Response to prejunctional adenosine receptors is dependent on stimulus frequency

Abstract

PURPOSE. To characterize the adenosine receptor modulation of norepinephrine (NE) release from sympathetic neurons in the isolated rabbit iris/ciliary body. METHODS. Iris/ciliary bodies were isolated from New Zealand White rabbits and incubated in the presence of 3 H-NE. Norepinephrine release was elicited by field stimulation with varied frequencies from 5 to 30 Hz. The effects of adenosinergic and a 2 adrenergic compounds on NE release were determined and compared. RESULTS. At a stimulation frequency of 5 Hz, the addition of the adenosine A 1 agonist CHA did not significantly alter 3 H-NE release. However, in the presence of the a 2 adrenergic antagonist yohimbine, the addition of CHA produced a dose-related reduction in 3 H-NE release. The EC 50 for this reduction was 14 nM. At a stimulus frequency of 20 Hz, the addition of CHA alone (10 - 6 M) produced a significant reduction in 3 H-NE release of 41%. The EC 50 s for the adenosine A 1 agonists CHA-and R-PIA-induced suppression of 3 H-NE release at 20 Hz were 32 and 24 nM, respectively. The adenosine A 2 agonist CV-1808 did not alter 3 H-NE release at stimulation frequencies of 5 or 20 Hz. Pretreatment of tissues with the adenosine A 1 antagonist CPT or pertussis toxin reversed the suppression of 3 H-NE release induced by CHA. Comparison of the inhibitory responses of CHA to the a 2 adrenergic agonist UK-14,304 at stimulus frequencies of 5 to 30 Hz demonstrated that this adenosine A 1 agonist was effective only in suppressing NE release at frequencies of 20 Hz or greater. In contrast, the a 2 adrenergic agonist UK-14,304 was most effective in reducing NE release at 5 Hz. CONCLUSIONS. These results provide evidence that adenosine agonists inhibit 3 H-NE release in the iris/ciliary body via prejunctional adenosine A 1 receptors linked to G i/o -protein. However, the expression of this response was dependent on the frequency of neuronal stimulation. Hence, prejunctional adenosine A 1 receptors may act to selectively limit high-frequency neurotransmission.