Neuroscience Letters, 134 (1992) 161-164 161 © 1992 Elsevier Scientific Publishers Ireland Ltd. All rights reserved 0304-3940/92/$ 03.50 NSL 08292 Caudal ventrolateral medullary projections to the nucleus of the solitary tract in the cat Stefanie Roder and John Ciriello Department of Physiology, Health Science Centre, University of Western Ontario, London, Ont. (Canada) (Received 14 August 199 I; Revised version received 24 September 1991; Accepted 27 September 1991) Key words: AI noradrenergic cell group; Commissural nucleus; Cardiovascular reflex pathway; Baroreceptor reflex; Central regulation of the circu- lation The projections of neurons, in and around the A 1 noradrenergic cell group of the caudal ventrolateral medulla (VLM), to nucleus of the solitary tract (NTS) were studied in the cat using the anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L). PHA-L was microiontophoresed into the region of the A1 noradrenergic cell group and after a 7-17 day survival period animals were sacrificed and brainstem sections were processed for PHA-L or tyrosine hydroxylase (TH) immunoreactivity. PHA-L injections within the region of the AI cell group resulted in labelled fibers with their presumptive terminal boutons primarily in the ipsilateral commissural and medial subnuclei of NTS. A light projection to the ipsilateral parvo- cellular, lateral and ventrolateral subnuclei of the NTS complex was also observed. These data demonstrate that neurons in the region of the AI noradrenergic cell group project to regions of NTS that receive cardiovascular afferent inputs and suggest that VLM may influence the activity of neurons in NTS involved in the reflex regulation of the circulation. It has recently been suggested that ventrolateral medulla (VLM) functions as an important component of a neuronal circuit which alters baroreceptor reflex exci- tability [4, 6, 18]. Lesions of VLM in the region of the A1 noradrenergic cell group have been shown to de- crease the gain of the baroreceptor reflex [19]. These re- sults have been interpreted to suggest that the A 1 norad- renergic neurons normally act to facilitate the barorecep- tor reflex [19]. Evidence for the existence of A 1 noradre- nergic projections to the nucleus of the solitary tract (NTS) complex is limited. In the rabbit, Blessing et al. [2] observed that after lesions of the dorsomedial medulla the fluorescence intensity of the A1 group of catecholamine neurons was increased, and swollen axons could be seen coursing from the caudal VLM towards the lesions on the same side. However, after lesions that destroyed most of the A1 cells, no detectable decrease in the number of fluorescent catecholamine nerve endings in NTS was seen [2]. Most of the evidence for a projec- tion to the NTS complex from the caudal VLM comes from retrograde transport studies in the rat [11, 14-16], in which the injections of the tracer not only covered Correspondence." J. Ciriello, Department of Physiology, Health Sci- ences Centre, University of Western Ontario, London, Ont. Canada N6A 5C1. Fax: (l) 519-661-3827. most of the NTS complex but also adjacent areas. Elec- trophysiological evidence of a direct VLM projection to NTS exists in the cat [4]. However, a detailed mapping of which subnuclei in NTS receive a caudal VLM input was not provided in this study. Therefore, the present study was done to investigate the distribution of fibers and terminal boutons in the NTS complex originating in the region of the A1 cell group of the VLM using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in the cat. A preliminary account of these data has been presented elsewhere [13]. Experiments were done in 6 adult cats (2.5-5.0 kg) of either sex, under pentobarbital sodium anesthesia (Som- notol, M.T.C. Pharmaceuticals, Hamilton, Canada; 35 mg/kg, i.p.). The head of the animal was fixed in a Kopf stereotaxic instrument, flexed approximately 45 ° for- ward and the dorsal surface of the brainstem was exposed at the level of the atlantooccipital junction. A glass micropipette (internal tip diameter 10-50 pm) filled with 2.5% PHA-L in 10 mM potassium phosphate-buf- fered saline (PBS; pH 7.2-7.4) was lowered stereotaxi- cally into the caudal VLM using obex as the point of reference. The PHA-L was iontophoresed as previously described by Gerfen and Sawchenko [9]. The current (5 pA, cathodal) was delivered through a Grass PSIU stim- ulus isolation unit using 7 s pulses every 14 s for 20-30 min. The animals were allowed to recover from the anes-