Post-weaning housing conditions influence the behavioural effects of cocaine and d-amphetamine

Post-weaning social isolation can induce profound and long lasting effects on an animal’s behaviour. The present study investigated the influence of post-weaning housing conditions on the sensitivity of rats to the behavioural effects of d-amphetamine and cocaine. The locomotor stimulant effects of both drugs were compared following acute and chronic administration. The influence of post-weaning housing conditions on the effects of d-amphetamine and cocaine on responding for food and for a conditioned reinforcer were also examined. Isolated rats showed enhanced locomotor activity on exposure to a novel environment. This difference was further exaggerated following administration of d-amphetamine (0.5 mg/kg) and cocaine (5 mg/kg). Isolated, but not enriched, rats exhibited sensitisation to the locomotor activating effects of repeated administration of a dose of 0.5 mg/kg d-amphetamine, whilst both groups sensitised equally to a dose of 1.0 mg/kg d-amphetamine. Rearing conditions did not affect sensitisation to cocaine (5, 10 mg/kg). Isolated rats exhibited a higher rate of responding for a conditioned stimulus and for food on a progressive ratio schedule of reinforcement, both of which were enhanced to a greater extent in isolates following administration of cocaine (5 mg/kg) and d-amphetamine (0.5 mg/kg). These results suggest that isolation rearing induces an enhancement in sensitivity to both the locomotor stimulant and reinforcing properties of amphetamine and cocaine. Received: 12 June 1996 / Final version: 17 October 1996     

Enhanced acquisition of discriminative approach following intra-amygdala d-amphetamine

This study examined the role of the mesoamygdaloid dopamine projection in stimulus-reward learning. Bilateral post-session intra-amygdala microinjections of d-amphetamine were carried out in rats during training in a discriminative approach task known to be sensitive to experimental manipulations of the amygdala. The experiment consisted of two phases: discriminative approach training, and a subsequent assessment of instrumental conditioned reward efficacy. During discriminative approach training, subjects were trained to associate a neutral stimulus with 10% w/v sucrose reward. Each trial consisted of a 1-s light stimulus followed by a 5-s presentation of the sucrose reward. Approach behaviour into the recess housing sucrose reward was measured during each trial. Inappropriate approach behaviour (approach outside of the trial periods) was punished by delaying the next trial. Intra-amygdala d-amphetamine (10 μg/side) enhanced the rate of acquisition of discriminative approach behaviour. This effect was most evident early during training (sessions 2–4) and by the tenth session both groups had reached similar asymptotic performance. Horizontal and vertical activity increased slightly across sessions, but there was no indication of a differential effect of d-amphetamine. Thus, intra-amygdala microinjections of d-amphetamine enhanced selectively the acquisition of the stimulus-reward association. During a subsequent test of instrumental conditioned reward, presentation of the conditioned light stimulus was made contingent upon performance of a novel lever-pressing response (probability 0.5). Responding on a second, control lever was without programmed consequences. Sucrose reward was not available at any point, and subjects were tested drug-free. In both groups the conditioned stimulus was found to possess significant conditioned rewarding efficacy. Extraneous behaviour was increased in the d-amphetamine group but the rewarding properties of the conditioned stimulus were unaltered. These findings demonstrate that the mesoamygdaloid dopamine projection modulates the acquisition of a stimulus-reward association, but is apparently without subsequent effect on the rewarding efficacy of a conditioned stimulus. Received: 24 October 1996/Final version: 28 February 1997     

Post-session sulpiride infusions within the perifornical region of the lateral hypothalamus enhance consolidation of associative learning

Whilst neurons within the lateral hypothalamus are well known to be responsive to the presentation of previously learned associative stimuli, the consolidation of a Pavlovian association is thought to depend in large part upon other brain regions, including the amygdala. The present study addressed this assumption directly, by examining the effect of post-session infusions of sulpiride within the lateral hypothalamus upon the acquisition of a conditioned approach response in an appetitive differential conditioning task. Subjects were exposed to an initially neutral stimulus (CS⁺), which immediately preceded the availability of a 10% sucrose reward (US). A second, control stimulus (CS⁻) was also presented, but never in close temporal proximity to the US. The number and duration of alcove approaches were recorded. Immediately following each training session, subjects were infused bilaterally with sulpiride (0, 0.5, 5 μg) in the vicinity of the perifornical region of the lateral hypothalamus. Sulpiride dose-dependently enhanced the rate of acquisition of a conditioned approach response to presentation of the CS⁺, but was without affect upon approach behaviour during CS⁻ or US presentations. Thus, 0.5 μg sulpiride facilitated at an early stage (session 2 onwards) the number of alcove approaches to the CS⁺, while 5 μg sulpiride enhanced to a greater extent the duration of conditioned approach, particularly during later sessions. A subsequent locomotor test using 0.5 mg/kg d-amphetamine indicated that repeated infusions of the higher dose sulpiride (5 μg), but not the lower dose (0.5 μg), resulted in behavioural sensitisation to administration of the psychomotor stimulant. Acquisition of a novel conditioned instrumental response was not affected by previous exposure to sulpiride. These data suggest that dopamine-sensitive neurons within the lateral hypothalamus may play a significant role in the acquisition of appetitive Pavlovian associations. Received: 2 January 1998/Final version: 16 April 1998     

Differential effects of ondansetron and α-flupenthixol on responding for conditioned reward

Previous experiments have suggested that 5-HT₃ antagonists such as ondansetron may alter reward-related behaviour that is dependent in part upon raised mesolimbic dopamine activity. However, the evidence for this is far from conclusive. One major behavioural role of dopamine is in the control of behaviour elicited by conditioned rewarding stimuli. To date, the effects of 5-HT₃ antagonists on this function of mesolimbic dopamine have not been examined. Two experimental procedures were employed to examine the effects of ondansetron (10 and 100 μg/kg) on the acquisition of responding for conditioned reward, and on the response potentiating effect of intra-accumbens d-amphetamine (10 μg). These effects were compared to those elicited by the dopamine antagonist α-flupenthixol (0.1 mg/kg). In the first procedure, rats were trained to associate food pellet delivery with a conditioned stimulus (CS). Rats subsequently allowed to respond on a lever delivering this CS, and on an inactive lever, showed a greater preference for the lever delivering the CS, indicating that this CS functioned as a conditioned reward (CR). Ondansetron administered during the conditioning phase did not alter subsequent responding for the CR, but α-flupenthixol induced a small but significant reduction in responding on the CR lever. These results suggest that blockade of dopamine receptors, but not 5-HT₃ receptors interfere with the learning of stimulus reward relationships. In the second procedure, d-amphetamine injected into the nucleus accumbens markedly potentiated responding for CR. Ondansetron at 10 μg/kg induced a small attenuation of this effect, without altering responding in its own right. However, at a higher dose (100 μg/kg) ondansetron plus amphetamine treatment significantly enhanced responding on the inactive lever. At both doses, the net effect of ondansetron was to produce a subtle impairment in the allocation of responses such that the differential responding on the CR versus NCR lever was diminished. In contrast to these effects α-flupenthixol significantly attenuated d-amphetamine’s selective enhancement of responding for conditioned reward, as well as impairing the ability of the conditioned reward to elicit and maintain behaviour. These results confirm the role of dopamine in responding for conditioned reward, and suggest a possible modulators role for 5-HT₃ receptors in this process. However, the effects of ondansetron on the acquisition of, and responding for, conditioned reward are clearly different from those induced by blockade of dopamine receptors. Received:4 December 1996/Final version:30 April 1997     

Activation of 5-HT1B receptors in the nucleus accumbens reduces amphetamine-induced enhancement of responding for conditioned reward

Previously, we have demonstrated that 5-hydroxytryptamine (5-HT) injected into the nucleus accumbens attenuates the potentiating effects of d-amphetamine on responding for conditioned reward (CR). The present studies examined the 5-HT receptor involved in this effect by investigating the effects of 5-HT agonists with differing affinities for 5-HT₁ and 5-HT₂ receptors on d-amphetamine-induced potentiation of responding for CR. Rats were trained to associate a light/tone stimulus (subsequently the CR) with water delivery. In a test phase, they were allowed access to a lever delivering the CR, and an inactive (NCR) lever. Responding on the CR lever was greater than responding on the NCR lever, indicating that the light/tone stimulus functioned as a CR. Responding for the CR was selectively potentiated by injections of d-amphetamine (10 μg) into the nucleus accumbens. This effect was reduced by injections into the nucleus accumbens of 5-CT (0.5 and 1 μg), RU24969 (10 μg), CP93,129 (1.25 and 2.5 μg) but not by DOI (10 μg) or 8-OH-DPAT (5 μg). The lower doses of 5-CT and CP93,129 did not reduce baseline responding for CR, or responding for water in a separate group of animals, indicating that the effects of these drugs were behaviourally selective. The higher doses abolished the CR effect, and in the case of 5-CT and RU24969 also reduced responding for water. All of the effective drugs share in common the ability to stimulate 5-HT_1B receptors, albeit with differing selectivities. The effect of CP93,129, the most selective of the 5-HT_1B agonists, to inhibit the response-potentiating effect of d-amphetamine was reversed by the5-HT_1B/1D antagonist GR127935 (3 mg/kg). The results indicate that activation of 5-HT_1B receptors within the nucleus accumbens attenuates the effects of a dopamine-dependent behaviour, and that activation of these receptors can oppose the behavioural effects of elevated mesolimbic dopamine transmission. Received: 22 April 1998/Final version: 28 July 1998     

Interoceptive Pavlovian conditioning with nicotine as the conditional stimulus varies as a function of the number of conditioning trials and unpaired sucrose deliveries

In rats, the pharmacological (interoceptive) effects of nicotine can serve as a signal (conditional stimulus) in a Pavlovian (classical) conditioning task. In this task, nicotine administration (0.4 mg base/kg, subcutaneous) is typically paired with intermittent access to a liquid sucrose unconditional stimulus; sucrose is withheld on saline sessions. An increase in sucrose receptacle entries (goal tracking) on nicotine sessions indicates conditioning. Given our limited understanding of the functional relationships controlling conditioned responding to a nicotine conditional stimulus, the present research examined nicotine's sensitivity to several manipulations shown to affect the conditioned responding in more widely studied Pavlovian conditioning tasks that use exteroceptive conditional stimuli: number of nicotine conditional stimulus-sucrose unconditional stimulus pairings per session (0, 3, 9, 18, or 36) and the impact of sucrose deliveries in saline sessions. Differential goal tracking developed in fewer sessions and asymptotic conditioned responding magnitude was greater with more nicotine-sucrose pairings. Further, goal tracking was more resistant to extinction (unconditional stimulus withheld) with more conditional-unconditional stimulus pairings during the acquisition phase. The discrimination was not acquired when sucrose presentations (9 or 18) also occurred during saline sessions. Furthermore, expression of the discrimination was disrupted when sucrose was presented in saline sessions; this disruption resulted from goal tracking in saline sessions. These results are consistent with the notion that nicotine-evoked goal tracking results from interoceptive conditioning processes.     

Facilitation of sexual behavior in male rats following d-amphetamine-induced behavioral sensitization

The present study investigated the effect of sensitization, induced by repeated injections of d-amphetamine, on sexual behavior in the naive male rat tested in a drug-free state. Injections of either d-amphetamine (1.5 mg/kg, IP) or saline were given every other day for a total of ten injections, and this regimen induced behavioral sensitization of locomotor activity in drug-treated rats. After a 3-week post-drug period, d-amphetamine-treated rats exhibited facilitated sexual behavior, as indicated by shorter latencies to mount and intromit, and a greater percentage of rats copulating. These rats also exhibited a general increase in the amount of copulation. Furthermore, sensitized rats displayed a facilitated acquisition of sexual behavior (i.e. mount and intromission latency <300 s for 3 consecutive days). After repeated sexual experience, rats pre-treated with d-amphetamine also showed an augmented increase in level changes made in anticipation of the presentation of a receptive female. Finally, enhanced sexual behavior was independent of the environment in which repeated administration of d-amphetamine occurred, indicating that facilitation was not a consequence of conditioned associations between drug and test environment. These results demonstrate that behavioral sensitization due to repeated psychostimulant administration can “cross-sensitize” to a natural motivated behavior, such as sex. Furthermore, the subsequent facilitation of anticipatory sexual behavior (i.e. level changes) after repeated experience in rats previously treated with d-amphetamine suggests that behavioral sensitization can influence incentive learning. Received: 10 June 1998/Final version: 7 August 1998     

Enhanced appetitive conditioning following repeated pretreatment with d-amphetamine

The behavioural response to psychomotor stimulants is augmented with repeated exposure to these drugs. Enhanced stimulated dopamine overflow within the nucleus accumbens and amygdala has been found to accompany this behavioural sensitization. In the present experiment, rats received 2 mg/kg d-amphetamine or 1 ml/kg physiological saline once per day for 5 days. Five days later, a behavioural assay confirmed that prior repeated d-amphetamine treatment markedly enhanced the locomotor activating effects of a d-amphetamine (0.5 mg/kg, i.p.) challenge. Training on a Pavlovian conditioning task began six days subsequently. In Stage 1, a stimulus (light or tone, S-) was presented negatively correlated with a sucrose reward. In Stage 2, presentation of the alternative counterbalanced stimulus (light or tone, S+) was paired with the availability of a 10% sucrose solution. There were no differences between the two groups in their response to the the S- stimulus. However, sensitized animals showed a selective enhancement in the acquisition of conditioned responding to S+, relative to vehicle-injected controls. No differences in behaviour were recorded during the prestimulus periods, nor during presentations of sucrose. Levels of activity within the operant chamber extraneous to alcove approach were also similar in both groups of animals. The conditioned instrumental efficacy of S+, relative to S- was assessed in Stage 3, in which stimulus availability was made contingent on a novel lever-pressing response. Both groups showed a similar preference for the S+ over the S- stimulus. Hence, rats sensitized by prior repeated d-amphetamine showed enhanced appetitive Pavlovian conditioning, without subsequent effect on conditioned reward efficacy. These data are discussed in light of possible changes in mesoamygdaloid dopamine functioning.     

Enhanced stimulus-reward learning by intra-amygdala administration of a D3 dopamine receptor agonist

The amygdala is considered to be a critical neural substrate underlying the formation of stimulus-reward associations, and is known to receive substantial innervation from dopaminergic neurons located within the ventral mesencephalon. However, relat- ively little is known about the function of the mesoamygdaloid dopamine projection in stimulus-reward learning. Recently, we have found post-session intra-amygdala microinjections of d-amphetamine to enhance appetitive Pavlovian conditioning as assessed in a discriminative approach task. In the present study, we have examined the effects of dopamine receptor agonists possessing relative selectivity for the D₁, D₂ and D₃ receptor subtypes in order to examine more fully the role of the mesoamygdaloid dopamine projection in stimulus-reward learning. Thus, subjects were trained to associate an initially neutral stimulus (CS⁺) with 10% sucrose reward (US). A second, control stimulus (CS⁻) was also presented but never paired with sucrose reward. In order to measure specifically the conditioned response to CS⁺/CS⁻ presentation, responding during CS and US presentations was measured separately. Immediately following each training session, subjects received bilateral intra-amygdala infusion of 0.1, 1 or 10 nmol/side of SKF-38393, quinpirole or 7-OH-DPAT. Infusions of SKF-38393 or quinpirole were without effect on CS⁺ approach. However, post-session intra-amygdala infusions of 7-OH-DPAT enhanced selectively CS⁺ approach in a dose-dependent fashion. No dose of any drug affected CS⁻approach, US behaviours, or measures of extraneous behaviour. Subsequent acquisition of a novel conditioned instrumental response was also unaffected. Thus, the present data indicate a selective involvement of the D₃ dopamine receptor subtype in the modulation of stimulus-reward learning by the mesoamygdaloid dopamine projection. Received: 12 December 1996 / Final version: 9 April 1997     

Injection of 5-HT into the nucleus accumbens reduces the effects ofd-amphetamine on responding for conditioned reward

Injection ofd-amphetamine into the nucleus accumbens potentiates responding for stimuli paired with a primary reward. A previous study showed that this potentiating effect ofd-amphetamine on responding for conditioned reward (CR) was attenuated by peripherally injectedd-fenfluramine, a 5-hydroxytryptamine (5-HT) releaser and re-uptake inhibitor. The present experiments further examined the effects of manipulating 5-HT function within the nucleus accumbens on responding for CR, and on the potentiation of CR responding following intra-accumbens injection ofd-amphetamine. Water deprived rats were trained to associate a compound stimulus with water delivery during a conditioning phase. During a test phase water was not delivered, but the compound stimulus was delivered according to a random ratio 2 schedule following a response on one of two levers. Rats responded at a higher rate on the lever delivering this CR.d-Amphetamine (10 g) injected into the nucleus accumbens enhanced responding on the CR lever. Co-injections of 5-HT (5 and 10 g) into the nucleus accumbens abolished the response-potentiating effect ofd-amphetamine but were without effect on the base-line level of responding for CR. This reduction by 5-HT of the response potentiating effect ofd-amphetamine was prevented by prior treatment with the 5-HT receptor antagonist metergoline (1 mg/kg). Responding for water was not altered by 5-HT and so the effects of 5-HT on responding for CR cannot be due to a change in the motivation to seek the primary reward. Thus, elevating 5-HT activity within the nucleus accumbens antagonises the effects ofd-amphetamine on responding for CR within the nucleus accumbens. These results suggest that 5-HT within the nucleus accumbens may play an important role in mediating incentive motivation by modulating dopaminergic neurotransmission.     

Enhanced behavioural control by conditioned reinforcers following microinjections of d-amphetamine into the nucleus accumbens

Stimulant drugs have been shown to enhance the control over behaviour exerted by stimuli previously correlated with primary reinforcers, termed conditioned reinforcers (CR). Experiment 1 examined the possible neuroanatomical specificity of the enhancement of conditioned reinforcement following intracerebral injections ofd-amphetamine. Thirsty rats were trained to associate, a light with water. In the test phase, water was no longer presented but the light (CR) was intermittently produced by responding on one of two novel levers. Rats with bilateral guide cannulae aimed at the nucleus accumbens, posterior caudate nucleus, or medio-dorsal nucleus of the thalamus received four counterbalanced microinfusions ofd-amphetamine (10, 20, 30 g/2 l) or vehicle (control) over 4 test days. There was a dose-dependent selective increase in responding on the lever that produced the light (CR) with intra-accumbensd-amphetamine infusions. Quantitatively similar, but much more variable effects were found with intra-caudate infusions and no effects following intra-thalamicd-amphetamine. Experiment 2 provided evidence that the enhanced control over responding by a CR with intra-accumbensd-amphetamine is behaviourally specific. Three groups of rats received a compound tone — plus —light stimulus that was positively, negatively or randomly correlated with water during training. Intra-accumbensd-amphetamine produced selective increases in responding only if the contingent stimulus had been positively correlated. The results suggest that the nucleus accumbens may play an important role ind-amphetamine's enhanced control over behaviour exerted by conditioned reinforcers.     

Effects ofd-fenfluramine and metergoline on responding for conditioned reward and the response potentiating effect of nucleus accumbensd-amphetamine

These studies investigated the effects of the 5-hydroxytryptamine (5-HT) releaser, and re-uptake inhibitor,d-fenfluramine, and the non-selective 5-HT receptor antagonist metergoline, on responding for conditioned reward (CR), and on the potentiation of responding for CR following amphetamine injected into the nucleus accumbens. Water deprived rats were trained to associate a compound stimulus with water delivery during a conditioning phase. During a test phase, water was not delivered but the compound stimulus was delivered according to a random ratio 2 schedule following a response on one of two levers; responding on the other lever was not reinforced. Overall, rats responded at a higher rate on the lever delivering the CR.d-Amphetamine (1, 3 and 10 µg) injected into the nucleus accumbens dose-dependently enhanced responding on the CR lever. Treatment withd-fenfluramine (0.5 and 1 mg/kg) reduced responding for the CR, and abolished the potentiating effect ofd-amphetamine. Responding on the inactive lever was also reduced by 1 mg/kg but not 0.5 mg/kgd-fenfluramine. The reduction ofd-amphetamine's effect on responding for CR was prevented by prior treatment with the 5-HT receptor antagonist metergoline (1 mg/kg). Control experiments showed that changes in thirst and motor performance, as well as deficits in learning ability, cannot account for the effects ofd-fenfluramine in this paradigm. In a separate experiment, 1 mg/kg metergoline failed to enhance responding for CR, and to augment the response potentiating effect of a low dose (2 µg) ofd-amphetamine injected into the nucleus accumbens. Thus, elevating brain 5-HT activity appears to reduce the ability of secondary reinforcing stimuli to elicit and maintain behaviour, and antagonizes the effects of enhanced dopamine activity within the nucleus accumbens. However, reduced 5-HT function induced by blockade of 5-HT_1/2 receptors does not appear to influence responding for CR, or the response potentiating effect ofd-amphetamine. These results suggest that 5-HT may play an important role in mediating incentive motivation.     

Effects of intra-amygdala R(+) 7-OH-DPAT on intra-accumbens d-amphetamine-associated learning I. Pavlovian conditioning

We have previously obtained evidence that the mesoamygdaloid dopamine projection modulates the acquisition of a conditioned response (CR) elicited by presentation of a conditioned stimulus (CS) predicting the availability of a natural (sucrose) reward. This property was found to be dependent upon D₃, but not D₁ or D₂, dopamine receptor activation. The aim of the present study was to determine whether the mesoamygdaloid dopamine projection is similarly involved in the acquisition of a drug-associated CR. Thus, two groups of rats with guide cannulae aimed at the nucleus accumbens and amygdala were trained using a Pavlovian conditioning procedure in which an initially neutral CS was paired with a computer-controlled, bilateral intra-accumbens infusion of d-amphetamine (the unconditioned stimulus; US). Conditioning sessions were conducted in standard operant chambers, with each session consisting of a single CS-US trial. For one group of rats, CS presentation was positively correlated with the drug US (Paired group), while for the second group CS and US presentations were negatively correlated (Unpaired group). During training, locomotor activity was recorded and was utilised as the measure both of the unconditioned (UR) and conditioned response (CR). A within-subjects design was utilised to investigate the effect of post-session bilateral intra-amygdala administration of R(+) 7-OH-DPAT on the development of the drug-associated CR. Hence, both Paired and Unpaired groups were exposed to two different CSs which were presented on alternate sessions. Post-session bilateral intra-amygdala administration of R(+) 7-OH-DPAT (10 nmol) followed sessions in which one CS was presented, while intra-amygdala vehicle followed sessions in which the alternate CS was presented. The development of a CR occurred only in the presence of a CS that had been positively correlated with presentation of the drug US. Post-session, intra-amygdala administration of R(+) 7-OH-DPAT enhanced the acquisition of this CR. However, R(+) 7-OH-DPAT was without effect upon the unconditioned response to intra-accumbens d-amphetamine. Our previous data indicate a comparable effect of R(+) 7-OH-DPAT on conditioning to a CS associated with a non-drug, natural reward. Therefore, taken together, these findings suggest that D₃ dopamine receptors within the amygdala modulate specifically the acquisition of Pavlovian conditioned responses, regardless of whether drug or natural rewards constitute the US. Received: 28 November 1997/Final version: 9 April 1998     

Amygdala and hippocampus control dissociable aspects of drug-associated conditioned rewards

Limbic innervation of the nucleus accumbens via the ventral subiculum/hippocampus and basolateral area of the amygdala has been shown to determine dissociable aspects of behaviour controlled by stimuli associated with natural rewards. However, the respective contributions of the ventral subiculum and amygdala to behaviour governed by drug-associated stimuli remain to be determined. Experiments consisted of two phases: drug-stimulus training, and subsequent stimulus-only testing. Initial training sessions were of two alternating forms. During drug sessions, responding upon one lever resulted in an infusion of 1 μg d-amphetamine into the nucleus accumbens, whilst during saline sessions d-amphetamine was replaced with saline. Each infusion (drug or saline) was preceded with either a light, or tone. Responding upon a control lever had no programmed consequences. Following training, the levers were retracted, and instead two novel vertical bars were extended from the chamber ceiling. Movement of one bar produced the drug-associated stimulus, whilst the alternative bar produced the saline-associated stimulus. Infusions of the AMPA receptor antagonist CNQX into the ventral subiculum or basolateral area of the amygdala (0, 0.2, 2.0 nmol) were made immediately before the start of each session. Intra-basolateral area of the amygdala CNQX reduced responding upon the drug-associated stimulus bar, but at the same time increased responding upon the saline-associated stimulus bar. By contrast, intra-ventral subiculum CNQX reduced drug-associated stimulus responding selectively. Neither manipulation affected levels of activity within the operant chamber extraneous to the bar-pushing response. Hence, the basolateral area of the amygdala appeared to have determined the degree of discriminative control exerted over behaviour by the drug-associated stimulus, whilst the ventral subiculum is suggested to have determined the efficacy of the conditioned reward. Received: 24 October 1996/Final version: 11 December 1996     

The role of mesolimbic and nigrostriatal dopamine in latent inhibition as measured with the conditioned taste aversion paradigm

Repeatedly presenting a non-reinforced stimulus normally retards conditioning to this stimulus when it is coupled to a reinforcer. This phenomenon is called latent inhibition. Since latent inhibition is disturbed after systemic administration of amphetamine, the present study investigated the role of the mesolimbic and nigrostriatal dopamine terminal fields in latent inhibition using a conditioned taste aversion (CTA) paradigm. In this paradigm, a 5% sucrose solution was used as the test stimulus and lithium chloride (LiCl) as the CTA inducing drug. The degree of CTA was assessed by measuring the sucrose preference in a two-bottle sucrose/water choice paradigm 24 h after the LiCl injection. Since conditioned taste aversion has so far not been used to evaluate the role of dopamine in latent inhibition, we first studied the effects of systemic application of amphetamine. The results show that intraperitoneal injections of 0.25 or 0.5 mg/kg d-amphetamine sulphate (given at preexposure and conditioning) significantly disrupted latent inhibition, by selectively reducing sucrose preference in the preexposed group. This could not be attributed to a reduced sucrose intake during preexposure or to a conditioned taste aversion effect of amphetamine itself. In experiment 2 local bilateral administration of 10 μg/0.5 μl amphetamine into the nucleus accumbens or the dorsal striatum was given in the pre-exposed and the conditioning phase, after which the rats were allowed to drink for a fixed period of time. The results show a significant reduction in latent inhibition after intrastriatal, but not after intra-accumbens injections of amphetamine. Intra-accumbens injections of amphetamine, however, significantly reduced fluid intake during preexposure and conditioning. In experiment 3, we therefore repeated this experiment, but allowed the animals to drink only a restricted amount of liquid during preexposure and conditioning. Again the results show a disruption of latent inhibition after intrastriatal, but not intra-accumbens injections of amphetamine. These experiments emphasize the importance of the nigrostriatal dopamine system in the disruption of latent inhibition, at least when using the conditioned taste aversion paradigm. A possible mechanism by which the dorsal striatum might influence latent inhibition is discussed. Received: 22 November 1995 /Final version: 20 August 1996     

Differential effects of intra-accumbens and systemic amphetamine on latent inhibition using an on-baseline,within-subject conditioned suppression paradigm

Latent inhibition (LI) is a phenomenon in which repeated, non-reinforced presentation of a stimulus retards subsequent conditioning to that stimulus. Several recent experiments have suggested that LI is abolished following acute, low doses of amphetamine given during pre-exposure and conditioning, and this effect has been attributed to amphetamine-induced changes in dopamine levels in the nucleus accumbens. Experiments 1 and 2 examined the effects of two doses of intra-accumbensd-amphetamine (10 µg/µl and 3 µg/µl) on LI in an on-baseline, within-subject conditioned suppression paradigm. There was no effect of either dose on LI, but a significant disinhibition of conditioned suppression resulted in a retardation of learning. In experiment 3 the effects of a low dose of systemicd-amphetamine (0.5 mg/kg) on latent inhibition were examined. The results replicated the abolition of LI found in previous studies, and demonstrated enhanced post-shock suppression in amphetamine-treated animals. These data provide no evidence for the involvement of the mesolimbic dopamine system in LI.     

The effects of pipradrol on the acquisition of responding with conditioned reinforcement: A role for sensory preconditioning

Previous studies have shown that pipradrol enhances the acquisition of responding with conditioned reinforcement. The present experiments replicated this finding and assessed the possible role of nonspecific stimulus change, feeding in the test environment and procedure consisted of three phases: Operant rates of pressing two levers, one of which produced a 3-s tone, were measured in the pre-exposure phase; the tone was paired with food in the four conditioning sessions; and conditioned reinforcement was demonstrated in the test phase by a relative increase in pressing the tone lever. A group (N=8) receiving food but no tone during the conditioning phase also increased responding on the tone lever in the test phase and this effect was enhanced in a group (N=8) receiving pipradrol prior to the test. A group (N=8) receiving neither tones nor pellets during conditioning and pipradrol prior to test failed to show a change in lever bias, ruling out the possibility that pipradrol produced a nonspecific enhancement of responding for stimulus change. It was hypothesized that the conditioned reinforcement effect seen in the food-alone groups resulted from sensory preconditioning. According to this hypothesis, the tone was conditioned to environmental stimuli during the pre-exposure phase; subsequent presentation of pellets resulted in a learned association between environmental stimuli and food, leading to increased responding for the tone in the test phase. The results of two control experiments supported this view. One group (N=8) received no tone in the pre-exposure phase, pellets alone in the conditioning phase and then pipradrol prior to test. The second group (N=8) received pellets alone in the conditioning phase, but under altered environmental stimulus conditions, and then was given pipradrol prior to test. Neither group showed evidence of conditioned reinforcement. It was concluded that pipradrol enhanced acquisition of responding with conditioned reinforcement even if the conditioned reinforcing stimulus was established with sensory preconditioning procedures.     

Effect of d-amphetamine on prepulse inhibition of the startle reflex in humans

Rationale: Prepulse inhibition of the startle reflex (PPI) is attenuated in animals after administration of d-amphetamine and other drugs that stimulate mesolimbic dopamine activity. Objective: The aim of the present study was to evaluate the effects of d-amphetamine (20 mg) on a variety of psychophysiological and subjective measures, including PPI, in humans. Method: Thirty-six participants (18 women) participated in a double-blind, placebo controlled, repeated measures study. In one session, participants received d-amphetamine (20 mg) orally, and in the other session, participants received an identical appearing placebo. Participants were assessed at 60, 90, and 120 min after ingestion with a 5-min block of startle trials (six control trials and six prepulse trials) followed by subjective measures of stimulation and mood. Results: d-Amphetamine increased subjective measures of stimulation and euphoria, attenuated PPI, and increased heart rate, relative to placebo treatment. Conclusions: The effect of d-amphetamine on the subjective measures was substantial and consistent over time, while the effect on PPI was only observed at 90 min after ingestion, and the effect on heart rate was limited to 90 and 120 min after ingestion. Received: 22 June 1998/Final version: 23 November 1998     

Effects of intra-amygdala R(+) 7-OH-DPAT on intra-accumbens d-amphetamine-associated learning II. Instrumental conditioning

Rats were trained to associate an initially neutral conditioned stimulus (CS) with a response-independent, intra-accumbens infusion of d-amphetamine (the unconditioned stimulus; US). Elsewhere, we have reported that as a result of this training, presentations of the CS alone elicited a conditioned response consisting of increased locomotor activity and that acquisition of this conditioned response was enhanced by post-session, intra-amygdala infusion of the dopamine D₃ receptor preferring agonist, R(+) 7-OH-DPAT. Here, in this same group of animals, we have examined the conditioned rewarding properties of the drug-associated CS by determining its ability to support the acquisition of a novel instrumental response in the absence of drug reward. Thus, rats were presented with two novel levers. Presentation of the drug-associated CS was made contingent upon depression of one of the levers (CR lever), while responding upon the other lever (NCR lever) had no programmed consequences. Preferential responding upon the lever delivering the drug-associated CS was observed despite a 6-week interval between CS-US training and the conditioned reward test. Intra-accumbens administration of d-amphetamine (0–20?μg) increased the control over behaviour exerted by the CS, increasing CR, but not NCR lever responding. In contrast, rats that received three pairings of an intra-accumbens infusion of d-amphetamine in combination with intra-amygdala infusion of R(+) 7-OH-DPAT, 3 weeks prior to testing, displayed similar rates of response upon both levers and were insensitive to the potentiation of responding for conditioned reward following intra-accumbens d-amphetamine. However, intra-accumbens d-amphetamine stimulated locomotor activity in a similar, dose-related manner in both groups. In this way, rats that had received intra-accumbens infusion of d-amphetamine in combination with intra-amygdala infusion of R(+) 7-OH-DPAT appeared exactly like control group rats, for which the CS had been paired with intra-accumbens d-amphetamine on a negative basis only. A locomotor activity test indicated that one behavioural consequence of intra-amygdala administration of R(+) 7-OH-DPAT was the reduction of the unconditioned locomotor response resulting from intra-accumbens administration of d-amphetamine. Hence, the present data demonstrate that the conditioned rewarding properties of a drug-associated CS are specific to the CS-US association and are relatively insensitive to decay over time. However, the rewarding properties of a drug-associated CS were selectively abolished following activation of amygdala D₃ receptors during presentation of the drug reward. Potential explanations for this effect are discussed, including the possibility that intra-amygdala R(+) 7-OH-DPAT reduced the incentive value of the US.     

The development of addiction to d-amphetamine in an animal model: same principles as for alcohol and opiate

We have established a rat model that reflects the course of development of alcohol and opiate addiction. The present study with d-amphetamine aimed to define general principles in the development of an addiction. Male rats had a continuous free choice between d-amphetamine solutions (100, 200 and 400 mg/l) and water for 47 weeks. An initial intake of high doses of d-amphetamine during the first weeks of drug choice was followed by an individually stable pattern of drug consumption of moderate drug doses. During this period of controlled consumption (from week 10 to week 40), the voluntary intake of d-amphetamine depended on individual factors (dominant rats: 0.37 ± 0.02 mg/kg per day, subordinate rats: 0.57 ± 0.05 mg/kg per day) and environmental variables (group housing: 0.21 ± 0.02 mg/kg per day, single housing: 0.41 ± 0.03 mg/kg per day). Beginning with week 41, voluntary d-amphetamine consumption progressively increased (1.9 ± 0.2 mg/kg per day in week 47), although the experimental conditions remained unchanged. Drug intake during a retest (free choice as before) after 6 months of drug deprivation revealed that the rats had persistently lost their control over drug intake and were no longer able to adjust drug taking to internal and external conditions. These addicted rats took very high drug doses, even when all d-amphetamine solutions but not water were adulterated with bitter tasting quinine (6.6 ± 0.6 mg/kg per day; age-matched controls: 0.37 ± 0.04 mg/kg per day). Forced intake of d-amphetamine for 47 weeks (7.1 ± 0.3 mg/kg per day) via the drinking fluid caused physical dependence (hyperreactivity during withdrawal) but did not lead to drug addiction (voluntary intake in the retest with adulteration: 0.42 ± 0.04 mg/kg per day). Both the temporal development and the prerequisites of psychostimulant addiction were in principle the same as for alcohol and opiates. Received: 3 April 1998/Final version: 26 August 1998