AIMTo explore the role of histaminergic receptors in the nucleus tractus solitarius (NTS) in the responses of carotid baroreflex (CBR) performance to the intracerebroventricular (ICV) injection of histamine (HA).

METHODSThe left and right carotid sinus regions were isolated from the systemic circulation in 18 Wistar rats anesthetized with pentobarbital sodium. The intracarotid sinus pressure (ISP) was altered in a stepwise manner. ISP-mean arterial pressure (MAP) relationship curve and its characteristic parameters were constructed by fitting to the logistic function with five parameters. We observed the changes in CBR performance induced by ICV HA and the effects of pretreatment with HA receptors antagonists into the NTS on the responses of CBR to HA.

RESULTSICV injection of HA (100 ng) significantly shifted the ISP-MAP relationship curve upwards and moved the middle part of ISP Gain relationship curve downwards, and reduced the MAP range and maximum gain (Gmax), but increased the threshold pressure (TP), saturation pressure(SP) and ISP at Gmax (ISP(Gmax)). The pretreatment with H1 or H2 receptors antagonist, chlorpheniramine (CHL, 0.5 microg) or cimetidine (CIM, 1.5 microg) into the NTS, could obviously diminish the above-mentioned changes in CBR performance induced by HA, but the effect of CIM was less remarkable than that of CHL.

CONCLUSIONThe intracerebroventricular administration of HA results in a rapid resetting of CBR and a decrease in reflex sensitivity, and the histaminergic receptors in the NTS (H1 and H2 receptors), especially H1 receptors might play an important role in the responses of CBR to HA, and furthermore, the effects of the central HA on CBR might be related to a histaminergic descending pathway from the hypothalamus to NTS.