μ-opioid receptor modulation of calcium channel current in periaqueductal grey neurons from C57B16/J mice and mutant mice lacking MOR-1

1 The actions of opioid receptor agonists on the calcium channel currents (I-Ba) Of acutely dissociated periaqueductal grey (PAG) neurons from C57B16/J mice and mutant mice lacking the first exon of the mu-opioid receptor (MOR-1) were examined using whole cell patch clamp techniques. These effects were compared with the GABA(B)-receptor agonist baclofen. 2 The endogenous opioid agonist methionine-enkephalin (met-enkephalin, pEC(50) 6.8, maximum inhibition 40%), the putative endogenous mu-opioid agonist endomorphin-1 (pEC(50) 6.2, maximum inhibition 35%) and the mu-opioid selective agonist DAMGO (Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol enkephalin, pEC(50) 6.9, maximum inhibition 40%) inhibited I-Ba in 70% of mouse FAG neurons. The inhibition of I-Ba by each agonist was completely prevented by the mu-receptor antagonist CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2). The delta-opioid receptor agonists DPDPE ([D-Pen(2,5)]-enkephalin, 1 mu M) and deltorphin II (1 mu M), and the kappa-opioid receptor agonist U-69593 (1-10 mu M), did not affect I-Ba in any cell tested. 3 The GABA(B) agonist baclofen inhibited I-Ba in all neurons (pEC(50) 5.9, maximum inhibition 42%). 4 In neurons from the MOR-1 deficient mice, the mu-opioid agonists met-enkephalin, DAMGO and endomorphin-1 did not inhibit 4, whilst baclofen inhibited I-Ba in a manner indistinguishable from wild type mice. 5 A maximally effective concentration of endomorphin-1 (30 mu M) partially (19%), but significantly (P<0.005), occluded the inhibition of I-Ba normally elicited by a maximally effective concentration of met-enkephalin (10 mu M). 6 This study indicates that mu-opioid receptors, but not delta- or kappa-opioid receptors, modulate somatic calcium channel currents in mouse FAG neurons. The putative endogenous mu-agonist, endomorphin-1, was a partial agonist in mouse FAG neurons.