Neural mechanisms of tolerance to the effects of cocaine.

Chronic use of cocaine in high doses can produce tolerance as assessed by various behavioral, neurochemical, cellular and molecular measures in specific brain regions. Tolerance to cocaine is indicated by drug discrimination and intracranial self-stimulation models, which show the development of tolerance after approximately 1 week of frequent cocaine treatment, with recovery after a similar period of cocaine abstinence. Tolerance to the reinforcing properties of cocaine depends on dose, duration and frequency of cocaine self-administered by experimental animal or human subjects. The mechanism underlying this effect may involve an absolute or relative attenuation of dopamine response to cocaine challenge after frequent or repeated treatment in the nucleus accumbens (NAc). Similarly, afferent and efferent NAc circuits exhibit reduced metabolic activity, which lasts throughout the early period of withdrawal following repeated treatment. Attenuation of immediate early gene response also occurs, which might be related to a functional desensitization of dopamine D1-like receptors. Furthermore, intracellular adaptive responses to chronic cocaine exposure induce striatal dynorphin expression decreasing the behavioral potency of subsequent drug treatment. Thus, a combination of various pharmacodynamic mechanisms and the attenuation of dopamine response induced by sufficient dose, duration and frequency of cocaine exposure ultimately invoke the transient development of tolerance to the reinforcing effects of cocaine.

Tolerance to the discriminative stimulus and reinforcing effects of ketamine.

In order to examine whether tolerance develops to the discriminative stimulus and reinforcing effects of ketamine, rats were trained either to discriminate ketamine (10mg/kg) from saline or to self-administer ketamine (1.1mg/kg/injection), and then treated with chronic ketamine (32mg/kg), administered i.p. every 8 hours for 7 days. No shift in the dose-response curve for either paradigm was obtained following this chronic regimen. However, following a 2-week rest period in which animals had no exposure to ketamine, the dose-response curve was shifted two-fold to the left, indicating increased sensitivity to the drug. Reinstatement of training shifted the dose-response curve back to the right in both paradigms. These results suggest that tolerance to the discriminative stimulus and reinforcing effects of ketamine develops during training. Examination of the self-administration training data support this assumption, since inter-reinforcer time decreases, reflecting an increase in ketamine intake over training sessions.

Dopamine release during cocaine self-administration in rats: effect of SCH23390.

This experiment tested the hypotheses that: (1) self-administration of cocaine would produce an increase in dopamine (DA) oxidation current in the nucleus accumbens (n. acc.); and (2) a faster rate of cocaine intake in the presence of a D1 receptor antagonist would produce a greater increase in DA levels. Rats trained to self-administer cocaine under a fixed-ratio 2 schedule were implanted with stearate-modified graphite paste electrodes bilaterally in the n. acc. The effect of pretreatment with the D1 receptor antagonist, SCH23390, on the DA oxidation current associated with self-administration of cocaine (1 mg/inj.) or saline was investigated using chronoamperometry. Pretreatment with SCH23390 produced a 2-fold increase in the amount of cocaine intake. This in turn resulted in a 2-fold increase in the DA oxidation current recorded in the n. acc. Pretreatment with SCH23390 did not, however, produce any significant change in either the number of saline injections received or the DA oxidation current recorded during saline self-administration. These findings show that cocaine increases DA oxidation currents in the n. acc., and that both the rate of cocaine self-administration and the magnitude of these currents increase even further following SCH23390. The results also imply that the baseline rate of cocaine self-administration does not result in the occupation of all possible DA transporter sites.

Comparison of changes in extracellular dopamine concentrations in the nucleus accumbens during intravenous self-administration of cocaine or d-amphetamine.

Changes in extracellular concentrations of dopamine (DA) were measured in vivo in the nucleus accumbens of the rat during intravenous self-administration of either cocaine (0.25, 0.5, 1.0mg/infusion) or d-amphetamine (0.05, 0.1, 0.2mg/infusion). Drug intake was limited to 12 self-administered infusions per session for each drug/dose combination. Changes in extra-cellular DA concentrations were measured by two different techniques: chronoamperometry in conjunction with chronically-implanted stearate-modified carbon paste electrodes, or intracerebral microdialysis with off-line analyses using high performance liquid chromatography with electrochemical detection (HPLC-ED). Significant increases in extracellular DA concentrations were observed with both in vivo techniques during self-administration of each dose of cocaine or d-amphetamine. For each drug, the magnitude of change during the first hour of the test session was comparable across doses. However, the change observed over the first 2h period, as measured by microdialysis and HPLC-ED, revealed a dose effect for cocaine, but no dose-response effect for d-amphetamine. The duration of the drug-induced elevation was increased significantly as a function of dose with both cocaine and d-amphetamine. Data from the microdialysis experiments indicated that the high dose of d-amphetamine (0.2mg/infusion) produced a significantly greater increase in extracellular DA concentrations in the nucleus accumbens than did the high dose of cocaine (1.0mg/infusion), but that comparable changes were induced by doses of 0.1mg/infusion of d-amphetamine and 1.0mg/infusion of cocaine, respectively. Each dose of both psychostimulant drugs also produced a significant decrease in dihydroxyphenylacetic acid (DOPAC) levels. The latter finding indicated that the electrochemical signal measured in these studies was not due to the oxidation of DOPAC. These results confirm that self-administration of cocaine or d-amphetamine by the rat is accompanied by a significant increase in extracellular DA concentrations in the nucleus accumbens. The fact that two different psychomotor stimulant drugs of abuse have qualitatively similar neurochemical correlates when self-administered, adds credence to the hypothesis that their reinforcing properties are related to dynamic changes in DA concentrations in the ventral striatal region of the brain.

Discrimination of ethanol and of diazepam: differential cross-tolerance.

These experiments tested the hypothesis that cross-tolerance between ethanol (EtOH) and diazepam would occur in a drug discrimination paradigm. One group of rats (n = 28) was trained to discriminate EtOH (1.0g/kg, i.p.) from vehicle; another group of rats (n = 10) was trained to discriminate diazepam (5.6mg/kg, i.p.) from vehicle. Subjects were trained using a two-lever choice procedure where food was delivered under a fixed-ratio 10 schedule of reinforcement. In rats trained to detect EtOH, both EtOH (0.1-1.78g/kg) and diazepam (0.32-10mg/kg) dose dependently substituted for EtOH. Chronic administration of EtOH (6.8g/kg/12h for 7 days) resulted in 3-fold tolerance to EtOH and 6-fold cross-tolerance to the ability of diazepam to substitute for EtOH; chronic administration of diazepam (20mg/kg/8h for 7 days) failed to confer cross-tolerance to EtOH nor did it produce tolerance to the ability of diazepam to substitute for EtOH. In rats trained to detect diazepam, diazepam (0.56-10mg/kg) but not EtOH (0.1-1.78g/kg) substituted for diazepam. Chronic administration of diazepam (20mg/kg/8h for 7 days) produced 3-fold tolerance to diazepam; in contrast, chronic administration of EtOH (6.8g/kg/12h for 7 days) failed to confer cross-tolerance to diazepam. The dissociation of the cross-substitution and cross-tolerance patterns between EtOH and diazepam suggests that the population of benzodiazepine receptors that mediates substitution of diazepam for EtOH differs from the population of benzodiazepine receptors that mediates substitution of diazepam for diazepam.

Effects of m-cholinoceptor antagonists on the neuromuscular junction.

The effects of four m-cholinoceptor antagonists and d-tubocurarine on carbachol- or nicotine-induced contractions of frog rectus abdominus muscle were investigated. The n-cholinoceptor-mediated contractile responses of frog rectus abdominis muscles were inhibited competitively by d-tubocurarine whereas m-cholinoceptor antagonists inhibited them noncompetitively. Antagonistic potencies for m-cholinoceptor antagonists, assessed by pD'2 values, indicated a rank order of potency which was 4-DAMP > AF-DX 116 > atropine > pirenzepine. The antagonistic potencies of m-cholinoceptor antagonists on electrically induced twitch responses of rat phrenic nerve-diaphragm preparation were also examined. These responses were blocked by 4-DAMP, but pirenzepine had no effect. However, they were enhanced by AF-DX 116 or atropine. Inhibitory action of 4-DAMP in this preparation was suggested to be due to noncompetitive blockade of postjunctional n-cholinoceptors.

Epithelial modulation of antigen-induced tracheal smooth muscle contractions in actively sensitized guinea-pigs.

Coaxial bioassay system (guinea-pig trachea and rat anococcygeus muscle as donor and bioassay organs, respectively) and tracheal open ring preparations from ovalbumin-sensitized guinea-pigs were used to investigate the role of the epithelium in response to the antigen challenge. Ovalbumin induced concentration-dependent relaxations in the phenylephrine precontracted rat anococcygeus muscle suspended in the lumen of sensitized guinea-pig tracheal tube preparations in the presence of either indomethacin or mepacrine. Removal of the epithelium abolished ovalbumin-induced relaxation responses. In tracheal open ring preparations, epithelium removal shifted the ovalbumin concentration-response curve to the left in both the control and mepacrine-treated experiments. Indomethacin significantly potentiated the ovalbumin-induced contractions in epithelium-intact preparations, while in epithelium-denuded ones no significant change was observed. In the presence of indomethacin, the maximum contractile responses were significantly higher in epithelium-intact preparations than in epithelium-denuded ones. These results provide direct evidence for the release of epithelium-derived relaxant factor(s), which is (are) neither a cyclooxygenase nor a lipoxygenase product, in response to the antigen challenge. In addition, the observation of the increased maximum contraction to antigen in epithelium-intact preparations in the presence of indomethacin, suggests the involvement of contractile influences derived from the epithelium in antigen-induced responses of airways.