Differential effect of buspirone and diazepam on negative contrast in one-way avoidance learning

Psychopharmacological characterisation of the successive negative contrast effect in rats

RATIONALE

Successive negative contrast (SNC) describes a change in the behaviour of an animal following a downshift in the quantitative or qualitative value of an expected reward. This behavioural response has been hypothesised to be linked to affective state, with negative states associated with larger and/or prolonged shifts in behaviour.

OBJECTIVE

This study has investigated whether different psychopharmacological treatments have dissociable actions on the SNC effect in rats and related these findings to their actions on different neurotransmitter systems and affective state.

METHODS

Animals were trained to perform a nose-poke response to obtain a high-value food reward (four pellets). SNC was quantified during devalue sessions in which the reward was reduced to one pellet. Using a within-subject study design, the effects of acute treatment with anxiolytic, anxiogenic, antidepressant and dopaminergic drugs were investigated during both baseline (four pellets) or devalue sessions (one pellet).

RESULTS

The indirect dopamine agonist, amphetamine, attenuated the SNC effect whilst the D1/D2 antagonist, alpha-flupenthixol, potentiated it. The antidepressant citalopram, anxiolytic buspirone and anxiogenic FG7142 had no specific effects on SNC, although FG7142 induced general impairments at higher doses. The α2-adrenoceptor antagonist, yohimbine, increased premature responding but had no specific effect on SNC. Results for the anxiolytic diazepam were mixed with one group showing an attenuation of the SNC effect whilst the other showed no effect.

CONCLUSIONS

These data suggest that the SNC effect is mediated, at least in part, by dopamine signalling. The SNC effect may also be attenuated by benzodiazepine anxiolytics.

Behavioral neuroscience of psychological pain

Pain is a common word used to refer to a wide range of physical and mental states sharing hedonic aversive value. Three types of pain are distinguished in this article: Physical pain, an aversive state related to actual or potential injury and disease; social pain, an aversive emotion associated to social exclusion; and psychological pain, a negative emotion induced by incentive loss. This review centers on psychological pain as studied in nonhuman animals. After covering issues of terminology, the article briefly discusses the daily-life significance of psychological pain and then centers on a discussion of the results originating from two procedures involving incentive loss: successive negative contrast-the unexpected devaluation of a reward-and appetitive extinction-the unexpected omission of a reward. The evidence reviewed points to substantial commonalities, but also some differences and interactions between physical and psychological pains. This evidence is discussed in relation to behavioral, pharmacological, neurobiological, and genetic factors that contribute to the multidimensional experience of psychological pain.

Serotonergic mechanisms in addiction-related memories

Drug-associated memories are a hallmark of addiction and a contributing factor in the continued use and relapse to drugs of abuse. Repeated association of drugs of abuse with conditioned stimuli leads to long-lasting behavioral responses that reflect reward-controlled learning and participate in the establishment of addiction. A greater understanding of the mechanisms underlying the formation and retrieval of drug-associated memories may shed light on potential therapeutic approaches to effectively intervene with drug use-associated memory. There is evidence to support the involvement of serotonin (5-HT) neurotransmission in learning and memory formation through the families of the 5-HT(1) receptor (5-HT(1)R) and 5-HT(2)R which have also been shown to play a modulatory role in the behavioral effects induced by many psychostimulants. While there is a paucity of studies examining the effects of selective 5-HT(1A)R ligands, the available dataset suggests that 5-HT(1B)R agonists may inhibit retrieval of cocaine-associated memories. The 5-HT(2A)R and 5-HT(2C)R appear to be integral in the strong conditioned associations made between cocaine and environmental cues with 5-HT(2A)R antagonists and 5-HT(2C)R agonists possessing potency in blocking retrieval of cocaine-associated memories following cocaine self-administration procedures. The complex anatomical connectivity between 5-HT neurons and other neuronal phenotypes in limbic-corticostriatal brain structures, the heterogeneity of 5-HT receptors (5-HT(X)R) and the conflicting results of behavioral experiments which employ non-specific 5-HT(X)R ligands contribute to the complexity of interpreting the involvement of 5-HT systems in addictive-related memory processes. This review briefly traces the history of 5-HT involvement in retrieval of drug-cue associations and future targets of serotonergic manipulation that may reduce the impact that drug cues have on addictive behavior and relapse.

Successive positive contrast in one-way avoidance behavior with Roman low-avoidance rats

The inbred Roman High- (RHA-I) and Roman Low-Avoidance (RLA-I) rats, psychogenetically selected for rapid (RHA-I) vs. extremely poor (RLA-I) acquisition of two-way active avoidance, exhibit a lower or a higher level of fearfulness, respectively, that can be observed in many laboratory anxiety models. The present study analyzed the performance of female RLA-I and RHA-I rats in a successive positive contrast situation induced during one-way avoidance learning. Three groups of RLA-I and three of RHA-I rats (1-30, 30-30 and 1-1 groups, the numbers stand for the time spent in the safe compartment during the first and second phase of training) were trained to avoid an electric foot-shock administered in a "danger" compartment, by running from this compartment to a "safe" one. Only RLA-I rats showed a significant positive contrast effect, in such a way that the reinforcement increase from the lower (1 s spent in safety) to the higher reward (30 s) led to a response enhancement, surpassing the performance of rats trained with the low (1-1 s) or the high (30-30 s) reward from the beginning of training. The results are discussed in the context of an opponent process theory based upon the interaction between the motivational strength of fear and the incentive value of relief taking place during one-way avoidance learning.

Differential effects of diazepam infused into the amygdala and hippocampus on negative contrast

Behavioral suppression is observed when animals shift from a high to a lower magnitude of reward in comparison to animals that continuously receive the lower magnitude reward. As previously reported, systemic administration of benzodiazepines promotes recovery from this negative contrast. This study aimed to assess where the neural substrate(s) located in the limbic areas for diazepam to induce such recovery effects on negative contrast. With food-deprived rats, the negative contrast procedure was conducted by comparing a group consuming a 32% sucrose solution which was shifted to 4% with a group consuming only 4% sucrose throughout the experiment. Represented mainly by a decreased number of licks, the negative contrast effects were clearly shown in the control groups receiving the vehicle. Systemic injection of diazepam dose-dependently reduced this contrast. Further, this negative contrast effect was significantly attenuated by local infusion of diazepam (30 microg) into the amygdala, but no such effect was confirmed when diazepam was infused into the hippocampus. Together, the present study shows that a reliable anti-contrast effect can be induced by diazepam administration peripherally or locally infused into the amygdala. These data indicate that the amygdala is involved in the recovery effects of benzodiazepines on consummatory negative contrast.

5-HT system and cognition

The study of 5-hydroxytryptamine (5-HT) system has benefited from the identification, classification and cloning of multiple 5-HT receptors (5-HT1 to 5-HT7). Growing evidence suggests that 5-HT is important in learning and memory and all its receptors might be implicated in this. Actually, 5-HT pathways, 5-HT reuptake site/transporter complex and 5-HT receptors show regional distribution in brain areas implicated in learning and memory. Likewise, the stimulation or blockade of presynaptic 5-HT1A, 5-HT1B, 5-HT(2A/2C) and 5-HT3 receptors, postsynaptic 5-HT(2B/2C) and 5-HT4 receptors and 5-HT uptake/transporter sites modulate these processes. Available evidence strongly suggests that the 5-HT system may be important in normal function, the treatment and/or pathogenesis of cognitive disorders. Further investigation will help to specify the 5-HT system nature involvement in cognitive processes, pharmacotherapies, their mechanisms and action sites and to determine under which conditions they could operate. In this regard, it is probable that selective drugs with agonists, neutral antagonist, agonists or inverse agonist properties for 5-HT1A, 5-HT(1B/1D), 5-HT(2A/2B/2C), 5-HT4 and 5-HT7 receptors could constitute a new therapeutic opportunity for learning and memory alterations.

Pre- or post-training injection of buspirone impaired retention in the inhibitory avoidance task: involvement of amygdala 5-HT1A receptors

The present study investigated the effect of buspirone on memory formation in an aversive learning task. Male Wistar rats were trained on the inhibitory avoidance task and tested for retention 1 day after training. They received peripheral or intra-amygdala administration of buspirone or other 5-HT1A drugs either before or after training. Results indicated that pretraining systemic injections of buspirone caused a dose-dependent retention deficit; 5. 0 mg/kg had a marked effect and 1.0 mg/kg had no effect. Post-training injections of the drug caused a time-dependent retention deficit, which was not due to a state-dependent effect on retrieval. When training in the inhibitory avoidance task was divided into a context-training phase and a shock-training phase, buspirone impaired retention only when administered in the shock-training phase, suggesting that the drug influenced memory processing of affective events. Further results indicated that post-training intra-amygdala infusion of buspirone or the 5-HT1A agonist 8-hydroxy-di-n-propylaminotetralin (8-OH-DPAT) caused a time-dependent and dose-dependent retention deficit. Post-training intra-amygdala infusion of the 5-HT1A antagonist WAY100635 (N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)-N-(2-pyridyl) cyclohexane carboxamine maleate) attenuated the memory-impairing effects of buspirone. These findings suggest that buspirone may modulate memory storage processes in the inhibitory avoidance task through an action on amygdaloid 5-HT1A receptors.

Successive negative contrast in one-way avoidance: effect of thiopental sodium and chlorpromazine

The successive negative contrast effect on one-way avoidance was induced by shifting rats from a large reward (30 s spent in the safe compartment after completion of the avoidance response, pre-shift phase) to a small reward (1 s, post-shift phase). Under these conditions, the previously learned avoidance response deteriorated (negative contrast) when compared to a control group for which 'safe time' remained constant throughout the experimental situation (1 s). Thiopental sodium at a dose of 5 or 10 mg/kg, but not at 1, 2, 15 or 20 mg/kg i.p., abolished the negative contrast effect, and did not affect performance of the one-way avoidance task. Similar results were found when rats were treated with diazepam (1 mg/kg i.p.). Chlorpromazine at a dose of 0.5 or 1 mg/kg i.p. did not affect negative contrast, although at higher doses (2 or 3 mg/kg) there was an increase in the number of trials needed to reach the criterion for learning the avoidance response. This increase was evident in both pre-shift and post-shift phases, although only in the experimental situations involving a low level of reinforcement (1 s in the safe compartment). On the basis of these results, we tentatively suggest that the successive negative contrast effect in one-way avoidance in rats can be considered a useful pharmacological animal model for research into anxiety.