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Generation of a physiological sympathetic motor rhythm in the rat following spinal application of 5‐HT

When applied in vitro to various CNS structures 5‐HT and/or NMDA have been observed to generate rhythmic nervous activity. In contrast, reports of similar in vivo actions are relatively rare. Here we describe a physiological sympathetic motor rhythm regulating the thermoregulatory circulation of the rat tail (T‐rhythm; 0.40–1.20 Hz) that can be elicited following intrathecal (i.t.) application of 5‐HT to an in situ‘isolated’ spinal cord preparation (anaesthetized rats spinalized at T10–T11 and cauda equina cut). i.t. injections were delivered to L1 as sympathetic neuronal activity to the tail (SNAT) arises from preganglionic neurones at T11–L2. SNAT was abolished after spinal transection (n= 18) and it did not return spontaneously. The administration of 5‐HT (250 nmol) generated rhythmic sympathetic discharges (n= 6). The mean frequency of the T‐like rhythm during the highest level of activity was 0.88 ± 0.04 Hz which was not significantly different from the T‐rhythm frequency observed in intact animals (0.77 ± 0.02 Hz; P > 0.05 n= 16). In contrast, NMDA (1 μmol) generated an irregular tonic activity, but it failed to generate a T‐like rhythm (n= 9), even though the mean levels of activity were not significantly different to those produced by 5‐HT. However, 5‐HT (250 nmol) applied after NMDA generated a T‐like rhythm (0.95 ± 0.11 Hz, n= 6). Our observations support the idea that 5‐HT released from rostral ventromedial medullary neurones, known to innervate sympathetic preganglionic neurones, can induce sympathetic rhythmic activity.