DISTRIBUTION OF BRAIN-STEM PROJECTIONS FROM SPINAL LAMINA-I NEURONS IN THE CAT AND THE MONKEY

The distribution of terminal projections in the brainstem from lamina I neurons in the spinal dorsal horn was investigated with the anterogr ade tracer Phaseolus vulgaris-leucoagglutinin in the cat and the cynom olgus monkey. Iontophoretic injections made with physiological guidanc e were restricted to lamina I or to laminae I-III in the cervical (C6- 8) or lumbar (L6-7) enlargement. The distribution of terminal labeling was essentially identical in the cat and the monkey, although consist ently of greater intensity in the monkey. Terminations were observed i n the solitary nucleus, the dorsomedial medullary reticular formation, the entire rostrocaudal extent of the ventrolateral medulla, the locu s coeruleus, the subcoerulear region and the Kolliker-Fuse nucleus, th e lateral and medial portions of the parabrachial nucleus, the cuneifo rm nucleus, the ventrolateral and lateral portions of the periaqueduct al gray, and the intercollicular nucleus. Lamina I terminations were g enerally bilateral in the medulla but more dense contralaterally in th e pens and mesencephalon. The density and laterality of labeling in th e medulla varied between cases independently from that in the pens and mesencephalon, suggesting that the lamina I projections to these regi ons may originate from different subsets of neurons. A clear topograph ic organization was observed only in the lateral column of the periaqu eductal gray, where lumbar lamina I terminations were found caudal to cervical terminations. These observations indicate that spinal lamina I neurons project to a variety of brainstem sites involved in autonomi c (cardiovascular, respiratory) and homeostatic processing and the con trol of behavioral state. These projections provide an afferent substr ate for spino-bulbospinal somatoautonomic reflex arcs activated by noc iceptive, thermoreceptive activity and for a spino-bulbo-hypothalamic relay of such activity by cells in the caudal ventrolateral medulla. T hese observations support the general concept that lamina I projection s distribute modality-selective sensory information relevant to the ph ysiological status and maintenance of the tissues and organs of the en tire organism. (C) 1995 Wiley-Liss, Inc.