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Adenosine inhibits voltage-dependent Ca²⁺ currents in rat dissociated supraoptic neurones via A₁ receptors

Jun Noguchi and Hiroshi Yamashita

1. The modulation of voltage-dependent Ca²⁺ currents (I_Ca) by adenosine was investigated in magnocellular neurones acutely dissociated from the rat hypothalamic supraoptic nucleus (SON) by using the whole-cell patch-clamp technique.

2. Adenosine dose dependently and reversibly inhibited I_Ca elicited by depolarizing voltage steps from a holding potential of -80 mV to potentials ranging from -30 to +20 mV. The mean (± s.e.m.) maximum inhibition rate was 36·1 ± 4·1 % (n = 6) at -20 mV and the EC₅₀ was 9·8 × 10^-7 M (n = 6).

3. The inhibition of I_Ca by adenosine was completely reversed by the selective A₁ receptor antagonist 8-cyclopentyl theophylline (CPT), and was mimicked by the selective A₁ receptor agonist N ⁶-cyclohexyladenosine (CHA).

4. The inhibition by CHA was strongly reduced when I_Ca was inhibited by -conotoxin GVIA, a blocker of N-type Ca²⁺ channels.

5. The adenosine-induced inhibition of I_Ca was largely reversed by a depolarizing prepulse to +150 mV for 100 ms, which is known to reverse the inhibition of Ca²⁺ channels mediated by G-protein subunits.

6. The adenosine receptor-mediated inhibition of I_Ca was not abolished by intracellularly applied preactivated pertussis toxin (PTX).

7. Using immunohistochemistry, G_z-like immunoreactivity (a PTX-resistant inhibitory G-protein) was observed throughout the SON.

8. These results suggest that adenosine modulates the neuronal activity of SON neurones by inhibiting N-type voltage-dependent Ca²⁺ channels via A₁ receptors which are coupled to PTX-resistant G-proteins.

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