Influence of gonadal hormones on social, sexual, emergence, and open field behaviour in the rat (Rattus norvegicus)

Preoptic and hypothalamic regulation of multi-tiered, chronologically arranged female rat sexual behavior

As in many mammalian behaviors, sexual behavior exhibits structure. Each modular components of the structure, that are linked together over time, occur in probabilistic manner. Endocrine milieu, in particular sex hormones, define the probability to synchronize the behavior with the production of gametes. Developmental experience and environmental cues affect the hormonal milieu of the brain. This is especially true in female mammals, in which ova mature with certain intervals along with ovarian secretion of sex hormones. Estrogens secreted by mature ovarian follicles support both affiliative and executive components of female sexual behavior. In the absence of the ovarian steroids, females avoid males when possible, or antagonize and reject males when put together. Female sexual behavior is intimately linked with the estrous cycle in many species such that females are only receptive for a brief period at the estrus stage surrounding ovulation. Thus, in the rat, females strongly influence the outcome of mating encounter with a male. Affiliative or solicitatory behavior shown by females in estrus leads to the female adapting the lordosis posture, which is characterized by hindleg postural rigidity and lordotic dorsiflexion of the spine, in response to touch-pressure somatosensory stimuli on the skin of the flanks, rump-tail base, perineum region given by male partner. The posture facilitates intromission and consequently fertilization. Although dependence on estrogens is the most important feature of female rat sexual behavior, cervical probing combined with palpation of the hindquarter skin acts as a supranormal stimulus to elicit lordosis. Thus, lordosis behavior is a hub of multi-tiered, chronologically arranged set of behaviors and estrogen appear to alter excitability of neural network for lordosis.

Behavioral Disorders in Mares with Ovarian Disorders, Outcome after Laparoscopic Ovariectomy: A Case Series

Owner complaints of estrus-related behavior in mares are a common cause of referral for laparoscopic ovariectomy. Granulosa cell tumors are a common neoplastic condition affecting the equine ovary, causing behavioral changes at rest and reduced performance. The reported success rate of ovariectomy in treating behavioral disorders is 64-86%. The aim of this study was to retrospectively evaluate the long-term follow-up of laparoscopic ovariectomy in mares in our case series, focusing on the owner's perspective of the behavior of the mares after surgery. In addition, the histopathological features of the removed ovaries were investigated. The clinical records of mares that underwent laparoscopic ovariectomy between 2015 and 2022 were retrospectively reviewed. Owners complaining of poor behavior were interviewed about the main behavioral problem leading to referral and its eventual resolution after surgery. Eleven mares were included. The most common complaints were increased sensitivity on both flanks (10/11, 91%) and general riding problems (9/11, 82%). In 5/11 cases (45%), both ovaries were removed by laparoscopic ovariectomy. Histopathologic findings consistent with GCT/GTCT were found in five out of six examined ovaries (five granulosa theca cell tumors, GTCT). According to owner interviews, the scores assigned to each behavior improved significantly after surgery, regardless of histological findings. Although many factors can influence the behavior of horses, granulosa cell tumors (GCTs) proved to be a common cause and, as reported by the owners, ovariectomy resulted in improvement or complete resolution of the abnormal behavior.

Ability to share emotions of others as a foundation of social learning

The natural habitats of most species are far from static, forcing animals to adapt to continuously changing conditions. Perhaps the most efficient strategy addressing this challenge consists of obtaining and acting upon pertinent information from others through social learning. We discuss how animals transfer information via social channels and what are the benefits of such exchanges, playing out on different levels, from theperception of socially delivered information to emotional sharing, manifesting themselves across different taxa of increasing biological complexity. We also discuss how social learning is influenced by different factors including pertinence of information for survival, the complexity of the environment, sex, genetic relatedness, and most notably, the relationship between interacting partners. The results appear to form a consistent picture once we shift our focus from emotional contagion as a prerequisite for empathy onto the role of shared emotions in providing vital information about the environment. From this point of view, we can propose approaches that are the most promising for further investigation of complex social phenomena, including learning from others.

Early estradiol exposure masculinizes disease-relevant behaviors in female mice

Most psychiatric disorders show a sex bias in incidence, symptomatology, and/or response to treatment. Males are more susceptible to neurodevelopmental disorders including autism spectrum disorder and attention-deficit activity disorder, while women are more prone to major depressive disorder and anxiety disorders after puberty. A striking difference between males and females in humans and other mammals is that males undergo a process of brain masculinization due to the early exposure to gonadal hormones. In rodents, this developmental organization of the brain is essential for adult males to express the appropriate sexual behaviors in the presence of a receptive female. Our goal was to determine whether this process of brain masculinization influences behaviors relevant to psychiatric disorders. To this aim, we studied sex differences and the effect of neonatal 17β-estradiol benzoate treatment of female mice on different disease-relevant behaviors. Our analysis includes postnatal behavior, juvenile play, and adult tests for sociability, repetitive behaviors, anxiety, and depression. Our results show that the sex differences observed in exploration, repetitive behaviors, and depression-related behaviors are largely reduced when females are neonatally treated with 17β-estradiol benzoate. These results suggest a role of neonatal sex steroids in the development of disease-relevant behaviors and provide evidence supporting a role for perinatal exposure to estrogens and androgens on the development and manifestation of psychiatric disorders.

Female rats release a trapped cagemate following shaping of the door opening response: Opening latency when the restrainer was baited with food, was empty, or contained a cagemate

Research on pro-social rat behaviour is growing within the fields of comparative psychology and social neuroscience. However, much work remains on mapping important variables influencing this behaviour, and there is even disagreement on whether this behaviour is empathetically motivated and correctly labelled pro-social, or whether the behaviour is motivated by social contact. The present study used the helping behaviour paradigm where a rat can release a familiar cagemate from a restrainer. Prior to testing with a trapped cagemate, restrainer door opening was trained and baseline opening latencies when the restrainer was empty or baited with food were established. The findings show that the first-time release occurred sooner than in previous research and that rats used a previously demonstrated response to release the trapped cagemate. Further, rats opened the restrainer door more often and with shorter latencies when the restrainer contained a cagemate than when the restrainer was empty, but less often and with longer latencies than when the restrainer contained food. The test of whether illumination levels affect door-opening included in the study showed no effects.

Estradiol-sensitive projection neurons in the female rat preoptic area

Electrical stimulation of the preoptic area (POA) interrupts the lordosis reflex, a combined contraction of back muscles, in response to male mounts and the major receptive component of sexual behavior in female rat in estrus, without interfering with the proceptive component of this behavior or solicitation. Axon-sparing POA lesions with an excitotoxin, on the other hand, enhance lordosis and diminish proceptivity. The POA effect on the reflex is mediated by its estrogen-sensitive projection to the ventral tegmental area (VTA) as shown by the behavioral effect of VTA stimulation as well as by the demonstration of an increased threshold for antidromic activation of POA neurons from the VTA in ovariectomized females treated with estradiol benzoate (EB). EB administration increases the antidromic activation threshold in ovariectomized females and neonatally castrated males, but not in neonatally androgenized females; the EB effect is limited to those that show lordosis in the presence of EB. EB causes behavioral disinhibition of lordosis through an inhibition of POA neurons with axons to the VTA, which eventually innervate medullospinal neurons innervating spinal motoneurons of the back muscle. The EB-induced change in the threshold or the axonal excitability may be a result of EB-dependent induction of BK channels. Recordings from freely moving female rats engaging in sexual interactions revealed separate subpopulations of POA neurons for the receptive and proceptive behaviors. Those POA neurons engaging in the control of proceptivity are EB-sensitive and project to the midbrain locomotor region (MLR). EB thus enhances lordosis by reducing excitatory neural impulses from the POA to the VTA. An augmentation of the POA effect to the MLR may culminate in an increased locomotion that embodies behavioral estrus in the female rat.

Effects of 17beta-estradiol on responses of viscerosomatic convergent thalamic neurons in the ovariectomized female rat

Ovarian hormones have been shown to exert multiple effects on CNS function and viscerosomatic convergent activity. Ovariectomized (OVX) female rats were used in the present study to examine the long-term effects of proestrus levels of 17beta-estradiol (EB) delivered by a 60-day time-released subcutaneous pellet on the response properties of viscerosomatic convergent thalamic neurons. In addition, avoidance thresholds to mechanical stimulation for one of the convergent somatic territories, the trunk, was assessed using an electro-von Frey anesthesiometer before and at the end of the 6-wk post-OVX/implant period prior to the terminal electrophysiological experiments, which were done under urethane anesthesia. Rats implanted with an EB-containing pellet, relative to placebo controls, demonstrated 1) altered thalamic response frequencies and thresholds for cervix and vaginal but not colon stimulation; 2) some response variations for just the lateral group of thalamic subnuclei; and 3) altered thalamic response frequencies and thresholds for trunk stimulation. Thalamic response thresholds for trunk pressure in EB versus placebo rats were consistent with the avoidance thresholds obtained from the same groups. In addition, EB replacement affected visceral and somatic thresholds in opposite ways (i.e., reproductive-related structures were less sensitive to pressure, whereas somatic regions showed increased sensitivity). These results have obvious reproductive advantages (i.e., decreased reproductive organ sensitivity for copulation and increased trunk sensitivity for lordosis posturing), as well as possible clinical implications in women suffering from chronic pelvic pain syndromes and/or neuropathic pain.

The brain as the engine of sex differences in the organization of movement in rats

Sex differences in the kinematic organization of non-reproductive behavior are often relegated to byproducts of sex differences in body morphology. We review evidence showing not only that male and female rats organize their posture and stepping differently during a variety of actions, but that these differences arise from sex differences in the organization of movement in the central nervous system (CNS). Indeed, the expression and choice of sex-typical patterns of movement can be altered by CNS injury. The pattern of hormonal regulation of these sex differences is also not organized as commonly held theory would predict. As expected, males castrated shortly after birth are female-typical in their motor organization. Females ovariectomized at birth, however, are male-typical in their patterns of movement. Thus, female-typical patterns of movement organization are not the default form, but rather are dependent on the effects of gonadal steroids to feminize the developing CNS. The implications of these findings are discussed with regards to our understanding of the evolution of sex differences in CNS anatomy and behavior both for animals and humans.

Effect of oestrous-cycle stage on the response of mares in a novel object test and isolation test

In various species, sex, hormonal treatments and oestrous-cycle stage have been shown to affect the animal's response in behavioural tests. Few such studies have been performed in the horse. The main aim of the present study was to investigate whether oestrous-cycle stage affects mares' response to a novel object test and isolation test and, in part, to study whether mares, assumed to suffer from oestrous-related behavioural problems, respond differently in these tests when compared with controls. Twelve mares were tested twice, in oestrus and dioestrus, in a crossover design. Seven behavioural and two heart rate variables were measured for the novel object test and two heart rate variables for the isolation test. Oestrous-cycle stage and whether a mare was classified as a 'problem' mare did not affect the mare's response. However, test order, i.e. the cycle stage a mare was tested in first, affected its reaction. This effect could partly be explained by significant differences between test occasions 1 and 2 in three behavioural variables and one heart rate variable (p < 0.05) in the novel object test. The mares explored the novel object more and had a higher mean heart rate in the first test. Exploring the novel object more could largely be attributed to those mares tested in dioestrus first, perhaps indicating that the mares in oestrus were less receptive to the novel object. The reason for the differences between test occasions could be an effect of learning or habituation.

Temperament in the rat: sex differences and hormonal influences on harm avoidance and novelty seeking

This study showed large and consistent individual differences in 64 rats (32 males) in the hole board and canopy test, which are considered to measure exploration and anxiety, respectively. Nonestrous females were more active than males and nose poked more in the hole board. In the canopy test, nonestrous females, compared with males, showed greater intraindividual variability in time spent outside the protective canopy. Estrous females spent significantly more time outside the canopy. Gonadectomy reduced nose poking in males and hole board locomotion in both sexes. Principal-components analysis disclosed 2 temperamental dimensions reflecting harm avoidance and novelty seeking. More males had high levels of psychometric harm avoidance, and fewer males than females had a low-harm-avoidance/high-novelty-seeking, sanguine profile.

Low-dose perinatal diethylstilbestrol exposure affected behaviors and hypothalamic estrogen receptor-α-positive cells in the mouse

To estimate the potential risk of perinatal exposure to estrogenic endocrine disrupters, pregnant female mice received daily oral administration of diethylstilbestrol (DES; either 0.3 or 3 microg/kg body weight) dissolved in corn oil from gestation days 11 to 17 and from postnatal days 2 to 6. Multiple behaviors that are sexually dimorphic were examined, and the numbers of estrogen receptor-alpha and tyrosine hydroxylase-immunoreactive (ER-IR and TH-IR) cells in some brain loci related to these behaviors were investigated. Perinatal exposure to DES caused significantly enhanced open-field activity in both males and females and significantly poorer passive avoidance performance in males. In addition, a significant increase in the number of ER-IR cells in the ventromedial hypothalamic nucleus (VMH) was demonstrated for the first time. The DES-induced increases in the sexual and aggressive behaviors, although statistically nonsignificant, and the increase in the number of ER-IR cells did not agree with those obtained in previous studies using high-dose DES, which suggests that DES may have a different effect on these endpoints depending on the dose used. The relationship between the increase in ER-IR cells and behavioral changes should be further examined.

The circadian Clock mutation increases exploratory activity and escape-seeking behavior

Disturbances of circadian rhythms are associated with many types of mood disorders; however, it is unknown whether a dysfunctional circadian pacemaker can be the primary cause of altered emotional behavior. To test this hypothesis, male and female mice carrying a mutation of the circadian gene, Clock, were compared to wild-type mice in an array of behavioral tests used to measure exploratory activity, anxiety, and behavioral despair. Female Clock mutant mice exhibited significantly greater activity and rearing in an open field and a greater number of total arm entries in the elevated plus maze. In addition, female Clock mutant mice spent significantly more time swimming in the forced swim test than wild-type mice on both days of a 2-day test. Male Clock mutant mice also exhibited increased exploration of the open field and increased swimming in the forced swim test; however, behavioral changes were less robust in Clock mutant males compared to Clock mutant females. These changes in behavior were not dependent on the expression of a lengthened free-running period but were more or less striking depending on the testing conditions. These data indicate that the Clock mutation leads to increased exploratory behavior and increased escape-seeking behavior, and, conversely, does not result in increased anxiety or depressive-like behavior. These results suggest that the Clock gene is involved in regulating behavioral arousal, and that Clock may interact with sex hormones to produce these behavioral changes.

Long-lasting changes in morphine-induced locomotor sensitization and tolerance in Long-Evans mother rats as a result of periodic postpartum separation from the litter: a novel model of increased vulnerability to drug abuse?

Daily postpartum separations from the litter produce enduring changes in anxiety and sensitivity to the antinociceptive effects of morphine in Long-Evans dams. We tested whether postpartum experience alters sensitivity to the effects of morphine on locomotor activity. Dams were tested 4-6 weeks after their pups were weaned, and had one of the following backgrounds: daily separation from the litter on postpartum days 2-14 for either 3 h (prolonged separation-LS) or 15 min (brief separation-BS), or no separation (nonhandled control-NH). After 2 consecutive days (B1-2) of baseline activity measurements, subjects were tested daily after s.c. injections of either morphine (10 mg/kg) or saline for 7 days and again on day 10. Beginning 5 days later, saline and 1.0-10 mg/kg of morphine were tested in all dams. On B1, LS and BS dams habituated slower than NH controls, yielding higher horizontal counts. LS dams failed to habituate across baseline days and were more active than other dams on B2. Sensitization, a progressive increase in horizontal activity, was more rapid and robust in LS and BS dams compared to NH animals. LS was the only group that developed tolerance to morphine-induced decreases in vertical activity. In LS dams with the history of morphine treatment, injection of saline resulted in higher horizontal activity and center time compared to saline-treated counterparts, indicative of conditioning. Among animals with a history of saline treatment, LS dams were more sensitive to morphine challenges than BS and NH dams. As a result of the robust and long-lasting increases in the ability of morphine to induce behavioral sensitization in litter-separated dams, periodic postpartum separation may represent a new animal model of increased vulnerability to substance abuse.

Estrogen-induced remodeling of hypothalamic neural circuitry

For decades, sexual behavior has been a valuable model system for behavioral neuroscientists studying the neural basis of motivated behaviors. One striking example of a change in motivation is the binary switch in sexual receptivity that occurs during the estrous cycle in female rats. Investigations of the neural basis of this change in behavior have fundamentally advanced our understanding of both behaviorally relevant neural pathways and basic mechanisms of steroid action in the brain. These advances have made this behavioral model system a staple of neuroendocrinology. A challenge that remains before us, given our current understanding of the circuitry and chemistry, is to develop a coherent model of how neural plasticity in the hypothalamus contributes to the dependence of this behavior on motivational state. This review will focus on the ventromedial nucleus of the hypothalamus, especially its ventrolateral subdivision. First, the anatomical, neurochemical, and functional aspects of the macro- and microcircuitry of this brain region will be discussed, followed by a discussion of the likely mechanisms of estrogen action within the ventrolateral VMH. Then, the evidence for estrogen-induced neural plasticity will be considered, including a comparison with the effects of estrogen on synaptic organization in other brain regions. Finally, a working model of neural plasticity within the ventrolateral VMH microcircuitry will be presented as a starting point for future experiments to verify or, more likely, revise and expand.

Neuronal activity in female rat preoptic area associated with sexually motivated behavior

Single unit activities were recorded from 31 neurons in the preoptic area (POA) of female rats engaging in sexual interactions. Concurrent videotape recordings were used to establish a relationship between neuronal activity and particular behavioral events. In 14 of the 31 neurons, the firing rate changed in association with bouts of sexual activity. The remaining 17 fired with more variability regardless of episodes of sexual interactions. Peri-event histograms identified four types of neurons: type 1 (n=4) increased their firing rate when the female rats initiated proceptive behavior; type 2 (n=4) showed a brief activation when the male mounted; type 3 (n=4) fired in response to intromission, and type 4 (n=2) were inhibited prior to and throughout the display of lordosis reflex. Type 1 neurons fired at significantly higher rates during the solicitatory period, from the initiation of solicitatory locomotion to the male mounts. Their activity was suppressed when the males mounted successfully with intromission. Types 1-3 neurons were recorded from the transitional region between the medial and lateral POAs. Type 4 neurons were located more medially in the medial POA. Systemic injection of pimozide, a dopamine receptor blocker, diminished firing in type 1 neurons and abolished proceptivity. The firing pattern in type 1 neurons appeared to embody the motivational state of the animal with an implication for a consummatory value of penile intromission. Visceral or somatosensory inputs may be responsible for short bursts in types 2 and 3 neurons. Type 4 neurons behaved exactly as if they inhibit the execution of the lordosis reflex. The results showed separate sets of POA neurons each specifically associated with proceptive and receptive components of female rat sexual behavior.

Estradiol accelerates extinction of lithium chloride-induced conditioned taste aversions through its illness-associated properties

Estradiol accelerates extinction of LiCl-induced conditioned taste aversions when it is present during a period that starts 2-3 days after acquisition and extends throughout extinction (before and during extinction). It has been suggested that estradiol acts before, not during, extinction and that its effect on extinction is associated with its illness-inducing properties. This hypothesis is based on previous work which shows an attenuation of conditioned taste aversion learning when rats are exposed to illness-inducing agents during a period that starts 2 days after acquisition and ends 2 days before extinction trials are initiated. Four experiments were designed to test elements of this hypothesis. The first two experiments demonstrated that if an estradiol-filled Silastic capsule is implanted before extinction of a LiCl-induced aversion, when the conditioned taste is not present, it accelerates extinction, but if it is implanted during extinction, when the conditioned taste is present, it prolongs extinction. The third experiment showed that the same dose of estradiol that accelerates extinction of a LiCl-induced aversion was effective in producing a conditioned taste aversion when it was present for 18 h after consumption of a novel sucrose solution. The fourth experiment indicated that serum levels of estradiol were elevated during the 18 h. These results are consistent with the hypothesis that the acceleration of extinction by estradiol is associated with its illness-inducing properties. It is suggested that estradiol acts on neural areas that mediate illness information and that one of these areas, the area postrema is necessary for estradiol to accelerate extinction of a LiCl-induced aversion.

Estradiol accelerates extinction of a conditioned taste aversion in female and male rats

Exogenous testosterone treatment prolongs extinction of conditioned taste aversions and estradiol treatment prevents testosterone from prolonging extinction in both gonadectomized males and females. Estradiol could require the presence of testosterone for its effect or its action alone could accelerate extinction. The first series of experiments were designed to test the hypothesis that estradiol accelerates extinction when it is given in the absence of testosterone. The results showed that estradiol accelerates extinction of conditioned taste aversions in the absence of testosterone in gonadectomized Sprague-Dawley females and Fischer 344 females and males. The second series of experiments were designed to determine whether estradiol and testosterone differ in the temporal requirements for their opposite effects on extinction. The results showed that estradiol can accelerate extinction when it is present before and during acquisition (from 8 days before until 3 days after acquisition) or when it is present before and during extinction (from 2 days after acquisition, which was 23 days before extinction, until extinction trials were terminated). This is in contrast to a previous finding that testosterone prolongs extinction only when it is present before and during extinction. The following two hypotheses were suggested to account for the temporal effects of estradiol on extinction of conditioned taste aversions: (1) the presence of estradiol during acquisition reduces the effectiveness of LiCl through its action on the opioid system, and the presence of estradiol during extinction activates a neural pathway, such as that associated with activity levels, that accelerates extinction of passive avoidance tasks in general or (2) the presence of estradiol before, not during, acquisition or extinction accelerates extinction because of its illness-inducing properties. Most of the evidence supports the second hypothesis.

The effects of gonadal steroids on brain stimulation reward in female rats

The present study examined possible estrogen and/or progesterone effects on the mesolimbic dopamine (DA) system using brain stimulation reward (BSR). It is well known that BSR with electrical stimulation of the medial forebrain bundle (MFB) depends on the functioning of the mesolimbic DA system. If estrogen affects this system in a manner similar to its effects on the nigrostriatal DA system, reward measures would be expected to vary across the estrous cycle. Cycling female rats were trained to bar press for electrical stimulation to the MFB. Animals were tested at each stage of the estrous cycle, after ovariectomy and 4, 24, 48, 72 and 96 h after hormone replacement with estradiol (10 micrograms, s.c.), estradiol and progesterone (0.5 mg, s.c.), or oil (s.c.). The rewarding value of the stimulation and the maximum rate of bar pressing increased during estrus, but not during proestrus or metestrus, as compared with diestrus. Hormone replacement had differing effects on reward and motor performance. Motor performance increased 4 and 24 h after estrogen alone and 24 h after estrogen with the addition of progesterone 4 h before testing. The rewarding value of the stimulation increased only 24 h after estrogen together with an injection of progesterone 4 h before testing. These results indicate that gonadal steroids affect the functioning of the mesolimbic DA system.

Galanin microinjected into the medial preoptic nucleus facilitates female- and male-typical sexual behaviors in the female rat

Galanin (GAL) microinjected within the sexually dimorphic medial preoptic nucleus (MPN) facilitates male-typical sexual behaviors in the male rat, a response that requires the presence of testosterone. As in the male, GAL-immunoreactive cells located within the MPN of the female also concentrate gonadal steroids and become less immunoreactive after gonadectomy. Thus, to investigate sexual behaviors in the female and to determine whether effects are comparable to those obtained in the male, GAL was microinjected unilaterally within the MPN of female rats. We report that GAL stimulated female-typical lordosis behavior after estrogen priming, and that the effect was not due to general arousal as measured by nonspecific locomotor activities. In a separate experiment, GAL microinjected within the MPN dose-responsively increased mount frequencies and decreased mount latencies in testosterone-primed females. A higher dose of testosterone was required in females for this stimulation of male-typical sexual behavior than required in a previous experiment in males.

Infradian alteration of circadian rhythms in owl monkeys (Aotus lemurinus griseimembra): an effect of estrous?

Long-term recordings of locomotor activity, feeding activity and core temperature carried out in 7 male and 7 female adult owl monkeys (Aotus lemurinus griseimembra) revealed sex-specific infradian alterations in the level of these circadian functions when the monkeys were housed under lighting conditions which neither inhibited nor enhanced (i.e., "masked,") their circadian activity rhythms. Such nonmasking lighting conditions were: constant dim light (LL) at 0.1-0.5 lx, photoperiods consisting of 0.5 h light (L) at 80 lx and 23.5 h darkness (D) at 0.5 lx, and skeleton photoperiods consisting of two 80-lux light pulses of 0.5 h applied at intervals of 12:12 h and 9:15 h, respectively. In 5 of the female and none of the male owl monkeys, the amount of locomotor activity per day or per circadian cycle, increased at an average interval of 14.0 +/- 2.3 days to 198.4 +/- 48.2%, while the feeding activity was concomitantly reduced to 53.7 +/- 11.2%, and the core temperature level dropped by 0.3 +/- 0.1 degrees C, as compared to the respective preceding level of these parameters. The period of this infradian periodicity superimposed on the circadian rhythms corresponds approximately to the ovarian cycle length of Aotus (6,7). As food deprivation for one day resulted in a drop in core temperature is probably the result of reduced food intake. The infradian rise in activity and the decrease in food intake might be an effect of the ovarian cyclic variation in estrogen level.

Diametrically opposite effects of estrogen on the excitability of female rat medial and lateral preoptic neurons with axons to the midbrain locomotor region

Electrical stimulation of the midbrain locomotor region (MLR) in 76 ovariectomized, urethan-anesthetized female rats elicited antidromic action potentials in 252 preoptic neurons. Thresholds and refractory periods for the activation ranged from 60 to 1550 microA and 1.3 to 5.0 ms, respectively. The probability distribution for the peak-to-peak amplitude (2-14 mV) or the overall duration (0.7-4.4 ms) was bell-shaped, whereas that for the latency (1.8-33.5 ms) was distinctively bimodal with a division at 12.0 ms. Two groups of preoptic neurons of a similar soma size therefore project to the MLR presumably via different routes. In 121 neurons with latencies < or = 12.0 ms, estrogen lowered the antidromic activation thresholds (nested analysis of variance, P < 0.02), but 131 neurons with latencies > 12.0 ms had their thresholds increased (P < 0.005) and refractory periods prolonged (P < 0.02) by estrogen. Even though both overlapped in part, many potentials with the shorter latencies were recorded from the medial part of the lateral preoptic area (mLPO), lateral to the recording sites of the longer-latency potentials in the medial preoptic area (MPO). The observed antagonistic effects of estrogen on the two groups of preoptic neurons with axons to the MLR may contribute to increased locomotor activity in female rats in estrus.

Sex differences in open-field behavior in response to the beta-carboline FG 7142 in rats

Sex differences in animal models of anxiety and depression that employ external stimuli have been previously reported. This study examined the effect of gender on pharmacologically induced anxiety in rats coupled with novel handling, injection, and activity in an open-field test. The anxiogenic compound FG 7142 significantly decreased male open-field exploratory at 5, 10, and 20 mg/kg, while rearing behavior was decreased only at the 20 mg/kg dose. Female rats were more resistant to the effects of FG 7142 on open-field exploratory behavior, decreasing open-field activity, and rearing behavior only after the administration of a dose of 40 mg/kg. In addition, a significant sex difference was observed in the open-field activity of rats injected with the vehicle control. Male rats were less active but showed a similar level of rearing behavior when compared to female rats. The greater sensitivity of male rats to the activity-suppressant effects of FG 7142 could explain the sex differences observed in several other animal models of anxiety and depression.

Regional sex differences in spine density along the apical shaft of visual cortex pyramids during postnatal development

Dendritic spines from the apical shaft of layer V pyramids were counted on Golgi-stained sections of the monocular subfield of the primary visual cortex of 10-, 20-, 40- and 60-day-old male and female rats. Dendritic segments located in layer IV and at 100-300 microns from the soma had a significantly higher spine content in 10-day-old females when compared to males. This sex difference was extended to outer dendritic segments with increasing age, and became restricted to dendritic segments of outer layers (II-III) located at 400-550 microns from the perikaryon in 40-day-old rats. Sex differences in spine content finally disappeared by day 60. These results show the existence of specific laminar and temporal sex differences in the development of dendritic spines in the apical shaft of visual cortex pyramids.

Development of sex differences in the response of spiny mouse pups to adult male odors

Using a three-choice preference test, olfactory-mediated investigatory activity in response to adult male urine odor was examined in a precocially active rodent, the spiny mouse (Acomys cahirinus) aged between 3-26 days. Temporally related sex differences were seen in the time spent in the presence of the odors of father's or unfamiliar adult male's urine, or distilled (control) water. Neither male nor female pups discriminated between odors from the father and strange adult males. After the first olfactory test, when the pups were aged between four and six days, male pups strongly preferred to stay in the vicinity of urine odors of adult males, whereas female pups avoided odors of adult males and remained in the enclosure with the control odor source. To our knowledge this is the first time that such a behavioral sex difference related to olfaction has been shown to occur in young rodent pups. We suggest that the sexually dimorphic response of the pups is associated with the development of later sex differences in behavior.

Estrous cycle-dependent variation in amphetamine-induced behaviors and striatal dopamine release assessed with microdialysis

Estrous cycle-dependent variation in amphetamine (AMPH)-stimulated behaviors and striatal dopamine (DA) release were determined for freely moving rats undergoing intracerebral microdialysis. There was greater AMPH-stimulated striatal DA release in the striatum of rats in estrus, during the period from 20 min to 100 min post-AMPH, compared with the response of rats in diestrus (P less than 0.05). Animals in estrus also showed less general activity and greater intensity of stereotyped behaviors than did animals in diestrus (P less than 0.05). Independent experiments examining estrous cycle-dependent differences in AMPH-stimulated striatal DA release in vitro also found that AMPH-stimulated striatal DA release was greater on estrus than on diestrus (P less than 0.05). Thus, striatal DA release and behaviors induced by AMPH are modulated by ovarian hormones. The heightened responsiveness to AMPH during estrus suggests that estrogen and/or progesterone potentiate the striatal DA response to AMPH.

Effects of maternal hormones on 'timidity' and attraction to pup-related odors in female rats

The present study was designed to determine whether changes in 'emotionality' and responses to odorants that occur in the postpartum rat are due to the same configuration of hormones that facilitate the expression of maternal behavior. Ovariectomized females were implanted with silastic capsules containing progesterone and estradiol or cholesterol for a 19-21 day period and were tested 1 or 7 days later for emergence behavior, ambulation in the open-field and responses to nesting-material containing different odorants. All females were given two tests in which clean nest-material was presented and two with lactating nest material. In comparison to cholesterol animals hormonally primed animals emerged more rapidly into the open-field, crossed more squares while in the field and spent more time around the odor stimulus. Moreover, hormonally primed animals crossed more squares in the region around the odor source when lactating nest odor was present than when it was not and, under the lactating nest odor condition, crossed more squares in the stimulus region than did any other group. This study suggests that the Bridges' regimen of progesterone and estradiol reduces general 'timidity,' reflected in open-field performance, and increases attraction to pup-related odors. These hormone-induced behavioral changes may well contribute to the heightened maternal responsiveness also produced by these hormones.

Sex steroids modulate motor-correlated increases in cerebellar discharge

Adult female rats implanted with a microelectrode drive unit were trained to walk on a computer-controlled treadmill apparatus (10 s on every 20 s for 2 h) during recording of single Purkinje neurons in the paravermal area of the anterior cerebellum. Vigorous increases in the firing rate of individual units were found to be correlated with movement of specific limbs in particular stages of the step cycle during treadmill locomotion. Both spontaneous and motor-evoked discharge of individual Purkinje neurons were monitored before and after s.c. injection of either 17 beta estradiol (E2, 100 ng/kg) or progesterone (P, 50 micrograms). The percent increase in firing rate during locomotion versus rest was determined as a measure of the evoked:spontaneous discharge ratio. Drug-induced changes in this ratio indicate differential effects on the individual parameters, rather than simple excitatory or inhibitory effects. For all neurons tested, E2 augmented the movement-evoked discharge over pre-E2 control levels. The onset for this effect occurred at 15 min post-steroid, with a peak response noted at 30-35 min post-steroid. By 60-90 min, a partial recovery of the evoked:spontaneous ratio was noted, although absolute increases in both parameters were still observed, indicating long-term effects on neuronal activity. These effects were independent of the stage of the estrous cycle. In contrast, P decreased absolute firing rates of Purkinje cells during stationary and locomotor phases. However, the evoked:spontaneous ratio was decreased to an even greater degree. The latency for this effect was 9-12 min, with recovery to control levels of response seen at 30 min post-steroid. This response was typical of cells tested on estrus and diestrus 1. Cells tested on proestrus or diestrus 2, when E2 levels are increasing, were not modulated by P using the above paradigm.

Locally applied estrogens potentiate glutamate-evoked excitation of cerebellar Purkinje cells

Ongoing studies in our laboratory have demonstrated that systemically administered sex steroids 17 beta-estradiol (E2) and progesterone (P) alter cerebellar Purkinje cell responses to the amino acid neurotransmitter glutamate (Glu) in the urethane-anesthetized, ovariectomized adult rat. In the present study, we have examined the effects of locally pressure ejected E2 (0.5 microM) on Purkinje cell responsiveness to microiontophoretically applied Glu. The inactive stereoisomer of E2, 17 alpha-E2 (0.5 microM), estradiol benzoate (EB, 0.5 microM), and estrone (E1, 0.5 microM) were also tested (vehicle: 0.01% propylene glycol-saline, pH 7.4). Extracellular activity of single Purkinje neurons was recorded using multibarrel glass micropipets. Spontaneous firing rate and neuronal responses to microiontophoretic pulses (10 s every 40 s at 10-50 nA) of Glu were examined before, during and after continuous local pressure application of the steroids (1-5 psi, 10-15 min). Local E2 administration increased Glu response by 86% within 2-3 min after the onset of steroid application, with no recovery apparent by 30 min after termination of steroid administration. As such, local E2 application mimicked the effect of systemic injection of this steroid. The inactive estrogen isomer, 17 alpha-E2, failed to significantly enhance Glu responsiveness. Both EB and E1, however, significantly potentiated Glu responsiveness in a manner similar to locally applied E2. In addition, EB administration produced long-lasting increases in background discharge, unlike E2, and eventual recovery of Glu responses to control pre-steroid levels. In summary, this study provides a demonstration of local sex steroid actions on neuronal responsiveness in a model extrahypothalamic CNS area. These effects were specific, as the inactive 17 alpha-E2 isomer did not alter neuronal physiology. The results presented here suggest that the neuronal effects of systemic estrogen may be mediated by local actions of E2 or E1.

A paradigm for determination of direct drug-induced modulatory alterations in Purkinje cell activity during treadmill locomotion

This paper describes a paradigm employing chronic single unit recording techniques and videotape analysis of treadmill locomotion in order to determine drug-induced modulation of sensorimotor neuronal activity. Animals implanted with a chronic headstage microdrive unit and an indwelling jugular cannula are trained to walk on a treadmill (10 s on, 10 s off). Characteristically, cerebellar Purkinje cells recorded 1-1.5 mm from midline, exhibit increased rates of discharge in phase with movement of specific limbs during a particular stage of the step-swing cycle, as assessed by video analysis of locomotor patterns. Drug-induced alterations in this movement-correlated discharge relative to changes in the spontaneous firing rate can then be determined to assess drug-induced neuromodulatory effects beyond general non-specific excitatory or inhibitory actions.

Ontogeny of sex differences in open-field ambulation in the rat

The effects of age and gonads were studied in rats subjected to open-field tests, during which ambulation behavior was recorded. Subjects were three groups of male and female rats: sham-operation on day 1 and day 21; gonadectomy on day 1 and sham-operation on day 21; and sham-operation on day 1 and gonadectomy on day 21. Half of each group were tested in a circular open field (3 min/day, 3 consecutive days) on days 28-30; the others were tested on days 47-49. Representatives of both batches were tested again in a square open field on days 76-78. There was a sex difference in ambulation at 77 days, but not at earlier ages. In animals gonadectomized on day 1 or day 21 the sex difference in adulthood failed to occur, because castration caused the males to ambulate as much as sham-operated and ovariectomized females. On the basis of our results and reports in the literature it is suggested that testicular secretions around puberty have an organizing effect on ambulation behavior. The intact adult male rat ambulates less than the adult female and this difference persists after castration in adulthood. Castration well before puberty prevents the development of the adult sex difference.

Sex and strain related effects of melatonin and 5-methoxytryptophol on open field behaviour in paired mice

The effects of two methoxyindoles, melatonin (MEL) and 5-methoxytryptophol (5-MTL), on partner investigation and other aspects of pair behaviour in the open field, were observed on male and female mice of the albino TO, and the pigmented C57BL/6 strains. Both indole derivatives are major secretory products of rodent (and human) pineal glands. They were administered by i.p. injection (100 micrograms/kg) to both members of the pair 1 h before each 10 min open field exposure. The testing took place on 3 consecutive days. There were no significant differences in behaviour over days, and group means were used in subsequent data analysis (MANOVA and Dunnet's test). Males, more socially active than the females in both strains, responded to MEL by a reduction in contact time; 5-MTL had the same effect on C57 males (TO males showed no change). Females, on the other hand, did not respond to MEL, and increased their social contact seeking after 5-MTL. There was no consistent effect of either indole on locomotion, and erratic (TO strain only) effects on defaecation. The data are discussed in terms of possible actions of pineal indoles on determinants of social behaviour.

The influence of sex, sexual condition and age on the exploratory behaviour of wild wood mice (Apodemus Sylvaticus L.)

From April 1978 to April 1980 the exploratory behaviour of more than 1000 wild wood mice was studied in a modified open field test situation. The influence of sex, sexual condition and age on the exploratory activity was investigated. Sufficient data being available, the exploratory activity of both sexually active and sexually inactive adult males and adult females, of both sexually active and sexually inactive subadult males and subadult females and of both sexually inactive juvenile males and females was compared by means of a one way analysis of variance for each two-month period. In other analyses the influence of age of adult males and adult females on the exploratory behaviour was further examined. No differences could be found in the exploratory activity displayed by males or females and by sexually active or sexually inactive animals. Also a clear influence of age was absent. The interindividual variation was very large.

Pineal indols and testosterone affect exploratory activity of male rats

The testosterone level has an inverse relation to activity in the open-field test. This is more important in red light than in white light. Pineal indols do not disturb this action. Some of these results are consistent with the assumption that androgens play a role on the exploratory activity of adult subjects.

Seasonal changes in open-field behavior in wild male meadow voles (Microtus pennsylvanicus)

Open-field behavior of free-living meadow voles was measured in males held overnight in the laboratory. Movement variables were positively correlated, and had negative associations with grooming and freezing. Parameters including activity, freezing, urinating, and grooming showed annual fluctuations related to the reproductive season. Together with the results of a previous study showing castration of wild voles results in altered open-field behavior, these results emphasize the role of testicular hormones in influencing this behavior. Factor analysis identified an activity component accounting for 39% of the variation, but other parameters were little simplified by this procedure. Most factors cycled annually, and significant variation was found in all eight factors during the 4-year sample. Open-field behavior varied between different reproductive and age segments of the population, and may be related to population processes such as dispersal.

Effects of gonadectomy and exogenous gonadal steroids on sex differences in open field behaviour of adult rats

Two experiments were carried out to investigate the contribution of gonadal hormones to the expression of sex differences in open field behaviour of adult female and male rats. In the first experiment rats were gonadectomized or sham-operated in adulthood and tested in the open field 1, 2, 3, 4, 5 or 6 weeks later (3 min/rat on 3 consecutive days during the light period of the day). It was found that following gonadectomy the well known sex difference in ambulation and rearing behaviour (females more than males), as well as in defaecation (females less than males) remained. Ovariectomy caused a decrease in ambulation and rearing, with an increase in defaecation, whereas castration had no significant effect on ambulation and rearing, but also increased defaecation. Also an overall increase in ambulation and rearing was found: animals tested 5 and 6 weeks following the operation were significantly more active than animals tested at 1-4 weeks after the operation. In the second experiment female and male rats were gonadectomized and 7 weeks later they received a silastic implant containing testosterone (T), dihydrotestosterone (DHT), oestradiol (E2), DHT plus E2, or nothing (control condition). Four weeks after the implantation the animals were tested in the open field (3 min/rat on 3 consecutive days). An overall sex difference was found for ambulation (females more than males) and for defaecation (females less than males). The only effect of hormone administration was found in DHT-treated animals: lowest ambulation and highest defaecation compared to other hormone and control animals. No differences were found between the T, E2, DHT plus E2, and control animals. From these data it was concluded that endogenous gonadal steroids seem to play a minor role in the expression of sex differences in adult open field behaviour in the rat.

Effects of neonatal testosterone and progesterone on open-field behaviour in the rat

Rats treated on the day of birth with progesterone (50 üg) or testosterone pro-pionate (200 üg) or the oil injection vehicle alone were tested in the open-field on four consectuve days at 45 days and 85 days of age. Averages across treatments showed that females ambulated more and reared more than males at both ages, that they groomed more than males at 45 days of age, and defaecated less at 85 days of age. Progesterone treatment significantly reduced defaecation in males at 45 days of age, and reduced grooming in both sexes. At 85 days of age progesterone significantly increased activity in females. Testosterone-treated animals of both sexes groomed significantly less than same-sex controls at 45 days of age, whereas at 85 days of age activity scores were significantly reduced only in females although testosterone treated males were less active on 2 test days and more active on 1. Early postnatal treatment with progesterone appeared to feminise male rats, and testosterone to masculinise female rats. Both hormones also altered the behaviour of opposite sexed rats, indicating that male rats may be further masculinised by exogenous testosterone and female rats further feminised by progesterone. Progesterone may have acted as an anti-androgenic agent by blocking gonadal and adrenal androgens in males and adrenal androgens in females.