Effects and interactions of naloxone and amphetamine on self-stimulation of the prefrontal cortex and dorsal tegmentum

Nicotine as an Addictive Substance: A Critical Examination of the Basic Concepts and Empirical Evidence

The present review is a critical analysis of the concepts behind and the empirical data supporting the view that tobacco use represents an addiction to nicotine. It deals with general aspects of the notion of addiction, while concentrating on specific problems associated with incorporating nicotine into current frameworks. The notion of addiction suffers from unprecedented definitional difficulties. The definitions offered by various authorities are very different, even contradictory. Definitions that reasonably include nicotine are so broad and vague that they allow many trivial things, such as salt, sugar, and watching television, to be considered addictive. Definitions that exclude the trivia also exclude nicotine. The addiction hypothesis, in general, is strongly shaped by views that certain drugs bring about a molecular level subversion of rationality. The main human evidence for this is verbal reports of smokers who say that they can't quit. On the other hand, the existence of many millions of successful quitters suggests that most people can quit. Some smokers don't quit, but whether they can't is another matter. The addiction hypothesis would be greatly strengthened by the demonstration that any drug of abuse produces special changes in the brain. It has yet to be shown that any drug produces changes in the brain different from those produced by many innocuous substances and events. The effects of nicotine on the brain are similar to those of sugar, salt, exercise, and other harmless substances and events. Apart from numerous conceptual and definitional inadequacies with the addiction concept in general, the notion that nicotine is addictive lacks reasonable empirical support. Nicotine does not have the properties of reference drugs of abuse. There are so many findings that conflict so starkly with the view that nicotine is addictive that it increasingly appears that adhering to the nicotine addiction thesis is only defensible on extra-scientific grounds.

Effects of naloxone on amphetamine induced striatal dopamine release in vivo: a microdialysis study

The opiate antagonist naloxone (NX) alters amphetamine (AMPH) induced behaviors including locomotor activity, rearing and stereotypy. However, the exact nature of the NX induced alteration of AMPH induced behaviors is controversial, with some studies using high (5-40 mg/kg) doses of NX reporting an inhibition, and others using low (< or = 1-2 mg/kg) doses observing a potentiation. As these behaviors are mediated by AMPH induced dopamine (DA) release, the effect of NX on the latter was examined by microdialysis in an effort to resolve the controversy. Saline and NX pretreated groups subsequently administered AMPH were compared in vivo across nine separate 10 min intervals as well as by grouped intervals. NX alone (0.8 mg/kg) and saline exerted statistically equivalent effects on striatal DA release with the exception of the fifth interval, where a small but significant increase was seen after NX. On the other hand, the same dose of NX significantly enhanced AMPH induced striatal DA release relative to saline pretreated animals during each of four separate intervals, from 30 to 70 minutes following AMPH (1.5 mg/kg), and across all nine intervals combined. NX pretreatment (0.8 mg/kg) followed by a higher dose of AMPH (3.0 mg/kg) produced a significantly greater cumulative effect on DA release than saline pretreatment over the last six combined intervals (30-90 min) and over two grouped intervals (30-50 min and 40-60 min inclusive). However, a comparison of single rather than paired or grouped intervals revealed no significant differences. Previous studies have also examined the effect of NX on AMPH induced striatal DA release using in vivo microdialysis. However, the doses used were invariably high (5 mg/kg) and the results on striatal DA release always inhibitory. The present results suggest that NX potentiates AMPH induced striatal DA release when lower doses of NX are used. These results combined with those of previous studies also suggest that NX exerts a biphasic effect on AMPH induced DA release, with lower doses potentiating release and higher doses inhibiting release. This is close agreement with behavioral observations and may be due to the effect of low versus high doses of NX on intraterminal calcium influx.

Influence of naloxone upon motor activity induced by psychomotor stimulant drugs

Naloxone, an opioid receptor antagonist, attenuates a wide range of behavioral effects of d-amphetamine, such as the stimulation of motor activity. To investigate the pharmacological selectivity of the naloxone/amphetamine interaction, we assessed the effects of naloxone (5.0 mg/kg SC) upon motor activity induced in rats by a range of psychomotor stimulant drugs with a mechanism of action either similar to or different from that of d-amphetamine. Each of the drugs tested caused dose-dependent increases in both gross and fine activity. Naloxone attenuated the gross but not the fine activity response to d- and l-amphetamine, but had no influence upon the other catecholamine-releasing drugs, methamphetamine and phendimetrazine. In contrast, naloxone increased the gross but not the fine activity response to the catecholamine uptake inhibitors cocaine and mazindol, but had no effects upon the motor response to methylphenidate. The responses to other stimulant drugs (apomorphine, caffeine, scopolamine) were unaffected by naloxone pretreatment. The present findings extend the range of conditions under which naloxone and, by inference, endogenous opioid systems, modulate the behavioral response to psychomotor stimulants. However, the differential effects of naloxone upon the motor response to individual stimulant drugs support previous suggestions of fundamental differences in mechanisms of action among these compounds.

Ventral tegmental self-stimulation selectively induces opioid peptide release in rat CNS

Intracranial self-stimulation (ICS) is thought to activate neuronal systems involved in processing natural reinforcing agents. Metabolic mapping studies have previously demonstrated a subset of CNS structures specifically engaged by ICS in animals receiving stimulation actively vs. passively. Since opiates are known to enhance ICS behavior and presumably its reinforcing properties, the current study addressed the question of the role of opioid peptides as mediators of ICS. Rats were trained on a fixed ration (FR) 20 schedule of responding maintained by ICS. Following response stabilization, rats were assigned either to an active or a corresponding yoked stimulation group at 1 of 2 schedules of reinforcement (i.e., FR1-YFR1, FR20-YFR20, or sedentary control), and opioid peptide release was inferred from in vivo receptor occupancy. Autoradiographic analyses identified 3 groups of structures. Treatment-induced alterations in occupancy were seen in the medial dorsal nucleus of the thalamus, basolateral amygdala, ventral pallidum, medial habenula, dorsal raphe, posterior hypothalamus, substantia nigra pars compacta, agranular preinsular cortex, and zona incerta. Depending upon the structure, peptide release was dependent upon stimulus contingency (active vs. yoked) and/or schedule (FR1 vs. FR20). Evidence for ICS-induced inhibition of peptide release was found in the habenula and preinsular cortex. Nine additional structures, all components of, or receiving projections from, the limbic system, revealed complex interactions between ICS treatment and the electrode side. Finally, a widespread ipsilateral increase in receptor binding was seen rostrally from the cingulate, olfactory tubercle, and nucleus accumbens, along the lateral hypothalamus and hippocampus, and extending caudally to the substantia nigra and ventral tegmentum. These later effects appear to be related to stimulation-induced changes in blood flow and subsequent ligant presentation increases. Collectively, these data point towards the ability of rewarding brain stimulation to activate discrete neuronal opioid systems contingent upon specific behavioral as well as stimulus conditions.

Naloxone reduces amphetamine-induced stimulation of locomotor activity and in vivo dopamine release in the striatum and nucleus accumbens

This study tested the possibility that naloxone (NX), an opioid antagonist, reduces the behavioral effects of amphetamine (AMPH) in rats by attenuating the dopaminergic response to AMPH. In the first experiment, adult, male rats were injected SC with either NX (5.0 mg/kg) or saline and 30 min later received doses of AMPH (0.0, 0.1, 0.4, 1.6, and 6.4 mg/kg) cumulatively at 30-min intervals. Gross locomotor counts following AMPH administration were significantly lower for rats pretreated with NX than for rats pretreated with saline. In the second experiment, the same drug treatments were given while performing microdialysis in either the striatum (STR) or nucleus accumbens (NACC). STR rats treated with vehicle showed a larger percentage increase in DA levels following AMPH treatment than did NACC rats treated with vehicle. NX pretreatment did not affect dopamine concentrations in either brain region. However, compared to pretreatment with saline pretreatment with NX significantly decreased the dopaminergic response to AMPH in the STR. There was no difference between the two groups in the peak dopaminergic response to AMPH in the NACC, but there was a significant AMPH x treatment x time interaction due to differences between the groups during the later portion of the response to 6.4 mg/kg AMPH. There was also a difference in locomotor activity following AMPH treatment between NX- and saline-treated subjects during dialysis. These findings suggest that a decrease in the dopaminergic response to AMPH is the mechanism by which NX attenuates behavioral stimulant effects of AMPH. In addition, there is a difference between the STR and NACC in dopaminergic responsiveness to AMPH.

Schedule-controlled brain self-stimulation: has it utility for behavioral pharmacology?

We review evidence that schedule-controlled intracranial self-stimulation (ICSS) has properties in common with conventional reinforcements, such as food and water, but unlike the latter, animals will respond for ICSS for long periods of time at a near-constant rate. Schedule-controlled ICSS has proven to be more sensitive to drug-induced changes than has ICSS on a continuous reinforcement schedule, and it permits a more fine-grained analysis of the pattern of responding that results in the reinforcement. Evidence is accumulating that the schedule of ICSS itself leads to neurochemical changes in areas of the brain, such as the nucleus accumbens, in which reward processes occur. Results obtained from schedule-controlled ICSS would complement those obtained by drug self-administration studies which generally use intermittent reinforcement. A systematic examination of ICSS schedules at different brain sites would greatly facilitate our interpretation of drug effects and this would have utility for behavioral pharmacology.

Cocaine facilitation of prefrontal cortex self-stimulation: a microstructural and pharmacological analysis

A novel self-stimulation methodology involving a fixed-interval (FI-5 s) schedule of reinforcement, microanalysis and threshold evaluation was used to investigate the effects of cocaine on rats lever pressing for electrical stimulation of the prefrontal cortex. Cocaine (15 mg/kg) increased medial prefrontal cortex (MPC) self-stimulation rates under FI-5 by a mean of 269% and reduced current thresholds for self-stimulation. A similar facilitation was evident with self-stimulation of the sulcal prefrontal cortex. Microanalysis showed that cocaine decreased inter-response times and post-reinforcement pauses, increased responding in the second and third quartiles of the inter-reinforcement interval (IRI) and decreased responding in the fourth IRI quartile. Schedule control of responding was still evident following cocaine despite the profound facilitation of response rates. Increased response rates were seen up to 48 h following a single dose of cocaine, suggesting sensitization of the PFC reinforcement substrate. The acute effects of cocaine on MPC self-stimulation were completely reversed by the dopamine (DA) D1 antagonist SCH 23390 0.02 mg/kg) and the D2 antagonist raclopride (0.3 mg/kg) but not by naloxone (0.5 mg/kg). These results are consistent with previous studies demonstrating the PFC as part of the neural substrate mediating cocaine reward. Further, these results implicate DA receptors in the reinforcing properties of both cocaine and MPC self-stimulation.

Naloxone blockade of amphetamine place preference conditioning

Amphetamine and naloxone were examined in place conditioning, in order to study possible interactions between endogenous opioids and catecholamines in reinforcement. After initial preferences were determined, animals were conditioned with amphetamine alone (1.0 mg/kg SC), naloxone alone (0.02, 0.2 or 2.0 mg/kg SC) or combinations of amphetamine plus naloxone. A reliable, long-lasting preference for the compartment associated with amphetamine was observed, reflecting the reinforcing properties of this drug. No preference or aversion was observed in animals that received saline in both compartments. Naloxone (0.02, 0.2 and 2.0 mg/kg) produced a dose-dependent place aversion; while the lowest dose had effects similar to saline, the higher doses produced significant place aversions. Naloxone, at all three doses examined, prevented the ability of amphetamine to produce a place preference. Thus, the lowest dose of naloxone, having no effects alone in place conditioning was still able to block the reinforcing effects of amphetamine. These results suggest that the reinforcing effects of amphetamine are dependent on activation of opiate receptors, and provide further evidence that interactions between endogenous opioids and catecholamines may be important in reinforcement.

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.

Effects of opiate antagonists and their quaternary analogues on nucleus accumbens self-stimulation

Naloxone and naltrexone were compared with their quaternary analogues naloxone methobromide and naltrexone methobromide for efficacy in suppressing intracranial self-stimulation behavior. These quaternary analogues effectively block opiate receptors in the periphery, but since they do not readily cross the blood-brain barrier they have little effect on central receptors. Rats with electrodes in the nucleus accumbens were trained to self-stimulate in daily 60-min sessions. Naloxone (0.2, 2.0 and 20 mg/kg) and naltrexone (20 mg/kg) potently suppressed self-stimulation behavior. In contrast, neither naloxone methobromide (0.2 and 20 mg/kg) nor naltrexone methobromide (20mg/kg) had any significant effects on this behavior. These results suggest that blockade of peripheral opiate receptors alone is insufficient to suppress self-stimulation, and therefore support the idea that opiate antagonists suppress self-stimulation by blockade of central receptors that mediate reinforcement.

Naloxone suppression of self-stimulation is independent of response difficulty

The action of the opiate antagonist naloxone on relatively easy (nose-poke) and relatively difficult (lever-press) self-stimulation behaviors was compared, in order to determine if opiate antagonists suppress self-stimulation by interfering with the ability of the animal to respond, or by reducing the reinforcement value of the stimulation. Naloxone (0.2, 2.0 and 20 mg/kg) significantly suppressed both nose-poking and lever-pressing self-stimulation rates, and the degree of suppression was virtually identical for both tasks at all doses examined. If naloxone had interfered with the ability of the animal to respond, then lever-pressing--which requires more motor output than nose-poking--should have been more suppressed than nose-poking. The results suggest that opiate antagonists do not interfere with the ability of the animal to respond, and are therefore consistent with the hypothesis that these drugs reduce the reinforcement value of the stimulation.

The effects of amfonelic acid alone and in combination with naloxone on brain-stimulation reward

Thresholds for rewarding brain stimulation delivered to the medial forebrain bundle-lateral hypothalamus were determined by means of a rate-free psychophysical method. Amfonelic acid (AFA), an indirect dopamine agonist, alone caused a significant dose-dependent lowering of the rewarding threshold. Although naloxone treatment by itself did not significantly alter the reward threshold, it blocked AFA's threshold lowering effect in every animal at one or more of the dose combinations of the two drugs tested. Naloxone was found to be more effective in blocking the threshold lowering actions of a higher (1 mg/kg) dose as opposed to a lower (0.25 mg/kg) dose of AFA. Since a lowering of threshold for rewarding intracranial stimulation is a model for drug-induced euphoria, the findings presented here indicate that AFA may have abuse potential. Furthermore, these results suggest that endogenous opioid systems may begin to modulate the effects of AFA on the reward system only when a certain level of activation of dopaminergic systems is reached.

Effects of naltrexone on nucleus accumbens, lateral hypothalamic and ventral tegmental self-stimulation rate-frequency functions

Rats trained to lever-press for electrical stimulation of the nucleus accumbens, lateral hypothalamus, or ventral tegmental area, were tested with a range of stimulation frequencies to assess the effects of naltrexone (2.5, 5.0, 10.0, 20.0 mg/kg, i.p.) during sessions beginning 15 or 45 min after injection. Naltrexone, when effective, shifted the rate-frequency functions to the right; the magnitude of the effect depended on site of stimulation and on the delay after injection. The greatest effect was observed with stimulation of the nucleus accumbens and the least with stimulation of the ventral tegmental area. There was a greater attenuation of responding during the late test sessions than during the early ones. The time course of naltrexone's effect on brain stimulation reward was determined for the highest dose by measuring a rat's rate of responding over a 3 h period in sessions with immediate access (5-min delay) or delayed access (45-min delay) to stimulation. The greatest decreases in responding were observed 45, 65, and 85 min after injection and the delay in access made little difference. The fact that the drug was more effective 45 min after injection explains some of the inconsistencies in the literature; the fact that its effectiveness was independent of early exposure to stimulation would suggest pharmacological rather than experiential factors as the explanation of the delayed effectiveness.

Opiate antagonists and rewarding brain stimulation

This review examines the literature on the effects of opiate antagonists on brain stimulation (ICSS) reward. Antagonists should have predictable effects if endogenous opioids modulate ICSS. Naloxone is the antagonist most often used, and it has produced inconsistent results in some ICSS paradigms. When schedules of intermittent reinforcement are used, however, naloxone reliably reduces the rate of responding. It reverses the effects of opiate agonists on ICSS behavior, and it also attenuates the effects of psychomotor stimulants, such as amphetamine. The results produced by naloxone are consistent with a modulatory effect of endogenous opioid systems on reward, and suggest that the opiate and dopamine systems together exert significant control over ICSS. Further research is needed to characterize better the actions of the antagonists on ICSS behavior, and productive research directions are proposed. Data obtained in future studies might suggest how the endogenous opioid systems modulate both natural and brain stimulation reward.

Brain stimulation of the ventral tegmental area attenuates footshock escape: an in vivo autoradiographic analysis of opiate receptors

An in vivo autoradiographic technique was employed to visualize discrete neuroanatomical changes in opiate receptor binding as a result of aversive footshock (FS) and rewarding electrical brain stimulation (ICS). Footshock-induced escape responding was shown to be attenuated by the simultaneous presentation of non-contingent ICS. Rats were divided into 4 groups (n = 6) receiving ICS, FS, ICS + FS or neither stimulus in an escape paradigm. During the final behavioral test session, rats were injected with 0.002 mg/kg [3H]diprenorphine ( [3H]Dpr) and subsequently prepared for autoradiography. Results indicated two groups of brain areas distinguishable by their treatment-induced changes in [3H]Dpr binding. One group of areas included the nucleus accumbens, claustrum, claustrocortex, perirhinal cortex and ventral tegmental area. These structures showed increased binding due to both FS and ICS. The other group consisted of the diagonal band of Broca, bed nucleus of the stria terminalis, lateral hypothalamus-medial forebrain bundle and amygdala. In these regions, an increase in binding ipsilateral to the electrode was observed in animals receiving ICS with no apparent effect of FS. These results demonstrate that non-contingent ICS may not be strictly aversive and suggest an anatomic, opioid-sensitive basis for both a rewarding and aversive component of this stimulus. It appears, further, that ICS can inhibit the release of endogenous opioid peptides in areas along the mesotelencephalic dopamine pathways, possibly to regulate the activity of neurons conveying reward information. Finally, the observed changes in opiate receptor binding may indicate a mechanism for ICS to produce both drive and reward.

The interaction of d-amphetamine and naloxone differs for rats trained on separate fixed-interval or fixed-ratio schedules of reinforcement

The effects of d-amphetamine and naloxone were investigated using two groups of rats trained on either an FR30 or F12 schedule of reinforcement. Amphetamine (0.1-1.0 mg/kg), and naloxone (1.0 and 10 mg/kg) administered separately reduced responding on the FR procedure in a dose-dependent manner. The combined administration of naloxone with amphetamine had an additive suppressive effect on responding. The same doses of amphetamine and naloxone, when given separately, did not significantly depress responding in the FI procedures. However, naloxone/amphetamine combinations produced a marked inhibition of lever-pressing. Naloxone did not alter the characteristic pattern of responding engendered by amphetamine in this schedule, as measured by the quarter-life and Index of Curvature. It appears that the type of procedure used is a critical factor in demonstrating the effects of naloxone on behavior, and the nature of naloxone/amphetamine interactions.

Acquisition of intracranial self-stimulation in medial prefrontal cortex of rats facilitated by amphetamine

Two groups of rats were trained to lever press for intracranial self-stimulation (ICSS) in the medial prefrontal cortex (mPFC) using a uniform amount of stimulation for all animals. One group acquired the lever pressing task very gradually during saline pretreatment but dramatically improved its rate of acquisition during the third week of training when pretreated with d-amphetamine (0.5 mg/kg). Administration of amphetamine to the other group of rats before each of the first five training sessions greatly facilitated acquisition of the ICSS task, and a significant improvement in performance over the saline control group appeared on the third day of training. After ICSS performance had stabilized, testing the animals revealed a significant amphetamine-induced increase in rate over the dose range of 0.25 to 1.0 mg/kg. These effects of amphetamine suggest that ICSS in mPFC is sensitive to changes in catecholamine neurotransmission during both the acquisition and maintenance of this behavior.

Opioid-receptor blockade reduces nose-poke self-stimulation derived from medial entorhinal cortex

Rats were trained to nose-poke for intracranial self-stimulation (SS) with electrodes unilaterally implanted in the medial entorhinal cortex. The acute effects of naloxone (NX; 0.1-10 mg/kg, IP) on a continuous reinforcement schedule were determined. Reductions in the self-stimulation rates occurred only at moderate doses (median of individual changes = -36% at 1 and 5 mg/kg), whereas the high dose (10 mg/kg) was ineffective. None of the doses influenced operant behavior. These results are consistent with the hypothesis that endogenous opioid-opiate receptor mechanisms play a modulatory role in SS reward. Considering that NX was administered systemically the action of the drug on reinforcement levels may be mediated by a site distinct from the locus of stimulation.

Effects of opioid antagonists and their quaternary derivatives on locomotor activity and fixed ratio responding for brain self-stimulation in rats

Rats were implanted with stimulating electrodes aimed at the midbrain-central gray area (MID-CG) and trained to lever press for brain stimulation (ICSS) on a fixed ratio:30 (FR:30) schedule of reinforcement. When response rates were stable, animals were administered either naloxone hydrochloride, naltrexone hydrochloride, naloxone methobromide or naltrexone methobromide in a dose range of 0.1-30 mg/kg. Fifteen minutes after the subcutaneous administration of either drug or vehicle, animals were tested for 45 min in the ICSS procedure and changes in response rates following drug administration were compared with those following vehicle administration. Both naloxone and naltrexone hydrochloride produced graded decreases in responding over the entire dose range, while naloxone and naltrexone methobromide did not alter response rates at any dose level. In a separate testing procedure, 30 mg/kg naloxone and naltrexone hydrochloride produced modest reductions in motor activity, while the methobromide derivatives did not. These results demonstrated that the fixed ratio procedure was sensitive to changes in responding for ICSS produced by opioid antagonists, and this effect depends upon the entry of these opioid antagonists into the brain.

Effects of intracranial self-stimulation on brain opioid peptides

The neurochemical system(s) underlying brain stimulation reward (ICSS) has been investigated for many years. The catecholamine hypothesis is currently most accepted with predominant emphasis on the role of dopamine. The present report examines the role of three opioid peptides--Methionine and Leucine Enkephalin (ME and LE) and beta-Endorphin (beta-E) in this behavior. Peptide levels from pituitary, hypothalamus and whole brain were determined by independent RIAs and analyzed according to treatment: low, moderate and high ICSS responders, sham controls, animals receiving nonspecific stimulation, and naloxone--with and without ICSS. Not only did naloxone reduce ICSS from high responders by 74%, it also was able to reduce peptide levels--most notably for ME and beta E in most regions. Additionally, the effects of ICSS on endorphin levels was found to be related to the rate category of responding. Since endorphins are known to interact with dopamine systems, it is therefore considered likely that the endogenous opioid peptides play an important role in ICSS either directly or indirectly via their influence on catecholamine systems.

Opioid peptides and self-stimulation of the medial prefrontal cortex in the rat

The possible involvement of opioid peptides as part of the neurochemical substrates of self-stimulation (SS) in the medial prefrontal cortex (MPC) of the rat was investigated in two different groups of rats bilaterally implanted with monopolar electrodes in the MPC. In the first group, morphine (5, 10, and 20 micrograms) and an enkephalin analogue (BW 180) (5, 10, 20 and 40 micrograms) were injected through cannulae implanted into the lateral ventricles (IV). In the second group, naloxone (0.04, 0.4, and 1.6 micrograms) and morphine (5, 10 and 20 micrograms) were injected through cannulae implanted into the MPC, 1.5 mm above the tip of the stimulating electrodes. In the first group, spontaneous motor activity (SMA) was measured as a control for non-specific effects (sedation or motor dysfunction). In the second group SS, contralateral to the microinjected side, served as control. SS and SMA were were measured 1 and 2 h postinjection. One hour after IV injection of morphine SS was not affected, although SMA was decreased. Two hours postinjection, on the contrary, SS was increased while SMA remained decreased. Similar effects were found with IV microinjections of BW 180. Naloxone, intraperitoneally injected, reversed all these effects. Naloxone or morphine injected intracerebrally (MPC) produced no changes in SS either in the injected or in the contralateral side, which served as control. The present results suggest that the effects found with IV injections of opioids on SS of the MPC are indirect (through activation of other brain areas) and not mediated by a direct action on the neurochemical substrates underlying this behaviour in the MPC.

Increasing the work requirements lowers the threshold of naloxone for reducing self-stimulation in the midbrain of rats

Rats were trained to lever-press for intracranial self-stimulation (ICSS) with electrodes in the midbrain central gray area. The effects of naloxone (0.1-30.0 mg/kg, SC) on a continuous reinforcement (CRF) schedule were determined. Rats were then re-trained on higher fixed-ratio (FR) schedules, and naloxone was re-tested at FR: 5, 10, 15 and 20. Only moderate reductions in lever-pressing rates were obtained at the highest dose of naloxone under CRF and FR: 5 schedules. In contrast, pronounced, dose-dependent reductions in ICSS rates occurred at FR: 10, 15 and 20. The time-course for this reduction at FR: 20 was consistent with an opiate-antagonistic action of naloxone. The modest decrease in locomotor activity produced by naloxone in a matched group of control rats was not sufficient to account for the effects on ICSS. The threshold of naloxone for reducing the rate of ICSS lever-pressing was lowered by increasing the effort and/or time requirement for each reinforcement.

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.