EVIDENCE THAT P2X PURINOCEPTORS MEDIATE THE EXCITATORY EFFECTS OF ALPHA-BETA METHYLENE-ADP IN RAT LOCUS-COERULEUS NEURONS

Extracellular and whole-cell patch clamp recordings were used to study the excitatory responses elicited by purine nucleotides in pontine sl ices of the rat brain containing the locus coeruleus (LC). The P2 puri noceptor agonists, alpha beta-methyleneadenosine 5'-triphosphate (alph a beta meATP) and adenosine 5'-O-(2-thiodiphosphate) (ADP beta S), and a novel purinoceptor agonist, alpha beta-methyleneadenosine 5'-diphos phate (alpha beta meADP), elicited concentration-dependent increases i n the spontaneous firing rate over the concentration range (1-300 mu M ). On vagus nerve or dorsal root preparations alpha beta meADP (100 mu M) had no agonist activity. In the presence of both alpha beta meATP (300 mu M), ADP beta S (300 mu M) elicited a further and significant i ncrease in the firing rate of the LC neurones, whilst neither alpha be ta meATP nor alpha beta meADP (300 mu M) elicited a further response. The P2 purinoceptor antagonists, suramin (100 mu M) and pyridoxalphosp hate-6-azophenyl-2',4'-disphonic acid (PPADS, 30 mu M), markedly atten uated responses to all three agonists. Whole-cell recording of membran e current showed that, at - 60 mV, alpha beta meATP and alpha beta meA DP (both 100 mu M) elicited inward currents of a similar magnitude, wh ilst the inward currents elicited by a lower concentration of ADP beta S (30 mu M) were larger and faded in the presence of this agonist. In the presence of tetrodotoxin and a combination of other neurotransmis sion blockers, both alpha beta meATP and alpha beta meADP still produc ed inward currents. Based on the known selectivity of the agonists use d in this study, there appear to be two distinct P2 purinoceptor types present on neurones in the LC, which correspond to the P2X and P2Y ty pes. The responses elicited by alpha beta meADP appear to be mediated through a putative P2X purinoceptor, although further work is required to determine which P2X receptor subtype(s) are involved. (C) 1998 Els evier Science Ltd. All rights reserved.