Biphasic effects of amphetamine on striatal dopamine dynamics

S(+)-Amphetamine (AMPH) was administered to rats over the dose range 0.15-10.0 mg/kg (1.74(3)mumoles/kg) at various times before sacrifice. 15 min before sacrifice rats received an intraventricular injection of 3H-tyrosine, and the accumulation of striatal 3H-dopamine (DA) was assessed as an index of striatal DA formation and utilization. 3H-DA formation exhibited a biphasic dose response to AMPH. At 16 and 31 min after AMPH administration, 3H-DA accumulation increased linearly in response to AMPH to a maximal rate of 200% of control values at 1.0 mg/kg, then declined to less than 50% of control values at AMPH doses of 5 and 10 mg/kg. As a function of time, low doses of AMPH (to 2.0 mg/kg)) only increased 3H-DA accumulation, whereas high doses of AMPH (greater than 2.5 mg/kg) only decreased 3H-DA accumulation. All doses of AMPH increased endogenous levels of striatal DA. The data are discussed in terms of compensatory adaptive mechanisms of the nigrostriatal dopaminergic pathway.     

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

Summary 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 -methyl-m-tyrosine (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 shortterm depletion of striatal DA produced by MMT and the long-term depletion of striatal DA produced by amphetamine in iprindole-treated rats by an effect on the nerve impulsemediated release of vesicular transmitter is discussed.     

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.     

Attenuation of intravenous amphetamine reinforcement by central dopamine blockade in rats

Norepinephrine (NE) and dopamine (DA) receptor blockade differentially affected amphetamine self-administration. DA blockade (pimozide, 0.0625 to 0.5 mg/kg, or (+)-butaclamol, 0.0125 to 0.1 mg/kg) caused periods of increased rate of responding for amphetamine which were followed, in the case of higher doses, by response cessation. The response cessation produced by 0.5 mg/kg pimozide was not reversed by non-contingent amphetamine injections until well after the peak effect of the pimozide was over. When access to amphetamine injections was delayed until 4 h after animals received 0.5 mg/kg pimozide, rate of responding was elevated. Thus DA seems to be critically involved in mediation of the reinforcing effects of amphetamine. Alpha-NE blockade with phentolamine (2.5–10 mg/kg) produced dose-related decreases in responding; blockade with phenoxybenzamine (1.25–10 mg/kg) had no effect. Beta-NE blockade with l-propranolol (2.5–10 mg/kg) decreased responding, although probably not through a beta-blocking action. The effects of phentolamine and propranolol do not appear to result from attenuation of the reinforcing effects of amphetamine.     

Changes in striatal dopamine metabolism during precipitated morphine withdrawal

Precipitation of withdrawal in morphine tolerant/dependent rats by either naloxone or the partial agonist ZK 48491 caused a significant increase in the contration of striatal DA, which persisted for at least 1 h. During the same time the probenecid-induced accumulation of HVA and DOPAC was reduced in the striatum in relation to probenecid-treated tolerant/dependent controls. 20 min after precipitation of withdrawal by naloxone, the striatal concentration of 3-methoxytyramine was decreased by about 40%, while the activity of the DA metabolizing enzymes, MAO and COMT, remained unchanged. Naloxone-precipitated withdrawal was, further, found to delay the depletion of striatal DA caused by inhibition of synthesis 90 min after alpha-methyl-p-tyrosine treatment. All these results provide evidence for a decreased release of DA from the striatum during precipitated morphine withdrawal.     

Effects of amphetamine and amfonelic acid on the disposition of striatal newly synthesized dopamine

A comparison of the in vivo biochemical actions of the psychotomimetic central stimulants, d-amphetamine (d-AMPH) and amfonelic acid (AFA), on the metabolism of rat striatal newly synthesized [3H]dopamine (DA) was made by pulse labeling with [3H]tyrosine. No evidence for the formation of the alcoholic DA metabolites [3H]3-methoxy-4-hydroxyphenylethanol (MOPET) or [3H]3,4-dihydroxyphenylethanol (DOPET) was found in control or drug-treated animals. Both [3H]3,4-dihydroxyphenylacetic acid (DOPAC) and [3H]homovanillic acid (HVA) concentrations were increased by AFA in the presence of haloperidol, while [3H]DA content was decreased. In contrast, d-AMPH, in the presence of haloperidol, decreased [3H]DOPAC and increased [3H]DA, even in monoamine oxidase-blocked rats. Thus monoamine oxidase inhibition did not appear to be a major factor in the action of amphetamine to increase [3H]DA, but cannot be excluded as a contributing factor to the lowering of [3H]DOPAC. Similar actions of d-AMPH were seen on preformed DA. Amphetamine may release newly synthesized DA in such a way that some of the released DA enters the neuronal storage system.     

Differential effects of D- and L-amphetamine and methylphenidate on rat striatal dopamine biosynthesis

Methylphenidate and the optical isomers of amphetamine have differential effects, in vivo and in vitro on rat striatal synaptosomal dopamine (DA) biosynthesis. Whereas the systemic administration of D- or L-amphetamine produces a dose-dependent decrease in synaptosomal DA synthesis, with ED₅₀'s of 1.1 and 3.5 mg/kg, respectively, methylphenidate, at doses which produce comparable degrees of stereotyped behavior, has no effect on this measure of DA biosynthesis. In vitro, D- and L-amphetamine exhibit a three-fold difference in ED₅₀'s (1 × 10⁻⁶ M and 3 × 10⁻⁶ M, respectively) with respect to both activation of striatal synaptosomal DA synthesis, and to reversal of reserpine-induced inhibition of DA synthesis. In contrast, although in vitro methylphenidate can partially activate DA synthesis, it does not overcome the reserpine-induced inhibition. The data are discussed in terms of the possible differential mechanisms by which these two types of stimulants may exert their effects on stereotypy.     

Long-lasting dopamine receptor up-regulation in amphetamine-treated rats following amphetamine neurotoxicity

Amphetamine (A) (9.2 mg/kg, IP), in combination with iprindole (I) (10.0 mg/kg, IP), caused long-lasting dopamine (DA) depletions in striatum (−49%, 4 weeks) but not in nucleus accumbens following one A/I injection. Striatal DA had recovered by 4 months. DA receptors (DAr) were up-regulated: 1) behavioral responses to a DA receptor agonist (apomorphine) were significantly elevated. These included apomorphine-induced locomotor activity (+ 103% and + 160%, on weeks 3 and 10) and apomorphine-induced stereotypy (day 10). 2) B_max for [³H]spiroperidol binding to striatal D₂ DAr (12 weeks) increased (+53%, week 12.). Injection of the DAr neuromodulator cyclo(leucyl-glycyl) (8 mg/kg/day × 4 days, SC) reversed the B_max increase. Thus toxicity (DA depletion) following high-dose amphetamine appears to induce compensatory changes in DAr. This DAr upregulation may explain the lack of abnormal movements despite enduring DA depletion. Additionally, the A/I paradigm as an animal model of long-lasting DAr up-regulation, could be used to screen neuromodulatory agents, like CLG, that might treat disorders (e.g., tardive dyskinesia and schizophrenia) thought to involve up-regulated DAr.     

Long-term effects of a priming dose and short-term infusion of amphetamine on striatal dopamine neurons in rats

A priming dose (15 mg/kg i.p.) of (+)-amphetamine sulfate combined with a 16 h subcutaneous infusion of 1.36 mg of the drug per hour to rats via its release from osmotic minipumps produced marked decreases in striatal dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and the synaptosomal uptake of [3H]DA which lasted for at least 12 weeks. The decrease in striatal DA persisted through 24 weeks after drug treatment. In contrast, striatal levels of DOPAC, HVA and synaptosomal DA uptake recovered to control levels by 24 weeks after amphetamine treatment.     

On a prime role for newly synthesized dopamine in striatal function

Rats were given either the tyrosine hydroxylase inhibitor, alpha-methyltyrosine (alphaMT), in doses of 10 or 250 mg/kg or the neuroleptic, haloperidol (0.25 mg/kg). Other rats received both drugs (haloperidol 30 min after alphaMT). This dose of haloperidol alone caused only a slight, gradually developing catalepsy, while alphaMT alone caused none. The combination quickly caused a strong catalepsy. Striatal dopamine (DA) stores were only minimally depleted at the time of catalepsy potentiation. Th e marked elevation of striatal homovanilluc acid concentration seen after haloperidol administration was greatly inhibited by alphaMT pretreatment. It is concluded that the marked potentiation of haloperidol-induced catalepsy by alpha MT is related to the absence of newly synthesized DA rather than to an exhausted main DA pool and that newly synthesized DA has a greater role in striatal function than does DA of the main striatal storage pool.     

Further studies on the long-term depletion of striatal dopamine in iprindole-treated rats by amphetamine

(+) Amphetamine was more potent than (?) amphetamine in causing persistent depletion of striatal dopamine in iprindole-treated rats. This effect of amphetamine was not mimicked by EXP561, a structurally related compound that is more potent than amphetamine as an inhibitor of dopamine uptake. The depletion of striatal dopamine at 1 week after amphetamine injection in iprindole-treated rats was prevented by amfonelic acid, an inhibitor of uptake into dopamine neurons. The depletion of dopamine by amphetamine was prevented when amfonelic acid was given at the same time as amphetamine or as long as 4 hr after amphetamine but not when amfonelic acid was given 24–48 hr after amphetamine. Amfonelic acid antagonized the depletion of dopamine by amphetamine but not the depletion of serotonin by p-chloroamphetamine; fluoxetine antagonized the depletion of serotonin by p-chloroamphetamine but not the depletion of dopamine by amphetamine. Pretreatment with α-methyltyrosine to block dopamine synthesis antagonized the persistent depletion of dopamine by amphetamine, but pretreatment with an inhibitor of monoamine oxidase to increase the dopamine concentration had no effect. The possibility that prolonged release of dopamine from intraneuronal storage granules leads to deleterious effects on dopamine neurons is discussed.     

Tolerance to the increase of striatal homovanillic acid elicited by several anorectic drugs

Several anoretic drugs affect dopamine metabolism in the rat striatum, increasing the concentration of homovanillic acid (HVA). However l- and d-amphetamine and mazindol develop a tolerance to increase. On the contrary, the effect of fenfluramine and S 992 is not reduced by sub-chronic treatments. Moreover, a cross-tolerance to this biochemical effect develops between amphetamine and mazindol, but pretreatment with fenfluramine or with S 992 does not induced cross-tolerance to amphetamine.     

Long-term effects of postnatal amphetamine treatment on striatal protein kinase A activity, dopamine D(1)-like and D(2)-like binding sites, and dopamine content

The purpose of the present study was to determine whether exposure to amphetamine during the preweanling period would alter dopaminergic functioning in the dorsal striatum of adult rats. In three experiments, we assessed the effects of repeated amphetamine treatment on striatal protein kinase A (PKA) activity, dopamine (DA) D₁-like and D₂-like binding sites, and DA content. Rats were pretreated with saline or amphetamine (2.5 mg/kg, ip) for 7 consecutive days starting on postnatal day (PD) 11. At PD 90, rats were killed and their dorsal striata (i.e., caudate–putamen) were removed and frozen until time of assay. Amphetamine pretreatment produced long-term reductions in both striatal PKA activity and DA content. Early amphetamine exposure also resulted in an upregulation of D₂-like binding sites, while leaving D₁-like binding sites unaffected. It is likely that the upregulation of D₂-like binding sites was stimulated by the persistent decline in striatal DA levels. Although speculative, it is possible that excess striatal D₂-like receptors were responsible for inhibiting PKA activity through actions on the cAMP signal transduction pathway. The behavioral relevance of these amphetamine-induced neurochemical changes has not yet be determined.     

The effect of chronic administration and withdrawal of amphetamine on cerebral dopamine receptor sensitivity

Mice with a 6-hydroxydopamine induced unilateral nigro-striatal lesion received (+)-amphetamine sulphate (2.5–20 mg/kg) over a 3-month period by daily incorporation into the drinking water. During this period the circling response to apomorphine hydrochloride (0.01–0.5 mg/kg, s.c.) was increasingly suppressed in comparison to control animals, while spontaneous locomotor activity increased. Following drug withdrawal the circling response to apomorphine remained suppressed two months later. However, spontaneous locomotor activity was also reduced up to 1 month following drug removal.The dopamine content of the lesioned side of the forebrain was 25% of the intact side in control animals and was not further reduced by amphetamine administration. The dopamine content of the intact forebrain was reduced by 43% during amphetamine administration and remained 18% depressed 1 month following drug withdrawal. No changes in 5-hydroxytryptamine or noradrenaline concentrations were observed in either the intact or lesioned side.This data, while showing that chronic amphetamine treatment can induce persistent changes in dopamine receptor sensitivity, can be interpreted in terms of increased striatal receptor sensitivity or as a decreased response of dopamine receptors in the nucleus accumbens.     

Endurance training effects on striatal D2 dopamine receptor binding and striatal dopamine metabolites in presenescent older rats

Endurance training is associated with higher binding of 3H-spiperone to striatal D2 dopamine receptors of rats sacrificed 48 h following the last exercise bout (Gilliam et al. 1984). In the present study we investigated the effects of endurance training in presenescent older rats on the relationship between steady-state levels of DA and its metabolites in striatum versus the affinity and density of striatal D2 DA receptors. Citrate synthase activity of the gastrocnemius-plantaris muscle was 29.06±2.27 mole/g wet wt in 21-month-old trained rats versus 22.88±1.13 mole/g wet wt in 21-month-old untrained animals.DOPAC levels and DOPAC/DA ratios were greater in the old controls. Endurance training was associated with lower DOPAC levels in the 21-month-old animals. Thus, endurance training may postpone selectively changes in DA metabolism over a portion of the lifespan.As expected, the number of D2 DA binding sites was reduced with age (6 months B _max:429±21 fmoles/mg protein; 21 months:355±20) with no change in affinity. The B_max of old runners was significantly higher (457 ± 38 fmoles/mg protein) than that of old controls. Thus, endurance training appears to exert a protective effect on D2 dopamine receptors during the lifespan. Taken together, the present results suggest that there may be a possible reciprocal relationship between changes in DA metabolites and DA binding as a function of exercise in presenescent older rats, and that endurance training may decelerate the effects of age both on nigrostriatal dopamine neurons and on striatal D2 dopamine receptors during a portion of the lifespan.     

Influence of luteinizing hormone releasing hormone (LHRH) on the behavioral effects of amphetamine in rats

The influence of luteinizing hormone releasing hormone (LHRH) on the behavioral effects induced by several doses of D-amphetamine (0.25, 0.5, 1.0 and 2.0 mg/kg IP) was studied. A dose response relation was previously established for the effects of LHRH (50, 100 and 200 micrograms/kg SC) on acquisition and retention of conditioned avoidance responses (CARs). The neuropeptide impaired acquisition and improved retention of CARs, without modifying spontaneous motor activity. Pretreatment with 100 micrograms/kg of LHRH antagonizes the enhancement in acquisition of CARs due to D-amphetamine 0.5, 1.0 and 2.0 mg/kg, the impairment in retention induced by amphetamine 1.0 and 2.0 mg/kg, and the hypermotility and the increased rearing behavior induced by amphetamine 1.0 and 2.0 mg/kg. These results suggest that brain catecholamines, particularly dopamine, could play a role in the behavioral effects of LHRH. Interactions between LHRH and central dopaminergic mechanisms are discussed.     

Specific antagonism by dopamine inhibitors of items of amphetamine induced aggressive behaviour

d-Amphetamine in a dose of 15 mg/kg elicits both aggressive activities and stereotyped sniffing, licking and biting of the cage in mice. A selective inhibition of the aggressive activities (without general sedation of the mice) was obtained by small doses of the neuroleptics spiramide and trifluperazine, indicating that this behaviour was mediated by increased activity of dopamine in the brain. This indication was supported by experiments with noradrenaline blocking agents and inhibitors of the synthesis of dopamine and noradrenaline.     

Blockade of the central effects of d-amphetamine sulfate by amantadine hydrochloride

Amantadine hydrochloride, unexpectedly, was found to block certain effects of d-amphetamine sulfate. In mice pretreated with amantadine, 150 mg/kg, d-amphetamine, 2 mg/kg or 5 mg/kg, failed to produce hyperactivity. This pretreatment also protected aggregated mice from the lethal effects of d-amphetamine, 30 mg/kg. Both d-amphetamine, 15 mg/kg, and chlorpromazine hydrochloride, 10 mg/kg, caused elevations in the homovanillic acid (HVA) concentrations in the caudate nucleus of mice, and amantadine pretreatment blocked this response to d-amphetamine but not that to chlorpromazine. Due to the many similarities in the pharmacological, behavioral, biochemical and clinical effects of amantadine and d-amphetamine, they may act at the same receptor and the observed antagonism may be due to a competitive blockade so that d-amphetamine fails to reach its site of action.     

Sensitization versus tolerance to the dopamine turnover-elevating effects of haloperidol: the effect of regular/intermittent dosing

Recent clinical research suggests that particular patterns of changes in presynaptic dopamine (DA) turnover accompany the therapeutic response to neuroleptics. We sought to determine whether daily versus weekly dosing of haloperidol for 3 weeks produced distinct effects on DA, dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) concentrations in multiple brain areas. Daily dosing favored the development of tolerance to the DA-turnover elevating effects of haloperidol in the striatum and nucleus accumbens. Weekly dosing favored the development of sensitization in the striatum, posterior olfactory tubercle, and ventral tegmental area. These results suggest that dosing schedules may determine, at least in part, the effects of chronic neuroleptic administration on presynaptic DA function.     

Mazindol and amphetamine as inhibitors of the uptake and releasers of 3H-dopamine by rat striatal synaptosomes

Summary The effects of mazindol, amphetamine and fentluramine on uptake and release of ³H-DA by synaptosomes were studied in different systems.In in vitro incubations of ³H-DA with synaptosomes isolated from the caudate nucleus of the rat, mazindol inhibited the uptake of the radioactivity more potently than did amphetamine.When the synaptosomes were isolated from the caudate nuclei of rats treated in vivo with either mazindol or amphetamine, the uptake of ³H-DA during in vitro incubation was lower with synaptosomes of amphetamine-treated rats than with those of mazindol-treated rats.When synaptosomes of untreated rats were prelabelled with ³H-DA and incubated in the presence of amphetamine or of mazindol, amphetamine caused a greater releaseoof radioactivity than did mazindol.Fenfluramine was without activity in all these systems.In spite of the quantitative differences, both amphetamine and mazindol appear to have similar effects on uptake and release of dopamine, and this may account for their analogous pharmacological profile.Supported by C.N.R. grant N. 75.00620.04.115.2380