David S. Segal

Long-term administration of d-amphetamine: Progressive augmentation of motor activity and stereotypy ☆

Abstract The competitive relationship between d-amphetamine induced stereotypy and locomotor activity indicates the importance of their concurrent evaluation, especially during chronic studies. Repeated injection of 0.5, 1.0, 2.5, 5.0, or 7.5 mg/kg d-amphetamine for 36 successive days, in rats continuously exposed to the experimental chambers, produced a progressive augmentation in stereotypy and/or locomotion (depending on dose) during the 3–4 hr interval following injections (post-injection phase). In contrast, dark phase locomotor activity (8–20 hr after each daily injection) was maximally reduced (30–40% of controls) after the first injection of either 5.0 or 7.5 mg/kg d-amphetamine and gradually declined to this level with repeated injection of 1.0 and 2.5 mg/kg. Carry-over of both the post-injection augmentation and dark phase reduction of locomotion was revealed during amphetamine retest 8 days following discontinuation of daily d-amphetamine injections. Possible mechanisms underlying these behavioral alterations are discussed.

Effects of methylphenidate on extracellular dopamine, serotonin, and norepinephrine: comparison with amphetamine.

Abstract: Methylphenidate promotes a dose‐dependent behavioral profile that is very comparable to that of amphetamine. Amphetamine increases extracellular norepinephrine and serotonin, in addition to its effects on dopamine, and these latter effects may play a role in the behavioral effects of amphetamine‐like stimulants. To examine further the relative roles of dopamine, norepinephrine, and serotonin in the behavioral response to amphetamine‐like stimulants, we assessed extracellular dopamine and serotonin in caudate putamen and norepinephrine in hippocampus in response to various doses of methylphenidate (10, 20, and 30 mg/kg) that produce stereotyped behaviors, and compared the results with those of a dose of amphetamine (2.5 mg/kg) that produces a level of stereotypies comparable to the intermediate dose of methylphenidate. The methylphenidate‐induced changes in dopamine and its metabolites were consistent with changes induced by other uptake blockers, and the magnitude of the dopamine response for a behaviorally comparable dose was considerably less than that with amphetamine. Likewise, the dose‐dependent increase in norepinephrine in response to methylphenidate was also significantly less than that with amphetamine. However, in contrast to amphetamine, methylphenidate had no effect on extracellular serotonin. These results do not support the hypothesis that a stimulant‐induced increase in serotonin is necessary for the appearance of stereotyped behaviors.

Behavioral studies following lesions of the mesolimbic and mesostriatal serotonergic pathways

The behavior of rats with selective lesions of either the dorsal (B7), median (B8), or lateral (B9) raphe nuclei was compared to that of sham-lesioned controls in a variety of experimental situations. As described previously, the extent of damage to the midbrain raphe nuclei was determined by fluorescence histochemistry, and the tryptophan hydroxylase and tyrosine hydroxylase activities of 6 forebrain regions were measured for each rat. None of the lesions affected tyrosine hydroxylase activity. Lesions of B7, which reduced tryptophan hydroxylase in the striatum, thalamus, cortex, and hypothalamus, had no significant effect on any of the behavioral measures. Lesions of B9, although twice as large, neither reduced forebrain tryptophan hydroxylase significantly nor affected any of the behavioral variables. However, B8 lesions, which reduced hippocampal, septal, cortical, and hypothalamic tryptophan hydroxylase, had behavioral effects similar to those reported after combined raphe lesions parachlorophenylalanine. Median raphe-lesioned rats were hyperactive when placed in a novel environment and throughout the dark phase of the light/dark cycle. With respect to locomotor activity, B8-lesioned rats were also hyper-responsive to amphetamine. When placed in a stabilimeter and subjected to repeated air puff stimuli, rats with B8 lesions exhibited larger startle responses. Furthermore, only B8-lesioned animals perseverated when given two unreinforced trials in a Y-maze. All these histologic, biochemical, and behavioral variables were assessed individually for all 39 animals, and a multivariate correlational analysis incorporating the data of this and the preceding paper is presented here. These experiments suggest that the mesolimbic serotonergic pathway originating in B8 subserves some of the inhibition necessary to dampen behavioral responsivity.

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.

Repeated cocaine administration induces behavioral sensitization and corresponding decreased extracellular dopamine responses in caudate and accumbens

Behavioral and brain regional dopamine responses to cocaine (10 mg/kg) were concurrently monitored using in vivo microdialysis in freely-moving rats pretreated with 4 daily injections of saline or cocaine (10 mg/kg). Repeated cocaine produced a behavioral sensitization characterized by a downward oriented locomotor activation profile. In contrast, both caudate and nucleus accumbens dopamine responses were significantly diminished in the drug-pretreated group. These results, obtained following two days of drug withdrawal, differ from previous reports of an enhanced dopamine response after longer withdrawal intervals. While the duration of withdrawal may play an important role in the quantitative features of the dopamine response to subsequent stimulant administration, these results suggest that an enhanced dopamine response may not be required for the expression of behavioral sensitization. A compensatory increase in the dopamine uptake carrier, resulting from chronic cocaine-induced uptake blockade, is discussed as a possible mechanism underlying the reduced dopamine response.

Effects of apomorphine and amphetamine on patterns of locomotor and investigatory behavior in rats

Rats were tested in a Behavioral Pattern Monitor after various doses of either amphetamine or apomorphine in order to characterize their behavioral profiles, including patterns and sequences of holepokes, rearings and locomotor movements. To enable direct comparisons between the behavioral effects of the two stimulants, doses and times for each drug were selected with which locomotor hyperactivity was the predominant behavioral response. Although both drugs increased the total amount of locomotor activity, amphetamine induced a relatively varied behavioral profile while apomorphine induced repetitive behavior with a restricted range of responses. These contrasting effects of the stimulants were interpreted as reflective of their differing modes of action with regard to central dopaminergic systems. It is suggested that, in the dose range used, the release of dopamine by amphetamine is coupled to neuronal firing and therefore this release increases behavioral activity without altering the normal response repertoire of the animal. Conversely, the direct agonist action of apomorphine results in a restricted and perseverative behavioral pattern because its activation of forebrain dopamine receptors is independent of the normal physiological pattern of dopaminergic neuronal firing.

Multiple daily amphetamine administration: behavioral and neurochemical alterations

In rats, multiple daily amphetamine injections (2.5 milligrams per kilogram of body weight, injected subcutaneously every 4 hours for 5 days) resulted in a progressive augmentation in response, characterized by a more rapid onset and an increased magnitude of stereotypy. By contrast, offset times of both the stereotypy and the poststereotypy hyperactivity periods were markedly shortened. When the animals were retested with the same dose of amphetamine 8 days after the long-term treatment was discontinued, the time of offset of the stereotypy and hyperactivity phases had recovered to values found with short-term amphetamine treatment, whereas the more rapid onset of stereotypy persisted. Brain monoamine and amphetamine concentrations and tyrosine hydroxylase activity were determined in comparably treated rats at times corresponding to the behavioral observations. The behavioral data indicate that enhanced responsiveness to amphetamine following its repeated administration may contribute to the development of amphetamine psychosis.

Relevance of pharmacokinetic parameters in animal models of methamphetamine abuse

Although the behavioral consequences of methamphetamine (METH) abuse have been extensively documented, a more precise and thorough understanding of underlying neurobiological mechanisms still requires the use of animal models. To study these biochemical processes in experimental animals requires consideration for the broad range of human METH abuse patterns and the many factors that have been identified to profoundly influence the behavioral and neurochemical effects of exposure to METH‐like stimulants. One potentially critical issue relates to pharmacokinetic differences between the species. In this review, METH plasma pharmacokinetic profiles after single and multiple dose intravenous METH administration are compared for the rat and human. Significant differences in elimination half‐life between the two species (t1/2: rat—70 min, human—12 h) result in markedly dissimilar profiles of METH exposure. However, the plasma profile of a human METH binge pattern can be approximated in the rat by increasing METH dose frequency. Consideration of METH pharmacokinetics in animal models should permit a closer simulation of the temporal profile of METH exposure in the human CNS and should provide further insight into the mechanisms contributing to the addiciton and psychopathology associated with METH abuse. Synapse 39:161–166, 2001.

Regional extracellular norepinephrine responses to amphetamine and cocaine and effects of clonidine pretreatment

Microdialysis in behaving animals was used to characterize the hippocampus (HP) and prefrontal cortex (PFC) norepinephrine (NE) responses to amphetamine (AMPH) and cocaine (COC). NE exhibited regionally similar dose- and time-dependent increases to each drug. However, peak NE concentrations were approximately 2-fold greater at behaviorally similar doses of AMPH compared with COC. To examine the role of noradrenergic impulse flow in the mechanism(s) by which these stimulants enhance extracellular NE, groups of animals were pretreated with the alpha 2 autoreceptor agonist, clonidine (CLON), prior to stimulant administration. CLON (50 micrograms/kg) administration completely blocked the NE response to both 20 and 30 mg/kg COC. By contrast, CLON decreased the NE response to 0.5 mg/kg AMPH by 75%, but became progressively less effective on the response as the dose was increased to 1.75 and 5.0 mg/kg. CLON also had no effect on the caudate dopamine responses to either AMPH or COC, consistent with the presumed specificity of this drug for alpha 2 receptors and suggesting the absence of any significant pharmacokinetic interactions. These results indicate that COC acts an uptake blocker at NE-containing neurons and suggest that AMPH increases extracellular NE through two consequences of its interaction with the neuronal transport carrier: (1) reuptake blockade which predominates at lower doses; and (2) release which becomes more prevalent at higher doses. Behavioral analyses revealed effects of CLON which varied as a function of stimulant and dose.

Stimulant actions in rodents : Implications for attention-deficit/hyperactivity disorder treatment and potential substance abuse

Most evidence supports the continued use of stimulants as the best available pharmacotherapy for the treatment of children with attention-deficit/hyperactivity disorder (ADHD), but little is known about possible enduring behavioral and neuroadaptational consequences of long-term stimulant exposure. Although a variety of preclinical studies, particularly those using methylphenidate (MP), have attempted to address these issues, most of these studies have used procedures that might not adequately simulate clinical treatment conditions, and results have not been entirely consistent. In particular, the rationale for selection of MP doses that simulate clinical exposure has not been well defined. We suggest that the use of more appropriate treatment conditions, including doses that result in plasma drug levels comparable to therapeutic levels, will provide a more accurate model for adequately assessing the therapeutic mechanisms and potential long-term consequences of stimulant psychotherapy in the treatment of ADHD.