PERIAQUEDUCTAL GRAY-MATTER PROJECTION TO THE PARABRACHIAL NUCLEUS IN RAT

The efferent projections from the periaqueductal gray matter (PAG) to the parabrachial nucleus (PB) were studied in the rat following microi njections of the anterograde axonal tracer Phaseolus vulgaris-leucoagg lutinin (PHA-L) into restricted regions of the PAG. The dorsomedial an d dorsolateral PAG columns project almost exclusively to the superior lateral PB subnucleus, whereas the lateral and ventrolateral PAG colum ns project to five lateral PB sites: dorsal lateral subnucleus, medial and lateral crescent areas (which flank the dorsal lateral PB subnucl eus), central lateral subnucleus (rostral portion), and superior later al subnucleus. The PAG region lying near the cerebral aqueduct project s to five lateral PB sites: external lateral subnucleus (inner subdivi sion), medial and lateral crescent areas, central lateral subnucleus ( rostral portion), and dorsal lateral subnucleus. The internal lateral PB subnucleus, which projects exclusively to the intralaminar thalamic nuclei, and the Kolliker-Fuse nucleus were not innervated by the PAG. T he PAG selectively innervates individual PB subnuclei that may be p art of the spino-parachio-forebrain pathway. All PAG columns, includin g the aqueductal region, project to the superior lateral PB subnucleus , a presumed nociceptive relay site that receives inputs from multiple spinal cord regions (laminae I, V, and VIII) and projects to the vent romedial and retrochiasmatic hypothalamic areas-two regions that have been implicated in complex goal-directed behavior (e.g., food intake a nd reproductive function). Earlier studies demonstrated that the dorsa l lateral and external lateral PB subnuclei (inner division) receive o verlapping inputs from the superficial dorsal horn (laminae I and II) and the nucleus tractus solitarius, and both PB subnuclei send project ions to limbic forebrain areas (e.g., hypothalamus, preoptic region, a mygdala). Because the PAG projects to both of these PB subnuclei, this projection system possibly functions as a behavioral state-dependent filter system that modulates ascending nociceptive and/or visceral inf ormation as it is relayed through the PB to forebrain sites. (C) 1998 Wiley-Liss, Inc.