Estrogen-induced suppression of female rat forebrain neurons with axons to ventral midbrain

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.

Neural circuits regulating sexual behaviors via the olfactory system in mice

Reproduction is essential for any animal species. Reproductive behaviors, or sexual behaviors, are largely shaped by external sensory cues exchanged during sexual interaction. In many animals, including rodents, olfactory cues play a critical role in regulating sexual behavior. What exactly these olfactory cues are and how they impact animal behavior have been a central question in the field. Over the past few decades, many studies have dedicated to identifying an active compound that elicits sexual behavior from crude olfactory components. The identified substance has served as a tool to dissect the sensory processing mechanisms in the olfactory systems. In addition, recent advances in genetic engineering, and optics and microscopic techniques have greatly expanded our knowledge of the neural mechanisms underlying the control of sexual behavior in mice. This review summarizes our current knowledge about how sexual behaviors are controlled by olfactory cues.

Effects of pelvic, pudendal, or hypogastric nerve cuts on Fos induction in the rat brain following vaginocervical stimulation

In the female rat, genitosensory input is conveyed to the central nervous system predominantly through the pelvic, pudendal, and hypogastric nerves. The present study examined the relative contribution of those three nerves in the expression of Fos immunoreactivity within brain regions previously shown to be activated by vaginocervical stimulation (VCS). Bilateral transection of those nerves, or sham neurectomy, was conducted in separate groups of ovariectomized, sexually-experienced females. After recovery, females were primed with estrogen and progesterone and given either 50 manual VCSs with a lubricated glass rod over the course of 1 h. VCS increased the number of neurons expressing Fos immunoreactivity in the medial preoptic area, lateral septum, bed nucleus of the stria terminalis, ventromedial hypothalamus, and medial amygdala of sham neurectomized females. Transection of the pelvic nerve reduced Fos immunoreactivity in the medial preoptic area, bed nucleus of the stria terminalis, ventromedial hypothalamus, and medial amygdala, whereas transection of the pudendal nerve had no effect. In contrast, transection of the hypogastric nerve increased Fos immunoreactivity in the medial preoptic area and lateral septum, whereas transaction of the pelvic nerve increased Fos immunoreactivity in the lateral septum, following VCS. All females given VCS, except those with pelvic neurectomy, displayed a characteristic immobility during each application. These data confirm that the pelvic nerve is largely responsible for the neural and behavioral effects of VCS, and support a separate function for the hypogastric nerve.

Implications of immediate-early gene induction in the brain following sexual stimulation of female and male rodents

Induction of immediate-early genes (IEGs), such as c-fos, has been widely used to mark the activation of brain regions following different types of sexual stimulation and behavior. A relatively common set of hormone-concentrating basal forebrain and midbrain structures in female and male rodents is activated by copulatory stimulation, in particular, stimulation of sensory nerves that innervate the penis or vagina/cervix, olfactory or pheromonal stimuli, and conditioned sexual incentives. These regions include the preoptic area, lateral septum, bed nucleus of the stria terminalis, paraventricular hypothalamus, ventromedial hypothalamus, medial amygdala, ventral premammillary nuclei, ventral tegmentum, central tegmental field, mesencephalic central gray, and peripeduncular nuclei. Regions that do not contain classic intracellular steroid receptors, such as the ventral and dorsal striatum or cortex, are also activated. IEGs have also been colocalized with cytoplasmic proteins like GnRH and oxytocin, and have been used in conjunction with retrograde tracers to reveal functional pathways associated with different sexual behaviors. Steroid hormones can also alter the ability of sexual stimulation to induce IEGs. Despite the many similarities, some differences in IEG induction between sexes have also been found. We review these findings and raise the question of what IEG induction in the brain actually means for sexual behavior, that is, whether it indicates the perception of sexual stimulation, commands for motor output, or the stimulation of a future behavioral or neuroendocrine event related to the consequences of sexual stimulation. To understand the role of a particular activated region, the behavioral or neuroendocrine effects of lesions, electrical stimulation, drug or hormone infusions, must also be known.

Differential induction of Fos in the female rat brain following different amounts of vaginocervical stimulation: modulation by steroid hormones

Vaginocervical stimulation (VCS), produced either by copulation with intromission or by manual stimulation of vagina and cervix with a glass rod, induces neuroendocrine and behavioral responses that are critical for female reproduction in many species. We and others have shown that Fos mRNA and protein are induced within different estrogen-concentrating and -non-concentrating regions of the female rat brain following copulation with intromission and manual VCS. In the present study, we investigated the amount of distributed VCS required to induce Fos immunoreactivity within estrogen-concentrating regions of the medial preoptic area, lateral septum, bed nucleus of the stria terminalis, ventromedial hypothalamus, medial amygdala, and mesencephalic central gray, and whether estrogen and progesterone could alter the threshold or pattern of induction. Ovariectomized rats were administered estradiol benzoate (10 micrograms) 48 h and progesterone (500 micrograms) 4 h before receiving either 0, 1, 5, 10, 20, 30, 40, or 50 manual VCSs with a lubricated glass rod. Ovariectomized hormone control rats received injections of the sesame oil vehicle 48 and 4 h before VCS. All rats were sacrificed 75 min after the first VCS. Fos immunoreactivity was induced differentially by VCS within the different regions, and the hormones either augmented, inhibited, or had no effect on the induction. These data demonstrate that cells within different estrogen-concentrating regions of the female rat brain are differentially sensitive to VCS, and that steroid hormones can either increase or decrease the amount of Fos induced by different amounts of VCS. Different brain regions may participate in gating the sensory information of VCS into different behavioral and neuroendocrine events.

Axon-sparing lesion of the preoptic area enhances receptivity and diminishes proceptivity among components of female rat sexual behavior

Stereotaxic infusion of ibotenic acid deleted neurons in the medial preoptic area (POA) in the ovariectomized female rats. A well-circumscribed lesion was infiltrated by astrocytes; local axons of passage were spared. Following estrogen priming and progesterone supplement, the females with the lesion had higher lordosis quotients than the vehicle-infused controls, when males successfully mounted them. On the other hand, the treatment did not induce solicitation in females with the lesion nor reduced their rejection of male partners. Meanwhile, gradual and persistent suppression of the lordosis reflex followed electrical stimulation through electrodes placed in the POA lesion. Except that the females with the POA lesion needed less estrogen to obtain comparable prestimulation quotients with the controls, the lesioned and control animals responded similarly to the stimulation. Because an adjunct neural transection dorsal to the POA lesion abolished the stimulus-bound suppression of lordosis, the effect was due to the activation of axons of passage that presumably descend from the septum. It is concluded that the POA is the major target for estrogen in eliciting proceptive behavior; local POA neurons as well as septal efferents appear to inhibit the lordosis reflex, the principal receptive component in female rat sexual behavior.

Dihydrotestosterone-sensitive neurons in the male rat ventromedial hypothalamus

Electrical stimulation of the midbrain central gray caused antidromic activation in 273 neurons in and around the hypothalamic ventromedial nucleus in 40 urethan-anesthetized male rats. The latency (range: 2-39 ms) and threshold (100-1,600 microA) of the responses were compared among intact males and orchidectomized males, which received either no treatment, daily injections of testosterone propionate or dihydrotestosterone, or a combined estradiol benzoate and progesterone injection. Recordings were from comparable neuronal pools, because probability distribution for the latency as well as histological localization of each response were not different among groups. Orchidectomy decreased the mean threshold, and either treatment reversed the effect. Dihydrotestosterone increased the mean threshold by reducing the number of cells with thresholds below 600 microA, to a level as in intact males. Estrogen reduced the number of cells responding at 300-900 microA, but at lower thresholds, fell short of replicating the distribution in intact males. Testosterone reinstated the distribution as in intact males. It was concluded that different subgroups of ventromedial hypothalamic neurons with specific sensitivity to metabolites of testosterone project to the central gray.

Developmental effect of testosterone on estrogen sensitivity of the rat preoptic neurons with axons to the ventral tegmental area

The neonatal effect of testosterone on neuronal sensitivity to estrogen was examined in the medial preoptic area of the rat when adult. Electrical stimulation of the ventral tegmental area induced antidromic action potentials in 683 neurons in 106 urethane-anesthetized animals which consisted of 38 males castrated on the day of birth (day 1), 30 females androgenized by testosterone injection on day 5, and 38 normal females. All females were used after ovariectomy when adult. Histological localization of the antidromically activated neurons was similar in all the animal groups. The latency jump in 40% of the potentials indicated that activated preoptic axons terminate in the stimulation site. The latency for activation was in the range 1.6-43.7 ms and the threshold was as low as 60 microA. The absolute refractory period did not exceed 1700 microseconds. Estrogen increased the threshold among 278 cells in the normal females and 181 cells in the neonatally castrated males. Preoptic neurons in the androgenized females (n = 224) differ from those in others in that their threshold, regardless of estrogen treatment, was at the ceiling values seen in the other groups in the presence of estrogen. Estrogen also prolonged the absolute refractory period in the females but not in the androgenized females. The reduced excitability and the lack of estrogen sensitivity may be responsible for the insensitivity of the androgenized females to feminine actions of estrogen, that may presumably underlie a sex difference in certain behavioral functions.

Suppression of the lordosis reflex of female rats by efferents of the medial preoptic area

In freely moving, estrogen- and progesterone-treated ovariectomized female rats, electrical stimulation of the medial preoptic area, which had been isolated dorsally from the septum by a horizontal knife cut (roof cut), caused immediate interruption of the lordosis reflex, a principal component of female reproductive behavior in this species. Neither the knife cut nor the stimulation affected their proceptive interactions with the males. Lordosis was interrupted in a graded manner in response to increased stimulus intensity, with a threshold at 30 microA. The optimal frequency was at 50-100 Hz. Lordosis was reinstated promptly by the termination of current application. The rapid time course distinguished the stimulation effect in the rats with the roof cut from that in the intact control animals, which was slow in its onset and recovery. In the sham-operated animals, an adjunct bilateral cut of the stria terminalis was sufficient to restore the rapid response as in the roof-cut animals. The roof-cut animals were sensitive to lower doses of estrogen than those without the knife cut; therefore, the preoptic area appears to be a separate entity from the septum in the inhibitory control of lordosis. The elimination of facilitatory neural components for this reflex, which enter the preoptic area dorsally via the stria terminalis, might be responsible for the prompt and exaggerated stimulation effect in the roof-cut animals.

Interruption of the lordosis reflex of female rats by ventral midbrain stimulation

The lordosis reflex, dorsiflexion of the vertebral column, is an estrogen-dependent, essential element of female sexual behavior in rodents. Unilateral electrical stimulation of the midbrain ventral tegmental area through a chronically implanted electrode in freely moving, estrogen-primed ovariectomized female rats caused a rapid and strong suppression of the lordosis reflex in response to either male mounts or manual cutaneous stimuli. The interruption occurred in a graded manner to increased stimulus intensity, with a threshold at 30 microA. The optimal frequency was at 75-125 Hz. After the termination of electrical stimulation, lordosis performance returned promptly to the pre-stimulation level. No aversive response accompanied the blockade of lordosis. Electrical stimulation specifically blocked lordosis, without disrupting the proceptive components of female sexual behavior. In 10 animals tested, concomitant injection of dopamine receptor blocker pimozide tended to offset the effects of electrical stimulation in 2 cases. Interruption of the lordosis reflex might be mediated by projections from the ventral tegmental area, which activate a descending pathway inhibitory to the lordosis reflex arc at or below the lower brain stem.

Differential regulation of estrogen-dependent sexual development of rat brain by growth factors

Intraventricular infusion of antiserum to nerve growth factor (ANGF), but not that to insulin, epidermal growth factor nor normal rabbit serum, resisted estrogen-induced behavioral defeminization in the female rat neonates. A significant number of the ANGF-treated rats showed lordosis as adults despite neonatal estrogen, but positive feedback of estrogen on serum luteinizing hormone was absent. Sexual phenotype in behavioral and gonadotropic functions may be under differential development regulation.