Regional haemodynamic effects of μ‐, δ‐, and κ‐opioid agonists microinjected into the hypothalamic paraventricular nuclei of conscious, unrestrained rats

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Abstract
1 The cardiovascular effects of bilateral injection into the hypothalamic paraventricular nuclei of selective μ-, δ-, and κ-opioid receptor agonists were investigated in conscious, unrestrained Wistar Kyoto rats, chronically instrumented with pulsed Doppler flow probes for measurement of regional haemodynamics. 2 The selective μ-agonist [D-Ala2, MePhe4, Gly5ol]enkephalin (DAMGO), injected bilaterally into the hypothalamic paraventricular nuclei (0.01-1.0 nmol), caused increases in blood pressure, tachycardias, vasoconstriction in renal and superior mesenteric vascular beds and substantial vasodilatation in the hindquarter vascular bed. 3 The administration of increasing doses (0.01–5.0 nmol) of the selective δ-agonist [D-Phe2,5]enkephalin (DPDPE) or the selective κ-agonist, U50488H into the paraventricular nuclei (PVN) had no significant effect on blood pressure, heart rate, or regional haemodynamics. 4 Together, the present results are further evidence of a role for opioid peptides, especially acting at μ-receptors in the PVN, in the central regulation of the cardiovascular system, whereas a role for opioid peptides, acting at δ- and κ-receptors in the PVN, seems less obvious from the present results.