3-Benzisothiazolylpiperazine derivatives as potential atypical antipsychotic agents.

A series of substituted phenethyl derivatives of 3-benzisothiazolylpiperazine incorporating potent D2 and 5-HT2A antagonist activity was investigated as an approach to a novel atypical antipsychotic agent. The in vitro profile of 8e from this series is a combination of D2 receptor affinity comparable to the typical antipsychotic agent haloperidol and a 5-HT2A/D2 ratio comparable to the atypical agent clozapine. In vivo 8e possesses activity consistent with an efficacious antipsychotic agent with less tendency to induce extrapyramidal side effects in man.

Ziprasidone (CP-88,059): a new antipsychotic with combined dopamine and serotonin receptor antagonist activity.

Ziprasidone (CP-88,059) is a combined 5-HT (serotonin) and dopamine receptor antagonist which exhibits potent effects in preclinical assays predictive of antipsychotic activity. Whereas the compound is a dopamine antagonist in vitro and in vivo, its most potent action is antagonism of 5-HT2A receptors, where its affinity is an order of magnitude greater than that observed for dopamine D2 sites. Laboratory and clinical findings have led to a hypothesis that antagonism of 5-HT2A receptors in the brain limits the undesirable motor side effects associated with dopamine receptor blockade and improves efficacy against the negative symptoms of schizophrenia. Ziprasidone possesses an in vitro 5-HT2A/dopamine D2 receptor affinity ratio higher than any clinically available antipsychotic agent. In vivo, ziprasidone antagonizes 5-HT2A receptor-induced head twitch with 6-fold higher potency than for blockade of d-amphetamine-induced hyperactivity, a measure of central dopamine D2 receptor antagonism. Ziprasidone also has high affinity for the 5-HT1A, 5-HT1D and 5-HT2C receptor subtypes, which may further enhance its therapeutic potential. The prediction of antipsychotic efficacy without severe motor side effects is supported by the relatively weak potency of ziprasidone to produce catalepsy in animals, contrasted with its potent antagonism of conditioned avoidance responding and dopamine agonist-induced locomotor activation and stereotypy. The compound is well tolerated in animals at doses producing effective dopamine antagonism in the brain. Ziprasidone should be a valuable addition to the treatment of psychotic disorders.

Synthesis, in vitro binding profile, and autoradiographic analysis of [3H]-cis-3-[(2-methoxybenzyl)amino]-2-phenylpiperidine, a highly potent and selective nonpeptide substance P receptor antagonist radioligand.

The synthesis of a highly potent and selective NK1 receptor antagonist radioligand, [3H]-cis-3-[(2-methoxybenzyl)amino]-2-phenylpiperidine (6a) is described. The in vitro binding pharmacology and autoradiographic distribution of 6a in guinea pig brain following peripheral administration are also reported.

1-Naphthylpiperazine derivatives as potential atypical antipsychotic agents.

The design and synthesis of a series of potential atypical antipsychotic agents based on the structure of 1-naphthylpiperazine are described. The incorporation of dopamine antagonist activity into the parent structure was achieved with heterocyclic surrogates for the catechol moiety of dopamine. Compound 4b from this series showed a biochemical profile that translated to behavioral activity in the rat predictive of an antipsychotic agent with a low propensity to cause extrapyramidal side effects in man.

Activity and distribution of binding sites in brain of a nonpeptide substance P (NK1) receptor antagonist.

CP-96,345, a nonpeptide substance P antagonist, is selective for the tachykinin NK1 receptor. The compound binds to a single population of sites in guinea pig brain and potently inhibits substance P-induced excitation of locus ceruleus neurons. CP-96,345 should be a useful tool for studying the action of substance P in the central nervous system.

Synthesis, in vitro binding profile, and central nervous system penetrability of the highly potent 5-HT3 receptor antagonist [3H]-4-(2-methoxyphenyl)-2-[4(5)-methyl-5(4)-imidazolylmethyl]thiazole.

4-(2-Methoxyphenyl)-2-[4(5)-methyl-5(4)-imidazolylmethyl]thiazole (5) is a highly potent member of a structurally novel series of selective serotonin-3 receptor antagonists. The synthesis of tritiated 5 and its binding profile in neuroblastoma-glioma 108-15 cells are described. Furthermore, in vivo studies in rat with this radioligand indicate that it effectively penetrates the blood-brain barrier upon peripheral administration. Thus, 5 should be a useful pharmacological tool for both in vitro and in vivo studies of this class of compounds.

In vivo opioid receptor occupation in the rat brain following exercise.

The effects of prolonged swim-stress (2 h and 1 h) upon brain opioid receptor binding of tritiated [3H]diprenorphine were investigated in male Sprague-Dawley rats. This was accomplished by injecting the label intravenously immediately following the swim, then allowing 20 min for tracer washout from non-specific binding sites, sacrificing the animal, dissecting the brain into several discrete areas (medulla-pons, mid-brain, mesolimbic, caudate, thalamus, and hypothalamus), and subsequently preparing homogenates from each brain area. Data were obtained from scintillation counting of the homogenates. A separate support experiment measured circulating beta-endorphin endorphin like immunoreactivity immediately following 2 h of swim-stress. Blood-borne beta-endorphin levels were significantly enhanced by the swim. Additionally, [3H]diprenorphine binding was insignificantly elevated following the 1-h swim and significantly greater in 5 of 6 brain areas examined subsequent to the 2-h swim. Greater availability of opioid receptors to allow enhanced binding of [3H]diprenorphine may have been caused by decreased competition for available receptors from endogenously produced peptides or possibly by alterations in receptor-binding characteristics. These proposed explanations await further investigation. As a result of these studies, we conclude: exercise-induced enhancement of peripheral beta-endorphin probably does not have a supraspinal action; and prolonged swim-stress apparently alters opioid receptor occupancy in the rat brain, and this effect may be dependent upon exercise duration.

In vivo autoradiography: visualization of stress-induced changes in opiate receptor occupancy in the rat brain.

A method of in vivo autoradiography was utilized which allows the visualization of local changes in opiate receptor occupation in the intact rat brain. The method is based on the exclusion of [3H]diprenorphine binding in areas in which the release of endogenous opiate peptides is increased by behavioral manipulation. The technique lends itself to the use of film autoradiography, allowing the mapping of relative levels of functional receptor occupancy throughout the whole brain. Prolonged intermittent footshock and forced swims in cold water (two stress-inducing manipulations which are known to release endogenous opiates) were found to cause highly significant decreases in specific high-affinity [3H]diprenorphine binding, as measured by liquid scintillation counting. These changes were unaccompanied by corresponding changes in non-specific binding and were not related to local changes in blood flow. A prolonged non-stressful swim in warm water caused no changes in [3H]diprenorphine binding. The use of tritium-sensitive film autoradiography allowed the resolution of these decreases to the level of individual nuclei. Differences in specific binding were found to be greatest in the periaqueductal gray, the reticular formation, and in midline-intralaminar thalamic nuclei, all of which have been implicated in the modulation of pain sensation.

Metabolic mapping of the brain during rewarding self-stimulation.

Local rates of cerebral glucose utilization were measured in rats by the quantitative 2-deoxy-D-[14C]glucose autoradiographic method during electrical stimulation of the ventral tegmental area. Rats trained in intracranial self-stimulation showed a pattern of changes in forebrain metabolic activity distinctly different from the pattern seen in rats stimulated by the experimenter. These findings provide information about the distribution of local cerebral activity specific to reinforced instrumental behavior.

Changes in local cerebral glucose utilization during rewarding brain stimulation.

The quantitative 2-deoxy[14C]glucose method was used to determine local cerebral glucose utilization in unrestrained rats responding (lever-press) for rewarding electrical stimulation to area A10 (ventral tegmental area) and in similarly implanted inactive controls. Self-stimulation was associated with significant increases in metabolic activity, highly circumscribed in the ventral tegmental area, that continued rostrally within a rather compact zone of activity through the medial forebrain bundle, extending via the diagonal band of Broca to the level of the preoptic area. In the forebrain terminal areas bilateral increases in local cerebral glucose utilization were noted in the nucleus accumbens, lateral septum, hippocampus, and the mediodorsal nucleus of the thalamus. Ipsilateral (i.e., side of stimulation) increases in glucose utilization were noted in the bed nucleus of the stria terminalis, the basolateral and central amygdaloid nuclei, and the medial prefrontal cortex. Caudal to the stimulation site, increases in glucose utilization were found in the midline dorsal raphe, the ipsilateral pontine gray, medial parabrachial nucleus, and the locus coeruleus. Significant bilateral increases were noted in various sensory and motor areas. These results indicate that rather than a diffuse pattern of activity, rewarding brain stimulation is associated with discrete activation of specific neuronal projection fibers and selective terminal sites.

Neurobehavioral evidence for mesolimbic specificity of action by clozapine: studies using electrical intracranial self-stimulation.

The ability of chronic treatment with the atypical neuroleptic clozapine to induce functional dopaminergic hypersensitivity in laboratory rats was assessed. The intracranial electrical self-stimulation paradigm, known to be sensitive to changes in functional dopaminergic sensitivity, was used. Animals with electrodes in the ventral tegmental nucleus (mesolimbic dopamine cell body area) showed a marked increase in self-stimulation rate following 3 weeks of chronic clozapine. This increase was similar in magnitude and duration to that shown by animals given 3 weeks of chronic haloperidol. In contrast, animals with electrodes in the substantia nigra (nigrostriatal dopamine cell body area) showed no change in self-stimulation rate following 3 weeks of chronic clozapine. These data are interpreted in the light of previous suggestions that clozapine and other atypical neuroleptics may possess functional selectivity for the mesolimbic dopamine system.

Behavioral and biochemical aspects of neuroleptic-induced dopaminergic supersensitivity: studies with chronic clozapine and haloperidol.

Rats were chronically injected with saline, clozapine, or haloperidol and tested for alterations in dopamine (DA)-mediated behavior, DA receptor binding, and both acetylcholine (ACH) concentration and choline acetylase activity. Behaviorally, chronic haloperidol significantly enhanced apomorphine-induced chewing and sniffing stereotypies, associated with DA nigrostriatal activation, while clozapine selectively enhanced apomorphine locomotor activity and cage-floor crossing, behavior associated with DA mesolimbic activation. Biochemically, chronic haloperidol significantly enhanced 3H-spiroperidol binding in striatum and in mesolimbic loci (nucleus accumbens/olfactory tubercle) while chronic clozapine failed to produce such enhancement. Acute haloperidol induced an initial decrease in striatal ACH concentration followed by a return of ACH to normal levels within 1 week. There was no change in choline acetylase activity during the same time interval. These findings suggest that haloperidol may inhibit DA mechanisms in both the nigrostriatal may inhibit DA mechanisms in both the nigrostriatal and mesolimbic systems, but that the effect of clozapine on DA mechanisms may be specific to mesolimbic rather than striatal structures. At the same time, the lack of effect of clozapine on 3H-spiroperidol binding may indicate that behaviorally important changes in DA sensitivity can develop independent of changes in post-synaptic DA receptors. The pattern of cholinergic changes with chronic haloperidol suggests that the increase in striatal DA receptor number seen with chronic treatment re-establishes DA inhibition of cholinergic firing within the striatum.

Interaction of opiates with dopamine receptors: receptor binding and behavioral assays.

We examined the hypothesis that opiates act as dopamine (DA) receptor-blocking agents thereby inducing a compensatory increase in DA receptor density during chronic administration, and that increased receptor density could account for the behavioral hypersensitivity to DA agonists seen after treatment with opiates. Morphine and methadone did not inhibit the specific binding of 3H-spiroperidol to DA receptors in vitro, nor did they decrease affinity or apparent receptor density in the striatum when administered acutely in vivo in behaviorally effective doses. In contrast, neuroleptics had the expected inhibitory effect in both these experiments. Stereotypy and locomotion in response to apomorphine were measured before and after a 3-week treatment with saline or methadone. About half the methadone-treated rats showed significant increases over predrug baselines in stereotypy or locomotion, as did a few saline-treated animals. However, in those animals showing enhanced stereotypy or locomotion, DA receptor density was not elevated in striatum or mesolimbic areas respectively. These results indicate that opiates do not act as antagonists at DA receptor sites, and that changes in DA receptor density cannot account for opiate-induced behavioral hypersensitivity.

Pentobarbital induces a naloxone-reversible decrease in mesolimbic self-stimulation threshold.

The effects of sodium pentobarbital and naloxone were tested on intracranial self-stimulation (ICSS) in rats implanted with electrodes in the ventral tegmental area. Threshold for ICSS was determined using a rate-independent current titration paradigm. A low dose of pentobarbital (5 mg/kg) did not have a significant effect on ICSS thresholds, while a high dose (20 mg/kg) rendered the subjects too ataxic to respond reliably in the operant task. An intermediate dose (10 mg/kg) induced a highly significant lowering of threshold (17% below saline baseline levels) without apparent deterioration in response capability. The concurrent administration of naloxone (2 mg/kg) significantly reversed the pentobarbital-induced threshold decrease, while naloxone treatment alone had no effect on the ICSS threshold.

Increase in mesolimbic electrical self-stimulation after chronic haloperidol: reversal by L-DOPA or lithium.

After chronic neuroleptic drug treatment, an increase in electrical intracranial self-stimulation (ICSS) rate is seen from electrodes in the A10 dopaminergic nucleus. This increase, which persists for approximately 3 weeks following drug withdrawal, is believed to represent a behavioral manifestation of drug-induced dopaminergic synaptic supersensitivity. Chronic L-DOPA caused a partial reversal of haloperidol-induced ICSS increase. Lithium carbonate, given concurrently with the haloperidol, partially prevented the development of ICSS supersensitivity. It is concluded that dopaminergic synaptic sensitivity has a two-way modulatory capability.

Morphine differentially affects ventral tegmental and substantia nigra brain reward thresholds.

In order to differentiate the roles of the nigrostriatal and mesolimbic-mesocortical dopamine systems in the action of opiates on dopaminergically mediated intracranial self-stimulation (ICSS), the effects of chronic morphine administration and acute naloxone administration on ICSS were tested in rats with electrode placements in the substantia nigra pars compacta (A-9) and the ventral tegmentum (A-10). Acute morphine (5.0 mg/kg SC) did not affect ICSS thresholds of rats with electrodes in the A-9 nucleus when tested 1, 3, 5, and 23 hours after administration. With repeated daily administration, though, these animals showed increases in thresholds which grew progressively larger with each day of morphine treatment. This threshold elevation was not reversed by naloxone given 0.5 hour after the final morphine treatment on the fifth day. In contrast, acute morphine significantly lowered self-stimulation thresholds in rats with A-10 placements. Tolerance to this facilitatory effect was evident with chronic administration. Naloxone attentuated the lowering of threshold caused by opiate administration in these A-10 animals. The present data suggest a specificity of action of opiates on different brain systems subserving reward and reinforcement. These findings also suggest that the mesolimbic-mesocortical system may play an important role in mediating the rewarding properties of morphine.

Selective inhibition of mesolimbic behavioral supersensitivity by naloxone.

Rats trained on ventral tegmental intracranial self-stimulation (ICSS) were unaffected by acute injection of naloxone (2 mg/kg), while acute morphine (5 mg/kg) increased ICSS rate by 46%. Chronic haloperidol (1 mg/kg/day for 21 days) produced mesolimbic dopaminergic supersensitivity, as seen by 42% increase in ICSS. Acute naloxone partially blocked this supersensitivity (to 13% over basal), while acute morphine augmented it (66% over basal). These findings suggest that enkephalinergic systems may be involved in neuroleptic-induced dopaminergic behavioral supersensitivity.

Enhancement of self-stimulation behavior in rats and monkeys after chronic neuroleptic treatment: evidence for mesolimbic supersensitivity.

The effect of chronic neuroleptic drug treatment on self-stimulation of the mesolimbic dopamine system was tested. Rats with electrodes implanted into the ventral tegmental nucleus (A10 cell body area) were treated with haloperidol for three weeks. Afterwards, the rats showed a 35% increase in self-stimulation rate, as compared to pre-drug control rates. This increase persisted for three weeks after drug withdrawal before returning to baseline rates. Rats treated for three weeks with the atypical neuroleptic, clozapine, also showed an increase, the duration and magnitude of which was similar to that seen in the haloperidol group. In addition, four rhesus monkeys with electrodes in the nucleus accumbens (one of the terminal projection areas of the A10 mesolimbic dopamine system) were given a three week treatment with haloperidol, after which all animals showed a significant, long-lasting decrease in self-stimulation threshold, as measured by a rate-independent reward paradigm. Taken together, these results suggest the induction of receptor supersensitivity in the mesolimbic dopamine system by long-term treatment with neuroleptic drugs.