Effects of medial preoptic hypothalamus anterior lesions on three kinds of behavior in the rat: Intermale aggressive, male-sexual, and mouse-killing behavior

A neural circuit for male sexual behavior and reward

Male sexual behavior is innate and rewarding. Despite its centrality to reproduction, a molecularly specified neural circuit governing innate male sexual behavior and reward remains to be characterized. We have discovered a developmentally wired neural circuit necessary and sufficient for male mating. This circuit connects chemosensory input to BNSTprTac1 neurons, which innervate POATacr1 neurons that project to centers regulating motor output and reward. Epistasis studies demonstrate that BNSTprTac1 neurons are upstream of POATacr1 neurons, and BNSTprTac1-released substance P following mate recognition potentiates activation of POATacr1 neurons through Tacr1 to initiate mating. Experimental activation of POATacr1 neurons triggers mating, even in sexually satiated males, and it is rewarding, eliciting dopamine release and self-stimulation of these cells. Together, we have uncovered a neural circuit that governs the key aspects of innate male sexual behavior: motor displays, drive, and reward.

A hypothalamic pathway that suppresses aggression toward superior opponents

Aggression is costly and requires tight regulation. Here we identify the projection from estrogen receptor alpha-expressing cells in the caudal part of the medial preoptic area (cMPOAEsr1) to the ventrolateral part of the ventromedial hypothalamus (VMHvl) as an essential pathway for modulating aggression in male mice. cMPOAEsr1 cells increase activity mainly during male-male interaction, which differs from the female-biased response pattern of rostral MPOAEsr1 (rMPOAEsr1) cells. Notably, cMPOAEsr1 cell responses to male opponents correlated with the opponents' fighting capability, which mice could estimate based on physical traits or learn through physical combats. Inactivating the cMPOAEsr1-VMHvl pathway increased aggression, whereas activating the pathway suppressed natural intermale aggression. Thus, cMPOAEsr1 is a key population for encoding opponents' fighting capability-information that could be used to prevent animals from engaging in disadvantageous conflicts with superior opponents by suppressing the activity of VMHvl cells essential for attack behaviors.

Functional Connectivity Differences Between Two Culturally Distinct Prairie Vole Populations: Insights Into the Prosocial Network

BACKGROUND

The goal of this study was to elucidate the fundamental connectivity-resting-state connectivity-within and between nodes in the olfactory and prosocial (PS) cores, which permits the expression of social monogamy in males; and how differential connectivity accounts for differential expression of prosociality and aggression.

METHODS

Using resting-state functional magnetic resonance imaging, we integrated graph theory analysis to compare functional connectivity between two culturally/behaviorally distinct male prairie voles (Microtusochrogaster).

RESULTS

Illinois males display significantly higher levels of prosocial behavior and lower levels of aggression than KI (Kansas dam and Illinois sire) males, which are associated with differences in underlying neural mechanisms and brain microarchitecture. Shared connectivity 1) between the anterior hypothalamic area and the paraventricular nucleus and 2) between the medial preoptic area and bed nucleus of the stria terminalis and the nucleus accumbens core suggests essential relationships required for male prosocial behavior. In contrast, Illinois males displayed higher levels of global connectivity and PS intracore connectivity, a greater role for the bed nucleus of the stria terminalis and anterior hypothalamic area, which were degree connectivity hubs, and greater PS and olfactory intercore connectivity.

CONCLUSIONS

These findings suggest that behavioral differences are associated with PS core degree of connectivity and postsignal induction. This transgenerational system may serve as powerful mental health and drug abuse translational model in future studies.

Cellular Composition of the Preoptic Area Regulating Sleep, Parental, and Sexual Behavior

The preoptic area (POA) has long been recognized as a sleep center, first proposed by von Economo. The POA, especially the medial POA (MPOA), is also involved in the regulation of various innate functions such as sexual and parental behaviors. Consistent with its many roles, the MPOA is composed of subregions that are identified by different gene and protein expressions. This review addresses the current understanding of the molecular and cellular architecture of POA neurons in relation to sleep and reproductive behavior. Optogenetic and pharmacogenetic studies have revealed a diverse group of neurons within the POA that exhibit different neural activity patterns depending on vigilance states and whose activity can enhance or suppress wake, non-rapid eye movement (NREM) sleep, or rapid eye movement (REM) sleep. These sleep-regulating neurons are not restricted to the ventrolateral POA (VLPO) region but are widespread in the lateral MPOA and LPOA as well. Neurons expressing galanin also express gonadal steroid receptors and regulate motivational aspects of reproductive behaviors. Moxd1, a novel marker of sexually dimorphic nuclei (SDN), visualizes the SDN of the POA (SDN-POA). The role of the POA in sleep and other innate behaviors has been addressed separately; more integrated observation will be necessary to obtain physiologically relevant insight that penetrates the different dimensions of animal behavior.

Relative role of neural substrates in the aggressive behavior of rats

Early animal studies have shown an association between aggression and brain dysfunction. The goal of the present study was to compare the effects of lesions of different parts of brain on aggression in rats. Adult rats (n = 40, weighing 200-260 g) were randomly divided into four groups of ten animals each and subjected to lesions of the septum (Group I), medial preoptic area (Group II), medial accumbens (Group III), and bed nucleus of stria terminalis (Group IV), using stereotaxy apparatus. Aggression toward an unfamiliar male intruder was observed before and after the lesion. The aggression score of each animal was recorded three times before lesion and averaged for use in analysis. Analysis of variance (ANOVA) was applied for finding homogeneity of the groups. Postoperative scores were also similarly recorded and summarized as mean +/- standard deviation. Pre- and post-lesion scores were compared using the t test. The scores were significantly reduced in Group I, II, and III, but increased in Group IV. We can conclude that the septum, medial preoptic area, medial accumbens, and bed nucleus of stria terminalis, by virtue of their interconnections, influence aggressive behavior.

Brain mechanisms of aggressive behavior: An updated review

During the 25 years since a motivational systems model was proposed to explain the brain mechanisms of aggressive behavior (D.B. Adams. Brain mechanisms for offense, defense, and submission. Behav. Brain. Sci. 2, (1979a) 200-241) considerable research has been carried out. Updating the model in the light of this research requires several changes. A previous distinction between submission and defense systems is abandoned and, instead, it is proposed that two distinct subsets of the defense motivational mechanism may be recognized, one for anti-predator defense and the other for consociate defense. Similarly, the offense motivational mechanism is now considered to have at least two subsets, one mediating territorial and the other competitive fighting. Data continue to indicate that the defense motivational mechanism is located in the midbrain central gray and adjoining tissue. Also data tend to support the hypothesis that the offense motivational mechanism is located in the hypothalamus at the level of the anterior hypothalamus. Consideration is also given to a motivational system for patrol/marking which is related to aggressive behavior. Research is reviewed that bears on the neural structure of motivating and releasing/directing stimuli and motor patterning mechanisms of offense, defense and patrol/marking, as well as the location of learning and hormonal effects, and attention is given to how the model can be tested.

Maternal behaviors: central integration or independent parallel circuits? Theoretical comment on Popeski and Woodside (2004)

N. Popeski and B. Woodside (2004) report that the injection of L-NAME into the 3rd ventricle, which would suppress the synthesis of nitric oxide, disrupts both maternal retrieval of pups and maternal aggression in postpartum rats. These findings are discussed around the question of whether a single central integrative site regulates retrieval and maternal aggression or whether independent neural mechanisms, each dependent upon nitric oxide, regulate the 2 behaviors. A case is made for each point of view. Future research utilizing direct injections of L-NAME into specific neural sites is needed to resolve this important question.

Sexual and social experience is associated with different patterns of behavior and neural activation in male prairie voles

Monogamous prairie voles (Microtus ochrogaster) show mating-induced aggression towards conspecific strangers. This behavior is both selective and enduring. The present study was designed to investigate the behavioral conditions for the emergence of selective aggression (by varying prior experience with a female and identity of intruders) and the limbic activation in response to an intruder (by mapping regional staining for c-fos) in male prairie voles. In a first experiment, males that mated with a female for 24 h exhibited aggression towards a male intruder and had more Fos-immunoreactive (Fos-ir) cells in the medial amygdala (AMYGme) and medial preoptic area (MPO) relative to males that cohabited with a female without mating or that had no prior exposure to a female. Cohabited males did not become aggressive. However, these males along with mated males had an increased number of Fos-ir cells in the lateral septum (LS) and the bed nucleus of the stria terminalis (BST) relative to males without prior exposure to a female. In a second experiment, mated males exhibited more offensive aggression to a male intruder but more defensive aggression to a female intruder. Both types of aggression, however, induced an increase in the number of Fos-ir cells in the AMYGme. In addition, Fos-ir staining in the BST was induced selectively in response to a male intruder and a similar trend was found in the LS. Exposure to a male or female intruder did not increase Fos-ir staining in the MPO. Taken together, our data suggest the neural substrates activated by social/sexual activity and involved in response to intruders. The AMYGme was involved in processing intruder-related cues and/or in the regulation of aggressive response to both male and female intruders. The BST and LS were modulated by social experience with a female (mating or cohabitation) and were responsive to male-related cues even in the absence of aggression. Finally, the MPO was activated at different magnitudes by social or sexual experience but did not respond to intruder-related cues as measured by Fos-ir.

Efferent connections of the hypothalamic "aggression area" in the rat

The efferent connections of the hypothalamic area of the rat, where attack behaviour can be elicited by electrical stimulation, were studied using iontophoretic injections of Phaseolus vulgaris-leucoagglutinin. Specificity for the hypothalamic "attack area" was investigated by comparison with efferents of hypothalamic sites outside the attack area. The hypothalamic attack area consists of the intermediate hypothalamic area and the ventrolateral pole of the ventromedial hypothalamic nucleus. Fibres from the hypothalamic attack area, as well as fibres from several other hypothalamic sites, form diffuse fibre "streams" running rostrally or caudally. Many varicosities that are found on the fibres suggest, that these fibres are capable of influencing many brain sites along their way. Projection sites were found throughout the brain. In the comparison between attack area efferents and controls, many overlapping brain sites were found. Hypothalamic efferents preferentially originating in the largest part of the attack area, i.e. the intermediate hypothalamic area, were found in the mediodorsal and parataenial thalamic nuclei. Within the septum, a spatial organization of hypothalamic innervation was found. Fibres from the attack area formed specialized "pericellular baskets" in the dorsolateral aspect of the intermediate part of the lateral septal nucleus. Fibres from other hypothalamic sites were found in other septal areas and did not form these septal baskets. Within the mesencephalic central gray, fibres from the attack area were found specifically in the dorsal part and dorsal aspect of the lateral part of the central gray. Physiological and pharmacological studies have shown that several brain sites are involved in different aspects of aggressive behaviour. Some of these areas, as for instance the dorsomedial thalamic nucleus, septum and central gray, are innervated by efferents from the hypothalamic attack area, whereas other sites, like ventral premammillary nucleus and ventral tegmental area, are not. It is concluded from the present findings, that a number of brain sites, that are known to be involved in agonistic behaviour, receive hypothalamic information preferentially from the hypothalamic attack area through diffusely arranged varicose fibres. The function of each connection in the regulation of specific behaviours remains to be further investigated.

Intracranial androgenic activation of male-typical behaviors in house mice: motivation versus performance

Castrated male mice were bilaterally implanted with 27 ga cannulae containing testosterone into either the septum, medial preoptic area (MPO), or corticomedial amygdala. One additional group of castrates received no hormone and another received only systemic testosterone via subcutaneous silastic capsules. All males were subsequently tested for ultrasonic mating vocalizations, urine marking, mounting behavior, aggression and gender preference, all of which are androgen-dependent, male-typical behaviors. In general castrates receiving no hormone performed these behaviors at low levels and animals receiving systemic testosterone performed the behaviors at normal male-typical levels. Ultrasonic vocalizations in response to female urine were activated by MPO implants. Urine marking in response to female urine appeared to be partially activated only with MPO implants. Very little mounting or fighting were seen in the brain implanted groups. Gender preference (for females over males) was restored with MPO implants and appeared to be partially activated with septal implants. The seminal vesicles of the castrates receiving brain implants were not significantly different from those receiving no hormone indicating that little or no implanted hormone was exiting the brain into general circulation. The implications of these findings for the neuroanatomy of sexual motivation and performance are discussed.

Aromatase activity in the preoptic area differs between aggressive and nonaggressive male house mice

Treatment with testosterone (T) or estradiol (E2) facilitates intraspecific aggressive behavior in adult rodents. Brain aromatization of T to E2 appears to be involved in facilitation of fighting behavior. In the present study we measure the in vitro brain aromatase activity (AA) in the preoptic area (POA), amygdaloid nuclei (Am), ventromedial hypothalamus (VMH), and parietal cortex (CTX) from two strains of adult male house mice, which were genetically selected for territorial aggression, based upon their attack latencies (short attack latency: SAL; long attack latency: LAL). The results reveal a higher AA in the POA of nonaggressive LAL males, as compared to aggressive SAL animals. The POA AA is, thus, inversely correlated with aggressiveness. The AA levels in both the VMH and Am do not differ significantly between strains. Furthermore, a differential brain area-specific AA distribution exists: POA > VMH AA in LAL, whereas POA < VMH in SAL. In both selection lines, the Am exhibits the highest levels of AA, as compared to the other investigated areas. Kinetic studies revealed that the aromatase Km is similar in both strains. The results indicate that the strain difference in AA is specific to the POA, but is not necessarily positively correlated with circulating plasma T levels. Other factors, in addition to androgen, are probably involved in the regulation of POA aromatase. We suggest that a higher neural androgen receptor sensitivity exists in the POA of nonaggressive LAL males, resulting in higher adult POA AA, despite lower concentrations of circulating T.

Hormone-dependent aggression in male and female rats: experiential, hormonal, and neural foundations

Hormone-dependent aggression in both male and female rats includes the distinctive behavioral characteristics of piloerection and lateral attack. In males the aggression is dependent on testicular testosterone and is commonly known as intermale aggression. In females, the aggression is most commonly observed as maternal aggression and is dependent on hormones whose identity is only beginning to emerge. The present review examines the experiential events which activate hormone-dependent aggression, the relation of the aggression to gonadal hormones, and the neural structures that participate in its modulation. In males and females, the aggression is activated by cohabitation with a conspecific of the opposite sex, by competitive experience, and by repeated exposure to unfamiliar conspecifics. In the female, the presence of pups also activates aggression. In both males and females, hormones are necessary for the full manifestation of the aggression. The essential hormone appears to be testosterone in males and a combination of testosterone and estradiol in females. The information available suggests the neural control systems for hormone-dependent aggression may be similar in males and females. It is argued that hormone-dependent aggression is behaviorally and biologically homologous in male and female rats.

Intracranial androgenic and estrogenic stimulation of male-typical behaviors in house mice (Mus domesticus)

Two experiments in house mice (Mus domesticus) examined the neural sites at which steroid hormones activate the following male-typical behaviors: 70 kHz ultrasonic mating vocalizations in response to stimulus females or their urine, urinary marking in response to stimulus males or stimulus females, mounting of estrous females, and intermale aggression. In the first experiment, four groups of castrated males received bilateral intracranial implants of testosterone (T) into either the septum (SEPTUM), medial preoptic area (MPO), anterior hypothalamus (AHA), or ventromedial hypothalamus (VMH). Two control groups received subcutaneous silastic capsules of T (TSIL) or empty silastic capsules (BSIL). The TSIL males performed all behaviors at male-typical levels while the BSIL males were unresponsive. MPO males emitted ultrasonic mating vocalizations at high levels while few vocalizations were seen in males of the other brain implant groups. The VMH, AHA, and MPO males urine marked at higher levels than the BSIL males but did not exhibit the high levels of the TSIL males. Mounting was observed only in MPO and TSIL males. Aggression was rare in males from any of the brain implant groups. In the second experiment, the hormone activity of the implants was increased by using testosterone propionate (TP) or a 50% mixture of estradiol (E2) and cholesterol. The six groups were SEPTUMTP, SEPTUME2, MPOTP, MPOE2, TPSIL, and BSIL. The TPSIL males performed all behaviors at male-typical levels while the BSIL males were unresponsive. TP was effective at restoring vocalizations and urine marking only when placed in the MPO; however, E2 was effective at both sites. Again aggression and mounting were less evident in the brain implanted males. In conclusion, implants of T or TP were effective in restoring ultrasonic mating vocalization when placed into the MPO. MPO implants of T and TP were also effective in stimulating urine marking, although VMH and AHA implants also showed some effectiveness. The restorative effects of E2 were not localized which is probably related to the greater hormonal activity of this treatment. Comparisons of the properties of the various brain implants to restore more than one behavior were discussed.

Cortisol exerts site-, context- and dose-dependent effects on agonistic responding in hamsters

Abstract Site-, context- and dose-dependent actions of intrahypothalamic cortisol administration on the agonistic behaviors of adult male golden hamsters (n = 128 dyads) were examined. When cortisol-treated animals were tested in paired encounters with aggressive cholesterol-treated opponents, chronic (>/= 24 h) cortisol treatment (1 mm implants) induced significant (P < 0.05) submission in three medial hypothalamic areas (anterior hypothalamic area > medial preoptic area > ventromedial hypothalamus), but aggression in the paraventricular nucleus or third ventricle. In contrast, chronic cortisol treatment in the anterior hypothalamic area resulted in high levels of aggression during paired encounters with submissive opponents, and during territorial aggression tests with juvenile male intruders. Acute (>/= 20 min) cortisol treatment in the anterior hypothalamic area (100 nl injections) induced significant submission after 10(-2) M, but significant aggression after 10(-6) M microinjections in paired encounters with aggressive vehicle-injected opponents. These findings suggest glucocorticoid-sensitive mechanisms within the anterior hypothalamus modulate aggressive responding during intrasexual social encounters.

The pre- and postnatal influence of hormones and neurotransmitters on sexual differentiation of the mammalian hypothalamus

A number of brain structures and a great number of brain functions have been shown to be sexually dimorphic. It has also been shown that development and differentiation of these structures and functions proceeds during a critical pre- and postnatal period of increased susceptibility, and is controlled by gonadal steroids and neurotransmitter substances. The brain of male and female mammals seems to be still undifferentiated before the period of increased susceptibility to gonadal steroids and neurotransmitters starts. Feminization of brain structure and functions, e.g., establishment of the cyclic LH-surge mechanism and the expression of lordosis behavior, seems to depend on the moderate interaction of estrogens with the developing nervous system. Defeminization and masculinization of brain functions seem to be established during interaction of the developing nervous system with androgens, which have to be converted, at least in part, into estrogens. Structural differentiation of the male brain, e.g., the sexually dimorphic nucleus of the preoptic area (SDN-POA), seems to be exclusively estrogen-dependent, during differentiation of male brain functions, however, estrogens may be supportive, rather than directive, to the primary action of androgens. The molecular mechanisms of sexual differentiation of the brain are not yet fully understood. It seems, however, that the priming action of gonadal steroids during the period of increased susceptibility is either mediated by neurotransmitters, or neurotransmitters modulate the priming action of gonadal steroids. In particular, the adrenergic, the serotoninergic, the cholinergic, and possibly the dopaminergic system were shown to have strong influences on sexual differentiation of brain structure and functions. In contrast to the great number of available studies on the influence of gonadal steroids on sexual differentiation of the brain, there are rather few studies available concerning the influence of neurotransmitter systems. The available results are partly contradictory, so that an interpretation must be done with caution and will leave plenty of room for speculation. Postnatal application of compounds which stimulate or inhibit adrenergic activity mainly affected the neural control of gonadotropin secretion, and had only minor influences on differentiation of behavior patterns. It seems, however, that adrenergic participation in the differentiation of the center for cyclic gonadotropin release is very complex and stimulatory and inhibitory components may operate simultaneously. Activation or inhibition of beta-adrenergic receptors during postnatal development was shown to impair the responsiveness of the center for cyclic gonadotropin release to gonadal steroids, and impairs the expression of ejaculatory behavior in male rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Eltoprazine, a drug which reduces aggressive behaviour, binds selectively to 5-HT1 receptor sites in the rat brain: an autoradiographic study

Eltoprazine, a phenylpiperazine derivative, selectively reduces offensive aggression in animal models. The present study was designed to localize and characterize the binding sites of [3H]eltoprazine in the rat brain and to compare the distribution of these sites with the distribution of [3H]5-HT binding sites. The binding of [3H]eltoprazine to whole tissue sections was saturable and revealed an apparent dissociation constant (Kd) of 11 nM. Autoradiographic studies demonstrated a widespread distribution of [3H]eltoprazine binding sites throughout the brain. Specific [3H]eltoprazine binding was completely displaced by 5-HT; conversely, unlabelled eltoprazine reduced [3H]5-HT binding to the levels of non-specific binding. The overall distribution of [3H]eltoprazine binding sites showed a strong resemblance to the location of 5-HT1 binding sites labelled with [3H]5-HT. Yet, regions enriched in 5-HT1A and 5-HT1C sites (e.g. dentate gyrus and choroid plexus, respectively) revealed relatively more [3H]5-HT binding as compared to [3H]eltoprazine binding, whereas [3H]eltoprazine binding was more pronounced in 5-HT1B receptor dense areas such as the dorsal subiculum, substantia nigra, ventral pallidum and globus pallidus. Displacement of [3H]eltoprazine with various selective serotonergic drugs demonstrated binding of [3H]eltoprazine to 5-HT1 receptor subtypes. The pharmacological and anatomical data indicate that eltoprazine binds to 5-HT1A, 5-HT1B and to a lesser extent to 5-HT1C binding sites in the rat brain. These results emphasize the important role of serotonin in the regulation of offensive aggression and suggest that eltoprazine may serve as a new tool to study the involvement of central 5-HT1 receptors in the expression of this behaviour.

Neurochemical profile of eltoprazine

In this paper we present the neurochemical profile of eltoprazine, a drug that specifically inhibits offensive aggression. Eltoprazine interacts selectively with serotonin (5-HT) receptor subtypes (Ki-values for 5-HT1A, 5-HT1B and 5-HT1C receptors are 40, 52 and 81 nM respectively). Affinity for other neurotransmitter receptors is much lower (Ki-values greater than 400 nM) than for 5-HT1 receptors. The selective interaction with 5-HT1 receptor subtypes is confirmed by in vitro autoradiographic studies using radiolabelled eltoprazine. The overall distribution of [3H]eltoprazine bears a strong resemblance to the localization of 5-HT1 binding sites labelled by [3H]5-HT, although some differences are observed. Eltoprazine (1 microM) inhibits the forskolin stimulated c-AMP production in hippocampus slices of the rat, indicating an agonistic action on the 5-HT1A receptor. The K+ stimulated release of 5-HT from rat cortex slices is inhibited by eltoprazine (pD2 = 7.8). The maximal response, however, was clearly less than that of the full agonist 5-HT, indicating partial agonistic activity on the 5-HT1B receptor (alpha = 0.5). Eltoprazine has a weak antagonistic action (IC50 = 7 microM) on the 5-HT1C receptor as revealed by inhibition of the 5-HT-induced accumulation of inositol phosphates in the choroid plexus of the pig. In vivo, eltoprazine reduces 5-HIAA levels in the striatum, without affecting the 5-HT levels. Eltoprazine also reduces the 5-HT synthesis rate as shown by 5-HTP accumulation after decarboxylase inhibition. These data indicate that eltoprazine acts as a 5-HT agonist in vivo in a dose range that affects aggressive behaviour (0.3-3 mg/kg p.o.). Taken together from a variety of neurochemical studies there is strong evidence both in vitro and in vivo that the pharmacological actions of eltoprazine can be attributed to an interaction with the 5-HT system, most probably via a (partial) agonistic action on 5-HT1A and 5-HT1B receptors.

Lesions of the SDN-POA inhibit sexual behavior of male Wistar rats

Discrete bilateral lesions in the SDN-POA of sexually naive adult male rats were found to decrease the number of animals ejaculating and/or to increase latencies to the first mount, intromission and ejaculation. The deleterious effects of the lesions disappeared after 4 tests for sexual behavior but were reinstated when the males were tested under suboptimal conditions, i.e., when they were tested with a marginally receptive female or when they had only limited access to the stimulus female. It was subsequently shown that males with a bilaterally lesioned SDN-POA still showed an increase in plasma testosterone. LH and prolactin levels in response to sexual stimulation. Effects of the lesions on scent marking were not found. Together with previous data indicating that SDN-POA-lesions disrupt masculine sexual behavior in females, these data are taken as evidence that the SDN-POA plays a role in the regulation of masculine sexual behavior. The data further suggest that previously reported negative results of SDN-POA-lesions on masculine sexual behavior in male rats might be attributed to the use of sexually experienced instead of sexually inexperienced animals.

Medial accumbens lesions attenuate testosterone-dependent aggression in male rats

Male hooded rats were castrated and implanted with testosterone-filled Silastic tubes appropriate for maintaining a normal average serum testosterone concentration. They were then given lesions of the medial accumbens nucleus or sham lesions. Twenty-four hours postoperatively each male was housed with a female. Beginning 7 days following pairing and continuing once each week for 4 weeks, each lesioned or sham-lesioned male was observed for aggression toward an unfamiliar male intruder. On the day following each test of aggression toward an unfamiliar male, each lesioned and sham-lesioned male was assessed for defensiveness toward an experimenter. Rats with medial accumbens lesions displayed significantly less aggression toward an unfamiliar male intruder during each of the weekly tests than did sham-lesioned animals. The attenuation was most pronounced in animals with lesions damaging the posterior part of the medial accumbens nucleus (also designated as anterior portion of the bed nucleus of the stria terminalis) in the region of the crossover of the anterior commissure. Although medial accumbens lesions are known to make individually housed rats hyperdefensive toward an experimenter, lesion-induced hyperdefensiveness was not observed in the pair-housed animals in the present experiment. It is argued that the medial accumbens/bed nucleus of the stria terminalis area is an important region in the anterior forebrain for the modulation of hormone-dependent aggression.

Effects of lesions of the sexually dimorphic nucleus on sexual behavior of testosterone-treated female Wistar rats

Discrete bilateral lesions were placed into the sexually dimorphic nucleus (SDN) of the medial preoptic area (MPOA) of ovariectomized female Wistar rats, chronically treated with testosterone (T). Effects of these lesions upon masculine and feminine sexual behavior were studied by comparing the results of pre- and postoperative tests, using sham-operated and unoperated females as controls. Bilaterally-lesioned and, to a lesser extent, unilaterally-lesioned females, showed a marked and significant reduction of masculine sexual behavior (i.e., mounting), especially in the first postoperative tests. Feminine sexual responses, i.e., receptive and proceptive behavior, although slightly lower in bilaterally-lesioned females, did not change significantly. Sexual partner preference, operationalized as the choice between a receptive female and a sexually active male, remained unaffected by the lesions. Plasma levels of testosterone were similar in the various groups. It is concluded that the SDN may be functionally implicated in the control of masculine sexual behavior in T-treated females.

Vasopressin receptor blockade in the anterior hypothalamus suppresses aggression in hamsters

Although the anterior hypothalamus has been implicated in the control of aggression in various rodent species, little is known about the neurochemical mechanisms mediating this control. It has been established that flank marking, which occurs with high frequency during agonistic encounters in hamsters, is dependent upon vasopressin-sensitive neurons in the anterior hypothalamus. The present study was undertaken to determine whether intraspecific aggression in this species is similarly influenced by vasopressin in this area of the hypothalamus. Adult male hamsters, surgically implanted with guide cannulae aimed at the anterior hypothalamus, were microinjected with three different concentrations of the V1-receptor antagonist d(CH2)5Tyr(Me)AVP or a vehicle control of 0.9% NaCl. Sixty minutes after each microinjection a smaller male hamster was introduced into the home cage of the treated hamster. The resident hamsters showed a significant dose-dependent reduction in the number of biting attacks on the intruders over the 10 minute test period. The V1-receptor antagonist also caused a significant increase in the resident hamster's latencies to attack the intruder. However, the resident hamsters' total contact time with the intruder was unaffected by drug treatment suggesting that the reduction of aggression was not due to a generalized effect upon social behavior. The specificity of the drug treatment was further supported by the observation that it did not affect resident hamsters' sexual motivation or ability to mount a receptive female. These data suggest that vasopressin-sensitive neurons in the anterior hypothalamus are involved in the control of intraspecific aggression in male hamsters.

Intermale social aggression in rats: suppression by medial hypothalamic lesions independently of enhanced defensiveness or decreased testicular testosterone

Medial hypothalamic lesions or sham lesions were made in castrated adult male rats with subcutaneous implants of testosterone-filled silastic capsules. Seven days following surgery all animals were given a test of defensiveness (reactivity) toward an experimenter. The following day, groups composed of one lesioned male rat, one sham-lesioned male rat, and one intact female rat were placed in large cages. Beginning two weeks later, unfamiliar intruders were introduced into each colony on a weekly basis and the aggressive behavior of the residents recorded. All 12 of the sham-lesioned animals but only 2 of 12 lesioned animals displayed substantial intermale social aggression toward intruders. Analysis of individual elements of intermale social aggression indicated that the lesioned animals were deficient in attack, bite, and piloerection but not in on-top behavior. The deficit in intermale social aggression was not correlated with defensiveness toward the experimenter or body weight of the lesioned animals. It is argued that the medial hypothalamus plays a role in the modulation of intermale social aggression which is independent of its role in modulating defensiveness or testosterone production. These results also demonstrate that intermale social aggression develops even when testosterone levels are held relatively constant by replacing testicular testosterone with an artificial testosterone source.

Intermale social aggression: reinstatement in castrated rats by implants of testosterone propionate in the medial hypothalamus

Male hooded rats were castrated, subcutaneously implanted with testosterone-filled silastic tubes, and individually housed with an intact adult female rat. An unfamiliar male intruder was introduced into each colony on a weekly basis and the aggressive behavior of the resident male was recorded. When the intermale social aggressive behavior of the resident male toward the intruder reached a high level in terms of a composite aggression score, the subcutaneous testosterone tubes were removed. Weekly tests of aggression toward unfamiliar intruders continued until the aggression of the resident male dropped to a low level for two successive weeks in terms of our composite aggression score. Bilateral implants of pellets of testosterone propionate were then made into the medial hypothalamus or adjacent tissue. A control group was implanted with cholesterol pellets into the medial hypothalamus. During four weekly tests following the implant, rats with testosterone propionate implants in the medial hypothalamus showed increases in lateral attacks, lateral attack duration, bites, and piloerection. The increase in aggression was not consistently displayed by animals with testosterone propionate implants dorsal or anterior to the medial hypothalamus or by animals with cholesterol implants in the medial hypothalamus. These results suggest that the medial hypothalamus or closely adjacent tissue contains testosterone-sensitive neural circuitry modulating intermale social aggression.

Identification of medial preoptic neurons that concentrate estradiol and project to the midbrain in the rat

Fluorescent dye retrograde tracing was combined with steroid hormone autoradiography to study the midbrain projections of the estrogen-concentrating neurons in the preoptic region of the rat brain. Microinjections of the dyes DAPI, true blue, or a mixture of DAPI and primuline were made into the ventral tegmental area and into the midbrain central gray of ovariectomized, adrenalectomized 2-3-month-old female rats; 3 or 4 days later these animals were injected with [3H]estradiol; the brains were then processed for autoradiography. After exposures of from 3 to 12 months, the autoradiograms were developed and examined for reduced silver grains under cell nuclei (indicating binding of [3H]estradiol) and retrogradely transported fluorescent dye in the cytoplasm (indicating an efferent projection to the midbrain). Numerous [3H]estradiol-concentrating neurons in the medial preoptic region were found to send their axons to the medial midbrain. The largest numbers of estrogen target neurons that were afferent to the ventral tegmental area and to the midbrain central gray were found in the medial preoptic nucleus, in the surrounding medial preoptic area, and in the ventral bed nucleus of the stria terminalis. Double-labeled neurons were also identified in the preoptic suprachiasmatic area, in the lateral preoptic area, and in the rostral anterior hypothalamic area. Thus, a subset of the gonadal steroid target cells of the preoptic region have long projections to the medial midbrain, and a subset of the medial preoptic neurons that project to the ventral tegmental area and to the midbrain central gray concentrate estrogen. Behaviors (for example, maternal behavior, male copulatory behavior, and wheel-running) that are regulated by estrogen action in the medial preoptic region may be controlled by the direct estrogen-sensitive pathway to the medial midbrain revealed in this study.

Sex behavior and the sexually dimorphic hypothalamic nucleus in male Zucker rats

To determine whether or not impaired male sex behavior in obese male Zucker rats is accompanied by any anatomical alterations in a hypothalamic area implicated in the control of sex behavior, 6 lean and 5 obese male Zucker rats were studied behaviorally and anatomically at 14 months of age. Obese males showed markedly decreased male sex behavior relative to lean males, in spite of serum levels of testosterone and testicular weights comparable to those of lean rats. Obese rats had significant decreases in brain weight and volumes of sexually dimorphic nuclei per g of brain, relative to lean rats; volumes per g brain of other structures (paraventricular and suprachiasmatic nuclei) were not different between groups. It is suggested that an incomplete expression of sexually dimorphic features of the preoptic area-anterior hypothalamus, due perhaps to an impaired process of perinatal brain androgenization, may contribute to decreased male sex behavior in adult obese rats.

Intermale social aggression: suppression by medial preoptic area lesions

The intermale social aggressive behavior of male rats cohabiting with a female rat was quantitatively scored weekly in response to the introduction of an unfamiliar intruding male. Resident male rats whose aggressiveness toward an intruder reached a criterion level were subjected to either sham lesions or bilateral lesions in the region of the medial preoptic area. The lesioned rats continued to exhibit levels of piloerection and lateral attack that were not significantly lower than those of sham-lesioned animals. However, the lesioned animals did emit significantly fewer bites and spent significantly less time in the "on-top" position than did sham-lesioned animals. The lesioned animals also displayed significantly less sexual behavior than the sham-lesioned animals but were not different in terms of defensiveness toward the experimenter. It is suggested that bilateral lesions in the region of the medial preoptic area cause a decrease in the intensity of intermale social aggression but do not prevent external stimuli from eliciting the aggression.