Modulation of cardiovascular and electrocortical activity through serotonergic mechanisms in the nucleus tractus solitarius of the rat

The nucleus tractus solitarius (NTS) is an integral part of the baroreceptor reflex arc. Thus, stimulation of the NTS elicits changes in arterial pressure and heart rate as well as in numerous other physiologic parameters including electrocortical activity. Serotonin (5-HT), which has been implicated in cardiovascular and electrocortical control, is present in nerve terminals within the NTS. Therefore, this study was designed to determine whether 5-HT may effect that control within the NTS. Serotonin injected into the NTS of anesthetized rats produced marked changes in the EEG, arterial pressure, and heart rate. EEG activity changed from irregular 1-5 Hz, 350-500 microV waves with an overlying 13-15 Hz, low voltage rhythm to a regular, 5 Hz, 250-300 microV rhythm. The dose-dependent cardiovascular changes were maximal at a dose of 400 pmol which produced a fall of mean arterial pressure of 48 +/- 2 mm Hg from a baseline of 96 +/- 4 mm Hg and of heart rate of 90 +/- 9 bpm from a baseline of 400 +/- 18 bpm (n = 6; P less than 0.001). Both the cardiovascular and EEG effects of 5-HT injected into the NTS were blocked by the prior injection of the 5-HT antagonist metergoline at the same site. However, the bilateral microinjection of metergoline into the NTS did not affect the baroreceptor reflex. Thus, although serotonergic mechanisms in the NTS may be involved in the modulation of electrocortical and cardiovascular activity, they are not integral to the baroreceptor reflex arc.