Binding characteristics of [3H]guanfacine to rat brain alpha-adrenoceptors. Comparison with [3H]clonidine

The use of reaction time distributions to study attention in male rats: the effects of atomoxetine and guanfacine

RATIONALE

Norepinephrine (NE) is involved in the control of sustained attention. Studies of sustained attention in humans include measures of reaction time (RT) and RT variability (RTV). The present study tested the role of NE using components of the RT distribution in rats in a manner thought to be similar to human studies of RTV.

OBJECTIVES

This study tested the effects of increased synaptic NE (atomoxetine (ATX)) and α-2 receptor binding (guanfacine) on attentional lapses in rats.

METHODS

Male Sprague-Dawley rats (n = 20) were trained and tested in a two-choice RT task (2CRTT). Atomoxetine dose (saline, 0.1, 0.5, 1.0 mg/kg, i.p.), guanfacine dose (saline, 0.01, 0.1, 0.3 mg/kg, i.p.), and distractors were manipulated in three experiments. RT was divided into initiation time (IT) and movement time (MT). Analyses of distribution mode (peak) and deviation from the mode (skew) were then performed.

RESULTS

ATX and guanfacine had no effect on IT mode, reduced IT devmode, and increased MT mode. When distractors were introduced, ATX again improved devmode, but a lack of interaction between ATX and distractor indicated that ATX did not prevent distractor-induced impairments.

CONCLUSIONS

IT devmode is a measure of distribution skew thought to reflect lapses of attention. The effects of ATX on IT devmode suggest that increased synaptic NE reduces attentional lapses. These findings are consistent with human reports of reduced RTV after ATX administration. The same pattern of results with guanfacine suggests that the effects of increased NE are due in part to binding at α-2 noradrenergic receptors.

Antihypertensive activity of 2[(2-hydroxyphenyl)amino]-4(3H)-quinazolinone

Two 2-substituted 4(3H)-quinazolinones were synthesized through an easy one-step synthetic procedure. One of them (1), which was synthesized by the reaction of 1-(2-hydroxyphenyl)-2-thiourea and isatoic anhydride, contained a guanidino moiety and showed antihypertensive activity following cumulative intravenous administration to anesthetized spontaneously hypertensive rats. The other compound (2) does not contain the guanidino group and did not exhibit antihypertensive activity.

Characterization of flufylline, fluprofylline, ritanserin, butanserin and R 56413 with respect to in-vivo alpha 1-,alpha 2- and 5-HT2-receptor antagonism and in-vitro affinity for alpha 1-,alpha 2- and 5-HT2-receptors: comparison with ketanserin

The experimental drugs butanserin (R 53393), ritanserin (R 55667), R 56413, flufylline (Sgd 195/78) and fluprofylline (Sgd 144/80) were evaluated with respect to their antagonism at postjunctional alpha 1- and alpha 2-adrenoceptors and 5-HT2-receptors in pithed rats. Moreover, affinity for [3H]mianserin, [3H]prazosin and [3H]yohimbine binding sites was assessed using rat brain preparations. In all experiments ketanserin was taken as a reference compound. It is concluded that of the compounds investigated butanserin is the most potent and selective alpha 1-adrenoceptor antagonist, whereas ritanserin was found to be a potent and selective 5-HT2-antagonist. Of the other compounds, fluprofylline was a very selective though not very potent alpha 1-adrenoceptor antagonist. The other compounds were less active and less selective in this respect.

Animal pharmacology of guanfacine

The pharmacologic data obtained from animal experiments with guanfacine, a novel, centrally acting antihypertensive agent, are reviewed. When given orally, guanfacine lowers systemic blood pressure in conscious DOCA-NaCl-hypertensive rats, Grollman rats and spontaneously hypertensive rats in a dose-dependent manner. It is also effective in renal hypertensive cats. Guanfacine reduces blood pressure in cats, rabbits and rats after injection into the lateral cerebral ventricle and in dogs after infusion into the vertebral artery at intravenously ineffective doses. Vagally mediated reflex bradycardia in dogs is enhanced. The preganglionic splanchnic (sympathetic) nerve activity is reduced in cats. In rats, guanfacine reduces the noradrenaline turnover in the brain stem. All these findings indicate a central site of action. Peripheral alpha-adrenoceptor stimulant properties of guanfacine have been demonstrated in various studies. In addition to postsynaptic stimulant effects, presynaptic guanfacine-induced inhibition of sympathetic heart nerve stimulation is antagonized by rauwolscine but not by prazosin, indicating a highly preferential alpha 2-agonistic presynaptic action of the drug. In receptor binding studies using rat cortex membranes and human platelets, guanfacine exhibited a high selectivity for alpha 2 adrenoceptors. Guanfacine has the advantage over other centrally acting antihypertensives of being less sedative and causing no rebound hypertension after discontinuation of treatment. The latter is mainly due to its pharmaco-kinetic properties.

Alpha 1-adrenoceptor-mediated vasoconstriction in vivo to enantiomers of SK & F 89748-A

The pressor activity of the 1-enantiomer of SK & F 89748-A, 1,2,3,4-tetrahydro-8-methoxy-5-(methylthio)-2-naphthalenamine, in pithed normotensive rats was found comparable with that of 1-phenylephrine. The d-enantiomer was half as potent. The log dose-pressor effect curves for d- and 1-SK & F 89748-A were not influenced by reserpine treatment (2 X 5 mg/kg i.p., -48 and -24 h), were virtually unaffected by yohimbine (1 mg/kg i.v., -15 min) but were markedly shifted to the right by prazosin (0.1 mg/kg i.v., -15 min) and phentolamine (1 mg/kg i.v., -15 min). Similar observations were made for the 1-enantiomer in pithed cats. It is concluded that d- and 1-SK & F 89748-A are potent, directly acting highly selective agonists of (vascular) postjunctional alpha 1-adrenoceptors. Potency and selectivity were equally pronounced for both enantiomers. The currently available selective agonists of alpha 1-adrenoceptors, including the optical isomers of SK & F 89748-A, cannot distinguish between alpha 1- and alpha 2-adrenoceptors. This conclusion is based on binding affinity since these affinities are linearly correlated as shown by radioligand displacement experiments.

Evidence against the involvement of a noradrenergic mechanism in the release by diazepam of novelty-induced hypophagia in rats

The involvement of noradrenergic processes in both the behavioral suppression and release of food intake induced by diazepam, was investigated using the novelty-induced inhibition of food consumption model. Clonidine (7.5-60 micrograms/kg i.p.) and another alpha 2-noradrenergic receptor agonist, guanfacine (0.25-1 mg/kg i.p.), increased the food intake of rats placed in an unfamiliar situation. The effects of clonidine (15 micrograms/kg) were antagonized by yohimbine (0.5-2 mg/kg i.p.). Clonidine (7.5 micrograms/kg) was unable to enhance the efficacy of diazepam (0.5 and 1 mg/kg i.p.) in stimulating food consumption, whereas yohimbine (0.5-2 mg/kg) did not reduce and even increased the potency of diazepam (2 mg/kg) to facilitate food intake. These findings suggest that although noradrenergic processes (and in particular alpha 2-adrenoreceptors) are involved in the hyponeophagia model they do not mediate the releasing effect exerted by benzodiazepines on novelty-induced suppression of food intake.

Guanfacine and clonidine: antihypertensive and withdrawal characteristics after continuous infusion and its interruption in the spontaneously hypertensive and normotensive rat

In conscious unrestrained spontaneously hypertensive and normotensive rats, prepared with permanently indwelling abdominal aortic catheters, the effects of blood pressure and heart rate of a 12-day continuous subcutaneous infusion of guanfacine (10 mg/kg/day) and clonidine (500 micrograms/kg/day) and sudden interruption of these treatments were studied. Both drugs significantly and consistently reduced the mean arterial pressure and heart rate throughout the infusion period in the SH rats, but not in the normotensive animals. The magnitude of the effects of both drugs in the SH rat were similar. Following withdrawal of treatment with guanfacine, a discontinuation syndrome was evoked, much less severe than that observed after suspension of the infusion with clonidine. The withdrawal syndromes were characterized by an overshoot of heart rate and a period of blood pressure lability. In spite of the ineffectiveness of guanfacine and clonidine to reduce blood pressure and heart rate consistently in the normotensive rat, similar withdrawal patterns as those found in the SH rat were observed. These findings are in general agreement with the results previously found in clinical studies in hypertensive patients. The spontaneously hypertensive rat may prove a suitable animal model for pre-clinical studies of discontinuation symptoms after cessation of treatment with antihypertensive drugs.