Activation of P2Y1 nucleotide receptors induces inhibition of the M-type K+ current in rat hippocampal pyramidal neurons

We have shown previously that stimulation of heterologously expressed P2Y(1) nucleotide receptors inhibits M-type K+ currents in sympathetic neurons. We now report that activation of endogenous P2Y(1) receptors induces inhibition of the M-current in rat CA1/CA3 hippocampal pyramidal cells in primary neuron cultures. The P2Y(1) agonist adenosine 5'-[beta-thio] diphosphate trilithium salt (ADP beta S) inhibited M-current by up to 52% with an IC50 of 84 nM. The hydrolyzable agonist ADP (10 mu M) produced 32% inhibition, whereas the metabotropic glutamate receptor 1/5 agonist DHPG [(S)-3,5-dihydroxyphenylglycine] (10 mu M) inhibited M-current by 44%. The M-channel blocker XE991 [10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone dihydrochloride] produced 73% inhibition at 3 mu M; neither ADP beta S nor ADP produced additional inhibition in the presence of XE991. The effect of ADP beta S was prevented by a specific P2Y(1) antagonist, MRS2179 (2'-deoxy-N'-methyladenosine-3', 5'-bisphosphate tetra-ammonium salt) (30 mu M). Inhibition of the M-current by ADP beta S was accompanied by increased neuronal firing in response to injected current pulses. The neurons responding to ADP beta S were judged to be pyramidal cells on the basis of (1) morphology, (2) firing characteristics, and (3) their distinctive staining for the pyramidal cell marker neurogranin. Strong immunostaining for P2Y(1) receptors was shown in most cells in these cultures: 74% of the cells were positive for both P2Y(1) and neurogranin, whereas 16% were only P2Y(1) positive. These results show the presence of functional M-current-inhibitory P2Y(1) receptors on hippocampal pyramidal neurons, as predicted from their effects when expressed in sympathetic neurons. However, the mechanism of inhibition in the two cell types seems to differ because, unlike nucleotide-mediated M-current inhibition in sympathetic neurons, that in hippocampal neurons did not appear to result from raised intracellular calcium.