Prenatal exposure to methylmercury alters locomotor activity of male but not female rats

In the present study the neurotoxic effects of a low dosage (0.5 mg/kg per day) of methylmercury (MeHg) on the developing nervous system were investigated. Pregnant rats were treated with MeHg from day 7 of pregnancy to day 7 of lactation. Locomotor activity (locomotion, rearing, and motility) and spatial learning ability were tested in the offspring at 6 months of age. The expression of tyrosine hydroxylase (TH) was examined by immunohistochemistry and in situ hybridization. A significant decrease in spontaneous motility and rearing was observed only in the MeHg-treated male rats. After administration of a low dose of d-amphetamine (0.5 mg/kg) no differences could be observed between control and MeHg-treated male rats, suggesting that changes in dopaminergic transmission were involved. However, no change in TH messenger RNA expression was observed. No changes in spatial learning acquisition or memory were shown in MeHg-treated rats. Taken together, these findings show that during development a very low dosage of MeHg exerts neurotoxic effects detectable in adulthood, and that susceptibility is gender-dependent. Received: 9 October 1996 / Accepted: 20 March 1997     

Handling alters cocaine-induced activity in adolescent but not adult male rats

The developmental period of adolescence is one that is characterized by increased levels of stress and vulnerability to drugs. Pre-test handling is an experimental manipulation that is used to acclimate animals prior to behavioral testing and exposure to a novel environment. Therefore, the present study was conducted in order to address the issue of pre-test handling of adolescent and adult male rats on subsequent cocaine-induced locomotor activity upon presentation to a novel environment. On days one through four, postnatal day (PND) 41-44 or PND 56-59, respectively, animals were handled b.i.d. for three minutes. On the fifth day, PND 45 or PND 60, animals were administered 30 mg/kg/ip cocaine or saline and immediately placed in a novel environment where locomotor activity was measured for 30 minutes. Cocaine increased locomotor activity similarly in all non-handled animals, regardless of age. Interestingly, adolescent animals expressed a differential effect when handled prior to an acute cocaine administration. Specifically, handling increased cocaine-induced locomotor activity in adolescent but not adult animals. These findings indicate that adolescent males that have been acclimated to the handling procedure experience significantly more behavioral reactivity than do adults to a high dose of cocaine upon exposure to a novel environment.     

Peripubertal stress of male,but not female rats increases morphine-induced conditioned place preference and locomotion in adulthood

Animal studies demonstrate that peripubertal social stress markedly increases the risk for subsequent substance use in adulthood. However, whether non-social stress has a similar long-term impact is not clear, and whether male and female animals show different sensitivity to peripubertal non-social stress has not been examined. In the present study, we addressed these issues by introducing two non-social stressors (elevated platform and predator odor 2,5-Dihydro-2,4,5-trimethylthiazoline) to male and female Wistar rats during adolescence (postnatal days 28–30, 34, 36, 40, and 42), then tested reward-related behaviors during adulthood, including morphine-induced conditioned place preference (CPP, 1 mg/kg morphine or 5 mg/kg morphine) and hyperlocomotor activity (5 mg/kg morphine). We found that adult male rats, but not females who were exposed to peripubertal non-social stressors showed enhanced morphine-induced CPP. Moreover, morphine-induced increase in locomotor activity was also significantly increased in adult male rats, but not in females. These results indicate that peripubertal exposure to repeated non-social stress may enhance sensitivity to the rewarding effects of opioids in adulthood in a sex-dependent manner, with males being even more sensitive than females in this regard.     

Tickling in juvenile but not adult female rats conditions sexual partner preference

Female rats display a conditioned partner preference for males that bear odors paired with different types of rewarding unconditioned stimuli (UCS). Here we examined whether tickling constitutes a rewarding UCS that supports the development of partner preferences. In Experiment 1, we tested the possibility that odors associated with a tickling UCS in prepubescent rats would induce a conditioned partner preference in adulthood. Two groups were formed with 31-day-old, single-housed females, tickled for 6 min daily for 10 days, by a hand that wore a scented glove (almond or lemon). At 47 days of age, females were ovariectomized (OVX), hormone-primed (EB+P), and tested for sexual partner preference with two scented stud males (one almond and one lemon). In each group, females displayed a sexual preference toward males bearing the odor paired with tickling, as observed with longer visits, more solicitations, hops & darts, and receiving more intromissions and ejaculations from the preferred male. In Experiment 2, we used 3-month old, OVX, hormone-primed rats conditioned every 4 days for 10 trials. In contrast to juvenile females, adult females failed to prefer males that bore the odor paired with tickling but instead preferred the novel male. These results suggest that tickling has opposite age-dependent effects in the conditioning of partner preference. Tickling in juvenile females appears to act as a rewarding UCS, whereas in adult females it may act as an aversive UCS. Further research is needed to understand brain mechanisms that might account for such differences.     

Repeated estradiol administration alters different aspects of neurogenesis and cell death in the hippocampus of female, but not male, rats

Estradiol has been shown to have neuroprotective effects, and acute estradiol treatment enhances hippocampal neurogenesis in the female brain. However, little is known about the effects of repeated administration of estradiol on the female brain, or about the effects of estradiol on the male brain. Gonadectomized male and female adult rats were injected with 5-bromo-2-deoxyuridine (BrdU) (200 mg/kg), and then 24 h later were given subcutaneous injections of either estradiol benzoate (33 mug/kg) or vehicle daily for 15 days. On day 16, animals were perfused and the brains processed to examine cells expressing Ki-67 (cell proliferation), BrdU (cell survival), doublecortin (young neuron production), pyknotic morphology (cell death), activated caspase-3 (apoptosis), and Fluoro-Jade B (degenerating neurons) in the dentate gyrus. In female rats, repeated administration of estradiol decreased the survival of new neurons (independent of any effects on initial cell proliferation), slightly increased cell proliferation, and decreased overall cell death in the dentate gyrus. In male rats, repeated administration of estradiol had no significant effect on neurogenesis or cell death. We therefore demonstrate a clear sex difference in the response to estradiol of hippocampal neurogenesis and apoptosis in adult rats, with adult females being more responsive to the effects of estradiol than males.     

Age at group formation alters behavior and physiology in male but not female CD-1 mice

In the laboratory environment, rodents are usually housed in unisexual groups, which are assembled after weaning. Housing of unfamiliar subjects has been described, however, as a stressful social setting for rodents and other mammals. Aim of the present study was to evaluate whether the age at which house mice are grouped might affect their behavior and physiology. Male or female unisexual groups were formed at different ages: at weaning, i.e., before puberty (JUV); at adolescence, i.e., after puberty (AD); and controls were raised with siblings since birth (CON). Results show that age at group formation induced several behavioral and physiological alterations in males but not in females. Specifically, when compared to controls, JUV males showed higher aggression, smaller preputial gland, and a marked reduction of neophobia in the free exploratory paradigm. Fewer changes occurred in the AD males, which showed reduced neophobia in the free exploratory paradigm and, when adults, a reduction in body weight. Females were not affected by the experimental treatment. Surprisingly, the basal corticosterone assessed at the nadir was lower for both males and females JUV and AD respect to CON. In conclusion, it is clear that mixing groups at different ages has profound effects on mouse behavior and physiology.     

Neonatal social isolation alters both maternal and pup behaviors in rats

The development of emotional behavior is dependent on the early experiences of the infant and the quality of maternal care. In these experiments, the effects of social isolation during the preweaning period on both pup behavior and maternal responsivity were examined. In the first study, the number of ultrasonic vocalizations (USVs) emitted after brief maternal separation was measured in neonatal rats with differing histories of social isolation. The social isolation procedure consisted of 5 days of daily separation from the dam and littermates for either 3 or 6 hr. At both ages tested, socially isolated pups vocalized significantly less than control pups. In the second study, the effects of prior isolation either daily for 5 previous days (Chronic Isolation) or for 4 hr prior to testing (Acute Isolation) were examined in a T-maze choice test. Pup vocalizations in the presence of the dam and dams' maternal behavior were assessed. When the dam was confined to the start box or during the maternal free access period, both Chronic and Acute Isolates vocalized less than pups that had never left the home nest. Dams spent more time with and licked and groomed more frequently and for a longer time both Chronic and Acute Isolates compared to pups that had always been with dams in the home nest. These results suggest that early isolation experience can alter subsequent responses to separation stress in neonatal rats and that maternal behavior is sensitive to the prior experiences of offspring.     

Estradiol reduces seizure-induced hippocampal injury in ovariectomized female but not in male rats

Estrogens protect ovariectomized rats from hippocampal injury induced by kainic acid-induced status epilepticus (SE). We compared the effects of 17beta-estradiol in adult male and ovariectomized female rats subjected to lithium-pilocarpine-induced SE. Rats received subcutaneous injections of 17beta-estradiol (2 microg/rat) or oil once daily for four consecutive days. SE was induced 20 h following the second injection and terminated 3 h later. The extent of silver-stained CA3 and CA1 hippocampal neurons was evaluated 2 days after SE. 17beta-Estradiol did not alter the onset of first clonus in ovariectomized rats but accelerated it in males. 17beta-Estradiol reduced the argyrophilic neurons in the CA1 and CA3-C sectors of ovariectomized rats. In males, estradiol increased the total damage score. These findings suggest that the effects of estradiol on seizure threshold and damage may be altered by sex-related differences in the hormonal environment.     

Conditioned partner preference in female rats for strain of male

Female rats show conditioned place preference following paced copulation, and we have recently demonstrated that pairing almond odor with paced copulation induces a conditioned partner preference for almond-scented males. The present study examined whether cues of two different strains of male (albino and pigmented) induce a conditioned partner preference for the strain of male associated with paced copulation. Ovariectomized, hormone-primed Wistar (W) or Long-Evans (LE) female rats received 10 conditioning trials at 4-day intervals. In the Wistar-pacing group females copulated with W males in a chamber bisected by a 4-hole partition that only the female could pass through. Four days later, they copulated with LE males without the partition. The Long-Evans-pacing group received the opposite association. In the final preference test all females chose freely between two males tethered in opposite corners of an open field, one W and one LE. Regardless the strain of male, females displayed more solicitations toward the pacing-related male, and most of the females received their first ejaculation from that male. The preference was facilitated if the pacing-related male was of the same strain as the female. These results suggest that female rats have an unconditioned preference for males of the same strain, but this preference can be switched towards males of a different strain if that male is associated with the sexual reward induced by paced copulation.     

Hippocampal long-term potentiation is reduced in mature compared to young male rats but not in female rats

Aging is associated with a decline in cognitive function which could be due to reduced synaptic plasticity. Hippocampal long-term potentiation (LTP) is an activity-dependent form of increased transmission efficacy at synapses that is considered the basis for some forms of learning and memory. We studied the N-methyl-d-aspartic acid (NMDA) receptor-dependent LTP in the CA1 region of hippocampus in young (2 months) and mature (8 months) male and female rats. We have found that in young male rats the tetanus increased the magnitude of excitatory post-synaptic potentials to 204+/-10% of basal while in mature male rats the magnitude of the LTP was significantly lower reaching only 153+/-11% of basal. This decrease did not occur in female rats. Similar changes occurred in the content of the NMDA receptor subunits NR1 and NR2A in hippocampus. The amount of both subunits was reduced significantly (15-16%) in hippocampus of 8-month-old compared with 2-month-old male rats. This decrease was not observed in female rats. Moreover, there is a significant correlation between the content of NR1 subunit and the magnitude of the potentiation. These data suggest that some of the neurobiological changes induced in hippocampus by aging are different in males and females.     

Combined estrogen-progestogen but not progestogen-only oral contraceptive alters glucose tolerance and plasma lipid profile in female rats

Women are exposed to sex steroids in several formulations such as oral contraceptives (OCs) and hormone replacement therapies. Estrogen is believed to have cardiometabolic protection effect; but its beneficial effects have recently been queried. The aim of the present study was to clarify whether or not the altered glucose tolerance and plasma lipid profile was associated with OC and due to the estrogenic or progestogenic-component and if that was dose-dependent in 7-8 weeks old female rats. Rats were divided into vehicle-treated (control), high dose combined OC-treated (HCOC; receiving 1.5μg ethinyl estradiol/15.0μg norgestrel), low dose combined OC-treated (LCOC; receiving 0.15μg ethinyl estradiol/1.5μg norgestrel), high dose progestogen OC-treated (HOC; receiving 35.0μg levonorgestrel) and low dose progestogen OC-treated (LOC; receiving 3.5μg levonorgestrel) groups. Rats were given (p.o.) vehicle (distilled water), HCOC, LCOC, HOC and LOC daily for 6 weeks. When compared with the controls, HCOC treatment led to significant decreases in glucose tolerance and plasma high-density lipoprotein-cholesterol. However, HCOC-treated and LCOC-treated groups had significantly higher plasma triglyceride levels when compared with the control group. Fasting blood glucose, plasma total cholesterol, and low-density lipoprotein-cholesterol were comparable among groups. Body weight gain appeared to be attenuated by OC treatments, particularly in LOC-treated rats. In conclusion, our findings demonstrate that combined estrogen-progestogen but not progestogen-only OC use resulted in impaired glucose tolerance that was associated with increased triglyceride and decreased high-density lipoprotein-cholesterol. The effects on glucose tolerance and high-density lipoprotein-cholesterol were dose-dependent while that on triglyceride was not.     

Prenatal morphine exposure differentially alters learning and memory in male and female rats

The present study tested the hypothesis that exposure to morphine on prenatal days 11-18 impairs performance on tasks requiring learning and memory in adult male and female rats. In Experiment 1, a symmetrical maze was used to measure learning. In Experiment 2, an eight-arm radial maze was used to assess working spatial memory. The results of Experiment 1 demonstrated that prenatal morphine exposure reduces the time needed to complete the trials, but does not affect the accuracy of performance in male rats. In contrast, prenatal drug treatment had no effects on either the time or the accuracy of performance in female rats. In Experiment 2, both male and female morphine-exposed rats needed more time to complete regular trials (no delay) than controls; however, morphine exposure in male rats did not affect performance on tasks requiring memory, measured with delay trials, but hindered it in ovariohysterectomized (OVX) female rats. In OVX females, replacement injections of both estrogen and progesterone restored the impairment of performance on delay trials produced by prenatal morphine exposure. Thus, the present study demonstrates that prenatal morphine exposure differentially alters performance of adult male and female rats on tasks requiring learning and spatial memory.     

Gonadal hormones differentially modulate cocaine-induced conditioned place preference in male and female rats

There is accumulating evidence that suggests there are sex differences in behavioral and subjective responses to cocaine. However, it is not known whether differences in cocaine reward contribute to sex differences in these responses or whether gonadal hormones affect the rewarding properties of cocaine. In the present study, conditioned place preference (CPP), a measure of non-contingent reward, was used to determine the effects of endogenous gonadal hormones and of estrogen and progesterone replacement on cocaine reward. Neurochemical measurements were also taken to identify monoaminergic substrates which underlie the behavioral phenotype. Although both intact and gonadectomized male and female rats showed a significant CPP for cocaine, ovariectomy attenuated the magnitude of CPP. These alterations coincided with a decrease in serum levels of corticosterone. In ovariectomized rats, pretreatment with progesterone inhibited cocaine CPP while estrogen plus progesterone potentiated the magnitude of CPP. Additionally, gonadectomy and ovarian hormone replacement in female rats affected serotonin/dopamine levels and turnover ratios in the ventral tegmental area and nucleus accumbens shell. While no effects of castration were observed, ovariectomy decreased levels of dopamine and serotonin in the ventral tegmental area. In females, progesterone replacement increased levels of serotonin and dopamine in the ventral tegmental area, while estrogen plus progesterone replacement increased dopamine levels in the nucleus accumbens. Collectively, these results indicate that ovarian hormones may influence cocaine reward by altering monoaminergic systems, which, in turn, may contribute to the current sex disparities in overall cocaine use.     

Foster mother care but not prenatal morphine exposure enhances cocaine self-administration in young adult male and female rats

The present study was designed to investigate cocaine self-administration in adult male and female rats exposed prenatally to morphine. Pregnant dams were injected two times a day with either saline, analgesic doses of morphine or no drug at all (controls) on gestation Days 11-18. One day after birth, litters were cross-fostered such that control dams were paired with one another and their litters were crossed; saline- and morphine-treated dams were paired and half of each saline litter was crossed with half of each morphine litter. Thus, each mother (control, saline, and morphine) raised half of her own and half of the adopted litter. At the age of 60 days, males and females were trained first to lever press for sucrose pellets and then for cocaine. Once the lever-pressing behavior was learned and baseline level of this activity was established, animals received a cocaine (.5 mg/kg per infusion) reward for each correct response on the active lever during the next 9-day session. The data demonstrate that adult control, saline- and morphine-exposed male rats self-administer cocaine at a similar rate independent of their prenatal treatment. Adult female rats self-administer cocaine at a higher rate than male rats. Further, saline- and morphine-exposed females in diestrus self-administer more than females in proestrus phase of the estrous cycle, while control females show no such differences. In addition, fostering induces increase in cocaine self-administration in all groups of male rats regardless of prenatal drug exposure. In females, the only fostering-induced increase is in prenatally saline-exposed female rats raised by morphine-treated foster mother. Thus, our results suggest that the prenatal drug exposure does not induce changes in lever-pressing behavior for cocaine reward in adult male and female rats, but it sensitizes the animals to postnatal stimuli such as gonadal hormones and/or rearing conditions that result in increased drug self-administration.     

Intervention with environmental enrichment after experimental brain trauma enhances cognitive recovery in male but not female rats

An increasing amount of evidence suggests a possible implication of oxidative stress in the pathophysiology of schizophrenia. Oxidized low-density lipoproteins (oxLDL) have been reported to be capable of eliciting neurocytotoxicity. On the other hand, paraoxonase (PON1), an arylesterase, plays a role in protection against oxidative modifications of LDL and is considered to be one of the antioxidant enzymes. Thus, we investigated the genetic association between a functional polymorphism (Gln192Arg) of the human PON1 gene and schizophrenia in 244 patients and 177 controls. No significant association between the polymorphism and schizophrenia was observed. In addition, our results revealed that there was no association between the genotypes of the polymorphism and any demographic characteristics of patients such as gender, age, age at onset, or current neuroleptic dosage. Our results suggest that the Gln192Arg polymorphism of the PON1 gene may not be involved in the susceptibility to schizophrenia.     

Exposure to PCB 77 affects partner preference but not sexual behavior in the female rat

In rats, exposure to the polychlorinated biphenyl congener 3, 4, 3′, 4′-tetrachlorobiphenyl (PCB 77) affects the brain and behavior of the offspring as well as the maternal behavior of the dams. In the present study, a cross-fostering design was used to examine the effects of pre- and/or postnatal exposure to PCB 77 on sexual behavior and partner preference in female rats, and to determine the role of altered maternal behavior in the mediation of these effects. Pregnant rats were treated with oil or PCB dissolved in oil (2 mg/kg b.w.) on gestation days 6-18 and then given pups that had been exposed to either the oil vehicle or PCB during gestation. As adults, the female offspring were tested for partner preference (that is, whether they preferred to spend time with a sexually receptive female or a sexually active male) and sexual behavior. None of the treatments affected female sexual behavior. However, both double exposure and postnatal exposure diminished the animals' preference for a male over a female stimulus, but partner preference was not affected by prenatal exposure alone. There were no significant correlations between the changes in partner preferences due to PCB exposure and the amount of maternal grooming and licking received by the treated litters. Thus, female partner preference is affected by early PCB exposure, and the effects depend upon whether the exposure is in utero or via lactation and may be independent of any effects of the PCB on maternal care.     

Dihydrotestosterone stimulates mounting behavior but not lordosis in female rats

The ability of androgens to stimulate masculine sexual behavior is thought to depend on the aromatization of such androgens to estrogens. In this scheme, reduced androgens such as dihydrotestosterone (DHT) which cannot be aromatized, are thought to exert major peripheral but little or no central nervous system influences on the display of masculine sexual behavior. Further, an early report that DHT can induce lordosis, an estrogen (E) dependent behavior, led to a notion that DHT may effect behavior through metabolic intermediates such as 5α-androstane-3β, 17β-diol (ADIOL) which then binds to estrogen receptors eliciting the E-dependent lordotic response. The present study reexamined and compared the relative effectiveness of a range of DHT dosages in stimulating a characteristic masculine (mounting) and feminine (lordosis) sexual behavior. Adult ovariectomized rats were randomly assigned to either 250 μg or 1 mg daily injections of DHT or DHTP. Other animals received OIL injections or crystalline DHT delivered by two different lengths (20 mm or 40 mm) of Silastic capsules. Animals were tested once weekly (for 5 weeks) for mounting behavior (20 minute test). Then animals were tested thrice (once weekly) for lordosis 4 hrs after the addition of 500 μg Progesterone (P). Finally, all females were tested for lordotic potential to respond to 10 μg EB plus P. 1 mg DHT or DHTP dosages and the 40 mm-Silastic condition significantly increased mounting behavior over that of lower dosages and OIL controls. A significant correlation existed between mounting frequency and circulating level of serum DHT. Treatment with DHTP was not different than DHT in eliciting mounting behavior. Lordosis was not enhanced by any treatment with DHT or DHTP over that of controls, although all females were capable of lordotically responding to EB. These data demonstrate that DHT can induce mounting behavior, but not lordosis, suggesting that whatever action DHT has may not occur via estrogen or estrogen receptors. A role for androgen and androgen receptors upon mounting behavior is discussed.     

Gonadectomy attenuates turning behavior produced by electrical stimulation of the nigrostriatal dopamine system in female but not male rats

Rotational behavior induced by electrical stimulation of ascending dopamine neurons is used as a behavioral model to investigate gender and hormonal influences on extra-hypothalamic dopamine systems. Steroid hormones influence the metabolism of many dopaminergic drugs, and therefore this approach avoids the complications inherent in drug-induced behavior models of dopamine activity. We found that gonadectomy of female, but not male, rats severely attenuates electrical stimulation-induced rotational behavior. This suggests that some female gonadal steroid hormone(s) may modulate the activity of ascending dopamine neurons, while male gonadal hormones do not.