Michael F. Sugrue

Effects of acutely and chronically administered antidepressants on the clonidine-induced decrease in rat brain 3-methoxy-4-hydroxyphenylethylene-glycol sulphate content.

Abstract A study was undertaken of the effects of acutely and chronically administered agents on rat brain MHPGSO 4 content and on the ability of a low dose of clonidine (25 μg/kg) to lower brain MHPGSO 4 levels. This effect of clonidine is attributed to an activation of presynaptic α-adrenoceptors. The agents studied were desipramine and nisoxetine, both inhibitors of NA uptake, Org 6582, a specific inhibitor of 5-HT uptake, and iprindole and mianserin, two atypical antidepressants. In all studies, a dose of 10 mg/kg was used. Acutely administered mianserin increased rat brain MHPGSO 4 levels and prevented the clonidine-induced reduction. The clonidine-induced fall in MHPGSO 4 levels was also absent following the acute administration of desipramine or nisoxetine. However, the observed effect cannot be unequivocally attributed to a blockade of presynaptic α-adrenoceptors since both drugs decreased basal levels of MHPGSO 4 . For chronic studies drugs, other than Org 6582, were injected every 12 h for 14 days and experiments were undertaken 12 h after the last injection. Org 6582 was injected once daily for 14 days and experiments undertaken 24 h after cessation of administration. Chronic mianserin and nisoxetine increased MHPGSO 4 levels. Only chronic desipramine blocked the clonidine-induced fall, the phenomenon developing between 5 and 9 days of chronic desipramine administration. This study indicates that, under the experimental conditions employed, the ability of chronic desipramine to elicit subsensitivity of presynaptic α-adrenoceptors does not extend to the other four agents studied.

Changes in rat brain monoamine turnover following chronic antidepressant administration

Changes in rat brain monoamine turnover were studied following the chronic administration of five agents which markedly differ in their patterns of monoamine uptake inhibition. Compounds (10 mg/kg, i.p.) were injected once daily for 14 days and experiments undertaken 24 h after the last injection. Chronic administration of desipramine or mianserin elevated brain MOPEG-SO4 content and the α-MT-induced reduction in brain NA levels was enhanced by chronic desipramine. either antidepressant altered turnover of brain DA or 5-HT. Steady state levels of brain 5-HIAA or striatal levels of DOPAC or HVA were also unchanged. Chronically administered Org 6582, a selective inhibitor of 5-HT uptake, decreased basal and attenuated the probenecid-induced increase iin brain 5-HIAA levels. Chronic Org 6582 had no effect on NA or DA turnover and on the levels of MOPEG-SO4, DOPAC or HVA. Neither maprotiline nor chlorimipramine altered turnover of NA, DA or 5-HT or levels of metabolites. Thus, in contrast to the acute situation, chronically administered desipramine increases rat brain NA turnover. Conversely, acute and chronic Org 6582 administration yield similar findings, viz. a decrease in turnover. These observations suggest that rat brain 5-HT systems are more resistant than NA systems to adaptive changes following a prolonged inhibition of monoamine uptake.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.

L-662,583 is a topically effective ocular hypotensive carbonic anhydrase inhibitor in experimental animals

1 L‐662,583 was a potent inhibitor in vitro of purified, human erythrocyte carbonic anhydrase II, possessing an IC50 of 0.7 nm. The IC50 values for MK‐927, acetazolamide and methazolamide were 13.0 nm, 10.8 nm and 21.2 nm, respectively. 2 A 1 h pretreatment with one 50 μl drop of a 0.1% solution of L‐662,583 blocked carbonic anhydrase activity in a homogenate of the iris + ciliary body of albino rabbits by 63%. Similar treatment with 0.1% suspensions of acetazolamide and methazolamide elicited inhibitions of 30% and 20%, respectively. This ex vivo model indirectly assesses the ability of an agent to enter the rabbit eye. 3 Concentrations of L‐662,583 in the cornea, aqueous humour and iris + ciliary body of albino rabbits were determined by h.p.l.c. at predetermined times after the instillation (one drop of 50 μl) of a 2% solution of L‐662,583. Peak levels for cornea (47.4 μg g−1), aqueous humour (4.51 μg ml−1) and iris + ciliary body (9.61 μg g−1) occurred at 0.5, 2 and 1 h after instillation, respectively. 4 The experimentally elevated intraocular pressure of the right eye of rabbits, induced by prior intraocular injection of α‐chymotrypsin, was maximally decreased by 4.5 mmHg, 6.2 mmHg and 9.8 mmHg after the instillation (one drop of 50 μl) of 0.01%, 0.1% and 0.5% solutions of L‐662,583, respectively. All three concentrations lowered intraocular pressure at all time points from 1 h up to and including 5 h, the last recorded time point. The unilateral instillation of L‐662,583 (0.5%) into the contralateral, left eye failed to lower the elevated intraocular pressure of the untreated, right eye. This finding indicates that the site of action of topically applied L‐662,583 in this paradigm is local. The ocular normotensive, albino rabbit was much less susceptible than the ocular hypertensive rabbit to the intraocular pressure lowering effect of topically applied L‐662,583, with a 2% solution maximally decreasing intraocular pressure by 2.3 mmHg. 5 Unilateral ocular hypertension was elicited in the right eye of sedated, cynomolgus monkeys by argon laser‐induced photocoagulation of the trabecular meshwork. The instillation (one drop of 50 μl) of L‐662, 583 (2%) significantly lowered the elevated intraocular pressure of the right eye at all time points from 1 h up to and including 5 h. The maximum decline was 8.3 mmHg at 3 h and this represented a reduction of 23% from the corresponding baseline value of 36.8 mmHg. The intraocular pressure of the hypertensive, right eye was maximally decreased by 4.1 mmHg and 4.8 mmHg after the instillation of 0.5% and 1% solutions of L‐662,583, respectively. Like the rabbit, the normotensive eye of cynomolgus monkeys was more resistant than the hypertensive eye to the ocular hypotensive action of L‐662, 583, as indicated by the inability of 0.5% and 1% solutions of the agent to lower intraocular pressure. L‐662,583 (2%) maximally reduced the intraocular pressure of normotensive monkey eyes by 2.4 mmHg at 2 h. 6 L‐662,583 is structurally different from MK‐927, a carbonic anhydrase inhibitor that lowers the intraocular pressure of glaucoma patients following the instillation of a 2% solution. These preclinical observations indicate that L‐662,583, like MK‐927, is a water‐soluble carbonic anhydrase inhibitor which, on topical administration, lowers intraocular pressure by virtue of an action confined to within the eye.

A study on the ocular and extraocular pharmacology of metipranolol

Metipranolol (Betamann) is a clinically efficacious ocular hypotensive drug and preclinical experiments were undertaken to characterize this beta-adrenoceptor antagonist. In vitro beta1- and beta2-adrenoceptor antagonism was evaluated using the guinea pig atrium and the rat uterus, respectively. The respective pA2 values were 8.3 and 8.4. Topical metipranolol, 0.3% and 0.6%, blocked both the hypotension (beta2-mediated) and the tachycardia (beta1-mediated) elicited in ganglion-blocked, conscious rabbits by isoproterenol, 0.5 μg/kg, i.v. The Ki for displacement of3H-dihydroalprenolol binding to rabbit iris + ciliary body homogenates was 34 nM. The effect of metipranolol, 0.3% and 0.6%, was not particularly striking on the intraocular pressure (IOP) of conscious, normotensive rabbits. However, the elevated IOP of the alpha-chymotrypsinized rabbit eye was significantly decreased (maximum reduction of 5.8 mm Hg) following the instillation of metipranolol, 0.3%. IOP recovery in conscious rabbits following hyperosmotic challenge (i.v. infusion of a 20% NaCl solution) was not decreased by a 1-h pretreatment with a 0.3% solution. In contrast, a significant reduction of 41% was elicited by a 0.6% solution. Hence, the ocular hypotensive effect of metipranolol may result from decreased aqueous humor inflow. Metipranolol, 0.6%, was devoid of effect on rabbit pupil diameter. However, corneal local anesthesia was elicited in the rabbit by both 0.3% and 0.6% solutions of the drug, the effect being more marked with the higher concentration.

Dorzolamide, a 40-Year Wait

Dorzolamide, on the basis of its pharmacological profile and lack of undesirable side effects in safety assessment studies together with the fact that it could be formulated in solution at 2%, underwent extensive clinical studies.

Chapter 9. Antiglaucoma Agents

Publisher Summary Recent advances in the development of anti-glaucoma drugs are discussed in this chapter. The ability of systemically administered β-adrenoceptor antagonists to lower intraocular pressure (IOP) in man has been demonstrated first with propanolol. Topically instilled timolol has been reported to decrease IOP in both experimental animals and glaucomatous patients. Timolol is a nonspecific 8-blocker that neither causes ocular irritation nor displays local anesthetic activity. Timolol is no longer the sole β-blocker used to treat glaucoma; befunolol, carteolol, and metipranolol have been introduced recently. Blockade of either α 1 - or α 2 -adrenoceptors can lower IOP. With regard to α-blockers, the ocular hypotensive effect of topical prazosin in rabbits was shown to result from decreased aqueous humor (AH) formation rather than from changes in systemic blood pressure. Adrenergic antagonists clearly lower IOP. Paradoxically, adrenergic agonists also lower IOP as evidenced by the ocular hypotensive activity of topically administered epinephrine, norepinephrine, and isoproterenol. Bromocriptine, given either orally or topically, decreased IOP in normal volunteers without altering either pupil diameter or plasma prolactin level. Systemically administered carbonic anhydrase inhibitors (CAIs), of which acetazolamide is the prototype, have been used to treat glaucoma for over three decades. By inhibiting CA in the ciliary process, CAIs reduce AH formation and, hence, lower IOP. Topically administered forskolin, a naturally occurring diterpene, lowers IOP in both experimental animals and man. Thus, the traditional treatment for glaucoma has been topically administered pilocarpine or systemic CAIs. But the armamentarium of the physician has been enormously enhanced in the last decade by the introduction of new topical therapeutics, such as timolol and dipivefrin.

Characterization of [3H]forskolin binding sites in the iris-ciliary body of the albino rabbit

[3H]Forskolin binding sites were identified using membranes prepared from the iris-ciliary body of adult, albino rabbits. Scatchard analysis of saturation binding experiments demonstrated that [3H]forskolin bound to a single population of high affinity sites. The Kd and Bmax values were 8.7 +/- 0.9 nM and 119.0 +/- 30.9 fmol/mg prot. using membranes prepared from frozen tissue and 17.0 +/- 6.2 nM and 184.4 +/- 47.2 fmol/mg prot. using fresh tissue. The binding of [3H]forskolin was magnesium-dependent. The Bmax was enhanced by sodium fluoride and Gpp(NH)p, a nonhydrolyzable guanine nucleotide analog. Forskolin was the most potent inhibitor of [3H]forskolin binding; two commercially-available analogs were weaker inhibitors. In an adenylate cyclase assay, there was the same rank order of potency to enhance enzyme activity. Based upon binding affinities, magnesium-dependence, sensitivity to sodium fluoride and Gpp(NH)p, rank order of potencies of analogs and correlation of binding with adenylate cyclase activity, these studies suggest that the [3H]forskolin binding site in the iris-ciliary body is similar to the binding site in other tissues.

The ability of salbutamol and theophylline to suppress immediate allergic conjunctivitis in the guinea pig

Topically administered salbutamol was extremely effective in suppressing immediate allergic conjunctivitis in the guinea pig; a dose as low as 0.1% elicited 98% inhibition. Topical pretreatment with 1% propranolol completely blocked the suppressant action of 0.1% salbutamol. This was also the case after systemic propranolol (1 mg/kg SC); the beta-adrenoceptor antagonist itself has no effect on antigen-induced inflammation. The effect of 0.1% salbutamol was unaltered by pretreatment with the specific beta1-adrenoceptor antagonist betaxolol (1 mg/kg SC). In marked contrast, the suppressant action of 0.1% salbutamol was profoundly inhibited by pretreatment with the selective beta2-adrenoceptor antagonist ICI-118,551 (0.5 mg/kg SC). The experiments employing beta-adrenoceptor antagonists unequivocally demonstrate that the salbutamol suppression of immediate allergic conjunctivitis in the guinea pig is mediated via the activation of beta2-adrenoceptors. The methylxanthine phosphodiesterase inhibitor theophylline was active after oral administration, 50 mg/kg eliciting an 80% inhibition. Theophylline was inactive topically at 1% and 5%, but this could be due to the fact that the compound was insoluble at these concentrations. Thus, procedures that elevate cyclic-AMP levels suppress immediate hypersensitivity reactions in the guinea pig conjunctiva. Whether or nor this offers an alternative approach to treat allergic conjunctivitis in humans remains to be determined.