Thermal stress modulates temporal patterns of responding on a multiple DRL-FR schedule

Acute stress differentially alters reward-related decision making and inhibitory control under threat of punishment

Acute stress has various effects on cognition, executive function and certain forms of cost/benefit decision making. Recent studies in rodents indicate that acute stress differentially alters reward-related decisions involving particular types of costs and slows choice latencies. Yet, how stress alters decisions where rewards are linked to punishment is less clear. We examined how 1 h restraint stress, followed by behavioral testing 10 min later altered action-selection on two tasks involving reward-seeking under threat of punishment in well-trained male and female rats. One study used a risky decision-making task involving choice between a small/safe reward and a large/risky one that could coincide with shock, delivered with a probability that increased over blocks of trials. Stress increased risk aversion and punishment sensitivity, reducing preference for the larger/risky reward, while increasing decision latencies and trial omissions in both sexes, when rats were teste. A second study used a "behavioral control" task, requiring inhibition of approach towards a readily available reward associated with punishment. Here, food pellets were delivered over discrete trials, half of which coincided with a 12 s audiovisual cue, signalling that reward retrieval prior to cue termination would deliver shock. Stress exerted sex- and timing-dependent effects on inhibitory control. Males became more impulsive and received more shocks on the stress test, whereas females were unaffected on the stress test, and were actually less impulsive when tested 24 h later. None of the effects of restraint stress were recapitulated by systemic treatment with physiological doses of corticosterone. These findings suggest acute stress induces qualitatively distinct and sometimes sex-dependent effects on punished reward-seeking that are critically dependent on whether animals must either choose between different actions or withhold them to obtain rewards and avoid punishment.

Task-Dependent Differences in Operant Behaviors of Rats With Acute Exposure to High Ambient Temperature: A Potential Role of Hippocampal Dopamine Reuptake Transporters

Behavioral or cognitive functions are known to be influenced by thermal stress from the change in ambient temperature (Ta). However, little is known about how increased Ta (i.e., when the weather becomes warm or hot) may affect operant conditioned behavior and the neural substrates involved. The present study thus investigated the effects of high Ta on operant behaviors maintained on a fixed-ratio 1 (FR1) and a differential reinforcement for low-rate responding 10 s (DRL 10-s) schedule of reinforcement. The rats were randomly assigned to three groups receiving acute exposure to Ta of 23°C, 28°C, and 35°C, respectively, for evaluating the effects of high Ta exposure on four behavioral tests. Behavioral responses in an elevated T-maze and locomotor activity were not affected by Ta treatment. Regarding operant tests, while the total responses of FR1 behavior were decreased only under 35°C when compared with the control group of 23°C, those of DRL 10-s behavior were significantly reduced in both groups of 28°C and 35°C. Distinct patterns of inter-response time (IRT) distribution of DRL behavior appeared among the three groups; between-group differences of behavioral changes produced by high Ta exposure were confirmed by quantitative analyses of IRT data. Western blot assays of dopamine (DA) D1 and D2 receptor, DA transporter (DAT) and brain-derived neurotrophic factor (BDNF) were conducted for the sample tissues collected in six brain areas from all the subjects after acute high Ta exposure. Significant Ta-related effects were only revealed in the dorsal hippocampus (dHIP). In which, the DAT levels were increased in a Ta-dependent fashion that was associated with operant behavior changes under high Ta exposure. And, there as an increased level of D1 receptors in the 28°C group. In summary, these data indicate that the performance of operant behavior affected by the present high Ta exposure is task-dependent, and these changes of operant behaviors cannot be attributed to gross motor function or anxiety being affected. The regulation of dHIP DAT may be involved in this operant behavioral change under high Ta exposure.

Chronic intermittent cold stress and serotonin depletion induce deficits of reversal learning in an attentional set-shifting test in rats

RATIONALE

Chronic stress perturbs modulatory brain neurotransmitter systems, including serotonin (5-HT), and is a risk factor for psychiatric disorders such as depression. Deficits in cognitive flexibility, reflecting prefrontal cortical dysfunction, are prominent in such disorders. Orbitofrontal cortex (OFC) has been implicated specifically in reversal learning, a form of cognitive flexibility modulated by 5-HT.

OBJECTIVES

The objectives of the study were (1) to assess the effects of chronic intermittent cold (CIC) stress, a potent metabolic stressor, on performance of rats in an attentional set-shifting test (AST), and (2) to assess a possible role for serotonin in CIC-induced deficits and test the effects of acute serotonin reuptake blockade.

MATERIALS AND METHODS

Male Sprague-Dawley rats were exposed to CIC stress (14 days x 6 h/day at 4 degrees C) before testing on the AST. In subsequent experiments, brain 5-HT was depleted in naïve rats with para-chlorophenylalanine or 5-HT release was increased acutely in CIC-stressed rats with citalopram (5 mg/kg, s.c.) given 30 min prior to the first reversal task. Microdialysis was used to assess CIC-induced changes in 5-HT release in OFC during testing.

RESULTS

CIC-stressed rats exhibited a selective impairment on the first reversal task in the AST. 5-HT depletion induced a similarly selective deficit in reversal learning. The CIC-induced impairment in reversal learning was attenuated by acute 5-HT reuptake blockade. 5-HT release was reduced in OFC of CIC-stressed rats during behavioral testing.

CONCLUSIONS

The CIC stress-induced impairment of cognitive flexibility may involve dysregulation of 5-HT modulatory function in OFC. Such deficits may thus model relevant symptoms of neuropsychiatric disorders that respond positively to SSRI treatment.

Appetitive operant behavior and free-feeding in rats exposed to acute stress

This study investigated whether appetitive operant food reinforcement or free-feeding behavior in rats, food-restricted to 85% of body weight, was disrupted by exposures to 3 h of restraint stress or by 3 h of restraint plus water immersion stress (RWI). Rats were trained under a 3-cycle 10-min time-out-10-min time-in fixed-ratio 15 (FR15) schedule of food reinforcement. Free-feeding was measured in a 3-cycle 10-min food jar-out-10-min food jar-in test conducted in the operant chambers. Three hours of restraint stress did not significantly affect response rate or food reinforcement in the operant FR15 task or in the free-feeding condition. In contrast, 3 h of RWI completely abolished operant food reinforcement and suppressed response rate, whereas free-feeding was significantly reduced but not abolished in food-restricted, hungry rats. We conclude that acute restraint stress in food-restricted, hungry rats does not affect their appetite or motor ability to lever-press for food nor did it affect their ability to feed in a free-feeding situation. In contrast, RWI stress may have induced a motor impairment, or some other aspect of motivation independent of hunger, that disrupted their performance in the operant FR15 task.