Summary
The effects of microinjecion of opioid receptor agonist and antagonist into mesencephalic nucleus dorsalis raphe, were studied on mean arterial pressure and heart rate to elucidate the nature and role of these opioid receptors in cardiovascular regulation. Microinjection of morphine (5 μg and 10 μg) into nucleus dorsalis raphe elicited both inhibitory and excitatory cardiovascular responses respectively, while microinjection of opioid receptor antagonist, naloxone (10 μg) failed to produce any significant cardiovascular responses. However, local pretreatment with naloxone blocked both inhibitory and excitatory responses of graded doses of morphine. These opioid receptors seem to be localised in the neurons of the nucleus since microinjection of morphine into neural structures adjoining nucleus dorsalis raphe failed to induce any cardiovascular responses. Furthermore, the dose or morphine (2 μg) which was ineffective when microinjected into nucleus dorsalis raphe, produced inhibitory cardiovascular responses after pretreatment with LM 5008, a 5-HT uptake blocker. Similarly, the excitatory cardiovascular responses of morphine microinjection were blocked by spinal cord transection (C1) and p-CPA, guanethidine and piperoxan pretreatments, while bilateral cervical vagotomy failed to do so. Thus, it is likely that the inhibitory cardiovascular responses of morphine are mediated directly through stimulation of opioid receptors present in the neurons of nucleus dorsalis raphe while the excitatory responses to higher dose of morphine, appear to be due to a release of noradrenaline which in turn modulates the activity of neurons by acting on a adrenoceptors.
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Saxena, A., Saksena, A., Vrat, S. et al. Presence of opioid receptors in mesencephalic nucleus dorsalis raphe concerned in cardiovascular regulation in cats. Naunyn-Schmiedeberg's Arch Pharmacol 336, 81–86 (1987). https://doi.org/10.1007/BF00177755
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DOI: https://doi.org/10.1007/BF00177755