enkephalin--ala(2)-mephe(4)-gly(5)- and methoctramine

The sphenopalatine ganglion (SPG) neurons represent the parasympathetic branch of the autonomic nervous system involved in controlling cerebral blood flow. In the present study, we examined the coupling mechanism between mu (mu) opioid receptors (MOR) and muscarinic acetylcholine receptors (mAChR) with Ca(2+) channels in acutely dissociated adult rat SPG neurons. Successful MOR activation was recorded in approximately 40-45% of SPG neurons employing the whole cell variant of the patch-clamp technique. In addition, immunofluorescence assays indicated that MOR are not expressed in all SPG neurons while M(2) mAChR staining was evident in all neurons. The concentration-response relationships generated with morphine and [d-Ala2-N-Me-Phe4-Glycol5]-enkephalin (DAMGO) showed IC(50) values of 15.2 and 56.1 nM and maximal Ca(2+) current inhibition of 26.0 and 38.7%, respectively. Activation of MOR or M(2) mAChR with morphine or oxotremorine-methiodide (Oxo-M), respectively, resulted in voltage-dependent inhibition of Ca(2+) currents via coupling with Galpha(i/o) protein subunits. The acute prolonged exposure (10 min) of neurons to morphine or Oxo-M led to the homologous desensitization of MOR and M(2) mAChR, respectively. The prolonged stimulation of M(2) mAChR with Oxo-M resulted in heterologous desensitization of morphine-mediated Ca(2+) current inhibition, and was sensitive to the M(2) mAChR blocker methoctramine. On the other hand, when the neurons were exposed to morphine or DAMGO for 10 min, heterologous desensitization of M(2) mAChR was not observed. These results suggest that in rat SPG neurons activation of M(2) mAChR likely modulates opioid transmission in the brain vasculature to adequately maintain cerebral blood flow.

Topics: Animals; Calcium Channels; Cells, Cultured; Diamines; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Ganglia, Parasympathetic; GTP-Binding Protein alpha Subunits, Gi-Go; Male; Membrane Potentials; Morphine; Muscarinic Agonists; Narcotics; Neurons; Neurotransmitter Agents; Oxotremorine; Parasympatholytics; Rats; Rats, Wistar; Receptor, Muscarinic M2; Receptors, Opioid, mu

The involvement of striatal cholinergic neurons in the release of dopamine (DA) elicited by the mu-opioid receptor agonist DAGO ([D-Ala2, NMePhe4-Gly5(ol)]enkephalin) was explored. The striatal release of DA was measured by microdialysis in rats anesthetized with chloral hydrate. When infused in the striatum, through the microdialysis probe, DAGO increased the extracellular levels of DA. The previous injection in striatum of AF 64-A, a toxin for cholinergic neurons, or the concomitant infusion of the M2-muscarinic antagonist methoctramine abolished the effect of DAGO on the DA release. It is concluded that stimulation of mu-opioid receptors, by inhibiting the acetylcholine release which stimulates tonically M2-muscarinic receptors likely associated with dopaminergic nerve endings, indirectly increases the striatal DA release.

Topics: 3,4-Dihydroxyphenylacetic Acid; Analgesics; Animals; Aziridines; Choline; Corpus Striatum; Diamines; Dopamine; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; gamma-Aminobutyric Acid; Homovanillic Acid; Infusions, Parenteral; Interneurons; Kinetics; Male; Microdialysis; Neurotoxins; Parasympatholytics; Rats; Rats, Sprague-Dawley; Receptors, Opioid, mu; Synaptosomes