Selective enhancement in SHR of hypotension and bradycardia caused by NTS-injected serotonin.

To examine whether serotonergic mechanisms in the nucleus tractus solitarius (NTS) become altered by hypertension, responses to serotonin (5-HT) or L-glutamate injected into the NTS were compared in anesthetized rats. Because isotonic saline had appreciable effects whereas artificial cerebrospinal fluid did not, artificial cerebrospinal fluid was routinely used as the vehicle. Microinjections of 5-HT or L-glutamate always reduced mean pressure, heart rate, and renal nerve activity. Depressor and bradycardic responses to 5-HT were consistently more pronounced in spontaneously hypertensive rats than in either regular Wistar or Wistar-Kyoto rats, but similar responses elicited with L-glutamate did not differ between rat groups. By contrast, attendant inhibition of renal nerve activity was the same in all rats, thereby suggesting that it either is not a good indicator of sympathetic activity or does not contribute to the hypotensive effects of 5-HT. Our results are compatible with the interpretation that hypotensive responses to 5-HT were enhanced because serotonergic mechanisms for cardiovascular regulation in the NTS were sensitized in spontaneously hypertensive rats.