Effect of repeated methamphetamine pretreatment on freezing behavior induced by conditioned fear stress

Dopamine D2-like receptors on conditioned and unconditioned fear: A systematic review of rodent pharmacological studies

Growing evidence supports dopamine's role in aversive states, yet systematic reviews focusing on dopamine receptors in defensive behaviors are lacking. This study presents a systematic review of the literature examining the influence of drugs acting on dopamine D2-like receptors on unconditioned and conditioned fear in rodents. The review reveals a predominant use of adult male rats in the studies, with limited inclusion of female rodents. Commonly employed tests include the elevated plus maze and auditory-cued fear conditioning. The findings indicate that systemic administration of D2-like drugs has a notable impact on both innate and learned aversive states. Generally, antagonists tend to increase unconditioned fear, while agonists decrease it. Moreover, both agonists and antagonists typically reduce conditioned fear. These effects are attributed to the involvement of distinct neural circuits in these states. The observed increase in unconditioned fear induced by D2-like antagonists aligns with dopamine's role in suppressing midbrain-mediated responses. Conversely, the reduction in conditioned fear is likely a result of blocking dopamine activity in the mesolimbic pathway. The study highlights the need for future research to delve into sex differences, explore alternative testing paradigms, and identify specific neural substrates. Such investigations have the potential to advance our understanding of the neurobiology of aversive states and enhance the therapeutic application of dopaminergic agents.

65 years of research on dopamine's role in classical fear conditioning and extinction: A systematic review

Dopamine, a catecholamine neurotransmitter, has historically been associated with the encoding of reward, whereas its role in aversion has received less attention. Here, we systematically gathered the vast evidence of the role of dopamine in the simplest forms of aversive learning: classical fear conditioning and extinction. In the past, crude methods were used to augment or inhibit dopamine to study its relationship with fear conditioning and extinction. More advanced techniques such as conditional genetic, chemogenic and optogenetic approaches now provide causal evidence for dopamine's role in these learning processes. Dopamine neurons encode conditioned stimuli during fear conditioning and extinction and convey the signal via activation of D1-4 receptor sites particularly in the amygdala, prefrontal cortex and striatum. The coordinated activation of dopamine receptors allows for the continuous formation, consolidation, retrieval and updating of fear and extinction memory in a dynamic and reciprocal manner. Based on the reviewed literature, we conclude that dopamine is crucial for the encoding of classical fear conditioning and extinction and contributes in a way that is comparable to its role in encoding reward.

Effects of Methamphetamine Exposure on Fear Learning and Memory in Adult and Adolescent Rats

Methamphetamine (meth) use is often comorbid with anxiety disorders, with both conditions predominant during adolescence. Conditioned fear extinction is the most widely used model to study the fear learning and regulation that are relevant for anxiety disorders. The present study investigates how meth binge injections or meth self-administration affect subsequent fear conditioning, extinction and retrieval in adult and adolescent rats. In experiment 1, postnatal day 35 (P35-adolescent) and P70 (adult) rats were intraperitoneally injected with increasing doses of meth across 9 days. At P50 or P85, they underwent fear conditioning followed by extinction and test. In experiments 2a-c, P35 or P70 rats self-administered meth for 11 days then received fear conditioning at P50 or P85, followed by extinction and test. We observed that meth binge exposure caused a significant disruption of extinction retrieval in adult but not adolescent rats. Interestingly, meth self-administration in adolescence or adulthood disrupted acquisition of conditioned freezing in adulthood. Meth self-administration in adolescence did not affect conditioned freezing in adolescence. These results suggest that intraperitoneal injections of high doses of meth and meth self-administration have dissociated effects on fear conditioning and extinction during adulthood, while adolescent fear conditioning and extinction are unaffected.

Long-term effects of neonatal methamphetamine exposure on cognitive function in adolescent mice

Exposure to methamphetamine during brain development impairs cognition in children and adult rodents. In mice, these impairments are greater in females than males. Adult female, but not male, mice show impairments in novel location recognition following methamphetamine exposure during brain development. In contrast to adulthood, little is known about the potential effects of methamphetamine exposure on cognition in adolescent mice. As adolescence is an important time of development and is relatively understudied, the aim of the current study was to examine potential long-term effects of neonatal methamphetamine exposure on behavior and cognition during adolescence. Male and female mice were exposed to methamphetamine (5 mg/kg) or saline once a day from postnatal days 11 to 20, the period of rodent hippocampal development. Behavioral and cognitive function was assessed during adolescence beginning on postnatal day 30. During the injection period, methamphetamine-exposed mice gained less weight on average compared to saline-exposed mice. In both male and female mice, methamphetamine exposure significantly impaired novel object recognition and there was a trend toward impaired novel location recognition. Anxiety-like behavior, sensorimotor gating, and contextual and cued fear conditioning were not affected by methamphetamine exposure. Thus, neonatal methamphetamine exposure affects cognition in adolescence and unlike in adulthood equally affects male and female mice.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

Involvement of medial prefrontal cortex neurons in behavioral and cardiovascular responses to contextual fear conditioning

To explore the ventral medial prefrontal cortex (vMPFC) involvement in behavioral and autonomic fear-conditioned responses to context, vMPFC synaptic transmission was temporarily inhibited by bilateral microinjections of 200 nL of the nonselective synapse blocker CoCl(2) (1 mM). Behavioral activity (freezing, motor activity and rearing) as well as evoked cardiovascular responses (arterial pressure and heart rate) was analyzed. Rats were pre-exposed to the footshock chamber (context) and shock stimulus was used unconditioned stimulus. During re-exposure to context, conditioned rats spent 80% of the session in freezing while non-conditioned rats (no shock group) spent less than 15% of the session time in freezing. Conditioned rats had significantly lower activity scores than non-conditioned animals. Exposure to context increased mean arterial pressure (MAP) and heart rate (HR) of both groups. MAP and HR of the conditioned animals were markedly increased and remained at a high and stable level, whereas MAP and HR increases in non-conditioned animals were less pronounced and declined during the session. CoCl(2) microinjected in the vMPFC significantly reduced freezing and attenuated MAP and HR increase of the conditioned group. Cobalt-induced vMPFC inhibition also significantly reduced MAP and HR increase observed in non-conditioned animals, without any behavioral changes. The effect of vMPFC acute ablation on MAP and HR did not seem to be specific to the fear response because they were also evident in non-conditioned animals. The results indicate that vMPFC integrity is crucial for expression of fear-conditioned responses to context, such as freezing and cardiovascular changes, suggesting that fear-conditioned responses to context involve cortical processing prior to amygdalar output. They also indicate a cardiovascular response observed during re-exposure of non-conditioned rats to the context is completely dependent on vMPFC integrity.

Ontogeny of the adrenal response to (+)-methamphetamine in neonatal rats: the effect of prior drug exposure

We examined the ontogeny of the corticosterone response to (+)-methamphetamine in neonatal rats. In experiment-1, animals were injected with 10 mg/kg of (+)-methamphetamine or saline and plasma corticosterone levels were examined in separate groups 30 or 105 min later on postnatal day (P) 1, 3, 5, 7, 9, 11, 13, 15, 17, or 19. The adrenal response to methamphetamine was best described by a U-shaped function with the nadir of corticosterone release occurring between P7 and P13. Experiment-2 was similar except that the effect of four consecutive days of exposure to (+)-methamphetamine (four times daily at 2 h intervals with 10 mg/kg) was assessed with a single final dose early on the fifth day (i.e. P1-5, 3-7, 5-9, 7-11, 9-13, 11-15, 13-17, 15-19). The 30 min corticosterone response after multiple methamphetamine doses was augmented compared to single exposures, with the exception of the two earliest dosing intervals ending on P5 and P7, where the responses were lower. In addition, at 105 min, the levels of corticosterone were attenuated relative to a single drug administration. With the exception of animals receiving methamphetamine from P15 to P19, thymus weights were unaffected. The data demonstrate that (+)-methamphetamine is a robust activator of corticosterone release in developing animals and this release is extensively modified by age and previous drug exposure.

Effects of cannabidiol and diazepam on behavioral and cardiovascular responses induced by contextual conditioned fear in rats

Cannabidiol (CBD) is a non-psychotomimetic compound from Cannabis sativa that induces anxiolytic-like effects similar to diazepam in animal models of innate aversive behavior. However, the effects of CBD contextual conditioned fear have not been studied. Therefore, the aim of this work was to compare the behavioral and cardiovascular effects of CBD and diazepam, a prototype anxiolytic, in animals submitted to a contextual conditioned fear paradigm. Male Wistar rats were submitted to a 10min conditioning session (six footshocks, 2.5 mA, 3s, delivered at pseudo-random intervals). The behavioral and cardiovascular responses to the context were measured 24h later in a 10 min test session. Diazepam (2.5 mg/kg), FG-7142 (8 mg/kg), a benzodiazepine inverse agonist, or CBD (10 mg/kg) were administered i.p. before the test session. Conditioned rats submitted to the aversive context exhibited more freezing behavior and a larger increase in blood pressure and heart rate as compared to non-conditioned animals. These effects were attenuated by CBD and diazepam in the conditioned animals. These drugs did not have any effect in non-conditioned rats. FG-7142 treatment failed to change the behavioral and cardiovascular responses to the aversive context. In conclusion, the results suggest that CBD has anxiolytic-like properties similar to those of diazepam in a rat model of conditioned fear to context.

Chronic stress augments the long-term and acute effects of methamphetamine

There is growing evidence that exposure to stress alters the acute effects of abused drugs on the CNS. However, it is not known whether stress augments the longer-term neurotoxic effects of psychostimulant drugs, such as methamphetamine. Methamphetamine at high doses decreases forebrain dopamine concentrations. The current study tested the hypothesis that 10 days of unpredictable stress augmented striatal dopamine depletions 7 days following four injections of either 7.5 or 10 mg/kg methamphetamine (1 injection every 2 h). Furthermore, to assess the effects of chronic stress on immediate responses to methamphetamine, extracellular striatal dopamine and methamphetamine concentrations, and rectal temperature were monitored during the methamphetamine injection regimen. Seven days following either a 7.5 mg/kg or 10 mg/kg methamphetamine injection regimen, male rats exposed to unpredictable stress showed greater depletions in striatal dopamine tissue content compared with non-stressed controls injected with methamphetamine. Stressed rats had increased hyperthermic responses and dopamine efflux in the striatum during the methamphetamine injections when compared with non-stressed control rats. Moreover, stressed rats had an increased mortality rate (33%) compared with non-stressed controls (16.7%) following four injections of 10 mg/kg methamphetamine. The enhanced acute and longer-term effects of methamphetamine in stressed rats was not due to a greater concentrations of methamphetamine in the striatum, as extracellular levels of methamphetamine during the injection regimen did not differ between the two groups. In summary, exposure to 10 days of chronic unpredictable stress augments longer-term depletions of dopamine in the striatum, as well as acute methamphetamine-induced hyperthermia and extracellular dopamine levels. These findings suggest that chronic stress increases the responsiveness of the brain to the acute pharmacological effects of methamphetamine and enhances the vulnerability of the brain to the neurotoxic effects of psychostimulants.

Acute withdrawal from repeated cocaine treatment enhances latent inhibition of a conditioned fear response

Psychostimulant-induced locomotor sensitization and disrupted latent inhibition (LI) of a classically conditioned association are two paradigms that have been widely studied as animal behavioural models of psychosis. In this study we assessed the effects of withdrawal from the repeated intermittent administration of cocaine on LI of a conditioned fear response. Animals which were either preexposed (PE) to a tone conditioned stimulus (CS) or naive to the tone (i.e. non-preexposed: NPE) subsequently experienced 10 pairings of the tone CS with footshock. Afterwards, both groups received five daily injections of cocaine (20 mg/kg, i.p.) or saline. After 3 days of withdrawal from drug treatment, animals were tested for conditioned freezing to the context of the footshock chamber, and 1 day later, for conditioned freezing to the tone CS. Cocaine-sensitized animals exhibited markedly enhanced LI compared to saline-treated animals, due to the fact that NPE-cocaine animals spent more time freezing during the tone CS than NPE-saline animals, whereas PE-cocaine animals showed a tendency toward reduced freezing compared to the saline groups. While these results suggest the presence of increased anxiety in cocaine-withdrawn NPE animals, the absence of this effect in cocaine-withdrawn PE rats indicates that cocaine withdrawal also influences the retrieval of previously learned information.

Haloperidol catalepsy consolidation in the rat as a model of neuromodulatory integration

Haloperidol, a non-selective D(2) dopamine antagonist, both in vitro (1 microM) and in vivo (2.5 mg/kg i.p.), induced a long-term potentiation of K(+)-induced Ca(2+)-dependent release of endogenous noradrenaline and dopamine in rat brain cortical slices, by increasing the content of noradrenaline and dopamine known to be controlled by dopamine auto- and heteroreceptors. Haloperidol administration (2.5 mg/kg i.p.) evoked catalepsy and increased the content of noradrenaline and dopamine in the same structures of the brain. Haloperidol catalepsy consolidated without any additional learning and could be retrieved up to two weeks later by placing the animals in the test box. The catalepsy is disordered and retrieved only in the test box. The catalepsy was more intense on day 14 than on day 7. Injection of haloperidol immediately after conditioning evened the reflex retrieval on the following days. Moreover, learning increased the intensity of catalepsy in animals tested on the day of injection. Repeated testing of the reflex on the following days led to specific modifications of catalepsy retrieval. Pre-conditioned rats exhibited maximal catalepsy when tested immediately after being placed in the test box. These results suggest that both the processes of long-term potentiation and catalepsy consolidation are mediated by the same type of receptors, long-term modulation-inducing receptors. Endogenous neuromodulators, acting non-specifically or diffusely via their respective long-term modulation-inducing receptors, can initiate and consolidate generalized states which form the basis for emotional and motivational states.

Anxiogenic-like effect of acute and chronic fluoxetine on rats tested on the elevated plus-maze

The selective serotonin reuptake inhibitor fluoxetine (FLX) is widely prescribed for depression and anxiety-related disorders. On the other hand, enhanced serotonergic transmission is known to be classically related to anxiety. In this study, the effects of acute (5.0 mg/kg) and chronic (5.0 mg/kg, 22 days) FLX were investigated in both food-deprived and non-deprived rats tested in the elevated plus-maze. Significant main effects of the three factors (drug, food condition and administration regimen) were observed, but no interaction between them. The administration of either acute or chronic FLX resulted in an anxiogenic effect, as detected by a significant reduction in the percentage of time spent in the open arms and in the percentage of open arm entries. Food deprivation yielded an anxiolytic-like profile, probably related to changes in locomotor activity. The administration regimen resulted in an anxiolytic profile in chronically treated rats, as would be expected after 22 days of regular handling. The anxiogenic action of acute FLX is consistent with both its neurochemical and clinical profile. The discrepancy between the anxiogenic profile of chronic FLX and its therapeutic uses is discussed in terms of possible differences between the type of anxiety that is measured in the plus-maze and the types of human anxiety that are alleviated by fluoxetine.

Effects of footshock stress on regional brain monoamine metabolism and the acquisition of conditioned freezing in rats previously exposed to repeated methamphetamine

1. Effects of footshock stress on regional brain DA and 5-HT metabolism and the acquisition of conditioned fear in male Wistar-King rats previously exposed to repeated MA were examined. 2. MA-pretreated rats exhibited more freezing than control rats in the conditioned fear test. 3. The HVA levels in the striatum was elevated by footshock only in MA-treated rats but not saline-treated rats. Furthermore, MA-treated rats showed increased metabolism of DA in the mPFC, even when placed in the shock chamber without shocks. 4. The brain serotonergic responsiveness to fear conditioning was also attenuated by repeated administration of MA. 5. These results suggest that not only the brain DA systems but also the brain 5-HT systems play an important role for the development of MA-induced emotional hypersensitivity to stress.