Long-term d-amphetamine in rats: lack of change in post-synaptic dopamine receptor sensitivity

Basal local cerebral glucose utilization is not altered after behavioral sensitization to quinpirole

Sensitization to psychostimulants results in a behavioral response of a greater magnitude than that produced by a given single dose. Previously, we have shown that sensitization to the D(2)/D(3) dopamine receptor agonist quinpirole produces alterations in quinpirole-stimulated local cerebral glucose utilization (LCGU) in ventral striatal and limbic cortical regions. To determine whether basal neuronal activity is altered in the sensitized animal, this study examined the effects of a sensitizing course of quinpirole on basal neuronal activity using the [(14)C]-2-deoxyglucose (2-DG) method in rats with verified sensitization. Adult, male Long-Evans rats (n = 7 or 10/group) were subjected to 10 injections of quinpirole (0.5 mg/kg, s.c.) or saline administered every 3rd day. Sensitization was verified on the basis of locomotor activity. The 2-DG procedure was performed in freely moving rats 3 days after the last quinpirole injection. LCGU was determined by quantitative autoradiography. No alterations in basal LCGU were detected in quinpirole-sensitized rats compared to those treated with saline. The present finding suggests that either the basal activity of very discrete populations of neurons is affected by sensitization to quinpirole that are not likely to be detected by the 2-DG method, or that the neurobiological changes that result in the sensitized behavioral response affect only stimulated, but not basal, neuronal activity.

Behavioral sensitization to amphetamine is not accompanied by a decrease in the number of c-Fos containing cells in the striatum

The expression of c-Fos-like immunoreactivity (FLI) and chronic Fos-related antigen-like immunoreactivity (FRALI) accompanying behavioral sensitization to amphetamine was assessed in male rat striatum. Animals were treated for four days with amphetamine (A; 5 mg/kg) or vehicle (V) and challenged with A or V on the fifth day. The number of FLI-positive cells in the striatum was enhanced in V-A and A-A groups as compared to control (V-V), while the number of FRALI-positive cells in the striatum was enhanced in the A-V and A-A groups as compared to control. These results suggest that the absence of a decrease in the number of striatal FLI-positive cells accompanying chronic amphetamine treatment is not due to antibody cross-reactivity with chronic FRAs, and that behavioral sensitization to amphetamine is not accompanied by a change in the number of striatal cells expressing c-Fos.

Effects of subchronic amphetamine or amfonelic acid on rat brain dopaminergic and serotonergic function

Repeated doses of direct or indirect CNS stimulants are known to cause behavioral hypersensitivity. The biochemical basis for hypersensitization remains unclear. Since the dopaminergic system uses a large storage pool that is only slowly mobilized to releasable sites, a change in this relationship may underlie the biochemical changes leading to increased responsiveness to stimulants. To test this hypothesis, rats were first tested with low doses of 2.5 mg/kg amphetamine or 1.0 mg/kg amfonelic acid (AFA) for their locomotor response, then 5.0 mg/kg amphetamine or 2.5 mg/kg AFA were injected daily for 7 days and the rats retested with the lower doses of amphetamine or AFA, respectively. Both drugs produced hypersensitivity, but the cataleptic response to acute dopamine (DA) receptor blockade by haloperidol was unaltered. The ability of haloperidol to increase DA metabolism was unaltered and the ability of acute AFA to synergize with haloperidol was similar in the striatum of stimulant and saline treated rats, but reduced in the medial prefrontal cortex of both AFA and d-amphetamine treated rats. Additional rats had DA2 receptor sensitivity measured in the striatum and frontal cortex, but no significant differences were found. Only amphetamine caused a significant decrease in frontal cortex serotonin type 2 receptors. Since there was no alteration in the ability of AFA to increase neurogenic release of DA in the striatum and a decrease occurred in prefrontal cortex, an increase in the storage to functional pool exchange in the nigrostriatal and mesocortical DA containing neurons seems unlikely. In contrast, both the amphetamine and AFA treatment groups had their brain 5HT and 5HIAA levels reduced by about 50%. This suggests that changes in other transmitter systems may have a permissive effect allowing exaggerated responses to excessive DA release.

D-2 but not D-1 dopamine agonists produce augmented behavioral response in rats after subchronic treatment with methamphetamine or cocaine

A study was performed to examine behavioral response to a challenge of selective dopamine D-1 and D-2 agonists in rats previously sensitized by subchronic administration of methamphetamine or cocaine. Rats in three groups received repeated injections (IP) of saline, methamphetamine (4 mg/kg/day) or cocaine (20 mg/kg/day), respectively, for 14 days. After an abstinence period of 7-13 days, all groups were challenged with either a selective D-1 agonist (SKF 38393) or D-2 agonists (quinpirole or RU 24213). The ability of SKF 38393 (6 mg/kg or 18 mg/kg) to produce grooming behavior did not differ significantly among the saline-, methamphetamine- and cocaine-treated groups. In contrast, quinpirole (1 mg/kg) and RU 24213 (3 mg/kg) produced more intense stereotypy consisting of rearing, sniffing and repetitive head movement in the two psychostimulant-treated groups than in the saline-treated group. Such augmented response to selective D-2 agonists was observed even after a 1-month abstinence period. These results suggest that the enduring behavioral sensitization induced by two pharmacologically distinct psychostimulant agents, methamphetamine and cocaine, occurs through a common neurobiological mechanism of lasting supersensitivity in postsynaptic D-2, but not D-1 dopamine receptors.

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.

An analysis of the 'tolerance' which develops to analgetic electrical stimulation of the midbrain periaqueductal grey in freely moving rats

Electrical stimulation of the ventral midbrain periaqueductal grey (PAG) elicits an opioidergic antinociception against noxious heat and pressure in freely moving rats. Recurrent stimulation was associated with a gradual decline and eventual loss of this stimulation-produced antinociception (SPA). This could be reinstated by an increase in current intensity and this reinstatement was preventable by naloxone. The current intensity--antinociception (dose--response) curve was shifted to the right in recurrently stimulated rats and parallel to that in naive animals. The loss of SPA upon repetitive simulation did not represent a conditioning phenomenon. Thus, tolerant rats exposed to all cues which accompanied stimulation revealed no (compensatory) hyperalgesic response--but rather a slight antinociception. Further, SPA recovered spontaneously in tolerant rats. Moreover, 'extinction' by repeated exposure to all cues accompanying stimulation did not restore or accelerate the recovery of SPA in tolerant animals. Tolerant rats showed no depletion in midbrain PAG or other CNS or hypophyseal pools of beta-endorphin, Met-enkephalin or dynorphin indicating that a depletion of endogenous opioid peptides does not underlie the tolerance which develops to stimulation. In fact recurrently stimulated rats did not show any of the pronounced effects upon CNS pools of opioid peptides which are seen with long-term stress. Moreover, repetitively stimulated rats revealed no indications of stress as judged by a diversity of stress-sensitive parameters; basal nociceptive threshold, core temperature, ingestive behaviour, body weight, adrenal weight and hypophyseal secretion of beta-endorphin and prolactin. The data offer two major conclusions. Firstly, the gradual loss of analgesia upon recurrent stimulation of the midbrain PAG does not reflect a generalized debilitation or stress and neither a conditioning phenomenon nor a depletion of pools of endogenous opioid peptides. Rather it closely corresponds to the pharmacological definition of tolerance and may reflect a process occurring at the level of the opioid receptor and coupled processes. This finding explains the cross-tolerance which we observe recurrently stimulated rats to display to morphine. Secondly, this SPA is not a form of stress-induced analgesia and rats undergoing recurrent stimulation reveal no indications of stress as judged by biochemical, physiological and behavioural parameters.

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.

Latent inhibition is not affected by acute or chronic administration of 6 mg/kg dl-amphetamine

Latent inhibition (LI) is a behavioral paradigm in which animals learn to ignore a repeatedly presented stimulus not followed by meaningful consequences. We previously reported that LI was disrupted following the administration of 1.5 mg/kg dl-amphetamine. The present experiments investigated the effects of 6 mg/kg dl-amphetamine administration on LI in a conditioned emotional response (CER) procedure consisting of three stages: pre-exposure, in which the to-be-conditioned stimulus, tone, was repeatedly presented without reinforcement; conditioning, in which the pre-exposed stimulus was paired with shock; and test, where LI was indexed by animals' suppression of licking during tone presentation. The three stages were conducted 24 h apart. In Experiment 1, the drug was administered in a 2 X 2 design, i.e. drug-no drug in pre-exposure and drug-no drug in conditioning. LI was obtained in all conditions. In Experiment 2, animals were given either 5 days of 6 mg/kg amphetamine pretreatment and amphetamine in pre-exposure and conditioning or 7 days of saline. LI was not obtained under amphetamine, but this outcome reflected a state-dependency effect. In Experiment 3, animals received either 5 days of amphetamine pretreatment and amphetamine in pre-exposure, conditioning and test or 8 days of saline. LI was obtained in both the placebo and amphetamine conditions. Experiments 4a and 4b compared the effects of two drug doses, 1.5 (4a) and 6 mg/kg (4b), administered in pre-exposure and conditioning. LI was abolished with the 1.5 mg/kg dose but not with the 6 mg/kg dose.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.

The effect of dopamine receptor agonist treatment on haloperidol-induced supersensitivity in mice

Mice were pretreated with haloperidol (HP) (3-4 mg/kg/day in drinking water) or vehicle for 21 days. On the 25th day, HP-pretreated mice were supersensitive to the locomotor stimulant effects of apomorphine (after acute premedication with reserpine and alpha-methyl-p-tyrosine). This behavioural supersensitivity was accompanied by a 25-39% increase in the number of [3H]-spiperone binding sites in the striata of HP-pretreated mice. Short-term repeated administration of the dopamine (DA) agonist drugs d-amphetamine and L-DOPA during the HP withdrawal phase (days 22, 23 and 24) had no effect on either measure of DA receptor supersensitivity. In contrast, the administration of apomorphine on days 22, 23 and 24 enhanced the HP-induced behavioural supersensitivity but decreased the HP-induced elevation of the number of [3H]-spiperone binding sites. Apomorphine treatment alone did not alter either measure. The results do not support the hypothesis that supersensitive DA receptors can be down-regulated by short-term treatment with DA agonist drugs and, moreover, indicate that important discrepancies may exist between behavioural and biochemical measures of DA receptor supersensitivity.

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.

Striatal dopamine receptor sensitivity after subchronic fencamfamine in the rat

Dopamine (DA) receptor sensitivity was assessed in the rat striatal system following subchronic treatment with fencamfamine or saline for 7 days (10 mg/kg i.p.). Seventy-two hours after the last injection the stereotyped behaviour and general activity induced by apomorphine or saline were evaluated. Apomorphine (2.0 mg/kg s.c.) induced a decrease of the stereotypic response when fencamfamine-pretreated animals were compared to saline-treated ones while apomorphine (0.02 mg/kg s.c.) failed to alter the general activity of animals, treated with fencamfamine or not. In biochemical experiments subchronic fencamfamine did not alter the effects of apomorphine (0.02 mg/kg s.c.) in reducing homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC) striatal levels, when compared to saline. In addition we observed a slight but significant reduction in the total dopamine receptor content in the striatum labelled by [3H]spiroperidol. These findings indicate that subchronic fencamfamine treatment leads to a desensitization of postsynaptic DA receptors in rat striatum.

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.

Chronic L-dopa treatment of rats and mice does not change the sensitivity of post-synaptic dopamine receptors

The effects of L-Dopa + benserazide (L-Dopa + B) treatment on pre- and postsynaptic dopamine (DA) receptors were studied. Mice treated once daily P.O. with L-Dopa (200 mg/kg) + B (50 mg/kg) or vehicle for 10 days were used on the 11th day. After premedication with reserpine and alpha-methyltyrosine (alpha-MT), apomorphine (0.5-2.0 mg/kg) produced locomotor stimulation which was of equal intensity in the 3 treatment groups, even when the treatment dose of L-Dopa was increased to 400 mg/kg per day. In contrast, low doses of apomorphine (0.1-0.5 mg/kg) produced locomotor depression in B- and vehicle-treated mice but not in L-Dopa + B-treated mice. In rats treated I.P. twice daily with L-Dopa (200 mg/kg) + B (50 mg/kg), B (50 mg/kg) or vehicle for 12 days, apomorphine produced an equivalent degree of stereotypy on the 13th day in each of the 3 treatment groups. There were no treatment group differences in the binding of [3H]-spiperone or [3H]-leu-enkephalin to rat striatal membranes. The data suggest that long-term L-Dopa + B treatment of mice and rats does not change the sensitivity of postsynaptic DA receptors but may affect the sensitivity of DA autoreceptors.

Effect of continuous systemic infusion of D-amphetamine on the sensitivity of nigral dopamine cells to apomorphine inhibition of firing rates

Using a single-unit recording technique, sensitivity of nigral dopamine neurons to the inhibitory effect of apomorphine was examined on various days after a 7-day pretreatment with osmotic minipumps containing D-amphetamine. Continuous systemic infusion of D-amphetamine produced a short-lasting subsensitivity of nigral neurons to apomorphine (0-2 days after D-amphetamine), followed by a supersensitivity on days 7, 8 and 9, post-amphetamine. The supersensitivity seems to persist at least for one week, suggesting one possible mechanism mediating long-term behavioral and biochemical changes following chronic high-dose amphetamine administration.

Chronic haloperidol and adrenergic receptor sensitivity in the rat

Rats were administered haloperidol (3-4 mg/kg/day) in their drinking water for 42 days, and experiments conducted on the seventh day of withdrawal. Anaesthetized haloperidol treated rats exhibited a similar mean blood pressure (BP) and heart rate (HR) response to control rats when challenged with phenylephrine (IV). When similarly pretreated rats were challenged with one of four possible doses of clonidine (IV), haloperidol treated rats were less sensitive than control rats to clonidine's hypertensive action, but there were no effects of treatment on the hypotensive (BP) effect of clonidine nor on its bradycardic effect. When one of six possible doses of tyramine was administered a similar mean BP response was seen in both treatment groups, but the positive HR response in the haloperidol-treated group was much less than in the vehicle-treated group. Atria isolated from haloperidol treated or control rats revealed a similar chronotropic response to noradrenaline and tyramine challenge. These data indicate that chronic haloperidol does not cause a generalized change in alpha-adrenergic receptor sensitivity. Nevertheless, it is clear that haloperidol has produced changes in the cardiovascular response of rats to these drugs.

Behavioural tolerance to amphetamine and other psychostimulants: the case for considering behavioural mechanisms

An hypothesis is presented about the nature of behavioural tolerance in animals to stimulant drugs. It is suggested that, in many behavioural procedures, tolerance is due to behavioural adaptation to those drug effects which cause disruption of ongoing rewarded behaviour. This unitary hypothesis accounts for the available data on tolerance and cross-tolerance to stimulants more effectively than all of the other more conventional explanations which are based upon dispositional or functional concepts, the most common of which are described, evaluated, and found to be inadequate. Furthermore, it is suggested that attempts to explain tolerance in terms of changes in synaptic functioning are subject to very considerable problems of interpretation and that an analysis of behavioural mechanisms may be of greater value in understanding the process of behavioural tolerance. Evidence for the basic behavioural hypothesis is outlined in some detail, and a theoretical justification presented for its major assumptions. Operant studies of chronic stimulant effects on behaviour have often produced very complex patterns of data, considerable differences being reported both between subjects and between studies. A speculative model is presented which attempts to account for this pattern of data in tolerance studies.

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.

Persistent effects of amphetamine on cerebellar Purkinje neurons following chronic administration

The spontaneous discharge of cerebellar Purkinje neurons was studied in rats after withdrawal from chronic treatment with amphetamine (2 mg/kg per day x 21 days). Discharge rates in withdrawn animals remained significantly lower than those of controls for up to 50 days. Disruption of the adrenergic input to these neurons from the locus coeruleus by treatment with propranolol, clonidine or reserpine, partially restored these discharge rates. Acute administration of amphetamine in amphetamine-withdrawn rats did not further depress Purkinje neurons discharge rate, whereas in a previous study in this lab, Purkinje neurons from naive animals were markedly slowed. Moreover, Purkinje neurons from amphetamine-withdrawn rats were also significantly less sensitive than controls to locally applied norepinephrine. These results demonstrate that chronic amphetamine can lead to very long-term changes in neuronal activity, and suggest that these changes may be mediated, in part, by the noradrenergic transmitter systems.