Ultrastructural analysis of ventrolateral periaqueductal gray projections to the A7 catecholamine cell group

Stimulation of neurons in the ventrolateral periaqueductal gray produces an tinociception that is mediated in part by pontine noradrenergic neurons. Pr evious light microscopic analysis provided suggestive evidence for ii direc t projection from neurons in the ventrolateral periaqueductal gray to norad renergic neurons in the A7 cell group that innervate the spinal cord dorsal horn. Therefore, the present ultrastructural study used anterograde tracin g combined with tyrosine hydroxylase immunoreactivity to provide definitive evidence that neurons in the ventrolateral periaqueductal gray form synaps es with the somata and dendrites of noradrenergic neurons of the A7 cell gr oup. Injections of the anterograde tracers biotinylated dextran amine or Ph aseolus vulgaris leucoagglutinin into the ventrolateral periaqueductal gray of Sasco Sprague-Dawley rats yielded a dense innervation in the region of the lateral pens containing the A7 cell group. Electron microscopic analysi s of anterogradely labeled terminals (n = 401) in the region of the A7 cell group indicated that approximately 10% of these formed plasmalemmal apposi tions to tyrosine hydroxylase-immunoreactive dendrites with no intervening astrocytic processes. About 23% of these were asymmetric synapses, 10% were symmetric synapses, and 67% did not exhibit clearly differentiated synapti c specializations. The majority of anterogradely labeled terminals (60%) fo rmed plasmalemmal appositions with dendrites and somata that lacked detecta ble tyrosine hydroxylase immunoreactivity. About 35% of these were symmetri c synapses, 9% were asymmetric synapses and 56% did not form synaptic speci alizations. Approximately 30% of all anterogradely labeled terminals displa yed features characteristic of axo-axonic synapses. The present results provide direct ultrastructural evidence to support the hypothesis that the analgesia produced by stimulation of neurons in the ven trolateral periaqueductal gray is mediated. in part. by activation of spina lly projecting noradrenergic neurons in the A7 catecholamine cell group. (C ) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.