Modulation of the pressor response elicited by carbachol and electrical stimulation of the amygdala by muscarinic antagonists in conscious rats

Abstract

1. The nature of the muscarinic receptor involved in mediating cardiovascular changes caused by unilateral microinjection of carbachol (5 nmol) into, and electrical stimulation (200–300 μA) of, the amygdaloid complex was investigated in conscious, unrestrained female Sprague-Dawley rats.

2. Unilateral microinjection of carbachol (5 nmol; n=6) and electrical stimulation (200–300 μA, 80 Hz, 30 s; n=4) caused a significant rise in blood pressure of 21±4 mmHg and 25±5 mmHg, respectively. These changes were associated with no overall effect on heart rate. The effects of electrical stimulation were found to be repeatable.

3. Pretreatment i.c.v. with pirenzepine (5–20 nmol; n=6–7 for each dose), dose-dependently inhibited the rise in blood pressure induced by carbachol, whereas AF-DX 116 (100 nmol; n=6) failed to have any effect on the carbachol-induced pressure response. Neither antagonist alone had any effect on resting baseline variables.

4. Unilateral microinjections of atropine sulphate (1–100 nmol; n=4–6 for each dose), pirenzepine (0.03–10 nmol; n=4 for each dose) or AF-DX 116 (10–60 nmol; n=4–5 for each dose), into the amygdala, dose-dependently inhibited the rise in blood pressure caused by electrical stimulation (200–300 μA). The ID₅₀ values were 1.05, 0.23 and 39.5 nmol, respectively. Although pirenzepine seemed to be more potent than atropine, this difference was not significant.

5. It is concluded that the rise in blood pressure elicited by unilateral microinjection of carbachol into, or electrical stimulation of, the amygdaloid complex is mediated by M₁-muscarinic receptors.

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