Pentobarbital induces a naloxone-reversible decrease in mesolimbic self-stimulation threshold

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

Naltrexone attenuates cue- but not drug-induced methamphetamine seeking: a possible mechanism for the dissociation of primary and secondary reward

The present study was aimed to clarify the role of the opioid system in the reinstatement of methamphetamine (METH)-seeking behavior in METH self-administering rats. Following 12 days of self-administration of METH, the replacement of METH with saline resulted in a gradual decrease in lever press responses (extinction). Under extinction conditions, METH-priming or re-exposure to cues previously paired with METH infusion markedly increased the responses (reinstatement of drug-seeking). Naltrexone administered 30 min before re-exposure to METH-associated cues attenuated reinstatement of drug-seeking behavior. On the other hand, administration of this antagonist had no effect on the reinstatement induced by METH-priming. We discussed these findings in relation with the dissociation of primary and secondary reward, suggesting that an opioid mechanism is responsible for this dissociation. Further, these results indicate the possibility of using naltrexone as an anti-relapse agent.

Systemic and intracerebroventricular administration of sodium barbital induced a place preference in rats

We have shown previously that 15 mg/kg pentobarbital induces a conditioned place preference (CPP), but it is unsuitable for intracranial administration. Since the long-acting barbiturate, sodium barbital, is soluble at a neutral pH, we tested whether it would induce a CPP when administered centrally. Furthermore, because barbital has a long duration of action, and because we obtained a significant CPP to systemically administered barbital using 30-minute conditioning trials, we tested whether longer conditioning trials would produce a more robust CPP. Using a three-compartment apparatus and an unbiased procedure, we found that systemic administration of barbital induced a significant CPP at 8 and 24 mg/kg, but not 2.7 or 72 mg/kg (i.p.). When rats were conditioned to 24 mg/kg barbital for conditioning trials of (1/2), 1, 3, or 6 hours, only the 30-min conditioning trial produced a CPP. Finally, 240 and 480 microg intracerebroventricular (ICV) barbital induced a significant CPP, but 60 or 120 microg did not. These findings suggest that: (1) like pentobarbital, barbital has reinforcing properties measured in the CPP test; (2) the CPP is impaired, rather than enhanced, by increasing the duration of drug-context pairing; and (3) the reinforcing effects of barbiturates are centrally mediated.

Reinforcing versus anticonvulsant drugs: effects on intracranial self-stimulation rate-frequency M50 indices

Drugs of abuse, such as amphetamine and morphine, produce reward-related shifts on intracranial self-stimulation (ICSS) thresholds. The facilitatory effects on ICSS thresholds of drugs that act through the GABAergic system, however, are reported to be attributed to their antiseizure and anticonvulsant effects, rather than their reinforcing effects. Using a rate-frequency ICSS paradigm, we examined the effects of amphetamine (a reinforcing drug of abuse that acts via the catecholaminergic system), pentobarbital (a GABA(A) receptor agonist and reinforcing barbiturate with anticonvulsant properties), and gabapentin (a nonspecific GABAergic agonist and anticonvulsant with low abuse potential) on ICSS M(50) indices. All three doses of amphetamine (0.5, 1.0, and 2.0 mg/kg) and pentobarbital (2.5, 5.0, and 10.0 mg/kg) significantly lowered rate-frequency M(50) values. Gabapentin, on the other hand, significantly raised rate-frequency M(50) values, albeit only at the highest dose administered (30 mg/kg). Our results indicate that shifts in ICSS M(50) values produced by pentobarbital are associated with the reinforcing, not the anticonvulsant, effect of pentobarbital. These results are consistent with the view that there is a common system underlying the reinforcing effects of drugs and ICSS reinforcement, and suggest that the reinforcing and anticonvulsant effects of GABA agonists are dissociable.

Facilitation of electrical brain self-stimulation behavior by abused solvents

Animal models are needed to study the abuse-related behavioral and pharmacological effects of inhaled solvents. Previous studies have suggested that intracranial self-stimulation techniques may be successfully adapted for testing the effects of solvent exposure. The present study aimed to assess the effects of toluene, cyclohexane, acetone, and petroleum benzine (a widely used mixture of hexanes and heptanes) in rats trained to lever press or nose-poke for electrical stimulation delivered through electrodes implanted into the medial forebrain bundle. It was found that toluene, cyclohexane, and benzine but not acetone, increased rates of responding, particularly at the lower stimulation intensities. In another set of experiments utilizing an auto-titration procedure, all tested solvents significantly reduced self-stimulation thresholds. However, only for toluene and benzine were these effects observed at the exposure levels that did not impair rates of operant performance. There may not be such a clear separation of effects for acetone and cyclohexane. Thus, toluene and benzine appear to selectively affect brain reward systems in a manner similar to that for most other abused drugs. Data from intracranial self-stimulation studies of solvents may be useful in abuse potential assessment of individual compounds and for examining neural and behavioral processes involved in inhalant abuse.

Parametric changes in response equilibrium during an intra-cranial self stimulation (ICSS) task: can reward value be assessed independently of absolute threshold?

Traditional ICSS methodologies have attempted to evaluate changes in the rewarding value of brain stimulation by assessing the lowest value of the stimulation that will support responding. However, orderly changes in suprathreshold indicants of hedonic magnitude such as titration point have been shown. In the present experiments, rats were trained to respond on two ICSS autotitration schedules in which every response on one lever produced stimulation of the medial forebrain bundle, and every Xth response decreased either the stimulation current or the stimulation frequency. At any time, a response on a second "reset" lever restored the stimulation current or frequency available on the stimulation lever to its starting level and operationally defined changes in "reward value". In order to study this titration point measure, two response requirements (responses/stepdown; step size) and two stimulation parameters (initial stimulation level; train duration) were systematically varied. Under both current and frequency titration schedules, data indicated that response rate and titration point remained stable over repeated trials and multiple testing days--parameters being constant. Across all conditions, compared to the frequency titration schedule, subjects responding under the current titration schedule showed significantly higher titration points and lower rates of responding. Indicating the independence of rate and titration point data, parametric manipulations did not affect titration point and rate data concurrently. Results support the conclusion that titration point is a relative measure of "reward value" that is generally independent of response rate, but that is affected by manipulations that alter the amount of stimulation available between "resets". Additional work is needed in order to determine the relationship between the magnitude of stimulation needed to maintain minimal responding and that needed to maintain response equilibrium in an autotitration task.

Facilitory effect of delta 9-tetrahydrocannabinol on hypothalamically induced feeding

Six male Lewis rats were tested for the effect of delta 9-tetrahydrocannabinol (delta 9-THC) on feeding evoked by electrical stimulation of the lateral hypothalamus. Treatment with delta 9-THC (0.4 mg/kg IP) decreased frequency threshold for feeding by 20.5% (+/- 4.3), causing a leftward shift in the function relating stimulation frequency to the latency to begin eating 45-mg food pellets upon stimulation onset; there was no change in the asymptotic performance that was approached with sufficiently high stimulation frequencies. Naloxone (1 and 2 mg/kg) reduced the facilitory effect of delta 9-THC, but did so at doses that can inhibit feeding in the no-drug condition. These data are consistent with evidence implicating endogenous opioids in feeding, and suggest (but do not confirm) that the facilitation of feeding by delta 9-THC may be mediated by endogenous opioids. The facilitation of stimulation-induced feeding by doses of delta 9-THC that have been found to facilitate brain stimulation reward is consistent with evidence suggesting common elements in the brain mechanisms of these two behavioral effects of medial forebrain bundle stimulation.

Opiate antagonists and self-stimulation: extinction-like response patterns suggest selective reward deficit

The present study investigated the response decrement patterns produced by opiate antagonists on intracranial self-stimulation behavior, in order to determine if these drugs affect the reinforcement value of the stimulation or interfere with the ability of the animal to respond. Male rats lever-pressed in 60-min sessions on a continuous reinforcement schedule for self-stimulation of the nucleus accumbens. Naloxone (2.0 and 20 mg/kg) and naltrexone (2.0 and 20 mg/kg) suppressed self-stimulation only after a significant delay, in an extinction-like response decrement pattern, mimicking the effects of reductions in current intensity (75% and 50% of baseline). The increasing behavioral effects characteristic of the extinction pattern were observed despite the fact that testing began after the time point at which maximal suppression of self-stimulation occurs with these drugs, and when brain concentrations of these drugs were declining. Since normal responding was observed for several minutes after the beginning of the session, the results may explain why long sessions are necessary to observe suppression of self-stimulation by opiate antagonists. The extinction-like pattern produced by these drugs suggests that opiate antagonists suppress self-stimulation by reducing the reinforcement value of the stimulation, rather than by interfering with the ability of the animal to respond. These findings are consistent with a role for endogenous opioid peptides in brain stimulation reward.

Anxiety, anxiolytics and brain stimulation reinforcement

The premise of this review is that neuronal substrates of anxiety are amenable to investigation using brain stimulation techniques. Anxiolytics such as meprobamate and the benzodiazepines may enhance intracranial self-stimulation (ICSS) behavior. Although demonstrated by numerous investigators, this effect shows considerable variability between and within laboratories. Some of this variability is explained by sedative/muscle relaxant effects, which are dissociable from drug-induced increases in ICSS and which may mask these increases. The anticonvulsant actions of anxiolytic drugs are unlikely to account for the increases in ICSS. Rather, anxiolytics appear to increase ICSS by attenuating concurrent aversive properties of stimulation. Consistent with this explanation, anxiolytic drugs attenuate escape from aversive dorsal tegmental stimulation. The neuronal substrates of this centrally mediated escape behavior differ from those mediating footshock-induced escape. Barbiturates also enhance ICSS, possibly due in part to an excitatory component that is not involved in benzodiazepine action. Inverse benzodiazepine agonists attenuate ICSS behavior in a manner that cannot be explained by nonspecific performance impairment. These substances, however, may not necessarily enhance stimulation-induced aversiveness. A strategy is proposed to integrate brain stimulation studies with molecular approaches to anxiety. Specifically, stimulation of sites associated with fear induction or fear reduction may selectively alter the release of endogeneous anxiogens or anxiolytic substances.

Discriminating between reward and performance: a critical review of intracranial self-stimulation methodology

Despite numerous pharmacological investigations of intracranial self-stimulation (ICSS), the substrates of this behavior have yet to be completely understood. In view of the likelihood that inadequate methodology has hindered the quest for these substrates, the present review was undertaken. Criteria for ICSS methodology should include not only the ability to discriminate reward from gross performance deficit, but also adequate capacity (ability to generate experimental data at a reasonable rate). For numerous reasons, bar-pressing on a continuous reinforcement schedule fails the first criterion despite its ease and rapidity. The use of partial reinforcement schedules may alleviate some of these shortcomings. Analysis of drug-induced response decrement patterns can discriminate gross motoric incapacity from other variables, although the question of subtle response maintenance deficits remains to be answered. Measurements of response rates using alternative operants do not differentiate reward and performance adequately. More promising, "rate-free" measures using locomotion as an operant include the two-platform method of Valenstein and the "locus of rise" method. Comparison of drug effects on ICSS with those on alternate tasks are fraught with pitfalls including the problems of assuring equivalent rates and patterns of responding. The use of differential electrode placements is ideally suited for neurochemically well-characterized drugs, particularly if "double dissociations" can be established during studies of multiple placements. Presentation of different current intensities or frequencies permits the compilation of rate-intensity functions, and drug-induced shifts in these functions have considerable analytical power. Self-regulation of current intensity constitutes a powerful tool that has yet to realize its full potential in the pharmacological study of ICSS. Extensive studies involving self-regulation of stimulation duration ("shuttlebox" studies) suggest that this method may be highly versatile despite several practical difficulties. It is concluded that at least six of these methods appear to do a reasonable job of excluding gross performance deficit. However, the possible influences of other factors, such as subtle response maintenance deficit, incentive or arousal, remain to be resolved in view of the multifactorial nature of ICSS. Multiple tests for ICSS drug or lesion studies are advocated whenever feasible, as no single test appears capable of resolving all theoretical complexities.

Effects of naloxone and naltrexone on the increased water intake and drinking duration in phenobarbitone-treated rats

In two experiments, phenobarbitone sodium (30 mg/kg) reliably enhanced water consumption and extended the duration of drinking in 24h water-deprived male rats. The opiate receptor antagonists naloxone (0.1-10 mg/kg) and naltrexone (0.1-10 mg/kg) both decreased water intake and reduced the duration of drinking. When the barbiturate was given in conjunction with either naloxone or naltrexone, phenobarbitone and the opiate antagonist exerted opposite effects on the two measures of drinking. While it was true that both opiate antagonists reduced water intake and drinking duration in barbiturate-treated animals, the barbiturate-induced enhancement of drinking was in no way modified by concurrent opiate antagonist treatment. Hence, the effects of phenobarbitone and of the two opiate antagonists upon the drinking measures appeared to be quite independent. There was no evidence, therefore, that the effects of phenobarbitone upon drinking were related to endogenous opioid mechanisms. The possible contrast between benzodiazepine- and barbiturate-induced hyperdipsia is briefly considered in the light of these results.