The light switch-off response as a putative rodent test of innate fear

Meta-Chlorophenylpiperazine-Induced Behavioral Changes in Obsessive-Compulsive Disorder Research: A Systematic Review of Rodent Studies

Meta-chlorophenylpiperazine (mCPP) was one of the first compounds used in clinical and preclinical studies that demonstrated the role of serotonin in Obsessive-Compulsive Disorder (OCD). This systematic review aimed to (a) identify publications that report in rodents the effects of mCPP relevant to OCD, (b) explore the methodological characteristics of these studies, and (c) summarize the profile of mCPP effects. A comprehensive literature search was performed using PubMed, Scopus, and Web of Science. Search terms were a combination of obsessive-compulsive disorder or OCD and meta-chlorophenylpiperazine or mCPP. Twenty-nine articles were included in the review. The years of publication ranged from 1993 to 2021. Most studies used adult male Wistar or Sprague-Dawley rats. The most frequent dose of mCPP was 1.0 mg/kg administered acutely, intraperitoneally. In general, available preclinical evidence suggests increased defensive and compulsive behaviors associated with a decreased locomotor activity. But other results besides these and the absence of significant mCPP effects were also observed. Among the factors that may contribute to the variability of mCPP effects, differences in methods are highlighted, such as characteristics of the species/strains studied, mCPP doses and treatment regimens used. The heterogeneity of the OCD-like behaviors evaluated and the interaction of mCPP with different receptors may also be critical variables for discrepancies in the findings with mCPP. The information described in this review may contribute to a better understanding of how mCPP-induced behavioral changes in rodents have been used to study OCD, highlighting the main challenges for future investigations in this field.

Decrease in the rewarding value of spatial novelty due to the contamination of the stimulus field with light - evidence from a free exploration test involving rats

It has been shown that rearranging the spatial properties of a familiar environment consistently elicits a positive response in rats directed toward the source of novelty. Previous studies have been conducted under red light or darkness. The purpose of this study was to test the effect of rearranging the spatial properties of a familiar environment in conjunction with a change in lighting conditions. The results have shown specific effects of the light presence and its intensity on different behavioral measures. We propose that this study provides a basis for hypothesizing a two-way mechanism of the behavioral response to light regulation in rats. The first is based on ON/OFF states. This level may be related to fundamental, evolutionarily early, emergent components of behavioral antipredator adaptations. Another level of behavioral regulation involves mechanisms sensitive to light intensity. These appear to be involved in the regulation of more advanced behavioral acts, such as exploratory responses. This may suggest that light intensity analysis may require the involvement of more advanced cognitive components in the behavioral regulation system.

Effects of Immediate Aversive Stimulation on Haloperidol-Induced Catalepsy in Rats

In animal models, the administration of the dopaminergic D2 antagonist haloperidol affects the nigrostriatal pathway, inducing catalepsy, a state of immobility similar to Parkinson’s disease (PD) bradykinesia and akinesia. In PD, the motor impairments are due to difficulties in selecting and executing motor actions, associated with dopamine loss in basal ganglia and cortical targets. Motor and affective limbic networks seem to be integrated via a striato-nigro-striatal network, therefore, it is not surprising that the motor impairments in PD can be influenced by the patient’s emotional state. Indeed, when exposed to aversive stimuli or life-threatening events, immobile patients are capable of performing sudden movements, a phenomenon known as paradoxical kinesia. Thus, the present study investigated the effects of unconditioned and conditioned aversive stimulation on haloperidol-induced catalepsy in rats. First, male Wistar rats received intraperitoneal administration of saline or haloperidol (1 or 2 mg/kg) and were evaluated in the catalepsy bar test to assess the cataleptic state induced by the different doses of haloperidol over time. Next, we evaluated the effects of two types of unconditioned aversive stimuli–100 lux light (1 and 20 s) or 0.6 mA footshock (1 s)–on the catalepsy. Finally, we evaluated the effects of light conditioned stimuli (Light-CS), previously paired with footshocks, on the cataleptic state. Catalepsy was observed following haloperidol 1 and 2 mg/kg administration. Exposure to footshocks, but not to light, significantly reduced step-down latency during the catalepsy test. Although unconditioned light did not affect catalepsy, paired Light-CS did reduce step-down latency. Here, we have provided evidence of face validity for the study of paradoxical kinesia. In addition to demonstrating that immediate exposure to an aversive stimulus is capable of disrupting the cataleptic state, our findings show that haloperidol-induced catalepsy seems to be differently influenced depending on the modality of aversive stimulation. Our data suggest that the selective recruitment of threat response systems may bypass the dysfunctional motor circuit leading to the activation of alternative routes to drive movement.

Short-term, low-dose fluoxetine prevents oestrous cycle-linked increase in anxiety-like behaviour in female rats

BACKGROUND AND AIMS

We sought a robust behavioural test that evoked increased anxiety-like behaviour during the late dioestrus phase of the oestrous cycle (similar to the premenstrual period in women) and tested whether this could be prevented by acute low-dose fluoxetine (FLX).

METHODS

Female Wistar rats in different stages of their cycle were exposed to four different tests of anxiety-like behaviour.

RESULTS

No oestrous cycle differences were detected in fear potentiated startle or conditioned freezing to an aversive context. In a light switch-off test where rats move from one compartment of a shuttle-box to the other to turn off an aversive light, females displayed enhanced responding in late dioestrus. During isolation restraint stress females in late dioestrus emitted three times more 22 kHz ultrasound vocalisations (USV) than at other cycle stages. Using the USV test, short-term administration of low-dose FLX (1.75 mg kg^-1, i.p.) designed to blunt the sharp fall in brain allopregnanolone concentration during late dioestrus but without affecting 5-HT systems, prevented the increase in isolation stress-evoked USVs.

CONCLUSIONS

The light switch-off and isolation restraint-induced USV tests evoke unconditioned adverse emotional responses that are ethologically relevant and sensitive to oestrous cycle stage. The USV test fulfils many criteria required of a model for premenstrual syndrome in women. Using the USV test, short-term administration of FLX to increase brain allopregnanolone concentration without affecting 5-HT systems prevented the increased USV responding in late dioestrus. Short-term low-dose FLX treatment may have potential to alleviate development of adverse premenstrual symptoms in women.

Intranasal administration of dopamine attenuates unconditioned fear in that it reduces restraint-induced ultrasound vocalizations and escape from bright light

BACKGROUND

Although substantial evidence suggests that dopamine (DA) enhances conditioned fear responses, few studies have examined the role of DA in unconditioned fear states. Whereas DA does not cross the blood-brain barrier, intranasally-applied dopamine reaches the brain directly via the nose-brain pathways in rodents, providing an alternative means of targeting DA receptors. Intranasal dopamine (IN-DA) has been demonstrated to bind to DA transporters and to increase extracellular DA in the striatum as well as having memory-promoting effects in rats. The purpose of this study was to examine the influence of IN-DA in three tests of fear/anxiety.

METHODS

The three doses of DA hydrochloride (0.03, 0.3, or 1 mg/kg) were applied in a viscous castor oil gel in a volume of 5 µl to each of both nostrils of adult Wistar rats prior to testing of (a) escape from a bright light, using a two-chamber procedure, (b) restraint-induced 22 kHz ultrasound vocalizations (USVs), and (c) exploratory behavior in the elevated plus-maze (EPM).

RESULTS

IN-DA dose-dependently reduced escape from bright light and the number of USV responses to restraint. It had no influence on the exploratory behavior in the EPM.

CONCLUSIONS

IN-DA application reduced escape behavior in two tests of unconditioned fear (escape from bright light and USV response to immobilization). These findings may be interpreted in light of the known antidepressant action of IN-DA and DA reuptake blockers. The results also confirm the promise of the nasal route as an alternative means for targeting the brain's dopaminergic receptors with DA.