Alpha-adrenoceptor-mediated inhibition of acetylcholine release in guinea-pig superior cervical ganglion

Adenosine analogs inhibit acetylcholine release and cyclic AMP synthesis in the guinea-pig superior cervical ganglion

The ability of adenosine agonists to modulate the electrically evoked release of acetylcholine (ACh) from [3H]choline preloaded guinea-pig superior cervical ganglia (SCG) was investigated. The adenosine A1-receptor selective agonist N6-cyclohexyladenosine (CHA) and 2-chloroadenosine (2-CADO) inhibited the evoked transmitter release, the effect being reversed by the A1-receptor selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and by sulmazole (SUL), which blocks both the A1-receptor and the adenylate cyclase inhibitory regulator Gi. In whole ganglia, CHA decreased both the basal and the forskolin (FSK)-stimulated cyclic AMP synthesis. The latter effect was again prevented by the A1 antagonist DPCPX. These results are compatible with the existence, in the guinea-pig SCG, of adenosine A1-receptors, part of which are located on the presynaptic nerve terminals mediating an inhibition of ACh release.

Beta 2-adrenoceptor stimulation inhibits ganglionic transmission in ferret trachea

To study the effect of beta-adrenoceptor stimulation on the transmission through airway parasympathetic ganglia, we used an in vitro nerve-muscle preparation of ferret trachea. Isometric muscle contractions were induced for 20 s every 2 min alternating with postganglionic activation by electrical field stimulation (EFS) or with preganglionic activation by direct nerve stimulation (DNS). The pulse frequency was always 12 Hz. A possible modulation of ganglionic transmission was assessed by comparing contractions induced by DNS to contractions induced by EFS. In 9 experiments isoprenaline initially reduced the response to EFS and DNS to the same extent, indicating an effect on postganglionic structures. After 20-30 min exposure the response to preganglionic activation by DNS was further reduced indicating an inhibition of ganglionic transmission. The concentration/response relationship for the ganglionic effect of isoprenaline was shifted about ten times to the right by 0.1 microM propranolol (n = 5) and by a beta 2-adrenoceptor-antagonist (0.1 microM ICI 118.551) (n = 8), whereas a beta 1-adrenoceptor-antagonist (10 microM practolol) had no, or only slight, effect. In two experiments pretreatment by 0.1 microM phenoxybenzamine for 20 min did not prevent ganglionic inhibition by 0.3 microM isoprenaline. We conclude that parasympathetic ganglia in ferret trachea have beta-adrenoceptors mediating inhibition of ganglionic transmission, and that these probably are of the beta 2 type. Tentatively these receptors could be operated upon by circulating catecholamines.

Alpha-2 adrenergic modulation of norepinephrine secretion in the perfused rabbit iris-ciliary body

Clonidine and other selective alpha-2 adrenergic agonists have been found to lower intraocular pressure in the eyes of rabbits and primates, including humans. It has been suggested that the ocular hypotensive response to alpha-2 agonists may be mediated, in part, by prejunctional inhibition of norepinephrine secretion at intraocular synapses. In this study, we have investigated the effects of adrenergic agonists and antagonists on field-stimulated, Ca++-dependent release of 3H-norepinephrine (3H-NE) from isolated, perfused rabbit iris-ciliary bodies and have utilized radioligand binding methods to identify prejunctional adrenoceptors in this tissue. Clonidine (10(-9)-10(-5) M) produced a dosage-dependent inhibition of stimulation-evoked 3H-NE secretion (EC50 approximately equal to 3 X 10(-8) M), but did not alter basal secretion. Other adrenergic agonists capable of activating alpha-2 adrenoceptors (e.g., epinephrine, norepinephrine and xylazine) also significantly depressed 3H-NE secretion, whereas selective alpha-1 adrenergic or beta adrenergic agonists were without effect. Clonidine-mediated inhibition of 3H-NE release was reversed by the selective alpha-2 antagonist yohimbine (10(-7) M), but was unaffected by prazosin or timolol. Yohimbine alone markedly enhanced 3H-NE secretion, indicating tonic activation of prejunctional alpha-2 adrenoceptors by endogenous released norepinephrine. Forskolin or 8-bromo-cAMP, which alone enhanced norepinephrine secretion, failed to attenuate the inhibitory responses to alpha-2 agonists. 3H-rauwolscine binding measurements showed a small decrease in alpha-2 receptor sites in iris-ciliary body membranes following surgical sympathetic denervation. It is concluded that the rabbit iris-ciliary body contains functional, prejunctional alpha-2 adrenoceptors which may play an autoregulatory role in vivo and contribute to the ocular effects of adrenergic drugs.

Quantitative evaluation of alpha- and beta-adrenoceptor modulation of [3H]choline release in guinea pig superior cervical ganglia

[3H] Overflow evoked by 5 min supramaximal preganglionic stimulation at 1 pps has been studied in isolated guinea pig superior cervical ganglion preparations preincubated with [3H]choline. At 15 microM norepinephrine (NE) reduced both the [3H]choline overflow and endogenous acetylcholine release by 59.4 and 54.1% respectively; the dose-response curve for NE inhibitory action is described. Evidence is given that endogenous catecholamines effectively reduce ACh release from the ganglia. After blocking the inhibitory action of endogenous NE, a significant beta-adrenoceptor-mediated facilitatory effect on ACh release could be observed. Preincubation of the ganglia with different combinations of alpha 1 and alpha 2 agonists (phenylephrine, 10 microM and clonidine, 1 microM respectively) and antagonists (prazosin, 10 microM and yohimbine, 3 microM) showed that the adrenoceptors involved in alpha-mediated NE inhibition of ACh output are exclusively of the alpha 2-type.

Some properties of adenosine 3',5'-cyclic monophosphate phosphodiesterase in the superior cervical ganglion of the guinea pig

Adenosine 3',5'-cyclic monophosphate (cAMP) phosphodiesterase activity in crude guinea-pig superior cervical ganglion homogenates was assayed under a variety of experimental conditions. Two forms of cAMP phosphodiesterase were found, one with high and the other with low affinity for the substrate. The Km values were about 1 and 110 microM respectively. Imidazole slightly but constantly stimulated the former enzyme form over a wide range of concentrations and 1-methyl-3-isobutylxanthine was a weak competitive inhibitor with a Ki value of 90 microM. Low affinity cAMP phosphodiesterase activity was increased by calmodulin and Ca2+. This stimulation was not observed in the presence of trifluoperazine, a specific inhibitor of calmodulin. On the other hand, neither [D-Ala2]met-enkephalinamide nor prostaglandin E2, alone or in combination, influenced high affinity cAMP phosphodiesterase.

Evidence that endogenous catecholamines can regulate acetylcholine release in a sympathetic ganglion

The objective of this study was to test whether activation of alpha-adrenoceptors by endogenously released catecholamines alters the release of acetylcholine (ACh) from the cat superior cervical ganglion. The alpha-adrenoceptor agonists noradrenaline and clonidine depressed evoked ACh release; this effect was concentration-dependent; it was apparent during preganglionic stimulation at 20 Hz, but not so at lower frequencies of stimulation. The inhibitory effect of noradrenaline on evoked ACh release was reversed by yohimbine, by phentolamine and, to a lesser extent, by prazosin. Thus, exogenous amines can depress evoked ACh release by an action on presynaptic alpha-adrenoceptors. To determine if activation of these receptors by endogenous amines inhibits ACh release, we tested whether the alpha-adrenoreceptor antagonists enhance ACh release. Yohimbine and phentolamine increased evoked ACh release during preganglionic stimulation at 20 Hz, but not during stimulation at 5 Hz, suggesting that endogenous, like exogenous, amine can depress evoked ACh release from preganglionic nerve terminals.

Forskolin suppresses sympathetic neuron function and causes ocular hypotension

Recent reports suggest that forskolin can produce ocular hypotension in laboratory animals and man by enhancing formation of cyclic AMP. This proposed mechanism of action implies that forskolin lowers intraocular pressure (IOP) by activating adenylate cyclase at some postjunctional site. Intra-arterial (i.a.) injections of forskolin (10, 33, 100 & 333 micrograms) into the cat nictitating membrane (CNM) preparation produced dose-related inhibition of contractions elicited by electrical stimulation of pre-and postganglionic sympathetic neurons. Interestingly, contractions elicited by i.a. norepinephrine were inhibited less than neuronally mediated contractions. Topically applied forskolin (0.5 mg) to the eyes of unilaterally sympathectomized (SX) rabbits and normal rabbits elicited ocular hypotension in the innervated eyes only. Forskolin (0.5 mg, topically) suppressed the rise in IOP induced by water loading in normal rabbits but was significantly less effective in SX rabbits. These results suggest that forskolin lowered IOP in rabbits and suppressed contraction of the electrically-stimulated CNM, in part, by inhibiting sympathetic neuronal function.