Enhanced responsiveness to intraventricular infusion of amphetamine following its repeated systemic administration

Continuous cocaine administration produces persisting changes in brain neurochemistry and behavior

Rats were administered either continuous cocaine, daily injections of cocaine, continuous amphetamine, or no drug for 5 days and then given a 30 day drug-free recovery period. When subsequently tested in open field, the daily cocaine injection animals were the most hyperactive whereas the cocaine pellet animals were the most fearful. In vitro autoradiography was then utilized to examine persisting changes in receptor binding for D2 ([3H]spiperone), D1 ([3H]SCH23390), benzodiazepine ([3H]flunitrazepam), 5-HT1 ([3H]5-HT), 5-HT2 ([3H]ketanserin), and muscarinic acetylcholine (ACh) receptors ([3H]QNB; quinuclidinyl benzilate). In the amphetamine pellet animals, there were large increases in [3H]spiperone binding in several dopamine (DA)-rich regions; these were accompanied by conversely decreased [3H]SCH23390 binding. Cocaine pellet animals showed a completely different pattern, with appreciable increases in [3H]flunitrazepam binding in DA-rich areas, cortex, and amygdala but decreased [3H]QNB binding in DA-rich areas, hippocampus, and amygdala. While cocaine injection animals showed elevated [3H]spiperone binding in caudate and substantia nigra, they had generally smaller changes in most brain regions than the other drug groups. These findings replicate and extend previous reports that continuous drug administration induces long-lasting alterations in brain chemistry, but indicate that continuous cocaine has enduring effects on different neurochemical systems from continuous amphetamine.

A silicone pellet for continuous cocaine: comparison with continuous amphetamine

An inexpensive silicone pellet is described for the continuous administration of cocaine for up to 5 days. Rats implanted with this pellet show minimal skin irritation and go through distinct behavioral stages, with an initial period of hyperactivity followed by motor stereotypies. Then, at 3-4 days after implantation, a variety of hallucinogen-like ("late-stage") behaviors appear, including limb flicks, sudden startle responses, and repetitive mid-air grasping movements. Compared to continuous d-amphetamine, continuous cocaine induces decreased motor stereotypies but heightened "late-stage" behaviors.

Sensitization to systemic amphetamine produces an enhanced locomotor response to a subsequent intra-accumbens amphetamine challenge in rats

Repeated amphetamine (AMPH) administration into the nucleus accumbens does not enhance (sensitize) the locomotor activity produced by a subsequent systemic AMPH challenge. We report here, however, that pretreatment with systemic injections of AMPH does produce a significant enhancement in the locomotor stimulant effects produced by intra-accumbens AMPH given 21 days after the last pretreatment injection of AMPH. These data support the hypothesis that neural adaptations in dopamine (DA) terminal fields are sufficient for the expression of AMPH sensitization, although an action on DA cell bodies may be required for the induction of AMPH sensitization.

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.

Evidence for multiple opiate receptor involvement in different phencyclidine-induced unconditioned behaviors in rats

Rats were used for comparing the behavioral response profiles of phencyclidine (PCP) and d,1-N-allylnormetazocine (NANM), two drugs that are proposed to exert their effects through the "PCP/sigma" receptor. Phencyclidine (1.0-5.0 mg/kg) and NANM (2.5-10.0 mg/kg) induced dose-related increases in locomotion, sniffing, repetitive head movements, non-object directed mouth movements, and ataxia. Both drugs also increased food and water consumption during the latter portion of the drug response. Ingestive behaviors induced by PCP (2.5 mg/kg), as with eating and drinking stimulated by the mu-opiate morphine (2.0 mg/kg), were blocked by a relatively low dose of the opiate antagonist naloxone (0.5 mg/kg). Multiple injections of PCP (2.5 mg/kg for 4 days) or NANM (10.0 mg/kg for 4 days) augmented several measures of behavioral activation, including horizontal locomotion, rearing, and nonfocused sniffing, but did not significantly change stereotyped behaviors or ataxia. Reciprocal cross-sensitization of locomotor activation is indicated by the finding that the response to a challenge injection of PCP (2.5 mg/kg) or to NANM (10.0 mg/kg) after 4 days of treatment with the other drug closely resembled the enhanced locomotor response observed after the chronic treatment. Phencyclidine and NANM thus appear to exert many of their effects on unconditioned behavior through common mechanisms, including interaction with sigma receptors. In addition, these findings are consistent with previous suggestions that a mu-opiate receptor system may modulate some effects of PCP.

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.

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.

Critical issues in assessing the behavioral effects of amphetamine

Amphetamine induces a behavioral syndrome in mammals that includes a variety of repetitive behaviors. An integral component of this syndrome in humans is the presence of a thought disturbance not unlike that manifest in idiopathic paranoid schizophrenia. The consistent pattern of behavioral changes produced by amphetamine across species, when considered in light of the psychosis it elicits in humans, has suggested to many that these drug-induced changes in animals may provide a model of the endogenous psychosis in humans. Amphetamine-induced changes in open-field behavior in the rat have been the most widely studied in attempts to formulate a model for investigating the neurobiological mechanisms underlying amphetamine psychosis and paranoid schizophrenia in humans and for testing the therapeutic efficacy of new antipsychotic drugs. The procedures used to assess the behavioral response to amphetamine, however, typically include rating scales or automated recordings that by their very nature ignore those components of the behavioral response that may be most critical for developing a viable animal model of the naturally occurring psychosis. Further, open-field behavior is often recorded during arbitrarily selected intervals without consideration for the multiphasic nature of the entire amphetamine response. We discuss how incomplete descriptive analyses of the amphetamine behavioral response in rats has led to confusion in the literature and describe behavioral research that is paradigmatic of the work we believe is most likely to eventuate in significant progress in the field.

Evidence that a behavioral augmentation following repeated amphetamine administration does not involve peripheral mechanisms

Repeated administration of amphetamine (AMPH) to rats results in an augmentation of the drug-induced locomotion and stereotypy. The studies reported below were directed at examining the potential role for some dispositional and peripheral sympathomimetic factors in mediating the enhanced stereotypy response. These included three factors associated with repeated AMPH administration: (1) the possible accumulation of AMPH in a peripheral mobilizable pool; (2) repeated sympathetic activation; and (3) AMPH metabolite-induced depletion of peripheral stores of norepinephrine. The approach utilized was to selectively reduce or mimic the peripheral actions of AMPH through the use of non-pharmacological or pharmacological manipulations which are relatively lacking in AMPH-like central stimulant effects. The results indicate that these factors cannot account for the augmentation of the behavioral response to AMPH and suggest that these behavioral alterations reflect changes in the responsiveness of brain mechanisms which mediate the behavioral effects of the drug.

Apparent tolerance to some aspects of amphetamine stereotypy with long-term treatment

Previous reports have demonstrated that long-term amphetamine treatment results in a progressive augmentation of locomotion and focused stereotypy in the rat. A series of experiments were conducted to determine whether an increase in dopamine receptor sensitivity is the primary mechanism underlying the behavioral alterations associated with multiple amphetamine injections. Detailed observations of the focused stereotyped behaviors produced by amphetamine revealed that although some components were enhanced with long-term treatment, others were reduced. Thus, whereas repeated administration of 2.5 mg/kg d-amphetamine produced a progressive increase in repetitive head and limb movements, long-term treatment with 5.5 mg/kg d-amphetamine resulted in a reduction of licking and biting behaviors (oral stereotypies). These results, which suggest that different mechanisms mediate the various components of focused stereotypy, argue against the supersensitivity hypothesis. In fact, the apparent tolerance that develops to oral stereotypies may reflect a decrease in dopamine receptor sensitivity since repeated amphetamine administration also reduces the oral stereotypies produced by 0.5 or 2.0 mg/kg apomorphine, a direct acting dopamine agonist. Thus, the behavioral alterations produced by repeated amphetamine injections cannot be explained solely by an increase in receptor sensitivity.