Yasuo Sakuma

Facilitation of the lordosis reflex of female rats from the ventromedial nucleus of the hypothalamus.

1. Effects of electrical stimulation of hypothalamic ventromedial nucleus (v.m.n.) on the lordosis reflex of female rats were examined in ovariectomized and oestrogen‐primed animals with chronically implanted electrodes. 2. Lordosis triggered either by manual cutaneous stimulation or by male mounting, was facilitated by electrical stimulation of the v.m.n. 3 A gradual increase in lordosis performance followed a relatively long period of stimulation; never less than 15 min and usually about 1 hr of stimulation was necessary for maximum facilitation. Following the termination of stimulation, the performance returned gradually to the control level during a 5‐‐8 hr period. 4. The optimal frequency of stimulation was between 10 and 30 Hz. Threshold for effective facilitation was, on the average, 12.5 microA. 5. Stimulation tended to induce larger facilitation when applied to the lateral side of v.m.n. 6. Pre‐treatment with oestrogen was necessary to obtain facilitation by v.m.n. stimulation. The threshold dosage of oestrogen was 2.5 microgram per animal. 7. Stimulation was effective in adrenalectomized rats, in dexamethasone‐primed animals, and in rats pre‐treated with exogenous progesterone. Thus, adrenal prodesterone release is not required for the v.m.n. facilitation of lordosis. 8. Medial preoptic stimulation with the same parameters suppressed the lordosis reflex. 9. The v.m.n. participates in the control of lordosis by a facilitatory output. The delay before facilitation implies that the v.m.n. is not in the direct reflex‐arc for the execution of lordosis. Rather, a summation or interaction process with an unusually long time course is involved.

Deficit in the lordosis reflex of female rats caused by lesions in the ventromedial nucleus of the hypothalamus.

1. The effect of electrolytic lesions of the ventromedial nucleus of the hypothalamus (v.m.n.) on the lordosis reflex has been investigated on ovariectomized female rats. Lesions were made through chronically implanted platinum‐iridium electrodes. 2. V.m.n. lesions did not disrupt lordosis immediately, but induced a gradual decline in the reflex. Lordosis performance reached it minimum no less than 12 hr after the lesion, and typically after 36‐‐60 hr. 3. The magnitude of the lordosis deficit was related to the amount of v.m.n. damage. Destruction of other hypothalamic regions was without appreciable relation to the deficit. Within v.m.n., lesion size in the lateral, but not medial portion was significantly correlated with lordosis deficit. 4. Because of the slow time courses of v.m.n. lesions and stimulation (Pfaff & Sakuma, 1978) effects, it is postulated that the v.m.n. is not part of the direct reflex‐arc for lordosis. Rather, neurones in v.m.n. are likely to exert a tonic hormone‐dependent bias on brain stem reflex paths for this behaviour.

Oxytocin-gaze positive loop and the coevolution of human-dog bonds

Gaze into my eyes Humans bond emotionally as we gaze into each others eyes—a process mediated by the hormone oxytocin. Nagasawa et al. show that such gaze-mediated bonding also exists between us and our closest animal companions, dogs (see the Perspective by MacLean and Hare). They found that mutual gazing increased oxytocin levels, and sniffing oxytocin increased gazing in dogs, an effect that transferred to their owners. Wolves, who rarely engage in eye contact with their human handlers, seem resistant to this effect. Science, this issue p. 333; see also p. 280 The human-dog bond is facilitated by the interaction of oxytocin feedback loops that emerged over the course of domestication. [Also see Perspective by MacLean and Hare] Human-like modes of communication, including mutual gaze, in dogs may have been acquired during domestication with humans. We show that gazing behavior from dogs, but not wolves, increased urinary oxytocin concentrations in owners, which consequently facilitated owners’ affiliation and increased oxytocin concentration in dogs. Further, nasally administered oxytocin increased gazing behavior in dogs, which in turn increased urinary oxytocin concentrations in owners. These findings support the existence of an interspecies oxytocin-mediated positive loop facilitated and modulated by gazing, which may have supported the coevolution of human-dog bonding by engaging common modes of communicating social attachment.

Sexually dimorphic expression of estrogen receptor β in the anteroventral periventricular nucleus of the rat preoptic area: Implication in luteinizing hormone surge

Striking sex difference was detected in the expression of estrogen receptor (ER) β mRNA and protein by nonisotopic in situ hybridization and immunohistochemistry in the anteroventral periventricular nucleus (AVPV) of the rat preoptic area. In females more than in males, a significantly larger number of ERβ mRNA-positive cells were visualized in the medial-most portion of the AVPV within 50 μm from the ependymal lining of the third ventricle. Rats of 7, 14, 21, 35, and 60 days of age (d 1 = day of birth) showed the sex difference. Orchidectomy of male neonates or estrogen treatment of female pups reversed the brain phenotype when examined on d 14. In the AVPV of adult females, ERα immunoreactivity colocalized in 83% of ERβ mRNA-positive cells. Tyrosine hydroxylase immunoreactivity colocalized in 18% of ERβ immunoreactive cells in d 21 females. Infusion of an ERβ antisense oligonucleotide into the third ventricle in the vicinity of the AVPV resulted in significantly longer days of successive estrus and a 50% reduction in the number of ERβ-immunoreactive cells in the AVPV. These findings provide support for the hypothesis that activation of ERβ in the AVPV is an important regulatory event in the female-typical induction of luteinizing hormone surge by estrogen.

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.

Modulation of the lordosis reflex of female rats by LHRH, its antiserum and analogs in the mesencephalic central gray.

Effects of intracerebral infusion of LHRH and related compounds on the lordosis reflex were examined in ovariectomized, estrogen-treated female rats. Lordosis, triggered either by manual cutaneous stimulation or by male mounting, was facilitated by the infusion of LHRH dissolved in saline into the mesencephalic central gray at midcollicular levels (CG). Infusion of an anti-LHRH globulin into the CG disrupted lordosis. Pretreatment with estrogen was necessary to obtain the behavioral facilitation by LHRH infusion. In separate experiments, placement of cannulae containing LHRH in an agar-saline gel in the CG also facilitated lordosis behavior. LHRH analogs applied to the CG by this method showed a clear dissociation between their behavioral effects and their reported effects on LH release. Cellular mechanisms involved in the mediation of LHRH effect in midbrain for the facilitation of lordosis may be different from those in gonadotrophs for LH release.

Importance of the medial amygdala in rat penile erection evoked by remote stimuli from estrous females.

Effects of medial amygdala lesions (MAL) were examined on rat penile erection in three different experimental situations. Only sexually vigorous males, as identified by preoperative mating tests, were used. Bilateral radiofrequency lesions were confined to the posterior medial amygdala, with little systematic damage to anterior medial amygdala or to adjacent structures. Lesion electrodes were withdrawn without current application in sham-operated animals (SHAM). After recovery for brain surgery, males were tested for (1) noncontact erection (NCE) that occurs when males were placed in proximity to inaccessible estrous females, (2) reflexive erection evoked in supine males by retraction of the penile sheath, and (3) copulatory behaviour with receptive females. In the NCE test, none of the MAL males showed penile erection during the 20 min observation, whereas 70% of the SHAM males showed it (P < 0.001). In contrast, no erectile dysfunction in the MAL males was detected in the other two tests. MAL males displayed more penile-body erections (flips) than SHAM males in the reflexive-erection test (P <0.05). In the copulation test, most of the MAL males achieved intromission, but their intromission ratio, a partial measure of erectile function, was marginally lower than that of SHAM males (P = 0.051). MAL males had longer intervals between intromissions (P < 0.001); as a result, none of them ejaculated during the 30 min period that followed the first intromission. The results suggest that the posterior medial amygdala plays an essential role in the regulation of NCE, and it may also contribute to the regulation of erection in other contexts.

Estrogen excites oxytocinergic, but not vasopressinergic cells in the paraventricular nucleus of female rat hypothalamus

Extracellular antidromic potentials were recorded from the paraventricular nucleus of the ovariectomized female rat hypothalamus following electrical stimulation of the neurohypophysis. Effects of estrogen-treatment were investigated after classifying the antidromically identified cells into tonically-firing, phasically-firing, or silent groups according to their patterns of spontaneous discharge. Estrogen significantly decreased the antidromic activation threshold and shortened the refractory period as well as the antidromic spike latency in the tonic-firing cells. We suggest that estrogen selectively and directly excited the tonically-firing, presumably oxytocinergic cells.

Neurons synthesizing gonadotropin-releasing hormone mRNA subtypes have multiple developmental origins in the medaka

The origins of the different populations of gonadotropin‐releasing hormone (GnRH)‐containing neurons in the brains of two genotypes (HO4C; HNI‐II) of medaka Oryzias latipes were analyzed at different stages of development (day 1 after fertilization through adulthood), by using oligonucleotide probes specific to salmon‐, seabream‐, and chicken II‐GnRH mRNA and antisera against specific GnRH peptides. Between the two genotypes, there was no difference in the site and time of GnRH expression or the final pattern of GnRH neuronal organization. In the adult fish of both sexes, salmon GnRH mRNA and peptide‐containing neurons were seen in the terminal nerve ganglia (nucleus olfactoretinalis; NOR) and chicken II‐GnRH mRNA and peptide‐containing neurons in the midbrain tegmentum. GnRH cells at the base of the olfactory placode (1–2 cells) and in the midbrain tegmentum were first seen in 1‐day‐old fish of both genotypes. On day 15, lightly immunoreactive GnRH cells were seen in the NOR of only HNI genotype. By day 30, GnRH expression in the NOR and in the midbrain was prominent. GnRH cells along the basal olfactory bulb and basal telencephalon were occasionally seen in animals 30 days or older. This developmental study shows differential distribution of salmon and chicken II‐GnRH mRNA subtypes and emphasizes their separate embryonic origins from the olfactory apparatus (salmon‐GnRH) and the ependymal cells of the third ventricle (chicken II‐GnRH). The absence of preoptic GnRH hybridization signals, immunoreactivity and the lack of GnRH fibers in the pituitary suggests that the preoptic GnRH neurons are distinct from the olfactory derived‐terminal nerve GnRH neurons, and that the GnRH neurites reported in the pituitary of teleost must be of preoptic origin. J. Comp. Neurol. 401:217–226, 1998.

Gonadal Steroid Action and Brain Sex Differentiation in the Rat

Gonadal steroids that establish sexually dimorphic characteristics of brain morphology and physiology act at a particular stage of ontogeny. Testosterone secreted by the testes during late gestational and neonatal periods causes significant brain sexual dimorphism in the rat. This results in both sex‐specific behaviour and endocrinology in adults. Sexual differentiation may be due to neurogenesis, migration or survival. Each mechanism appears to be uniquely regulated in a site‐specific manner. Thus, the volume of an aggregate of neurones in the rat medial preoptic area (POA), termed the sexually dimorphic nucleus of the POA (SDN‐POA), is larger in males than in females. The anteroventral periventricular nucleus (AVPV) is packed with neurones containing oestrogen receptor (ER)β in female rats but, in males, ERβ‐positive neurones scatter into the more lateral portion of the POA. POA neurones are born up to embryonic days 16–17 and not after parturition. Therefore, neurogenesis is unlikely to contribute to the larger SDN‐POA in males. DNA microarray analysis for oestrogen‐responsive genes and western blotting demonstrated site‐specific regulation of apoptosis‐ and migration‐related genes in the SDN‐POA and AVPV.