Cocaine and D-amphetamine induce changes in central beta-adrenoceptor sensitivity: effects of acute and chronic drug treatment.

The effects of acute and chronic treatment with psychomotor stimulants on specific binding of [3H]dihydroalprenolol to beta-adrenoceptors in rat brain were examined. At a dose of 10 mg/kg both acute and chronic treatment with cocaine and chronic treatment with D-amphetamine (10 mg/kg) caused increased binding of [3H]dihydroalprenolol. The molecular mechanism for this enhanced binding appears to be augmentation of the density of beta-adrenoceptors in rat brain. At a lower dose (5 mg/kg), however, chronic administration of D-amphetamine caused a decrease in the density of beta-adrenoceptors in rat brain. Chronic treatment with either D-amphetamine (10 mg/kg) or cocaine induced a marked increase in the magnitude of cyclic AMP accumulation in rat brain slices elicited by norepinephrine. Acute as well as chronic administration of D-amphetamine in vivo inhibited the temperature-dependent uptake of [3H]norepinephrine in rat brain synaptosomal homogenates, but no such inhibition was observed after chronic or acute treatment with cocaine. The results suggest that psychomotor stimulants induce beta-adrenoceptor supersensitivity which may be involved in the phenomenon of reverse tolerance and possibly psychosis in humans. The development of beta-adrenoceptor supersensitivity does not appear to be mediated through alterations in norepinephrine transport at the presynaptic sites.     

Down-regulation of rat β-adrenoceptors by clenbuterol or desipramine does not require chronic treatment: [H]CGP-12177 binding reveals rapid (24 hour) modulation

Desipramine (DMI, 15 mg/kg, s.c.) decreased [³H]CGP-12177-labelled cortical β-adrenoceptor density (B_max) by 30% upon chronic (14 day) treatment. However, even a single dose (in mg/kg) of DMI (15) or the β-adrenoceptor agonist, clenbuterol (20), induced a rapid (24 hour) and significant reduction of β-adrenoceptor B_max (−15%; p<0.01). Acute treatment with amitryptiline (10), clorgyline (1), fluoxetine (10), nomifensine (10) or maprotiline (20) had no significant effect on [³H]CGP-12177-labelled β-adrenoceptors, suggesting that rapid down-regulation may not be a general property of antidepressant drugs. None of the antidepressants altered the B_max of [³H]ketanserin-labelled 5-HT_2A receptors on acute treatment. These results show that β-adrenoceptor down-regulation by clenbuterol and DMI is not dependent on chronic treatment and may, therefore, be a poor correlate of the gradual onset of therapeutic efficacy seen clinically with antidepressant drugs.     

Effects of chronic antidepressant treatment of β-adrenoceptor subtype binding in the rat cerebral cortex and cerebellum

The effects of desipramine, tranylcypromine, and phenelzine on β-adrenoceptor subtype binding were measured in rat cerebral cortex and cerebellum. The drugs were administered to male Sprague Dawley rats for 28 d sc via 2ML4 Alzet osmotic minipumps (desipramine HCl 10, tranylcypromine HCl 1, phenelzine sulfate 5, 10 mg kg^?1/d). Binding of [³H]CGP 12177 was measured to determine total β-adrenoceptor density in each brain region. The relative densities of β₁ and β₂-adrenoceptors were directly calculated from competition analysis using ICI 89406 to displace [³H]CGP 12177. β₁-Adrenoceptor density was decreased in cortical tissue following each antidepressant treatment, but no effects on β₂-adrenoceptor density were evident. With cerebellar tissue, despite a higher density of β₂-adrenoceptors in this area, no effects of antidepressants on β₂-adrenoceptor density were evident. The present data confirm and extend previous reports of β₁- but not β₂-adrenoceoptor downregulation in brain following chronic antidepressant drug treatment.     

Nitric oxide synthesis inhibition attenuates haloperidol-induced supersensitivity.

Increased responsiveness to psychomotor stimulants can be produced by either chronic stimulant administration or by chronic dopamine receptor blockade. This study examined the role of nitric oxide in the development of neuroleptic-induced supersensitivity. N omega-nitro-L-arginine methyl ester hydrochloride (L-NAME) administered during chronic intraperitoneal (i.p.) haloperidol treatment (0.2 mg/kg/d for 14 d) was used to inhibit nitric oxide synthesis. Locomotor activity following a cocaine(i.p.) challenge injection (10 mg/kg) administered at 3 and 10 d after cessation of haloperidol treatment demonstrated the development of supersensitivity. Haloperidol animals pretreated with L-NAME i.p. (30 mg/kg) showed significantly less cocaine-stimulated locomotor activity on both tests than saline-pretreated animals. This finding suggests that nitric oxide is involved in haloperidol-induced supersensitivity and that a common neural mechanism may underlie the development of supersensitivity and stimulant-induced sensitization. This study also suggests that treatment with a nitric oxide synthesis inhibitor may decrease the side effects accompanying long-term treatment with antipsychotic medications.     

Long-term treatment with clonidine and α-receptors in the brain of Normotensive rats

The aim of the present study was to investigate whether or not changes in rat brain α-adrenoceptors take place during chronic treatment with a low dose of clonidine. Male Wistar normotensive rats were treated with clonidine (0.1 mg/kg) i.p. twice daily for 12 days. This treatment caused a significant increase in [³H]clonidine and in [³H]WB4101 binding, respectively, to α₂- and to α₂-adrenoceptors of the frontal cortex; the levels were 30% for [³H]clonidine and 20% for [³H]WB4101. The Scatchard analysis of data obtained in binding studies indicated that the enhanced binding of two ligands to membranes prepared from chronically clonidine-treated animals, was due to an apparent increase in the number of binding sites. These changes were seen 4 h after administration of the last treatment, before the appearance of the withdrawal syndrome. However, noradrenergic α₂-autoreceptors of synaptosomes, from the frontal cortex and hypothalamus of treated animals, were sensitive to the regulatory action of clonidine or of noradrenaline on the [³H]noradrenaline overflow elicited by high K⁺ as well as on the control animals. On the contrary, the α₂-receptors on the serotoninergic nerve terminals from the frontal cortex of treated animals were more sensitive than those of control animals to the action of clonidine or of noradrenaline in counteracting the [³H]5-hydroxytryptamine overflow elicited by high K⁺. These results suggests that during treatment with clonidine no autoreceptor hyposensitivity to the regulatory action of clonidine or noradrenaline on [³H]noradrenaline overflow elicited by high K⁺ takes place, but, as a consequence of the diminished noradrenaline availability at the synaptic cleft, the binding of [³H]WBA101 to α₁-receptors and of [³H]clonidine to pre- and postsynaptic α₂-receptors were significantly elevated in the frontal cortex, a brain areas where the α-₂-receptors are mainly postsynaptic. Thus, the neurotransmitter concentration in the synaptic cleft may be responsible for the trans-synpatic modulation of the α₂-adrenoceptor postsynaptic population. In fact, the α₂-adrenoceptors which are presynaptically located on the serotoninergic terminals, but are postsynaptic in relation to the noradrenergic neurons, also show increased sensitivity after chronic clonidine treatment.     

Chronic treatment with the potential antidepressant drug rolipram: the effect on the behavioural responses to adrenergic and dopaminergic receptor agonists with some biochemical correlates

Summary We studied the effect of acute and chronic treatment with rolipram, a potential antidepressant drug, on the behavioural responses induced by adrenergic and dopaminergic receptor agonists in mice and rats, and on (³H)prazosin and (³H)dihydroalprenolol binding to cortical membranes and whole brain noradrenaline and dopamine utilization in rats. Chronic, but not acute, administration of rolipram potentiated a behavioural response mediated through central ₁-adrenoceptors, attenuated an ₂-adrenoceptormediated response and inhibited a-adrenoceptor-mediated response. Neither treatment affected the behavioural responses to dopaminergic stimulants. Repeated treatment with rolipram decreased the density of cortical (³H)dihydroalprenolol, but not (³H)prazosin bindings sites, and reduced brain noradrenaline, but not dopamine utilization. These results suggest that chronic administration of rolipram induces the down-regulation of the central- and ₂-adrenoceptors and enhances the responsiveness of the central ₁-adrenoceptors with no apparent changes in the ₁-adrenoceptor density.     

Cocaine accumulates in dopamine-rich regions of primate brain after I.V. Administration: Comparison with mazindol distribution

Pharmacological and neurochemical evidence suggest that brain dopamine systems, and the dopamine transporter in particular, contribute significantly to the behavioral effects and reinforcing properties of cocaine. The first objective of this study was to determine whether the brain distribution of cocaine supports these conclusions. A high resolution neuroanatomical map of cocaine disposition in brain after i.v. administration was developed. [³H]Cocaine ([³H](?)-cocaine) was administered to squirrel monkeys (Saimiri sciureus) at a- trace dose (0.001 mg/kg) and at doses at or above the threshold for producing behavioral effects (0.1 mg/kg, 0.3 mg/kg). After 15 min, ex vivo autoradiography revealed the highest accumulation of [³H]cocaine in dopamine-rich brain regions, including the caudate nucleus, putamen, and nucleus accumbens/ olfactory tubercle. The norepinephrine-rich locus coeruleus, the hippocampus, and amygdala also accumulated large quantities of [³H]cocaine. Moderately high levels were found in the stria terminalis, medial septum, substantia nigra, and other regions. Lowest levels were found in the cerebellum. A high and positive correlation was established for the brain distribution of [³H]cocaine administered at trace or at behaviorally relevant doses (r: 0.94; P < 0.001). To determine whether radioactivity represented [³H]cocaine or its metabolic products, tissue extracts from brain regions with high levels of cocaine were subjected to thin layer chromatography using two solvent systems. In caudate-putamen, nucleus accumbens, cortex, and hippocampus, radioactivity comigrated with standard [³H]cocane. In substantia nigra, less than 70% of the radioactivity comigrated with [³H]cocaine, suggesting that cocaine metabolites are generated more rapidly in the substantia nigra than in other brain regions. The second objective was to determine the brain distribution of mazindol, a potent norepinephrine and dopamine transport inhibitor with low abuse liability in humans. The disposition of intravenously administered [³H]mazindol in brain (0.001 mg/kg, 0.007 mg/kg) was surveyed by ex vivo autoradiography. In sharp contrast to [³H]cocaine distribution, the highest accumulation of [³H]mazindol was localized in the norepinephrine-rich pineal gland, discrete regions of the hypothalamus (paraventricular nucleus, supraoptic nucleus), and the locus coeruleus. Moderately high levels were detected in the caudate-putamen, nucleus accumbens, and other regions. The following conclusions were drawn: (1) Although dopamine-rich brain regions are principal targets of cocaine after i.v. administration to the nonhuman primate, other prominent targets of cocaine (locus coeruleus, hippocampus, and amygdala) may contribute to the acute and chronic effects of cocaine. (2) [³H]Cocaine levels in primate brain regions are positively correlated with increased glucose utilization in rodent brain regions [Porrino (1993), Psychopharmacoloogy, 112:343-351]. (3) Higher levels of [³H]cocaine metabolites detected in the substantia nigra may indicate that cocaine's effects in this region are © 1994 Wiley-Liss, Inc.     

Alpha 1-adrenoceptor denervation supersensitivity in brain: physiological and receptor binding studies

Electrophysiological and radioligand binding methods were used to characterize noradrenergic denervation supersensitivity at alpha 1-adrenoceptors in rat thalamus. Denervation was accomplished either by intraventricular or intracerebral injection of the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA). In the physiological studies, the sensitivity of single lateral geniculate neurons to norepinephrine, carbachol, and serotonin was compared in sham and lesioned animals various times after 6-OHDA. Conducted in parallel were radioligand binding studies in which the density and affinity of thalamic alpha 1-adrenoceptors were measured with the specific antagonist [3H]prazosin. The results indicate that denervation produces a selective increase in the sensitivity of geniculate neurons to alpha 1-adrenergic stimulation and a concomitant increase in alpha 1-adrenoceptor density and agonist affinity.     

Cholecystokinin binding sites in the rat forebrain: effects of acute and chronic methamphetamine administration

Summary Using the in vitro quantitative receptor autoradiographical technique, changes in the binding parameters of [propionyl-³H] propionylated CCK-8 ([³H]pCCK-8) binding sites in the rat forebrain were investigated following acute and chronic administration of methamphetamine (MAP). The (Kd)app values of [³H]pCCK-8 binding sites in the frontal medial cortex and anterior cingulate cortex were significantly reduced after a single injection of 4 mg/kg MAP. On the other hand, chronic treatment (14 days) with MAP at this dose significantly decreased the B_max value of [³H]pCCK-8 binding sites in the anterior cingulate cortex accompanied by supersensitivity of locomotor effects to MAP. These findings suggest that dopamine (DA) neurons in these two regions are functionally related to intrinsic CCK-containing cortical neurons, and that CCK subsensitivity, perhaps due to an alteration in DA transmission, is involved in MAP sensitization. These findings may be relevant to the DA hypothesis of schizophrenia.     

Chronic treatments with tacrine and (−)-nicotine induce different changes of nicotinic and muscarinic acetylcholine receptors in the brain of aged rat

Summary. The present study evaluated effects of chronic treatment with tacrine and (−)-nicotine for 21 days on nicotinic and muscarinic acetylcholine receptors in the brains of old rats (24–25 months) by receptor autoradiography. The nicotinic receptor (nAChR) binding sites were measured by (−)-[³H]nicotine and [³H]epibatidine, and the muscarinic receptor (mAChR) binding sites by [³H]pirenzepine and [³H]AFDX 384. No change in (−)-[³H]nicotine binding was observed in all of the brain regions analysed following chroinic treatment with tacrine (10 mg/kg). Similarly, the [³H]epibatidine binding was not changed in most of the brain regions analysed except for a few brain regions where a decrease was observed in tacrine treated animals compared to control animals. Chronic treatment with (−)-nicotine (0.45 mg base/kg) significantly increased both (−)-[³H]nicotine and [³H]epibatidine bindings in every brain regions analysed except for the hippocampus when measured by (−)-[³H]nicotine. A significant decrease in [³H]AFDX 384 binding, but not in [³H]pirenzepine binding was observed in all of the brain regions analysed following the treatment with tacrine. These data suggest that chronic treatments with tacrine and (−)-nicotine differentially interfere and regulate the subtypes of nAChRs and mAChRs in the brain of aged rat. These data differ from what have been earlier observed in the brain of young adult rat following tacrine treatment, revealing some dynamic changes in receptor properties in the brain during aging. Received April 23, 2001; accepted September 4, 2001     

Alterations in the dopaminergic receptor system after chronic administration of cocaine

Several studies suggest that one of the most important factors contributing to cocaine dependence is an alteration in the actions of the neurotransmitter dopamine in the central nervous system. In order to understand some of the neuroreceptor consequences of cocaine administration, groups of rats were injected with cocaine (2 daily doses of 15 mg/kg) for 1 to 21 days. Binding of [³H]cocaine, [³H]SCH23390, [³H]raclopride, and [³H]BTCP in striatal and cortical tissue from the treated animals was compared to controls. [³H]Cocaine binding was increased by the drug in the striatum and cortex at days 14 and 21, respectively. The binding of [³H]SCH23390 to D₁ dopamine receptors was significantly increased at day 3 of cocaine exposure. In striatal membranes, [³H]BTCP binding to dopamine uptake sites was significantly increased after day 7, whereas binding in cortical membranes was increased from day 1. [³H]Raclopride binding to D₂ dopamine receptors remained unchanged throughout the study in both cortical and striatal tissues. These results indicate that repeated exposure to cocaine produces an upregulation (possible supersensitivity) in cortical D₁, cocaine, and DA-uptake sites which occurs in a time-dependent manner. These increases are coupled with an upregulation in striatal D₁, cocaine, and DA-uptake sites, without simultaneous changes in D₂ receptors. Thus, cocaine's effects are not uniformly distributed across all brain regions, but rather are focused within areas of the dopamine system. © 1993 Wiley-Liss, Inc.     

Expression of β-ADRENERGIC receptors inthe rat uterus : effects of puberty and oestrogen treatmentduring prepubertal development

The expression of β-adrenoceptors in the rat uterus has been analysed duringthe peripubertal transition and following acute and chronic oestradiol treatment duringprepubertal development. The distribution and density of β-adrenoceptors was assessedautoradiographically on cryostat tissue sections using [^fn2H]-dihydroalprenolol ([³H]-DHA). Binding sites were localised in all the ages andexperimental situations examined and showed the following intensity of labelling : endometrialepithelium > longitudinal muscle layer > circular myometrial layer > endometrial stroma.Competition experiments with the selective antagonists ICI 118,551 and atenolol, showed thatmost of the β-adrenoceptors in the uterus belong to the β₂ receptorsubclass. In prepubertal animals, the density of [³H]-DHA binding sites was extremelylow. Following puberty the density of binding sites showed a generalised increase. Acuteadmininstration of oestradiol at the end of the prepubertal period provoked an increase in thedensity of [³H]-DHA binding sites in all uterine regions, but the levels of labelling werelower than in peripubertal animals at proestrus and oestrus. Following chronic oestrogentreatment during postnatal development, oestradiol increased further the density of [³H]-DHA binding sites. Results are discussed considering both the endocrine and neuralchanges accompanying puberty and oestradiol treatment.     

Amygdala dopamine levels are markedly elevated after self- but not passive-administration of cocaine

The influence of cocaine on rat brain monoaminergic neurotransmitters (dopamine, serotonin, noradrenaline) and their metabolites, and on binding of [³H]WIN 35,428 and [³H]GBR 12,935 to the dopamine transporter was measured after 4 weeks of cocaine exposure. Cocaine (mean daily dose9.25 ± 0.48 mg/kg) was self-administered (responders) or passively received (yoked) during sessions which lasted for 1 h per day. As compared with the controls, mean dopamine and serotonin levels were significantly elevated (+107% and +47%, respectively) in amygdala of responders, but not of yoked rats, sacrificed 1 h after the last cocaine session. Dopamine and metabolite levels were normal in all other brain areas examined, including striatum, nucleus accumbens and medial prefrontal cortex, at both 1 h and 4 weeks withdrawal from cocaine. [³H]WIN 35,428 and [³H]GBR 12,935 binding were unaltered after cocaine exposure. These data provide additional support for the involvement of the amygdala in the acquisition of drug seeking behavior associated with cocaine self-administration.     

Long-lasting inhibition of in vivo cocaine binding to dopamine transporters by 3β-(4-Iodophenyl)Tropane-2-Carboxylic acid Methyl Ester: RTI-55 or βCIT

Cocaine analogs such as 3β-(4-iodophenyl)tropane-2β-carboxylic acid methyl ester (RTI-55 or βCIT) with a higher affinity for the dopamine transporter (DAT) may be potentially useful in interfering with cocaine's actions in brain. This study evaluates the time course of the effects of RTI-55 on cocaine binding in baboon brain using PET and [¹¹C]cocaine. [¹¹C]Cocaine binding was measured prior to, and 90 minutes, 24 hours, 4–5 days and 11–13 days after RTI-55 (0.3 mg/kg i.v.). Parallel studies with [³H]cocaine and RTI-55 (0.5 mg/kg i.v. or 2 mg/kg i.p.) were performed in the mouse. RTI-55 significalitly inhibited [¹¹C]cocaine binding at 90 minutes and 24 hours after administration.The half-life for the clearance of RTI-55 from the DAT was estimated to be 2 to 3 days in the baboon brain. In the mouse brain, RTI-55 significantly inhibited [³H]cocaine binding at 60 and 180 minutes after administration and recovery was observed at 12 hours. These results document long-lasting inhibition of cocaine binding by RTI-55 and corroborate that binding kinetics of RTI-55 in striatum observed in imaging studies with [123I]RTI-55 represents binding to DATs. © 1995 Wiley-Liss, Inc.     

Alteration of alpha and muscarinic receptors in rat brain and heart following chronic nicotine treatment

Adrenergic and muscarinic binding sites in 4 brain regions (cerebral cortex, corpus striatum, hypothalamus/thalamus and brainstem) and in heart ventricles were measured in rats chronically treated with nicotine added to the drinking water in doses ranging from 6 to 8 mg/kg/day, for 4 weeks. Control rats received only tap water. The nicotine treatment led to increases in the specific binding of both [³H]prazosin and [³H]clonidine in the cerebral cortex. An increase in [³H]prazosin binding was also observed in the hypothalamus/thalamus of nicotine-treated rats. These changes were all due to an increase of about 23% in B_max. In the brainstem and heart left ventricle, respectively, an increase and a decrease in the affinity of [³H]quinuclidinyl benzilate binding were observed. There were no changes of the binding parameters for the 3 radioligands in other regions tested, and no alteration of [³H]dihydroalprenolol binding was detected in any region examined. These results indicate that chronic administration of nicotine causes an increase in the density of α₁-and α₂-binding sites in some brain regions and reciprocal changes of the affinity of muscarinic binding sites in the brain and in the heart.     

α-Adrenergic regulation of androgen receptor concentration in the preoptic area of the rat

We examined the effect of the pharmacological disruption of the catecholaminergic system on the concentration of nuclear androgen receptor, as measured by the in vitro binding of methyltrienolone ([³H]R1881) to salt extracts of anterior pituitary (AP), preoptic are (POA) and medial basal hypothalamus (MBH). Treatment of gonadectomized male and female rats with the dopamine-ß-hydroxylase inhibitor, diethyldithiocarbamate (400 mg/kg b. wt.), 30 min before treatment with dihydrotestosterone (1 mg/animal) produced a decrease in the number of nuclear androgen receptor compared with saline-treated controls (P<0.05). This effect was specific for the POA and was not present 15 h after DHT treatment. There was no effect on cytosolic androgen receptor nor was there a drug effect on the apparent dissociation constant (K_d) of [³H]R1881 binding to hypothalamus-preoptic area cytosols. Treatment of intact males and castrated, testosterone-treated males with thea₁- anda₂-adrenergic antagonists, prazosin (5 mg/kg b. wt.) and yohimbine (2 mg/kg b. wt.), respectively, resulted in a significant decrease in the number of nuclear AR 2 h following drug treatment (P<0.05). There was no effect of the ß-adrenergic receptor antagonist propranolol (10 mg/kg b. wt.) when given to intact animals, nor was there an effect of idazoxan (5 mg/kg) when given to testosterone-treated animals. The effects of yohimbine and prazosin were restricted to the POA. None of the drugs competed with the binding of [³H]R1881 for the androgen receptor nor did they alter theK_d of cytosol or nuclear androgen receptor. These data provide evidence for an adrenergic interaction with the POA androgen receptor and suggest a role for catecholamines in modulating androgen sensitivity in the rat brain.     

Some effects of chronic antidepressant treatments on rat brain monoaminergic systems

Summary A range of established and putative antidepressant therapies were studied for the effect of their long-term administration on two facets of presynaptic monoaminergic functioning in rat brain, namely NE, DA, and 5-HT turnover and alpha₂-adrenoceptor sensitivity. Unless stated otherwise drugs (10 mg/kg) were injected i.p. twice daily for 14 days. ECT (100 mA for 1 s) was applied once daily for 10 days.Changes in turnover were indirectly assessed by measuring levels of metabolites. Brain levels of MHPG-SO₄ were unchanged by chronic amitriptyline, imipramine, nisoxetine (20 mg/kg), nortriptyline, salbutamol (5 mg/kg), and ECT. Amitriptyline elicited a slight, but significant, increase in brain DOPAC content. Brain levels of 5-HIAA were increased by amitriptyline, imipramine, salbutamol, and ECT. An overall view of the results indicates that no common pattern of change was elicited by the range of antidepressant therapies studied.Central alpha₂-adrenoceptor sensitivity was assessed by investigating the effect of various therapies on the ability of clonidine (25g/kg i.p.) to decrease rat brain MHPG-SO₄ content. The clonidine-induced fall was attenuated by desipramine, imipramine, and ECT. Amitriptyline, iprindole, mianserin, nisoxetine, nortriptyline, Org 6582 (10 mg/kg once daily), pargyline (25 mg/kg once daily), salbutamol, and trazodone were ineffective.The following chronic antidepressant therapies were investigated for their effect on rat frontal cortex³H-clonidine binding: amitriptyline, desipramine, imipramine, iprindole, mianserin, nisoxetine, nortriptyline, pargyline, salbutamol, and ECT. Chronic, but not acute, pargyline decreased³H-clonidine binding and this was due to a diminished number of binding sites.The induction of subsensitive presynaptic alpha₂-adrenoceptors in rat brain is not a property common to all forms of antidepressant therapies. Hence it cannot be the fundamental mode of action of antidepressants. No correlation exists between the changes in rat cortical³H-clonidine binding and the observed changes in the sensitivity of central presynaptic alpha₂- adrenoceptors.     

Brain dopamine transporter: gender differences and effect of chronic haloperidol

Gender differences and the effect of chronic haloperidol on the rat brain dopamine transporter is reported. The density of striatal dopamine transporter sites labelled with [³H]GBR 12935, and of substantia nigra dopamine transporter mRNA measured by in situ hybridization were higher in female compared to male rats whereas striatal D2 specific binding labelled with [³H]spiperone was not significantly higher. Daily haloperidol treatment (1 mg/kg, i.p.) for 21 days increased striatal [³H]spiperone specific binding but left unchanged striatal [³H]GBR 12935 binding density and affinity as well as substantia nigra dopamine transporter mRNA levels. A reduce clearance rate of dopamine in the striatum after acute and chronic haloperidol was previously reported; the present results indicate that this may occur without changes in the sites of dopamine transport or in gene expression of this transporter.     

An in vivo binding assay to determine central alpha(1)-adrenoceptor occupancy using [(3)H]prazosin.

An alpha(1) adrenoceptor (alpha(1)-AdR) assay using [(3)H]prazosin binding in mouse brain is described which allows in vivo determination of central alpha(1)-AdR occupancy for ligands with alpha(1)-AdR affinity. Binding of [3H]prazosin in rat and mouse brain membranes in vitro was used to characterise the pharmacological profile of alpha(1)-AdRs in order to determine any potential species variations. Saturation and displacement studies yielded comparable affinity and pharmacological profile for [(3)H]prazosin binding in mouse and rat brain homogenates. These studies confirmed the absence of species variation for ligands in central alpha(1)-AdR pharmacology which is in good agreement with previous studies in rat brain. Subsequently, in vivo binding of [(3)H]prazosin in mouse whole brain was used to measure the occupancy of a number of AdR ligands. Timecourse studies revealed that a [3H]prazosin (5 mu Ci/mouse) pretreatment time of at least 20 min following intravenous (i.v.) administration was required for optimal specific binding. Ligands were administered systemically 40 min prior to i.v. administration of radiolabel. The alpha(1)-adrenoceptor ligands prazosin (ED(50)=0.15 mg/kg i.p.), benoxathian (0.52 mg/kg i.p.) and phentolamine (51 mg/kg i.p.) were all able to block in vivo [(3)H]prazosin binding from mouse brain. In addition, receptor occupancy values for a number of compounds including haloperidol (ED(50)=0.83 mg/kg s.c.), clozapine (2.2 mg/kg s.c.) and MDL-100907 [R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol], (10 mg/kg s.c.)], which possess high to moderate affinity at alpha(1)-adrenoceptors, were also determined. These results suggest that in the mouse, [(3)H]prazosin binding can be used to measure in vivo receptor occupancy of ligands with affinity at central alpha(1)-adrenoceptors.     

Specific increase of hypothalamic α1-adrenoceptors in spontaneously hypertensive rats: Effect of hypotensive drug treatment

To study potential central adrenoceptor alterations in the hypertension, we have determined α₁, α₂ and β-adrenoceptors using [³H]WB4101, [³H]yohimbine and [³H]DHA in the brain regions of spontaneously hypertensive rats (SHR), stroke-prone SHR (SHRSP) and renal hypertensive rats. There was a significant increase in specific [³H]WB4101 binding only in the hypothalamus of SHR and SHRSP at 16–24 weeks of age compared to that of age-matched Wistar-Kyoto rats (WKY). Scatchard analysis revealed a 28–33% increase in the B_max value for hypothalamic [³H]WB4101 binding without a change in the K_d value, suggesting a change in the receptor density. An increased density of α₁-adrenoceptors was consistently observed in the prehypertensive (5 weeks) and developmental (10 weeks) stages of spontaneous hypertension. In contrast, there was no α₁-adrenoceptor alteration in the hypothalamus of rats with renal hypertension. The receptor alteration in the SHRSP hypothalamus was not abolished by a chronic hypotensive treatment which prevented the development of hypertension, thereby suggesting that an increased density of the α₁-adrenoceptors in spontaneous hypertension does not occur secondarily to the elevation of blood pressure. The SHRSP hypothalamus showed significantly lowered levels of noradrenaline. There was no change in specific binding of [³H]yohimbine and [³H]DHA in the brain regions of SHRSP, except the brainstem which showed a significant decrease in the [³H]yohimbine binding. Thus, the present study suggests an important role for hypothalamic α₁-adrenoceptors in the pathogenesis of spontaneous hypertension.