Sex differences in behavior and glutamic acid decarboxylase in Long Evans rats after prolonged social isolation beginning in adolescence.

Social isolation can have long-term effects on brain development and behavior and increases the risk of developing clinical conditions, including anxiety disorders. One modulator of the stress response is gamma-aminobutyric acid, an inhibitory neurotransmitter synthesized by glutamic acid decarboxylase (GAD). This study examined sex differences in behavior and GAD expression following prolonged social isolation beginning in adolescence in Long Evans rats. Males and females were equally divided into group-housed (GH) and socially isolated conditions on Postnatal Day 28 (n = 8 per group). Beginning 5 weeks later, tests were conducted for anxietylike behaviors (open-field test and elevated plus maze), social interactions (sociability test), and spatial memory (novel object location). Sex differences in behavior were observed, with GH females showing fewer anxietylike behaviors in the open-field test and elevated plus maze and spending more time with objects (sociability task) compared to GH males. Isolation had no effect on males but increased anxiety and reduced neophilic measures in females, removing sex differences. On the sociability task, all groups spent more time with novel rats compared to objects, suggesting social interest was retained after isolation. In the hippocampus, isolation reduced GAD in both sexes, and sex differences were seen (F > M). However, no group differences in behavior were observed in the hippocampal-dependent novel object location task. Our findings suggest that prolonged social isolation beginning in adolescence is anxiogenic for female Long Evans rats. Furthermore, sex and housing impact hippocampal GABA-ergic activity, which may have important implications in the treatment of anxiety disorders. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Prenatal exposure to valproic acid and treatment with intranasal oxytocin have sex-specific effects on behavior in Long Evans rats.

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social behaviors and communication. In rodents and humans, prenatal exposure to antiepileptic valproic acid is associated with an increased risk for autistic-like characteristics. One potential treatment is oxytocin, a prosocial neuropeptide that can be delivered intranasally. However, the sex-specific effects of valproic acid exposure and intranasal oxytocin treatment on behavior have not been fully explored. Pregnant Long Evans rats were administered valproic acid (500 mg/kg) or saline midday on gestational day 12, and after weaning, male and female pups were assigned to control (saline-saline), valproic acid-saline, or valproic acid-oxytocin groups. Oxytocin (0.8 IU/kg) or saline was delivered intranasally 30-60 min before tests for anxiety-like behaviors (elevated plus maze), social interactions (sociability) and sociosexual behaviors (partner preference, 50 kHz vocalizations and scent marking). Prenatal exposure to valproic acid resulted in sex-specific differences in behavior. When compared to controls, valproic acid males showed enhanced anxiety-like behaviors in adolescence and fewer scent marks in adulthood, while valproic acid females showed reduced sexual (partner) preference as adults. Intranasal oxytocin was anxiolytic for valproic acid males, but moderately anxiogenic for valproic acid females, and in both sexes it surprisingly impaired social interactions in the sociability test. Furthermore, intranasal oxytocin failed to improve sociosexual deficits in valproic acid rats. These findings highlight the importance of conducting preclinical studies in both sexes, and suggest that oxytocin may be an effective treatment in animal models with heightened anxiety-like behaviors.

The effects of repeated forced ethanol consumption during adolescence on reproductive behaviors in male rats.

Adolescence is a sensitive period of brain development when changes in hormone levels may have long-lasting effects on synaptic connections and behavior. In humans, alcohol consumption frequently begins during this critical period, although the impact of early exposure has not been fully examined. The current study was designed to investigate short- and long-term effects of repeated forced ethanol consumption during adolescence on emerging reproductive behaviors. Twenty-six young male Long-Evans rats were assigned to ethanol (Young EtOH, n = 12) or water (Young Control, n = 14) groups at postnatal day (P) 32, receiving a modified binge protocol of 3 g/kg of solution via gavage twice per week from P32 to P80. For comparison, another cohort of rats received a similar treatment paradigm in adulthood from P75-P133 (Adult EtOH, n = 8; Adult Control, n = 10). Reproductive behavior was assessed with tests for copulation, partner preference, and 50-kHz vocalizations during forced consumption (intoxication) and again after a 4-5 week period of abstinence. During forced consumption, the Young EtOH group showed significantly longer latencies on copulation tests than Young Controls, but these differences did not persist after abstinence. Different patterns were observed in Adult animals, who only showed significant, delayed impairments in the post-ejaculatory interval. Preference for sexually receptive females increased with sexual experience in both adolescent and adult rats, regardless of treatment during the forced consumption phase. However, after abstinence, the Young EtOH group showed a significantly reduced partner preference compared to the Young Control group, which may indicate long-term effects on sexual motivation. Additionally, during forced consumption the Young EtOH group tended to emit fewer ultrasonic vocalizations, perhaps reflecting impairments in sexual communication. Adult groups showed no differences in partner preference or vocalization tests at any time. Taken together, these findings indicate that repeated, intermittent ethanol exposure may have moderate effects on reproductive behavior that vary as a function of age. After abstinence, differences were only observed in the younger group, suggesting that the adolescent brain and behavior are more sensitive to ethanol exposure than the adult brain for sexual motivation and performance.

Comparing the relative amount of testosterone required to restore sexual arousal, motivation, and performance in male rats.

It is well established that male rat reproductive behaviors including sexual arousal, motivation, and performance are dependent on circulating levels of testosterone (T). The present study was designed to (1) compare the relative amount of T required to restore these different aspects of behavior in castrated rats, and (2) create an animal model for clinical populations with sexual impairments. Twenty-nine male Long-Evans rats were tested before and after castration for sexual performance (copulation), motivation (partner preference), and arousal (50 kHz ultrasonic vocalizations; measured together with scent marking). Sexual arousal was also inferred from copulation data. Rats were then assigned to one of four groups, and T was re-introduced via Silastic capsule implants varying in length and content: No T (empty capsules), Low T (2mm capsules), Medium T (5mm capsules), or High T (two 10mm capsules). The highest dose was intended to restore physiological levels. Results indicate that High T is required for 50 kHz vocalizations, while Medium T was sufficient for the restoration of copulation, partner preference, and scent marking. These data suggest that sexual arousal may be most sensitive to reductions in testosterone. The role of T levels in measures of generalized and specific (sexual) arousal is discussed in the context of other reproductive behaviors. Furthermore, because the Low T group showed impairments across all behaviors during post-implant tests, we propose that these animals may provide a good animal model for studying clinical conditions marked by reduced motivation and arousal, including Hypoactive Sexual Desire Disorder.

Microlesions of the ventromedial nucleus of the hypothalamus: effects on sociosexual behaviors in male rats.

Electrolytic microlesions aimed at the dorsomedial portion of the ventromedial nucleus (VMN) of the hypothalamus were generated, and effects on copulation, 50-kHz vocalizations, scent marking, and sexual motivation were measured. Male rats were tested before and after lesions, after castration, and after testosterone replacement. Three control groups were used: One received sham surgery, another received no surgery or testosterone replacement, and a 3rd received lesions primarily outside the VMN. VMN lesions produced impairments in testosterone's ability to restore ultrasonic vocalizations and scent marking, assessed with 2 different test methods. Copulation, sexual motivation, and weight gain were largely unaffected, although some differences were observed in copulatory efficiency. The authors conclude that the integrity of the VMN is important for full expression of sociosexual behaviors in male rats.

Androgen receptor blockade in the MPOA or VMN: effects on male sociosexual behaviors.

Previously, our laboratory has shown that androgen receptors in the medial preoptic area (MPOA) and ventromedial nucleus (VMN) are necessary for copulation in male rats. The present study examined whether these receptors are required for other sociosexual behaviors. In Experiment 1, different regions of the VMN were implanted with the antiandrogen hydroxyflutamide (OHF). We found that implants located in anterodorsal portions of the VMN were more effective at inhibiting the restoration of copulation than implants in the posteroventral VMN. In Experiment 2, a second set of male rats was pretested for copulation and other sociosexual behaviors and was castrated. Experimental animals then received Silastic capsules filled with testosterone (T) plus intracranial (IC) implants filled with OHF to selectively block androgen receptors in either the MPOA or VMN. We found that androgen receptor blockade in the MPOA inhibited the restoration of copulation but had no effect on other sociosexual behaviors. OHF directed at the VMN inhibited the restoration of copulation and 50-kHz vocalizations but had no effect on scent marking. Two tests were used to assay sexual motivation: partner preference and conditioned place preference (CPP). Both methods revealed impairments in sexual motivation in the VMN group but not in animals receiving OHF in the MPOA. Taken together, these data suggest that androgen receptors in the MPOA are essential for copulatory performance, while androgen receptors in the VMN are important for copulation, sexual motivation, and androgen-dependent vocalizations.

Effects of testosterone in the VMN on copulation, partner preference, and vocalizations in male rats.

The present study tested whether testosterone propionate (TP) implanted in the ventromedial nucleus (VMN) of the hypothalamus could initiate performance, motivational, or sociosexual components of sexual behavior in castrated male rats. Twenty-seven intact male Long Evans rats were pretested for copulation, partner preference, and 50-kHz vocalization and were subsequently castrated. Approximately 3 weeks after castration, males were retested to confirm that these behaviors had declined, and groups were assigned. Groups 1 and 2 were implanted with bilateral stainless steel cannulae directed at the VMN that were either filled with TP (TVMN group) or remained empty (Blank group). A third group (TSC) was implanted subcutaneously with two 10-mm Silastic capsules filled with testosterone. Restoration of behavior was measured for 2 weeks after implants. We found that copulation and 50-kHz vocalization were not restored by TP in the VMN alone. However, partner preference returned to preoperative levels in both the TVMN and TSC groups, indicating that TP in the VMN was sufficient to restore sexual motivation. Following behavioral testing, prostate glands and seminal vesicles were weighed and confirmed that TP did not leak into the periphery in the TVMN group. Immunostaining for androgen receptors also verified that TP spread was confined to the immediate area surrounding the cannula tip. These results suggest that androgen activation at the VMN is sufficient to induce the motivational components of male sexual behavior, whereas activation of other brain sites is required for copulation and ultrasonic vocalization.