Adenosine Postsynaptically Modulates Supraoptic Neuronal Excitability

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Abstract
Effects of adenosine on the excitability of supraoptic nucleus neurons were investigated in whole cell patch-clamp experiments conducted in horizontal slices of rat hypothalamus. Adenosine (10–100 μM) inhibited all neurons tested by reducing or abolishing spontaneous or evoked discharge. Large hyperpolarizations were seen, averaging -6.08 ± 0.83 mV below resting membrane potential, and action potential durations were significantly reduced by 134 ± 41 μs in the presence of 100 μM adenosine. The A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1 μM) blocked these effects, whereas the A1 agonists N6-cyclopentyladenosine (CPA) and N6-cyclohexyladenosine (CHA) mimicked the actions of adenosine. A2 receptor contributions to excitability were assessed by application of an A2 agonist, carboxamidoadenosine (CPCA). This resulted in membrane depolarizations (3.56 ± 0.65 mV) and maintenance of firing. The presence of endogenous adenosine in the slice was revealed by both the application of the adenosine uptake inhibitor dilazep (1–100 μM), which resulted in a strong inhibition of firing activity, and the application of DPCPX, which induced firing in cells silenced by negative current injection. We tested for postsynaptic actions of adenosine by blocking G protein activation via GDP-β-S infusion into recorded neurons. Under these conditions, the adenosinergic inhibition of firing and reduction of spike duration were blocked, suggesting the effects were mediated by postsynaptic adenosine receptors. That the effects on excitability could be due to direct activation of adenosine A1 receptors on supraoptic neurons was further explored immunocytochemically via the co-labeling of magnocellular neurons with polyclonal antibodies raised against the A1 receptors. It is concluded that adenosine, acting at postsynaptic A1 receptors, exhibits a powerful inhibitory influence on supraoptic magnocellular activity and is an important endogenous regulator of magnocellular neuroendocrine function.