Central alpha 1-adrenoceptors mediate the pressor response to intracerebroventricular injection of noradrenaline in unanesthetized rats.
The intracerebroventricular (i.c.v.) administration of noradrenaline (NA) caused dose-dependent blood pressure increases in unanesthetized rats with an ED50 of 35 nmol. Similar pressor responses were observed after the i.c.v. injection of the more selective alpha 1-agonists ST-91, methoxamine and phenylephrine with ED50 of 60, 155 and 575 nmol, respectively. The maximal pressor response was 57 +/- 3 mmHg. No tachyphylaxis was observed when injections of 37.5 nmol of NA was i.c.v.-injected at an interval of 24 hr. No significant differences were observed in the plasma content of NA and adrenaline at the peak of the pressor response to i.c.v.-injected NA when compared to i.c.v. injections of saline. The pressor effects of NA were blocked by the i.c.v. pretreatment with prazosin or yohimbine with ID50 of 0.9 and 29 nmol, respectively. Prazosin was 32-fold more potent than yohimbine in blocking the effect of i.c.v. NA, suggesting the involvement of alpha 1-adrenoceptors in the central mediation of the pressor response to NA. Intravenous injections of 13 nmol of prazosin or 90 nmol of yohimbine did not affect the pressor response to i.c.v. NA, further indicating the central nervous system nature of the response.
Department of Pharmacology, School of Medicine of Ribeirão Preto, Brazil.
F M
FM
Corrêa
V L
VL
Peres-Polon
D000818:Animals; D001794:Blood Pressure / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D007278:Injections, Spinal; D008297:Male; D008729:Methoxamine / Q000494:pharmacology; D009638:Norepinephrine / Q000494:pharmacology*; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects*; D015016:Yohimbine / Q000494:pharmacology
D018340:Receptors, Adrenergic, alpha-1; D015016:Yohimbine; D008729:Methoxamine; D009638:Norepinephrine; D011224:Prazosin