Sex differences in passive avoidance depend on the integrity of the central serotonergic system

Prelimbic and Infralimbic Prefrontal Regulation of Active and Inhibitory Avoidance and Reward-Seeking

Flexible initiation or suppression of actions to avoid aversive events is crucial for survival. The prelimbic (PL) and infralimbic (IL) regions of the medial prefrontal cortex (mPFC) have been implicated in different aspects of avoidance and reward-seeking, but their respective contribution in instigating versus suppressing actions in aversive contexts remains to be clarified. We examined mPFC involvement in different forms of avoidance in rats well trained on different cued lever-press avoidance tasks. Active/inhibitory avoidance required flexible discrimination between auditory cues signaling foot-shock could be avoided by making or withholding instrumental responses. On a simpler active avoidance task, a single cue signaled when a lever press would avoid shock. PL inactivation disrupted active but not inhibitory avoidance on the discriminative task while having no effect on single-cued avoidance. In comparison, IL inactivation broadly impaired active and inhibitory avoidance. Conversely, on a cued appetitive go/no-go task, both IL and PL inactivation impaired inhibitory but not active reward-seeking, the latter effect being diametrically opposite to that observed on the avoidance task. These findings highlight the complex manner in which different mPFC regions aid in initiating or inhibiting actions in the service of avoiding aversive outcomes or obtaining rewarding ones. IL facilitates active avoidance but suppress inappropriate actions in appetitive and aversive contexts. In contrast, contextual valence plays a critical role in how the PL is recruited in initiating or suppressing actions, which may relate to the degree of cognitive control required to flexibly negotiate response or motivational conflicts and override prepotent behaviors.SIGNIFICANCE STATEMENT Choosing to make or withhold actions in a context-appropriate manner to avoid aversive events or obtain other goals is a critical survival skill. Different medial prefrontal cortex (mPFC) regions have been implicated in certain aspects of avoidance, but their contributions to instigating or suppressing actions remains to be clarified. Here, we show that the dorsal, prelimbic (PL) region of the medial PFC aids active avoidance in situations requiring flexible mitigation of response conflicts, but also aids in withholding responses to obtain rewards. In comparison the ventral infralimbic (IL) cortex plays a broader role in active and inhibitory avoidance as well as suppressing actions to obtain rewards. These findings provide insight into mechanisms underlying normal and maladaptive avoidance behaviors and response inhibition.

Electric foot shock stress: a useful tool in neuropsychiatric studies

Electric foot shock is a complex stressor with both physical and emotional components. It has been employed as an important tool to develop diverse animal models in the field of psychopharmacology. The electric foot shock paradigm includes acute or chronic exposures of shocks of varying intensity and duration on an electrified grid floor in an electric foot shock apparatus. Research evidence reveals that foot shocks of varying intensity produce behavioral and neurochemical changes reflecting depression, anxiety, and post-traumatic stress disorder (PTSD) in humans. Animals generally do not habituate to foot shocks in comparison to other stressors, including loud noise, bright light, and hot and cold temperatures. Additionally, it offers an experimental advantage of control over intensity and duration; therefore, by varying its application parameters, different disorder models have been created. Electric foot shock fear conditioning-induced ultrasonic vocalization and fear-potentiated startle have been explored to develop models of anxiety and panic. Similarly, fear conditioning in the form of foot shock exposure followed by situational reminders has been used to develop a model of PTSD. Electric foot shock-induced conflict has been explored to develop operant conflict models (Geller-Seifter and Vogel tests), which in turn are pharmacologically validated to screen potential anti-anxiety agents. Inescapable electric shock-induced ‘learned helplessness’ mimics the symptomology of depression, and this phenomenon has been employed to develop the model of depression. The present review describes the pharmacologically validated models of anxiety, depression, and PTSD involving electric foot shock as an aversive stimulus.

Behaviour of a genetic mouse model of depression in the learned helplessness paradigm

RATIONALE

H/Rouen (displaying a helpless phenotype in the tail suspension test) mice exhibiting features of depressive disorders and NH/Rouen (displaying non-helpless phenotype) mice were previously created through behavioural screening and selective breeding. Learned helplessness (LH), in which footshock stress induces a coping deficit, models some aspects of depression in rodents, but so far, fewer LH studies have been performed in mice than in rats.

OBJECTIVES

To study H/Rouen and NH/Rouen in the LH paradigm.

RESULTS

When CD1 mice were submitted to footshock with various training durations and shock intensities, the most suitable parameters to induce a behavioural deficit were 0.3 mA and four training sessions. A significantly longer latency to escape shocks was found in male H/Rouen mice compared to male NH/Rouen mice. On the other hand, once shocked, NH/Rouen mice showed more severe coping deficits than H/Rouen mice. In addition, a sub-chronic treatment with fluoxetine lacked efficacy in NH/Rouen mice, whereas it improved performances in H/Rouen mice. We also found that a shock reminder at day 8, subsequent to inescapable shocks, maintained helplessness for 20 days. Finally, female H/Rouen mice responded to chronic fluoxetine administration after 10 days of treatment, while a 20-day treatment was necessary to improve the behavioural deficit in H/Rouen male mice.

CONCLUSION

H/Rouen and NH/Rouen lines displayed different despair-related behaviour in the LH paradigm. Fluoxetine had beneficial effects after sub-chronic or chronic but not acute treatment of H/Rouen mice, thus providing a pharmacological validation of the protocols.

Sex differences in aversive memory in rats: possible role of extinction and reactive emotional factors

Studies usually show better spatial learning in males and stronger emotional memory in females. Spatial memory differences could relate to diverse strategies, while dissimilar stress reactions could cause emotional memory differences. We compared male and female rats in two emotional (classical emotional conditioning and aversive discrimination memory) and two emotionally "neutral" tasks: (1) plus-maze discriminative avoidance, containing two open and two enclosed arms, one of which presenting aversive stimuli (light/noise). No differences were found in learning, retrieving, or basal emotional levels, while only male rats presented extinction of the task; (2) contextual fear conditioning--a cage was paired to mild foot shocks. Upon reexposure, freezing behavior was decreased in females; (3) spontaneous alternation--the animals were expected to alternate among the arms of a four-arm maze. No differences between genders were found and (4) open-field habituation was addressed in an arena which the rats were allowed to explore for 10 min. Habituation was similar between genders. Differences were found only in tasks with strong emotional contexts, where different fear responses and stress effects could be determinant. The lack of extinction of discriminative avoidance by females points out to stronger consolidation and/or impaired extinction of aversive memories.

Females do not express learned helplessness like males do

Women are more likely than men to suffer from stress-related mental disorders, such as depression. In the present experiments, we identified sex differences in one of the most common animal models of depression, that of learned helplessness. Male and female rats were trained to escape a mild footshock each day for 7 days (controllable stress). Each rat was yoked to another rat that could not escape (uncontrollable stress), but was exposed to the same amount of shock. One day later, all stressed rats and unstressed controls were tested on a more difficult escape task in a different context. Most males exposed to uncontrollable stress did not learn to escape and were therefore helpless. In contrast, most females did learn to escape on the more difficult escape task, irrespective of whether they had been exposed to controllable or uncontrollable stress. The sex differences in helplessness behavior were not dependent on the presence of sex hormones in adulthood, because neither ovariectomy of females nor castration of males abolished them. The absence of helplessness in females was neither dependent on organizational effects of testosterone during the day of birth, because masculinized females did not express helplessness as adults. Thus, sex differences in helplessness behavior are independent of gonadal hormones in adulthood and testosterone exposure during perinatal development. Learned helplessness may not constitute a valid model for depressive behavior in women, at least as reflected by the response of female rats to operant conditioning procedures after stressful experience.

Chronic administration of fluoxetine impairs inhibitory avoidance in male but not female mice

The effects of chronic administration of fluoxetine (20 mg/kg/day i.p.) on a one-trial step-through inhibitory avoidance task were investigated in male and female CD1 mice. In Experiment 1, treatment was administered for 21 days before the training session, whereas in Experiment 2, other subjects were subjected to the same treatment starting 24 h after the training session. The comparison of test versus training latencies showed memory deterioration with pre-training administration of fluoxetine (Experiment 1), which affected males but not females. Sex differences in this task were also observed in Experiment 1, with females showing a better performance. Sex differences were evident in controls as well as in treated animals. The locomotor activity of the animals was also tested in Experiment 1. Due to the absence of sex differences in the drug effects on this measure, the sex differences in the effects of fluoxetine on inhibitory avoidance were hardly attributable to non specific effects on locomotor activity. The lack of effect of post-training administration of fluoxetine (Experiment 2) constitutes additional support of the idea that the observed effect on inhibitory avoidance in Experiment 1 is specifically related to learning and memory.

Sex differences in behavioral and neurochemical profiles after chronic stress: role of housing conditions

This experiment sought to identify the extent housing conditions can differentially enhance or dampen the effects chronic restraint stress has on exploration and object memory in male and female rats. Subjects were either pair- or singly housed during stress (21 days of 6-h restraint) and maintained under those conditions during poststress behavior testing (7 days). Neurochemical analysis of neural tissue was accomplished by HPLC with electrochemical detection. Interactions between stress and housing conditions were found across both sexes. Stress was associated with less activity in the center of the forced open-field in both sexes. Stress also decreased the latency for males to enter the free open-field to female levels. Object recognition was greatly impaired in double-housed males but unaffected by stress or housing in females. Object location memory was impaired in stressed males if they were singly housed, and females performed as well as control males only if they were stressed. Both sexes generally showed increased in hippocampal (CA3) norepinephrine levels in their respective stress groups. Singly housed subjects had higher CA1 serotonin levels compared to double-housed subjects, whereas in the prefrontal cortex, a general sex difference was found with females having higher levels of serotonin and dopamine metabolites. These results show that stress affects limbic neurochemistry across sex, although only males exhibit stress-dependent decrements in object memory. Housing condition also has a profound effect on neurochemistry and male performance on object recognition. Thus, housing condition is a critical variable for male models of stress that can influence the extent the stress manipulation affects behavior. The differences observed across sex are further discussed in the context of behavioral inhibition.

Neonatal caffeine alters passive avoidance retention in rats in an age- and gender-related manner

Chronic administration of an adenosine receptor antagonist disturbs spatial learning and memory in adult mice and neonatal caffeine exposure results in long-term behavioral and biochemical sequelae in mice and rats. We thus postulated that early treatment with caffeine would have latent effects on learning and memory as measured in a passive avoidance paradigm. Rats were not handled or received caffeine (15-20 mg/kg/day) by gavage over postnatal days 2-6. At 28 or 70-90 days of age, rats were trained to avoid an electrified grid and tested for retention 24 h, 72 h, and 7 days later. At 28 days, caffeine-exposed rats required more trials to meet criterion than did control rats, regardless of gender. There was minimal effect on retention of either neonatal treatment or gender at this age. At 70-90 days, there was no effect of either gender or treatment on learning; however, there was a significant effect of gender (P < 0.05) on retention at 24 h that was more pronounced in neonatally caffeine-treated rats (P < 0.01). At 72 h, the effect of caffeine on retention differed between male and female rats. Neonatal caffeine exposure significantly improved retention in females (P < 0.01) and significantly decreased retention in males (P < 0.05). Thus, caffeine exposure limited to the first week of life resulted in alterations in passive avoidance retention that became apparent over pubertal development. These changes were a function of the gender of the animal.

Impulsivity as a confounding factor in certain animal tests of cognitive function

Performance in cognitive tasks which require the subject to wait and/or to process a large amount of information can be disrupted by an increase in impulsive-like behaviour. Accordingly, a decrease in impulsive-like behaviour can improve performance in such tasks. Conversely, impulsive-like behaviour may improve performance in cognitive tasks where simple and fast responses and/or only little information processing is required. Thus, impulsivity constitutes a confounding factor in studies of cognitive function. Impulsive-like behaviour may be modified by serotonergic (5-HT) activity, with underactivity in 5-HT neurotransmission increasing impulsivity and vice versa. Drug- or lesion-induced alteration in 5-HT neurotransmission may, therefore, constitute suitable tools to investigate the role of impulsivity in animal tests of cognitive function. Benzodiazepines also increase impulsive-like behaviour, possibly by decreasing 5-HT neurotransmission. Hence, the effects of modulation of 5-HT systems and of the benzodiazepine-binding site on performance in animals tests of cognitive function will be discussed. It is predicted that the effects of manipulations of serotonergic activity or of benzodiazepine administration depend upon the nature of the response required, and that these effects may be mediated through changes in impulse control.

Effects of p-chloroamphetamine on brain serotonin neurons

p-Chloroamphetamine (PCA) is a useful pharmacologic tool for selectively increasing brain serotonin function acutely by release of serotonin into the synaptic cleft. PCA produces behavioral, neurochemical and neuroendocrine effects believed due to serotonin release after doses in the range of 0.5-5 mg/kg. At higher doses and at longer times, PCA causes depletion of brain serotonin. The mechanisms of this depletion are not well understood but require the serotonin uptake carrier. Antagonism of PCA-induced depletion of brain serotonin is a useful means of assessing the ability of a compound to block the serotonin uptake carrier on brain serotonin neurons. PCA can also be used as a neurotoxic agent to deplete brain serotonin in functional studies, apparently by destroying some serotonergic nerve terminals. Used in this way, PCA has an advantage over 5,6- and 5,7-dihydroxytryptamines in being effective by systemic injection, and it affects brain serotonergic projections with a different neuroanatomic specificity than the dihydroxytryptamines.

Sex effects in defensive behavior: baseline differences and drug interactions

Female rats consistently show a pattern of differences in defensive behaviors compared to males which parallel the effects of exposure to a nonpainful threat stimulus (cat or cat odor) in the same tests and measures. These indications of greater defensiveness for females are particularly common in situations involving potential, as opposed to actual and present, threat, a factor which probably also reflects ceiling or floor effects in situations involving very intense defensiveness. In addition, pharmacological studies indicate sex differences in the effects of selective serotonin (5-HT) receptor agonists and antagonists on defensive responding. These findings indicate that sex effects must be considered in studies of the pharmacological control of defensive behaviors, and suggest that responsivity to sex effects may be an additional criterion for the suitability of animal models of anxiety.

Effect of lifelong hypocaloric diet on discrete memory of the senescent rat

The memory retention abilities of aged rats fed different diets were assessed in two different avoidance tasks. The standard passive avoidance procedure revealed an age-related memory impairment in old rats fed a standard diet (ST), whereas old rats fed a hypocaloric diet (HY) behaved similarly to young animals. To clarify whether this deficit could be attributed only to cognitive decay and not other factors, such as the tendency of old rats to prefer darkness to light more than young and adult animals, a multiple passive avoidance task was performed. This test offers rats the possibility to escape to a dark chamber in which they have never been shocked, and thus provides a means of checking factors other than memory retention abilities. All the old rats showed a more marked preference to escape to darkness compared to young and adult animals. However, senescent animals fed a ST diet had poor memory retention abilities compared to aged animals fed the HY diet, and young and adult rats. The results of this test confirmed the findings of the standard passive avoidance task.

Sex differences in the effects of inescapable footshock on central catecholaminergic and serotonergic activity

In two experiments sex differences in changes in central noradrenergic, dopaminergic and serotonergic activity were measured immediately after a 30-min session of inescapable footshocks. In Experiment 1 concentrations of noradrenaline, dopamine, serotonin and their major metabolites were determined in the frontal cortex, hypothalamus, amygdala, striatum, mesencephalon and the medulla-pons area. Inescapable shock increased the activity of all 3 transmitter systems, as evidence by increased metabolite concentrations in specific brain areas. Shock-induced increments in metabolite levels were larger in females than in males, especially for the serotonergic system. In addition, shock presentation resulted in a decrement in the noradrenaline content in most areas studied. In the frontal cortex, noradrenaline was reduced by inescapable shock in males but not in females. In Experiment 2, sex-dependent neurochemical consequences of predictable versus unpredictable shocks were studied in the frontal cortex and the medulla-pons area. Similar to Experiment 1, both brain parts showed large shock-induced increments in the activity of the catecholaminergic systems. Differential effects of predictable and unpredictable shock were not found (frontal cortex) or were rather small (medulla-pons) and appeared sex-dependent for serotonin in this area. The sex differences in neurochemical change found in the first experiment were largely replicated in the second experiment. The relevance of the observed sex differences in central neurotransmitter reactivity for sex differences in behavior is discussed.

Behavioral differences between male and female rats: effects of gonadal hormones on learning and memory

The organizational, activational and reorganizational effects of gonadal hormones have been extensively investigated with respect to sexual, aggressive and maternal behavior. It has thus been established that manipulations of gonadal hormones during critical periods in development functionally affect reproductive behavior. The effects of gonadal hormones on nonreproductive behavior are not immediately obvious because of the fact that the behavioral effects of gonadal hormones on learning and memory have been investigated in a large number of unrelated experimental procedures. The present paper provides an organized overview of these different experimental procedures, summarizes the most important findings and discusses some of the variables which determine the effects of manipulations in gonadal hormones on learning and memory in male and female rats.

Scopolamine and methylscopolamine differentially affect fixed-consecutive-number performance of male and female Wistar rats

Male and female Wistar rats were trained on a fixed-consecutive-number schedule in which a response on a food lever was followed by the presentation of reinforcement when at least three, but not more than seven responses had been completed on a work lever. Subjects were treated with different doses of the centrally acting cholinergic antagonist scopolamine hydrobromide or the more peripherally active cholinergic antagonist scopolamine methylbromide (0.08, 0.16 or 0.32 mg/ml/kg) once behavior had stabilized. Scopolamine hydrobromide and scopolamine methylbromide dose-dependently decreased response rates in males and females. Scopolamine methylbromide decreased response rates more than equivalent doses of scopolamine hydrobromide and the rate-suppressant effects of both drugs were more marked in males than in females. Scopolamine hydrobromide dose-dependently decreased response accuracy, but differences between males and females were not observed. Response accuracy also decreased after scopolamine methylbromide, but did not vary as a function of the dose of the drug. The decrease in response accuracy induced by both drugs was attributable to an increase in the percentage of trials with a premature switch from the work lever to the food lever. Both scopolamine hydrobromide and scopolamine methylbromide dose-dependently increased the number of premature switches. Differences between males and females were not observed. Administration of scopolamine hydrobromide and scopolamine methylbromide also decreased the number of obtained reinforcers in a dose-dependent manner. Females obtained significantly fewer reinforcers than males, while scopolamine methylbromide affected the number of obtained reinforcers to a larger extent than scopolamine hydrobromide.