Endocrine and behavioral traits in low-avoidance Sprague-Dawley rats

The modulatory role of serotonin-1A receptors of the basolateral amygdala and dorsal periaqueductal gray on the impact of hormonal variation on the conditioned fear response

Despite significant advances in the study of fear and fear memory formation, little is known about fear learning and expression in females. This omission has been proven surprising, as normal and pathological behaviors are highly influenced by ovarian hormones, particularly estradiol and progesterone. In the current study, we investigated the joint influence of serotonin (5-HT) neurotransmission and estrous cycle phases (low or high levels of estradiol and progesterone) on the expression of conditioned fear in a group of female rats that were previously divided according to their response to stressful stimuli into low or high anxiety-like subjects. The baseline amplitude of the unconditioned acoustic startle responses was high in high-anxiety female rats, with no effect on the estrous cycle observed. Data collected during the proestrus-estrus phase revealed that low-anxiety rats had startle amplitudes similar to those of high-anxiety rats. It is supposed that high-anxiety female rats benefit from increased estradiol and progesterone levels to achieve comparable potentiated startle amplitudes. In contrast, female rats experienced a significant decrease in hormone levels during the Diestrus phase. This decrease is believed to play a role in preventing them from displaying a heightened startle response when faced with strongly aversive stimuli. Data collected after 5-HT and 8-OH-DPAT were administered into the basolateral nuclei and dorsal periaqueductal gray suggest that 5-HT neurotransmission works with progesterone and estrogen to reduce startle potentiation, most likely by activating the serotonin-1A receptor subtype.

Revisiting the role of anxiety in the initial acquisition of two-way active avoidance: pharmacological, behavioural and neuroanatomical convergence

Two-way active avoidance (TWAA) acquisition constitutes a particular case of approach -avoidance conflict for laboratory rodents. The present article reviews behavioural, psychopharmacological and neuroanatomical evidence accumulated along more than fifty years that provides strong support to the contention that anxiety is critical in the transition from CS (conditioned stimulus)-induced freezing to escape/avoidance responses during the initial stages of TWAA acquisition. Thus, anxiolytic drugs of different types accelerate avoidance acquisition, anxiogenic drugs impair it, and avoidance during these initial acquisition stages is negatively associated with other typical measures of anxiety. In addition behavioural and developmental treatments that reduce or increase anxiety/stress respectively facilitate or impair TWAA acquisition. Finally, evidence for the regulation of TWAA acquisition by septo-hippocampal and amygdala-related mechanisms is discussed. Collectively, the reviewed evidence gives support to the initial acquisition of TWAA as a paradigm with considerable predictive and (in particular) construct validity as an approach-avoidance conflict-based rodent anxiety model.

Exposure to an obesogenic diet during adolescence leads to abnormal maturation of neural and behavioral substrates underpinning fear and anxiety

BACKGROUND

Post-traumatic stress disorder (PTSD) and obesity are highly prevalent in adolescents. Emerging findings from our laboratory and others are consistent with the novel hypothesis that obese individuals may be predisposed to developing PTSD. Given that aberrant fear responses are pivotal in the pathogenesis of PTSD, the objective of this study was to determine the impact of an obesogenic Western-like high-fat diet (WD) on neural substrates associated with fear.

METHODS

Adolescent Lewis rats (n = 72) were fed with either the experimental WD (41.4% kcal from fat) or the control diet. The fear-potentiated startle paradigm was used to determine sustained and phasic fear responses. Diffusion tensor imaging metrics and T2 relaxation times were used to determine the structural integrity of the fear circuitry including the medial prefrontal cortex (mPFC) and the basolateral complex of the amygdala (BLA).

RESULTS

The rats that consumed the WD exhibited attenuated fear learning and fear extinction. These behavioral impairments were associated with oversaturation of the fear circuitry and astrogliosis. The BLA T2 relaxation times were significantly decreased in the WD rats relative to the controls. We found elevated fractional anisotropy in the mPFC of the rats that consumed the WD. We show that consumption of a WD may lead to long-lasting damage to components of the fear circuitry.

CONCLUSIONS

Our findings demonstrate that consumption of an obesogenic diet during adolescence has a profound impact in the maturation of the fear neurocircuitry. The implications of this research are significant as they identify potential biomarkers of risk for psychopathology in the growing obese population.

Short- and long-term effects of risperidone on catalepsy sensitisation and acquisition of conditioned avoidance response: Adolescent vs adult rats

The effects of antipsychotic drugs (APDs) on the adolescent brain are poorly understood despite a dramatic increase in prescription of these drugs in adolescents over the past twenty years. Neuronal systems continue to be remodeled during adolescence. Therefore, when given in adolescence, antipsychotic drugs (APDs) have the potential to affect this remodeling. In this study we investigated the effects of chronic 22-day risperidone treatment (1.3mg/kg/day) in both adolescent and adult rats. We examined short- and long-term changes in behaviour (catalepsy, locomotion and conditioned avoidance response (CAR)), and dopaminergic and serotonergic neurochemistry in the striatum and the nucleus accumbens. Here, we report that, both during chronic treatment and after a lengthy drug-free interval, risperidone induced a sensitised cataleptic response regardless of the age of exposure. Selectively in adolescents, risperidone-induced catalepsy was inversely correlated with striatal dopamine turnover immediately after chronic treatment. After a drug-free interval, a significant proportion of rats with prior adolescent risperidone treatment also failed to acquire CAR to a defined criterion. Our data provide evidence that the same chronic risperidone treatment regimen can induce contrasting short- and long-term neural outcomes in the adolescent and adult brains.

Effects of oxytocin on background anxiety in rats with high or low baseline startle

RATIONALE

Oxytocin has antianxiety properties in humans and rodents. However, the antianxiety effects have been variable.

OBJECTIVES

To reduce variability and to strengthen the antianxiety effect of oxytocin in fear-potentiated startle, two experiments were performed. First, different amounts of light-shock pairings were given to determine the optimal levels of cue-specific fear conditioning and non-predictable startle (background anxiety). Second, the antianxiety effects of oxytocin were examined in rats with high and low pre-fear conditioning baseline startle to determine if oxytocin differentially affects high and low trait anxiety rats.

METHODS

Baseline pre-fear conditioning startle responses were first measured. Rats then received 1, 5, or 10 light-shock pairings. Fear-potentiated startle was then tested with two trial types: light-cued startle and non-cued startle trials. In the second experiment, rats fear conditioned with 10 light-shock pairings were administered either saline or oxytocin before a fear-potentiated startle test. Rats were categorized as low or high startlers by their pre-fear conditioning startle amplitude.

RESULTS

Ten shock pairings produced the largest non-cued startle responses (background anxiety), without increasing cue-specific fear-potentiated startle compared to one and five light-shock pairings. Cue-specific fear-potentiated startle was unaffected by oxytocin. Oxytocin reduced background anxiety only in rats with low pre-fear startle responses.

CONCLUSIONS

Oxytocin has population selective antianxiety effects on non-cued unpredictable threat, but only in rats with low pre-fear baseline startle responses. The low startle responses are reminiscent of humans with low startle responses and high trait anxiety.

Oxytocin reduces background anxiety in a fear-potentiated startle paradigm

Oxytocin reportedly decreases anxious feelings in humans and may therefore have therapeutic value for anxiety disorders, such as post-traumatic stress disorder (PTSD). As PTSD patients have exaggerated startle responses, a fear-potentiated startle paradigm in rats may have face validity as an animal model to examine the efficacy of oxytocin in treating these symptoms. Oxytocin (0, 0.01, 0.1, or 1.0 μg, subcutaneously) was given either 30 min before fear conditioning, immediately after fear conditioning, or 30 min before fear-potentiated startle testing to assess its effects on acquisition, consolidation, and expression of conditioned fear, respectively. Startle both in the presence and absence of the fear-conditioned light was significantly diminished by oxytocin when administered at acquisition, consolidation, or expression. There was no specific effect of oxytocin on light fear-potentiated startle. In an additional experiment, oxytocin had no effects on acoustic startle without previous fear conditioning. Further, in a context-conditioned test, previous light-shock fear conditioning did not increase acoustic startle during testing when the fear-conditioned light was not presented. The data suggest that oxytocin did not diminish cue-specific conditioned nor contextually conditioned fear, but reduced background anxiety. This suggests that oxytocin has unique effects of decreasing background anxiety without affecting learning and memory of a specific traumatic event. Oxytocin may have antianxiety properties that are particularly germane to the hyper-vigilance and exaggerated startle typically seen in PTSD patients.

Effects of administration of sodium glutamate during the neonatal period on behavior and blood corticosterone levels in male mice

Treatment of male DBA/2 mice with sodium glutamate (4 mg/g) on postnatal days 1, 3, 5, 7, and 9 induced reductions in the numbers of square crossings, vertical rearings, excursions to the center, and the time spent in the center in adulthood, as compared with a group of males given physiological saline at the same times. These measures showed no change as compared with intact animals. In the light-dark test, the time spent by mice in the light sector was greater after administration of sodium glutamate than after administration of physiological saline but did not differ from that in intact animals. In the acoustic startle reflex test, sodium glutamate decreased startle amplitude but had no effect on the magnitude of prestimulus inhibition. Sexual motivation in males decreased after sodium glutamate, physiological saline producing a tendency to decreased sexual motivation. Neonatal administration of sodium glutamate increased basal blood corticosterone in adult males by a factor of 4, while physiological saline had no effect on this measure. These results lead to the conclusion that neonatal administration of sodium glutamate decreases motor and investigative activity, anxiety, and sexual motivation in adult male mice and increases basal corticosterone. Physiological saline increased all these parameters apart from sexual motivation, though this was not associated with changes in basal corticosterone.

Extinction of a conditioned response in rainbow trout selected for high or low responsiveness to stress

Two lines of rainbow trout (Oncorhynchus mykiss) that exhibit divergent endocrine responsiveness to stressors also display disparate behavioral traits. To investigate whether the high-responding (HR) and low-responding (LR) fish also differ in cognitive function, the rate of extinction of a conditioned response was compared between the two lines. Groups of HR and LR fish were exposed to a paired conditioned stimulus (CS; water off) and unconditioned stimulus (US; confinement stressor). After exposure to 18 CS-US pairings, at least 70% of individuals of both lines acquired a conditioned response (CR) manifested as an elevation of blood cortisol levels on presentation of the CS only. Post-conditioning, the fish were tested by presentation of the CS at weekly intervals, for 4 weeks, with no further reinforcement, and the extinction of the CR in the two lines was compared. The decline in mean plasma cortisol levels after exposure to the CS over successive tests suggested that the CR was retained for a shorter period among the HR (<14 days) than LR fish (<21 days). The frequency of individuals within each line whose plasma cortisol levels indicated a stress response when exposed to the CS was significantly greater among the LR than HR fish at 14 and 21 days with no HR fish falling into this category at 21 days. At 28 days post-conditioning, there were no HR fish and only three LR fish were categorized as "stressed". These results suggest that there are differences in cognitive function between the two lines. Possible mechanisms underlying these differences are discussed.

Behavioral and cellular consequences of increasing serotonergic activity during brain development: a role in autism?

The hypothesis explored in this review is that the high levels of serotonin in the blood seen in some autistic children (the so-called hyperserotonemia of autism) may lead to some of the behavioral and cellular changes also observed in the disorder. At early stages of development, when the blood-brain Barrier is not yet fully formed, the high levels of serotonin in the blood can enter the brain of a developing fetus and cause loss of serotonin terminals through a known negative feedback function of serotonin during development. The loss of serotonin innervation persists throughout subsequent development and the symptoms of autism appear. A review of the basic scientific literature on prenatal treatments affecting serotonin is given, in support of this hypothesis, with an emphasis on studies using the serotonin agonist, 5-methoxytryptamine (5-MT). In work using 5-MT to mimic hyperserotonemia, Sprague-Dawley rats are treated from gestational day 12 until postnatal 20. In published reports, these animals have been found to have a significant loss of serotonin terminals, decreased metabolic activity in cortex, changes in columnar development in cortex, changes in serotonin receptors, and "autistic-like" behaviors. In preliminary cellular findings given in this review, the animals have also been found to have cellular changes in two relevant brain regions: 1. Central nucleus of the amygdala, a brain region involved in fear-responding, where an increase in calcitonin gene related peptide (CGRP) was found 2. Paraventricular nucleus of the hypothalamus, a brain region involved in social memory and bonding, where a decrease in oxytocin was found. Both of these cellular changes could result from loss of serotonin innervation, possibly due to loss of terminal outgrowth from the same cells of the raphe nuclei. Thus, increased serotonergic activity during development could damage neurocircuitry involved in emotional responding to social stressors and may have relevance to the symptoms of autism.

Sex and species differences in plasma oxytocin using an enzyme immunoassay

The neuropeptide hormone oxytocin (OT) is released peripherally and centrally and has been implicated in both physiology and behavior, especially sociosexual behaviors. Knowledge of OT levels in blood or other sources would be useful but these are rarely reported. Radioimmunoassay following extraction is the most commonly used method for measuring OT but is not ideal for use in small mammals in which blood volumes and concentrations of OT are low. Here we report a chemical and biological validation for a commercially available enzyme immunoassay for OT in unextracted plasma. In addition, comparisons of OT were made across species to allow comparison of the monogamous prairie vole (Microtus ochrogaster (Wagner, 1842)) to the polygynous Sprague Dawley rat. These species were chosen because OT plays a role in the formation of social bonds and we predicted that the highly social prairie vole would have higher plasma OT than the less social rat. Results of this comparison confirmed our hypothesis. Further, OT was significantly higher in females than in males in both species. Our results indicate that this enzyme immunoassay can be used to assay plasma OT in rodents and that the predicted correlations exist between plasma OT and gender as well as species-typical social behavior.

Improved conditioned avoidance learning by oxytocin administration in high-emotional male Sprague-Dawley rats

OBJECTIVE

To examine anti-stress-like properties of oxytocin as a means to improve conditioned avoidance learning in a low-performing, high-emotional, stock of Sprague-Dawley male rats.

METHODS

Adult male rats of two stocks of the Sprague-Dawley strain, designated Stock A and Stock B, were treated daily with oxytocin (1 mg kg(-1) s. c.) for 5 days preceding four daily conditioned avoidance acquisition sessions (approximately 20 trials per 15 min session). The Stock B animals were previously characterized as high-emotional based on [1] elevated plasma corticosterone, and lowered plasma oxytocin, levels and [2] decreased reaction time and an increased startle amplitude to an acoustic stimulation. Finally, [3] these animals were unable to acquire a conditioned avoidance response within 5 days of training.

RESULTS

The Stock A animals rapidly and statistically significantly acquired the avoidance behaviour within 4 days of daily training, whereas Stock B animals did not improve over this time period. The avoidance performance of Stock B animals was markedly and statistically significantly improved by the oxytocin pre-treatment, whereas the performance of Stock A animals was not affected by the same oxytocin treatment.

CONCLUSIONS

Pre-treatment with oxytocin markedly improved avoidance learning in the Stock B high-emotional animals. It is suggested that the improvement is due to previously demonstrated anti-stress-like properties of oxytocin, rendering the animals able to successfully cope with the demands of the conditioned avoidance situation.