Muscarinic inhibition of [3H]-noradrenaline release on rabbit iris in vitro: effects of stimulation conditions on intrinsic activity of methacholine and pilocarpine

Autonomic control of the eye

The autonomic nervous system influences numerous ocular functions. It does this by way of parasympathetic innervation from postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia, and by way of sympathetic innervation from postganglionic fibers that originate from neurons in the superior cervical ganglion. Ciliary ganglion neurons project to the ciliary body and the sphincter pupillae muscle of the iris to control ocular accommodation and pupil constriction, respectively. Superior cervical ganglion neurons project to the dilator pupillae muscle of the iris to control pupil dilation. Ocular blood flow is controlled both via direct autonomic influences on the vasculature of the optic nerve, choroid, ciliary body, and iris, as well as via indirect influences on retinal blood flow. In mammals, this vasculature is innervated by vasodilatory fibers from the pterygopalatine ganglion, and by vasoconstrictive fibers from the superior cervical ganglion. Intraocular pressure is regulated primarily through the balance of aqueous humor formation and outflow. Autonomic regulation of ciliary body blood vessels and the ciliary epithelium is an important determinant of aqueous humor formation; autonomic regulation of the trabecular meshwork and episcleral blood vessels is an important determinant of aqueous humor outflow. These tissues are all innervated by fibers from the pterygopalatine and superior cervical ganglia. In addition to these classical autonomic pathways, trigeminal sensory fibers exert local, intrinsic influences on many of these regions of the eye, as well as on some neurons within the ciliary and pterygopalatine ganglia.

Muscarinic receptor agonists and antagonists: effects on ocular function

Muscarinic agonists act mainly via muscarinic M₃ cholinoceptors to cause contraction of the iris sphincter, ciliary muscle and trabecular meshwork as well as increase outflow facility of aqueous humour. In the iris dilator, the effect of muscarinic agonists is species dependent but is predominantly relaxation via muscarinic M₃ receptors. In the conjunctiva, muscarinic agonists stimulate goblet cell secretion which contributes to the protective tear film. Muscarinic M₂ and M₃ receptors appear mainly involved. In the lens muscarinic agonists act via muscarinic M₁ receptors to produce depolarization and increase [Ca(2+)](i). All five subtypes of muscarinic receptor are present in the retina. In the developing retina, acetylcholine appears to limit purinergic stimulation of retinal development and decrease cell proliferation. In the adult retina acetylcholine and other muscarinic agonists may have complex effects, for example, enhancing light-evoked neuronal firing in transient ON retinal ganglion cells and inhibiting firing in OFF retinal ganglion cells. In the lacrimal gland, muscarinic agonists activate M₃ receptors on secretory globular acinar cells to stimulate tear secretion and also cause contraction of myoepithelial cells. In Sjögren's syndrome, antibodies to the muscarinic M₃ receptor disrupt normal gland function leading to xerophthalmia although the mechanism of action of the antibody is still not clear. Atropine and pirenzepine are useful in limiting the development of myopia in children probably by an action on muscarinic receptors in the sclera, although many other muscarinic receptor antagonists are not effective.

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.

Role of N- and L-type calcium channels in depolarization-induced activation of tyrosine hydroxylase and release of norepinephrine by sympathetic cell bodies and nerve terminals

Multiple types of voltage-activated calcium (Ca(2+)) channels are present in all nerve cells examined so far; however, the underlying functional consequences of their presence is often unclear. We have examined the contribution of Ca(2+) influx through N- and L- type voltage-activated Ca(2+) channels in sympathetic neurons to the depolarization-induced activation of tyrosine hydroxylase (TH), the rate-limiting enzyme in norepinephrine (NE) synthesis, and the depolarization-induced release of NE. Superior cervical ganglia (SCG) were decentralized 4 days prior to their use to eliminate the possibility of indirect effects of depolarization via preganglionic nerve terminals. The presence of both omega-conotoxin GVIA (1 microM), a specific blocker of N-type channels, and nimodipine (1 microM), a specific blocker of L-type Ca(2+) channels, was necessary to inhibit completely the stimulation of TH activity by 55 mM K(+), indicating that Ca(2+) influx through both types of channels contributes to enzyme activation. In contrast, K(+) stimulation of TH activity in nerve fibers and terminals in the iris could be inhibited completely by omega-conotoxin GVIA alone and was unaffected by nimodipine as previously shown. K(+) stimulation of NE release from both ganglia and irises was also blocked completely when omega-conotoxin GVIA was included in the medium, while nimodipine had no significant effect in either tissue. These results indicate that particular cellular processes in specific areas of a neuron are differentially dependent on Ca(2+) influx through N- and L-type Ca(2+) channels.

Pupil responses to stimulus color, structure and light flux increments in the rhesus monkey

Visual stimuli that isolate pupil color and pupil grating responses in human vision have been used to investigate the properties, of stimulus-specific pupil responses in the rhesus monkey. We measured and compared pupil responses to light flux increments, isoluminant chromatic stimuli, and gratings of equal and lower space-averaged luminance. The parameters investigated were luminance contrast and chromatic saturation. The results demonstrate clearly the existence of pupil color, pupil grating and pupil light reflex responses in the rhesus monkey. Comparison of pupil color and pupil grating responses of equivalent amplitude reveals similar onset response latencies. However, both are approximately 40 ms longer than the corresponding pupil light reflex latency. In general these pupil responses are qualitatively similar to those observed in humans. However, when compared to equivalent human data, pupil onset response latencies are some 80-100 ms shorter and the pupil shows more rapid recovery from constriction.

Effects of human recombinant interleukins on stimulation-evoked noradrenaline overflow from the rat perfused spleen

Experiments were carried out in the isolated spleen of the rat to study in a lymphoid organ the influence of interleukins (ILs) on noradrenaline release. Spleens were perfused with Tyrode's solution and the overflow of endogenous noradrenaline was determined by HPLC with electrochemical detection. Perivascular electrical stimulation (4 or 10 Hz, 20-28 mA, 2 min) caused an increase in noradrenaline overflow and in perfusion pressure, both of which were markedly reduced by perfusion with Ca(2+)-free solution, abolished by tetrodotoxin, unaffected by hexamethonium, and subject to alpha 2-adrenoceptor- and muscarinic receptor-mediated modulation as shown by the effects of rauwolscine and methacholine. Human recombinant IL-1 beta and IL-2 and mouse recombinant IL-2 10 ng/ml failed to affect the evoked overflow of noradrenaline after an exposure time of 15 min. In contrast, human recombinant IL-1 beta and IL-2 0.1 ng/ml reduced the evoked overflow after exposure for 80 min; the inhibition tended to increase 30 min later despite washout. Murine recombinant IL-2 1.2 ng/ml caused no change after contact with the tissue for 80 min but there was an inhibition 30 min later after washout. Human recombinant IL-6 (0.1 ng/ml) caused no significant change. The inhibitory effect of low concentrations of IL-1 beta and IL-2 supports the idea that locally produced mediators of the immune system may affect neuronal function.

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.

Developmental regulation of parasympathetic nerve density by sympathetic innervation in the tarsal smooth muscle of the rat

The developmental influence of sympathetic innervation on parasympathetic nerve density was investigated in the tarsal smooth muscle of the rat. Specificity of acetylcholinesterase staining as a marker for parasympathetic innervation was first determined by acute selective denervations. Excision of the ipsilateral superior cervical ganglion caused a 39% reduction in the density of acetylcholinesterase-positive nerves seven days later, indicating that sympathetic nerves contribute to cholinesterase-positive tarsal muscle innervation. Excision of the pterygopalatine ganglion concurrent with superior cervical ganglionectomy caused a virtually complete disappearance of acetylcholinesterase-positive innervation within seven days, indicating that non-sympathetic cholinesterase-positive fibers derive from the pterygopalatine ganglion and are presumed to be parasympathetic. Analysis of the control population indicated that parasympathetic nerve density did not vary significantly between males and females, between the superior and inferior muscles, or in rats studied at four and 12 months of age. The influence of sympathetic innervation on parasympathetic nerve density during postnatal development was examined by conducting surgical sympathectomies on postnatal day 5 and quantifying acetylcholinesterase-positive nerve density at four months of age. Neonatal sympathectomy caused a 46% reduction in cholinesterase-positive nerve density beyond that which occurred in acutely sympathectomized adult controls. It is concluded that sympathetic innervation is required for developing parasympathetic nerves to attain their normal density within the rat tarsal muscle. This finding is consistent with the idea that sympathetic nerves can exert positive effects on parasympathetic nerve outgrowth during development.

Muscarine receptor types mediating autoinhibition of acetylcholine release and sphincter contraction in the guinea-pig iris

The potencies of several muscarine receptor antagonists in blocking either the autoinhibition of acetylcholine release or the muscarinic contraction of the sphincter muscle upon acetylcholine release were investigated in the guinea-pig iris. The agonist at pre- or postjunctional muscarine receptors was acetylcholine released upon field stimulation (5.5 Hz, 2 min) of the irides preloaded with 14C-choline. The stimulation-evoked 14C-overflow was doubled in the presence of atropine 0.1 mumol/l but unaffected by the agonist (+/-)-methacholine (50 mumol/l). Thus, under the present stimulation conditions, the autoinhibition of acetylcholine release on the guinea-pig iris cholinergic nerves was nearly maximally activated. Isotonic contractions of the irides upon field stimulation consisted of a rapid, atropine (0.1 mumol/l)-sensitive peak phase followed by a sustained contraction which involved a cholinergic and a non-cholinergic stimulation of the sphincter muscle. The M2-selective antagonists methoctramine (10 mumol/l) and gallamine (100 mumol/l) increased both the 14C-overflow and the peak contractions evoked by field stimulation. In contrast, the M3-selective antagonist hexahydrosiladifenidol (0.1-10 mumol/l) failed to affect the evoked 14C-release but concentration-dependently (1-10 mumol/l) reduced the iris contractions. Pirenzepine (10 mumol/l) enhanced the evoked 14C-overflow and inhibited the peak contractions (0.1-10 mumol/l; maximal effect at 10 mumol/l). The low potency of the antagonist at both receptor sites indicates that an M1 muscarine receptor is not involved. The results are consistent with the idea of M2 muscarine receptors mediating autoinhibition of acetylcholine release in the guinea-pig iris and M3-like receptors inducing the contraction of the sphincter muscle.

Different muscarine receptors mediate the prejunctional inhibition of [3H]-noradrenaline release in rat or guinea-pig iris and the contraction of the rabbit iris sphincter muscle

To investigate the muscarine receptor type mediating inhibition of [3H]-noradrenaline release from the isolated rat and guinea-pig iris we have determined the potency of antimuscarinic drugs to antagonize the methacholine-induced inhibition of [3H]-noradrenaline overflow evoked by field stimulation (3 Hz, 2 min). The prejunctional apparent affinities were compared with those obtained for postjunctional muscarine receptors mediating the methacholine-induced contraction of the isolated rabbit iris sphincter muscle. Prejunctional apparent affinity constants of pirenzepine (6.67), himbacine (8.51), methoctramine (7.92), 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, 8.00), hexahydro-difenidol enantiomers (6.92, (R); 5.77, (S)) in the rat iris and methoctramine (7.58) in the guinea-pig iris indicate the presence of M2 receptors. Although the postjunctional affinity constants in the rabbit iris sphincter of methoctramine (5.93), gallamine (3.92), and 4-DAMP (9.07) confirm our previous suggestions of the presence of M3-like receptors, the results obtained with the hexahydro-difenidol enantiomers do not agree with that concept. The postjunctional affinity constants of the hexahydro-difenidol enantiomers were not different from the prejunctional values (6.86, (R); 5.55, (S)), indicating a similar and low degree of stereoselectivity for these stereoisomers at both receptor sites (14 and 17, (R)/(S)-ratios, respectively). Hence, the postjunctional muscarine receptor in the rabbit iris sphincter fails to exhibit the high degree of stereoselectivity observed for hexahydro-difenidol enantiomers at M3 receptors on other smooth muscles.

Effects of muscarinic receptor agonists and antagonists on alpha 2-adrenoceptors in rat brain

The specific binding of [3H]clonidine to alpha 2-adrenoceptors on neural membranes isolated from six brain areas was determined with rats treated for various periods of time with the muscarinic agonists, oxotremorine or pilocarpine, or with the muscarinic antagonists atropine, atropine methyl nitrate, scopolamine and scopolamine methyl bromide. Administration of pilocarpine, 10 mg/kg, twice daily i.p. for 1 and 14 days increased markedly the number of alpha 2-adrenoceptors on neural membranes from all six brain areas. In contrast, oxotremorine, 0.3 mg/kg, twice daily i.p., for 7 days decreased the number of alpha 2-adrenoceptors on membranes from all brain areas except the brainstem and caudate nucleus. Both atropine and scopolamine increased the density of alpha 2-adrenoceptors in specific brain areas. Neither atropine methyl nitrate nor scopolamine methyl bromide had an appreciable effect upon the specific binding of [3H]clonidine to neural membranes from most brain areas.

Presynaptic muscarinic receptors inhibiting endogenous noradrenaline release in the portal vein of the freely moving rat

1. In the portal vein of the freely moving unanaesthetized rat, the existence of presynaptically located inhibitory muscarinic receptors was investigated by use of the muscarinic agonist methacholine (MCh) 2. Infusion of MCh (0.3 micrograms min-1) did not significantly inhibit the endogenous noradrenaline (NA) overflow in portal plasma. However, after inducing high intra-synaptic concentrations of NA by blocking the presynaptic alpha 2-adrenoceptors with yohimbine (1 mg kg-1), MCh (0.3 microgram min-1) was able to reduce the yohimbine-induced enhanced NA overflow by 38%. 3. The MCh-induced inhibition was almost completely abolished after blockade of the presynaptic muscarinic receptors with atropine (0.6 mg kg-1). 4. During electrical stimulation of the portal vein nervous plexus the evoked NA overflow was strongly inhibited (95%) during MCh-infusion (0.3 microgram min-1). Again atropine (0.6 mg kg-1) was able to reverse this inhibition. 5. These results show the existence of presynaptic muscarinic receptors inhibiting endogenous NA overflow from the portal vein nervous plexus under conditions of enhanced sympathetic activity in the freely moving rat.

M2 muscarinic receptors on the iris sphincter muscle differ from those on iris noradrenergic nerves

The pre- and postjunctional affinity constants of a series of muscarinic antagonists were determined in guinea pig and rabbit irises. Field stimulation-evoked [3H]noradrenaline release from superfused isolated irises was concentration dependently inhibited by (+/-)-methacholine, confirming the presence on the iris noradrenergic nerves of prejunctional inhibitory muscarinic receptors. The affinity constants of the antagonists at the pre- and postjunctional receptors are compatible with the coexistence in the iris of two different M2 receptors: the cardiac (M2 alpha) subtype on the noradrenergic nerves and the smooth muscle (M2 beta) subtype on the iris sphincter muscle. The rank order of potency of the antagonists studied at the prejunctional site was: atropine greater than himbacine greater than AF-DX 116 greater than pirenzepine greater than hexahydrosiladifenidol. The order of potency at the postjunctional receptors mediating the methacholine-induced isotonic contraction of the isolated rabbit iris sphincter was: atropine greater than hexahydrosiladifenidol greater than pirenzepine greater than himbacine greater than AF-DX 116.

AF-DX 116 differentiates between prejunctional muscarine receptors located on noradrenergic and cholinergic nerves

Prejunctional affinity constants of the cardioselective muscarine receptor antagonist AF-DX 116 (11-[(2-[(diethyl-amino)methyl]-1-piperidinyl)acetyl]-5,11-dihydro-6 H-pyrido [2,3-b] [1,4] benzodiazepine-6-one) were determined for muscarine autoreceptors on cholinergic nerves of the guinea-pig ileum and for heteroreceptors on noradrenergic nerves of the rat heart and guinea-pig iris. AF-DX 116 antagonized with low affinity the muscarinic inhibition induced by arecaidine propargyl ester of the stimulation-evoked [3H]acetylcholine overflow (pA2 6.74) from the guinea-pig ileum. In contrast, AF-DX 116 was more potent in antagonizing the methacholine-induced inhibition of the stimulation-evoked [3H]noradrenaline overflow from rat heart (pA2 7.29) or guinea-pig iris (pA2 7.57). The data confirm previously reported differences between prejunctional muscarine heteroreceptors in the rat heart which belong to the cardiac subtype (M2 alpha or M2) and autoreceptors in the guinea-pig ileum that cannot be distinguished from the ileal subtype (M2 beta) or M3).