IDENTIFICATION OF LAMINA-V AND LAMINA-VII INTERNEURONS PRESYNAPTIC TOADRENAL SYMPATHETIC PREGANGLIONIC NEURONS IN RATS USING A RECOMBINANTHERPES-SIMPLEX VIRUS TYPE-1

Although indirect evidence suggests that the control of sympathetic pr eganglionic neurons is mediated to a great extent through interneurons , little is known about the location, morphology or neurotransmitter p henotype of such interneurons. This limitation seriously impedes our u nderstanding of spinal synaptic circuits crucial to control of arteria l pressure and other visceral functions. We used a highly neurotropic, minimally cytopathic recombinant herpes simplex virus type-1 to study spinal ''sympathetic'' interneurons labelled by trans-synaptic transp ort of the virus from the adrenal gland in rats. Approximately 120-320 infected neurons/rat were identified by immunocytochemical detection of the viral antigen. We distinguished between virus-infected pregangl ionic neurons and infected interneurons by (i) their location within t he spinal laminae, (ii) their size and shape and (iii) the presence or absence of immunoreactivity for the acetylcholine-synthesizing enzyme , choline acetyltransferase, a marker of sympathetic preganglionic neu rons. Virus-labelled sympathetic preganglionic neurons were found with in the known spinal preganglionic nuclei. Non-cholinergic, virus-label led neurons were located throughout lamina VII and in the ventral port ion of lamina V. These putative interneurons were found in the major s pinal preganglionic nuclei, usually intermingled with the preganglioni c neurons. Sometimes, they were located in clusters separate from the preganglionic neurons. The interneurons were approximately 15 mu m in diameter, smaller than the average preganglionic neuron (diameter=25 m u m), and had a few fine processes emanating from them. These non-chol inergic interneurons constituted approximately one-half of the populat ion of virus-infected neurons. In summary, with the use of a recombina nt herpes simplex virus, we identified a large number of non-cholinerg ic interneurons close to, or intermingled with, adrenal sympathetic pr eganglionic neurons. The neurotransmitter phenotype of these neurons r emains to be determined but they likely integrate much of the supraspi nal and primary afferent inputs to spinal preganglionic neurons that c ontrol arterial pressure and other visceral functions. (C) 1998 IBRO. Published by Elsevier Science Ltd.