Skeletal muscle afferent fibres release substance P in the nucleus tractus solitarii of anaesthetized cats

The nucleus tractus solitarii (NTS) is a medullary region involved in central transmission and processing of peripheral viscerosensory input (Spyer, 1994). Neural input from peripheral cardiorespiratory receptors (arterial baroreceptors, cardiac and pulmonary receptors, peripheral chemoreceptors) and supra-bulbar regions (hypothalamus, parabrachial nucleus, sensorimotor cortex) synapse in the NTS (Jordan et al. 1988; Felder & Mifflin, 1988; Ba-M'Hamed et al. 1996). In addition, neuroanatomical (Kalia et al. 1981; Nyberg & Blomqvist, 1984; Menetrey & Basbaum, 1987) and electrophysiological (Person, 1989; McMahon et al. 1992; Toney & Mifflin, 1994) studies have shown that somatic afferent fibres also project to and alter the discharge properties of NTS neurones. These findings suggest that the NTS may be a principal relay site for viscerosomatic sensory input. However, NTS neurones also contain an abundance of neuropeptides, including neuropeptide Y, angiotensin II, vasopressin, opioids, bradykinin, substance P and related tachykinins (Maley & Elde, 1982; Van Giersbergen & De Jong, 1992; Lawrence & Jarrott, 1996). Therefore, the NTS may perform integrative functions that are controlled in a highly complex and refined manner. The role of the tachykinin substance P in central cardiovascular regulation in the NTS has recently been an area of increasing interest. To date, there are several lines of evidence demonstrating that substance P may be involved in mediating central autonomic neurotransmission. First, biochemical and immunohistochemical evidence has demonstrated that the NTS contains substance P nerve fibres, terminals and receptors (Maley & Elde, 1982; Van Giersbergen et al. 1992; Sykes et al. 1994). Second, substance P immunoreactivity has been localized in the nodose and petrosal ganglia that contain cell bodies for primary baroreceptor, chemoreceptor and cardiopulmonary receptor afferent fibres (Helke et al. 1980, 1984). Third, in vivo microdialysis has shown that substance P is released from neurones in the NTS in response to electrical stimulation of aortic baroreceptor afferent fibres and activation of peripheral chemoreceptors by hypoxia (Lindefors et al. 1986; Morilak et al. 1988; Srinivasan et al. 1991). Fourth, localized microinjection of substance P in the NTS has been reported to evoke hypotension and bradycardia in rats (Hall et al. 1989; Chan et al. 1990). And finally, blockade of tachykinin receptors in the NTS attenuates the cardiovascular responses evoked by cardiac vagal receptors (Paton, 1998). Because some of these cardiovascular responses are similar to those evoked by the arterial baroreceptor reflex, it has been suggested that substance P may be the neurotransmitter for arterial baroreceptor afferent fibres (Gillis et al. 1981). However, it has also been suggested that substance P is not involved in modulating the arterial baroreceptor reflex (Feldman, 1995; Massari et al. 1998). Thus, although there is evidence that substance P is present and released by NTS neurones there remains debate as to whether substance P exerts a tonic modulatory action on central pathways involved in reflex autonomic control. Despite the lack of agreement over the action of substance P on central cardiovascular regulation, it has been reported that substance P is involved in the transmission of nociceptive and non-nociceptive stimuli in spinal pathways. For example, electrically induced contraction of the triceps surae releases substance P in the dorsal horn of the lower lumbar-upper sacral spinal cord (Wilson et al. 1993a,b). Selective blockade of substance P receptors in the spinal cord has also been reported to blunt the cardiovascular responses evoked by muscle contraction (Hill et al 1992; Wilson et al 1992). These findings suggest that substance P mediates the transmission of contraction-related afferent input in the dorsal horn of the spinal cord. Because these afferent fibres project to cardiovascular-related regions in the brainstem, including the NTS (Kalia et al. 1981; Nyberg & Blomqvist, 1984; Menetry & Basbaum, 1987), it is important to determine if neural input from contraction-sensitive skeletal muscle afferent fibres is a source of tachykininergic input. Furthermore, knowledge of the origin of substance P-releasing (SPergic) inputs to the NTS may provide insight into the peptidergic mechanisms in the lower brainstem that mediate the cardiovascular responses during muscular exercise. Two objectives are proposed in this study: (1) to determine if activation of skeletal muscle afferent fibres releases substance P in the NTS; and (2) to determine if the release of substance P is secondary to activation of cardiorespiratory receptors (arterial baroreceptors, cardiac receptors, pulmonary stretch receptors). We hypothesized that skeletal muscle contraction increases the extraneuronal concentration of substance P in the NTS independent of neural input transmitted by the carotid sinus and vagus nerves. A preliminary report of these findings has been published (Potts et al. 1997).