Potentiation of norepinephrine secretion by angiotensin II in the isolate rabbit iris-ciliary body

3H-noradrenaline release from mouse iris?ciliary body: role of presynaptic muscarinic heteroreceptors

Sympathetic neurotransmitter release and its modulation by presynaptic muscarinic heteroreceptors were studied in mouse iris-ciliary bodies. Tissue preparations were preincubated with (3)H-noradrenaline and then superfused and stimulated electrically. Firstly, experimental conditions were defined, allowing study of presynaptic sympathetic inhibition in mouse iris-ciliary body. If tissue was stimulated four times with 36 pulses/3 Hz, tritium overflow peaks were reliably and reproducibly measured. As expected, these stimulation conditions led to marked alpha(2)-autoinhibition as indicated by the release-enhancing effect of the alpha(2)-antagonists phentolamine and rauwolscine. To ensure autoinhibition-free (3)H-noradrenaline release, which is optimal for studying presynaptic sympathetic inhibition, alpha(2)-receptors were blocked in all subsequent experiments. Under these conditions, evoked tritium overflow was almost completely abolished in the presence of the sodium channel blocker tetrodotoxin, indicating a neuronal origin of (3)H-noradrenaline release. Secondly, muscarinic inhibition of (3)H-noradrenaline release was characterized using the conditions described above (36 pulses/3 Hz; phentolamine 1 muM and rauwolscine 1 muM throughout). The muscarinic receptor agonist oxotremorine M decreased evoked tritium overflow in a concentration-dependent manner with an IC(50) of 0.33 muM and maximal inhibition of 51%. The concentration-response curve of oxotremorine M was shifted to the right by the muscarinic antagonists ipratropium and methoctramine, whereas pirenzepine was ineffective. The observed rank order of antagonist potencies, ipratropium > methoctramine > pirenzepine, which is typical for the M(2) subtype, indicates that presynaptic muscarinic receptors on sympathetic axons of mouse iris-ciliary bodies are predominantly M(2). Finally, inhibition of (3)H-noradrenaline release by endogenously secreted acetylcholine was investigated. Longer pulse trains, 120 pulses/3 Hz and 600 pulses/5 Hz, were used and the cholinesterase inhibitor physostigmine was added to the superfusion medium to increase synaptic levels of endogenous acetylcholine. Under these conditions, ipratropium approximately doubled the evoked overflow of tritium, indicating that endogenously released acetylcholine can activate presynaptic muscarinic heteroreceptors. In conclusion, the present experiments establish measurement of the electrically induced release of (3)H-noradrenaline from mouse iris-ciliary bodies. As in other species, noradrenaline release in this preparation was subject to presynaptic muscarinic inhibition. Our results also indicate that the presynaptic muscarinic receptors on sympathetic axons in mouse iris-ciliary body are predominantly M(2). Moreover, these receptors can be activated by both exogenous agonists and endogenously released acetylcholine and, hence, may operate physiologically in the interplay between the parasympathetic and sympathetic nervous system.

Functional consequences of prejunctional receptor activation or blockade in the iris

The iris is innervated by nerves of the sympathetic, parasympathetic, and sensory nervous systems. The terminal nerve fibres are endowed with prejunctional receptors which modulate neurotransmitter release. Activation or blockade of prejunctional receptors by drugs may have an influence on iris smooth muscle tone. Several findings are in favour of the hypothesis that prejunctional receptors may be involved in regulation of iris smooth muscle tone and/or pathophysiological events. (i). Release of acetylcholine from parasympathetic nerves of guinea-pig iris sphincter evoked by electrical stimulation is subject to autoinhibition via prejunctional M2 muscarinic receptors, and the release can be enhanced by M2 selective antagonists such as methoctramine or gallamine. Concomitantly with the increased neurotransmitter release, the sphincter contraction is enhanced in the presence of M2 antagonists, since the postjunctional muscarinic receptors (presumably M3, or at least not M2) are not simultaneously blocked. Unlike the non-selective blocker atropine, M2 antagonists are not expected to cause mydriasis but rather miosis. (ii). Sensory nerves are involved in pathophysiological events following ocular irritation. Release of substance P and/or neurokinin A from sensory nerves of rabbit iris is followed by a non-adrenergic-non-cholinergic iris sphincter contraction (mediated by NK1 and NK3 receptors) which can be used to estimate sensory neurotransmitter release. Exocytotic release of the sensory neurotransmitters is inhibited by activation of alpha 2B-adrenoceptors and probably also via putative prejunctional imidazoline receptors. Alpha-adrenoceptors are stimulated by oxymetazoline and other imidazoline derivatives (which are agonists at imidazoline receptors) leading to a reduction of sensory neurotransmitter release, as evident from a decrease in evoked sphincter contraction. Imidazolines in eye drops may not only cause relief in ocular inflammation due to postjunctional vasoconstriction but also possibly due to a prejunctional effect, a reduction of sensory neurotransmitter release. Reinforcement of inflammation due to release of sensory neurotransmitters may thus be prevented.