Adenosine modulation of calcium channel currents and synaptic γ- aminobutyrate (GABA) release was investigated with whole cell voltage-clamp recordings in rat suprachiasmatic nucleus (SCN) and arcuate nucleus cultures (n = 94). In SCN cultures, ~70% of the neurons showed a reversible inhibition of whole cell barium currents on the application of adenosine or its analogues. Adenosine at 1 μM reduced the amplitude of the barium currents by ~27%. In contrast to the significant reduction in the amplitude, the rising and decaying phases of the barium currents, and the inverted bell shape of the current-voltage curve of the barium currents, were not changed by adenosine. The adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA; 100 nM) and the adenosine A2 receptor agonist N6-[2-(3,5- dimelhoxyphenyl)-ethyl] adenosine (DPMA; 100 nM) inhibited the barium currents by 21% and 16%, respectively, in SCN neurons, indicating both A1 and A2 receptor actions. The A1 receptor antagonist 8-cyclopentyl-1,3- dipropylxanthine (100 nM) significantly reduced the effect of CPA but did not change the effect of DPMA on the barium currents. In the presence of tetrodotoxin to block action potentials, the frequency, but not the amplitude, of miniature inhibitory postsynaptic currents was significantly reduced (46%) by 1 μM adenosine, suggesting a presynaptic mechanism of adenosine action. In support of this suggestion, the postsynaptic GABA receptor responses were not influenced by 1 μM adenosine in the majority of SCN neurons. Most solitary self-innervating SCN neurons in microisland cultures were GABAergic. In these cells, the evoked autaptic GABA release (inhibitory postsynaptic current) was significantly inhibited by adenosine (37%), CPA (27%), and DPMA (28%), indicating that both A1 and A2 receptors were present in presynaptic axons. Similar to the effect in SCN neurons, adenosine inhibited both barium currents and GABA release in arcuate neurons. The reduction of whole cell barium currents by adenosine (1 μM), CPA (100 nM), and DPMA (100 nM) was 24, 17, and 19%, respectively. In solitary self- innervating arcuate neurons, adenosine inhibited the evoked GABA release (inhibitory postsynaptic current) by ~48%. We conclude that both adenosine A1 and A2 receptors are present in the SCN and arcuate nucleus of the hypothalamus. Adenosine inhibits calcium currents and presynaptically reduces inhibitory GABA neurotransmission.
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