Hippocampal Inputs in the Prelimbic Cortex Curb Fear after Extinction.

In contrast to easily formed fear memories, fear extinction requires prolonged training. The prelimbic cortex (PL), which integrates signals from brain structures involved in fear conditioning and extinction such as the ventral hippocampus (vHIP) and the basolateral amygdala (BL), is necessary for fear memory retrieval. Little is known, however, about how the vHIP and BL inputs to the PL regulate the display of fear after fear extinction. Using functional anatomy tracing in male rats, we found two distinct subpopulations of neurons in the PL activated by either the successful extinction or the relapse of fear. During the retrieval of fear extinction memory, the dominant input to active neurons in the PL was from the vHIP, whereas the retrieval of fear memory, regardless of the age of a memory and testing context, was associated with greater BL input. Optogenetic stimulation of the vHIP-PL pathway after one session of fear extinction increased conditioned fear, whereas stimulation of the vHIP inputs after several sessions of extinction decreased the conditioned fear response. This latter effect was, however, transient, as stimulation of this pathway 28 d after extinction increased conditioned fear response again. The results show that repeated fear extinction training gradually changes vHIP-PL connectivity, making fear suppression possible, whereas in the absence of fear suppression from the vHIP, signals from the BL can play a dominant role, resulting in high levels of fear.SIGNIFICANCE STATEMENT Behavioral therapies of fear are based on extinction learning. As extinction memories fade over time, such therapies produce only a temporary suppression of fear, which constitutes a clinical and societal challenge. In our study, we provide a framework for understating the underlying mechanism by which extinction of fear memories fade by demonstrating the existence of two subpopulations of neurons in the prelimbic cortex associated with low and high levels of fear. Insufficient extinction and exposure to the context in which fear memory was formed promoted high fear neuronal activity in the prelimbic cortex, leading to fear retrieval. Extensive extinction training, on the other hand, boosted low fear neuronal activity and, as a result, extinction memory retrieval. This effect was, however, transient and disappeared with time.

Subharmonics and Chaos in Simple Periodically Forced Biomolecular Models.

This article uncovers a remarkable behavior in two biochemical systems that commonly appear as components of signal transduction pathways in systems biology. These systems have globally attracting steady states when unforced, so they might have been considered uninteresting from a dynamical standpoint. However, when subject to a periodic excitation, strange attractors arise via a period-doubling cascade. Quantitative analyses of the corresponding discrete chaotic trajectories are conducted numerically by computing largest Lyapunov exponents, power spectra, and autocorrelation functions. To gain insight into the geometry of the strange attractors, the phase portraits of the corresponding iterated maps are interpreted as scatter plots for which marginal distributions are additionally evaluated. The lack of entrainment to external oscillations, in even the simplest biochemical networks, represents a level of additional complexity in molecular biology, which has previously been insufficiently recognized but is plausibly biologically important.

Quorum-Sensing Synchronization of Synthetic Toggle Switches: A Design Based on Monotone Dynamical Systems Theory.

Synthetic constructs in biotechnology, biocomputing, and modern gene therapy interventions are often based on plasmids or transfected circuits which implement some form of "on-off" switch. For example, the expression of a protein used for therapeutic purposes might be triggered by the recognition of a specific combination of inducers (e.g., antigens), and memory of this event should be maintained across a cell population until a specific stimulus commands a coordinated shut-off. The robustness of such a design is hampered by molecular ("intrinsic") or environmental ("extrinsic") noise, which may lead to spontaneous changes of state in a subset of the population and is reflected in the bimodality of protein expression, as measured for example using flow cytometry. In this context, a "majority-vote" correction circuit, which brings deviant cells back into the required state, is highly desirable, and quorum-sensing has been suggested as a way for cells to broadcast their states to the population as a whole so as to facilitate consensus. In this paper, we propose what we believe is the first such a design that has mathematically guaranteed properties of stability and auto-correction under certain conditions. Our approach is guided by concepts and theory from the field of "monotone" dynamical systems developed by M. Hirsch, H. Smith, and others. We benchmark our design by comparing it to an existing design which has been the subject of experimental and theoretical studies, illustrating its superiority in stability and self-correction of synchronization errors. Our stability analysis, based on dynamical systems theory, guarantees global convergence to steady states, ruling out unpredictable ("chaotic") behaviors and even sustained oscillations in the limit of convergence. These results are valid no matter what are the values of parameters, and are based only on the wiring diagram. The theory is complemented by extensive computational bifurcation analysis, performed for a biochemically-detailed and biologically-relevant model that we developed. Another novel feature of our approach is that our theorems on exponential stability of steady states for homogeneous or mixed populations are valid independently of the number N of cells in the population, which is usually very large (N ≫ 1) and unknown. We prove that the exponential stability depends on relative proportions of each type of state only. While monotone systems theory has been used previously for systems biology analysis, the current work illustrates its power for synthetic biology design, and thus has wider significance well beyond the application to the important problem of coordination of toggle switches.

Cardiac Nerve Growth Factor Overexpression Induces Bone Marrow-derived Progenitor Cells Mobilization and Homing to the Infarcted Heart.

Reparative response by bone marrow (BM)-derived progenitor cells (PCs) to ischemia is a multistep process that comprises the detachment from the BM endosteal niche through activation of osteoclasts and proteolytic enzymes (such as matrix metalloproteinases (MMPs)), mobilization to the circulation, and homing to the injured tissue. We previously showed that intramyocardial nerve growth factor gene transfer (NGF-GT) promotes cardiac repair following myocardial infarction (MI) in mice. Here, we investigate the impact of cardiac NGF-GT on postinfarction BM-derived PCs mobilization and homing at different time points after adenovirus-mediated NGF-GT in mice. Immunohistochemistry and flow cytometry newly illustrate the temporal profile of osteoclast and activation of MMP9, PCs expansion in the BM, and liberation/homing to the injured myocardium. NGF-GT amplified these responses and increased the BM levels of active osteoclasts and MMP9, which were not observed in MMP9-deficient mice. Taken together, our results suggest a novel role for NGF in BM-derived PCs mobilization/homing following MI.

Matrix metalloproteinase (MMP) 9 transcription in mouse brain induced by fear learning.

Memory formation requires learning-based molecular and structural changes in neurons, whereas matrix metalloproteinase (MMP) 9 is involved in the synaptic plasticity by cleaving extracellular matrix proteins and, thus, is associated with learning processes in the mammalian brain. Because the mechanisms of MMP-9 transcription in the brain are poorly understood, this study aimed to elucidate regulation of MMP-9 gene expression in the mouse brain after fear learning. We show here that contextual fear conditioning markedly increases MMP-9 transcription, followed by enhanced enzymatic levels in the three major brain structures implicated in fear learning, i.e. the amygdala, hippocampus, and prefrontal cortex. To reveal the role of AP-1 transcription factor in MMP-9 gene expression, we have used reporter gene constructs with specifically mutated AP-1 gene promoter sites. The constructs were introduced into the medial prefrontal cortex of neonatal mouse pups by electroporation, and the regulation of MMP-9 transcription was studied after contextual fear conditioning in the adult animals. Specifically, -42/-50- and -478/-486-bp AP-1 binding motifs of the mouse MMP-9 promoter sequence have been found to play a major role in MMP-9 gene activation. Furthermore, increases in MMP-9 gene promoter binding by the AP-1 transcription factor proteins c-Fos and c-Jun have been demonstrated in all three brain structures under investigation. Hence, our results suggest that AP-1 acts as a positive regulator of MMP-9 transcription in the brain following fear learning.

Sexual performance and precontact 50-kHz ultrasonic vocalizations in WAG/Rij rats: effects of opioid receptor treatment.

WAG/Rij rats are genetically selected animals that model absence epilepsy in rats. Ultrasonic vocalizations and sexual behavior - both ethologically relevant markers of reward system functioning - are poorly described in this strain. The aim of our experiment was to investigate reward-dependent precontact 50-kHz vocalizations (PVs) and copulatory behavior as well as the effects of opioid receptor treatment on such behaviors in sexually experienced WAG/Rij males and rats from two control strains: Sprague-Dawley and Crl: Han Wistar. We analyzed the effects of the opioid receptor antagonist naltrexone (3 mg/kg) and the agonist morphine (1 mg/kg) administration. Additionally, we analyzed the initiation of copulation in sexually naïve males before drug treatment. A significantly lower number of sexually naïve WAG/Rij rats initiated copulation. Sexually experienced WAG/Rij males differed at the control session (after physiological saline treatment) compared with Sprague-Dawley rats: WAG/Rij rats displayed more 50-kHz precontact vocalizations and had longer mount and intromission latencies, longer ejaculation latency, longer postejaculatory latency to exploration, longer 22-kHz vocalization duration after ejaculation, and longer postejaculatory intromission latency. Compared with Crl: Han Wistar rats, WAG/Rij males displayed longer mount latency and shorter 22-kHz vocalization duration. Neither naltrexone nor morphine affected PVs in all groups. On the other hand, opioid receptor treatment differently influenced the number of intromissions required to achieve ejaculation and 22-kHz postejaculatory vocalization duration in WAG/Rij rats than in both control groups. This suggests functional differences in the opioid system in this strain. As a result of the number of males that initiated copulation as well as the number of intromissions to ejaculation and 22-kHz postejaculatory vocalizations which all depend on D1 receptor activation, we suggest that the proportion of opioid receptor to D1 receptors in WAG/Rij rats is different when compared with the control strains. The reward system of Wag/Rij rats with absence epilepsy is sensitive to social rewards (high level of precontact 50-kHz ultrasounds) although this strain displays a lower level of sexual motivation (longer mount latency) compared with other control strains. A lower number of sexually naïve rats initiating copulation and longer mount latency in sexually experienced males could suggest a moderate depressive-like syndrome in this strain of rats.

Nicotinamide mononucleotide synthetase is the key enzyme for an alternative route of NAD biosynthesis in Francisella tularensis.

Enzymes involved in the last 2 steps of nicotinamide adenine dinucleotide (NAD) cofactor biosynthesis, which catalyze the adenylylation of the nicotinic acid mononucleotide (NaMN) precursor to nicotinic acid dinucleotide (NaAD) followed by its amidation to NAD, constitute promising drug targets for the development of new antibiotics. These enzymes, NaMN adenylyltransferase (gene nadD) and NAD synthetase (gene nadE), respectively, are indispensable and conserved in nearly all bacterial pathogens. However, a comparative genome analysis of Francisella tularensis allowed us to predict the existence of an alternative route of NAD synthesis in this category A priority pathogen, the causative agent of tularaemia. In this route, the amidation of NaMN to nicotinamide mononucleotide (NMN) occurs before the adenylylation reaction, which converts this alternative intermediate to the NAD cofactor. The first step is catalyzed by NMN synthetase, which was identified and characterized in this study. A crystal structure of this enzyme, a divergent member of the NadE family, was solved at 1.9-A resolution in complex with reaction products, providing a rationale for its unusual substrate preference for NaMN over NaAD. The second step is performed by NMN adenylyltransferase of the NadM family. Here, we report validation of the predicted route (NaMN --> NMN --> NAD) in F. tularensis including mathematical modeling, in vitro reconstitution, and in vivo metabolite analysis in comparison with a canonical route (NaMN --> NaAD --> NAD) of NAD biosynthesis as represented by another deadly bacterial pathogen, Bacillus anthracis.

The elucidation of metabolic pathways and their improvements using stable optimization of large-scale kinetic models of cellular systems.

Metabolic engineering of cellular systems to maximize reaction fluxes or metabolite concentrations still presents a significant challenge by encountering unpredictable instabilities that can be caused by simultaneous or consecutive enhancements of many reaction steps. It can therefore be important to select carefully small subsets of key enzymes for their subsequent stable modification compatible with cell physiology. To address this important problem, we introduce a general mixed integer non-linear problem (MINLP) formulation to compute automatically which enzyme levels should be modulated and which enzyme regulatory structures should be altered to achieve the given optimization goal using non-linear kinetic models of relevant cellular systems. The developed MINLP formulation directly employs a stability analysis constraint and also includes non-linear biophysical constraints to describe homeostasis conditions for metabolite concentrations and protein machinery without any preliminary model simplification (e.g. linlog kinetics approximation). The framework is demonstrated on a well-established large-scale kinetic model of the Escherichia coli central metabolism used for the optimization of the glucose uptake through the phosphotransferase transport system (PTS) and serine biosynthesis. Computational results show that substantial stable improvements can be predicted by manipulating only small subsets of enzyme levels and regulatory structures. This means that while more efforts can be required to elucidate larger stable optimal enzyme level/regulation choices, no further significant increase in the optimized fluxes can be obtained and, therefore, such choices may not be worth the effort due to the potential loss of stability properties. The source for instability through saddle-node and Hopf bifurcations is identified, and all results are contrasted with predictions from metabolic control analysis.

The antitumorigenic response of neural precursors depends on subventricular proliferation and age.

Glioblastomas, the most aggressive primary brain tumors, occur almost exclusively in adult patients. Neural precursor cells (NPCs) are antitumorigenic in mice, as they can migrate to glioblastomas and induce tumor cell death. Here, we show that the antitumor effect of NPCs is age-dependently controlled by cell proliferation in the subventricular zone (SVZ) and that NPCs accumulating at a glioblastoma are diverted from their normal migratory path to the olfactory bulb. Experimentally induced cortical glioblastomas resulted in decreased subventricular proliferation in adult (postnatal day 90) but not in young (postnatal day 30) mice. Adult mice supplied fewer NPCs to glioblastomas and had larger tumors than young mice. Apart from the difference in proliferation, there was neither a change in cell number and death rate in the SVZ nor a change in angiogenesis and immune cell density in the tumors. The ability to kill glioblastomas was similar in NPCs isolated from young and adult mice. The proliferative response of NPCs to glioblastomas depended on the expression of D-type cyclins. In young mice, NPCs express the cyclins D1 and D2, but the expression of cyclin D1 is lost during aging, and in adult NPCs only cyclin D2 remains. In young and adult cyclin D2-deficient mice we observed a reduced supply of NPCs to glioblastomas and the generation of larger tumors compared with wild-type mice. We conclude that cyclin D1 and D2 are nonredundant for the antitumor response of subventricular NPCs. Loss of a single D-type cyclin results in a smaller pool of proliferating NPCs, lower number of NPCs migrating to the tumor, and reduced antitumor activity. Disclosure of potential conflicts of interest is found at the end of this article.

Diverse sensitivity of RHA/Verh and RLA/Verh rats to emotional and spatial aspects of a novel environment as a result of a distinct pattern of neuronal activation in the fear/anxiety circuit.

Psychogenetically selected Roman high (RHA/Verh) and Roman low (RLA/Verh) avoidance rats constitute a well-recognized model of diverse emotional reactivity. The two Swiss lines display marked behavioral and endocrine differences in reaction to a novel environment. In our study we found that these differences are accompanied by a distinct, line-specific pattern of neuronal activation within the fear/anxiety circuit. We have compared the c-Fos protein expression in the medial prefrontal cortex (mPFC), basolateral (BLA), central (CeA), medial (MeA), and cortical (CoA) nuclei of amygdala, paraventricular nucleus of the hypothalamus (PVN), and CA1, CA2, and CA3 fields of the hippocampus upon exposure to a novel situation of different stressorgeneity (open field with illuminated center, elevated plus maze, hole board test and acute restraint). Profound between-line differences in the sensitivity to emotional and spatial aspects of the behavioral challenge were observed for tests measuring spontaneous behavior. This effect seems to reflect different motivational factors driving the rat behavior, which clearly suggests that the diverse emotional reactivity of RHA/Verh and RLA/Verh rats is a result of different activation of the fear/anxiety circuit.

Immunobiochemical Reconstruction of Influenza Lung Infection-Melanoma Skin Cancer Interactions.

It was recently reported that acute influenza infection of the lung promoted distal melanoma growth in the dermis of mice. Melanoma-specific CD8+ T cells were shunted to the lung in the presence of the infection, where they expressed high levels of inflammation-induced cell-activation blocker PD-1, and became incapable of migrating back to the tumor site. At the same time, co-infection virus-specific CD8+ T cells remained functional while the infection was cleared. It was also unexpectedly found that PD-1 blockade immunotherapy reversed this effect. Here, we proceed to ground the experimental observations in a mechanistic immunobiochemical model that incorporates T cell pathways that control PD-1 expression. A core component of our model is a kinetic motif, which we call a PD-1 Double Incoherent Feed-Forward Loop (DIFFL), and which reflects known interactions between IRF4, Blimp-1, and Bcl-6. The different activity levels of the PD-1 DIFFL components, as a function of the cognate antigen levels and the given inflammation context, manifest themselves in phenotypically distinct outcomes. Collectively, the model allowed us to put forward a few working hypotheses as follows: (i) the melanoma-specific CD8+ T cells re-circulating with the blood flow enter the lung where they express high levels of inflammation-induced cell-activation blocker PD-1 in the presence of infection; (ii) when PD-1 receptors interact with abundant PD-L1, constitutively expressed in the lung, T cells loose motility; (iii) at the same time, virus-specific cells adapt to strong stimulation by their cognate antigen by lowering the transiently-elevated expression of PD-1, remaining functional and mobile in the inflamed lung, while the infection is cleared. The role that T cell receptor (TCR) activation and feedback loops play in the underlying processes are also highlighted and discussed. We hope that the results reported in our study could potentially contribute to the advancement of immunological approaches to cancer treatment and, as well, to a better understanding of a broader complexity of fundamental interactions between pathogens and tumors.

The effect of age on the dynamics and the level of c-Fos activation in response to acute restraint in Lewis rats.

Recent studies have reported an age-related increase of anxiety in rodents with a concomitant decrease in neuronal activity in some of the key structures of the fear/anxiety circuit. In the present study we present evidence that distinct parts of this circuit are differentially affected by age in Lewis rats. The effect of ageing is observed both at the actual level of neuronal activation and its time-course. While the structures belonging to the HPA axis react with a bigger neuronal activation and almost no change in the shape of dynamics curve in response to restraint, the structures involved in higher processing of emotional cues (amygdala and hippocampus) become deficiently activated with age despite their generally higher basal level of activation.

A computational procedure for optimal engineering interventions using kinetic models of metabolism.

The identification of optimal intervention strategies is a key step in designing microbial strains with enhanced capabilities. In this paper, we propose a general computational procedure to determine which genes/enzymes should be eliminated, repressed or overexpressed to maximize the flux through a product of interest for general kinetic models. The procedure relies on the generalized linearization of a kinetic description of the investigated metabolic system and the iterative application of mixed-integer linear programming (MILP) optimization to hierarchically identify all engineering interventions allowing for reaction eliminations and/or enzyme level modulations. The effect of the magnitude of the allowed changes in concentrations and enzyme levels is investigated, and a variant of the method to explore high-fold changes in enzyme levels is also analyzed. The proposed framework is demonstrated using a kinetic model modeling part of the central carbon metabolism of E. coli for serine overproduction. The proposed computational procedure is a general approach that can be applied to any metabolic system for which a kinetic description is provided.

Modulation of 22-khz postejaculatory vocalizations by conditioning to new place: Evidence for expression of a positive emotional state.

It has been assumed that the 22-kHz ultrasonic vocalizations (USVs) are emitted by adult rats as a result of a negative emotional state. However, emission of the 22-kHz vocalizations by male rats has been also observed following ejaculation, which has a high rewarding value as shown by a conditioned place preference test. These observations suggest that 22-kHz USVs may also occur in response to a positive emotional state. The aim of this study was to determine whether the postejaculatory 22-kHz USVs are related to conditioning processes. The 22 kHz USVs were recorded in Sprague-Dawley males in the postejaculatory refractory period during conditioning processes to a new chamber unrelated to copulation. During the first session in the clean recording chamber, males vocalized marginally and exhibited intensive rearing behavior. From the second to fourth sessions, vocalization duration increased and the number of rearing decreased. Following established conditioning process, odor cues from foreign males, but not the familiar ones, resulted in decreased duration of 22-kHz USVs and increased the number of rearing. On the other hand, in the presence of mating cues (copulatory chamber and presence of the female), males exhibited increased duration of postejaculatory 22-kHz USVs and reduced number of rearing. These results demonstrated that the conditioning to the cues, both unrelated and related to copulation, is important for evoking postejaculatory 22-kHz USVs as well as the relaxation state. Furthermore, these results confirmed the postejaculatory 22-kHz USVs' involvement in expression of the positive emotional state. (PsycINFO Database Record

Fundamental limitation of the instantaneous approximation in fold-change detection models.

The phenomenon of fold-change detection, or scale-invariance, is exhibited by a variety of sensory systems, in both bacterial and eukaryotic signalling pathways. It has been often remarked in the systems biology literature that certain systems whose output variables respond at a faster time scale than internal components give rise to an approximate scale-invariant behaviour, allowing approximate fold-change detection in stimuli. This study establishes a fundamental limitation of such a mechanism, showing that there is a minimal fold-change detection error that cannot be overcome, no matter how large the separation of time scales is. To illustrate this theoretically predicted limitation, the authors discuss two common biomolecular network motifs, an incoherent feedforward loop and a feedback system, as well as a published model of the chemotaxis signalling pathway of Dictyostelium discoideum.

Differential response of two subdivisions of lateral amygdala to aversive conditioning as revealed by c-Fos and P-ERK mapping.

Rats were subjected to two aversive learning protocols: either classical (fear conditioning) or instrumental (two-way active avoidance training). Next, immunocytochemical mapping of phosphorylated extracellular signal-regulated kinase (P-ERK) and c-Fos transcription factor protein was performed, and the expression pattern of both markers within the dorsal nucleus of lateral amygdala (LaD) was analyzed. As immunocytochemical studies revealed, aversive training induced ERK phosphorylation and c-Fos expression specifically in ventral but not dorsal tip of LaD. These data show for the first time molecular distinction between subdivisions of LaD as well as they also strengthen the idea of Repa that the neurons in the ventral tip of LaD are involved in storage of long-lasting changes associated with formation of fear memories.

Environmental manipulation differentially alters c-Fos expression in amygdaloid nuclei following aversive conditioning.

This study analyzes the impact of environmental complexity in adult rats on their emotional behavior and c-Fos expression (a transcription factor protein implicated in neuronal plasticity) in various subdivisions of amygdala, as well as selected parts of the thalamus and hypothalamus. The animals were housed for 60 days in either enriched or impoverished conditions and then one group of rats was tested in an open field test, and a second group of rats was treated to footshock-motivated aversive training. Two and 24 h later, the animals from the second group, along with the appropriate controls, were sacrificed and their brains were used for the immunocytochemical analysis of c-Fos levels. We found that this long-term environmental manipulation exerts significant effects on animals' emotionality and this behavioral difference is accompanied by the differential c-Fos activation (at 2 h after the aversive training) in the amygdala, a brain structure believed to subserve emotional reactions. On the other hand, no difference was found in c-Fos expression between both groups of animals in the thalamus and hypothalamus. At 24 h after the training, c-Fos levels were down to the values observed in naive rats that did not differentiate between enriched versus impoverished breeding conditions. These results may help to explain differential emotional aspects of behavior that arise following differential housing conditions of adult animals.

Local blockade of NMDA receptors in the rat prefrontal cortex increases c-Fos expression in multiple subcortical regions.

Ketamine, phencyclidine and MK801 are uncompetitive NMDA receptor (NMDAR) antagonists which are used widely to model certain features of schizophrenia in rats. Systemic administration of NMDAR antagonists, in addition to provoking an increase in c-Fos expression, leads to important neurochemical and electrophysiological changes within the medial prefrontal cortex (mPFC). Since the mPFC is considered to exert a top-down regulatory control of subcortical brain regions, we examined the effects of local infusion of the NMDAR antagonist, MK801, into the mPFC on the expression of c-Fos protein (widely used marker of neuronal activation) in several subcortical structures. The experiment was performed on freely moving rats, bilaterally implanted with guide cannulae in the prelimbic mPFC, infused with MK801 or saline. Bilateral administration of MK801 to the mPFC produced changes in the behavior (increased stereotypy and decreased sleep-like behavior) and complex changes in c-Fos protein expression with significant increases observed in the nucleus accumbens (core and shell), amygdala (basolateral and central nuclei), the CA1 field of the hippocampus, and mediodorsal and paraventricular thalamic nuclei, as compared to the saline group. Together, we demonstrate that blockade of NMDA receptors in the mPFC is sufficient to lead to behavioral abnormalities and increased c-Fos expression in many, but not all, of the subcortical structures examined. Our findings suggest that some of the behavioral abnormalities produced by uncompetitive NMDAR antagonists may result from aberrant activity in cortico-subcortical pathways. These data support an increasing body of literature, suggesting that the mPFC is an important site mediating the effects of NMDAR antagonists.

Blockade of androgen receptor in the medial amygdala inhibits noncontact erections in male rats.

Our previous work demonstrated that androgens in the medial amygdala (MeA) of castrated male rats maintained noncontact erections (NCEs), which occur during exposure to an inaccessible receptive female, for one week after implantation. The present experiments investigated the effects of implantation into the MeA of either flutamide (F), a blocker of androgen receptors, or of 1,4,6-androstatrien-3,17-dione (ATD), which blocks aromatization of testosterone. One day after implantation of F, fewer males displayed NCEs, and had longer latencies to the first NCE and fewer NCEs, and spent less total time in genital grooming, compared to the control group. ATD had only weak facilitative effects on some measures of NCEs. These results suggest that androgen receptors in the MeA play a major role in the regulation of NCEs and that the MeA is one of the neuronal structures that regulate male sexual arousal. Furthermore, it is sensitive to relatively fast changes in the level of androgen receptors stimulation.