Blood pressure following microinjection of somatostatin related peptides into the rat nucleus tractus solitarii

Abstract

The possible role of medullary somatostatin as a neurotransmitter involved in blood pressure regulation was examined. Blood pressure and heart rate were measured in pentobarbital anesthetized rats while pmol quantities of somatostatin-28 (SS28), SS28-(15–28) (somatostatin-14), SS28-(1–12) or SS28-1(1–10) were injected into the caudal nucleus tractus solitarii (NTS). Injection of artificial cerebrospinal fluid (ACSF; 500 nl/10 s) alone was followed by a small (2 mmHg) statistically insignificant decrease in blood pressure. SS28, SS28-(15–28) or SS28-(1–12) induced an immediate and statistically significant decrease in blood pressure as compared to vehicle alone. SS28-(1–10) had no significant effect. SS28 was tested at three doses. The lowest dose of SS28 (2.5 pmol) induced an 8 mmHg drop in blood pressure, the medium dose (25 pmol), an 11 mmHg drop and the highest dose (250 pmol) a 16 mmHg drop. The other peptide fragments were tested only at the higher 250 pmol dose. SS28-(1–12) induced a significantly (P < 0.01) less drop in blood pressure (10 mmHg) when compared to either SS28 or SS28-(15–28) (18–19 mmHg). The hypotensive and bradycardiac effects of SS28 were prevented by pretreatment with methylscopolamine (5 mg/kg i.p.) 10 min prior to SS28. The cardiovascular effects of intramedullary injected SS28 were also blocked with an ipsilateral NTS injection of yohimbine or tolazoline 10 min prior to SS28. These data indicate that the hypotension and bradycardia following the intramedullary injection of SS28 is mediated through the parasympathetic nervous system and that the C-terminal portion of SS28 and SS28-(15–28) is important for potency, but not obligatory, for efficacy. In addition, because two different adrenergic blocking agents can prevent the cardiovascular effects of somatostatin, central adrenergic mechanisms may be involved.