Decreased in vivo [3H]spiroperidol binding in rat brain after repeated methamphetamine administration

Effects of chronic methamphetamine on psychomotor and cognitive functions and dopamine signaling in the brain

Methamphetamine (MA) studies in animals usually involve acute, binge, or short-term exposure to the drug. However, addicts take substantial amounts of MA for extended periods of time. Here we wished to study the effects of MA exposure on brain and behavior, using an animal model analogous to this pattern of MA intake. MA doses, 4 and 8mg/kg/day, were based on previously reported average daily freely available MA self-administration levels. We examined the effects of 16 week MA treatment on psychomotor and cognitive function in the rat using open field and novel object recognition tests and we studied the adaptations of the dopaminergic system, using in vitro and in vivo receptor imaging. We show that chronic MA treatment, at doses that correspond to the average daily freely available self-administration levels in the rat, disorganizes open field activity, impairs alert exploratory behavior and anxiety-like state, and downregulates dopamine transporter in the striatum. Under these treatment conditions, dopamine terminal functional integrity in the nucleus accumbens is also affected. In addition, lower dopamine D1 receptor binding density, and, to a smaller degree, lower dopamine D2 receptor binding density were observed. Potential mechanisms related to these alterations are discussed.

Repeated methamphetamine administration differentially alters fos expression in caudate-putamen patch and matrix compartments and nucleus accumbens

Background

The repeated administration of psychostimulant drugs produces a persistent and long-lasting increase (“sensitization”) in their psychomotor effects, which is thought to be due to changes in the neural circuitry that mediate these behaviors. One index of neuronal activation used to identify brain regions altered by repeated exposure to drugs involves their ability to induce immediate early genes, such as c-fos. Numerous reports have demonstrated that past drug experience alters the ability of drugs to induce c-fos in the striatum, but very few have examined Fos protein expression in the two major compartments in the striatum—the so-called patch/striosome and matrix.

Methodology/Principal Findings

In the present study, we used immunohistochemistry to investigate the effects of pretreatment with methamphetamine on the ability of a subsequent methamphetamine challenge to induce Fos protein expression in the patch and matrix compartments of the dorsolateral and dorsomedial caudate-putamen and in the ventral striatum (nucleus accumbens). Animals pretreated with methamphetamine developed robust psychomotor sensitization. A methamphetamine challenge increased the number of Fos-positive cells in all areas of the dorsal and ventral striatum. However, methamphetamine challenge induced Fos expression in more cells in the patch than in the matrix compartment in the dorsolateral and dorsomedial caudate-putamen. Furthermore, past experience with methamphetamine increased the number of methamphetamine-induced Fos positive cells in the patch compartment of the dorsal caudate putamen, but not in the matrix or in the core or shell of the nucleus accumbens.

Conclusions/Significance

These data suggest that drug-induced alterations in the patch compartment of the dorsal caudate-putamen may preferentially contribute to some of the enduring changes in brain activity and behavior produced by repeated treatment with methamphetamine.

Ex vivo [11C]-(+)-PHNO binding is unchanged in animal models displaying increased high-affinity states of the D2 receptor in vitro

Dopamine (DA) D2 receptor supersensitivity has been linked to an increase in the density of the D2 high-affinity state as measured in vitro. The two- affinity-state model of the D2 receptor predicts that the ex vivo specific binding of [11C]-(+)-PHNO, an agonist radiotracer thought to bind selectively to the high-affinity state in vivo, should be increased in animal models that display in vitro increases in the proportion of receptors in the D2 high-affinity state. Here, we test this hypotheses by comparing the ex vivo SBR of [11C]-(+)-PHNO with that of the antagonist radiotracer [3H]-raclopride in three dopaminergically supersensitive rat models-AMPH-sensitized rats, rats withdrawn from chronic ethanol, and unilaterally 6-OHDA-lesioned rats-using ex vivo dual-radiotracer biodistribution studies. We find that in AMPH-sensitized rats and rats withdrawn from chronic ethanol treatment, models that exhibited approximately 4-fold increases in the D2 high-affinity state in vitro, the SBRs of [11C]-(+)-PHNO and [3H]-raclopride are unchanged relative to control rats. In unilaterally 6-OHDA-lesioned rats, we find that the increase in [11C]-(+)-PHNO SBR is no different than that observed for the antagonist radiotracer [3H]-raclopride (54% +/- 16% and 52% +/- 14%, respectively). In addition, the effect of acute AMPH pretreatment (4 mg/kg, i.v.) on the SBRs of [11C]-(+)-PHNO and [3H]-raclopride is equivalent in AMPH-sensitized (-38% +/- 12% and -36% +/- 8%, respectively) and in control rats (-40% +/- 11% and -38% +/- 7%). These data emphasize a significant discrepancy between in vitro and in vivo measures of D2 agonist binding, indicating that the two-affinity-state model of the D2 receptor may not apply veridically to living systems. The potential implications of this discrepancy are discussed.

Increased dopamine D2High receptors in amphetamine-sensitized rats, measured by the agonist [3H](+)PHNO

Repeated injections of amphetamine causes animals to become sensitized and supersensitive to DA. Previous work showed that the striata from such sensitized rats revealed a 3.5-fold increase in the density of D2(High) DA receptors, as measured by the guanine-nucleotide-sensitive component of [(3)H]raclopride binding. The present study was done to confirm these earlier findings by different methods and different ligands. The striata from amphetamine-sensitized rats showed an increase of 2.2-fold in the density of guanine-nucleotide-sensitive D2 receptors labeled by saturation experiments with [(3)H](+)PHNO. The proportion of D2(High) receptors was also found to increase 2.5-fold using the method of competition between DA and [(3)H]domperidone. The overall 2.2-3.5-fold increase of DA D2(High) receptors may explain why amphetamine-sensitized animals are much more sensitive to DA agonists, even though the total density of D2 receptors may apparently be unchanged or even decreased.

Amphetamine-sensitized animals show a marked increase in dopamine D2 high receptors occupied by endogenous dopamine, even in the absence of acute challenges

While a range of dopamine D(2)-related behaviors are exaggerated in amphetamine-sensitized animals, studies of the dopamine D(2) receptor have reported either no change or a decrease in dopamine D(2) receptor density--especially when measured using radioraclopride. We hypothesized that a decrease in D(2) receptors may actually be "apparent" and that these receptors may still be present, but are noncompetitively "occupied" by endogenous dopamine. Animals sensitized to amphetamine (and their saline controls) were examined 4 weeks after their last injection. We first measured the [(3)H]raclopride binding in vivo, and observed that sensitized animals showed a 29% lower level of raclopride binding in vivo, suggesting an apparently lower level of dopamine D(2) receptors. To assess the reason for this we examined the density of receptors (using Scatchard analysis in vitro) measured by [(3)H]raclopride in the presence and absence of guanilylimidodiphosphate. This guanine nucleotide converts the dopamine-bound high-affinity state of D(2) receptors into low-affinity states, thereby making measurable the absolute density of the sites. As previously reported, the amphetamine-sensitized animals showed a 31% lower number of D(2) receptors in conventional binding (B(max) 15.6 vs. 22.7 pmol/g). However, with the addition of guanilylimidodiphosphate there was an equalization of both groups (B(max) 25.9 vs. 25.6 pmol/g), revealing an additional 10.3 pmol/g in the sensitized animals, but only 2.9 pmol/g in saline controls. There were no changes in the dissociation constant of [(3)H]raclopride for the receptors. The nearly four-fold increase of dopamine D(2) receptors in the high-affinity state occupied by dopamine may explain why amphetamine-sensitized animals show almost an order of magnitude greater response to dopamine-releasing challenges or dopamine agonists, even though the absolute receptor number is unchanged and the "apparent" receptor number is decreased.

The behavioral and neurochemical effects of phencyclidine in humans and animals: some implications for modeling psychosis

Phencyclidine (PCP) produces a psychotic reaction in humans which closely resembles an acute episode of schizophrenia and has therefore been given an increasing amount of attention as a model for schizophrenia. The present article reviews the behavioral and neurochemical effects of PCP in both humans and animals. Where possible, comparisons are made between the effects of PCP and amphetamine. The merits of the dopamine versus NMDA/PCP receptor mediated expression of PCP-induced psychosis are discussed, as well as the importance of selecting behavioral models which are best suited to model the expression of psychosis, rather than the motor effects of psychotomimetics.

Effect of repeated methamphetamine administrations on dopamine and glutamate efflux in rat prefrontal cortex

Pretreatment with psychostimulants such as methamphetamine (METH) results in augmented mesostriatal dopamine transmission upon a challenge administration of the drug. This effect can be blocked by dopamine antagonists and excitatory amino acid antagonists. However, no direct comparisons have been made with respect to the effects of a low-dose pretreatment regimen of METH on impulse and transporter-mediated dopamine release or to what extent glutamate release is altered by a pretreatment regimen with METH. The purpose of this study was to examine dopamine and glutamate efflux in the prefrontal cortex and striatum in rats pretreated with METH following either high potassium (80 microM) infusion or after a systemic injection of a low dose of METH. Extracellular dopamine and glutamate concentrations in the prefrontal cortex and striatum were measured in vivo by microdialysis. Potassium infusion increased extracellular dopamine and glutamate concentrations to a greater extent in the prefrontal cortex than in the striatum of METH-pretreated rats compared to saline-pretreated controls. A low dose METH challenge significantly increased extracellular dopamine but not glutamate concentrations in both prefrontal cortex and striatum of all animals. Moreover, the acute METH-induced increased in cortical dopamine efflux was significantly greater in rats pretreated with METH. Overall, these data are the first evidence that repeated METH administrations can enhance cortical glutamate efflux and indicate that a low dose pretreatment regimen of METH enhances dopamine transmission in the prefrontal cortex through both transporter and depolarization-induced mechanisms.

Behavioral and neuroimaging studies on schizophrenia in Japan

Chronic methamphetamine psychotics became a major problem in Japan after World War II. On one hand, the clinical symptoms of chronic methamphetamine psychotics were noted to resemble those of schizophrenics quite closely but, on the other hand, their respective personality profiles appeared to differ. Recent research on exploratory eye movements has also shown such similarities and differences. In long-term remitted chronic methamphetamine psychotics, extremely small doses of drugs, or stress, can be enough to cause a recurrence of symptoms. This phenomenon resembles the recurrence of schizophrenia, and animal experiments and basic research are being actively conducted in order to elucidate the mechanism involved. Some 20 years ago, unique studies were conducted that objectively looked at the characteristic behavior of severe chronic schizophrenics in a playground situation. Exploratory eye movements can also be regarded as a kind of behavior and, in this paper, eye movement patterns made in response to a maze test are discussed. In addition, much research on brain imaging in schizophrenia is being conducted in Japan and several interesting studies have also been chosen by the authors for presentation in this paper.

The effect of repeated administration of methamphetamine on dopamine uptake sites in rat striatum

In this experiment, rats were injected intraperitoneally with 6 mg/kg methamphetamine (MAP) hydrochloride or the same volume of saline once daily for 14 days. Rats were decapitated after a 1-day, 4-day or 10-day withdrawal period. The number and affinity of [3H]mazindol binding sites in the striatum were measured. Their number in the MAP group decreased as compared with controls after a 1-day withdrawal period, but returned to the control level after a 4-day and 10-day withdrawal period. These results are believed to show that the decrease in dopamine uptake sites after repeated administration of MAP is transient and does not contribute to MAP-induced behavioral sensitization.

Changes in sensitivity of dopamine autoreceptors in rat striatum after subchronic treatment with methamphetamine

To determine the functional alterations of the release modulating striatal dopamine (DA) autoreceptors that might be associated with the behavioral sensitization to methamphetamine after pretreatment with methamphetamine, we investigated the effect of apomorphine and sulpiride on electrically evoked DA release from striatal slices of rats pretreated with methamphetamine. Apomorphine induced a dose-dependent inhibition of the DA release evoked from the striatal slices. Pretreatment with methamphetamine (6 mg/kg per day for 9 days) significantly reduced the inhibitory effect of apomorphine. On the other hand, sulpiride caused a dose-dependent increase in the DA release evoked from the striatal slices. The enhancement by sulpiride of the evoked DA release was diminished by pretreatment with methamphetamine. These results show that pretreatment with methamphetamine blunts the responsiveness of evoked DA release to the DA antagonists as well as to the DA agonists, indicating that DA autoreceptor subsensitivity is produced by pretreatment with methamphetamine.

Enhancement of dopamine release from striatal slices of rats that were subchronically treated with methamphetamine

The effect of dopamine (DA) uptake inhibitors (methamphetamine, nomifensine, and phenylethylamine) on the release of endogenous DA from striatal slices of rats pretreated with methamphetamine (6 mg/kg/day for 9 days) was investigated. The exposure of methamphetamine-pretreated rat striatal slices to a low concentration (10(-7) M, 5 X 10(-7) M) of methamphetamine caused a greater increase in DA efflux than that of saline-treated rat striatal slices. The drug-treated rats displayed an enhanced stereotyped behavioral response to a small dose of methamphetamine (1 mg/kg). Removal of Ca2+ from the superfusion medium did not affect the difference in the rates of methamphetamine (10(-7) M) induced DA release between methamphetamine-treated and saline-treated rat striatal slices. Nomifensine- and phenylethylamine-induced DA release from striatal slices was also enhanced by repeated administration of methamphetamine. On the other hand, there was no difference in K+-induced DA release between the two groups. Moreover, repeated administration of methamphetamine caused a significant increase in 3H-dopamine uptake in rat striatal synaptosomes. These results suggest that the behavioral sensitization produced by the repeated administration of methamphetamine is accompanied by an enhancement in the release of DA induced by methamphetamine, nomifensine, and phenylethylamine in vitro and is also accompanied by increased DA uptake into striatal synaptosomes.

Alteration of striatal dopaminergic functions implicated in methamphetamine-induced reverse tolerance in rats

Rats were injected repeatedly with methamphetamine (6 mg/kg per day, for 14 days). The effects of the repeated treatment on the change of behaviors and striatal DA metabolism induced by challenge with DA agonists were studied 10 days after the last injection. Repeated methamphetamine administration decreased the sedative effect of low dose apomorphine, and enhanced apomorphine-induced stereotyped behavior but reduced in after pretreatment with tetrabenazine. Striatal DA, HVA and DOPAC contents, and striatal gamma-butyrolactone-induced DA or DOPA accumulation were not altered by repeated methamphetamine treatment. The effect of apomorphine to decrease DA metabolite content and DA or DOPA accumulation was not changed either. As evidenced by experiments using alpha-MT, repeated methamphetamine administration increased DA utilization after methamphetamine or low dose apomorphine challenge, although it did not change DA utilization at the steady state (without drug challenge). These findings suggest that methamphetamine-induced reverse tolerance is accompanied by an increase of DA utilization resulting from the subsensitivity of DA utilization-modulating autoreceptors.

Evidence that alpha-methyl-p-tyramine is implicated in behavioural augmentation to amphetamine

Behavioural studies showed that administration of alpha-methyl-p-tyramine (AMT; 10 mg/kg i.p.) to rats 24 hr before treatment with d-amphetamine (AMPHET; 4 mg/kg i.p.) resulted in augmentation of AMPHET-induced stereotype activity. Parallel experiments involving electro-chemical estimation of dopamine metabolites in the striatum showed that the decrease in the concentration of homovanillic acid (HVA) produced by AMPHET (4 mg/kg) was enhanced in AMT (10 mg/kg) pretreated animals. These findings suggest that AMT derived from previous doses of AMPHET may play a role in the phenomena of behavioural augmentation observed after chronic administration of AMPHET.

Subchronic methamphetamine treatment selectively attenuates apomorphine-induced decrease in 3,4-dihydroxyphenylacetic acid level in mesolimbic dopaminergic regions

To investigate mechanisms of behavioral enhancement produced by repeated doses of amphetamines, the effects of apomorphine on 3,4-dihydroxyphenylacetic acid (DOPAC) and dopamine (DA) levels were examined in various brain regions of the rat on the 4th day of withdrawal after repeated administration of saline or methamphetamine (3 mg/kg, s.c.) twice daily for 14 days. Apomorphine (0.1 and 1.0 mg/kg, i.p.) produced a dose-dependent decrease in DOPAC levels and no effect on DA levels in the olfactory tubercle, nucleus accumbens, striatum, frontal and cingulate cortices of saline-treated animals. A decrease in DOPAC levels produced by a low dose of apomorphine was attenuated selectively in the olfactory tubercle and nucleus accumbens of methamphetamine-treated animals. A high dose of apomorphine produced a significant decrease in DOPAC levels in both regions. No such attenuation was obtained in the striatum and the frontal and cingulate cortices. These results suggest that subchronic methamphetamine may induce development of hyposensitivity of presynaptic DA receptors in the mesolimbic regions, which contribute to the behavioral enhancement produced by the drug.

In vivo stereospecific [3H]spiperone binding in rat brain: characteristics, regional distribution, kinetics and pharmacological properties

The time course of [3H]spiperone distribution in the three major pools (specifically and non-specifically membrane-bound and soluble) of different brain areas, was studied in rats given a tracer amount of the drug. In addition, the stereospecificity, dissociation kinetics and pharmacological nature of the in vivo bound [3H]spiperone were investigated. The data show that [3H]spiperone binding sites in the striatum, olfactory tubercles and hypophysis differ clearly from those of the cortical regions. In the prevalently dopaminergic areas the amount of ligand bound to membranes is, up to 24 h post-treatment, proportional to the total 3H present. However a more correct analysis of the data was obtained in all the experiments when membrane-bound was measured instead of total radioactivity. Thus assay of the in vivo specifically bound [3H]spiperone appears essential for a correct evaluation of the density, affinity, regional distribution, pharmacological nature and kinetics of the drug-receptor interaction.

Differential effects of chronic clorgyline and amfonelic acid on desensitization of striatal dopamine receptors

Chronic treatment of rats with the MAOI clorgyline significantly reduced the density (Bmax) of cortical beta-adrenergic receptors but did not alter either the Bmax or dissociation constant (Kd) of 3H-spiperone binding to striatal DA receptors. Clorgyline co-treatment also did not significantly affect either behavioral supersensitivity to apomorphine or the increase in 3H-spiperone binding induced by chronic haloperidol. In contrast, repeated treatment with the DA uptake inhibitor amfonelic acid elicited behavioral subsensitivity and reduced striatal 3H-spiperone binding. Furthermore, amfonelic acid co-treatment prevented haloperidol-induced behavioral and receptor binding changes. The possible relevance of these findings in relation to drug choice in clinical trials of receptor sensitivity modification are discussed.

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.

Quantification of in vivo spiperone binding in the rat striatum after lesions produced by kainate or decortication

The potential of in vivo spiperone binding as a tool for the detection and quantitative analysis of striatal dopamine (DA) receptor alterations was studied in rat brain lesioned in several ways. Two weeks after kainate (KA) injection a significantly higher radioactivity accumulation was observed in the lesioned striatum than in the contralateral structure after a tracer dose of [3H]spiperone. The difference was maximal 2 days after surgery and it was present for at least 4 weeks while it was reversed 11 weeks after KA injection. The radioactivity uptake (tracer dose of [3H]spiperone) measured 2 weeks after surgery could be specifically prevented in both KA-lesioned and contralateral striatum by haloperidol and N-n-propylnorapomorphine while non-dopaminergic drugs were almost without effect. More than 80% of the radioactivity accumulation was saturable in both contralateral (unlesioned) and KA-lesioned striatum, leaving a slightly higher non-saturable radioactivity level in the latter. One week after unilateral ablation of the cerebral cortex overlying the striatum only minor bilateral differences in striatal radioactivity content were found after a tracer dose of [3H]spiperone. No differences were present after 6-OHDA lesion of the nigrostriatal pathway. Striatal DA receptor densities (Bmax) were determined from the dose-dependency of total striatal spiperone accumulation. This relationship was assessed using cerebellar spiperone accumulation instead of dose. Thus a Bmax of about 75 fmol X mg-1 tissue was found in the striatum of control (unoperated) rats and contralateral to the striatal KA lesion while 2 weeks after surgery it was approximately 33 fmol X mg-1 in the KA-lesioned striatum. One week after unilateral decortication Bmax values of about 50 and 65 fmol X mg-1 were found ipsi- and contralaterally to the lesion respectively.

Behavioral sensitization and relative hyperresponsiveness of striatal and limbic dopaminergic neurons after repeated methamphetamine treatment

Rats were used in a study of the effects of repeated methamphetamine treatment on stereotyped behavior and striatal and limbic dopamine metabolism in response to challenge with the drug or other dopamine agonists. Repeated administration of d-methamphetamine (6 mg/kg per day for 3-14 days) produced long-term behavioral sensitization (augmented response to a challenge injection) not only to the compound (at 44-89 days after drug withdrawal) but also to apomorphine and nomifensine. Even a single injection of d-methamphetamine (6 mg/kg) enhanced the behavioral response to the drug. A challenge dose of d-methamphetamine (2 mg/kg) markedly increased dopamine turnover (lower dopamine and higher 3,4-dihydroxyphenylacetic acid levels, higher ratios of 3,4-dihydroxyphenylacetic acid over dopamine) in the striatum and mesolimbic area of the sensitized animals on day 15 of withdrawal from treatment repeated for 14 days with the drug (6 mg/kg per day). These findings demonstrate that behavioral sensitization induced by methamphetamine is accompanied by increased central dopaminergic transmission.

Reduction of 3H-spiroperidol binding in rat striatum and frontal cortex by chronic amphetamine: dose response, time course and role of sustained dopamine release

After 5 days of continuous treatment with d-amphetamine base in doses greater than 0.5 mg/kg/h maintained by subcutaneously implanted osmotic minipumps, specific binding of 3H-spiroperidol was reduced in rat striatum and frontal cortex as previously reported. These effects were dose-dependent at lower doses of amphetamine, whereas with higher doses an apparent ceiling for the reduction in binding was reached at approximately 70% of control values. Similarly, increasing the exposure time to amphetamine for up to 14 days only slightly augmented the reduction in 3H-spiroperidol binding already present after 5 days of treatment. In rats treated for 5 days with amphetamine, concomitant treatment with the dopamine (DA) synthesis inhibitor alpha-methyl-p-tyrosine prevented the decrease in 3H-binding in corpus striatum, and attenuated the decrease in frontal cortex. Furthermore, in rats with unilateral 6-hydroxydopamine lesions of the nigro-striatal DA tract, 5 days of chronic amphetamine had no significant effect on 3H-spiroperidol binding in the denervated striatal tissue. Since a major effect of amphetamine is to release DA from nerve terminals, these results indicate that the reduction of DA receptors by chronic amphetamine in the striatum is mediated by sustained release of DA.

Long-term facilitation of amphetamine-induced rotational behavior and striatal dopamine release produced by a single exposure to amphetamine: sex differences

Amphetamine (AMPH)-induced rotational behavior in non-lesioned rats and AMPH-stimulated dopamine (DA) release from striatal tissue fragments in vitro were used to study the long-term effects of a single injection of AMPH on activity in the mesostriatal DA system. A single injection of a low dose of AMPH (1.25 mg/kg) greatly enhanced the rotational behavior produced by a second injection of AMPH given 3-4 weeks later in intact female, ovariectomized female and castrated male rats. The effect of AMPH pretreatment in intact males differed from that in the other groups. When only 7-8 days separated the two test sessions both intact male and female rats showed sensitization of rotational behavior, but the magnitude of the change was greater in females. In addition, a single injection of 1.25 mg/kg of AMPH in vivo produced a long-lasting (3-5 weeks) enhancement of AMPH-stimulated DA release from striatal tissue in vitro. It is suggested that: (1) repeated injections of AMPH are not necessary to produce a long-lasting facilitation of behaviors mediated by the mesostriatal DA system; (2) gender and/or hormonal state influences the development of long-term changes in the mesostriatal DA system; and (3) changes in DA release from presynaptic terminals may contribute to the behavioral sensitization produced by stimulant drugs. The phenomena reported here may provide complementary in vitro and in vivo models for studying neuroplasticity in brain DA systems.

Effect of chronic phencyclidine or d-amphetamine treatment on [3H]spiroperidol binding in rat caudate-putamen and nucleus accumbens

Chronic treatment (20 days) of rats with d-amphetamine decreased the density, but had no effect on the apparent affinity of [3H]spiroperidol binding sites in caudate-putamen and nucleus accumbens. In contrast, chronic phencyclidine treatment had no effect either on the number or on the affinity of [3H]spiroperidol binding sites in these two brain regions.

Behavioural and biochemical changes following chronic administration of L-dopa to rats

Rats were given powdered diet containing L-DOPA (together with the peripheral decarboxylase inhibitor carbidopa) for a period of 6 months. The estimated daily intake was in the range 20-30 mg/kg. Initially, at 1 week and 1 month, L-DOPA-fed rats exhibited enhanced spontaneous locomotor activity, but this fell to within the control range by 3 and 6 months, although (+)-amphetamine-induced hyperactivity was greater at 6 months in L-DOPA-treated animals than in control rats. Six months after receiving L-DOPA in their diet rats showed enhanced stereotypy scores to a series of dopamine agonists administered acutely including (+)-amphetamine, nomifensine, L-DOPA, apomorphine and piribedil compared with the control animals. In another behaviour test L-DOPA administration reduced the cataleptic potency of both fluphenazine and haloperidol was increased. Biochemically 6 months treatment of rats with L-DOPA was associated with significantly increased plasma concentrations of L-DOPA, enhanced striatal levels of L-DOPA, dopamine and dopamine metabolites, enhanced specific binding (as indicated by increased Bmax values) of [3H] spiroperidol, [3H] ADTN and [3H] 5-HT to striatal membranes, and increased basal and dopamine-stimulated striatal adenylate cyclase activity. The results are discussed in the light of changes of sensitivity of cerebral dopamine receptors, an increase in receptor numbers, and the tolerance to L-DOPA which often develop in the treatment of Parkinson's disease.