BMY-14802, a sigma ligand and potential antipsychotic drug, reverses amphetamine-induced changes in neostriatal single-unit activity in freely moving rats

Antipsychotic-like properties of 5-alpha-reductase inhibitors

Recent evidence indicates that neuroactive steroids may participate in the pathogenesis of schizophrenia spectrum disorders, yet the mechanisms of this involvement are elusive. As 5-alpha-reductase (5AR) is the rate-limiting enzyme of one of the two major metabolic pathways in brain steroidogenesis, we investigated the effects of its blockade in several rat models of psychotic-like behavior. The 5AR inhibitor finasteride (FIN, 60 or 100 mg/kg, intraperitoneal, i.p.) dose- and time-dependently antagonized prepulse inhibition (PPI) deficits induced by apomorphine (APO, 0.25 mg/kg, subcutaneous, s.c.) and d-amphetamine (AMPH, 5 mg/kg, s.c.), in a manner analogous to haloperidol (HAL, 0.1 mg/kg, i.p.) and clozapine (CLO, 5 mg/kg, i.p.). Similar results were observed with the other 5AR inhibitors dutasteride (DUT, 40 or 80 mg/kg, i.p.) and SKF 105111 (30 mg/kg, i.p.). FIN (60 or 100 mg/kg, i.p.) also reduced hyperlocomotion induced by AMPH (1 or 3 mg/kg, s.c.) and attenuated stereotyped behaviors induced by APO (0.25 mg/kg, s.c.). Nevertheless, FIN (100 mg/kg, i.p.) did not reverse the PPI disruption induced by the N-methyl-d-aspartate receptor antagonist dizocilpine (0.1 mg/kg, s.c.). FIN (60-300 mg/kg, i.p.) induced no catalepsy in either the bar test or the paw test. Our results suggest that 5AR inhibitors elicit antipsychotic-like effects in animals and may be proposed as a putative novel target in the management of psychotic disorders.

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.

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.

Localization of motor- and nonmotor-related neurons within the matrix-striosome organization of rat striatum

Striatal neurons can be classified as movement- and nonmovement-related depending on their ability to change firing rate in close temporal association with spontaneous movement in an open-field arena. The present study assessed the location of these cell types within the compartmental organization of the striatum by combining single-unit recording techniques in freely moving rats with calbindin immunohistochemistry. Movement-related neurons were found predominately either in the matrix or along the matrix-striosome border. Most of these neurons were nonselective in that they increased activity whenever the animals changed from a quiet resting posture to any form of behavioral activation (e.g., grooming, locomotion, rearing). The remaining neurons in this group responded exclusively to movements of the head. Nonselective units discharged at a significantly slower rate than head-movement units during both quiet rest and periods of actual movement. Nonmovement-related neurons, which failed to show a reliable change in activity to overt behavior, comprised a relatively small portion of the neuronal sample but were also located in either the matrix or along the matrix-striosome border. Collectively, these results suggest that even though striatal neurons can be distinguished on the basis of their responsiveness to ongoing behavior in an open-field paradigm, such distinctions are not clearly linked to sites within the matrix or its striosomal borders.

Evidence for genetically mediated dysfunction of the central dopaminergic system in the stargazer rat

The stargazer rat is an autosomal recessive mutant (homozygous stg/stg) that displays abnormal behavior, characterized by stereotypic head-movement, circling, and a high level of ambulatory activity. Heterozygous (stg/+) littermates display normal spontaneous behaviors. In this study, stargazers and their unaffected littermates were compared in their behavioral responses to both stimulation and inhibition of dopamine D2/D3 receptors, using quinpirole and haloperidol. Stargazers were observed to yawn a significantly fewer number of times than littermates in response to (--)-quinpirole (50 mu g/kg, IP). Haloperidol (HAL 0.1 mg/kg and 0.3 mg/kg, SC) caused a decrease in stereotypic head-movement in the mutants that was both time- and dose-dependent. In normal littermates, HAL inhibited locomotor activity and produced catalepsy in a time- and dose-dependent manner. In stargazers, both doses of HAL inhibited locomotor activity to a similar degree as in the littermates. However, no catalepsy was detectable in the mutants using 0.1 mg/kg of HAL. A dose of 0.3 mg/kg HAL was only weakly cataleptogenic. Overall, the spectrum of abnormal behaviors expressed by the stargazers and the present evidence of D2/D3 receptor subsensitivity suggest that stargazers possess a genetically mediated dysfunction of the central dopaminergic system.

Phencyclidine-induced increases in striatal neuron firing in behaving rats: reversal by haloperidol and clozapine

Amphetamine and related drugs of abuse facilitate dopamine transmission in the striatum. This action is believed to underlie the increase in firing of striatal motor-related neurons after amphetamine administration in behaving rats. The present study extended this electrophysiological investigation to phencyclidine (PCP), a nonamphetamine psychomotor stimulant that acts primarily as a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) glutamate receptors. Like amphetamine, PCP (1.0, 2.5, or 5.0 mg/kg) increased the activity of striatal motor-related neurons concomitant with behavioral activation. These effects were blocked by subsequent administration of either 1.0 mg/kg haloperidol or 20.0 mg/kg clozapine, typical and atypical neuroleptics, respectively. Dizocilpine (MK- 801), another noncompetitive NMDA antagonist, mimicked the effect of PCP. Collectively, these results indicate that amphetamine and NMDA antagonists exert comparable effects on striatal motor-related neurons, suggesting that the response of these cells to psychomotor stimulants is regulated by a dopaminergic-glutamatergic influence.

Pharmacological comparison of antipsychotic drugs and sigma-antagonists in rodents

We compared antipsychotic drugs (haloperidol, chlorpromazine and clozapine) and sigma antagonists (remoxipride, cinuperone, alpha-(4-fluorophenyl)-4-(-fluoro-2-pyrimidinyl)-1-piperazine butanol (BMY 14802) and rimcazole) in the radio-ligand binding and behavioural experiments in rodents. A good correlation was established between the affinity of compounds at dopamine2-receptors in the striatum and their ability to block apomorphine-, amphetamine- and quipazine-induced behavioural effects in rodents. By contrast, no correlation was found between the behavioural effects of these drugs and their affinity at dopamine1-5-HT2- and sigma receptors. The rank order of potency among the studied antipsychotic drugs in the behavioural tests and at dopamine2-receptors was following: haloperidol >> chlorpromazine > or = clozapine. The effectiveness of chlorpromazine and clozapine was nearly similar against apomorphine-induced aggressiveness and yawning, whereas at 5-HT2-receptors clozapine was more active than chlorpromazine. The weak activity of sigma antagonists at dopamine2 receptors could be a possible reason why these compounds were less effective in the behavioural studies compared to antipsychotic drugs. However, the antagonism of remoxipride against apomorphine-induced stereotypy and aggressiveness is not related to its activity at sigma receptors, because the other sigma antagonists did not block these effects of apomorphine. It is probable that remoxipride exerts its action through blocking of dopamine2 receptors. In conclusion, the present study revealed only weak activity of sigma antagonists in the behavioural models widely used to study the antipsychotic drugs. Therefore, the antipsychotic activity of sigma antagonists is doubtful.

A methodology for determining the patch-matrix compartmental location of extracellular single-unit recordings in the striatum of freely moving rats

A methodology was developed to combine extracellular electrophysiological recording techniques in awake, behaving rats with immunohistochemical protocols to determine the placement of recording sites in the patch (striosome) or matrix (extrastriosome) regions of the striatum. The recording system includes a 3-barrel glass micropipette, which can be used to deposit Pontamine Sky Blue to mark a small number of neurons at the recording site. Subsequent immunostaining for calbindin allows the site to be localized within the patch-matrix organization. Other dyes or neuroanatomical probes can be ejected from other barrels of the recording pipette to label afferent and efferent structures. The methodology can be applied to many brain regions, providing for integrative studies of behavior and nervous system structure and function.

Cortical lesions attenuate the opposing effects of amphetamine and haloperidol on neostriatal neurons in freely moving rats

Neuronal activity was recorded from the neostriatum of freely moving rats at least 1 week following either sham or bilateral ablations of frontal and somatosensory cortex. In both groups of animals, the majority of neurons increased firing rate in close temporal association with spontaneous movement. No group differences emerged either with respect to baseline firing rates or open-field behavior. Following amphetamine administration, however, the excitatory response of motor-related neurons was suppressed in cortical-lesioned rats. A behavioral clamping procedure, which assessed neuronal activity during matched pre- and post-amphetamine behaviors, confirmed these results, suggesting that the amphetamine-induced changes in neuronal activity reflect a direct drug effect independent of behavioral feedback. In animals that received a subsequent injection of 1.0 mg/kg haloperidol, cortical lesions attenuated the ability of this neuroleptic to block both the behavioral and neuronal effects of amphetamine. Collectively, these results support mounting evidence for an important modulatory influence of cortical afferents on the amphetamine-induced excitation of neostriatal neurons and the reversal of this effect by haloperidol.

Neuronal activity in rabbit neostriatum during classical eyelid conditioning

Extracellular multiple- and single-unit recordings were made from the neostriatum of rabbits during classical eyelid conditioning. Neostriatal neurons processed information regarding the conditioned auditory stimulus (CS) and conditioned eyelid response (CR) as well as the unconditioned stimulus/response (US/UR). These data are consistent with previous reports that neostriatal neurons respond to movement and movement-related sensory stimuli. In most cases, neostriatal neurons increased activity to the US during the early phase of training, but to the CR as training progressed. A close temporal correlation was found between neuronal activity and CR onset with unit discharges typically preceding CR onset by 10-50 ms. The activity of some multiple and single units was monitored after injection of haloperidol, a neuroleptic and dopamine antagonist known to disrupt neostriatal function. Interestingly, haloperidol caused a greater disruption of CRs at low-intensity than at high-intensity CSs, but conditioning-related neuronal activity was disrupted equally at both intensities. These data are discussed in terms of a possible role for the neostriatum in eyelid conditioning.

Neuronal and behavioral correlates of intrastriatal infusions of amphetamine in freely moving rats

When injected systemically in rats, amphetamine routinely activates striatal neurons that increase firing rate in close temporal association with movement but suppresses nonmotor-related neurons. To assess the role of striatal mechanisms in these opposing effects, D-amphetamine (20 micrograms/microliters) was infused (10 microliters/h) directly into the striatum of awake, behaving rats and single-unit activity was recorded simultaneously at the infusion site. Intrastriatal amphetamine reliably activated motor-related, but suppressed nonmotor-related neuronal activity shortly after infusion onset. These changes in firing rate preceded overt behavioral changes, in most cases by several minutes. When they did emerge, behavioral responses were characterized mainly by focused sniffing and head bobbing. Interestingly, the strongest behavioral responses, as measured by onset latency and response magnitude, were likely to result from infusions into motor-related rather than nonmotor-related recording sites. Systemic injection of haloperidol (1.0 mg/kg) shortly after infusion offset suppressed both behavior and striatal neuronal activity. Control infusions of intrastriatal saline had no consistent effect on either striatal neuronal activity or behavior. Collectively, these results indicate that the divergence in firing rate between motor- and nonmotor-related striatal neurons reflects an intrinsic action of amphetamine in the striatum rather than a secondary effect of behavioral feedback. Moreover, the linkage of motor-related striatal areas with the strongest behavioral responses to amphetamine suggests important functional differences between motor- and nonmotor-related striatal neurons.

The involvement of D1 and D2 dopamine receptors in amphetamine-induced changes in striatal unit activity in behaving rats

Selective D1 (SCH-23390) and D2 (eticlopride and sulpiride) dopamine receptor antagonists were assessed for their ability to reverse the effects of 1.0 mg/kg D-amphetamine on excitatory motor-related neurons in the striatum of freely moving rats. SCH-23390 (0.125, 0.25, 0.5 and 1.0 mg/kg) rapidly and consistently blocked amphetamine-induced neuronal excitations as did eticlopride (0.25 and 1.0 mg/kg). In contrast, (-)-sulpiride (10, 20 and 40 mg/kg) failed to alter the neuronal response to amphetamine. Similarly, SCH-23390 and eticlopride also blocked the behavioral effects of amphetamine, but sulpiride did not. Collectively, these results support the involvement of D1 and D2 dopamine receptors in the excitatory effects of amphetamine on striatal neurons, but suggest caution in assessing the neuronal and behavioral effects of sulpiride.

Responses of rat striatal neurons during performance of a lever-release version of the conditioned avoidance response task

Neural activity was recorded from 218 sites in the striatum (caudate-putamen and nucleus accumbens) of rats trained on a lever-release version of the conditioned avoidance response (CAR) task, in which an auditory signal elicits a short-latency, forelimb withdrawal. > 80% of these recording sites showed task-related activity, including neurons that responded to the auditory stimulus (signal-related cells), the lever-release (response-related cells), or both of these events (signal/response-related cells). Histological analysis revealed a predominance of signal-related neurons in medial striatum, whereas lateral recording sites mainly showed response-related activity. Haloperidol (0.1 mg/kg s.c.), a widely used neuroleptic that impairs CAR performance, significantly attenuated task-related neural activity without altering the latency of the neural response or spontaneous firing rate. Collectively, these results, which demonstrate the usefulness of the lever-release CAR paradigm for assessing striatal function, suggest that the sensory and motor aspects of the CAR task are processed by different striatal regions. Moreover, haloperidol appears to disrupt the striatal processing of both sensory and motor information.