Prejunctional alpha 2-adrenoceptors and adenylyl cyclase regulation in the rabbit iris-ciliary body.

Agents that elevate intracellular cyclic AMP (cAMP) have been found to enhance the synaptic discharge of norepinephrine (NE) from sympathetic nerve terminals in the rabbit iris-ciliary body and other peripheral tissues. We explored the hypothesis that prejunctional alpha 2-adrenergic receptors that mediate feedback inhibition of NE release may be coupled to adenylyl cyclase inhibition. To indirectly monitor cAMP changes in sympathetic axon terminals, we analyzed the cAMP-mediated activation of tyrosine hydroxylase, a sympathetic marker protein that undergoes acute phosphorylation and activation by cAMP-dependent protein kinase A. Tyrosine hydroxylase activity was assayed in situ by incubation of rabbit iris-ciliary body tissue segments in buffered Krebs-Ringer solution containing the substrate tyrosine (100 microM) and the DOPA decarboxylase inhibitor brocresine (30 microM). Intraneuronal DOPA accumulation was quantified by HPLC with electrochemical detection. Tyrosine hydroxylase activity was increased approximately 2 fold by incubation with forskolin (10 microM) plus IBMX (0.5 mM) or with 8-Bromo-cAMP (3 mM). Simultaneous addition of the alpha 2-adrenergic agonist clonidine (1 microM) attenuated the response to forskolin/IBMX, but had no effect on the response to 8-Br-cAMP. Clonidine-mediated inhibition of the forskolin/IBMX response was abolished by treatment of tissues with N-ethylmaleimide (NEM), an alkylating agent that inactivates pertussis toxin-sensitive G proteins (Gi) that couple receptors to adenylyl cyclase inhibition. These findings suggest that prejunctional alpha 2-adrenoceptors in the rabbit iris-ciliary body are negatively coupled to adenylyl cyclase. This mechanism may contribute to autofeedback regulation of NE biosynthesis and release.