Gonadal hormones stimulate ultrasound production by female hamsters

Ultrasonic vocalizations in golden hamsters (Mesocricetus auratus) reveal modest sex differences and nonlinear signals of sexual motivation

Vocal signaling is one of many behaviors that animals perform during social interactions. Vocalizations produced by both sexes before mating can communicate sex, identity and condition of the caller. Adult golden hamsters produce ultrasonic vocalizations (USV) after intersexual contact. To determine whether these vocalizations are sexually dimorphic, we analyzed the vocal repertoire for sex differences in: 1) calling rates, 2) composition (structural complexity, call types and nonlinear phenomena) and 3) acoustic structure. In addition, we examined it for individual variation in the calls. The vocal repertoire was mainly composed of 1-note simple calls and at least half of them presented some degree of deterministic chaos. The prevalence of this nonlinear phenomenon was confirmed by low values of harmonic-to-noise ratio for most calls. We found modest sexual differences between repertoires. Males were more likely than females to produce tonal and less chaotic calls, as well as call types with frequency jumps. Multivariate analysis of the acoustic features of 1-note simple calls revealed significant sex differences in the second axis represented mostly by entropy and bandwidth parameters. Male calls showed lower entropy and inter-quartile bandwidth than female calls. Because the variation of acoustic structure within individuals was higher than among individuals, USV could not be reliably assigned to the correct individual. Interestingly, however, this high variability, augmented by the prevalence of chaos and frequency jumps, could be the result of increased vocal effort. Hamsters motivated to produce high calling rates also produced longer calls of broader bandwidth. Thus, the sex differences found could be the result of different sex preferences but also of a sex difference in calling motivation or condition. We suggest that variable and complex USV may have been selected to increase responsiveness of a potential mate by communicating sexual arousal and preventing habituation to the caller.

Chemosignals, hormones and mammalian reproduction

Many mammalian species use chemosignals to coordinate reproduction by altering the physiology and behavior of both sexes. Chemosignals prime reproductive physiology so that individuals become sexually mature and active at times when mating is most probable and suppress it when it is not. Once in reproductive condition, odors produced and deposited by both males and females are used to find and select individuals for mating. The production, dissemination and appropriate responses to these cues are modulated heavily by organizational and activational effects of gonadal sex steroids and thereby intrinsically link chemical communication to the broader reproductive context. Many compounds have been identified as "pheromones" but very few have met the expectations of that term: a unitary, species-typical substance that is both necessary and sufficient for an experience-independent behavioral or physiological response. In contrast, most responses to chemosignals are dependent or heavily modulated by experience, either in adulthood or during development. Mechanistically, chemosignals are perceived by both main and accessory (vomeronasal) olfactory systems with the importance of each system tied strongly to the nature of the stimulus rather than to the response. In the central nervous system, the vast majority of responses to chemosignals are mediated by cortical and medial amygdala connections with hypothalamic and other forebrain structures. Despite the importance of chemosignals in mammals, many details of chemical communication differ even among closely related species and defy clear categorization. Although generating much research and public interest, strong evidence for the existence of a robust chemical communication among humans is lacking.

The medial preoptic area is necessary for sexual odor preference, but not sexual solicitation, in female Syrian hamsters

Precopulatory behaviors that are preferentially directed towards opposite-sex conspecifics are critical for successful reproduction, particularly in species wherein the sexes live in isolation, such as Syrian hamsters (Mesocricetus auratus). In females, these behaviors include sexual odor preference and vaginal scent marking. The neural regulation of precopulatory behaviors is thought to involve a network of forebrain areas that includes the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA). Although MA and BNST are necessary for sexual odor preference and preferential vaginal marking to male odors, respectively, the role of MPOA in odor-guided female precopulatory behaviors is not well understood. To address this issue, female Syrian hamsters with bilateral, excitotoxic lesions of MPOA (MPOA-X) or sham lesions (SHAM) were tested for sexual odor investigation, scent marking, and lordosis. MPOA-X females did not investigate male odors more than female odors in an odor preference test, indicating that MPOA may be necessary for normal sexual odor preference in female hamsters. This loss of preference cannot be attributed to a sensory deficit, since MPOA-X females successfully discriminated male odors from female odors during an odor discrimination test. Surprisingly, no deficits in vaginal scent marking were observed in MPOA-X females, although these females did exhibit decreased overall levels of flank marking compared to SHAM females. Finally, all MPOA-X females exhibited lordosis appropriately. These results suggest that MPOA plays a critical role in the neural regulation of certain aspects of odor-guided precopulatory behaviors in female Syrian hamsters.

Effects on hamster vocalization and aggression of carbachol injections into the MPOA/AH

Recent work has shown that microinjections of the cholinergic agonist carbachol into the preoptic area (MPOA) increase 20-30 kHz ultrasonic vocalization in rats. This response could be interpreted as a quite specific and direct effect on a central mechanism for vocalization or ultrasound production. Alternatively, it could reflect a more general drug-induced increase in defensiveness or defensive aggression. Two studies were conducted to distinguish these possibilities. In both, MPOA carbachol treatments very similar to those in previous work (unilateral injections into the MPOA/AH of 1 microg of carbachol in 0.2 microl of saline) were administered to female hamsters. The first study focused on the rate of ultrasonic courtship vocalization, finding it to decrease after treatment. This result is inconsistent with the view that cholinergic agonists directly stimulate vocalization. In contrast, although it does not completely exclude an effect on a mechanism for vocalization, it provides at least suggestive support for an interpretation in terms of defensiveness: Because hamster ultrasounds convey sexual readiness rather than alarm or aggressiveness, they would be expected to decrease in the event of a drug-induced increase in an incompatible motivational or emotional state. The second study exploited the aggressiveness of hamsters to even more directly test carbachol's ability to increase defensiveness or defensive aggression. Its results showed that minimally-estrous female subjects bit their male pursuers more quickly if recently treated with carbachol rather than placebo. Taken together, both results support the existence of a cholinergic system that extends through the MPOA and regulates defensiveness or a related motivational or emotional state.

Estrogen receptor-alpha and -beta immunoreactive neurons in the brainstem and spinal cord of male and female mice: relationships to monoaminergic, cholinergic, and spinal projection systems

For many populations of estrogen-sensitive neurons it remains unknown how they are associated with central nervous system circuitries that mediate estrogen-induced modulation of behavioral components. With the use of double-labeling immunohistochemistry and tracing techniques, the relationships of estrogen receptor (ER)-alpha- and ER-beta-immunoreactive (IR) neurons in the mouse brainstem and spinal cord to monoaminergic, cholinergic, and spinal projection systems are explored. Similar distributions of ER-IR neurons were present in females and males, with differences in labeling intensity of ER-alpha immunoreactivity among males and estrogen-, and oil-treated females. Barrington's nucleus, the ventrolateral medulla, and the nucleus of the solitary tract contained spinal-projecting ER-alpha-IR neurons, whereas ER-alpha-IR neurons in the periaqueductal gray, parabrachial nucleus, and catecholaminergic A1 cell group received spinal input. Numerous tyrosine hydroxylase (TH)-IR ER-alpha-IR neurons were present in the ventral periaqueductal gray, nucleus of the solitary tract, A1 cell group, and lumbosacral cord. The dorsal raphe nucleus contained ER-alpha-IR and ER-beta-IR neurons that colocalized with serotonin (5HT), and the reticulotegmental nucleus contained 5HT-IR ER-alpha-IR neurons. Fibers IR for vesicular acetylcholine transporter (VAChT), TH, and 5HT were located among ER-alpha-IR neurons in the dorsal horn and spinal autonomic regions. Robust staining for TH and VAChT, but not 5HT, was present among ER-alpha-IR neurons in the lumbosacral lateral collateral pathway. Possible modulatory actions of estrogen on each of these ER-IR populations are discussed in the context of their specific function, including micturition, sexual behavior, ejaculation, cardiovascular and respiratory control, tactile and nociceptive sensory processing, anti-nociception, endocrine regulation, and feeding.

Projections from estrogen receptor-alpha immunoreactive neurons in the periaqueductal gray to the lateral medulla oblongata in the rhesus monkey

The periaqueductal gray (PAG) contains numerous estrogen receptor-alpha immunoreactive (ER-alpha IR) neurons that are distributed in a species-specific way. These neurons might modulate different types of behavior that are mediated by the PAG such as active and passive coping responses, analgesia, and reproductive behavior. In primates, it is not known whether ER-alpha IR PAG neurons represent local interneurons and/or neurons that project to brainstem areas that control these behaviors. In this double labeling study, we asked whether ER-alpha IR neurons in the PAG of the rhesus monkey project to the nucleus retroambiguus (NRA), an area in the ventrolateral caudal medulla oblongata that is involved in expiration, vocalization, and reproductive behavior. Tracer was injected into the caudal lateral medulla oblongata to retrogradely label PAG neurons, and ER-alpha was visualized immunohistochemically. Although ER-alpha IR neurons and NRA-projection neurons were present at similar levels of the PAG, their distributions hardly overlapped. ER-alpha IR PAG neurons that project to the lateral caudal medulla represented less than 2% of ER-alpha IR PAG neurons. These double-labeled neurons were mainly located in the ipsilateral caudal PAG. The cluster of neurons in the medial part of the lateral PAG that projects specifically to the NRA-region did not contain double-labeled cells. The results indicate that only a few ER-alpha IR PAG neurons project to the NRA-region. This might be related to the modest effects of estrogen on mating-related behavior in primates compared most other mammalian species. Remaining ER-alpha IR PAG neurons might act locally on other PAG neurons, or they might represent neurons that project to other areas. Furthermore, the finding that the distributions of ER-alpha IR neurons and neurons that project to premotor neurons in the NRA-region scarcely overlap illustrates that the PAG in primates is very highly organized into anatomically distinct regions compared with other species.

Cuts between the septum and preoptic area increase ultrasound production, lordosis, and body weight in female hamsters

Studies of the mechanisms for female-typical mating behavior have focused on the ventromedial hypothalamus, and on the decrements in lordosis caused by lesions of this structure. However, opposed changes of comparable size are produced, at least in rats, by horizontal cuts extending forward from the anterior commissure (anterior roof deafferentation, or ARD). This suggests the existence of a lordosis-inhibiting system of forebrain structures that may include the lateral septum and preoptic area. To test the generality of this system, ovariectomized hamsters in hormone-induced estrus were observed for levels of ultrasound production and lordosis during tests with male conspecifics. In addition, subjects were observed for lordosis responses to light manual stimulation. Upon the completion of these tests, subjects received control treatments or ARD prior to a second round of behavioral observations. These postoperative tests revealed clear ARD-stimulated increases in ultrasound production and body weight. In contrast, the facilitation of lordosis was more subtle, appearing in tests with manual stimulation, but not in response to males. These results, then, demonstrate some consistency across species in the effects of ARD. At the same time, however, they suggest species differences in the magnitude of these effects, in turn, suggesting species differences in the dependence of receptivity on forebrain lordosis-inhibiting mechanisms.

Hormonal regulation of sociosexual behavior in female mammals

The role of estrogen (E) and progesterone (P) in regulating the expression of agonistic, scent marking and proceptive displays is reviewed. In the intact sexually mature female, data indicate that agonistic, scent marking and proceptive patterns of behavior fluctuate significantly prior to and during the period of mating. The propensity to display a specific pattern of behavior is dependent upon the stimulus situation. Hence, agonistic behavior may be induced by the presence of a conspecific female whereas sexual behavior and proceptive responses will be elicited by a male. Administration of E to ovariectomized animals does not appear to produce significant effects on agonistic behavior and territorial scent-marking responses albeit in a few cases E appears to reduce agonistic tendencies toward male conspecifics. Exogenous E treatment, however, does appear to induce the occurrence of proceptive forms of scent-marking behavior as well as a variety of other proceptive responses which are further increased after P treatment. Importantly, the sequential administration of E and P facilitates both proceptive and copulatory responses in several rodent species. In addition, hormone implant studies indicate that sites in the brain which are sensitive to the hormonal facilitation of sexual receptivity concurrently facilitate proceptive behavior. On the basis of the current data, ovarian hormones appear to exert their strongest effects on producing behavioral displays which attract males prior to mating, facilitating sexual receptivity, and inducing concurrently proceptive responses which further enhance copulation and reproductive success.

Dissociation of hypothalamic effects on ultrasound production and copulation

Two studies were conducted to compare the brain mechanisms for copulation with those controlling other sexual behaviors, such as the ultrasonic calls that help hamsters and other rodents attract potential mates. Specifically, male and female golden hamsters were castrated and hormone-primed prior to being observed for rates of ultrasound production and levels of sex-typical copulatory behavior (mounts, intromissions and ejaculations in males; lordosis in females). Such tests were conducted before and after subjects received sham operations or bilateral lesions of the preoptic area (POA), anterior hypothalamus (AH) or ventromedial hypothalamus (VMN). The results confirmed previous work in showing the POA lesions decrease rates of intromission, ejaculation and ultrasound production, while VMH lesions decrease lordosis duration. More surprising was the tendency of VMN lesions to increase rates of ultrasonic calling by both males and females. For males, these effects identify differences between the neural circuits controlling copulatory and noncopulatory sexual behaviors. For females, they suggest a mechanism for the behavioral incompatibility of ultrasound production and lordosis. In particular, they raise the possibility that the suspension of ultrasonic calling that normally accompanies lordosis reflects an increase in VMN activity that simultaneously provokes lordosis and inhibits vocalization.

Effects of sex and reproductive state on acoustic responsiveness in hamsters

Semi-quantitative [14C]2-deoxyglucose (2DG) autoradiography was used to describe the responses of hamsters to 35 kHz mimics of the "ultrasounds" used for communication during mating. The first study examined the processing of ultrasounds and ambient noise by estrous females, some of which were deafened or hemideafened with plastic ear plugs. These data failed to reveal responses specific to the ultrasounds. However, lateralized responses to the ambient noise were apparent, especially in the hemideafened subjects. For the ventral cochlear nucleus (VCN), 2DG uptake was elevated contralateral to the plug and ipsilateral to the effective stimulus. In contrast, uptake by more rostral structures (dorsal n. of the lateral lemniscus = DNLL; ventral n. of the lateral lemniscus = VNLL; central n. of the inferior colliculus = CIC; medial geniculate n.) was elevated contralateral to the stimulus. A second experiment examined the responses of intact or castrated male and female hamsters to unilaterally presented ultrasounds and ambient noise. As before, relative levels of 2DG uptake differed across hemispheres for structures including the VCN, trapezoid body, VNLL, DNLL, and CIC. More surprisingly, intact females showed more 2DG uptake than males in the DNLL, auditory nerve, and lateral lemniscus. Females also tended to show elevated anterior hypothalamic uptake, but just in the hemisphere contralateral to the stimulus. These results suggest that male and female hamsters differ in acoustic responsiveness, and that this difference is mediated by hormonal effects at several brainstem components of the central auditory system.

Female sexual behaviour: roles of gonadal hormones in the Syrian hamster

The sexual behaviour of ovariectomized female hamsters was examined after treatment with oestrogen alone and after the addition of progesterone. Both the proceptive and receptive components of behaviour were recorded at four oestrogen dose levels; progesterone dose was kept constant. Oestrogen alone caused dose-dependent increases in proceptive behaviour and, at the higher dose levels, a small increase in receptivity. The addition of progesterone increased proceptive behaviour in females primed with the lower dose of oestrogen but not the higher. Receptivity was markedly increased in all animals but the intensity of lordosis was oestrogen-dose dependent.

Sagittal knife cuts in the near and far lateral preoptic area-hypothalamus reduce ultrasonic vocalizations in female hamsters

Although it has been reported that lesions of the medial preoptic area reduce ultrasonic vocalizations in female hamsters, the neural connections of this area important for ultrasound production have not been investigated. We, therefore, tested vocalizations in female hamsters following sagittal cuts along the medial preoptic area-medial anterior hypothalamic continuum (MPOA-MAH) at either the medial (Near Lateral or NL cuts) or lateral (Far Lateral or FL cuts) border of the medial forebrain bundle (MFB). Bilateral NL and FL cuts did not affect spontaneously occurring vocalizations or vocalizations occurring during contact with males, but were equally effective in substantially blocking increases in vocalizations following exposure to males. Another group of animals with a unilateral NL cut and a contralateral FL cut showed a vocalization deficit comparable to that in NL and FL groups, suggesting that NL and FL cuts severed a common pathway important for ultrasonic calling. These results indicate that the lateral connections of the MPOA-MAH are critical for ultrasonic vocalizations. Alternatively, a ventral pathway from the medial amygdala to the bed nucleus of the stria terminalis, one which crosses the plane of both NL and FL cuts, is discussed in relation to the deficits observed.

Delta-9-Tetrahydrocannabinol stimulates receptive and proceptive sexual behaviors in female hamsters

This experiment studied the effects of delta-9-tetrahydrocannabinol (THC) on lordosis responses and ultrasonic communication (measures of sexual receptivity and proceptivity, respectively) in female hamsters. Specifically, lordosis durations and rates of ultrasound production by estradiol-primed ovariectomized hamsters were observed following acute treatment with 1.5 mg/kg of THC, 500 micrograms of progesterone, or the injection vehicle. The results showed that THC can facilitate both lordosis and ultrasound production. Together with results from other laboratories, these data indicate that THC can stimulate female sexual behavior and suggest that this effect reflects a direct, nonhormonal, effect of THC on brain mechanisms for behavior.