Involvement of serotonin in the excitation of phrenic motoneurons evoked by stimulation of the raphe obscurus

Abstract

Short-latency averaged responses in the C5 phrenic nerves to electrical stimulation (2.5-80 microA; 5-80 Hz; 150 microseconds pulse duration) of raphe pallidus (RP) and raphe obscurus (RO) were investigated in anesthetized, paralyzed, and artificially ventilated cats. The responses to stimulation of RO were excitatory, whereas a mixture of inhibitory and excitatory responses of lesser magnitude were observed after stimulating in RP. The maximal response was obtained from the ventral part of RO and consisted of early and delayed excitatory responses that were of equal magnitude in both left and right C5 phrenic nerve roots. The mean latency for the early response was 2.5 +/- 0.1 msec and for the delayed response was 7.0 +/- 0.2 msec. Both responses were elicited during inspiratory phase stimulation, but only the delayed response was present during expiratory phase stimulation. The stimulus threshold of the early response was 5 microA; the delayed response was elicited at currents as small as 2.5 microA. Early and delayed responses were affected in different ways by increasing stimulus current and by increasing stimulus frequency. Intravenous administration of serotonin receptor antagonists methysergide (0.1-0.7 mg/kg), metergoline (33-244 micrograms/kg), and cinanserin (1.5-9.0 micrograms/kg) caused significant dose-related reductions in the magnitude of the delayed response, but did not significantly affect the early response. These data suggest that the early and delayed excitatory responses are mediated by different neuronal pathways. The early response does not involve serotonin release, while the later response is mediated at least in part by activation of a serotonergic pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonergic Neurons Activate Chemosensitive Retrotrapezoid Nucleus Neurons by a pH-Independent Mechanism

“…In brief, via their projections to RTN and other components of the breathing network such as the ventral respiratory group and phrenic motor neurons (Holtman et al, 1986(Holtman et al, , 1990a, serotonergic neurons may contribute to matching breathing intensity to the overall metabolic rate.…”

Modulation of respiratory activity of neonatal rat phrenic motoneurones by serotonin.

“…Stimulation of different nuclei within the raphe can produce either facilitation or inhibition of phrenic nerve activity that is at least partially mediated by 5-HT (Lalley, 1986a, b;Holtman, Dick & Berger, 1986. However, raphe nuclei stimulation produces similar changes in activity of medullary respiratory neurones concurrent with changes in phrenic nerve activity (Lalley, 1986a, b).…”

Behavioral correlates of the activity of serotonergic and non-serotonergic neurons in caudal raphe nuclei

“…Second, increased NRP or NRO activity induced by electrical or chemical stimulation facilitates trigeminal motoneurons (18), facilitates or depresses phrenic motoneurons (19)(20)(21), facilitates lumbar motoneurons (22,23), facilitates motor and secretory gastric preganglionic neurons in the dorsal motor nucleus of the vagus (12,24), and facilitates or depresses sympathetic preganglionic neurons to the heart and vessels (25,26).…”

“…Correspondingly, target structures present many serotonergic terminals (18,36). Artificial activation of the NRO was shown to increase the release of serotonin in target structures (37) and blockade of serotonin receptors with antagonists considerably reduces the effects of artificial activation of NRP and NRO on target neurons (18)(19)(20)(21)(22). Furthermore, the discharge of some serotonergic neurons in the NRP and NRO is associated with mastication and other orofacial behaviors and locomotion (30,31).…”