Primary afferent projections in a diver, the muskrat

Abstract

In a preliminary search for primary afferent connections involved in the diving response, cutaneous afferents from the nose were traced in muskrats and compared with those in rats, and with projections from the soft palate, posterior pharynx and larynx. Horseradish peroxidase (HRP) was injected into the skin or mucosa, under anesthesia. After 48 h survival, the deeply anesthetized animal was transcardially perfused and the brain was frozen and sectioned transversely in a cryostat. The sections were reacted for HRP according to standard techniques, using tetramethylbenzidine; alternate sections were Nissl stained. HRP-labeled structures were mapped using darkfield photomicrographs and camera lucida drawings. Cutaneous afferents from the nose in the muskrat project densely to layers I-II of the ventral and dorsolateral parts of the caudal subnucleus of the spinal trigeminal nucleus (Sp5C) and sparsely to layers V-VI of Sp5C, sparsely to the ventromedial part of the interpolar subnucleus of the spinal trigeminal nucleus (Sp5I), moderately to the oral subnucleus of the spinal trigeminal nucleus (Sp5O)--particularly the dorsomedial part, possibly overlapping with the nucleus of the solitary tract, and with processes of labeled cells of other lateral facial nucleus extending into ventromedial Sp5O,--moderately to the principal trigeminal nucleus (Pr5); and to the paratrigeminal nucleus (Pa5). Projections in the rat were the same, except that little or no labeling of layers V-VI of Sp5C, dorsomedial Sp5O, or Pa5 was present. Projections from the soft palate to layers I-II of rostral Sp5C, Sp5O, Sp5I, and Pr5 were similar to those from the nose in the muskrat. Heavy projections from the soft palate, and less dense projections from the posterior pharynx and larynx, to Pa5 also were found. Those regions receiving dense projections from the nose, overlapping projections from the various sites, and more highly developed projections from the nose in the muskrat than in the rat, are of particular interest for further investigation of the neural substrate underlying the diving response. The projections traced from the nose correspond particularly with nociceptive and thermoreceptive projections, which suggests that thermoafferent function may be involved in the elicitation of the diving response.