Gestational manganese intoxication and anxiolytic-like effects of diazepam and the 5-HT1A receptor agonist 8-OH-DPAT in male Wistar rats

In the present study, the effects of prenatal manganese (Mn) intoxication on the anxiolytic-like effects of diazepam and the 5-HT_1A receptor agonist R-(+)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT) were examined. Wistar dams were exposed to MnCl₂?4H₂O at 5, 000 ppm in the drinking water for the duration of pregnancy. On the day of parturition, Mn was discontinued as an additive in the drinking water. Control rats were derived from dams that consumed tap water and had no exposure to Mn. Male offspring were tested at the age of 12 weeks. The anxiolytic-like effect was assessed in an elevated plus maze device and with the Vogel conflict test. The benzodiazepine anxiolytic diazepam (5 mg/kg, ip) increased the percentage of time spent in open arms in control rats (in comparison to saline treatment) (p < 0.05); no such effect was seen in Mn-exposed rats. Conversely, the serotoninergic 5-HT_1A agonist 8-OH-DPAT (0.3 mg/kg, ip) increased the percentage of time spent in open arms in both experimental groups. In the Vogel drinking test, an anxiolytic-like effect was also observed in both test groups (in controls this was of borderline significance). In contrast, 8-OH-DPAT did not evoke an anxiolytic-like action in control or in Mn-exposed rats in the anticonflict test. In conclusion, findings indicate that gestational Mn exposure attenuated benzodiazepine-mediated anxiolytic-like effects but not those of the 5-HT_1A receptor agonist 8-OH-DPAT.     

Behavioral effects Of 8-OH-DPAT in chronically stressed male and female rats

The present study tested the hypothesis that chronic stress desensitizes serotonergic 5-HT(1A) receptors and alters behavioral changes following 5-HT(1A) agonist administration. Eating, acoustic startle response (ASR), and locomotor activity were measured in stressed and nonstressed male and female rats after 8-OH-DPAT administration. Stressed rats were paired and stressed by around-the-clock intermittent foot shock. Controllable stress (CS) rats could avoid/terminate shock for themselves and their yoked partners by pulling a ceiling chain, whereas their partners, the uncontrollable stress (UCS) rats, could not. Rats earned their entire daily ration of food by pressing a lever. In previous experiments, this paradigm was stressful, but not debilitating and rats continued to eat, groom, sleep, and avoid/escape greater than 99% of shock trials. Locomotor activity and ASR were measured in the present study after saline and 8-OH-DPAT administration (0.25 mg/kg, IP) before, 24 h, and 72 h after shock onset. 8-OH-DPAT only decreased food intake significantly in male and female rats after the first administration. Stress decreased food intake in both the CS and UCS rats, with UCS rats eating the least. However, the effects of stress and 8-OH-DPAT were not additive. 8-OH-DPAT significantly increased peak startle amplitude at 100 and 120 dB, and decreased latency to peak startle amplitude at 100 dB in male and female rats. In contrast, 8-OH-DPAT did not alter percent prepulse inhibition (%PPI) at 100 dB, but significantly decreased %PPI in males but not females at 120 dB. Stress did not have a consistent effect on ASR, but reduced %PPI in males, but not females. Neither stress nor 8-OH-DPAT significantly altered locomotor activity. Although the results do not show an increased sensitivity to 8-OH-DPAT in stressed rats, the unexpectedly weak effects of 8-OH-DPAT alone on the behavioral measures chosen limits the conclusions that can be drawn.     

Effect of the serotonin agonist 8-OH-DPAT on the sensorimotor system of the rat

8-Hydroxy-2-(di-n-propylamino)-tetralin hydrobromide (8-OH-DPAT, 2 mg/kg) is used to induce perseverative behavior in rats in a T-maze as a model for obsessive-compulsive disorder (OCD). Using the open-field test, radiant heat test, and the test with von Frey filaments, we examined whether alterations in sensorimotor functioning could contribute to the perseverative tendencies in this model by measuring differences in left versus right hind paw reactions after 8-OH-DPAT administration (2 mg/kg, sc). Also, the effect of repeated 8-OH-DPAT administration on sensorimotor functioning was tested every third day. 8-OH-DPAT administration induced a significantly decreased sensorimotor performance in the open-field test, an increased threshold for noxious thermal stimulation (increased withdrawal latency, WL, and decreased elevation time, ET) in the radiant heat test, and a decreased nociceptive threshold for mechanical stimulation in the test with von Frey filaments. All changes in sensorimotor functioning were similar for left and right hind paws suggesting that, these changes as measured with the tests in the present study, are not likely to contribute to the perseverative behavior of rats in a T-maze. Further, repeated administration of 8-OH-DPAT had no effect in the radiant heat test and the test with the Frey filaments, but produced a tolerance effect in the open-field test.     

Differential effects of the serotonin1A agonist, 8-OH-DPAT,on masculine and feminine sexual behavior of the ferret

Administration of the serotonin (5-HT)_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) facilitates the expression of masculine sexual behavior in male and female rats as well as in male rhesus monkeys and inhibits lordosis behavior in female rats. In the present study the effects of 8-OH-DPAT on masculine coital and feminine proceptive and receptive behaviors were evaluated in the ferret, a carnivore. Doses of 8-OH-DPAT (0.1 or 0.2 mg/kg) that facilitate masculine sexual behavior in rats inhibited masculine sexual behavior in castrated, estradiol benzoate (EB)-treated male ferrets. Lower doses of 8-OH-DPAT (5 or 10 µg/kg) had no effect on the expression of masculine sexual behavior in either males or females. In contrast to the female rat, administration of 8-OH-DPAT significantly facilitated receptive behaviors in ovariectomized, EB-treated female ferrets. None of the doses of 8-OH-DPAT tested modified proceptive behaviors of gonadectomized, EB-treated male or female ferrets, as assessed in a T-maze in which the subjects could choose to approach either a castrated, sexually inactive male or a castrated, testosterone-primed stud male. Thus whereas the 5-HT_1A receptor agonist 8-OH-DPAT facilitates masculine sexual behavior and inhibits lordosis in the rat, it inhibits masculine sexual behavior and facilitates receptivity in the ferret. The different effects of 8-OH-DPAT observed in these two species may reflect differences in the neural control of their masculine coital and feminine receptive responses, respectively.     

Development of apomorphine-induced aggressive behavior: comparison of adult male and female Wistar rats

The development of apomorphine-induced (1.0 mg/kg s.c. once daily) aggressive behavior of adult male and female Wistar rats obtained from the same breeder was studied in two consecutive sets. In male animals, repeated apomorphine treatment induced a gradual development of aggressive behavior as evidenced by the increased intensity of aggressiveness and shortened latency before the first attack toward the opponent. In female rats, only a weak tendency toward aggressiveness was found. In conclusion, the present study demonstrates gender differences in the development of the apomorphine-induced aggressive behavior and indicates that the female rats do not fill the validation criteria for use in this method.     

Suppression of lordosis behavior by the putative 5-HT receptor agonist 8-OH-DPAT in the rat

The administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), inhibited the lordosis induced by estradiol benzoate or estradiol benzoate plus progesterone in ovariectomized rats. There was no facilitation of lordosis by 8-OH-DPAT in animals pretreated with a threshold dose of estradiol benzoate. The results are consistent with the view that 8-OH-DPAT is an agonist at 5-HT receptors and provide further support for an inhibition role of central 5-HT in the mediation of lordosis behavior.     

Para-chlorophenylalanine prevents feeding induced by the serotonin agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT)

The effects of para-chlorophenylalanine pre-treatment (PCPA, 150 mg/kg IP daily for 3 days) on feeding and stereotyped behaviour elicited by the serotonin agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) in rats were investigated. PCPA depleted brain serotonin and 5-hydroxyindoleacetic acid concentrations by 90% and increased feding during a 2-h day-time test. 8-OH-DPAT (60–4000 μg/kg SC) increased food intake in control animals but decreased in in PCPA-treated animals during the 2-h test. PCPA treatment had no effect on 8-OH-DPAT-induced locomotion or serotonin-related stereotyped behaviour (i.e. forepaw treading, headweaving, wet dog shakes, etc). Since PCPA prevents the operation of pre-synaptic serotonergic mechanisms, the failure of 8-OH-DPAT to increase food intake in PCPA-treated rats suggests that 8-OH-DPAT-induced hyperphagia is autoreceptor mediated.     

Cocaine discrimination and time-course effects in male and female Wistar rats.

Previously, sex differences have been observed in the behavioral effects of acute and chronic cocaine administration. In the present experiment, male and female rats were trained to discriminate intraperitoneal injections of 10.0 mg/kg cocaine from its vehicle. It was hypothesized that the subjective effects of cocaine might differ between male and female rats. It was further hypothesized that generalization gradients between male and female rats might differ as a function of the time since cocaine administration. In addition, we were interested to see whether multiple generalization gradients could be determined within the same experimental session. For that purpose, two different types of generalization tests were conducted in extinction, one in which subjects were tested both 10 min and 30 min following cocaine administration (vehicle, 1.0, 3.0, 5.6, 10 or 17 mg/kg) and one in which subjects were only tested 30 min after cocaine administration. The generalization gradients obtained 30 min following drug administration were shifted to the right of the gradient obtained 10 min following drug administration. The two 30-min gradients were not different from one another, showing that multiple generalization gradients can be obtained within the same experimental session.     

Evidence for differential effects of 8-OH-DPAT on male and female rats in the Anxiety/Defense Test Battery

The Proxemics/Activity test and the Eat/Drink test, two components of the Anxiety/Defense Test Battery, were developed to measure defensive reactions to situations associated with a natural predator (cat). In the present studies the behavioral effects of 8-OH-DPAT treatment (0.01–1.0 mg/kg, SC) were entirely consistent with anxiety/fear reduction. These effects included an increase in time spent near the cat compartment, and a complimentary decrease in time spent farthest from this compartment, together with an increase in transits and locomote behavior. 8-OH-DPAT (1.0 mg/kg) also increased eat frequencies and durations (highly preferred food) both during and following cat presentation, without influencing drinking. This finding is discussed with reference to previous findings with 8-OH-DPAT in studies assessing both food intake and anxiolysis. Interestingly, 8-OH-DPAT was more potent in a majority of its effects in female subjects, a finding consistent with recent neurochemical data. These findings provide important behavioral evidence for a sexual differentiation in 5-HT function, and support the case for greater emphasis on female subjects in animal models of anxiety.Supported by NIH MH42803 and RCMI Grants RR03061 and RR01825     

Cocaine-induced conditioned taste aversions in male and female Wistar rats

After an initial period of adaptation to 20 min per day of limited water availability, male and female Wistar rats were allowed access to water or a 0.1% sodium-saccharin solution. Saccharin exposures were followed by the subcutaneous administration of 0, 5, 10 or 20 mg/kg cocaine for different groups of rats. Four pairings of the saccharin solution with cocaine administration resulted in a consistent decrease in saccharin consumption only in female subjects injected with the largest dose of cocaine (20 mg/kg). Choice testing in which subjects could choose between two drinking tubes, one containing water, the other one containing the saccharin solution, was then conducted during extinction. During four of such experimental sessions, subjects which had previously been injected with vehicle mostly consumed the saccharin solution or showed a position bias. Conditioned taste aversions were not only observed in the group of female subjects injected with 20 mg/kg cocaine, but also in males previously treated with 20 mg/kg cocaine. In addition, compared to vehicle control groups, males and females injected with 5 and 10 mg/kg cocaine tended to avoid the saccharin solution in favor of regular water. It is suggested that previous failures to obtain consistent cocaine-mediated taste aversions may have been a function of the experimental procedures used to assess cocaine's efficacy in inducing conditioned taste aversions.     

Scopolamine differentially disrupts the behavior of male and female Wistar rats in a delayed nonmatching to position procedure

Evidence is available that pharmacological interference with the cholinergic system may disrupt behavior in experimental procedures designed to investigate learning and memory processes. Recently it has been suggested that the cholinergic system may be sexually dimorphic. The present experiment was designed to investigate whether or not manipulation of the cholinergic system differentially affected memory processes in both sexes. Male and female Wistar rats were exposed to a delayed nonmatching to position procedure and were challenged with increasing doses of scopolamine hydrobromide (a central and peripheral muscarinic receptor blocker) and scopolamine methyl bromide (which does not pass the blood-brain barrier). Response accuracy decreased in both sexes as the delay interval duration increased. Behavioral differences between saline-treated males and females were not observed. Response accuracy decreased dose-dependently after subjects were injected with scopolamine hydrobromide. Response accuracy also decreased after treatment with scopolamine methyl bromide, but to a smaller extent. Males showed less accurate responding after treatment with either drug than females. These results provide behavioral evidence for the hypothesis that cholinergic functioning may differ between the sexes.     

Inhibition of sexual behavior by dopamine antagonist or serotonin agonist drugs in castrated male rats given estradiol or dihydrotestosterone

Four experiments were performed to determine whether the activational effects of two behaviorally active neural metabolites of testosterone, estradiol (E₂) and 5α-dihydrotestosterone (DHT), on male rats' sexual behavior possibly result from the action of either steroid at dopaminergic or serotoninergic synapses. In Experiment 1 lower doses of the dopamine receptor antagonist, spiperone, were needed significantly to reduce mounting and intromission rates in castrated males implanted with silastic capsules containing E₂ as opposed to DHT. However, in Experiment 2 increasing doses of another dopamine receptor blocker, clozapine, were equally effective in suppressing males' sexual behavior, regardless of whether they were implanted with E₂ or DHT, suggesting that these testosterone metabolites may both normally contribute to the activation of masculine sexual behavior by enhancing dopaminergic neurotransmission. In Experiment 3 administering increasing doses of the serotonin reuptake blocker, fluoxetine, caused an equal suppression of sexual behavior in castrated males implanted with E₂ of DHT. In Experiment 4 no differential suppressive effects of the serotonin receptor agonist, 5 methoxy-N,N-dimethyltryptamine were obtained in castrated rats implanted with E₂ or DHT. It is suggested that these testosterone metabolites may both contribute to the activation of masculine sexual behavior by suppressing activity at serotoninergic synapses.     

Anticonflict effect of the putative serotonin receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)

The putative 5-HT agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) produced in rats an increase in the number of shocks accepted in a modified Vogel's conflict test design. Subchronic pretreatment with p-chlorophenylalanine (PCPA) similarly caused release of the punished behavior. This anticonflict effect of PCPA was antagonized by both 5-hydrotryptophan and 8-OH-DPAT. Thus in naive animals 8-OH-DPAT exerting anticonflict effects acted like a 5-HT antagonist, whereas in subchronically PCPA-pretreated animals with presumably supersensitive 5-HT receptors, 8-OH-DPAT decreasing the number of accepted shocks acted like a 5-HT agonist.     

Antagonism by lisuride and 8-OH-DPAT of 5-HTP-induced prolongation of the performance of male rat sexual behavior

Both lisuride and 8-OH-DPAT dose dependently antagonized the 5-HTP-induced inhibition of male rat sexual behavior. The increase in the number of intromissions and/or the ejaculation latency produced by 5-HTP, 25 mg/kg i.p. (−60 min) in combination with the peripheral 5-HTP decarboxylase inhibitor benserazide, 25 mg/kg i.p. (−90 min), were antagonized by lisuride, 0.05–0.1 mg/kg i.p. (−15 min) and by 8-OH-DPAT, 0.025–0.05 mg/kg i.p. (−15 min). Thus, in this model lisuride and 8-OH-DPAT behave as 5-HT antagonists.     

Phthalate affect the reproductive function and sexual behavior of male Wistar rats

Phthalates are chemicals used in many industrial products (plastic toys, shampoos, soaps), and are suspected of inducing adverse effects on the male reproductive system. In the present study, we evaluated the effects of the plasticizer di-(2-ethylhexyl)-phthalate (DEHP) on the reproductive function and sexual behavior of male offspring rats, exposed in utero and during lactation (0, 20, 100 and 500 mg/kg per day by gavage). The effects produced clearly demonstrate the ability of DEHP to disrupt the androgen-regulated development of the male reproductive tract. Absolute and relative weights of androgen-dependent tissue organs (ventral prostate and seminal vesicle) were significantly reduced at the highest dose level tested (500 mg/kg per day). Impairment of male sexual behavior (500 mg/kg per day) was also observed. Moreover, the reduction in daily sperm production and epididymal sperm counts observed after administration of the highest dose suggests an impairment of the spermatogenic processes. Most of the adverse effects reported here were observed both during puberty and during adulthood, indicating permanent effects of in utero and lactational DEHP exposure.     

Effects of perinatal ethinyl estradiol exposure in male and female Wistar rats

Perinatal exposure to endocrine disrupting chemicals with estrogenic activity can adversely affect reproductive development, but few studies evaluating estrogen-sensitive endpoints have been performed in Wistar rats. Therefore, time-mated Wistar rats (n = 10) were gavaged during gestation and lactation with 0, 5, 15 or 50 μg/kg bw/day of ethinyl estradiol.This potent estrogen was found to induce an increased number of nipples and reduced ovary weight in female offspring. Malformations of female genitalia were found in young as well as adult offspring, as an increased AGD was seen at birth and a deeper urethral slit length was seen in adulthood. In prepubertal male offspring, estrogen-regulated gene expression in ventral prostate was increased dose-dependently and a decreased ventral prostate weight was seen at 15 μg/kg.Female external sexual characteristics and prostate development were found to be targets for exposure to estrogenic compounds and may be of interest in studies on estrogenic environmental compounds.     

The effects of long-term treatment with 8-OH-DPAT on the lordosis response and hypothermia in female rats.

The effects of long-term treatment with a low dose of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on steroid hormone-dependent copulatory behaviour in female rats, the lordosis response, and on the hypothermic response of female rats were studied. Female rats were treated for 15 days, once daily, and tested on days 1 and 15 of treatment. They received 25 micrograms/kg on test days and 50 micrograms/kg on all other days. Subsensitivity was not induced to the inhibitory effect of 8-OH-DPAT on the lordosis response. In contrast, however, the acute effect of 8-OH-DPAT on body temperature was abolished by the long-term treatment. The results presented indicate that the induction of subsensitivity to the effects of 8-OH-DPAT is not primarily dependent on the pre- or post synaptic locus of action. Our data suggest that hormonal mechanisms are involved.     

Involvement of hypothalamic serotonin in activation of the sympathoadrenomedullary system and hypothalamo-pituitary-adrenocortical axis in male Wistar rats.

Infusion of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (2.5-20 micrograms in 1 microliter during 15 min), into the paraventricular nucleus of the hypothalamus (PVN) in the rat dose dependently increased plasma adrenaline and corticosterone concentrations, without affecting plasma noradrenaline concentrations. The highest dose also increased plasma glucose levels significantly. The results suggest that both the sympathoadrenomedullary system and the hypothalamo-pituitary-adrenocortical axis are activated after stimulation of 5-HT1A receptors in the PVN.     

Respiratory responses to intravenous infusion of sodium lactate in male and female Wistar rats.

In patients with panic disorder or premenstrual dysphoria, anxiety attacks can be triggered by intravenous administration of sodium lactate. Since respiratory symptoms, such as hyperventilation and shortness of breath, are characteristic features of spontaneous as well as lactate-induced panic, an involvement of central or peripheral chemoreceptors in this reaction has been suggested. In the present study, we examined to what extent intravenous infusion of sodium lactate influences respiratory parameters in freely moving male and female Wistar rats. Prompted by clinical reports suggesting that the susceptibility to spontaneous and lactate-induced anxiety may be influenced by the menstrual cycle, we also investigated if the effect of lactate on respiration in female rats is estrus cycle-dependent. Male and ovariectomized female rats exposed to sodium lactate displayed a larger increase in respiratory rate than rats given an infusion of saline. In intact female rats, the response to lactate infusion was significantly more pronounced in the diestrus phase than in the proestrus/estrus phase of the cycle. It is concluded that sodium lactate is a respiratory stimulant in rat, and that this effect is influenced by female sex steroids.     

Estradiol modulation of the hyperphagia induced by the 5-HT1A agonist, 8-OH-DPAT.

Ovariectomized rats were primed with sesame oil or estradiol benzoate followed 48 h later by either sesame oil or progesterone. Four hours later, rats were treated with either saline or 0.25 mg/kg 8-hydroxy-2-9(di-n-propylamino)tetralin (8-OH-DPAT). Rats were allowed to eat for 4 h after this final treatment. Animals in all hormonal conditions showed hyperphagia following 8-OH-DPAT. However, the hyperphagia was significantly attenuated by pretreatment with estradiol benzoate. There was no effect of progesterone on the hyperphagic response. These results suggest that previous findings of an estrous cycle modulation of the hyperphagic response to 8-OH-DPAT arise from the modulatory effects of estradiol, and not progesterone, during the female reproductive cycle.