Presynaptic alpha 2-adrenoceptor and kappa-opiate receptor occupancy promotes closure of neuronal (N-type) calcium channels.
Synaptosomes prepared from rat cerebral cortex by homogenization in isotonic sucrose and centrifugation on four-step discontinuous percoll density gradients were loaded with the fluorescent indicator fura-2 to allow measurement of intrasynaptosomal free calcium concentrations [( Ca2+]i). Incubation of fura-2 loaded synaptosomes with either the kappa-opiate agonist U-50,488H (0.1-100 microM) or the alpha 2-adrenoceptor agonist clonidine (0.1-100 microM), resulted in a dose-dependent reduction in [Ca2+]i and these changes were completely antagonised by prior inclusion of naloxone (20 microM) or idazoxan (RX781094) (2 microM) respectively. When the 1,4-dihydropyridine Ca2+-channel blocker nifedipine (1 microM) was incubated with synaptosomes for 1 min, there was a 17.0% decrease in [Ca2+]i and when it was combined with either U-50,488H (1 microM) or clonidine (1 microM) there was a reduction in [Ca2+]i of 35.0 and 48.1% respectively i.e. the effects were additive. The increases in the depression of [Ca2+]i produced by these drug combinations were antagonised by the inclusion of naloxone (20 microM) or idazoxan (2 microM) which resulted in decreases in free [Ca2+]i of 26.5 and 14.1% respectively. These data indicate that the effects of clonidine and U-50,488H are not mediated by L-type Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)
Adamson
P
P
Department of Biochemistry, Institute of Psychiatry, London, U.K.
Xiang
J Z
JZ
Mantzourides
T
T
Brammer
M J
MJ
Campbell
I C
IC
D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D000818:Animals; D001572:Benzofurans; D002118:Calcium / Q000494:pharmacology; D015220:Calcium Channels / Q000378:metabolism*; D003000:Clonidine / Q000494:pharmacology; D004146:Dioxanes / Q000494:pharmacology; D016257:Fura-2; D019329:Idazoxan; D066298:In Vitro Techniques; D008297:Male; D008978:Mollusk Venoms / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D011759:Pyrrolidines / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D011957:Receptors, Opioid / Q000378:metabolism*; D017473:Receptors, Opioid, kappa; D013569:Synapses / Q000378:metabolism; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism; D020866:omega-Conotoxin GVIA
D001572:Benzofurans; D015220:Calcium Channels; D004146:Dioxanes; D008978:Mollusk Venoms; D011759:Pyrrolidines; D011942:Receptors, Adrenergic, alpha; D011957:Receptors, Opioid; D017473:Receptors, Opioid, kappa; D009270:Naloxone; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D020866:omega-Conotoxin GVIA; D009543:Nifedipine; D003000:Clonidine; D002118:Calcium; D016257:Fura-2; D019329:Idazoxan