Brain ResearchBulletin. Vol. 22, pp. 501-509. Q Pergsmon Press plc, 1989.Printedin the U.S.A. 0361-9230/89 $3.00+ .OO Brainstem Projecting Neurons in the Rat Basal Forebrain: Neurochemical, Topographical, and Physiological Distinctions From Cortically Projecting Cholinergic Neurons KAZUE SEMBA, PETER B. REINER, EDITH G. McGEER AND HANS C. PIBIGER divisive of ~eur~lo~ic~l Sciences, department of Psyc~~at~ University of British Columbia, Vancouver, BC V6T I W 5 Canada Received 4 April 1988 SEMBA, K., P. B. REINER, E. G. MffiEER AND H. C. FIBIGER. Brainstem projecting neurons in the rat b~saf~orebroin: Neurochemical, topographical, and physiological distinctions from cortically projecting cholinergic neurons. BRAIN RES BULL 22(3) 501-509, 1989. -Magnocellular regions of the basal forebrain contain cholinergic neurons that project to the cerebral cortex. Neurons in the same basal forebrain regions innervate the brainstem. The present study investigated whether these brainstem projecting neurons are cholinergic, project also to the cortex, and share similar physiological properties as cortically projecting neurons. Data with retrograde tracing from various regions of the pons, medulla, and cortex combined with choline acetyhransferase immunofluorescence indicated that: 1) brainstem projecting neurons are usually segregated from cortically projecting and/or cholinergic neurons in the basaf forebrain, 2) virtuaily no brainstem projecting neurons in the basal forebrain are cholinergic, and 3) only rarely do basal forebrain neurons have axon collaterals that project to both cortex and brainstem. Extracellutar recordings from basal forebrain neurons confirmed the paucity of axonal collateralization and the topographic segregation between cortically and brainstem projecting basal forebrain neurons, and, in addition, showed that brainstem projecting neurons have a slower mean conduction velocity than cortically projecting neurons. These observations suggest that basal forebrain neurons projecting to the brainstem (pans, medulla) and the cortex represent separate cell populations in terms of projections, neurotransmitter content, distribution, and physiological properties. Basal forebrain Brainstem Choiinergic Retrograde tracing Immuno~uo~~ence Extracellular recordings THE presence of brainstem projecting neurons in the magnocel- lular regions of the basal forebrain was reported as early as 1975 by Divac on the basis of retrograde labeiling following large ho~eradish peroxidase injections into the brainstems of primate and rat (10). He also discussed the possibility that some basal forebrain neurons project to both cortex and brainstem. While relatively little attention has since been paid to the descending projections from the magnocellular basal forebrain, major ad- vances have been made with respect to the ascending projections of the basal forebrain, these firmly establishing that large neurons in basal forebrain project widely to the cerebral cortex, and that the majority of these neurons are cholinergic (4, 34, 49, 53). In addition, cholinergic projections have been demonstrated from the basal forebrain to extracortical structures including the interpedun- cular nucleus (3,52), tbalamus (17, 22,42), amygdala (8, 29, 51), and olfactory bulb (34, 53, 54). In view of these findings, the presence of brainstem projecting neurons in the magnocellular basal forebrain raised the possibility that at least some of the brainstem projecting neurons in this region are cholinergic, and may also project to the cortex. The present study addressed these questions by using both anatomical and physiological techniques. The results suggest that, on the whole, basal forebrain neurons projecting to the cortex, and those projecting to the pons and medulla represent two separate cell populations in terms of distribution, neurotransmitter content, and physiological proper- ties. Preliminary results have been reported (38). METHOD For anatomical experiments, 69 male Wistar rats, 150-300 g in body weight, were used. An additional 28 male Wistar rats, 250-300 g, were used for physiological experiments. Retrograde WGA-HRP Tracing Under chtoral hydrate (400 mglkg. IP) or pentob~bital (50 zyxwvut 501