The effects of continuous cocaine dose on the induction of behavioral tolerance and dopamine autoreceptor function

Comparing phasic dopamine dynamics in the striatum, nucleus accumbens, amygdala, and medial prefrontal cortex

Midbrain dopaminergic neurons project to and modulate multiple highly interconnected modules of the basal ganglia, limbic system, and frontal cortex. Dopamine regulates behaviors associated with action selection in the striatum, reward in the nucleus accumbens (NAc), emotional processing in the amygdala, and executive functioning in the medial prefrontal cortex (mPFC). The multifunctionality of dopamine likely occurs at the individual synapses, with varied levels of phasic dopamine release acting on different receptor populations. This study aimed to characterize specific aspects of stimulation-evoked phasic dopamine transmission, beyond simple dopamine release, using in vivo fixed potential amperometry with carbon fiber recording microelectrodes positioned in either the dorsal striatum, NAc, amygdala, or mPFC of anesthetized mice. To summarize results, the present study found that the striatum and NAc had increased stimulation-evoked phasic dopamine release, faster dopamine uptake (leading to restricted dopamine diffusion), weaker autoreceptor functioning, greater supply levels of available dopamine, and increased dopaminergic responses to DAT blockade compared to the amygdala and mPFC. Overall, these findings indicate that phasic dopamine may have different modes of communication between striatal and corticolimbic regions, with the first being profuse in concentration, rapid, and synaptically confined and the second being more limited in concentration but longer lasting and spatially dispersed. An improved understanding of regional differences in dopamine transmission can lead to more efficient treatments for disorders related to dopamine dysfunction.

The nuclear transcription factor CREB: involvement in addiction, deletion models and looking forward

Addiction involves complex physiological processes, and is characterised not only by broad phenotypic and behavioural traits, but also by ongoing molecular and cellular adaptations. In recent years, increasingly effective and novel techniques have been developed to unravel the molecular implications of addiction. Increasing evidence has supported a contribution of the nuclear transcription factor CREB in the development of addiction, both in contribution to phenotype and expression in brain regions critical to various aspects of drug-seeking behaviour and drug reward. Abstracting from this, models have exploited these data by removing the CREB gene from the developing or developed mouse, to crucially determine its impact upon addiction-related processes. More recent models, however, hold greater promise in unveiling the contribution of CREB to disorders such as addiction.

Augmented behavioral response and enhanced synaptosomal calcium transport induced by repeated cocaine administration are decreased by calcium channel blockers

Recent studies suggest that calcium influx via L-type calcium channels is necessary for psychostimulant-induced behavioral sensitization. In addition, chronic amphetamine upregulates subtype Cav1.2-containing L-type calcium channels. In the present studies, we assessed the effect of calcium channel blockers (CCBs) on cocaine-induced behavioral sensitization and determined whether the functional activity of L-type calcium channels is altered after repeated cocaine administration. Rats were administered daily intraperitoneal injections of either flunarizine (40 mg/kg), diltiazem (40 mg/kg) or cocaine (20 mg/kg) and the combination of the CCBs and cocaine for 30 days. Motor activities were monitored on Day 1, and every 6th day during the 30-day treatment period. Daily cocaine administration produced increased locomotor activity. Maximal augmentation of behavioral response to repeated cocaine administration was observed on Day 18. Flunarizine pretreatment abolished the augmented behavioral response to repeated cocaine administration while diltiazem was less effective. Measurement of tissue monoamine levels on Day 18 revealed cocaine-induced increases in DA and 5-HT in the nucleus accumbens. By contrast to behavioral response, diltiazem was more effective in attenuating increases in monoamine levels than flunarizine. Cocaine administration for 18 days produced increases in calcium uptake in synaptosomes prepared from the nucleus accumbens and frontal cortex. Increases in calcium uptake were abolished by flunarizine and diltiazem pretreatment. Taken together, the augmented cocaine-induced behavioral response on Day 18 may be due to increased calcium uptake in the nucleus accumbens leading to increased dopamine (DA) and serotonin (5-HT) release. Flunarizine and diltiazem attenuated the behavioral response by decreasing calcium uptake and decreasing neurochemical release.

Kappa-opioid agonist U69593 potentiates locomotor sensitization to the D2/D3 agonist quinpirole: pre- and postsynaptic mechanisms

To assess whether the development and expression of behavioral sensitization to the dopamine D2/D3 agonist quinpirole (QNP) is influenced by coadministration of the kappa opioid receptor agonist U69593, rats received every 3-4 days for a total of 10 treatments an injection of U69593 (0.3 mg/kg) together with an injection of either a postsynaptic (0.5 mg/kg) or a presynaptic dose of QNP (0.05 mg/kg); locomotor activity was measured after each treatment. Control rats were injected as appropriate with QNP, U69593, and vehicle/saline. Following chronic treatment, dose-response profiles to QNP were obtained to assess the expression of sensitization; the effect of U69593 on locomotor activity in animals already sensitized to QNP was also assessed. Results showed that cotreatment of U69593 with a postsynaptic dose of QNP doubled the speed and magnitude of sensitization to QNP, while U69593 cotreatment with a presynaptic dose of QNP switched the effects of QNP from locomotor depression to locomotor sensitization. However, U69593 cotreatment with a presynaptic dose of QNP changed a different set of measures of sensitization than did cotreatment with a postsynaptic dose of the dopamine agonist. Together, findings suggest that sensitization to QNP is not a unitary phenomenon but has components that are relatively independent, mediated by distinct pre- and postsynaptic mechanisms and modulated by kappa receptor activity.

The effects of continuous cocaine dose, treatment, and withdrawal duration on the induction of behavioral tolerance and dopamine autoreceptor function

The present experiment evaluated the interactions between continuous cocaine dose, duration of administration, and duration of withdrawal on the induction of behavioral tolerance and changes in dopamine autoreceptor (DA) function. In the current experiments, rats were exposed to a pretreatment regimen involving the continuous administration of 0, 5, or 20 mg/kg/day cocaine for either 3 or 7 days. All subjects were then withdrawn from the pretreatment regimen for 1 or 7 days. For the experiments examining behavioral tolerance, the subjects received 15.0 mg/kg ip cocaine. For the experiments examining alterations in DA function, the subjects received a 0.063 mg/kg ip quinpirole injection, followed 5 min later by a 15.0 mg/kg ip cocaine injection. For all experiments, the subjects were placed in activity monitors, and ambulation was measured for 60 min. The results indicated that all continuous cocaine durations induced significant changes in cocaine-induced behavior at the 1-day withdrawal period. However, for tolerance to be exhibited on the 7-day withdrawal period, either high-dose or long-duration continuous cocaine had to be administered. This tolerance was associated with an increase in DA sensitivity. However, the change in DAs was dose- or duration-dependently related to tolerance. Overall, the literature suggests that behavioral tolerance following continuous cocaine administration may be mediated by multiple, time-dependent mechanisms that operate in an all-or-none manner.

Differential Modulation of Clock Speed by the Administration of Intermittent Versus Continuous Cocaine

The roles that psychostimulant sensitization and tolerance play in temporal perception in the seconds-to-minutes range were assessed in rats. Cocaine (20 mg/kg/day) was administered for 2 weeks either intermittently via daily injections (induces sensitization) or continuously via an osmotic minipump (induces tolerance). Interval timing was evaluated throughout administration and withdrawal. Injections of cocaine caused immediate, proportional, leftward shifts in peak times, indicating an increase in the speed of an internal clock. These shifts grew progressively larger with repeated administration, indicating that stimulant-induced increases in clock speed can be sensitized. Continuous cocaine administration produced no reliable effects. These results suggest that the mechanisms of sensitization may play a considerable role in drug-induced alterations of the perception of time.

Presynaptic regulation of dopaminergic neurotransmission

The development of electrochemical recordings with small carbon-fiber electrodes has significantly advanced the understanding of the regulation of catecholamine transmission in various brain areas. Recordings in vivo or in slice preparations monitor diffusion of catecholamine following stimulated synaptic release into the surrounding tissue. This synaptic 'overflow' is defined by the amount of release, by the activity of reuptake, and by the diffusion parameters in brain tissue. Such studies have elucidated the complex regulation of catecholamine release and uptake, and how psychostimulants and anti-psychotic drugs interfere with it. Moreover, recordings with carbon-fiber electrodes from cultured neurons have provided analysis of catecholamine release and its plasticity at the quantal level.

Evidence of cross-tolerance between behavioural effects of nicotine and cocaine in mice

RATIONALE

Studies have reported that chronic exposure to nicotine does not alter the effects of cocaine on locomotor activity, and vice versa. However, the apparent lack of effect of one drug on the behavioural response to the other may be due to an exclusive focus on locomotor activity as the target behaviour.

OBJECTIVE

To test whether repeated pretreatment with nicotine causes tolerance or sensitization to cocaine's effects on diverse behaviours: locomotion, rearing, grooming, and immobility. Similarly, the effects of repeated cocaine treatment on the acute response to nicotine were also tested.

METHODS

Mice were pretreated with 14 injections of nicotine (0.3 mg/kg), cocaine (5 mg/kg) or saline, the injections being given once daily, except for three breaks of two days each. Two days after the final pretreatment injection, mice were given a challenge injection of saline, cocaine (3 or 5 mg/kg) or nicotine (0.3 or 1 mg/kg), and observed in a large test cage for 40 min using a time-sampling procedure.

RESULTS

Repeated administration of either drug produced some tolerance to subsequent challenge with the same dose of the drug. Prior nicotine exposure significantly attenuated cocaine-induced decreases in grooming and increases in rearing, but did not significantly affect other behaviours. In contrast, prior cocaine exposure failed to alter nicotine's effects on any behaviour.

CONCLUSIONS

Cross-tolerance between nicotine and cocaine (but not vice-versa) can be demonstrated if several behaviours are observed; measures of locomotor activity are less sensitive to the effect. The asymmetrical pattern of cross-tolerance may be due to differential inhibition of dopamine uptake by the two drugs.

The effects of continuous 5-HT(3) receptor antagonist administration on the subsequent behavioral response to cocaine

A functional down-regulation of central serotonin3 (5-HT(3)) receptors represents a partial mechanism of the tolerance to cocaine induced by the continuous administration of cocaine. Blocking this down-regulation by co-administering continuous cocaine and daily injections of 5-HT(3) receptor antagonists blocks the development of tolerance. The present experiment evaluated the ability of continuously administered 5-HT(3) receptor antagonists, to induce sensitization (reverse tolerance) to the behavioral effects of cocaine, based on the hypothesis that chronic blockade of 5-HT(3) receptors should induce an up-regulation of these receptors. In all experiments, rats received a 14 day pretreatment involving the continuous administration of tropisetron (0.0, 1.0, 4.0, or 8.0 mg/kg/day) or LY 278584 (0.001, 0.01, or 0.1 mg/kg/day). The rats were withdrawn for 7 days from this pretreatment regimen. On day 7 of withdrawal from the pretreatment regiment, the rats received a 0.0, 7.5, or 15.0 mg/kg i.p. cocaine challenge. Ambulatory behavior was automatically recorded for 60 min. Both continuous tropisetron and LY 278584, opposite to the initial hypothesis, induced tolerance, and not sensitization, to the behavioral effects of cocaine. The results clearly indicate that central 5-HT(3) receptors are critical for the effects of chronic cocaine administration.

The effects of continuous cocaine duration on the induction of behavioral tolerance and dopamine autoreceptor function

The current experiment evaluated the duration-dependent nature of the induction of behavioral tolerance and changes in dopamine autoreceptor function by continuously administering cocaine for different durations. For all experiments, rats were exposed to a pretreatment regimen involving the continuous administration of 40 mg/kg/day cocaine. The pretreatment regimen lasted 3, 7, or 14 days. All subjects were then withdrawn from the pretreatment regimen for 7 days. The subjects were placed in activity monitors and ambulation measured. In experiment 1, the subjects were challenged with 0.0, 7.5, or 15.0 mg/kg i.p. cocaine on day 7 of withdrawal from the continuous cocaine administration regimen. The results indicated that all continuous cocaine durations induced significant tolerance to the 7.5 and 15.0 mg/kg cocaine challenge, relative to the control group. However, the magnitude of tolerance was not duration dependent. In experiment 2, the subjects were challenged with 0.063 or 0.125 mg/kg quinpirole. The results indicated that the 0.063 mg/kg quinpirole challenge inhibited activity in both pretreatment groups, while the 0.125 mg/kg quinpirole challenge enhanced behavior in the saline control, but not the cocaine, pretreatment group. In experiment 3, the subjects were challenged with the same doses of quinpirole in combination with 7.5 mg/kg i.p. cocaine. Both quinpirole challenge doses inhibited cocaine-induced hyperactivity. The results suggest that the induction of tolerance by continuous cocaine administration is not duration-dependent. Continuous cocaine administration did induce dopamine autoreceptor supersensitivity. Different continuous cocaine durations may induce differential degrees of dopamine autoreceptor supersensitivity.

Modulation of pre- and postsynaptic dopamine D2 receptor function by the selective kappa-opioid receptor agonist U69593

The repeated administration of selective kappa-opioid receptor agonists prevents the locomotor activation produced by acute cocaine administration and the development of cocaine-induced behavioral sensitization. Previous studies have shown that dopamine (DA) D2 autoreceptors modulate the synthesis and release of DA in the striatum. Evidence that kappa agonist treatment downregulates DA D2 receptors in this same brain region has recently been obtained. Accordingly, the present studies were undertaken to examine the influence of repeated kappa-opioid receptor agonist administration on pre- and postsynaptic DA D2 receptor function in the dorsal striatum using pre- and postsynaptic receptor-selective doses of quinpirole. Rats were injected once daily with the selective kappa-opioid receptor agonist U69593 (0.16-0.32 mg/kg s.c.) or vehicle for 3 days. Microdialysis studies assessing basal and quinpirole-evoked (0.05 mg/kg s.c.) DA levels were conducted 2 days later. Basal and quinpirole-stimulated locomotor activity were assessed in a parallel group of animals. The no-net flux method of quantitative microdialysis revealed no effect of U69593 on basal DA dynamics, in that extracellular DA concentration and extraction fraction did not differ in control and U69593-treated animals. Acute administration of quinpirole significantly decreased striatal DA levels in control animals, but in animals treated with U69593, the inhibitory effects of quinpirole were significantly reduced. Quinpirole produced a dose-related increase in locomotor activity in control animals, and this effect was significantly attenuated in U69593-treated animals. These data reveal that prior repeated administration of a selective kappa-opioid receptor agonist attenuates quinpirole-induced alterations in DA neurotransmission and locomotor activity. These results suggest that both pre- and postsynaptic striatal DA D2 receptors may be downregulated following repeated kappa-opioid receptor agonist administration. Synapse 39:343-350, 2001. Published 2001 Wiley-Liss, Inc.

The dopamine D2/D3 antagonist DS121 potentiates the effect of cocaine on locomotion and reduces tolerance in cocaine tolerant rats

To explore the significance of dopamine (DA) autoreceptors in cocaine tolerance and cocaine induced locomotor activity rats were treated with saline and cocaine (40 mg/kg per day via osmotic minipump; normal and cocaine tolerant rats, respectively). Injections of DS121 (0-7 mg/kg, i.p.; S(-)-3-(3-(cyanophenyl)-N-n-propylpiperidine), a DA D2/3 and autoreceptor preferring antagonist, either alone (i.e. DS121 + saline injection) or in combination with cocaine (7.5 mg/kg, i.p.) were also given. DS121 (+ saline) increased locomotor activity in both saline and cocaine pump (CP) treated animals. DS121 also potentiated the effect of cocaine on locomotor activity; this effect was greatest in CP (tolerant) animals. It is concluded that DS121 can increase locomotor activity and that this effect is greatest when the DA tone is high, that is when cocaine is present, suggestive of a presynaptic mechanism. Furthermore, because DS121 potentiation of cocaine induced locomotor activity is greatest in tolerant animals it is concluded that supersensitive DA autoreceptors underlie this effect. These data further support our previous data, which show that DA autoreceptors are sensitized after continuous cocaine (minipump) treatment.