Medullary Serotonergic Neurons and Peripheral Vasomotor Tone during Active Sleep

Activation of 5‐HT 1A receptors in the medullary raphé with 8‐OH‐DPAT (DPAT) attenuates stress induced increases in heart rate (HR) and brown adipose tissue (BAT) activity. Although some thermoregulatory (Treg) mechanisms such as shivering and BAT thermogenesis are suspended during active sleep (AS), there is less information about state related changes in peripheral vasomotor tone (PVT). Since medullary serotonergic (5‐HT) neurons become quiescent during AS, and appear to modulate some Treg responses, they might play a role in the attenuation of Treg activity during AS. We therefore hypothesized that in a cool environment, shivering and PVT would decrease on transition from QS to AS and that activating 5‐HT 1A receptors in the PGCL, an extra‐raphé, medullary 5‐HT neuron group, would result in less shivering and PVT during QS compared to control QS periods. Piglets were instrumented to measure EEG, EOG, nEMG, BP, HR, and ear capillary blood flow (Fear) (laser Doppler). The animals were studied in a cool environment in an attempt to maximize Treg responses, including shivering and peripheral vasoconstriction. Integrated nEMG (iEMG) activity, and Fear, an index of PVT, were compared during paired periods of QS and AS, and during QS before and after local unilateral dialysis of DPAT into the PGCL. On transition from QS to AS, iEMG, HR, and BP decreased and Fear increased. During QS, after DPAT dialysis, there were decreases in visible shivering and a decrease in iEMG compared to pre‐dialysis values, but increases in Fear were not observed. We conclude that active peripheral vasoconstriction is suspended during AS. 5‐HT neurons in the PGCL appear to modulate shivering, but not PVT during cooling and AS.