Typical and atypical neuroleptics are potent antagonists at alpha 1-adrenoceptors of the dorsal lateral geniculate nucleus. An electrophysiological study

Peripheral and central adrenoceptor modulation of the behavioural effects of clozapine in the paw test

1. In rats, the atypical neuroleptic, clozapine, has been found to increase the hindlimb retraction time but not the forelimb retraction time, in the paw test. These parameters have predictive validity for the antipsychotic efficacy and extrapyramidal side-effects of drugs, respectively. The present study analysed to what extent drugs acting on adrenoceptors affect the behavioural effect of clozapine in the paw test. 2. The alpha 1-adrenoceptor agonist, ST 587 but not the peripherally working alpha 1-agonist, methoxamine, decreased the effect of clozapine on the hindlimb retraction time. The alpha 1-antagonist phenoxybenzamine increased this effect of clozapine, and blocked the effect of ST 587 on clozapine at low doses. Only the combination of phenoxybenzamine with clozapine produced an increase in forelimb retraction time. 3. The alpha 2-adrenoceptor agonist, clonidine, decreased the effect of clozapine on the hindlimb retraction time. This effect was neither antagonized by the alpha 2-antagonist rauwolscine nor by the alpha 1-antagonist phenoxybenzamine. Rauwolscine or the peripherally working alpha 2-antagonist L-659,066 did not influence the effect of clozapine on the hindlimb retraction time. The forelimb retraction time was not affected by any of the drug combinations. 4. In contrast to the beta 2-adrenoceptor agonist, clenbuterol, which was ineffective, the peripherally acting beta-agonist, (-)-isoprenaline, increased the effects of clozapine on the hindlimb retraction time. The beta-antagonist, (-)-propranolol as well as the peripherally acting beta-antagonist, nadolol decreased this effect of clozapine. Low doses of the peripherally acting beta 1-antagonist, atenolol, as well as low doses of the beta2-antagonist, ICI-118,551, decreased the effect of clozapine. A low dose of nadolol blocked the effect of (-)-isoprenaline on clozapine. Only the combination of clenbuterol with clozapine produced an increase in forelimb retraction time.5. It is concluded that blockade of central alpha l-adrenoceptors plays an important role in the effect of clozapine on the hindlimb retraction time. Furthermore, the effect of clozapine on the hindlimb retraction time is strongly modulated by peripheral beta 1- and/or beta 2-adrenoceptors. Given the predictive validity of the paw test, the presented data suggest that the alpha 1-adrenoceptor antagonist properties of clozapine are important for its therapeutic effects, but not for its lack of extrapyramidal side-effects.

Prazosin modulates the firing pattern of dopamine neurons in rat ventral tegmental area

Previous studies have indicated a noradrenergic modulation of midbrain dopamine cell activity. The effects of systemic administration of the alpha 1-adrenoceptor antagonist prazosin and the alpha 2-antagonist idazoxan on midbrain dopamine cell firing were now studied with extracellular recording from single dopamine neurons in the ventral tegmental area of chloral hydrate-anaesthetized male rats. Prazosin (0.15-0.6 mg/kg i.v.) dose dependently decreased burst firing and regularized the firing pattern of dopamine neurons, while the firing rate was unaffected. The prazosin-induced effects were abolished by pretreatment with reserpine. Idazoxan (0.5-2.0 mg/kg i.v.) increased firing rate and burst firing and made the firing pattern less regular, probably by increasing adrenergic transmission via blockade of presynaptic alpha 2-adrenoceptors. The effects of idazoxan were blocked by prazosin. The present results indicate that noradrenergic neurons modulate the dopamine cell firing pattern via excitatory postsynaptic alpha 1-adrenoceptors. This mechanism might be involved in the pathogenesis and pharmacological treatment of schizophrenia.

Noradrenergic modulation of midbrain dopamine cell firing elicited by stimulation of the locus coeruleus in the rat

Electrical stimulation techniques were employed in the chloral hydrate anaesthetized male rat to evaluate if the pontine noradrenergic nucleus locus coeruleus can influence the activity of midbrain dopamine neurons in the ventral tegmental area and zona compacta, substantia nigra. Single-pulse locus coeruleus stimulation evoked an excitation, followed by an inhibition, of the electrical activity of single midbrain dopamine neurons. Neither of these responses were observed in animals pretreated with reserpine, implicating noradrenaline as a mediator. The alpha 1-adrenoceptor antagonist prazosin decreased the excitation, while other adrenoceptor antagonists were without general effect. Burst-type stimulation produced only a more long-lasting inhibition. The influence from the locus coeruleus on midbrain dopamine neurons could be important in behavioural situations involving novelty and reward, and might also be of importance for the actions of psychotropic drugs.

Antagonism of the hypothermic effect of clozapine in mice by centrally-active alpha 2-adrenergic antagonists and alpha 1-adrenergic agonists

Hypothermia induced by either clozapine or clonidine in mice was blocked by the alpha 2-adrenergic antagonists yohimbine, idazoxan, CH-38083, SKF 86466, and L-657,743. These effects were dose related, and the ID50 values for inhibition of clozapine- or clonidine-induced hypothermia were fairly comparable. The order of potency for blocking clonidine-induced hypothermia was: L-657,743 greater than CH-38083 greater than yohimbine greater than idazoxan greater than SKF 86466. A very similar blockade hierarchy for clozapine-induced hypothermia was observed, with the order of the two most effective compounds being reversed. Hypothermia induced by either compound was not blocked by the peripherally-acting, selective alpha 2-adrenergic antagonist, L-659,066, indicating that blockade by the other compounds occurred centrally. The centrally-acting, alpha 1-adrenergic agonists St 587, cirazoline, and SKF 89748 were very effective in blocking the response to clozapine, but ineffective in antagonizing clonidine-induced hypothermia. The ED50 values for the blockade of this response to clozapine, however, did not correlate with their reported potencies in stimulating either peripheral or central alpha 1-adrenergic receptors. This indicates that clozapine-induced hypothermia in mice is not a suitable model for evaluating the properties of central alpha 1-adrenergic compounds. Moreover, since the clonidine-induced hypothermia is not influenced by alpha 1-adrenergic agonists, this paradigm is preferable to clozapine-induced hypothermia in the assessment of alpha 2-adrenergic antagonism The ability of alpha 2-adrenergic antagonists to block clozapine-induced hypothermia may result from the central overflow of norepinephrine, which is known to be brought about by this group of compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between clozapine and a lipophilic alpha 1-adrenergic agonist

Acute intraperitoneal injection of clozapine produced marked hypothermia and ataxia in Swiss-Webster mice. These two effects were almost completely blocked by the lipophilic alpha 1-adrenergic agonist, St 587, but not by the peripherally-acting alpha 1 agonist methoxamine. It was inferred that these effects of clozapine are central in origin and probably resulted from alpha 1 adrenergic blockade. However, since prazosin, a selective alpha 1-adrenergic antagonist did not elicit either hypothermia or ataxia in mice it became clear that the alpha 1 adrenergic blocking effect of clozapine is not entirely responsible for these effects, but has a major contributory role in their production. Both clozapine and prazosin inhibited the d-amphetamine-induced locomotor stimulation in mice. St 587 did not significantly reduce this amphetamine-blocking effect of clozapine. It was inferred that this response to d-amphetamine involving the release of mesolimbic dopamine is distinct from the other two St 587-sensitive responses. The hypothermic and ataxic effects of clozapine developed complete tolerance after just four days of treatment, but ten days of such treatment was required for the development of tolerance to the amphetamine-blocking effect of clozapine. The possible relationships between St 587-sensitive and insensitive effects of clozapine and its antipsychotic property are discussed.

Alpha adrenoceptor modulation of the jaw-opening reflex

In the present study, the nature, role and location of the alpha-adrenoceptors, modulating the tooth pulp stimulation (TPS)-evoked jaw-opening reflex (JOR), was investigated. Clonidine, a lipophilic alpha 2-adrenoceptor agonist, rapidly, reversibly and significantly elevated the threshold of the digastric electromyogram (dEMG) evoked by tooth pulp stimulation which was antagonized by yohimbine or piperoxane (two alpha 2-adrenoceptor antagonists) but not by prazosin (an alpha 1-adrenoceptor antagonist). 4-Hydroxyclonidine and ST-91 non lipophilic alpha 2 adrenoceptor agonists, were ineffective on the tooth pulp stimulation-evoked threshold of the digastric electromyogram. Clonidine was ineffective upon digastric muscle stimulation (DMS)-evoked threshold of the digastric electromyogram of the curarized digastricus and mylohyoid (motor) nerve stimulation (MNS)-evoked digastric EMG. It is concluded that modulation by clonidine of the tooth pulp stimulation-evoked jaw-opening reflex is devoid of a peripheral efferent component and that such modulation is predominantly expressed through interactions with centrally located alpha 2 adrenoceptors.

Enhancement of central transmission to sympathetic preganglionic neurons by phosphodiesterase inhibitors and its prevention by clonidine

The effects of 3 phosphodiesterase inhibitors, aminophylline, isobutylmethylxanthine (IBMX), and RO 20-1724, were tested on descending intraspinal and spinal reflex transmission to sympathetic preganglionic neurons in unanesthetized spinal cats. Sympathetic discharges, recorded from upper thoracic preganglionic white rami, were evoked by stimulation (0.1 Hz) of descending excitatory pathways in the cervical dorsolateral funiculus (intraspinal) or of adjacent intercostal nerves (spinal reflex). Each phosphodiesterase rapidly and markedly enhanced transmission through intraspinal pathways but only slowly and modestly enhanced transmission through spinal reflex pathways. Pretreatment with a methyltyrosine-reserpine combination, chlorpromazine, or prazosin markedly restricted the enhancement of intraspinal transmission by IBMX to levels typically produced on spinal reflex pathways. Clonidine markedly depressed transmission through both pathways and prevented enhancement by the phosphodiesterase inhibitors. Yohimbine or tolazoline antagonized the depressant effects of clonidine and restored the ability of the phosphodiesterase inhibitors to enhance transmission. Somatic spinal reflexes were not affected by the phosphodiesterase inhibitors. The results suggest that descending norepinephrine pathways to sympathetic preganglionic neurons activate adenylate cyclase to generate cyclic AMP which increases neuronal excitability, possibly by phosphorylating membrane proteins. Clonidine appears to depress neuronal excitability by inhibiting adenylate cyclase through activation of alpha 2-adrenergic receptors.

Heterogeneity of dog-liver glutathione S-transferases. Evidence for a unique temperature dependence of the catalytic process

Dog liver glutathione S-transferase activities are associated with five cytosolic proteins and to approximately 1.5% with microsomal proteins determined on the basis of activity conjugating to 1-chloro-2,4-dinitrobenzene. The four major cytosolic enzymes were purified to apparent homogeneity by sequential use of ion-exchange, hydrophobic, hydroxyapatite and affinity chromatography. The isolated transferases are binary combinations of three classes of subunits: alpha (Mr = 26,000), beta (Mr = 27,000), gamma (Mr = 28,500). They were classified by roman numerals assigned in order of increasing isoelectric point as DI alpha gamma (pI 6.4), DII alpha alpha (pI 6.9), DIII beta gamma (pI 8.1), and DIV beta gamma (pI 8.7). Additionally, traces of conjugating activity may be attributed to a, beta monomeric or dimeric protein with cationic character. The differences in catalytic specificity, temperature and pH dependence of activity, and sensitivity and kinetic response to inhibitory ligands may reflect the intrinsic structural heterogeneity of the transferases. At physiological glutathione concentrations DI alpha gamma accounted for roughly 60% of the total 1-chloro-2,4-dinitrobenzene-conjugating activity, the rank order of activity being DI alpha gamma greater than DII alpha alpha greater than DIV beta gamma greater than DIII beta gamma. The glutathione-dependent denitration of organic nitrates seems to be restricted to the cationic enzymes, whereas 1,2-dichloro-4-nitrobenzene-conjugating activity is exclusively associated with the anionic transferases, DI alpha gamma much greater than DII alpha alpha. Arrhenius plots from initial rate experiments performed over a range of temperatures (15-40 degrees C) exhibit an upward bend for DI alpha gamma, an apparently constant slope for DII alpha alpha and DIII beta gamma, and a downward bend for DIV beta gamma.

Electrophysiological assay and characterization of central adrenoceptors: techniques and neuropharmacology

Several single unit electrophysiological studies that have investigated central adrenoceptors are reviewed. The techniques and paradigms employed to electrophysiologically assay such adrenoceptors are discussed. Several regions of the brain, e.g., the nucleus locus coeruleus, the dorsal raphe nucleus, the lateral geniculate nucleus, and the cerebellar Purkinje neurons, are examined in detail, with reference to the nature of the adrenoceptor(s) located on these neurons. From the studies reviewed, it can be concluded that single unit electrophysiological recordings provide a valuable and powerful assay for adrenoceptors. Modification of this technique to study adrenoceptors from awake (behaving) or chronically treated animals is likely to result in significant advances in our understanding of mechanisms contributing to neuroreceptor plasticity.

Chlorpromazine and trifluoperazine potentiate the action of adenosine on rat cerebral cortical neurons

The effect of chlorpromazine and trifluoperazine on adenosine-evoked depressions of the spontaneous firing of rat cerebral cortical neurons has been studied. When applied by iontophoresis from multiple barrelled micropipettes, in amounts that did not affect neuronal firing rates, both substances enhanced the depressant actions of adenosine. Trifluoperazine did not enhance the depressant actions of the uptake resistant analog, adenosine 5'-N-ethylcarboxamide. Intravenously administered chlorpromazine and trifluoperazine (1 mg/kg) enhanced the depressant actions of iontophoretically applied adenosine. Caffeine antagonized the depressant effect on the firing of cortical neurons observed with larger applications of trifluoperazine. These findings support the conclusion, drawn from previous studies on the effects of phenothiazines on the uptake of adenosine by brain synaptosomes, that inhibition of adenosine uptake and potentiation of the actions of endogenously released adenosine may be a significant factor in the generation of the central actions of the phenothiazines.

Critical issues in assessing the behavioral effects of amphetamine

Amphetamine induces a behavioral syndrome in mammals that includes a variety of repetitive behaviors. An integral component of this syndrome in humans is the presence of a thought disturbance not unlike that manifest in idiopathic paranoid schizophrenia. The consistent pattern of behavioral changes produced by amphetamine across species, when considered in light of the psychosis it elicits in humans, has suggested to many that these drug-induced changes in animals may provide a model of the endogenous psychosis in humans. Amphetamine-induced changes in open-field behavior in the rat have been the most widely studied in attempts to formulate a model for investigating the neurobiological mechanisms underlying amphetamine psychosis and paranoid schizophrenia in humans and for testing the therapeutic efficacy of new antipsychotic drugs. The procedures used to assess the behavioral response to amphetamine, however, typically include rating scales or automated recordings that by their very nature ignore those components of the behavioral response that may be most critical for developing a viable animal model of the naturally occurring psychosis. Further, open-field behavior is often recorded during arbitrarily selected intervals without consideration for the multiphasic nature of the entire amphetamine response. We discuss how incomplete descriptive analyses of the amphetamine behavioral response in rats has led to confusion in the literature and describe behavioral research that is paradigmatic of the work we believe is most likely to eventuate in significant progress in the field.