Inhibition of adenylate cyclase in bovine ciliary process and rabbit iris ciliary body to alpha 2-adrenergic agonists

Effects of a topical alpha2 adrenergic agonist on ciliary blood flow and aqueous production in rabbits

The relationship between ciliary perfusion and aqueous production is poorly understood. It was recently reported that aqueous production decreases when ciliary blood flow is reduced by lowering the ocular perfusion pressure, and hypothesized that drug-induced reduction of ciliary blood flow would also decrease aqueous production. In the present study, we test this hypothesis with an alpha2 adrenergic agonist (brimonidine) formulated for topical application. When used acutely, brimonidine decreases intraocular pressure (IOP) by suppressing aqueous production, although its mechanism of action is unclear. The experiments were performed in four groups of anesthetized rabbits (n=33) in which the following variables were measured: ocular mean arterial pressure (OMAP), IOP, orbital venous pressure (OVP), aqueous flow, ciliary blood flow, ciliary oxygen tension (PO2), episcleral venous pressure (EVP), carotid blood flow and heart rate. The measurements were made before and after brimonidine (0.15%, 40 microl) was applied to the cornea. Brimonidine decreased IOP (-33%, p<0.01), aqueous flow (-39%, p<0.01), ciliary blood flow (-37%, p<0.01), EVP (-42%, p<0.01) and ciliary PO2 (-32%, p<0.05). We conclude that topical brimonidine is a ciliary vasoconstrictor, and that alpha2 adrenergic agonist-induced decreases in ciliary blood flow decrease aqueous production.

A placebo-controlled study of lofexidine in the treatment of children with tic disorders and attention deficit hyperactivity disorder

This study evaluated the efficacy and safety of Lofexidine in treating children with tic disorders and attention deficit hyperactivity disorder (ADHD). Subjects from a specialty tic disorders clinic were randomly assigned to receive 8 weeks of treatment with lofexidine or placebo under double-blind conditions. Follow-up visits occurred every 2 weeks for safety monitoring and dose adjustment. Fourty-four medication-free subjects (41 boys and three girls; mean age of 10.4 years) with ADHD, combined type, and a tic disorder participated. After 8 weeks of treatment, lofexidine was associated with a mean improvement of 41% in the total score on the teacher-rated ADHD Rating Scale compared to 7% improvement for placebo. Eleven of 22 subjects who received lofexidine were blindly rated on the Clinical Global Scale-Improvement as either much improved or very much improved compared to none of 22 subjects who received placebo. The mean score on the parent-rated hyperactivity index improved by 29% in the lofexidine group and 18% in the placebo group, which was not a significant difference. On the Continuous Performance Test, commission errors decreased by 25% and omission errors by 20% in the lofexidine group, compared with increases of 33% in commission errors and of 36% in omission errors in the placebo group. Tic severity decreased by 27% in the lofexidine group, compared to 0% in the placebo group. One lofexidine subject with sedation withdrew at week 4. Lofexidine was associated with insignificant decreases in blood pressure and pulse. Lofexidine appears to be a safe and effective treatment for children with tic disorders and ADHD.

Naphazoline-induced suppression of aqueous humor pressure and flow: involvement of central and peripheral alpha(2)/I(1) receptors

The objective of this study was to examine the ocular hydrodynamic effects of topically and centrally administered naphazoline, alone and following pretreatment with pertussis toxin (PTX) and alpha(2)/I(1)receptor antagonists. Topically and intracisternally administered naphazoline was examined for its ability to alter intraocular pressure (IOP) of rabbits in the absence and presence of receptor antagonists (rauwolscine, efaroxan) and a G(i/o)ribosylating agent PTX. In addition, the topical effects of naphazoline on pupil diameter and aqueous humor flow rate were evaluated. Topical unilateral application of naphazoline (7.5, 25 and 75 micro g; 25 micro l) elicited an ipsilateral dose-dependent mydriasis (2, 4 and 5.5 mm) that peaked at 2 hr with a duration of up to 5 hr. The IOP decreases induced by naphazoline were bilateral and dose-dependent (3, 6 and 10 mmHg); the response peaked at 1 hr and lasted for up to 5 hr. Pretreatment with efaroxan (250 micro g) elicited significantly greater antagonism of the ocular hypotensive response to naphazoline than did rauwolscine (250 micro g) suggesting an involvement of imidazoline (I(1)) receptors. Intracisternal application of naphazoline (3.3 micro g) also produced bilateral reductions (6 mmHg) of IOP that were immediate (10 min post drug) and lasted for approximately 2 hr. In PTX-pretreated (2.5 micro g kg(-1), i.a.) rabbits, the ocular hypotensive effects of naphazoline by both routes (topically and centrally) were attenuated by 50--65%. In addition to producing ocular hypotension, topical application of naphazoline (75 micro g; 25 micro l) caused significant reduction, from 2.8 to 1.5 micro l min(-1), in aqueous humor flow. These in vivo data indicate that, regardless of route of administration, alteration of aqueous humor flow by naphazoline was induced by the activation of alpha(2)and I(1)receptors. The ocular hypotensive effects produced by central administration did not result in sedation, therefore, there is the suggestion that central alpha(2)adrenergic receptors were stimulated minimally by naphazoline. Thus, these data suggest that ocular hypotensive effects and suppression of aqueous humor flow rate by naphazoline are mediated, in part, by alpha(2)and/or central I(1)at both central (brain) and peripheral (eye) sites. Moreover, these data indicate that the receptors are linked to PTX-sensitive G((i/o))proteins.

Multiple receptor activation elicits synergistic IP formation in nonpigmented ciliary body epithelial cells

We have examined the interaction between muscarinic and alpha(2)-adrenergic receptor activation on inositol phosphate (IP) formation in the nonpigmented cells of the ciliary body epithelium (NPE cells) of the rabbit. We have compared these changes with those previously observed in the intracellular free Ca(2+) concentration. Whereas muscarinic receptor activation causes an increase in intracellular Ca(2+) and IP formation, activation of alpha(2)-receptors does not significantly increase either intracellular Ca(2+) or IPs over basal levels. However, simultaneous activation of muscarinic and alpha(2)-adrenergic receptors with the specific agonists carbachol and UK-14304 produces massive Ca(2+) increases and results in a synergistic increase in IP formation. This synergistic IP formation is inhibited by both muscarinic and alpha(2)-adrenergic receptor antagonists as well as by pertussis toxin and an inhibitor of phospholipase C. IP formation is predominantly independent of intracellular Ca(2+), because it is decreased but not prevented by blocking the entry of Ca(2+) with LaCl(3) or chelating intracellular Ca(2+) with 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Thus synergistic IP formation underlies, at least in part, the synergistic increase in intracellular Ca(2+) resulting from simultaneous activation of muscarinic and alpha(2)-adrenergic receptors.

Adrenergic and imidazoline receptor-mediated responses to UK-14,304-18 (brimonidine) in rabbits and monkeys. A species difference

Brimonidine is a relatively selective alpha-2 adrenoceptor agonist that is being developed for the treatment of glaucoma. Because brimonidine is chemically related to clonidine and has affinity for the nonadrenergic imidazoline receptor, its ocular effects may be unrelated to alpha-2 receptor activation. The objective of this study was to determine the pharmacology of the intraocular pressure (IOP) response to brimonidine in rabbits and monkeys and the side effects (miosis, cardiovascular depression) in monkeys. Conscious albino rabbits and cynomolgus monkeys were pretreated topically with the following receptor antagonists: rauwolscine (alpha-2), idazoxan (alpha-2, imidazoline), SKF 105854 (vascular postjunctional alpha-2), and prazosin (alpha-1). Intraocular pressure, pupil size, or blood pressure/heart rate was monitored noninvasively for 6 hours following dosing. Binding experiments were performed using [3H]brimonidine in membrane preparations from rabbit iris/ciliary body and from monkey cerebral cortex and brain stem. In rabbits, the ocular hypotensive response to brimonidine was unilateral and was inhibited by rauwolscine > idazoxan >> SKF 105854 = prazosin; this ranked order of potency correlated with displacement of [3H]brimonidine in the rabbit iris/ciliary body. In monkeys, brimonidine decreased IOP bilaterally and suppressed cardiovascular function suggesting a CNS site of action. Intraocular pressure and cardiovascular responses to brimonidine were inhibited by idazoxan >> rauwolscine > SKF 105854 = prazosin; a similar profile was obtained for displacement of [3H]brimonidine in monkey brain tissue. Both rauwolscine and idazoxan inhibited the miotic response to brimonidine in monkeys. Taken together, these results indicate that brimonidine stimulates an ocular alpha-2 adrenoceptor to decrease IOP in the rabbit and a CNS imidazoline receptor to decrease IOP, blood pressure, and heart rate in the cynomolgus monkey. The miotic response in the monkey is mediated by an alpha-2 adrenoceptor. The alpha-1 and vascular postjunctional alpha-2 adrenoceptors do not appear to play a role in mediating these responses.

Cyclic adenosine monophosphate stimulation of mucociliary activity in the upper airways in vivo

Xanthine derivatives are known to accelerate mucociliary transport in the lower airways, probably by preventing degradation of cyclic adenosine monophosphate (cAMP) and thereby increasing its intracellular concentration. The purpose of this study was to investigate the effects of cAMP on mucociliary activity in the upper airways. The effect on the mucociliary activity in the rabbit maxillary sinus of the xanthine derivatives theophylline and enprophylline was compared to that of the cAMP analog dibutyryl cAMP. The compounds were administered into the maxillary artery, and the response was recorded with a photoelectric technique. Infusions of theophylline (1.0 and 10 mg/kg) increased mucociliary activity (22.8% +/- 5.9%, n = 6, and 21.6% +/- 4.9%, n = 7, p < .05, respectively). Infusions of enprophylline (1.0 and 10.0 mg/kg) accelerated mucociliary activity (at the highest dosage tested, 24.3% +/- 4.1%). Infusions of dibutyryl cAMP (0.1 and 1.0 mg/kg) stimulated mucociliary activity, with the maximum increase (20.1% +/- 3.0%, n = 13, p < .05) being observed at a dosage of 0.1 mg/kg. The infused substances increased mucociliary activity within 1 minute after the start of the infusion, the duration of the response being approximately 20 minutes for theophylline, 22 minutes for enprophylline, and 12 minutes for dibutyryl cAMP. The present results support the view that cAMP is involved in regulating mucociliary activity in the upper airways. It remains to be elucidated whether xanthines such as theophylline and enprophylline are beneficial in upper airway disease in which mucociliary function is impaired (eg, chronic sinusitis).

Ocular alpha 2-receptor subclasses and antiglaucoma efficacy

An overview of the ocular hypotensive actions of some alpha 2-agonists with imidazoline structures is presented. These agents inhibit isoproterenol-stimulated adenylate cyclase (AC) activity in ciliary process membrane through a Na+ and GTP-dependent mechanism. Receptor binding studies with the alpha 2-agonist radioligand [125I] p-iodoclonidine ([125I]PIC) in rabbit ciliary body membranes indicate that the alpha 2-receptor subtype is alpha 2A. Gpp(NH)p and NaCl dose-dependently decreased the number of [125I]PIC binding sites by shifting the receptor-G protein complexes from the high affinity state to the low affinity state for agonist binding. This is consistent with the observations that inhibition of AC was Na+ and GTP dependent. The NaCl and Gpp(NH)p effects on binding appeared to be through different mechanisms. The alpha 2-receptor in ciliary process thus appears to be an alpha 2A-receptor that is negatively coupled to the AC-cAMP generating system.

Inhibition of cAMP production by alpha 2-adrenoceptor stimulation in rabbit retina

Adenylate cyclase activity in rabbit retinal homogenates can be stimulated directly by forskolin or through a receptor-mediated mechanism by vasoactive intestinal peptide (VIP). In contrast the alpha 2-adrenoceptor agonists clonidine and UK-14,304 reduce the basal cAMP level slightly. This was more evident following application of forskolin and VIP where the decrease of cAMP caused by clonidine and UK-14,304 is dose-dependent. The alpha 2-adrenoceptor agonist response is blocked by pertussis toxin and is insensitive to the phosphodiesterase inhibitor, isobutylmethylxanthine, suggesting the involvement of a Gi-protein. Clonidine and UK-14,304 attenuation of elevated cAMP levels can be inhibited by the alpha 2-receptor antagonist yohimbine and phentolamine but not by the specific alpha 1-receptor antagonist, prazosin. Serotonergic, cholinergic and beta-adrenergic receptor antagonists were without effect. The results demonstrate that alpha 2-adrenergic receptors in the retina exert inhibitory effects on adenylate cyclase activity mediated by an inhibitory guanine nucleotide regulating protein.

Stimulatory and inhibitory cyclic AMP responses in rabbit ciliary processes after cervical ganglionectomy

Cyclic AMP production in response to agonists which act at a variety of receptors to either stimulate or inhibit cyclic AMP production has been studied in intact, dissected ciliary processes from rabbit eyes after unilateral surgical removal of the cervical ganglion. Cyclic AMP responses to stimulatory ligands vasoactive intestinal peptide (VIP), isoproterenol, and forskolin and inhibitory agonists neuropeptide Y (NPY), the synthetic somatostatin analogue SMS 201-995, and alpha-adrenergic agents were investigated in tissues from normal eyes and compared to the same responses in tissues from sympathetically denervated eyes. Neither stimulated cyclic AMP production nor inhibition of stimulated cyclic AMP production was significantly different in tissues from denervated vs. normal eyes. Inhibition of VIP-stimulated cyclic AMP production by epinephrine and paraaminoclonidine in tissues from both normal and denervated eyes was blocked by the alpha 2-adrenergic antagonist yohimbine but not by the alpha 1-adrenergic antagonist prazosin. These data indicate that the VIP, NPY, somatostatin, and alpha 2- and beta 2-adrenergic receptors which regulate cyclic AMP production in rabbit ciliary processes are postjunctional and suggest that ligands known to modulate cyclic AMP levels in this tissue may exert effects on aqueous humor formation independently of adrenergic innervation.