The amygdala and male reproductive functions: I. Anatomical and endocrine bases

Hedonic processing in humans is mediated by an opioidergic mechanism in a mesocorticolimbic system

It has been hypothesized that the pleasure of a reward in humans is mediated by an opioidergic system involving the hypothalamus, nucleus accumbens and the amygdala. Importantly, enjoying the pleasure of a reward is distinct from incentive salience induced by cues predicting the reward. We investigated this issue using a within subject, pharmacological challenge design with the opioid receptor antagonist naloxone and fMRI. Our data show that blocking opioid receptors reduced pleasure associated with viewing erotic pictures more than viewing symbols of reward such as money. This was paralleled by a reduction of activation in the ventral striatum, lateral orbitofrontal cortex, amygdala, hypothalamus and medial prefrontal cortex. Crucially, the naloxone induced activation decrease was observed at reward delivery, but not during reward anticipation, indicating that blocking opioid receptors decreases the pleasure of rewards in humans.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Parasite-augmented mate choice and reduction in innate fear in rats infected by Toxoplasma gondii

Typically, female rats demonstrate clear mate choice. Mate preference is driven by the evolutionary need to choose males with heritable parasite resistance and to prevent the transmission of contagious diseases during mating. Thus, females detect and avoid parasitized males. Over evolutionary time scales, parasite-free males plausibly evolve to advertise their status. This arrangement between males and females is obviously detrimental to parasites, especially for sexually transmitted parasites. Yet Toxoplasma gondii, a sexually transmitted parasite, gets around this obstacle by manipulating mate choice of uninfected females. Males infected with this parasite become more attractive to uninfected females. The ability of T. gondii to not only advantageously alter the behavior and physiology of its host but also secondarily alter the behavior of uninfected females presents a striking example of the 'extended phenotype' of parasites. Toxoplasma gondii also abolishes the innate fear response of rats to cat odor; this likely increases parasite transmission through the trophic route. It is plausible that these two manipulations are not two distinct phenotypes, but are rather part of a single pattern built around testosterone-mediated interplay between mate choice, parasitism and predation.

Pathology and pathophysiology of the amygdala in epileptogenesis and epilepsy

Acute brain insults, such as traumatic brain injury, status epilepticus, or stroke are common etiologies for the development of epilepsy, including temporal lobe epilepsy (TLE), which is often refractory to drug therapy. The mechanisms by which a brain injury can lead to epilepsy are poorly understood. It is well recognized that excessive glutamatergic activity plays a major role in the initial pathological and pathophysiological damage. This initial damage is followed by a latent period, during which there is no seizure activity, yet a number of pathophysiological and structural alterations are taking place in key brain regions, that culminate in the expression of epilepsy. The process by which affected/injured neurons that have survived the acute insult, along with well-preserved neurons are progressively forming hyperexcitable, epileptic neuronal networks has been termed epileptogenesis. Understanding the mechanisms of epileptogenesis is crucial for the development of therapeutic interventions that will prevent the manifestation of epilepsy after a brain injury, or reduce its severity. The amygdala, a temporal lobe structure that is most well known for its central role in emotional behavior, also plays a key role in epileptogenesis and epilepsy. In this article, we review the current knowledge on the pathology of the amygdala associated with epileptogenesis and/or epilepsy in TLE patients, and in animal models of TLE. In addition, because a derangement in the balance between glutamatergic and GABAergic synaptic transmission is a salient feature of hyperexcitable, epileptic neuronal circuits, we also review the information available on the role of the glutamatergic and GABAergic systems in epileptogenesis and epilepsy in the amygdala.

Amygdaloid lesion-induced obesity: relation to sexual behavior, olfaction, and the ventromedial hypothalamus

Lesions of the amygdala have long been known to produce hyperphagia and obesity in cats, dogs, and monkeys, but only recently have studies with rats determined that the effective site is the posterodorsal amygdala (PDA)-the posterodorsal medial amygdaloid nucleus and the intra-amygdaloid bed nucleus of the stria terminalis. There is a sex difference; female rats with PDA lesions display greater weight gain than male rats. In the brains of female rats with obesity-inducing PDA lesions, there is a dense pattern of axonal degeneration in the capsule of the ventromedial hypothalamus (VMH) and other targets of the stria terminalis. Transections of the dorsal component of the stria terminalis also result in hyperphagia and obesity in female rats. Similar to rats with VMH lesions, rats with PDA lesions are hyperinsulinemic during food restriction and greatly prefer high-carbohydrate diets. The PDA is also a critical site for some aspects of rodent sexual behavior, particularly those that depend on olfaction, and the pattern of degeneration observed after obesity-inducing PDA lesions is remarkably parallel to the circuit that has been proposed to mediate sexual behavior. Medial amygdaloid lesions disrupt the normal feeding pattern and result in impaired responses to caloric challenges, and there is evidence that these behavioral changes are also due to a disruption of olfactory input. With its input from the olfactory bulbs and connections to the VMH, the PDA may be a nodal point at which olfactory and neuroendocrine stimuli are integrated to affect feeding behavior.

Sexually dimorphic synaptic organization of the medial amygdala

The medial amygdala is important in social behaviors, many of which differ between males and females. The posterodorsal subnucleus of the medial amygdala (MeApd) is particularly sensitive to gonadal steroid hormones and is a likely site for gonadal hormone regulation of sexually dimorphic social behavior. Here we show that the synaptic organization of the MeApd in the rat is sexually dimorphic and lateralized before puberty. With the use of whole-cell voltage-clamp recording and quantitative electron microscopy, we found that, specifically in the left hemisphere, prepubertal males have approximately 80% more excitatory synapses per MeApd neuron than females. In the left but not the right MeApd, miniature EPSC (mEPSC) frequency was significantly greater in males than in females; mEPSC amplitude was not sexually dimorphic. Paired-pulse facilitation of EPSCs, an index of release probability, also was not sexually dimorphic, suggesting that greater mEPSC frequency is caused by a difference in excitatory synapse number. Electron microscopy confirmed that the asymmetric synapse-to-neuron ratio and the total asymmetric synapse number were significantly greater in the left MeApd of males than of females. In contrast to results for excitatory synapses, we found no evidence of sexual dimorphism or laterality in inhibitory synapses. Neither the frequency nor the amplitude of mIPSCs was sexually dimorphic or lateralized. Likewise, the number of symmetric synapses measured with electron microscopy was not sexually dimorphic. These findings show that the excitatory synaptic organization of the left MeApd is sexually differentiated before puberty, which could provide a sexually dimorphic neural substrate for the effects of hormones on adult social behavior.

Exposure to female rats produces differences in c-fos induction between sexually-naïve and experienced male rats

Sexual incentive-induced Fos-like immunoreactivity (Fos-Li) within six neural regions implicated in male sexual behavior was investigated in both sexually-naïve and experienced male rats. Sexual experience was limited to one copulation culminating in ejaculation 24 h prior to testing. On test-day, subjects were placed within a cylindrical arena for 15 min on the opposite side of a perforated, Plexiglas partition from one of three targets: an uninhabited area, a non-estrous female, or an estrous female. Then 1 h later, each subject was sacrificed and its brain prepared for subsequent immunocytochemical staining. Analyses revealed a main effect of target stimulus on c-fos expression within the nucleus accumbens shell and core of male subjects. In addition, sexually-experienced subjects demonstrated significantly more Fos-Li within the nucleus accumbens shell in response to an estrous female versus a non-estrous female. There was also greater estrous cue-induced Fos-Li in the nucleus accumbens shell of experienced subjects when compared to naïve subjects. These data support previous suggestions implicating the nucleus accumbens in the generation of male sexual motivation. In addition, copulatory experience, even when limited to one ejaculation, seems to mediate long-term changes in the response properties of nucleus accumbens neurons that may reflect the value enhancement of primary female incentives.

Photoperiod-dependent response to androgen in the medial amygdala of the Siberian hamster, Phodopus sungorus

The medial nucleus of the amygdala (MeA) is a steroid-sensitive region that has been implicated in the expression of behaviors such as mating and aggression. The male Siberian hamster (Phodopus sungorus) uses light cues to regulate its reproductive neuroendocrine system, reducing androgen synthesis in the autumn and increasing it in the spring. There is also evidence that short photoperiods reduce the sensitivity of the brain to the behavioral effects of androgen. The authors tested the hypothesis that MeA neurons are less responsive to androgen in short photoperiods by comparing the regional volume and average soma size of the four MeA subnuclei (anterodorsal [MeAD], anteroventral [MeAV], posterodorsal [MePD], and posteroventral) in adult male hamsters that had been castrated and then implanted with capsules containing either testosterone (T) or nothing. Animals from each group were housed in either long or short photoperiods for 15 weeks. MeAD and MeAV somata displayed photoperiod-dependent responses to androgen, increasing in size after T treatment only in long days. In contrast, the average soma size and the regional volume of the MePD subnucleus were significantly larger in T-treated males regardless of photoperiod. The authors conclude that photoperiod influences the sensitivity of the MeA to androgen.

Regulation by the medial amygdala of copulation and medial preoptic dopamine release

The medial preoptic area (MPOA) is a critical integrative site for male copulatory behavior in most vertebrate species. Extracellular dopamine (DA) is increased in the MPOA of male rats immediately before and during copulation. DA agonists microinjected into the MPOA of male rats facilitate and DA antagonists inhibit sexual behavior. A major source of input to the MPOA is the medial amygdala (MeA), which processes and relays olfactory information to the MPOA. We now report that microinjections of a DA agonist into the MPOA of animals with excitotoxic lesions of the amygdala restored copulatory ability that was lost after the lesions. Moreover, radio-frequency lesions of the MeA impaired copulation and blocked the increases in extracellular DA seen in animals with sham lesions during exposure to a receptive female and during copulation. Thus, both copulatory ability and the MPOA DA response, during exposure to a receptive female and during copulation, are facilitated by input from the MeA to the MPOA.

Signs of sexual behaviour are not increased after subchronic treatment with LHRH in young men

Apart from its action as gonadotropin releasing factor, luteinizing hormone-releasing hormone (LHRH) is a potent regulator of sexual behaviour in animals. The present study aimed to assess a similar role of LHRH for sexuality in humans. In a double-blind placebo-controlled and randomized study, effects of human LHRH after acute (400 microg) and subchronic (800 microg/day over 2 weeks) intranasal administration were evaluated in 20 young and healthy men. Sexual desire and activity was assessed by a diary, ratings of women's attractiveness, a modified version of the Stroop colour naming task and a short term memory task. Effects on sexuality were contrasted with those on eating motivation and general neurocognitive functioning, the latter being assessed in addition by tasks of divergent thinking and a motor perseveration test. None of the measures of sexual desire and activity indicated any effect of LHRH, neither after acute nor after subchronic treatment. Unexpectedly, the diary indicated a significant increase in 'food intake' towards the end of the 14-day LHRH treatment. Enhanced colour naming performance on the Stroop task (independently of whether sex, food or neutral stimuli were used) in conjunction with an increased motor perseveration after LHRH points to a general effect on cognitive function towards stronger focussing of cortical processing. While overall the data show discrete central nervous changes after LHRH, a particular influence on sexuality after acute or subchronic intranasal administration in healthy men was not detected.

Retinohypothalamic projections in the common marmoset (Callithrix jacchus): A study using cholera toxin subunit B

Retinal projections in vertebrates reach the primary visual, accessory optic, and circadian timing structures. The central feature of the circadian timing system is the principal circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. The direct projections from the retina to the SCN are considered the entrainment pathway of the circadian timing system. In this study, unilateral intravitreal injections of cholera toxin subunit B were used to trace the retinal projections to the marmoset hypothalamus. The retinohypothalamic tract reaches the ventral suprachiasmatic nucleus bilaterally, as anticipated from previous studies. However, labeled fibers were found in several other hypothalamic regions, such as the medial and lateral preoptic areas, supraoptic nucleus, anterior and lateral hypothalamic areas, retrochiasmatic area, and subparaventricular zone. These results reveal new aspects of retinohypothalamic projection in primates and are discussed in terms of their implications for circadian as well as noncircadian control systems.

Tail pinch induces fos immunoreactivity within several regions of the male rat brain: effects of age

Brief, intermittent stressors, such as low-level foot shock or tail pinch, induce a general excitement and autonomic arousal in rats that increases their sensitivity to external incentives. Such stimulation can facilitate a variety of behaviors, including feeding, aggression, sexual activity, parental behavior, and drug taking if the appropriate stimuli exist in the environment. However, the ability of tail pinch to induce general arousal and incentive motivation appears to diminish with age. Here we report on the ability of tail pinch to induce Fos immunoreactivity within several brain regions as a function of age. Young (2-3 months) and middle-aged (12-13 months) male rats were administered either five tail pinches (one every 2 min), one tail pinch, or zero (sham) tail pinches (n = 4 per stimulation condition). Rats were sacrificed 75 min following the onset of stimulation, and their brains were prepared for immunocytochemical detection of Fos protein. Fos immunoreactivity was induced by one and five tail pinches in several brain regions, including the anterior medial preoptic area (mPOA), paraventricular nucleus of the hypothalamus (PVN), paraventricular nucleus of the thalamus (PV-Thal), medial amygdala (MEA), basolateral amygdala (BLA), lateral habenula (LHab), and ventral tegmental area (VTA), of young rats compared with those that received zero tail pinches. In contrast to young rats, middle-aged rats had significantly less Fos induced by one and five tail pinches in the mPOA, PVN, MEA, BLA, and VTA, but an equivalent amount induced in the LHab. Fos immunoreactivity was not found within the medial prefrontal cortex, nucleus accumbens, striatum, lateral septum, or locus coeruleus in either young or old rats. Tail pinch appears to activate regions of the brain known to be involved in behavioral responses to both incentive cues and stressors. The lower level of cellular reactivity to tail pinch in middle-aged rats suggests a diminished neural responsiveness to incentives and stressors.

Lesions of the ventral striatum mimic the effect of olfactory bulbectomy to prevent short photoperiod-induced testicular regression in golden hamsters

Bilateral olfactory bulbectomy (BX) or bilateral transection of the rostral lateral olfactory tract (LOT) at the level of the anterior olfactory nucleus markedly increases gonadotropin secretion and prevents the testicular regression associated with maintenance on short photoperiod in golden hamsters. In an effort to further elucidate the neural tracts involved in this influence on gonadotropin secretion, lesions were placed in several potential pathways. Hamsters underwent sham surgery (SH), bilateral BX, or electrolytic or radiofrequency lesions of the: medial nucleus of the amygdala (MeX) caudal LOT just rostral to the medial nucleus of the amygdala (LOTX); or ventral striatum (VSX). Lesions were either bilateral or unilateral with contralateral olfactory bulbectomy. All animals were then placed on short photoperiod (LD 10:14) for 10 weeks and testicular size and body weight were assessed at weekly intervals. Lesion placement was assessed in brain sections stained with cresyl violet and animals with misplaced lesions were excluded. The following represent the number of animals in each group undergoing testicular regression in response to short photoperiod: SH: 32/35; BX: 8/31 (P < 0.01 vs. SH); MeX: 5/5; caudal LOTX: 8/9 and VSX: 3/8 (P < 0.05 vs. SH). Serum LH, FSH and testosterone at the end of the study correlated with the testicular regression data. These results suggest that the tonic inhibitory effect of the olfactory bulbs on gonadotropin secretion is mediated by fibers that exit the LOT rostral to the amygdala and project medially, either passing through or synapsing in the ventral striatum.

Pericapillary and distant axon terminals in the nuclei of the cat amygdala: a morphometric study

According to some ultrastructural studies, the pericapillary axon terminals in the central nervous system (CNS) are functionally connected with the capillary vessel wall. Thus, it may be expected that the population of pericapillary axon terminals will be morphologically distinct from the terminals at a further distance from the capillary walls. To test this hypothesis, morphometrical analysis of 3,048 axon terminals was performed, comparing terminals situated in the close vicinity of the capillary vessel with those at a distance from the vessels in the lateral, basal, medial, central and cortical nuclei of the amygdaloid body of eight cats. The cross-sectional area and circumference of each identified axon terminal profile were measured, and the shape of synaptic vesicles and the presence of synaptic contacts and granular vesicles were recorded. The statistical evaluation of results was performed by means of the Newman-Keuls' test, Wilcoxon's test, Fisher's contingency-table test and the test for two coefficients of structure. The morphometric examination revealed two ultrastructurally distinct groups of axon terminals, pericapillary and distant terminals, in all the nuclei of the amygdaloid body. The differentiating features were the shape of the synaptic vesicles, the number of synaptic contacts, and the size of the axon terminals. These results further support the hypothesis of a functional connection between axon terminals and the capillary vessel wall in the CNS.

Neurotrophic effects of testosterone on the medial nucleus of the amygdala in adult male rats

Our previous reports of major sex differences in the substance P-immunoreactive (SPir) innervation of the medial posterior divisions of the bed nucleus of the stria terminalis (BST) and medial nucleus of the amygdala in rats raised the question of the hormonal regulation of this innervation. We now report the results of two experiments which examined the effects of castration of adult males on the SPir innervation of these regions. In experiment 2 we asked whether castration might also alter the cytoarchitecture of these regions. In experiment 1 three groups; sham operated (Sham), castrated (C) and castrated plus testosterone (C+T) were examined at each of the three survival periods (2, 4 and 8 weeks) post castration. Animals of the C+T groups each received a 45 mm silastic implant of testosterone sc at the time of castration to maintain testosterone levels postoperatively. Castration produced a consistent and highly significant decrease in the area of dense SPir fiber staining in the posterior medial amygdala which became greater with increasing survival. By 8 weeks the area of staining was 42% smaller in group C as compared to the matched sham-operated group. Smaller decreases were seen in the size of the dense field of SPir fibers in the posterior part of the dorsomedial BST. Testosterone implants maintained the size of the SPir fields of fibers in both the medial amygdala and BST, as the areas of staining in the C+T groups were not significantly different from those in the Sham groups at any of the 3 survival times. In experiment 2 we measured the area and optical density of SPir fiber staining in the medial amygdala and medial BST at 8 weeks post-castration. In addition, we measured the size of the cell groups within these regions using cresyl-stained sections. As in experiment 1, at 8 wks following castration there was a marked decrease in the area of dense SPir staining in both the BST and medial amygdala. The sizes of the dense fields of fibers were reduced by approximately 23% in the BST and by 40% in the posterior medial amygdala. Castration also significantly reduced the optical density of staining within the medial amygdala. The major finding of experiment 2 is that castration affects the cytoarchitecture as well as the SPir staining in these areas. In the BST, the cell group BSTMPM receives most of the dense SPir innervation.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebral glucose utilization during conditioned sexual arousal

Local cerebral glucose utilization was investigated in male rats during conditioned sexual arousal. Increased glucose utilization was found in three amygdaloid nuclei after exposure to a stimulus associated with exposure to a sexually active female. No changes were observed in areas known to be of crucial importance for the expression of consummatory aspects of sexual behavior. These results corroborate and extend previous results showing a dissociation between the expression of appetitive and consummatory aspects of sexual behavior at a neural level.

Changes in male copulatory behavior after sexual exciting stimuli: effects of medial amygdala lesions

A paradigm was developed to investigate how precoital sexual arousal affects parameters of sexual behavior in male rats. Estrous females in a wire mesh cage were used to induce sexual arousal before the sexual interaction test. In control procedures, males were presented in a wire mesh cage or else there was no stimuli at all. The results indicate that ejaculation latency is consistently reduced after preexposure to a female, but not after preexposure to a male, showing that the effect is specific for precoital sexual arousal. Other parameters were affected by precoital sexual arousal in some, but not in all experiments. Reductions in intromission latency moreover, were observed after both preexposure to a male and preexposure to a female, indicating that general social excitement affects this parameter. Preexposure to females for 10 minutes or 3 hours produced similar results. It was subsequently found that medial amygdala-lesioned (AME) animals differed from sham-lesioned (SHAM) controls with respect to their reaction to precoital sexual arousal. The results show that AME-lesioned animals, in contrast to SHAM-animals, do not show reduced ejaculation latencies after preexposure to an estrous female. The results are in line with the idea that AME-lesioned animals are deficient in the assimilation of information on sexual exciting stimuli.

Melatonin modifies the spontaneous multiunit activity recorded in several brain nuclei of freely behaving rats

Melatonin, a pineal hormone, released photoperiodically, was administered systemically in rats, previously implanted with semimicroelectrodes into six different brain structures. The multiunit electrical activity of these structures was recorded for 10 min before and 60 min after melatonin administration in unanesthetized, freely moving rats. Different melatonin doses (100, 200, 500, and 1000 micrograms/kg) produced changes in the electrical activity of all tested structures. However, amygdala, rostral hypothalamus and mesencephalic reticular formation showed the most important changes. The main effect induced by melatonin was a dose-related decrease of the spontaneous electrical activity. The significance of these effects is discussed within the context of the behavioral and endocrinological effects of melatonin.

Sex difference in the substance P-immunoreactive innervation of the medial nucleus of the amygdala

Discrete fields of substance P-immunoreactive fibers are present within the posterior dorsal division of the medial nucleus of the amygdala and the posterior medial bed nucleus of the stria terminalis in adult male and female rats. We previously reported a sex difference in the extent of this innervation of the bed nucleus. In the present study, we have replicated our earlier finding and found an equally dramatic sex difference in the amygdala. Morphometry revealed that the areas of dense staining in both the medial amygdala and the medial bed nucleus were more than twice as large in male brains.

Recovery of olfactory behavior following removal of accessory olfactory bulb in adult rat

It has been shown that male rats demonstrate a preference for the odor of the female rat. This preference has been suggested to be a function of the accessory olfactory pathway. Thus, changes in preference of the male rat for the odor of the female were examined following accessory olfactory bulb (AOB) removal. Preference decreased to about 30% of that seen in the intact rat during the 10 days following AOB removal. Thereafter, it increased gradually over one month survival time up to approximately 70% of the level demonstrated by the intact rat. The rats whose main olfactory bulb and AOB were removed did not show any recovery at any survival time. The time course of the behavioral recovery after AOB removal was compared with the time course of the recovery of synaptic density which was observed in the medial amygdaloid nucleus after AOB removal. The relationship between functional recovery and reorganization of synaptic connections is discussed.

Bilateral transection of the lateral olfactory tract but not removal of the vomeronasal organs inhibits short-photoperiod-induced testicular regression in golden hamsters

It is now known that removal of the olfactory bulbs increases basal gonadotropin secretion and prevents short-photoperiod-induced testicular regression in Syrian hamsters. The experiments described in the present paper were an attempt to determine which neuronal systems associated with the olfactory bulbs are responsible for this influence on the reproductive neuroendocrine axis. In the first experiment, removal of the vomeronasal organ failed to influence gonadotropin secretion or testes weight in hamsters on long or short photoperiod, suggesting that the vomeronasal-accessory olfactory pathway is not individually responsible for the effect of the olfactory bulbs on gonadotropin secretion. In the second experiment, bilateral transection of the lateral olfactory tracts (LOT) did prevent short-photoperiod-induced testicular regression and the associated decrease in gonadotropin secretion. Since the nervus terminalis is confined to the surface of the medical olfactory bulb pathway, the results of LOT transection indicate that the nervus terminalis, which itself contains gonadotropin releasing hormone, does not mediate the influence of the olfactory bulbs on gonadotropin secretion. These results further suggest that the olfactory bulb influence on gonadotropin secretion is due to neural connections to the pyriform cortex, entorhinal cortex or amygdala.

Effects of ATD on male sexual behavior and androgen receptor binding: a reexamination of the aromatization hypothesis

The aromatization hypothesis asserts that testosterone (T) must be aromatized to estradiol (E2) to activate copulatory behavior in the male rat. In support of this hypothesis, the aromatization inhibitor, ATD, has been found to suppress male sexual behavior in T-treated rats. In our experiment, we first replicated this finding by peripherally injecting ATD (15 mg/day) or propylene glycol into T-treated (two 10-mm Silastic capsules) or control castrated male rats. In a second experiment, we bilaterally implanted either ATD-filled or blank cannulae into the medial preoptic area (MPOA) of either T-treated or control castrated male rats. With this more local distribution of ATD, a lesser decline in sexual behavior was found, suggesting that other brain areas are involved in the neurohormonal activation of copulatory behavior in the male rat. To determine whether in vivo ATD interacts with androgen or estrogen receptors, we conducted cell nuclear androgen and estrogen receptor binding assays of hypothalamus, preoptic area, amygdala, and septum following treatment with the combinations of systemic T alone. ATD plus T, ATD alone, and blank control. In all four brain areas binding of T to androgen receptors was significantly decreased in the presence of ATD, suggesting that ATD may act both as an androgen receptor blocker and as an aromatization inhibitor. Competitive binding studies indicated that ATD competes in vitro for cytosol androgen receptors, thus substantiating the in vivo antiandrogenic effects of ATD. Cell nuclear estrogen receptor binding was not significantly increased by exposure to T in the physiological range. No agonistic properties of ATD were observed either behaviorally or biochemically. Thus, an alternative explanation for the inhibitory effects of ATD on male sexual behavior is that ATD prevents T from binding to androgen receptors.