"Classical" and "atypical" antipsychotic drugs: differential antagonism of amphetamine- and apomorphine-induced alterations of spontaneous neuronal activity in the neostriatum and nucleus accumbens

Schizophrenia: Antipsychotics and drug development

The introduction of chlorpromazine and the work that ensued provided the foundation to reposition schizophrenia as a biological illness. The present paper follows the evolution of antipsychotics and their shift from 'typical' to 'atypical'. Atypicality is reviewed in reference to its original definition, clozapine's role, and developments that now leave the concept's utility in question. In a similar fashion, drug development is reviewed in the context of the illness' multiple symptom domains, as well as differences captured by clinical staging and phenotyping. Collectively, the evidence argues for a more nuanced approach to drug development that aligns with the illness' heterogeneity and complexity. Just as 'atypical' as a descriptor for antipsychotics may be outdated, it may be time to set aside the notion of developing drugs that treat 'schizophrenia'.

Electrophysiological characterization of dopamine neuronal activity in the ventral tegmental area across the light-dark cycle

Direct evidence that dopamine (DA) neurotransmission varies during the 24 h of the day is lacking. Here, we have characterized the firing activity of DA neurons located in the ventral tegmental area (VTA) using single-unit extracellular recordings in anesthetized rats kept on a standard light-dark cycle. DA neuronal firing activity was measured under basal conditions and in response to intravenous administration of increasing doses of amphetamine (AMPH: 0.5, 1, 2, 5 mg/kg), apomorphine (APO: 25, 50, 100, 200 µg/kg) and melatonin (MLT: 0.1, 1, 10 mg/kg) at different time intervals of the light-dark cycle. DA firing activity peaked between 07:00 and 11:00 h (3.5 ± 0.3 Hz) and between 19:00 and 23:00 h (4.1 ± 0.7 Hz), with lowest activity occurring between 11:00 and 15:00 h (2.4 ± 0.2 Hz) and between 23:00 and 03:00 h (2.6 ± 0.2 Hz). The highest number of spontaneously active neurons was observed between 03:00 and 06:00 h (2.5 ± 0.3 neurons/track), whereas the lowest was between 19:00 and 23:00 h (1.5 ± 0.2 neurons/track). The inhibitory effect of AMPH on DA firing rate was similar in both phases. The inhibitory effect of low dose of APO (25 μg/kg, dose selective for D2 autoreceptor) was more potent in the dark phase, whereas APO effects at higher doses were similar in both phases. Finally, MLT administration (1 mg/kg) produced a moderate inhibition of DA cell firing in both phases. These experiments demonstrate the existence of an intradiurnal rhythmic pattern of VTA DA neuronal firing activity and a higher pharmacological response of D2 autoreceptors in the dark phase.

Neonatal medial prefrontal cortex lesion enhances the sensitivity of the mesoaccumbal dopamine system

Neurodevelopmental models of schizophrenia posit that early brain damage leads to dys- or misconnection effects possibly altering synaptic transmission in brain sites distal of the lesion. We tested the hypothesis that neonatal medial prefrontal cortex (mPFC) lesions affect the sensitivity of the mesoaccumbal dopamine (DA) system. Using extracellular single-unit recordings combined with systemic application of the DA agonist apomorphine, followed by the D2 receptor antagonist haloperidol or the D1 receptor antagonist SCH23390, we compared electrophysiological properties of nucleus accumbens core and shell neurons after bilateral excitotoxic lesions of mPFC induced at postnatal day 7 or in adult rats. Whereas animals with adult mPFC lesions showed an altered discharge pattern within the core region, neonatal mPFC lesions altered the discharge pattern within the shell region. Subcutaneous administration of apomorphine (4 mg/kg) reduced accumbal firing rate in 77% of all neurons. Onset and magnitude of apomorphine-induced inhibition of neuronal activity was faster and stronger in rats with neonatal but not adult mPFC lesions in both core and shell regions. Apomorphine-induced inhibition was partially reversed by 0.1 mg/kg haloperidol only in core region of neonatal lesioned rats. Apomorphine-induced excitation of neuronal activity (in 21% of all neurons) was reversed by the D1 receptor antagonist SCH23390 (0.1 mg/kg) in all excited neurons. These data support the hypothesis that neonatal but not adult lesions of mPFC alter cortico-striatal networks and suggest that disturbance of mPFC development leads to neurodevelopmental changes in mesoaccumbal DA system during adulthood.

Acute effects of 3,4-methylenedioxymethamphetamine on striatal single-unit activity and behavior in freely moving rats: differential involvement of dopamine D1 and D2 receptors

3,4-Methylenedioxymethamphetamine (MDMA) is a widely abused amphetamine derivative that increases dopamine (DA) and serotonin release via a reverse transport mechanism. Changes in the activity of striatal neurons in response to increased DA transmission may shape the behavioral patterns associated with amphetamine-like stimulants. To determine how the striatum participates in MDMA-induced locomotor activation, we recorded the activity of >100 single units in the striatum of freely moving rats in response to a dose that increased motor activation (5.0 mg/kg). MDMA had a predominantly excitatory effect on neuronal activity that was positively correlated with the magnitude of locomotor activation. Categorizing neurons according to baseline locomotor responsiveness revealed that MDMA excited significantly more neurons showing movement-related increases in activity compared to units that were non-movement-related or associated with movement-related decreases in activity. Further analysis revealed that the drug-induced striatal activation was not simply secondary to the behavioral change, indicating a primary action of MDMA on striatal motor circuits. Prior administration of SCH-23390 (0.2 mg/kg), a D(1) antagonist, resulted in a late onset of MDMA-induced locomotion, which correlated positively with delayed neuronal excitations. Conversely, prior administration of eticlopride (0.2 mg/kg), a D(2) antagonist, completely abolished MDMA-induced locomotion, which paralleled its blockade of MDMA-induced excitatory neuronal responses. Our results highlight the importance of striatal neuronal activity in shaping the behavioral response to MDMA, and suggest that DA D(1) and D(2) receptors have distinct functional roles in the expression of MDMA-induced striatal and locomotor activation.

Integrative role for serotonergic and glutamatergic receptor mechanisms in the action of NMDA antagonists: potential relationships to antipsychotic drug actions on NMDA antagonist responsiveness

NMDA receptor antagonists worsen symptoms in schizophrenia and induce schizophrenic-like symptoms in normal individuals. In animals, NMDA antagonist-induced behavioral responses include increased activity, head weaving, deficits in paired pulse inhibition and social interaction, and increased forced swim immobility. Repeated exposure to NMDA antagonists in animals results in behavioral sensitization-a phenomenon accentuated in rats with dopaminergic neurons lesioned during development. In keeping with an involvement of serotonin and glutamate release in NMDA antagonist action, selected behaviors induced by NMDA antagonists are minimized by 5-HT(2A) receptor antagonists and mGLU2 receptor agonists. These observations provide promising new approaches for treating acute NMDA antagonist-induced psychosis. Further, acute atypical antipsychotic drugs also minimize NMDA antagonist actions to a greater degree than typical antipsychotics. However, because knowledge concerning acute versus chronic effectiveness of various antipsychotic drugs against NMDA antagonist neuropathology is limited, future studies to define more fully the basis of their differences in efficacy after chronic treatment could provide an understanding of their actions on neural mechanisms responsible for the core pathogenesis of schizophrenia.

Effects of risperidone, clozapine and haloperidol on extracellular recordings of substantia nigra reticulata neurons of the rat brain

Risperidone has proven to be effective as an antipsychotic drug and has fewer extrapyramidal side-effects than classic neuroleptics. In addition to its dopamine D2 receptor antagonistic properties, this antipsychotic agent is a potent 5-HT2 receptor antagonist. The atypical antipsychotic, clozapine, also possesses both dopamine D2 and 5-HT2 receptor affinity next to affinities for other receptors. To gain an insight in the consequences for basal ganglia activity of treatment with these atypical neuroleptics vs. typical neuroleptics, the effects of cumulative doses of risperidone, clozapine and haloperidol on the firing rate of substantia nigra reticulata neurons were studied. Extracellular recordings were performed in chloralhydrate-anaesthetized male Wistar rats. Both risperidone (50-3200 micrograms/kg i.v.) and clozapine (100-6400 micrograms/kg i.v.) dose dependently decreased substantia nigra reticulata activity maximally to 70% of the basal activity. With both treatments, a dose of 800 micrograms/kg was significantly effective. In contrast, haloperidol (12.5-800 micrograms/kg i.v.) gradually induced a slight increase in substantia nigra reticulata activity, which was identical to the substantia nigra reticulata activity after saline treatment. Therefore, these results indicate that typical and atypical neuroleptics affect differentially the output of the basal ganglia in the substantia nigra reticulata. To evaluate the involvement of 5-HT2 receptors in the effect of risperidone, the 5-HT2 receptor agonist, quipazine (0.5 mg/kg i.p.), was administered 15 min preceding risperidone treatment. A 4-fold higher dose of risperidone was needed to significantly affect the substantia nigra reticulata firing rate. Thus, the 5-HT2 component of the effect of risperidone is, at least partly, responsible for the difference in effect on substantia nigra reticulata neurons in comparison to haloperidol.

Phasic firing of single neurons in the rat nucleus accumbens correlated with the timing of intravenous cocaine self-administration

To examine potential neural mechanisms involved in cocaine self-administration, the activity of single neurons in the nucleus accumbens of rats was recorded during intravenous cocaine self-administration. Lever pressing was reinforced according to a fixed-ratio 1 schedule. On a time base comparable to the interinfusion interval, half the neurons exhibited phasic firing patterns time locked to the cocaine reinforced level press. For almost all neurons, this pattern consisted of a change in firing rate postpress, typically a decrease, followed by a reversal of that change. The postpress change was closely related to the lever press. Typically, it began within the first 0.2 min postpress and culminated within the first 1.0 min postpress. For a small portion of responsive neurons, the reversal of the postpress change was punctate and occurred within 1-3 min of either the last lever press or the next lever press so that firing was stable during much of the interinfusion interval. For the majority of neurons, the reversal was progressive; it began within 2 min after the previous level press, and it was not complete until the last 0.1-2.0 min before the next lever press. The duration of this progressive reversal, but not of the postpress change, was positively correlated with the interinfusion interval. Thus, in addition to exhibiting changes in firing related to the occurrence of self-infusion, the majority of neurons also exhibited progressive changes in firing related to the spacing of infusions. In a structure that has been shown to be necessary for cocaine self-administration, such a firing pattern is a likely neurophysiological component of the mechanism that transduces declining drug levels into increased drug-related appetitive behavior. It is, thus, a neural mechanism that may contribute to compulsive drug-maintained drug taking.

Amygdaloid neurons respond to clozapine rather than haloperidol in behaving rats pretreated with intra-amygdaloid amphetamine

Single-unit activity was recorded from the amygdaloid complex in freely moving rats during an infusion of amphetamine directly into the recording site. Relative to the quiet resting period prior to the infusion, amphetamine routinely increased neuronal activity within 5-15 min after infusion onset, and this response continued for at least another 30 min. It was generally accompanied by marked increases in sniffing, rearing, locomotion, and grooming as well as by a tendency to turn to the ipsilateral side. Haloperidol and clozapine, typical and atypical antipsychotic drugs, respectively, were then tested in their ability to reverse these neuronal and behavioral effects. Both antipsychotics were administered subcutaneously at behaviorally effective doses within 10 min after termination of the amphetamine infusion. Haloperidol (1.0 mg/kg) failed to reverse the amphetamine-induced increase in amygdaloid neuronal activity and required more than 20 min to exert a partial blockade of the accompanying behavioral activation. Clozapine (10.0 mg/kg), in contrast, blocked the excitatory effects of amphetamine on all tested neurons and also blocked most amphetamine-induced behaviors within 10 min. Taken together, these results, which support other lines of electrophysiological evidence, point to the amygdala as a critical site in the differential behavioral effects of typical and atypical antipsychotic drugs.

Striatal single-unit responses to amphetamine and neuroleptics in freely moving rats

Single-unit recordings from 50 striatal neurons in freely moving rats revealed generally low activity (< 3 spikes/sec) during resting behavior and movement-related excitations in most (n = 36) neurons. While activating behavior, d-amphetamine (1.0 mg/kg, sc) usually excited and inhibited motor- and nonmotor-related neurons, respectively, relative to resting baseline firing rates. A behavioral clamping analysis, which controlled for neuronal effects secondary to behavior, yielded results suggesting a primary, amphetamine-induced excitation of striatal motor-related neurons. Haloperidol (0.1-1.0 mg/kg) strongly inhibited behavior and neuronal activity when injected 30 min after amphetamine. Clozapine (5.0-30.0 mg/kg) inhibited only selected behaviors, but reliably produced haloperidol-like reversals of amphetamine-induced neuronal excitations. A literature review revealed that the neuronal results in behaving animals differ markedly from the inhibitory striatal responses to amphetamine and the excitatory responses to dopamine antagonists often found in immobilized or anesthetized rat preparations. These contrasting, preparation-dependent results support a model based on drug interactions with a proposed neuromodulatory function of striatal dopamine, which is to facilitate or attenuate the activity of neurons receiving, respectively, substantial, or little excitatory afferent input.

Contribution of the amygdala and nucleus accumbens to ventral pallidal responses to dopamine agonists

Neurons recorded from ventral pallidum/substantia innominata (VP) of the basal forebrain respond to dopaminergic agonists that activate either the D1 or D2 the receptor subtype. Major afferent systems to the VP originate within amygdaloid nuclei (AMN) and the nucleus accumbens (NA). Since both the AMN and the NA are dopaminoceptive, the present study sought to analyze the contribution of these afferent systems to VP responses to dopaminergic agonists. Single VP neurons were electrophysiologically recorded in vivo from chloral hydrate-anesthetized rats, and the following determinations were made. 1) Effects of pharmacologic inactivation of an afferent system were assessed by monitoring VP neurons during intracerebral microinjections of the local anesthetic procaine, administered directly into either the AMN or the NA. 2) With procaine-induced VP rate changes used to indicate an afferent influence on the recorded neuron, VP responses to apomorphine (an agonist that acts at D1 and D2 receptor subtypes), SKF38393 (a D1 agonist), or quinpirole (a D2 agonist) were determined and compared with responses in rats not receiving the procaine pretreatment. Following pharmacologic inactivation of either the AMN or the NA, approximately 80% of the VP neurons monitored demonstrated rate changes, illustrating that spontaneous neuronal firing in the Vp is dependent on tonically input systems. Following afferent cessation, responses to apomorphine and quinpirole remained intact, suggesting that the AMN or NA is not necessary for VP responding to the systemic administration of dopaminergic agonists that act at D2 receptors. In contrast, the number of neurons that responded to SKF38393 was diminished follow intra-AMN (but not intra-NA) procaine. This suggests that D1-induced VP responses are mediated, at least in part, via the AMN.

Neuroleptics increase C-FOS expression in the forebrain: Contrasting effects of haloperidol and clozapine

The mechanisms by which the atypical neuroleptic clozapine produces its therapeutic effects in the treatment of schizophrenia without causing the extrapyramidal side effects that are characteristic of most antipsychotic drugs remain unclear. Recently, a single injection of the typical antipsychotic haloperidol has been shown to increase c-fos expression in the striatum [Dragunow et al. (1990) Neuroscience 37, 287-294]. C-fos is a proto-oncogene that encodes a 55,000 mol. wt phosphoprotein, Fos, which is thought to assist in the regulation of "target genes" containing an AP-1 binding site. Because a wide variety of physiological and pharmacological stimuli increase c-fos expression, it has been proposed that Fos immunohistochemistry might be useful in mapping functional pathways in the central nervous system. The present experiments examined some potential neuroanatomical differences in the actions of clozapine and haloperidol by comparing their effects on c-fos expression in the medial prefrontal cortex, nucleus accumbens, striatum and lateral septum. The effects of the selective dopamine receptor antagonists SCH 23390 (D1) and raclopride (D2) were also examined. Haloperidol (0.5, 1 mg/kg) and raclopride (1, 2 mg/kg) produced large increases in the number of Fos-containing neurons in the striatum and nucleus accumbens. SCH 23390 (0.5, 1 mg/kg) reduced the number of Fos-positive neurons in the nucleus accumbens and striatum, and had no effect in the other regions. Neither haloperidol nor raclopride increased the number of Fos-positive neurons in the medial prefrontal cortex. Haloperidol, but not raclopride, produced a modest increase in c-fos expression in the lateral septal nucleus. Clozapine (10, 20 mg/kg) was without effect in the striatum; however, it significantly increased the number of Fos-positive neurons in the nucleus accumbens, medial prefrontal cortex and lateral septal nucleus. Destruction of mesotelencephalic dopaminergic neurons with 6-hydroxydopamine abolished the increase in Fos expression in the nucleus accumbens and striatum produced by haloperidol and raclopride, and also blocked the clozapine-induced increase in the nucleus accumbens. However, the inductive effects of clozapine and haloperidol on c-fos expression in the lateral septal nucleus and of clozapine in the medial prefrontal cortex were not affected by the 6-hydroxydopamine lesions. These results suggest that clozapine's unique therapeutic profile may be related to its failure to induce Fos in the striatum as well as its idiosyncratic actions in the lateral septum and medial prefrontal cortex. The effects of clozapine in these latter regions do not appear to be mediated by dopaminergic mechanisms.

Chronic ascorbate potentiates the effects of chronic haloperidol on behavioral supersensitivity but not D2 dopamine receptor binding

Ample behavioral evidence suggests that ascorbate parallels the action of haloperidol, a widely used neuroleptic. To determine the extent to which this parallel extends to chronic treatment, 21 days of exposure to ascorbate (100 or 500 mg/kg) alone or combined with haloperidol (0.5 mg/kg) were assessed on stereotyped behavior and neostriatal D2 dopamine receptor binding in rats. Our results indicate that when challenged with the dopamine agonist, apomorphine (0.5 mg/kg), animals chronically treated with haloperidol or high-dose ascorbate alone display a supersensitive sniffing response relative to controls, while animals chronically treated with the combination of haloperidol and high-dose ascorbate display a further potentiation of sniffing relative to the haloperidol groups. In addition, [3H]spiperone saturation studies showed, as expected, an up-regulation of striatal D2 dopamine receptors in rats treated with haloperidol as reflected by a change in receptor density (Bmax) but not affinity (KD). Ascorbate treatment, however, had no effect on D2 receptor density or the distribution of [3H]apomorphine in whole brain. Even though chronic treatment with the haloperidol-high-dose-ascorbate combination produced an up-regulation of striatal D2 dopamine receptors, this treatment did not cause a further up-regulation relative to haloperidol alone nor did it have any effect on [3H]apomorphine distribution. Taken together, these findings indicate that although chronic ascorbate produces behavioral supersensitivity to apomorphine through central mechanisms, they appear to differ from those induced by chronic haloperidol.

Clozapine: an atypical neuroleptic

Since it was synthesized in 1960, much has been written about clozapine. Although a number of its properties are those of a neuroleptic, it displays marked differences from classical antipsychotics to the extent that it is currently listed as an atypical neuroleptic. A classical neuroleptic has been defined in man according to its antipsychotic properties, accompanied by extrapyramidal effects, and in animals according to its cataleptic properties, its ability to antagonize apomorphine and amphetamine stereotypies and to suppress the conditioned avoidance response. Moreover, the classical neuroleptic exerted depressive and anhedonic effects in most conditioning schedules. With clozapine, most of these properties are no longer strictly in force to the point that they call in question the validity of the tests carried out to detect the potential of neuroleptics. This article attempts to compare the characteristics of clozapine with those of classical neuroleptics from a toxicological, neuropharmacological, psychopharmacological and clinical point of view.

Clozapine antagonism of D-1 and D-2 dopamine receptor-mediated behaviors

When tested in rats supersensitive to dopamine agonists, the atypical neuroleptic clozapine displayed pharmacological properties expected of both a D-1 and D-2 receptor antagonist. The locomotor response induced by the D-1 receptor agonist SKF-38393 in neonatal-6-hydroxydopamine (6-OHDA)-lesioned rats was reversed in a dose-related fashion, although a complete blockade of this behavior was not observed indicative for only a partial antagonism of D-1 receptor function. Clozapine also blocked the self mutilation resulting from L-dihydroxyphenylalanine (L-DOPA) administration to neonatal-6-OHDA-lesioned rats, an effect previously linked to D-1 receptor activation. At higher doses, clozapine blocked the locomotor activity elicited by the D-2 agonist LY-171555 in adult-6-OHDA-lesioned rats. Therefore, the action of clozapine on D-1 as well as D-2 receptor-mediated behaviors contributes to its pharmacological effects. The ability of clozapine to stop self-mutilatory behavior in neonatal-6-OHDA-lesioned rats suggests that this drug might be an effective treatment for self-injurious behavior associated with the Lesch-Nyhan syndrome and mental retardation.

Clozapine and haloperidol in the amygdaloid complex: differential effects on dopamine transmission with long-term treatment

Single-unit recording techniques and liquid chromatography with electrochemical detection were used to measure the effects of neuroleptic pretreatment on the efficacy of dopamine transmission in the amygdaloid complex. Rats received twice-daily injections of clozapine (10.0 mg/kg), haloperidol (1.0 mg/kg), or vehicle for 6 days. During withdrawal from haloperidol, but not clozapine, amygdaloid neurons were significantly more responsive to iontophoretic application of dopamine. Neither drug altered the neuronal response to norepinephrine or acetylcholine. Tolerance developed to the ability of haloperidol to increase the amygdaloid level of 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine metabolite, but such an effect did not occur with clozapine. In fact, clozapine pretreatment led to a significant increase in both DOPAC and dopamine levels in the amygdaloid complex. These results suggest that a differential action of these drugs on dopamine transmission may explain their differential effects on postsynaptic dopamine receptors.

Differential response of amygdaloid neurons to clozapine and haloperidol: effects of repeated administration

Rats were pretreated with saline or with behaviorally equivalent doses of clozapine (10.0 mg/kg) or haloperidol (1.0 mg/kg) twice daily for six consecutive days. On the following day, amygdaloid neurons in clozapine-pretreated rats responded to a challenge injection of this drug with a significantly greater increase in firing rate than saline controls. In contrast, amygdaloid neurons generally remained unresponsive to haloperidol even when pretreatment with this drug was extended to 13 days. Neither clozapine nor haloperidol pretreatment, however, altered the response of amygdaloid neurons to d-amphetamine administered after a four-day washout period. Amphetamine inhibited amygdaloid activity to a comparable extent in all rats. Taken together, these results implicate the amygdaloid complex as an important site of action of clozapine and related antischizophrenic drugs.

Unilateral dopamine depletions attenuate the response of striatal neurons to systemic amphetamine in both hemispheres

Neuronal activity was recorded bilaterally from the striatum of intact control animals and rats pretreated 10-15 days earlier with a unilateral intranigral injection of 6-hydroxydopamine. Following isolation of single unit discharges, each group was challenged with intravenous injections of 0.2 mg kg-1 d-amphetamine administered at 2-min intervals. Striatal neurons from intact control animals responded to amphetamine with equal numbers of inhibitions and excitations. In contrast, the predominant response in the animals with lesions was no response at all even with a total cumulative dose of 2.0 mg kg-1. Approximately 50% of the neurons in each striatum of the rats with unilateral lesions failed to respond to amphetamine despite a greater than 98% difference in dopamine levels between hemispheres. The remaining neurons in these animals responded as in intact controls. These results suggest that some functional change occurs following unilateral dopamine depletion that acts to decrease the response of neurons to amphetamine in both the intact and the dopamine-depleted striatum.

Nigral reticulata neurons: potentiation of responsiveness to amphetamine with long-term treatment

Single-unit activity was recorded in the substantia nigra pars reticulata of rats in response to intravenous challenge injections of D-amphetamine. The animals were pretreated with saline or 5.0 mg/kg D-amphetamine twice daily for 6 consecutive days. Whereas the large majority of saline controls (6 of 8) showed no consistent responses to amphetamine at doses up to 2.0 mg/kg, amphetamine pretreated rats (7 of 10) responded with a progressive increase in firing rate. Both groups of animals responded to a subsequent injection of 5.0 mg/kg clozapine with a depression of firing rate. The remaining control rats were inhibited by amphetamine and this aberrant response was enhanced with long-term treatment. In these unusual cells, clozapine accelerated firing rate. Taken together, these results indicate that unlike dopaminergic neurons in the compacta region of the nigra, reticulata neurons increase their responsiveness to amphetamine with repeated administration.

A computer-supported method for analyzing behavioral observations: studies with stereotypy

The present report describes an observational method for quantifying behavior including drug-induced stereotypy, and employs an electronic data-collecting device with microcomputer hardware and software support. This method generates interval scale data, thus permitting the application of powerful parametric statistics, and also allows examination of discrete response topographies. For example, the method permits parametric comparisons among drug doses, treatment groups, as well as over time. The effects of the atypical antipsychotic drug thioridazine on apomorphine-induced stereotypy were used to illustrate the utility of the method. Thioridazine was found, among other effects, to potentiate apomorphine-induced gnawing and licking, while blocking sniffing.

Apomorphine-induced inhibition of neostriatal activity is enhanced by lesions induced by 6-hydroxydopamine but not by long-term administration of amphetamine

Single-unit activity was recorded in the neostriatum of locally-anesthetized, immobilized rats which had received either a unilateral injection of 6-hydroxydopamine (6-OHDA) into the nigro-neostriatal dopamine pathway, or repeated injections (twice daily for 6 days) of saline, 1.0, 5.0 or 10.0 mg/kg (+)-amphetamine. A staircase regimen of apomorphine was administered intravenously to each rat to determine if the long-term administration of amphetamine changed the sensitivity of postsynaptic dopamine receptors in a way comparable to that produced by 6-hydroxydopamine. In all groups, the most frequent response to apomorphine was inhibition of neostriatal activity. In saline-treated rats, most units were moderately excited by small doses of apomorphine (0.0025-0.02 mg/kg) and then inhibited by doses exceeding 0.04 mg/kg. In rats with lesions induced by 6-hydroxydopamine, apomorphine caused significantly more inhibition than in saline-treated animals. By contrast, neuronal responses in amphetamine-treated rats were not significantly different from those of saline controls. These results indicate that the long-term administration of amphetamine produces an augmentation of behavior by some mechanism other than an increase in the sensitivity of postsynaptic dopamine receptors.

Effects of antipsychotic drugs on the long-term effects of amphetamine on nigro-striatal dopamine neurons in iprindole-treated rats

The decrease in striatal dopamine (DA) at 1 week after the administration of a single injection of (+)-amphetamine sulfate (9.2 mg/kg) to iprindole-treated (10 mg/kg of iprindole hydrochloride) rats was prevented by haloperidol (0.2 mg/kg), sulpiride (32 mg/kg), pimozide (2 mg/kg), chlorpromazine hydrochloride (3.5 mg/kg), fluphenazine 2-hydrochloride (0.25 mg/kg) and (+)-butaclamol hydrochloride (1 mg/kg) but not by (-)-butaclamol hydrochloride (1 mg/kg) or clozapine (40 mg/kg). The same dose of sulpiride did not significantly attenuate the rotational behavior induced by the administration of (+)-amphetamine sulfate (9.2 mg/kg) to iprindole-treated rats with unilateral aspiration lesions of the striatum. The concentration of amphetamine in the brains of iprindole-treated rats at 8 h after (+)-amphetamine sulfate (9.2 mg/kg) administration was not altered by the coadministration of haloperidol (1 mg/kg), sulpiride (32 mg/kg), pimozide (2 mg/kg) or clozapine (40 mg/kg). Recovery of striatal DA after depletion by alpha-methyl-m-tyrosine (alpha MMT) (50 mg/kg) was facilitated by haloperidol (0.2 mg/kg) and sulpiride (32 mg/kg) but not by clozapine (40 mg/kg). The possibility that neuroleptic drugs antagonize both the short-term depletion of striatal DA produced by alpha MMT and the long-term depletion of striatal DA produced by amphetamine in iprindole-treated rats by an effect on the nerve impulse-mediated release of vesicular transmitter is discussed.

Inhibition of drug-induced circling by GABA-ergic activity in the nucleus accumbens

Injections of GABA (125-500 micrograms in 1 microliter) or muscimol (40-200 ng in 1 microliter) into the nucleus accumbens markedly reduced the amphetamine-induced circling of rats with unilateral 6-hydroxydopamine (6-OHDA)-induced lesions of nigrostriatal dopamine neurons. The potency of muscimol was approximately 10(4) times that of GABA. Muscimol exerted a similar effect when injected in a smaller volume (0.5 microliter) at a slower rate (0.11 microliter/min), a procedure which was shown to reduce the spread of injected [3H]muscimol. Intra-accumbens injections of subconvulsive doses of picrotoxin had no effect on amphetamine-induced circling. Intra-accumbens muscimol (40 ng, 0.5 microliter, 0.11 microliter/min) also reduced the contralateral circling evoked by apomorphine in rats with unilateral 6-OHDA-induced lesions of the nigrostriatal pathway and bilateral 6-OHDA-induced lesions of the mesolimbic dopamine terminals. These results suggest that GABA-ergic activity in the nucleus accumbens exerts an inhibitory influence on drug-induced circling. At least part of this action is at a step beyond the release of dopamine from mesolimbic terminals.

Shift from inhibition to excitation in the neostriatum but not in the nucleus accumbens following long-term amphetamine

Neuronal responses to 1.0 or 5.0 mg/kg D-amphetamine were recorded simultaneously in the neostriatum and nucleus accumbens of rats pretreated twice daily with these doses or with saline for 6 consecutive days. In all groups, the number of neurons responding to a challenge injection of either dose of amphetamine with an overall excitation or inhibition was not significantly different. During the first 30-60 min of the drug response, however, neurons in the neostriatum of amphetamine-pretreated rats responded with a significant increase in firing rate compared to saline controls. In the nucleus accumbens, on the other hand, tolerance developed to the inhibition produced by 1.0 mg/kg D-amphetamine, whereas the responses produced by 5.0 mg/kg were not significantly altered by long-term treatment. Liquid chromatography with electrochemical detection revealed that pretreatment with 5.0 mg/kg D-amphetamine produced a slight, but significant, reduction of dopamine and norepinephrine levels in the neostriatum. Catecholamine levels were not significantly altered in the nucleus accumbens by either dose. These electrophysiological and neurochemical changes are discussed in relation to the known involvement of these sites in the dose-dependent behavioral alterations that accompany repeated amphetamine injections.

Tolerance to amphetamine-induced inhibition of neuronal activity in the central amygdaloid nucleus

Rats were pretreated twice daily for 5 consecutive days with saline or 2.5 mg/kg d-amphetamine. Approximately 12 hr after the last injection, neuronal activity was recorded bilaterally from the central amygdaloid nucleus (CAN) and the animals were challenged every 2 min with 0.2 mg/kg d-amphetamine or with increasing incremental doses of apomorphine. In saline controls, all CAN neurons were inhibited by the 5th amphetamine injection and two-thirds were suppressed by 0.64 mg/kg apomorphine. In amphetamine-pretreated animals, on the other hand, the majority of CAN neurons failed to respond even by the 10th amphetamine injection and less than one-third were inhibited by apomorphine even at a dose of 2.56 mg/kg. These results indicate that tolerance develops to the inhibitory effects of d-amphetamine in the CAN and that this effect is mediated, at least in part, by a decrease in the sensitivity of postsynaptic dopamine receptors.

Differential actions of classical and atypical neuroleptics on mouse nigrostriatal neurons

The classical neuroleptics haloperidol, perphenazine and chlorpromazine increase both dopamine metabolism and release in the mouse striatum. The atypical agents clozapine and thioridazine increase dopamine metabolism with no increase in release. At high doses, sulpiride increases both dopamine metabolism and release. These data suggest that atypical neuroleptics act to inhibit dopamine release and indicate that sulpiride may not be an atypical agent.

Antipsychotic drug effects on the behavior of squirrel monkeys differentially controlled by noxious stimuli

The effects of several antipsychotic compounds were examined on two types of behavioral performances of squirrel monkeys. Both behaviors occurred simultaneously and were maintained separately by different schedules using noxious stimuli. Steady rates of responding were maintained when a chain pulling response postponed electric shock delivery (avoidance schedule). Concurrently, positively accelerated rates of responding were maintained on a lever where the first response after 3 min produced electric shock (fixed-interval 3-min schedule). The effects of the different drugs depended both upon whether the behavior postponed or presented shock and on the particular drug. Chlorpromazine (0.001-0.03 mg/kg), haloperidol (0.001-0.01 mg/kg), molindone (0.001-0.03 mg/kg) and thiothixene (0.001-0.03 mg/kg) increased slightly or had no effect on responding under the shock-postponement schedule at doses that decreased responding maintained by shock presentation. The effects of clozapine, a clinically effective antipsychotic compound, differed markedly from the other antipsychotic drugs. Clozapine (0.01-1.0 mg/kg) increased responding maintained by the presentation of shock at doses that decreased responding under the shock-postponement schedule. Higher doses of these drugs decreased responding under both schedules and, with the exception of clozapine, resulted in increased frequency of shocks under the postponement schedule.

Differential sensitivity to amphetamine following long-term treatment with clozapine or haloperidol

When rats were pretreated for 8 consecutive days with 2.0 mg/kg haloperidol, injection of 2.5 or 5.0 mg/kg d-amphetamine 2 or 6 days later resulted in a larger increase in oral behaviors and a more prolonged period of focused sterotypy than in saline-pretreated controls. This increased sensitivity to amphetamine is consistent with the effects of a chronic haloperidol-induced increase in dopamine receptor sensitivity. In contrast, long-term treatment with either d-amphetamine or clozapine produced complex changes in the multiphasic behavioral response to amphetamine, which cannot be explained solely by a shift in the sensitivity of dopamine receptors.

Differential actions of classical and atypical antipsychotic drugs on spontaneous neuronal activity in the amygdaloid complex

Classical antipsychotic drugs such as haloperidol produce akinesia and catalepsy, whereas clozapine and related atypical antipsychotics fail to elicit these behaviors even at relatively high doses. Despite these behavioral differences, a cataleptic dose of haloperidol (2.0 mg/kg) produces changes in neuronal activity in the neostriatum and nucleus accumbens comparable to those produced by a non-cataleptic dose of clozapine (20.0 mg/kg). To further elucidate the brain mechanisms underlying the differential behavioral response to these drugs, an electrophysiological analysis was extended to neurons in the rat amygdaloid complex. Whereas an intraperitoneal injection of 2.0 mg/kg haloperidol generally failed to alter the firing rate of amygdaloid neurons, 20.0 mg/kg clozapine typically produced a prolonged increase in activity. Similarly, clozapine, but not haloperidol, reversed the depression of firing rate produced by 1.0 mg/kg d-amphetamine. The results suggest that neurons in the amygdaloid complex are more responsive to antipsychotic drugs devoid of extrapyramidal side effects than to antipsychotics which elicit parkinsonian-like motor dysfunctions.