Pharmacological profile of the clonidine-induced inhibition of vasodepressor sensory outflow in pithed rats: correlation with alpha(2A/2C)-adrenoceptors

Abstract

Background and purpose: Resistance blood vessels are innervated by sympathetic and primary sensory nerves, which modulate vascular tone through the release of noradrenaline and calcitonin gene-related peptide (CGRP), respectively. Moreover, electrical stimulation of the perivascular sensory outflow in pithed rats results in vasodepressor responses which are mainly mediated by CGRP release. The present study has investigated the role of alpha(2)-adrenoceptors in the inhibition of these vasodepressor responses.

Experimental approach: 144 pithed male Wistar rats were pretreated with hexamethonium (2 mg kg(-1) min(-1)) followed by i.v. continuous infusions of either methoxamine (15 and 30 microg kg(-1) min(-1)) or clonidine (3, 10 and 30 microg kg(-1) min(-1)). Under these conditions, electrical stimulation (0.56-5.6 Hz; 50 V and 2 ms) of the spinal cord (T(9)-T(12)) resulted in frequency-dependent decreases in diastolic blood pressure.

Key results: The infusion of clonidine (10 microg kg(-1) min(-1)), as compared to those of methoxamine (15 or 30 microg kg(-1) min(-1)), inhibited the vasodepressor responses to electrical stimulation without affecting those to i.v. bolus injections of alpha-CGRP (0.1-1 microg kg(-1)). This inhibition by clonidine was: (i) antagonized by 300 microg kg(-1) rauwolscine (alpha(2A/2B/2C)), 300 and 1000 microg kg(-1) BRL44408 (alpha(2A)), or 10 and 30 microg kg(-1) MK912 (alpha(2C)); and (ii) unaffected by 1 ml kg(-1) saline, 100 microg kg(-1) BRL44408, 3000 and 10,000 microg kg(-1) imiloxan (alpha(2B)) or 3 microg kg(-1) MK912.

Conclusions and implications: The inhibition produced by 10 microg kg(-1) min(-1) clonidine on the vasodepressor (perivascular) sensory outflow in rats may be mainly mediated by prejunctional alpha(2A)/alpha(2C)-adrenoceptors.

Figure 1

Original experimental tracings illustrating the vasodepressor responses produced by electrical stimulation of the perivascular sensory outflow during the continuous infusions of: (a) methoxamine (15 μg kg⁻¹ min⁻¹) or (b) clonidine (10 μg kg⁻¹ min⁻¹) in pithed rats. Note that the vasodepressor responses obtained during the clonidine infusion are smaller than those obtained during the methoxamine infusion. Similar results were obtained with the total number of experiments (n=6 for methoxamine and clonidine).

Figure 2

Vasodepressor responses induced by electrical stimulation (a and b) or i.v. bolus injections of exogenous α-CGRP (c and d) induced during i.v. continuous infusions of 15 and 30 μg kg⁻¹ min⁻¹ methoxamine (left panel) or 3, 10 and 30 μg kg⁻¹ min⁻¹ clonidine (right panel) (n=6 each dose). ^*P<0.05 vs the corresponding control response.

Figure 3

Effect of i.v. bolus injections of saline (a; 1 ml kg⁻¹), rauwolscine (b; 300 μg kg⁻¹), BRL44408 (c; 300 μg kg⁻¹), imiloxan (d; 3000 μg kg⁻¹) or MK912 (e; 30 μg kg⁻¹) per se (n=6 each dose) on the electrically induced vasodepressor responses produced during an i.v. continuous infusion of methoxamine (15 μg kg⁻¹ min⁻¹) (n=6 for each group).

Figure 4

Effect of i.v. bolus injections of saline (a; 1 ml kg⁻¹), rauwolscine (b; 300 μg kg⁻¹), BRL44408 (c–e for, respectively, 100, 300 and 1000 μg kg⁻¹), imiloxan (f and g for, respectively, 3000 and 10000 μg kg⁻¹) or MK912 (h–j for, respectively, 3, 10 and 30 μg kg⁻¹) (n=6 each dose) on the inhibition induced by clonidine (cloni.; 10 μg kg⁻¹ min⁻¹) of the electrically induced vasodepressor responses. The control group represents that of animals receiving an i.v. continuous infusion of methoxamine (15 μg kg⁻¹ min⁻¹; shown for comparison). ^*P<0.05 vs the corresponding control response.