Autoradiographic localization of 5HT₁ binding sites in autonomic areas of the rat dorsomedial medulla oblongata

Abstract

Serotonin (5HT) binding sites in autonomic portions of the dorsomedial medulla oblongata of the rat were localized using autoradiographic techniques with radioactive ligands that express high affinity for the 5HT1 (3H‐5HT), 5HT1A (3H‐8OH‐DPAT), or 5HT1B (125I‐CYP with isoproterenol) receptor subtypes. 5HT1A sites were densely distributed in the nucleus tractus solitarius (NTS), with the highest densities localized to the interstitial subnucleus and the central subnucleus. 5HT1B sites were also found in the NTS, with the highest densities localized to the substantia gelatinosa subnucleus. The dorsal motor nucleus of the vagus nerve and nucleus ambiguus exhibited low densities of 5HT1B sites. However, the nucleus intercalatus, a cerebellar relay nucleus that also contains dendrites of vagal parasympathetic preganglionic neurons and receives autonomic forebrain afferent input, showed very dense 5HT1B sites. The promontorium, paratrigeminal islands, and the dorsomedial portion of the trigeminal nucleus (DM5), which are areas of viscerosomatic integration, exhibited high densities of both 5HT1A and 5HT1B sites. The area postrema contained low levels of both 5HT1A and 5HT1B sites. Visceral deafferentation via cervical vagotomy or nodose ganglionectomy caused a significant decrease in 5HT1A sites in the interstitial subnucleus of the NTS ipsilateral to the lesion. No changes were seen in 5HT1B sites. These studies suggest that 5HT1A and 5HT1B sites are involved in the processing of visceral sensory information in the NTS and associated areas. Based upon viscerotopic organization of the NTS, 5HT1A sites appear preferentially distributed in portions of the NTS that are associated with the coordination of swallowing, respiration, and cardiovascular function, while 5HT1B sites appear preferentially distributed in areas of the NTS associated with gastrointestinal, hepatic, pancreatic, and cardiovascular function. However, since these associations were not absolute and there was a great deal of overlap between the two sites, speculation regarding their specific functions in autonomic control must await pharmacological examination.

Mediation by 5‐HT3 receptors of an excitatory effect of 5‐HT on dorsal vagal preganglionic neurones in anaesthetized rats: an ionophoretic study

“…This suggests that although 5-HT3 receptors certainly can play a major role in mediating the excitatory action of 5-HT, the effects of 5-HT on DVN neurones may not be explained exclusively by an action on 5-HT3 receptors. Indeed, it has been previously demonstrated that 5-HTIA (Wang et al, 1995b) I 5-HT2 receptor antagonists (Wang et al, 1995a) can also attenuate the excitatory action of 5-HT on DVN neurones, and autoradiographic studies have demonstrated 5-HTIA, and 5-HT3 binding sites in the dorsal vagal nucleus (Dashwood et al, 1988;Manaker & Verderame, 1990;Pratt, et al, 1990;Thor et al, 1992;Steward et al, 1993;Leslie et al, 1994).…”

Impaired serotonergic regulation of heart rate may underlie reduced baroreflex sensitivity in an animal model of depression

“…Intracisternally applied selective 5-HT 1A or 5-HT 7 antagonists reduce baroreflex sensitivity (13). 5-HT 7 receptors are located only in the NTS (10); 5-HT 1A receptors are found in the NTS and NA (40). Iontophoretic application of the selective 5-HT 1A antagonist WAY-100635 onto CVPN in the NA attenuates their activation as a result of stimulation of cardiopulmonary afferents.…”

Gustatory System

“…Gustatory responses of neurons in the rostral Sol are modulated by substance P (Davis and Smith, 1997). Serotonin receptors are found in relatively high density in the anterior Sol and the adjacent oral trigeminal nucleus (Thor et al, 1992), and when applied iontophoretically to gustatory neurons in the Sol, 5-HT primarily produced sustained excitation (Thornton, Nicolaidis and Norgren, unpublished observations, 1992). Although some serotonergic axons are present in the gustatory nerves, most of 5HT innervation is thought to arise centrally.…”