Offense produced by chemical stimulation of the anterior hypothalamus of the rat

Directional Deep Brain Stimulation of the Posteromedial Hypothalamus for Refractory Intermittent Explosive Disorder: A Case Series Using a Novel Neurostimulation Device and Intraoperative Microdialysis

BACKGROUND

Intermittent explosive disorder (IED) is a psychiatric disorder characterized by recurrent outbursts of aggressive behavior. Deep brain stimulation (DBS) in the posteromedial nucleus of the hypothalamus (pHyp) is an alternative therapy for extreme cases and shows promising results. Intraoperative microdialysis can help elucidate the neurobiological mechanism of pHyp-DBS. We sought to evaluate efficacy and safety of pHyp-DBS using 8-contact directional leads in patients with refractory IED (rIED) and the accompanying changes in neurotransmitters.

METHODS

This was a prospective study in which patients with a diagnosis of rIED were treated with pHyp-DBS for symptom alleviation. Bilateral pHyp-DBS was performed with 8-contact directional electrodes. Follow-up was performed at 3, 6, and 12 months after surgery.

RESULTS

Four patients (3 men, mean age 27 ± 2.8 years) were included. All patients were diagnosed with rIED and severe intellectual disability. Two patients had congenital rubella, one had a co-diagnosis of infantile autism, and the fourth presented with drug-resistant epilepsy. There was a marked increase in the levels of gamma-aminobutyric acid and glycine during intraoperative stimulation. The average improvement in aggressive behavior in the last follow-up was 6 points (Δ: 50%, P = 0.003) while also documenting an important improvement of the Short Form Health Survey in all domains except bodily pain. No adverse events associated with pHyp-DBS were observed.

CONCLUSIONS

This is the first study to show the safety and beneficial effect of directional lead pHyp-DBS in patients with rIED and to demonstrate the corresponding mechanism of action through increases in gamma-aminobutyric acid and glycine concentration in the pHyp.

Hypothalamic Control of Conspecific Self-Defense

Active defense against a conspecific aggressor is essential for survival. Previous studies revealed strong c-Fos expression in the ventrolateral part of the ventromedial hypothalamus (VMHvl) in defeated animals. Here, we examined the functional relevance and in vivo responses of the VMHvl during conspecific defense. We found that VMHvl cells expressing estrogen receptor α (Esr1) are acutely excited during active conspecific defense. Optogenetic inhibition of the cells compromised an animal’s ability to actively defend against an aggressor, whereas activating the cells elicited defense-like behaviors. Furthermore, the VMHvl is known for its role in aggression. In vivo recording and c-Fos mapping revealed differential organization of the defense and aggression-responsive cells in the VMHvl. Specifically, defense-activated cells are concentrated in the anterior part of the VMHvl, which preferentially targets the periaqueductal gray (PAG). Thus, our study identified an essential neural substrate for active conspecific defense and expanded the function of the VMHvl.

Active defense against conspecific aggressors is essential for survival, but its underlying neural substrates remain largely unknown. Through a series of in vivo recordings and functional manipulations, Wang et al. demonstrate that cells expressing estrogen receptor α in a small medial hypothalamic nucleus are essential for defense against a bully.

Social instigation and repeated aggressive confrontations in male Swiss mice: analysis of plasma corticosterone, CRF and BDNF levels in limbic brain areas

Introduction:

Agonistic behaviors help to ensure survival, provide advantage in competition, and communicate social status. The resident-intruder paradigm, an animal model based on male intraspecific confrontations, can be an ethologically relevant tool to investigate the neurobiology of aggressive behavior.

Objectives:

To examine behavioral and neurobiological mechanisms of aggressive behavior in male Swiss mice exposed to repeated confrontations in the resident intruder paradigm.

Methods:

Behavioral analysis was performed in association with measurements of plasma corticosterone of mice repeatedly exposed to a potential rival nearby, but inaccessible (social instigation), or to 10 sessions of social instigation followed by direct aggressive encounters. Moreover, corticotropin-releasing factor (CRF) and brain-derived neurotrophic factor (BNDF) were measured in the brain of these animals. Control mice were exposed to neither social instigation nor aggressive confrontations.

Results:

Mice exposed to aggressive confrontations exhibited a similar pattern of species-typical aggressive and non-aggressive behaviors on the first and the last session. Moreover, in contrast to social instigation only, repeated aggressive confrontations promoted an increase in plasma corticosterone. After 10 aggressive confrontation sessions, mice presented a non-significant trend toward reducing hippocampal levels of CRF, which inversely correlated with plasma corticosterone levels. Conversely, repeated sessions of social instigation or aggressive confrontation did not alter BDNF concentrations at the prefrontal cortex and hippocampus.

Conclusion:

Exposure to repeated episodes of aggressive encounters did not promote habituation over time. Additionally, CRF seems to be involved in physiological responses to social stressors.

Sociality and oxytocin and vasopressin in the brain of male and female dominant and subordinate mandarin voles

The dominant-subordinate hierarchy in animals often needs to be established via agonistic encounters and consequently affects reproduction and survival. Differences in brain neuropeptides and sociality among dominant and subordinate males and females remain poorly understood. Here we explore neuropeptide levels and sociality during agonistic encounter tests in mandarin voles. We found that dominant mandarin voles engaged in higher levels of approaching, investigating, self-grooming and exploring behavior than subordinates. Dominant males habituated better to a stimulus vole than dominant females. Dominant males displayed significantly less oxytocin-immunoreactive neurons in the paraventricular nuclei and more vasopressin-immunoreactive neurons in the paraventricular nuclei, supraoptic nuclei, and the lateral and anterior hypothalamus than subordinates. Dominant females displayed significantly more vasopressin-immunoreactive neurons in the lateral hypothalamus and anterior hypothalamus than subordinates. Sex differences were found in the level of oxytocin and vasopressin. These results indicate that distinct parameters related to central nervous oxytocin and vasopressin are associated with behaviors during agonistic encounters in a sex-specific manner in mandarin voles.

The neurobiology of abnormal manifestations of aggression--a review of hypothalamic mechanisms in cats, rodents, and humans

Aggression research was for long dominated by the assumption that aggression-related psychopathologies result from the excessive activation of aggression-promoting brain mechanisms. This assumption was recently challenged by findings with models of aggression that mimic etiological factors of aggression-related psychopathologies. Subjects submitted to such procedures show abnormal attack features (mismatch between provocation and response, disregard of species-specific rules, and insensitivity toward the social signals of opponents). We review here 12 such laboratory models and the available human findings on the neural background of abnormal aggression. We focus on the hypothalamus, a region tightly involved in the execution of attacks. Data show that the hypothalamic mechanisms controlling attacks (general activation levels, local serotonin, vasopressin, substance P, glutamate, GABA, and dopamine neurotransmission) undergo etiological factor-dependent changes. Findings suggest that the emotional component of attacks differentiates two basic types of hypothalamic mechanisms. Aggression associated with increased arousal (emotional/reactive aggression) is paralleled by increased mediobasal hypothalamic activation, increased hypothalamic vasopressinergic, but diminished hypothalamic serotonergic neurotransmission. In aggression models associated with low arousal (unemotional/proactive aggression), the lateral but not the mediobasal hypothalamus is over-activated. In addition, the anti-aggressive effect of serotonergic neurotransmission is lost and paradoxical changes were noticed in vasopressinergic neurotransmission. We conclude that there is no single 'neurobiological road' to abnormal aggression: the neural background shows qualitative, etiological factor-dependent differences. Findings obtained with different models should be viewed as alternative mechanisms rather than conflicting data. The relevance of these findings for understanding and treating of aggression-related psychopathologies is discussed. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Genetics of aggression in voles

Prairie voles (Microtus ochrogaster) are socially monogamous rodents that form pair bonds-a behavior composed of several social interactions including attachment with a familiar mate and aggression toward conspecific strangers. Therefore, this species has provided an excellent opportunity for the study of pair bonding behavior and its underlying neural mechanisms. In this chapter, we discuss the utility of this unique animal model in the study of aggression and review recent findings illustrating the neurochemical mechanisms underlying pair bonding-induced aggression. Implications of this research for our understanding of the neurobiology of human violence are also discussed.

Anterior hypothalamic neural activation and neurochemical associations with aggression in pair-bonded male prairie voles

Male prairie voles (Microtus ochrogaster) display mating-induced pair bonding indicated by social affiliation with their female partners and aggression toward unfamiliar conspecifics. In the present study, we characterized their aggression associated with pair bonding and examined the related neuronal activation and neurochemical architecture. Males that were pair-bonded for 2 weeks displayed intense levels of aggression toward a female or male conspecific stranger but maintained a high level of social affiliation with their familiar female partners. These social interactions induced increases in neural activation, indicated by increased density of Fos-immunoreactive staining (Fos-ir) in several brain regions including the bed nucleus of the stria terminalis (BNST), medial preoptic area (MPOA), paraventricular nucleus (PVN), anterior cortical (AcA), and medial nuclei (MeA) of the amygdala. In the anterior hypothalamus (AH), increased density of Fos-ir staining was found specifically to be associated with aggression toward unfamiliar female or male strangers. In addition, higher densities of AH cells that were stained for tyrosine hydroxylase (TH) or vasopressin (AVP) were also labeled with Fos-ir in these males displaying aggression toward a conspecific stranger compared with males displaying social affiliation toward their female partner. Together, our results indicate that dopamine and vasopressin in the AH may be involved in the regulation of enduring aggression associated with pair bonding in male prairie voles.

Reduced aggression in mice lacking GABA transporter subtype 1

Dysregulation of the brain GABAergic system has been implicated in the pathophysiology of violence and aggression. As a key regulator of central GABAergic activity, dysfunction of the GABA transporter subtype 1 (GAT1) represents a potential mechanism mediating pathologic aggression. We provide evidence that GAT1-/- mice and GAT1+/- mice exhibit lower aggressive behavior both in home cage resident-intruder test and neutral arena resident-intruder test, compared to wild-type mice (GAT1+/+). The pharmacologic effects of the GAT1 inhibitor, tiagabine and the GABA(A) receptor antagonist, bicuculline have been assessed in GAT1+/+ mice: tiagabine inhibits attacks but bicuculline induces attacks. Compared to GAT1+/- and +/+ mice, the GAT1-/- mice displayed a normal circadian pattern of home cage activity, but more activity overall. Meanwhile, reduced testosterone concentration was found in GAT1-/- mice compared to GAT1+/+ mice but not in GAT1+/+ mice treated with tiagabine, suggesting that testosterone is not directly involved in GAT1 mediated aggressive behavior regulation. These results showed that GAT1 is an important target involved in the regulation of aggressive behavior in mice, and long-term dysfunction of GAT1 may also result in the alteration of testosterone secretion.

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.

5-HT1B receptors and aggression: A review

The serotonergic (5-HT) system in the brain is involved in the modulation of offensive aggressive behavior. The dogma that activity of the 5-HT system is inversely related to aggression is obsolete now. Research on the status of the 5-HT system before, during and after the execution of aggression is ongoing but has not yet led to a clear picture about the actual functional role of the 5-HT system, the more because state versus trait aggression seems to play a pivotal role in the outcome. Pharmacological challenges pinpoint 5-HT(1A) and 5-HT(1B) receptors as key players in the modulation of offensive aggression. This review emphasizes in particular the role of postsynaptic 5-HT(1B) (hetero) receptors as a premier site to modulate offensive aggression. Modulation of the firing and 5-HT release of the serotonergic neuron, via presynaptic 5-HT(1A) (auto) receptors, presynaptic 5-HT(1B) (auto) receptors and serotonergic transporters, may also have striking influences on aggression under certain conditions. Therefore, it is hypothesized that postsynaptic 5-HT(1B) (hetero) receptors directly influence the executive, consummatory phases of agonistic behavior, whereas presynaptic serotonergic feedback systems are particularly useful in the introductory (appetitive) phases of the agonistic behavioral complex.

Neurochemical characterization of hypothalamic neurons involved in attack behavior: glutamatergic dominance and co-expression of thyrotropin-releasing hormone in a subset of glutamatergic neurons

The electrical stimulation of a specific hypothalamic area rapidly evokes attacks in rats. Noteworthy, attack-related hypothalamic structures were identified in all species studied so far. The area has been extensively mapped in rats, and its anatomical connections have been studied in detail. However, technical difficulties precluded earlier the precise identification of the neural elements mediating the aggressive effects of stimulation. It now appears that a dense and distinct group of glutamatergic cells expressing vesicular glutamate transporter 2 mRNA extends over the entire hypothalamic attack area. Rostral parts overwhelmingly contained glutamatergic neurons. In more caudal parts, glutamatergic and fewer GABAergic neurons were found. The remarkable similarity in the distribution of hypothalamic attack area and glutamatergic cell groups suggests that these cells mediate the aggressive effects of stimulation. Surprisingly, thyrotropin releasing hormone mRNA was co-localized in a subset of glutamatergic neurons. Such neurons were present at all rostro-caudal levels of the hypothalamic attack area, except for that part of the hypothalamic attack area extending into the ventro-lateral part of the ventromedial hypothalamic nucleus. Earlier data on the projections of hypothalamic thyrotropin releasing hormone neurons suggest that this subpopulation plays a specific role in attack behavior. Thus, we identified three neuronal phenotypes in the hypothalamic structure that is involved in the induction of attacks: glutamatergic neurons co-expressing thyrotropin releasing hormone, glutamatergic neurons without thyrotropin releasing hormone, and GABAergic neurons dispersed among the glutamatergic cells. Assessing the specific roles and connections of these neuron subpopulations would contribute to our understanding of the mechanisms underlying attack behavior and aggression.

The effect of sub-chronic nandrolone decanoate treatment on dopaminergic and serotonergic neuronal systems in the brains of rats

Anabolic-androgenic steroids (AASs) are widely abused by adolescents, although persistent AAS use can cause several adverse physical and mental effects, including drug dependence. The first aim of the present study was to study the action of nandrolone decanoate on dopaminergic and serotonergic activities in the brains of rats. In order to evaluate the anabolic or toxic effects of the dosing regimens used, selected peripheral effects were monitored as well. Male Wistar rats were treated for 2 weeks. Injections containing nandrolone (5 and 20 mg/kg, i.m.) or vehicle were given once daily, 5 days a week. The levels of dopamine (DA), 5-hydroxytryptamine (5-HT) and their metabolites were assayed from dissected brain regions 3 days after the last injection. Blood was collected for chemical assays before, after 1 week treatment and at decapitation. Both doses of nandrolone significantly increased the levels of 3,4-dihydroxyphenylacetic acid (DOPAC), a metabolite of DA in the cerebral cortex, and the higher dose of nandrolone increased the concentrations of 5-HT in the cerebral cortex compared with the vehicle. In addition, after nandrolone treatment, the levels of hemoglobin and erythrocytes increased, and reticulocyte levels decreased. The results suggest that nandrolone at supraphysiological doses, high enough to induce erythropoiesis, induces changes in the dopaminergic and serotonergic neuronal system in the brains of rats. These phenomena may account to some of the observed central stimulatory properties that have been reported following AAS abuse.

Mouse grooming microstructure is a reliable anxiety marker bidirectionally sensitive to GABAergic drugs

Grooming is an important part of rodent behavioural repertoire, representing a complex hierarchically ordered cephalo-caudal sequence of patterns sensitive to stress and various drugs. Gamma-aminobutyric acid (GABA) is involved in the regulation of both anxiety and grooming behaviours. This study investigated the predictive validity of grooming behavioural microstructure as a marker of anxiety, by examining the effects of two GABAergic reference compounds, anxiolytic diazepam (0.1 and 0.5 mg/kg i.p.) and anxiogenic pentylenetetrazole (5 and 10 mg/kg i.p.) on mouse grooming. Our data suggest that the percentage of pattern transitions not fitting to the cephalo-caudal progression, and the percentage of interrupted grooming bouts are more reliable behavioural markers of stress bidirectionally sensitive to GABAergic anxiogenic and anxiolytic drugs, compared to the frequency and duration scores. Our study also confirms that detailed ethological analyses of grooming microstructure can be a useful tool in behavioural pharmacology of anxiety.

Fast positive feedback between the adrenocortical stress response and a brain mechanism involved in aggressive behavior

Aggressive behavior induces an adrenocortical stress response, and sudden stressors often precipitate violent behavior. Experiments in rats revealed a fast, mutual, positive feedback between the adrenocortical stress response and a brain mechanism controlling aggression. Stimulation of the aggressive area in the hypothalamus rapidly activated the adrenocortical response, even in the absence of an opponent and fighting. Hypothalamic aggression, in turn, was rapidly facilitated by a corticosterone injection in rats in which the natural adrenocortical stress response was prevented by adrenalectomy. The rapidity of both effects points to a fast, mutual, positive feedback of the controlling mechanisms within the time frame of a single conflict. Such a mutual facilitation may contribute to the precipitation and escalation of violent behavior under stressful conditions.

GABAergic function in Alzheimer's disease: evidence for dysfunction and potential as a therapeutic target for the treatment of behavioural and psychological symptoms of dementia

Alzheimer's disease (AD) is characterized by disruptions in multiple major neurotransmitters. While many studies have attempted to establish whether GABA is disrupted in AD patients, findings have varied. We review evidence for disruptions in GABA among patients with AD and suggest that the variable findings reflect subtypes of the disease that are possibly manifested clinically by differing behavioural symptoms. GABA, the major inhibitory neurotransmitter, has long been a target for anxiolytics, hypnotic sedatives, and anticonvulsants. We review the clinical use of GABAergic agents in treating persons with AD symptoms. While newer generation GABAergic medications are now available, they have yet to be evaluated among patients with AD.

Activation of hypothalamic angiotensin receptors produces pressor responses via cholinergic inputs to the rostral ventrolateral medulla in normotensive and hypertensive rats

We have previously reported that the angiotensin system in the anterior hypothalamic area (AHA) is enhanced in spontaneously hypertensive rats (SHR) and that this enhancement is involved in hypertension in SHR. In addition, acetylcholine (ACh) release is increased in the rostral ventrolateral medulla (RVLM) of SHR, which has also been shown to be involved in hypertension in SHR. In this study, we examined whether the enhanced angiotensin system in the AHA of SHR is related to the increase in cholinergic inputs to the RVLM. Electrical stimulation in the AHA produced a pressor response and an increase in firing rate of RVLM barosensitive neurons. These responses were inhibited and enhanced by RVLM application of the muscarinic receptor antagonist scopolamine and the cholinesterase inhibitor physostigmine, respectively. AHA stimulation also produced release of ACh in the RVLM. Microinjections of angiotensin II and carbachol into the AHA produced pressor responses. The pressor response to angiotensin II was inhibited by scopolamine microinjected into the RVLM, although this produced no effect on the response to carbachol. In SHR, although not in Wistar-Kyoto rats, microinjection of losartan into the AHA inhibited pressor responses to physostigmine. However inhibition was not observed in response to the directly acting muscarinic receptor agonist carbachol, injected into the RVLM. These findings demonstrate that angiotensin receptor activation or electrical stimulation in the AHA produce a pressor response via an increase in ACh release in the RVLM. In addition, the present study suggests that the enhanced angiotensin system in the AHA of SHR increases cholinergic inputs to the RVLM, which leads to increases in blood pressure.

Hypothalamic attack area-mediated activation of the forebrain in aggression

It is believed that aggressive attacks are activated by a downward stimulatory stream that includes the medial amygdala, hypothalamic attack area, and periaqueductal grey. However, the hypothalamic attack area (from which attacks can be induced by electrical stimulation) sends projections to the forebrain, the significance of which is unknown. Here we report that the unilateral stimulation of the hypothalamic attack area per se induced an unilateral c-Fos activation of most brain nuclei involved in attack, and that attacks occurred only when cortical regions were also activated, and the activation of the medial amygdala and hypothalamic attack area were bilateral. This suggests that the hypothalamic attack area not only transmits information to lower brain structures but also activates forebrain structures involved in attack.

Neural background of glucocorticoid dysfunction-induced abnormal aggression in rats: involvement of fear- and stress-related structures

Glucocorticoid hypofunction is associated with persistent aggression in some psychologically disordered human subjects and, as reported recently, induces abnormal forms of aggression in rats. Here we report on the effects of glucocorticoid hypofunction on aggression-induced neural activation. Rats were adrenalectomized, and implanted with low-release glucocorticoid pellets. After one week recovery, they were challenged by an unfamiliar intruder in their home-cage. Neural activation was studied by c-Fos protein immunocytochemistry. Aggressive encounters in controls induced c-Fos activation in all brain areas relevant for the control of aggression (cortex, amygdala, septum, hypothalamus, periaqueductal grey and the locus coeruleus). Very intense c-Fos activation was observed in the medial amygdala, the hypothalamic attack area and the periaqueductal grey matter which constitute a downward stimulatory stream that activates attack behaviour. The experimentally induced glucocorticoid hypofunction dramatically increased attacks targeted towards vulnerable parts of the opponent's body (mainly the head). This abnormal behaviour was not associated with changes in the activation of brain centres involved in the control of aggression. However, the activation of brain centres involved in both the stress response (the parvocellular part of the hypothalamic paraventricular nucleus) and fear reactions (central amygdala) were markedly increased. An acute glucocorticoid treatment abolished both behavioural and neural consequences of glucocorticoid hypofunction. Our data suggest that glucocorticoid hypofunction-induced abnormal forms of aggressiveness are related to increased sensitivity to stressors and fear-eliciting stimuli. This assumption is supported by the finding that fearful situations induce attack patterns in intact rats that are similar to those induced by glucocorticoid hypofunction.

Efferent connections of the anterior hypothalamic nucleus: a biocytin study in the cat

The efferent connections of the anterior hypothalamic nucleus (AH) were examined using biocytin as anterograde tracer in the cat. The results provide several new findings in addition to confirming earlier observations. In the hypothalamus, the AH projections terminated mainly in the medial regions which are related to the defensive, reproductive and feeding behaviors, and autonomic functions. Moreover, we found dense patches of the AH terminals in the medial preoptic area and ventromedial hypothalamic nucleus, which suggests the existence of modular connections between sub-regions of each nucleus. In addition, the AH projected to regions which may be related to the emotional and autonomic responses, i.e., such regions in the amygdala, midline thalamus, septum, subthalamus, and midbrain. The data suggest that the AH may play an important role in the autonomic functions and behaviors between animals, and thus may play a key role in the defensive behavior elicited in the medial preoptic area and ventromedial hypothalamic nucleus.

Neuropharmacology of brain-stimulation-evoked aggression

Evidence is reviewed concerning the brain areas and neurotransmitters involved in aggressive behavior in the cat and rodent. In the cat, two distinct neural circuits involving the hypothalamus and PAG subserve two different kinds of aggression: defensive rage and predatory (quiet-biting) attack. The roles played by the neurotransmitters serotonin, GABA, glutamate, opioids, cholecystokinin, substance P, norepinephrine, dopamine, and acetylcholine in the modulation and expression of aggression are discussed. For the rat, a single area, largely coincident with the intermediate hypothalamic area, is crucial for the expression of attack; variations in the rat attack response in natural settings are due largely to environmental variables. Experimental evidence emphasizing the roles of serotonin and GABA in modulating hypothalamically evoked attack in the rat is discussed. It is concluded that significant progress has been made concerning our knowledge of the circuitry underlying the neural basis of aggression. Although new and important insights have been made concerning neurotransmitter regulation of aggressive behavior, wide gaps in our knowledge remain.

The hypothalamus: cross-roads of endocrine and behavioural regulation in grooming and aggression

Anatomical and functional studies show that the hypothalamus is at the junction of mechanisms involved in the exploratory appraisal phase of behaviour and mechanisms involved in the execution of specific consummatory acts. However, the hypothalamus is also a crucial link in endocrine regulation. In natural settings it has been shown that behavioural challenges produce large and fast increases in circulating hormones such as testosterone, prolactin, corticotropin and corticosterone. The behavioural function and neural mechanisms of such fast neuroendocrine changes are not well understood. We suggest that behaviourally specific hypothalamic mechanisms, at the cross-roads of behavioural and endocrine regulation, play a role in such neuroendocrine changes. Mild stimulation of the hypothalamic aggressive area, produces stress levels of circulating prolactin, corticotropin, and corticosterone. Surprisingly luteinizing hormone does not change. This increase in stress hormones is due to the stimulation itself, and not caused by the stress of fighting. Similar increases in corticosterone are observed during electrical stimulation of the hypothalamic self-grooming area. The corticosterone response during self-grooming-evoking stimulation is negatively correlated with the amount of self-grooming observed, suggesting that circulating corticosterone exerts a negative feedback control on grooming. Earlier literature, and preliminary data form our laboratory, show that circulating corticosterone exerts a fast positive feedback control over brain mechanisms involved in aggressive behaviour. Such findings suggest that the hormonal responses caused by the activity of behaviourally specific areas of the hypothalamus may be part of a regulation mechanism involved in facilitating or inhibiting the very behavioural responses that can be evoked from those areas. We suggest that studying such mechanisms may provide a new approach to behavioural dysfunctions associated with endocrine disorders and stress.

Inhibitory effects evoked from the anterior hypothalamus are selective for the nociceptive responses of dorsal horn neurons with high- and low-threshold inputs

The aim of the present study was to examine the selectivity of descending control of nociceptive information in the spinal dorsal horn following neuronal activation at "pressor" sites in the anterior hypothalamus. Extracellular single-unit activity was recorded from 11 dorsal horn neurons in the lower lumbar spinal cord of anesthetized rats. Neurons selected for investigation were those that responded to noxious (pinch and radiant heat >46 degrees C) and nonnoxious (prod, stroke, and/or brush) stimulation within their cutaneous receptive fields on the ipsilateral hind paw. These are referred to as Class 2 neurons. Micropipettes were inserted stereotaxically into the anterior hypothalamus at sites where injection of the excitatory amino acid L-homocysteic acid (L-HCA) evoked increases in arterial blood pressure. The effects of microinjection of L-HCA at "pressor" sites in the anterior hypothalamus were then tested on the responses of Class 2 neurons to noxious and nonnoxious stimulation of their excitatory receptive fields. The high-threshold (pinch and/or radiant heat) responses of 7/7 Class 2 neurons tested were inhibited by an average of 66.3 +/- 8.8% (mean +/- SE) by neuronal activation at hypothalamic pressor sites. The low-threshold (prod) responses of 10/10 Class 2 neurons tested were not inhibited by neuronal activation at hypothalamic pressor sites; in 6 of these cells the response to low-intensity stimulation was increased by between 4 and 20%. Control injections of the inhibitory amino acid gamma-aminobutyric acid (GABA) at the same hypothalamic pressor sites had no significant effects on arterial blood pressure or neuronal activity. With regard to sensory processing in the spinal cord, these data suggest that descending inhibitory control that originates from neurons in pressor regions of the anterior hypothalamus is highly selective for nociceptive inputs to Class 2 neurons.