Medial Preoptic-anterior Hypothalamic Continuum Research Articles

Short‐Photoperiod Exposure Reduces Vasopressin (V1a) Receptor Binding But Not Arginine‐Vasopressin‐Induced Flank Marking in Male Syrian Hamsters

In Syrian hamsters, socially relevant information is communicated with a form of scent marking known as flank marking. There is substantial evidence that arginine-vasopressin acting on V1a vasopressin receptors (V1aR) in the medial preoptic-anterior hypothalamic continuum (MPOA-AH) regulates the expression of flank marking. Previous studies have shown that the expression of flank marking is also influenced by the circulating concentrations of gonadal hormones. In hamsters housed in long 'summer-like' photoperiods (i.e. >12.5 h of light/day), castration reduces flank marking and administration of testosterone restores precastration levels of flank marking. When exposed to short 'winter-like' photoperiods (i.e. <12.5 h of light/day), hamsters undergo gonadal regression and the circulating levels of testosterone decline. Surprisingly, flank marking induced during social encounters is not reduced in hamsters exposed to short photoperiods despite the low circulating concentrations of testosterone. In the present study, it was hypothesized that reductions in testosterone, caused by exposure to short photoperiod, would not reduce the ability of vasopressin to stimulate flank marking by its actions in the MPOA-AH. The amount of flank marking induced by vasopressin injected into the MPOA-AH did not significantly differ between hamsters housed in long and short photoperiods; however, short photoperiod-exposed males had significantly less V1aR binding in the MPOA than long photoperiod-exposed males. These results support the hypothesis that the sensitivity of the MPOA-AH to vasopressin is not reduced in short photoperiod-exposed males, despite decreases in serum testosterone. However, by contrast to our predictions, short photoperiod-exposed males have significantly reduced V1aR binding in the MPOA-AH compared to long photoperiod-exposed males.

Oxytocin inhibits aggression in female Syrian hamsters.

Dominant subordinate relationships are formed as the result of social conflict and are maintained at least in part by communication. At this time, little is known about the neural mechanisms that are responsible for coordinating the social behaviours (e.g. aggression) that occur in association with the formation and maintenance of these relationships. The purpose of the present study was to investigate the role of oxytocin (OXT) within the medial preoptic anterior hypothalamic continuum (MPOA-AH) in the control of aggression in female hamsters. OXT injected into the MPOA-AH immediately before testing significantly reduced the duration of aggression in a dose-dependent manner. Injection of an OXT antagonist 30 min before testing significantly increased the duration of aggression. In contrast, the duration of aggression was not altered when hamsters were tested either 30 min after injection of OXT or immediately following injection of an OXT-antagonist. These data support the hypothesis that OXT release within the MPOA-AH regulates social behaviours important in the formation and maintenance of dominant subordinate relationships in female hamsters.

The Effects of Serotonin Agonists on the Hypothalamic Regulation of Sexual Receptivity in Syrian Hamsters

One brain region that has been implicated in the regulation of lordosis is the medial preoptic-anterior hypothalamic continuum (MPOA-AH). Previous studies have suggested that this zone may be part of the circuit mediating the effects of serotonin (5-HT) on sexual receptivity. In the present experiments, we investigated the role of 5-HT 1a/7 and 5-HT 2 receptor subtypes in the MPOA-AH in the control of lordosis. In two experiments, either 5-HT 1a/7 or 5-HT 2 agonists were injected unilaterally into the MPOA-AH of ovariectomized, hormonally primed female hamsters. In the first experiment, microinjections of the 5-HT 1a/7 agonist 8-hydroxy-2,9-(di- n-propylamino)tetralin resulted in an attenuation of the lordosis posture by causing a decrease in the mean lordosis duration and an increase in the number of lordosis episodes over the entire testing period. In the second experiment, microinjections of the 5-HT 2b/2c agonist m-chlorophenylpiperazine did not result in any changes in sexual receptivity. However, microinjections of the 5-HT 2 agonist (2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl facilitated lordosis by increasing the mean lordosis duration and decreasing the number of lordosis episodes in the first 5 min of the testing period. These data indicate that serotonin may act in the MPOA-AH via 5-HT 1a/7 receptors to attenuate and 5-HT 2 receptors to facilitate sexual receptivity.

Social experience and social context alter the behavioral response to centrally administered oxytocin in female Syrian hamsters

The type of social behavior displayed by an individual is profoundly influenced by its immediate social environment or context and its prior social experience. Although oxytocin is important in the expression of social behavior in several species, it is not known if social factors alter the ability of oxytocin to influence behavior. The purpose of the present study was to test the hypothesis that social experience and social context alter the ability of oxytocin to regulate flank marking (a form of scent marking) in female Syrian hamsters. Oxytocin was microinjected into the medial preoptic anterior hypothalamic continuum (MPOA-AH) of socially experienced, dominant female hamsters which were then tested with either a subordinate partner, with a novel partner, or alone. Oxytocin induced flank marking in a dose-dependent manner but only when the experienced dominant hamsters were tested with their familiar, subordinate partners. Oxytocin did not induce flank marking when injected into socially naive female hamsters that were tested with an opponent or alone. In males, by contrast, oxytocin induced flank marking in dominant hamsters when they were tested with their subordinate partner or alone. These data support the hypothesis that social experience and social context interact to regulate the ability of oxytocin to stimulate flank marking by its actions in the MPOA-AH in female hamsters.

Vasopressin and sex differences in hamster flank marking

Vasopressin (AVP) within the medial preoptic-anterior hypothalamic continuum (MPOA-AH) plays an essential role in the control of flank marking in Syrian hamsters. Sex differences are found in the scent marking of many mammalian species, including hamsters. The first two experiments tested the hypothesis that sex differences in flank marking are the result of sex differences in the availability of AVP for release in several CNS sites. No support for this hypothesis was provided because neither immunohistochemical analysis nor radioimmunoassay of tissue punches revealed sex differences in AVP immunoreactivity in the MPOA-AH or other sites likely to be involved in flank marking. The third experiment, which tested the hypothesis that sex differences in flank marking are the result of sex differences in the sensitivity or response of the MPOA-AH to AVP, found no sex differences in the amount of flank marking stimulated by microinjection of AVP in the MPOA-AH. These data provide no support for the hypothesis that sex differences in vasopressinergic activity are responsible for sex differences in flank marking.

The medial preoptic area is involved in both sexual arousal and performance in male rats: re-evaluation of neuron activity in freely moving animals

A total of 74 single unit activities was recorded from the medial preoptic-anterior hypothalamic continuum (MPOA) during free copulatory behavior of male rats. Forty-six units (62.2%) showed changed activities during at least one phase of male copulatory movements; 26 units (35.1%) increased in the firing rate during pelvic thrusting; 32 units (43.2%) increased in activity during backward jumping immediately after intromission; only one unit (1.4%) showed decreased firing rate during thrusting and backward jumping; 12 units (16.2%) increased in activity during pursuit of a female; 19 units (25.7%) were suppressed during genital grooming. Furthermore, 67 units (90.5%) showed a significant change in activity throughout a series of copulatory behavior. From the introduction of a female up to ejaculation, relatively large number of units increased in the firing rate above the value during pre-introduction adaptation period. During postejaculatory interval, however, most units decreased in activity below the level during copulation. These results strongly suggest that the MPOA is involved in both sexual arousal and performance in male rats.

Norepinephrine Inhibits Vasopressin-Stimulated Flank Marking in the Syrian Hamster by Acting Within the Medial Preoptic-Anterior Hypothalamus

Syrian hamsters exhibit a form of scent marking behavior called flank marking. Flank marking, which is elicited during social contact with other hamsters and by the odors of other hamsters, communicates socially important information such as mate choice and dominance status. Vasopressinergic activity within the medial preoptic-anterior hypothalamic continuum (MPOA-AH) is essential for the expression of flank marking. In female hamsters, an inverse relationship exists between the expression of flank marking and sexual receptivity during the 4-day estrous cycle. Since norepinephrine (NE) appears to facilitate sexual receptivity, the present study investigated whether NE might have an inhibitory effect on flank marking by acting on Vasopressinergic activity within the MPOA-AH. Microinjection of 9.0 μM arginine vasopressin (AVP) into the MPOA-AH stimulated high levels of flank marking. Microinjection of 9.0 μM AVP combined with NE in concentrations of 4.0, 0.4 or 0.2 nM, drastically reduced or eliminated flank marking. In contrast, AVP in combination with 0.09, 0.04 or 0.004 nM NE produced no significant reductions in flank marking. In addition, microinjection of 9.0 μM AVP, in combination with epinephrine (4.0 nM), but not dopamine (4.0 nM), serotonin (4.0 nM) or neuropeptide Y (900 μM), significantly reduced AVP-induced flank marking. In male hamsters, microinjection of NE (4.0 nM) combined with AVP (9.0 μM) into the MPOA-AH was not found to inhibit AVP-stimulated flank marking. These results suggest that NE is involved in regulating the expression of flank marking during the estrous cycle by acting on Vasopressinergic activity within the MPOA-AH.

Microinjection of arginine-vasopressin into the periaqueductal gray stimulates flank marking in Syrian hamsters ( Mesocricetus auratus)

Syrian hamsters can communicate using a distinctive form of scent marking called flank marking. Vasopressin-sensitive neurons within the medial preoptic-anterior hypothalamic continuum (MPOA-AH) play a critical role in the control of this form of olfactory communication. Extrahypothalamic regions may also mediate hamster flank marking. Since the MPOA-AH and the periaqueductal gray (PAG) are reciprocally connected, the present study investigated whether PAG neurons are involved n the control of flank marking. The first study found that microinjection of vasopressin, but not oxytocin or saline, into the PAG induced high levels of flank marking in male ( n = 8) and female ( n = 5) hamsters ( P < 0.01). The second study demonstrated that microinjection of vasopressin into the PAG stimulated flank marking in a dose-dependent manner in both male ( n = 7) and female ( n = 11) hamsters ( P < 0.01). These data suggest that vasopressin-responsive neurons within the periaqueductal gray participate in the control of hamster flank marking.

Female reproductive function and sexually dimorphic prolactin secretion in rats with lesions in the medial preoptic-anterior hypothalamic continuum.

The role played by the medial preoptic area-anterior hypothalamus (MPOA-AH) in the regulation of reproductive function of the female rat was examined in the first part of the present study. Extensive bilateral lesions placed in different locations of the MPOA-AH continuum resulted in three different patterns of vaginal smears. Lesions located most rostrally and dorsally, sparing the suprachiasmatic nuclei (SCN), resulted in series of successive pseudopregnancies. Lesions located more caudally and ventrally, still sparing the SCN, eventuated in regular oestrus cycles with only occasional pseudopregnancies. MPOA-AH lesions encroaching upon the SCN were associated with persistent vaginal cornification. MPOA-AH-lesioned females exhibiting repeated pseudopregnancies were capable of mating, conceiving, and giving birth. Parturition in these females was uniformly delayed by 1 day and was associated with a variable incidence of stillbirth, cannibalism of newborn, or incomplete placentophagy. As a rule, MPOA-lesioned mothers did not retrieve pups but they did show crouching behaviour and some of them even secreted milk. Thus, while the MPOA subserves in the regulation of oestrous cyclicity and maternal behaviour, it appears to play no essential role in mating behaviour or in ovarian function during pregnancy and lactation. The pattern of prolactin secretion in MPOA-AH-lesioned male and female rats was evaluated in the second part of this study. Both intact and orchidectomized males were incapable of generating nocturnal prolactin surges following extensive bilateral ablation of the MPOA-AH. On the other hand, 6 out of 9 ovariectomized MPOA-AH-lesioned females exhibited spontaneous nightly surges of prolactin. The absence of the surges in the remaining 3 females was associated with invasion of the lesion into the SCN and/or structures rostral to the MPOA. During the light period, prolactin levels remained within the basal range in all lesioned females, irrespective of the presence or the absence of the nocturnal surge. These lesioned ovariectomized females, in contrast to nonlesioned females, were incapable of producing a diurnal (afternoon) surge of prolactin in response to administration of oestrogen, though their basal prolactin levels were much higher than the levels seen in oestrogen-treated, nonlesioned castrated males. These results support the view that the MPOA is inhibitory to the nocturnal prolactin surge while stimulatory to the diurnal surge. The results also suggest that the male rat is functionally devoid of both a 'diurnal-surge centre' residing in the MPOA, and a 'nocturnal-surge centre' located outside the MPOA.

Effect of lesion location within the medial preoptic-anterior hypothalamic continuum on maternal and male sexual behaviors in female rats.

Lesions of the medial preoptic-anterior hypothalamic continuum (MPOA-AH) are known to disrupt both maternal behavior and male sexual behavior in the rat. In order to test the hypothesis that the two behaviors involve different neural systems in the MPOA-AH, small bilateral lesions were made in different anterior-posterior locations in the MPOA-AH of maternal-sensitized, testosterone-treated female rats, and the effects of these lesions on maternal and male sexual behaviors were assessed. Lesions centering in the MPOA disrupted maternal behavior (pup retrieval, nest building, and nursing), with anterior MPOA lesions being more effective (on pup retrieval and nest building) than posterior MPOA lesions. Lesions centering in the AH had little or no effect on maternal behavior. By contrast, male sexual behavior (mounting) was strongly disrupted by lesions in either the MPOA or the AH, with lesions in the rostral AH being most effective.

Effect of lesion location within the medial preoptic-anterior hypothalamic continuum on maternal and male sexual behaviors in female rats.

Effect of lesion location within the medial preoptic-anterior hypothalamic continuum on maternal and male sexual behaviors in female rats.

Hypothalamic destruction and mouse aggression

Lesions of the medial preoptic-anterior hypothalamic continuum (MPAH) virtually eliminate the display of aggressive behavior in most male mice. The absence of fighting after MPAH damage is not accompanied by signs of impaired testicular functioning. Apparently, neural elements originating in and/or passing through MPAH are essential for the display of aggressive behavior in most male mice.

Facilitation of male rat copulatory behavior by electrical stimulation of the medial preoptic area.

Abstract Lesion studies have demonstrated that the medial preoptic area (MPO) plays a critical role in male rat copulatory behavior. The present study attempted to better localize the neural elements mediating this behavior pattern and to determine the influence of preoptic area stimulation on particular aspects of copulatory behavior. Monopolar stimulating electrodes were implanted in the medial preoptic-anterior hypothalamic continuum or the lateral preoptic-medial forebrain bundle region. The effects of stimulating a particular locus were measured by comparing each animal's behavior on repeated stimulation and control tests. All measures of copulatory activity taken were facilitated by medial, but not by lateral, preoptic stimulation. The most common change produced by MPO stimulation was a reduction in both the number of mounts and intromissions preceding ejaculation. Short-latency approach and mounting of the female and greatly reduced refractory periods were also seen in two MPO animals. No evidence of a post-stimulation inhibition of copulatory behavior resulting from the stimulation itself was seen in these two animals. Most animals in both the medial and lateral groups learned to self-stimulate (SS) using the same 30-sec trains of stimulation as used in earlier tests of copulatory activity. SS rate, stimulation-bound copulation, and degree of facilitation of ejaculation were positively correlated in MPO, but not in lateral preoptic animals.

Impairment of mating behavior in male rats following lesions in the preoptic-anterior hypothalamic continuum

Electrolytic lesions were made in the medial preoptic-anterior hypothalamic continuum, the lateral anterior hypothalamus, the dorsal part of the middle hypothalamus and in the septal nuclei of male rats. Sexual behavior was observed before and after the operations. Large lesions in the medial preoptic-anterior hypothalamic continuum permanently eliminated mating in all subjects. Small lesions within this region never completely abolished sexual behavior although 19 of 42 rats showed a temporary impairment. The sexual impairment was not related to a particular location of the lesion within the continuum. Treatment with testosterone propionate did not restore mating activity in any male with extensive lesions in the medial preoptic-anterior hypothalamic continuum. Animals with small lesions within the continuum, which had become sexually inactive after the operation, resumed mating after the hormone treatment. None of the operated animals showed any abnormalities in the testes or in the accessory organs. The medial preoptic-anterior hypothalamic continuum is situated intermediate between the limbic and olfactory telencephalon, on the one hand, and borders the gonadotropic mechanisms of the tuberal hypothalamic region. This position of the medial preoptic-anterior hypothalamic continuum appears to be quite compatible with the present physiological evidence that it is virtually involved in mating behavior.