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

Lack of behavioral tolerance by repeated treatment with taltirelin hydrate, a thyrotropin-releasing hormone analog, in rats

In order to determine whether acute tolerance develops by taltirelin hydrate ((-)-N-[(S)-hexahydro-1-methyl-2,6-dioxo-4-pyrimidinylcarbonyl]-l-histidyl-l-prolinamide tetrahydrate; taltirelin), a thyrotropin-releasing hormone (TRH) analog, we examined the motor behavior, TRH receptors and dopamine D(2) receptors following 2 weeks treatment in rats. Taltirelin selectively bound to TRH receptors and increased the spontaneous motor activity by a single administration, suggesting that the motor effect of taltirelin is mediated by TRH receptors. Following repeated treatment with TRH, there was a significant reduction in the increment of spontaneous motor activity. In contrast, after repeated treatment with taltirelin at a dose that increased the motor activity to a similar extent to TRH by a single administration, there was no apparent change in its motor effect. In accord with the motor activity, we found a significant reduction in the [(3)H]methyl-TRH binding to TRH receptors in the brain following repeated treatment with TRH but not taltirelin. However, the [(3)H]spiperone binding to dopamine D(2) receptors in the corpus striatum did not change by repeated taltirelin and TRH treatments. Thus, the down-regulation of TRH receptors would be a main cause of the behavioral tolerance. These results suggest that taltirelin hardly develops the behavioral tolerance due to the lack of down-regulation of TRH receptors.

The tonic/phasic model of dopamine system regulation: its relevance for understanding how stimulant abuse can alter basal ganglia function

The changes in dopamine system regulation occurring during stimulant administration are examined in relation to a new model of dopamine system function. This model is based on the presence of a tonic low level of extracellular dopamine that is released by the presynaptic action of corticostriatal afferents. In contrast, spike-dependent dopamine release results in a phasic, high concentration of dopamine in the synaptic cleft that is rapidly inactivated by reuptake. Tonic dopamine has the ability to down-modulate spike-dependent phasic dopamine release via stimulation of the very sensitive dopamine autoreceptors present on dopamine terminals. Stimulants are known to elicit locomotion and stimulate reward sites by releasing dopamine from terminals in the nucleus accumbens, which is followed by a rebound depression. It is proposed that the initial activating action of stimulants is caused by increasing the release of dopamine into the synaptic cleft to activate the phasic dopamine response. However, by interfering with dopamine uptake, stimulants also allow dopamine to escape the synaptic cleft, thereby depressing subsequent spike-dependent phasic dopamine release by increasing the tonic stimulation of the autoreceptor. In contrast, repeated stimulant administration is proposed to cause long-term sensitization by pharmacological disruption of a cascade of homeostatic compensatory processes. Upon drug withdrawal, the fast compensatory systems that were blocked by stimulants rapidly restore homeostasis to the system at a new steady-state level of interaction. As a consequence, the slowly changing but potentially more destabilizing compensatory responses are prevented from returning to their baseline conditions. This results in a permanent change in the responsivity of the system. Homeostatic systems are geared to compensate for unidimensional alterations in a system, and are capable of restoring function even after massive brain lesions or the continuous presence of stimulant drugs. However, the system did not evolve to deal effectively with repetitive introduction and withdrawal of drugs that disrupt dopamine system regulation. As a consequence, repeated insults to a biological system by application and withdrawal of drugs that interfere with its homeostatic regulation may be capable of inducing non-reversible changes in its response to exogenous and endogenous stimuli.

Individual differences in behavior following amphetamine, GBR-12909, or apomorphine but not SKF-38393 or quinpirole

Subjects that respond more to a novel environment show a greater locomotor response to drugs of abuse such as cocaine and amphetamine. The current study was performed to examine differences between high (HR) and low (LR) responding rats to a novel environment following administration of amphetamine, a selective dopamine uptake blocker (GBR-12909), a nonselective dopamine agonist (apomorphine), and selective dopamine D1 and D2/D3 agonists. A behavioral checklist and a rating scale were used to determine the behavioral arousal caused by administration of amphetamine (0, 0.5, 2.0, and 8.0 mg/kg), GBR-12909 (0, 1.25, 5.0, and 20.0 mg/kg), apomorphine (0, 0.1, 0.3, and 1 mg/kg), SKF 39393 (0, 2.5, 10, and 40 mg/kg), or quinpirole (0, 0.05, 0.5, and 5.0 mg/kg). The five drugs produced behavioral activation profiles distinct from each other. Following amphetamine administration, both HR and LR subjects showed dose dependent increases in behavioral arousal. The behaviors primarily affected were sniffing, locomotor activity, rearing, and oral activity. HR rats showed a greater overall behavioral response to amphetamine administration compared with LR rats and there were differences in specific behaviors between the two groups. Following GBR-12909 administration, all subjects showed dose dependent increases in sniffing, locomotor activity, and rearing. Differences between HR and LR were observed in sniffing, locomotor activity, and rearing behaviors. HR and LR both showed dose dependent increases in behavior following apomorphine administration. HR showed greater behavioral activation after apomorphine than LR.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo microdialysis reveals a diminished amphetamine-induced DA response corresponding to behavioral sensitization produced by repeated amphetamine pretreatment

In vivo microdialysis procedures were used to assess the effects of repeated amphetamine administration on behavior and regional brain DA dynamics in freely moving rats. Pretreatment with amphetamine (2.5 or 3.0 mg/kg) for 4-6 days did not alter baseline DA or its metabolites in caudate or accumbens 48 h or 6 days after the last injection. However, whereas this dosage regimen revealed a profound behavioral sensitization in response to challenge with amphetamine (2.5 mg/kg), including a more rapid onset and intensification of stereotypy, the DA response was significantly diminished in both brain regions. In addition, the ratio of caudate to accumbens DA, either before or after amphetamine challenge, was not altered by the pretreatment regimen. These results are consistent with our previous suggestion that there is a dissociation between the DA and behavioral responses to amphetamine, and therefore that other neurotransmitter systems and/or mechanisms significantly contribute to the amphetamine response profile. Furthermore, DA effects may represent only one, albeit critical, aspect in a time-dependent sequence of changes underlying stimulant sensitization.

Time-dependent changes in the sensitivity of dopamine neurons to low doses of apomorphine following amphetamine infusion

Changes in dopamine autoregulatory mechanisms following a chronic infusion of amphetamine were studied. Rats were infused with D-amphetamine (4.8 mg/day) for 7 days by osmotic minipumps, and were studied at various times after withdrawal. In contrast to no changes in the spontaneous firing rate of single dopamine cells, the potency of apomorphine in the substantia nigra was markedly decreased soon after withdrawal, followed by an increase 7 days later. The ventral tegmental area showed no changes in either spontaneous firing rate or sensitivity to apomorphine following a 7-day withdrawal. Moderate decreases in striatal and tubercle dopamine concentrations were not accompanied by any significant decrease in basal dopamine synthesis. Under intact impulse-flow, the sensitivity of terminal dopamine synthesis to low doses of apomorphine was decreased immediately following withdrawal; by Day 7 supersensitivity was observed. Direct assessment of terminal autoreceptors following a 7-day withdrawal revealed normo- and supersensitivity of these receptors in the striatum and olfactory tubercle, respectively. Possible mediating mechanisms as well as implications of these findings for the development of characteristic behavioral syndromes during and after amphetamine infusion are discussed.

Behavior, striatal and nucleus accumbens field potential patterns and dopamine levels in rats given amphetamine continuously

Behavioral, electrophysiological (field potential recordings, analyzed by computer) and biochemical techniques were employed before, during and after 7 days of continuous administration of amphetamine to rats. Significant changes were observed using all three protocols. Behavioral alterations were greatest on the second day of treatment and progressively normalized during the remainder of treatment. Electrophysiological changes in the striatum were significant only on the second day of treatment, while electrophysiological alterations in the nucleus accumbens were significant on all treatment and recording days, and the magnitude of the changes paralleled the pattern of overt behavioral changes. Levels of DA in the striatum progressively decreased from normal throughout the treatment, declining to less than half of the control level by the sixth day of treatment. In contrast, DA levels in the nucleus accumbens were augmented on the second day of treatment and progressively approached the control level as treatment continued, again paralleling behavior and changes in the electrophysiology of the nucleus accumbens. These results indicate that continuous administration of amphetamine in rats differentially affects electrical activity in the striatum and nucleus accumbens as well as concentrations of DA. In addition, these results have implications for the study of paranoid schizophrenia.

Opioid influence on some aspects of stereotyped behavior induced by repeated amphetamine treatment

Rats were administered repeated IP injections of dl-amphetamine (AMPH) according to a chronic escalating dose schedule (three doses per 24 hr, for four days, two days or one day). Animals treated for four days exhibited a diminished oral stereotypy in response to a challenge of 12 mg/kg AMPH or 2 mg/kg SC apomorphine (APO), 72 hr after withdrawal. Pretreatment with 2 mg/kg IP naloxone (NAL) during the period of chronic AMPH administration prevented the reduction in oral stereotypy induced by AMPH or APO. No differences were detected among the mean of stereotypy scores from the different treatments in response to a challenge dose of 6 mg/kg AMPH. Neurochemical data showed that NAL pretreatment reversed the depletion of striatal dopamine content induced by chronic AMPH. When repeated injections of AMPH were given only one day, the diminished stereotypy response to AMPH or APO was not observed. Animals treated simultaneously with 1 mg/kg IP morphine or 5 micrograms/kg IP beta-endorphin and repeated AMPH injections for one day, showed a reduced stereotyped response to AMPH or APO. These results suggest that opioid peptides are involved in the mechanisms underlying the decrease in oral behaviors following AMPH treatment.

Autoradiographic analysis of muscarinic cholinergic and serotonergic receptor alterations following methamphetamine treatment

Autoradiographic examination of the response of muscarinic cholinergic (M1 and M2) receptors to multiple doses of methamphetamine has been performed in several regions of the rat brain. Both muscarinic receptor subtypes were identified with [3H]-N-methylscopolamine, while M1 receptors were specifically labeled with [3H]-pirenzepine. No change in muscarinic receptors labeled with [3H]-pirenzepine was found in any of the brain regions examined following methamphetamine treatment; however, [3H]-N-methylscopolamine binding was significantly reduced (24-40%). These results indicate that M1 receptors remained unchanged after the drug treatment, while M2 receptors were reduced in many areas of the rat central nervous system following multiple high doses of methamphetamine. Five doses of methamphetamine (6-hour interval between doses) were required to elicit the receptor changes in all brain regions analyzed. Within 7 days after drug treatment, the receptor number returned to control values in the affected brain areas. Additionally, the response of serotonin (5-HT1 and 5-HT2) receptors to methamphetamine was examined and found to be reduced in a few brain areas analyzed. The receptor changes were accompanied by METH-induced decreases in tyrosine hydroxylase and tryptophan hydroxylase activities.

Methamphetamine-induced reduction in D1 and D2 dopamine receptors as evidenced by autoradiography: comparison with tyrosine hydroxylase activity

As determined by autoradiographic techniques, multiple high doses of methamphetamine elicited a reduction in dopamine receptor population (both D1 and D2) in several areas of the rat central nervous system. D1 receptors were labeled with the D1-selective antagonist, [3H]SCH 23390, and D2 receptors were labeled with the D2-selective neuroleptic, [3H]sulpiride. Scatchard analysis, obtained from saturation data in caudate-putamen, indicated that the receptor alterations were due to a decrease in the number of receptors (Bmax) without an apparent change in affinity (Kd). A time course demonstrated that five doses of methamphetamine were required to elicit significant changes in receptors in most brain areas examined. The onset of the receptor alterations in various brain regions correlated with the development of methamphetamine-induced depression of striatal tyrosine hydroxylase activity. In most brain areas, the dopamine receptors returned to normal within 7 days following methamphetamine.

Haloperidol prevents the methamphetamine-induced apomorphine subsensitivity of dopamine metabolism in rat striatum

Our previous experiment showed that the sensitivity of striatal dopaminergic metabolism to apomorphine was lowered after pretreating rats with methamphetamine. This study further demonstrated that haloperidol administered with methamphetamine prevented the effect of the methamphetamine pretreatment. Since gamma-butyrolactone was administered in combination with apomorphine so as to remove complications which might arise from the modification of impulse flow through the striatonigral feedback loop, it is possible that the observed effect of apomorphine upon striatal dopaminergic metabolism and its antagonism with haloperidol may be mediated by autoreceptors of dopaminergic axon terminals in rat striatum.

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.

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.

Studies on the mechanism of tolerance to methamphetamine

We have reported that the ability of high doses of methamphetamine to impair dopamine and serotonin synthesis in the rat brain is attenuated when animals are pretreated with gradually increasing doses of methamphetamine. To examine the mechanism of this tolerance phenomenon, the effect of methamphetamine on several neurochemical parameters was determined in naive and methamphetamine-pretreated rats. The elevation of nigral substance P concentrations by methamphetamine was attenuated in pretreated compared to naive rats. The methamphetamine-induced reduction in [3H]sulpiride binding in the rat neostriatum and nucleus accumbens was similarly attenuated in animals pretreated with methamphetamine. Determination of brain concentrations of methamphetamine and amphetamine revealed significantly lower concentrations of both compounds in the brains of pretreated compared to naive animals. The results indicate a reduction in the ability of methamphetamine to increase dopamine transmission in the brains of methamphetamine-pretreated rats. Furthermore, this effect appears to be due, at least in part, to a change in the disposition of methamphetamine in pretreated animals.

Down-regulation of dopamine D-2, 5-HT2 receptors and beta-adrenoceptors in rat brain after prolonged treatment with a new potential antidepressant, Lu 19-005

Lu 19-005 is a new phenylindan derivative with strong and equipotent inhibitory effect on dopamine (DA), noradrenaline (NA) and serotonin (5-HT) uptake. The adaptive effects of 2 weeks treatment with Lu 19-005, on receptor binding in vitro and on d-amphetamine responsiveness in vivo have been investigated in rats. One or 3 days after the final dose the number of beta-adrenoceptors and of 5-HT2 and DA D-2 receptors was decreased by 20-30%, whereas alpha 1-adrenoceptor number was slightly decreased only 1 day after withdrawal. The DA D-2 receptor number remained decreased at 7 days withdrawal, but returned to normal after another 3 days. The brain levels of DA, NA and 5-HT were not changed by 2 weeks' Lu 19-005 treatment. The down-regulation of DA D-2 receptors was accompanied by tolerance to d-amphetamine-induced hypermotility (after low doses) and stereotyped licking or biting (after a high dose). The tolerance to d-amphetamine-induced hypermotility was maximal at 3-5 days withdrawal time, and remained significant also 15 days after the last dose. An acute dose of Lu 19-005 did not modify the effects of d-amphetamine. The results are discussed in relation to the effect of prolonged treatment with other antidepressant drugs.

Effect of neurotensin, substance P and TRH on the regulation of dopamine receptors in rat brain

Rats were exposed for one week to either neurotensin (4 micrograms/h), substance P (3.3 micrograms/h), thyrotropin-releasing hormone (5 micrograms/h), or saline administered intracerebroventricularly via mini osmotic-pumps and either haloperidol (2.5 mg/kg i.p., 2 X daily) or vehicle control. The peptide treatments by themselves did not alter [3H]spiroperidol binding in either the nucleus accumbens or the striatum. Neurotensin, however, augmented the increase in [3H]spiroperidol binding caused by the haloperidol treatment in both the nucleus accumbens and striatum.

Iontophoretic evidence for subsensitivity of postsynaptic dopamine receptors following long-term amphetamine administration

Neuronal activity was recorded from the anteromedial neostriatum of rats pretreated twice daily for 6 consecutive days with saline or 5.0 mg/kg d-amphetamine. Glutamate was applied in both groups of animals to increase spontaneous firing rates. Iontophoretic application of increasing currents of DA (20-120 nA) produced a progressive inhibition of unit activity in control animals that was significantly reduced in rats pretreated with amphetamine. These results support the view that long-term amphetamine treatment reduces the sensitivity of postsynaptic DA receptors in the neostriatum.