Baclofen facilitates the extinction of methamphetamine-induced conditioned place preference in rats

The powerful, long-lasting association between the rewarding effects of a drug and contextual cues associated with drug administration can be studied using conditioned place preference (CPP). The GABA(B) receptor agonist baclofen facilitates the extinction of morphine-induced CPP in mice. The current study extended this work by determining if baclofen could enhance the extinction of methamphetamine (Meth) CPP. CPP was established using a six-day conditioning protocol wherein Meth-pairings were alternated with saline-pairings. Rats were subsequently administered baclofen (2 mg/kg i.p. or vehicle) immediately after each daily forced extinction session, which consisted of a saline injection immediately prior to being placed into the previously Meth- or saline-paired chamber. One extinction training cycle, consisted of six once-daily forced extinction sessions, mimicking the alternating procedure established during conditioning, followed by a test for preference (Ext test). CPP persisted for at least four extinction cycles in vehicle-treated rats. In contrast, CPP was inhibited following a single extinction training cycle. These data indicate that Meth-induced CPP was resistant to extinction, but extinction training was rendered effective when the training was combined with baclofen. These findings converge with the prior demonstration of baclofen facilitating the extinction of morphine-induced CPP indicating that GABA(B) receptor actions are independent of the primary (unconditioned) stimulus (i.e., the opiate or the stimulant) and likely reflect mechanisms engaged by extinction learning processes per se. Thus, baclofen administered in conjunction with extinction training may be of value for addiction therapy regardless of the class of drug being abused.     

Reversible inactivations of the cerebellum with muscimol prevent the acquisition and extinction of conditioned nictitating membrane responses in the rabbit

Lesions of the cerebellum severely impair the classically conditioned nictitating membrane response (NMR) in rabbits. Thus, the cerebellum is essential for the production of conditioned responses (CRs), either because it is actively involved in NMR conditioning or because damage to it causes motor or other general deficits. To distinguish between these alternatives, the cerebellum may be inactivated during training. Inactivation of the cerebellum during acquisition training might result in the absence of CRs on initial trials of subsequent training without the neuronal blockade. The blockade may have prevented learning but it may have produced other deficits that require time or further training to overcome. This problem can be addressed by inactivating the cerebellum during extinction training. If inactivation during extinction training results in the immediate production of CRs when training is resumed without the blockade, then it may be concluded that extinction learning was prevented by the blockade — the presence of CRs argues against any deficits not associated with learning. We used muscimol to inactivate the cerebellum and test its involvement in acquisition and extinction of NMR conditioning in the same subjects. We injected muscimol close to the interpositus nucleus of the cerebellum 1 h before each of four daily training sessions of delay conditioning. Almost no CRs were produced in these training sessions — there was little or no acquisition of NMR conditioning during cerebellar inactivation. The subjects were then trained for four daily sessions without injections of muscimol. There were no CRs on initial trials of the first session of retraining, but all subjects produced CRs by the end of this session. The subjects then received four daily sessions of extinction training with muscimol inactivation of the nuclei — no CRs were produced. Extinction training then continued for four daily sessions without muscimol inactivation. On the first of these sessions, all subjects immediately produced high levels of CRs. These responses then extinguished within and between sessions with characteristic beginning-of-session spontaneous recovery. There was little or no extinction of NMR conditioning during cerebellar inactivation. After inactivation, the muscimolinactivated subjects went on to acquire and extinguish NM responses at rates similar to those of appropriate controls. We conclude that cerebellar circuitry is essential for, and actively engaged in, both acquisition and extinction of this simple form of motor learning.     

Dissociable effects of lesions to the dorsal or ventral noradrenergic bundle on the acquisition, performance, and extinction of aversive conditioning

In six experiments we studied the effects of lesions to either the dorsal or ventral noradrenergic bundle on the acquisition and extinction of the conditioned emotional response (CER) as measured in a conditioned suppression paradigm. Infusions of the neurotoxin 6-hydroxydopamine (6-OHDA) into the trajectory of the dorsal noradrenergic ascending bundle (DNAB) impaired the acquisition of on-the-baseline and off-the-baseline conditioned suppression. The acquisition impairment for on-the-baseline conditioning was also shown to still be present when training did not commence until 8 weeks following central noradrenergic depletion. However, in rats previously trained on the CER, DNAB lesions did not affect performance. There was also a small resistance to extinction following on-, but not off-the-baseline conditioning. The acquisition impairment was shown not to be because of an altered sensitivity to the footshock. In contrast, infusions of 6-OHDA into the ventral noradrenergic ascending bundle (VNAB) had no effect upon the acquisition of the CER in an on-the-baseline procedure, but retarded its extinction to a much greater extent. The results here are discussed in terms of other acquisition deficits shown by rats with DNAB lesions, and with reference to Gray's "anxiety" and Mason's "selective attention" theories of locus coeruleus function.     

Amygdala inactivation blocks expression of conditioned memory modulation and the promotion of avoidance and freezing

Rats were exposed to shock-paired cues immediately after training on an appetitive preference task. Elevated levels of freezing in and active avoidance of the shock-paired compartment were observed, and memory for the appetitive task was improved when tested 24 hr later. Intra-amygdala muscimol injected before the posttraining exposure eliminated freezing, avoidance, and memory modulation. The blockade of both freezing and active avoidance, which involve competing behavioral tendencies, makes it unlikely that the amygdala itself generates either behavior. The elimination of conditioned memory modulation suggests that conditioned neurohormonal responses were blocked. These conditioned internal responses may comprise the intervening variable of "conditioned fear" and may promote observable behaviors, the form of which is determined by the environment in which they occur.     

Fear conditioning to tone, but not to context, is attenuated by lesions of the insular cortex and posterior extension of the intralaminar complex in rats

C. Shi and M. Davis (1999) recently reported that combined lesions of the posterior extension of the intralaminar complex (PINT) and caudal insular cortex (INS) block acquisition but not expression of fear-potentiated startle to discrete conditioned stimuli (CSs) and a footshock unconditioned stimulus (US) and proposed that PINT-INS projections to the amygdala constitute the essential US pathways involved in fear conditioning. The present study further tested this hypothesis by examining whether PINT-INS lesions block fear conditioning (as measured by freezing) to diffuse-context and discrete-tone CSs, and whether posttraining lesions with continued CS-US training result in extinction to the CSs. Posttraining lesions resulted in a selective attenuation of tone conditioning, but context conditioning was unaffected by pre- and posttraining lesions. These results do not support the view that the PINT-INS represent the essential US pathway in fear conditioning.     

Posttraining D1 receptor blockade impairs odor conditioning in neonatal rats.

Rat pups that were exposed to a novel anise odor paired with tactile stimulation (stroking the skin with a paint brush) received injections of either saline or the dopamine D1 receptor antagonist (+/-)-SKF 83566 (0.1 mg/kg) before conditioning or immediately after conditioning. Animals that received the drug either before or after training showed less approach to the conditioned odor during the testing period 24 hr later than did animals that received the vehicle. Posttraining administration of the D2 receptor antagonist spiperone (0.1 mg/kg) did not affect subsequent approach to the conditioned odor, suggesting a selective effect of D1 receptor blockade. The impairment in learning by the administration of (+/-)-SKF 83566 before conditioning was reversed by the injection of the dopamine receptor agonist apomorphine (0.1 mg/kg) immediately after conditioning. Posttraining D1 receptor activation appears necessary for normal odor conditioning in rat pups.     

Retention impairment by posttraining epinephrine: role of state dependency and of endogenous opioid mechanisms

Mice were trained in a step-through inhibitory avoidance task with a 0.6-mA, 60-Hz, 2-s footshock and were tested for retention 3 or 6 hr later. Posttraining intraperitoneal administration of a high dose (25.0 micrograms per mouse) of epinephrine (Epi) impaired retention; this effect was counteracted by another injection of the same dose of Epi given before retention testing either 3 or 6 hr after training. When administered before the 6-hr test but not the 3-hr test, however, Epi enhanced retention (i.e., above that of controls). The retention enhancement, but not the reversal of impairing effects of posttraining Epi, was antagonized by naltrexone (20.0 micrograms per mouse). Naltrexone, when administered alone, had no effect on retention when given before testing. However, posttraining administration of naltrexone produced an enhancement of retention detectable 6 but not 3 hr after training. Furthermore, posttraining naltrexone also blocked the impairing effect of posttraining Epi otherwise seen 6 hr after training. These results suggest that the impairment of retention caused by posttraining Epi is attributable to the induction of state dependency based on an Epi state. When the animals are tested 3 hr after training, this effect appears alone. But, when tested 6 hr after training, the Epi effect appears together with an opioid, presumably beta-endorphin-mediated, state dependency.     

Naloxone induces multiple effects on aversive Pavlovian conditioning in rabbits

A series of experiments examined the effects of intravenous naloxone treatment on aversive Pavlovian conditioning of eye-blink and heart rate responses, and related unconditioned behaviors, in rabbits. Naloxone treatment before testing attenuated bradycardiac orienting responses to tones used as conditioning stimuli. Naloxone also attenuated conditioned bradycardia when administered either before or after training sessions, but it potentiated conditioned bradycardia during extinction of discriminative conditioning. Naloxone did not influence acquisition or extinction of discriminative eye-blink conditioning or somatic or cardiac responses to shocks used as unconditioned stimuli, but it did decrease locomotor activity. Naloxone treatment immediately after training sessions facilitated acquisition of eye-blink responses. It was concluded that naloxone influences aversive Pavlovian conditioning in more than one way: (a) During training, it appears to alter reception and processing of signals but does not affect subsequent development of somatic responses to the Pavlovian conditioning contingency. (b) After training sessions, naloxone apparently affects consolidation of both somatic and autonomic conditioning. (c) Naloxone also appears to delay extinction of Pavlovian conditioning; this effect may similarly involve changes in a stimulus-processing mechanism or in memory functions, but it apparently does not involve changes in somatomotor responsitivity.     

Context modulation of memory for fear extinction in humans

Distinct memories are formed during fear conditioning and subsequent extinction. In animals, the expression of the latter is gated by the context. The recall of extinction memory after a long delay, and the contextual modulation thereof, has not been directly tested in humans. Mentally healthy volunteers underwent a 2-day fear conditioning and extinction protocol that examined the recall of extinction memory and its relationship to context. Conditioned stimuli were paired with an aversive electric shock in one visual context and extinguished in a different context. Extinction recall and renewal were examined 24 h after training. We found that skin conductance responses were small when the conditioned stimulus was presented in the extinction context, but responses were renewed when the conditioned stimulus was presented in the conditioning context. This finding demonstrates context dependency of extinction recall in humans.     

Picrotoxin enhances latent extinction of conditioned fear

Male CD1 mice received 20 pairings of tone and footshock (FS) or tone alone in an arm of a Y-maze on Day 1. On Day 2 either extinction (tone alone) or no extinction was followed immediately by saline or picrotoxin (0.5 or 1.0 mg/kg ip). Nonextinguished groups received only saline or picrotoxin (1.0 mg/kg ip) on Day 2. Other groups received saline or picrotoxin (1.0 mg/kg) 2 hr after extinction. On Day 3 all mice were placed in the Y-maze (with doors to all 3 alleys open), and total alley entries during a 2-min test session were recorded. Day 1 FS training resulted in reduced alley entries during the test session. Day 2 extinction session significantly attenuated the effects of the FS training. Day 3 performance of mice given picrotoxin (1.0 but not 0.5 mg/kg) immediately postextinction was comparable to that of mice not given FS on Day 1. The findings suggest that picrotoxin enhanced extinction of conditioned fear.     

Opioid receptors regulate retrieval of infant fear memories: effects of naloxone on infantile amnesia

The authors examined the role of the endogenous opioid system in infantile amnesia for contextual fear conditioning. Rats that were 18 days of age received an aversive footshock in a novel context. Rats displayed conditioned fear when tested 1 min after training but not 24 hr after training. Systemic injection of the opioid receptor antagonist naloxone prior to test, but not immediately after training, alleviated infantile amnesia. Naloxone also alleviated infantile amnesia when injected prior to test 7 days after training. These effects of naloxone were due to actions on central rather than peripheral opioid receptors and were not due to any tendency of the drug to produce fear or freezing. These results show that central opioid receptors regulate retrieval of fear memories in infant rats. ((c) 2006 APA, all rights reserved).     

Timing of extinction relative to acquisition: a parametric analysis of fear extinction in humans

Fear extinction is a reduction in conditioned fear following repeated exposure to the feared cue in the absence of any aversive event. Extinguished fear often reappears after extinction through spontaneous recovery. Animal studies suggest that spontaneous recovery can be abolished if extinction occurs within minutes of acquisition. However, a limited number of human extinction studies have shown that short interval extinction does not prevent the return of fear. For this reason, we performed an in-depth parametric analysis of human fear extinction using fear-potentiated startle. Using separate single-cue and differential conditioning paradigms, participants were fear conditioned and then underwent extinction either 10 min (Immediate) or 72 hr (Delayed) later. Testing for spontaneous recovery occurred 96 hr after acquisition. In the single cue paradigm, the Immediate and Delayed groups exhibited differences in context, but not fear, conditioning. With differential conditioning, there were no differences in context conditioning and the Immediate group displayed less spontaneous recovery. Thus, the results remain inconclusive regarding spontaneous recovery and the timing of extinction and are discussed in terms of performing translational studies of fear in humans.     

The reinstatement of amphetamine-induced place preference is long-lasting and related to decreased expression of AMPA receptors in the nucleus accumbens

A great deal of effort has been devoted to elucidating the psychopharmacology underlying addiction and relapse. Long-term neuroadaptations in glutamate transmission seem to be of great relevance for relapse to stimulant abuse. In this study, we investigated amphetamine-induced conditioned place preference during adolescence and the reinstatement of the conditioned behavior following a priming injection of the drug 1 day (adolescence), 30 days (early adulthood) and 60 days (adulthood) after the extinction test. The nucleus accumbens was dissected immediately after the reinstatement test to examine alterations in GluR1 and NR1 subunits of glutamatergic receptors. Our results showed that a priming injection of amphetamine was able to reinstate the CPP 1 and 30 days after extinction. However, it failed to reinstate the conditioned response after 60 days. GluR1 levels were decreased on days 1 and 30 but not on day 60 while NR1 levels were unaltered in the reinstatement test. Using a relapse model we found that reinstatement of amphetamine-induced conditioning place preference during adolescence is long lasting and persists through early adulthood. Decreased levels of GluR1 in the nucleus accumbens might be related to the reinstatement of amphetamine-induced conditioning place preference.     

On the resistance to extinction of fear conditioned to angry faces

The present study investigated whether, like fear conditioned to pictures of snakes and spiders, fear conditioned to angry faces resists extinction even after verbal instruction and removal of the shock electrode. Participants were trained in a differential Pavlovian fear conditioning procedure with angry face or happy face conditional stimuli (CSs). Prior to extinction, half the participants in each group were informed that no more unconditional stimuli would be presented and the shock electrode was removed. In the absence of this manipulation, participants showed resistance to extinction after training with angry face CSs, but not after training with happy face CSs. Instructed extinction and electrode removal abolished fear conditioning regardless of the emotion expressed by the CS faces. This finding suggests that fear conditioned to angry faces, like fear conditioned to racial out-group faces, is more malleable than fear conditioned to snakes and spiders.     

Long-term retention of the classically conditioned eyeblink response in rats

Retention of the classically conditioned eyeblink response in rats was tested with a conditioned stimulus (CS)-alone extinction test and 2 sessions of reacquisition training. Retention of the eyeblink conditioned response (CR) during both tests was highest 24 hr and 1 month after initial acquisition. Three months after initial acquisition, responding during the CS-alone test was at baseline, but there was significant savings during reacquisition. By 6 months after initial acquisition, the memory for the eyeblink CR was not expressed in either test. The group differences in retention, despite initial acquisition of the eyeblink CR to equal levels, suggest that rat eyeblink conditioning may provide a useful behavioral model for studying the neural processes underlying memory retention and loss.     

Inhibition by diazepam of the effect of additional training and of extinction on the retention of shuttle avoidance behavior in rats

Rats were submitted to a training and a test session in a shuttle avoidance task. In some groups, a second training session was interpolated 2 or 24 hr after the first session. In others, a session of extinction was interpolated 2 or 24 hr after the training session. When the interpolated task was 2 hr after training, training-test interval was 24 hr. When the interpolated task was 24 hr after training, training-test interval was 48 hr. The additional training enhanced, and the extinction depressed, retention test performance. Diazepam, given 30 min prior to the first (or only) training session enhanced the performance of avoidance responses in that session but inhibited it in the subsequent retention test. Diazepam given 90 min after training had no effect on retention. Diazepam given 30 min prior to either the additional training session or the extinction session did not affect performance in that session but cancelled their effects on retention test performance. The effects are related to the previously described prevention by diazepam of interfering effects on memory.     

Effects of ethanol on encoding, consolidation, and expression of extinction following contextual fear conditioning

Studies of contextual fear conditioning have found that ethanol administered prior to a conditioning session impairs the conditioned freezing response during a test session the next day. The present experiments examined the effects of ethanol on extinction, the loss of conditioned responding that occurs as the animal learns that a previously conditioned context no longer signals shock. Ethanol (1.5 g/kg) administered prior to single (Experiment 1) or multiple (Experiment 2) extinction sessions impaired extinction. Ethanol administered prior to a test session disrupted the expression of freezing after extinction (Experiments 3-5). There was some evidence that ethanol served as an internal stimulus signaling the operation of conditioning or extinction contingencies (Experiments 4-5). In Experiment 6, postsession injections of 1.5 g/kg ethanol had no effect on extinction with brief (3 min) or long (24 min) exposures to the context, but injections of 3 g/kg after long exposures impaired extinction. Together, these results indicate that ethanol affects extinction by acting on multiple learning and performance processes, including attention, memory encoding, and memory expression.     

Frequency-specific receptive field plasticity in the medial geniculate body induced by pavlovian fear conditioning is expressed in the anesthetized brain.

Fear conditioning modifies the processing of frequency information; receptive fields (RF) in the auditory cortex and the medial geniculate body (MGB) are altered to favor processing the frequency of the conditioned stimulus (CS) over the pretraining best frequency (BF) and other frequencies. This experiment was designed to determine whether brief conditioning in the waking state produces RF plasticity that is expressed under general anesthesia. Guinea pigs bearing electrodes in the MGB received 20 trials of tone-shock pairing in a single training session. RFs were determined with animals under ketamine anesthesia before conditioning and 1-3 hr and 24 hr after conditioning. Frequency-specific RF plasticity was evident for both postconditioning periods: The BF shifted toward or to the CS frequency, responses to the BF decreased, and responses to the CS increased. Broadly tuned cells developed greater RF plasticity than narrowly tuned neurons. The results demonstrate that the specific neuronal results of brief learning experiences can be expressed in the anesthetized brain.     

The unconditioned stimulus pre‐exposure effect in preweanling rats in taste aversion learning: Role of the training context and injection cues

The unconditioned stimulus pre‐exposure effect (US‐PE) refers to the interference paradigm in which acquisition of the conditioned response is retarded due to prior experience with the US. Most studies analyzing the psychological mechanisms underlying this effect have been conducted with adult rats. The most widely accepted hypothesis explains this effect as a contextual blocking effect. Contextual cues associated with the US block the conditioned stimulus (CS)‐US association during conditioning. The modulatory role of a context devoid of distinctive olfactory attributes is not observable until approximately PD23 in rats, including modulation of interference paradigms such as latent inhibition or extinction. In this study, we analyzed US‐PE in preweanling rats along with the role of the training context in this effect in terms of conditioned taste aversion preparation. Pre‐exposure to LiCl before conditioning retarded the acquisition of taste aversion. The US‐PE was observed in preweanling rats when, during pre‐exposure, subjects were exposed to the conditioning context, and this effect was not attenuated either by the administration of the US in a familiar environment (Experiment 1a), or by the presence of an alternative, more salient context during pre‐exposure (Experiment 1b). Additionally, the US‐PE was still observed when the route by which the US was administered was changed between the pre‐exposure and conditioning phases (Experiment 2a) as well as when the injection cues were removed during conditioning (Experiment 2b). These experiments show a strong US‐PE in preweanling rats and fail to support the contextual blocking hypothesis, at least in this stage of ontogeny. © 2012 Wiley Periodicals, Inc. Dev Psychobiol 55: 193–204, 2013     

The Effects of Directional Facial Cues on Electrodermal Conditioning to Facial Stimuli

The present two Experiments demonstrate that the orientation of angry facial expressions is a critical cue when faces serve as conditioned stimuli during aversive electrodermal conditioning. Experiment 1 demonstrated that responses conditioned to angry faces directed toward the subjects showed significant resistance to extinction whereas responses conditioned to angry faces directed away extinguished immediately during extinction. Experiment 2 was designed to investigate whether the orientation factor was effective only during extinction, i.e. whether the orientation was a performance variable, or if the observed effect originated from factors active during acquisition, i.e. if the orientation primarily influenced learning. When the orientation was shifted between the acquisition and the extinction phase, it was found that it did not matter what direction the faces had during acquisition; it was only during extinction that the faces needed to be directed toward the subjects to evoke persistent responding. These results suggest that the subjects acquired a conditioned response to the angry face regardless of its direction, but that this learning effect remained only when the angry face was directed toward the subject during extinction.