presynaptic to unlabeled, centrally placed dendrites. Much less frequently, immunoreactive ENK-containing boutons are found presynaptic to neuronal perikarya or vesicle-containing profiles. Thus, these results suggest that the dendrites of neurons intrinsic to the PAG are the most probable site of opiate action in the caudal ventral PAG.