Defense reaction elicited by microinjection of kainic acid into the medial hypothalamus of the rat: antagonism by a GABAA receptor agonist

Behavioral phenotype, intestinal microbiome, and brain neuronal activity of male serotonin transporter knockout mice

The serotonin transporter (5-HTT) plays a critical role in the regulation of serotonin neurotransmission. Mice genetically deficient in 5-HTT expression have been used to study the physiological functions of 5-HTT in the brain and have been proposed as a potential animal model for neuropsychiatric and neurodevelopmental disorders. Recent studies have provided evidence for a link between the gut-brain axis and mood disorders. However, the effects of 5-HTT deficiency on gut microbiota, brain function, and behavior remain to be fully characterized. Here we investigated the effects of 5-HTT deficiency on different types of behavior, the gut microbiome, and brain c-Fos expression as a marker of neuronal activation in response to the forced swim test for assessing depression-related behavior in male 5-HTT knockout mice. Behavioral analysis using a battery of 16 different tests showed that 5-HTT-/- mice exhibited markedly reduced locomotor activity, decreased pain sensitivity, reduced motor function, increased anxiety-like and depression-related behavior, altered social behavior in novel and familiar environments, normal working memory, enhanced spatial reference memory, and impaired fear memory compared to 5-HTT+/+ mice. 5-HTT+/- mice showed slightly reduced locomotor activity and impaired social behavior compared to 5-HTT+/+ mice. Analysis of 16S rRNA gene amplicons showed that 5-HTT-/- mice had altered gut microbiota abundances, such as a decrease in Allobaculum, Bifidobacterium, Clostridium sensu stricto, and Turicibacter, compared to 5-HTT+/+ mice. This study also showed that after exposure to the forced swim test, the number of c-Fos-positive cells was higher in the paraventricular thalamus and lateral hypothalamus and was lower in the prefrontal cortical regions, nucleus accumbens shell, dorsolateral septal nucleus, hippocampal regions, and ventromedial hypothalamus in 5-HTT-/- mice than in 5-HTT+/+ mice. These phenotypes of 5-HTT-/- mice partially recapitulate clinical observations in humans with major depressive disorder. The present findings indicate that 5-HTT-deficient mice serve as a good and valid animal model to study anxiety and depression with altered gut microbial composition and abnormal neuronal activity in the brain, highlighting the importance of 5-HTT in brain function and the mechanisms underlying the regulation of anxiety and depression.

Rearing behaviour in the mouse behavioural pattern monitor distinguishes the effects of psychedelics from those of lisuride and TBG

Psychedelics alter consciousness and may have potential for drug development. As psychedelics are likely therapeutically active, it is important to study their effects and mechanisms using preclinical models. Here, we examined the effects of phenylalkylamine and indoleamine psychedelics on locomotor activity and exploratory behaviour using the mouse Behavioural Pattern Monitor (BPM). DOM, mescaline, and psilocin reduced locomotor activity at high doses and influenced rearings, an exploratory behaviour, in a characteristic inverted U-shaped dose-response function. Pretreatment with the selective 5-HT_2A antagonist M100907 reversed the drug-induced alterations in locomotor activity, rearings, and jumps after systemic administration of DOM at low doses. However, holepoking at the full range of doses tested was not blocked by M100907. Administration of the hallucinogenic 5-HT_2A agonist 25CN-NBOH induced striking similarities in response to that to psychedelics; these alterations were significantly diminished by M100907, whereas the putatively non-hallucinogenic 5-HT_2A agonist TBG did not affect locomotor activity, rearings, or jumps at the most effective doses. The nonhallucinogenic 5-HT_2A agonist lisuride failed to increase rearing. The results of these experiments provide strong evidence that DOM-elicited increases in rearing are due to mediation by the 5-HT_2A receptor. Finally, discriminant analysis was able to distinguish all four psychedelics from lisuride and TBG based on behavioural performance alone. Thus, increased rearing in mice could provide additional evidence of behavioural differences between hallucinogenic and nonhallucinogenic 5-HT_2A agonists.

Modulation of tonic immobility by GABAA and GABAB receptors of the medial amygdala

Tonic immobility (TI) is a temporary state of profound motor inhibition associated with great danger as the attack of a predator. Previous studies carried out in our laboratory evidenced high Fos-IR in the posteroventral region of the medial nucleus of the amygdala (MEA) after induction of the TI response. Here, we investigated the effects of GABA_A and GABA_B of the MEA on TI duration. Intra-MEA injections of the GABA_A agonist muscimol and GABA_B agonist baclofen reduced TI response, while intra-MEA injections of the GABA_A antagonist bicuculline and GABA_B antagonist phaclofen increased the TI response. Moreover, the effects observed with muscimol and baclofen administrations into MEA were blocked by pretreatment with bicuculline and phaclofen (at ineffective doses per se). Finally, the activation of GABA_A and GABA_B receptors in the MEA did not alter the spontaneous motor activity in the open field test. These data support the role of the GABAergic system of the MEA in the modulation of innate fear.

μ-Opioid and 5-HT1A receptors in the dorsomedial hypothalamus interact for the regulation of panic-related defensive responses

The dorsomedial hypothalamus (DMH) and the dorsal periaqueductal gray (DPAG) have been implicated in the genesis and regulation of panic-related defensive behaviors, such as escape. Previous results point to an interaction between serotonergic and opioidergic systems within the DPAG to inhibit escape, involving µ-opioid and 5-HT1A receptors (5-HT1AR). In the present study we explore this interaction in the DMH, using escape elicited by electrical stimulation of this area as a panic attack index. The obtained results show that intra-DMH administration of the non-selective opioid receptor antagonist naloxone (0.5 nmol) prevented the panicolytic-like effect of a local injection of serotonin (20 nmol). Pretreatment with the selective μ-opioid receptor (MOR) antagonist CTOP (1 nmol) blocked the panicolytic-like effect of the 5-HT1AR agonist 8-OHDPAT (8 nmol). Intra-DMH injection of the selective MOR agonist DAMGO (0.3 nmol) also inhibited escape behavior, and a previous injection of the 5-HT1AR antagonist WAY-100635 (0.37 nmol) counteracted this panicolytic-like effect. These results offer the first evidence that serotonergic and opioidergic systems work together within the DMH to inhibit panic-like behavior through an interaction between µ-opioid and 5-HT1A receptors, as previously described in the DPAG.

Ventromedial hypothalamic neurons control a defensive emotion state

Defensive behaviors reflect underlying emotion states, such as fear. The hypothalamus plays a role in such behaviors, but prevailing textbook views depict it as an effector of upstream emotion centers, such as the amygdala, rather than as an emotion center itself. We used optogenetic manipulations to probe the function of a specific hypothalamic cell type that mediates innate defensive responses. These neurons are sufficient to drive multiple defensive actions, and required for defensive behaviors in diverse contexts. The behavioral consequences of activating these neurons, moreover, exhibit properties characteristic of emotion states in general, including scalability, (negative) valence, generalization and persistence. Importantly, these neurons can also condition learned defensive behavior, further refuting long-standing claims that the hypothalamus is unable to support emotional learning and therefore is not an emotion center. These data indicate that the hypothalamus plays an integral role to instantiate emotion states, and is not simply a passive effector of upstream emotion centers.

DOI:http://dx.doi.org/10.7554/eLife.06633.001

Animals have evolved a large number of ‘defensive behaviors’ to deal with the threat of predators. Examples include reptiles camouflaging themselves to avoid discovery, fish and birds swarming to confuse predators, insects releasing toxic chemicals, and humans readying themselves to fight or flee.

In mammals, defensive behaviors are thought to be mediated by a region of the brain called the amygdala. This structure, which is known as the brain's ‘emotion center’, receives and processes information from the senses about impending threats. It then sends instructions on how to deal with these threats to other regions of the brain including the hypothalamus, which pass them on to the brain regions that control the behavioral, endocrine and involuntary responses of the mammal.

For many years it has been thought that the role of the hypothalamus is to serve simply as a relay for emotion states encoded in the amygdala, rather than as an emotion center itself. However, Kunwar et al. have now challenged this assumption with the aid of a technique called optogenetics, in which light is used to activate specific populations of genetically labeled neurons. When light was used to directly activate neurons within the ventromedial hypothalamus in awake mice, the animals instantly froze and/or fled, just as they would when faced with a predator. Given that the optical stimulation had completely bypassed the amygdala, this suggested that the hypothalamus must be capable of generating this defensive response without any input from the amygdala.

The freezing and fleeing responses resembled the responses to a predator in a number of key ways. Mice chose to avoid areas of their cage in which they had received the stimulation, suggesting that—like a predator—these areas induced an unpleasant emotional state, perhaps akin to anxiety or fear. Freezing and fleeing persisted for several seconds after the stimulation had stopped, just as freezing and fleeing responses to predators do not immediately cease after the threat has gone. And finally, destroying the neurons targeted by the stimulation made mice less likely to avoid one of their main predators, the rat. It also made the animals less anxious.

Overall the results suggest that the hypothalamus may be more than simply a relay for the amygdala, and that ‘amygdala-centric’ views of emotion processing may need to be re-visited.

DOI:http://dx.doi.org/10.7554/eLife.06633.002

Dorsomedial hypothalamus CRF type 1 receptors selectively modulate inhibitory avoidance responses in the elevated T-maze

Corticotropin-releasing factor (CRF) plays a critical role in the mediation of physiological and behavioral responses to stressors. In the present study, we investigated the role played by the CRF system within the dorsomedial hypothalamus (DMH) in the modulation of anxiety- and panic-related responses. Male Wistar rats were administered into the DMH with CRF (125 and 250 ng/0.2 μl, experiment 1) or with the CRFR1 antagonist antalarmin (25 ng/0.2 μl, experiment 2) and 10 min later tested in the elevated T-maze (ETM) for inhibitory avoidance and escape measurements. In clinical terms, these responses have been respectively related to generalized anxiety and panic disorder. To further verify if the anxiogenic effects of CRF were mediated by CRFR1 activation, we also investigated the effects of the combined treatment with CRF (250 ng/0.2 μl) and antalarmin (25 ng/0.2 μl) (experiment 3). All animals were tested in an open field, immediately after the ETM, for locomotor activity assessment. Results showed that 250 ng/0.2μl of CRF facilitated ETM avoidance, an anxiogenic response. Antalarmin significantly decreased avoidance latencies, an anxiolytic effect, and was able to counteract the anxiogenic effects of CRF. None of the compounds administered altered escape responses or locomotor activity measurements. These results suggest that CRF in the DMH exerts anxiogenic effects by activating type 1 receptors, which might be of relevance to the physiopathology of generalized anxiety disorder.

Orexin-neuromodulated cerebellar circuit controls redistribution of arterial blood flows for defense behavior in rabbits

We investigated a unique microzone of the cerebellum located in folium-p (fp) of rabbit flocculus. In fp, Purkinje cells were potently excited by stimulation of the hypothalamus or mesencephalic periaqueductal gray, which induced defense reactions. Using multiple neuroscience techniques, we determined that this excitation was mediated via beaded axons of orexinergic hypothalamic neurons passing collaterals through the mesencephalic periaqueductal gray. Axonal tracing studies using DiI and biotinylated dextran amine evidenced the projection of fp Purkinje cells to the ventrolateral corner of the ipsilateral parabrachial nucleus (PBN). Because, in defense reactions, arterial blood flow has been known to redistribute from visceral organs to active muscles, we hypothesized that, via PBN, fp adaptively controls arterial blood flow redistribution under orexin-mediated neuromodulation that could occur in defense behavior. This hypothesis was supported by our finding that climbing fiber signals to fp Purkinje cells were elicited by stimulation of the aortic nerve, a high arterial blood pressure, or a high potassium concentration in muscles, all implying errors in the control of arterial blood flow. We further examined the arterial blood flow redistribution elicited by electric foot shock stimuli in awake, behaving rabbits. We found that systemic administration of an orexin antagonist attenuated the redistribution and that lesioning of fp caused an imbalance in the redistribution between active muscles and visceral organs. Lesioning of fp also diminished foot shock-induced increases in the mean arterial blood pressure. These results collectively support the hypothesis that the fp microcomplex adaptively controls defense reactions under orexin-mediated neuromodulation.

Chronic unpredictable mild stress alters an anxiety-related defensive response, Fos immunoreactivity and hippocampal adult neurogenesis

Previous results show that elevated T-maze (ETM) avoidance responses are facilitated by acute restraint. Escape, on the other hand, was unaltered. To examine if the magnitude of the stressor is an important factor influencing these results, we investigated the effects of unpredictable chronic mild stress (UCMS) on ETM avoidance and escape measurements. Analysis of Fos protein immunoreactivity (Fos-ir) was used to map areas activated by stress exposure in response to ETM avoidance and escape performance. Additionally, the effects of the UCMS protocol on the number of cells expressing the marker of migrating neuroblasts doublecortin (DCX) in the hippocampus were investigated. Corticosterone serum levels were also measured. Results showed that UCMS facilitates ETM avoidance, not altering escape. In unstressed animals, avoidance performance increases Fos-ir in the cingulate cortex, hippocampus (dentate gyrus) and basomedial amygdala, and escape increases Fos-ir in the dorsolateral periaqueductal gray and locus ceruleus. In stressed animals submitted to ETM avoidance, increases in Fos-ir were observed in the cingulate cortex, ventrolateral septum, hippocampus, hypothalamus, amygdala, dorsal and median raphe nuclei. In stressed animals submitted to ETM escape, increases in Fos-ir were observed in the cingulate cortex, periaqueductal gray and locus ceruleus. Also, UCMS exposure decreased the number of DCX-positive cells in the dorsal and ventral hippocampus and increased corticosterone serum levels. These data suggest that the anxiogenic effects of UCMS are related to the activation of specific neurobiological circuits that modulate anxiety and confirm that this stress protocol activates the hypothalamus-pituitary-adrenal axis and decreases hippocampal adult neurogenesis.

Acute restraint differently alters defensive responses and fos immunoreactivity in the rat brain

Results from a previous study show that rats exposed to acute restraint display anxiogenic-like behavior, evidenced by facilitation of avoidance responses in the elevated T-maze (ETM) model of anxiety. In contrast, escape responses were unaltered by stress exposure. Since ETM avoidance and escape tasks seem to activate distinct sets of brain structures, it is possible that the differences observed with acute restraint are due to particularities in the neurobiological mechanisms which modulate these responses. In the present study, analysis of fos protein immunoreactivity (fos-ir) was used to map areas activated by exposure of male Wistar rats to restraint stress (30 min) previously (30 min) to the ETM. Corticosterone levels were also measured in stressed and non-stressed animals. Confirming previous observations restraint facilitated avoidance performance, an anxiogenic result, while leaving escape unaltered. Performance of the avoidance task increased fos-ir in the frontal cortex, intermediate lateral septum, basolateral amygdala, basomedial amygdala, lateral amygdala, anterior hypothalamus and dorsal raphe nucleus. In contrast, performance of escape increased fos-ir in the ventromedial hypothalamus, dorsolateral periaqueductal gray and locus ceruleus. Both behavioral tasks also increased fos-ir in the dorsomedial hypothalamus. Restraint significantly raised corticosterone levels. Additionally after restraint, fos-ir was predominantly seen in the basolateral amygdala and dorsal raphe of animals submitted to the avoidance task. This data confirms that different sets of brain structures are activated by ETM avoidance and escape tasks and suggests that acute restraint differently alters ETM behavior and the pattern of fos activation in the brain.

NCS-1 in the dentate gyrus promotes exploration, synaptic plasticity, and rapid acquisition of spatial memory

The molecular underpinnings of exploration and its link to learning and memory remain poorly understood. Here we show that inducible, modest overexpression of neuronal calcium sensor 1 (Ncs1) selectively in the adult murine dentate gyrus (DG) promotes a specific form of exploratory behavior. The mice also display a selective facilitation of long-term potentiation (LTP) in the medial perforant path and a selective enhancement in rapid-acquisition spatial memory, phenotypes that are reversed by direct application of a cell-permeant peptide (DNIP) designed to interfere with NCS-1 binding to the dopamine type-2 receptor (D2R). Moreover, the DNIP and the D2R-selective antagonist L-741,626 attenuated exploratory behavior, DG LTP, and spatial memory in control mice. These data demonstrate a role for NCS-1 and D2R in DG plasticity and provide insight for understanding how the DG contributes to the origin of exploration and spatial memory acquisition.

GABA/benzodiazepine receptors in the ventromedial hypothalamic nucleus regulate both anxiety and panic-related defensive responses in the elevated T-maze

It has been shown that facilitation of GABA-mediated neurotransmission in the medial nucleus of the amygdala and the dorsal periaqueductal gray (dPAG) inhibits the escape, but not the inhibitory avoidance response generated in the elevated T-maze test of anxiety (ETM). These defensive behaviors have been associated with panic and generalized anxiety, respectively. Previous evidence indicates that the dorsomedial part of the ventromedial hypothalamus (VMHdm), which is interconnected with these two brain areas, is also part of the neurobiological substrate controlling escape behavior. In the present study, we investigated in male Wistar rats whether the intra-VMHdm injection of GABA-modulating drugs differently affect the two defensive tasks measured in the ETM. The results showed that the microinjection of the benzodiazepine (BZD) receptor agonist midazolam (10, 20 and 40 nmol), the GABA(A) receptor agonist muscimol (2, 4 and 8 nmol) or the GABA(B) receptor agonist baclofen (2, 4 and 8 nmol) impaired inhibitory avoidance and escape performance, an anxiolytic and panicolytic-like effect, respectively. On the other hand, local administration of the BZD inverse agonist FG 7142 (20, 40 and 80 pmol) facilitated both behaviors, suggesting anxiogenic and panicogenic-like effects. These results were not due to motor alterations, since the drugs did not affect exploratory behavior in an open field. The data suggest that GABA(A)/BZD and GABA(B) receptors within the VMHdm are involved not only in the control of panic-related, but also of anxiety-related behaviors.

Hypothalamic neural circuits regulating maternal responsiveness toward infants

A theoretical neural model is developed, along with supportive evidence, to explain how the medial preoptic area (MPOA) of the hypothalamus can regulate maternal responsiveness toward infant-related stimuli. It is proposed that efferents from a hormone-primed MPOA (a) depress a central aversion system (composed of neural circuits between the amygdala, medial hypothalamus, and midbrain) so that novel infant stimuli do not activate defensive or avoidance behavior and (b) excite the mesolimbic dopamine system so that active, voluntary maternal responses are promoted. The effects of oxytocin and maternal experience are included in the model, and the specificity of MPOA effects are discussed. The model may be relevant to the mechanisms through which other hypothalamic nuclei regulate other basic motivational states. In addition, aspects of the model may define a core neural circuitry for maternal behavior in mammals.

Contribution of the ventromedial hypothalamus to generation of the affective dimension of pain

The ventromedial hypothalamus (VMH) is a core structure underlying the generation of affective behaviors to threats. The prototypical threat to an individual is exposure to a noxious stimulus and the dorsomedial division of the VMH (dmVMH) receives nociceptive input. The present study evaluated the contribution of the dmVMH to generation of the affective reaction to pain in rats. Noxious tailshock elicits from rats vocalization afterdischarges (VADs) that have distinct spectrographic characteristics and are a validated model of the affective reaction to pain. VAD-like vocalizations (vocalizations with the same spectral characteristics of VADs) were elicited by stimulation (electrical or chemical) of the dmVMH. Stimulation in the vicinity of the dmVMH was ineffective in eliciting VADs. Manipulation of GABA(A) neurochemistry within the dmVMH altered the threshold for elicitation of VADs by dmVMH stimulation or tailshock. Administration of the GABA(A) antagonist bicuculline or the GABA(A) agonist muscimol into the dmVMH lowered and elevated VAD thresholds, respectively. These treatments did not alter thresholds of other tailshock elicited responses (vocalizations during tailshock or spinal motor reflexes). Bicuculline and muscimol administered into the dmVMH also elevated and lowered the asymptotic level of fear conditioning supported by dmVMH stimulation or tailshock. These findings demonstrate that the dmVMH contributes to the processing of pain affect and that the affective dimension of pain belongs to a broader class of sensory experience that represents threat to the individual.

Rearing on hind legs, environmental novelty, and the hippocampal formation

Many mammals spontaneously rear on their hind legs in response to novelty. The current paper is the first review of rearing behaviour, and is intended to collate findings from different perspectives that are not usually brought together. We suggest that rearing is a useful marker of environmental novelty, that the hippocampal formation is a crucial component of the system controlling rearing in novel environments, and that rearing is one of several ethological measures that can profitably be used to assess hippocampal learning and memory. Consideration is given to the following topics: the possible functions of rearing in information-gathering and escape behaviour; the modulation of rearing by various factors such as anxiety/ fear emotionality; comparative perspectives on rearing; neuroanatomical circuits involved in rearing with particular reference to the hippocampal formation and its afferents and efferents; and the role of the hippocampal formation in uncharted and mismatch environmental novelty. The review concludes with testable predictions about rearing, environmental novelty and the hippocampus.

Role of glutamate receptors in the ventromedial hypothalamus in the regulation of female rat sexual behaviors

Bilateral infusions of glutamate or kainate to the ventromedial hypothalamus (VMH) have been shown previously to produce a rapid inhibition of lordosis in estrogen-primed female rats induced by manual flank stimulation. The present study examined whether glutamate or selective ionotrophic glutamate receptor agonists can disrupt appetitive and consummatory sexual behaviors following bilateral infusions to the VMH of females during copulation with male rats. Ovariectomized, sexually experienced female rats were implanted bilaterally with guide cannulae aimed at the ventrolateral VMH. After recovery from surgery, females were primed with estradiol benzoate and progesterone and infused with different doses of glutamate, AMPA, NMDA or kainate (n=9-10 in each dose group) 3 min prior to a test with sexually vigorous males in bilevel chambers. Glutamate infusions decreased the display of hops and darts and produced a trend toward decreased lordosis. AMPA infusions decreased the display of solicitations, hops and darts, and lordosis. NMDA infusions decreased lordosis and increased defensive behaviors and pacing. Kainate infusions decreased solicitations, hops and darts, and lordosis, and increased defensive behaviors and pacing. These data indicate that the activation of glutamate receptors in the VMH is inhibitory for both appetitive and consummatory aspects of sexual behavior in the female rat.

Call type-specific differences in vocalization-related afferents to the periaqueductal gray of squirrel monkeys (Saimiri sciureus)

In a recent retrograde tracing study in the squirrel monkey, we found that regions in the midbrain periaqueductal gray (PAG) producing different call types when pharmacologically stimulated, receive their input largely from the same structures. The aim of the present study was to find out, whether there are quantitative differences in this input. For this reason, we counted retrogradely labeled neurons in various brain regions after injections of wheatgerm agglutinin-conjugated horseradish peroxidase (WGA-HRP) into three different vocalization-eliciting PAG sites: one site producing non-aversive contact calls (clucking); a second site producing slightly aversive social mobbing calls (cackling); and a third site producing highly aversive defensive threat calls (shrieking). Cell counting was carried out by the help of the optical fractionator technique. Six squirrel monkeys were used, two for each call type. In some regions, marked differences in the number of retrogradely labeled cells between the three call type groups occured. Such regions are the nucl. accumbens, preoptic area, posterior hypothalamus, anterior cingulate cortex, subcallosal gyrus and the nucl. striae terminalis. In some of these regions, the number of retrogradely labeled cells correlated positively (posterior hypothalamus) or negatively (preoptic area, nucl. striae terminalis) with the "aversiveness" of the elicited call type. Other regions of interest, e.g., the dorsomedial prefrontal and precallosal cortex, amygdala and hypothalamic regions surrounding the fornix, revealed no clear differences in their afferent projections to the different vocalization-eliciting PAG sites. The results make clear that distinct vocalization-controlling regions in the PAG receive a qualitatively similar but quantitatively differentiated input.

Role of glutamate ionotropic and benzodiazepine receptors in the ventromedial hypothalamic nucleus on anxiety

The dorsomedial part of the ventromedial hypothalamic nuclei (VMHdm) has been related to the modulation of defensive behavior in mammals. The objective of the present study was to test the hypothesis that administration into the VMHdm of midazolam, a benzodiazepine receptor full agonist, or AP7, a glutamate NMDA receptor antagonist, would produce anxiolytic effects in the elevated plus-maze (EPM) or the Vogel's punished licking tests. Male Wistar rats with unilateral cannulae aimed at the VMHdm received intra-cerebral injections of midazolam (15-60 nmol/0.25 microL), AP7 (0.2-2 nmol/0.3 microL) or saline and were submitted to the behavioral tests. Midazolam (30 nmol) increased the percentage of time spent in open arms of the EPM. AP7, on the other hand, decreased open and enclosed arm exploration. In the Vogel test, however, both midazolam (30-60 nmol) and AP7 increased the number of punished licks. Histological control experiments found no significant effects when the drugs were injected into the nearby lateral hypothalamic area. These results suggest that facilitation of gabaergic or antagonism of glutamatergic neurotransmission in the VMHdm can produce anxiolytic-like effects.

Different systems in the posterior hypothalamic nucleus of rats control theta frequency and trigger movement

Reduced frequency of theta activity is thought to compromise hippocampal function and so behavioural inhibition. The anxiolytic benzodiazepine chlordiazepoxide (CDP) reduces theta frequency when injected into the medial supramammillary nucleus (mSuM), posterior hypothalamic nucleus (PH) and dorsomedial hypothalamic nucleus (DMH). These hypothalamic effects on theta could underlie at least some behavioural effects of benzodiazepines. We have previously shown that in a fixed interval 60-s schedule (FI60), CDP injected into mSuM reduced both theta frequency and behavioural inhibition. The present experiments test the effect of injections into PH and DMH on theta and hippocampal-sensitive behaviour (FI60 and open field ambulation). Systemic CDP (5mg/kg i.p.) released, but PH/CDP (20microg in 0.5microl vehicle) suppressed FI responding, though they both reduced FI theta frequency. In the open field, both CDP i.p. and PH/CDP reduced ambulation, but only the systemic injection reduced ambulation theta frequency. Taken together with previous research, these results support a role for PH in the control of voluntary behaviour. They imply that this function may be suppressed, independently of theta, by benzodiazepines. An anxiolytic effect of PH/CDP in FI60 may, therefore, have been masked by a concurrent action of CDP on the PH motor system. DMH/CDP did not affect behaviour or theta in either experiment, despite the fact that this nucleus is involved in benzodiazepine mediation of risk assessment and the flight response. This suggests that, like the control of theta frequency by the hypothalamus, the neural mechanisms underlying anxiety are distributed in complex networks.

Role of glutamate ionotropic receptors in the dorsomedial hypothalamic nucleus on anxiety and locomotor behavior

The medial hypothalamus is proposed to play an important role in the modulation of defensive responses. Administration of a NMDA receptor antagonist (AP7) into the dorsomedial hypothalamic nucleus (DMH) of rats reduced exploratory behavior in the open field and elevated plus-maze (EPM), but failed to produce anxiolytic effects in the latter test. The objectives of the present work were to test the hypotheses that (i) AP7 injections into the DMH would also fail to induce anxiolytic effects in another model of anxiety, the Vogel's punished licking test; (ii) injection into the DMH of other glutamate ionotropic antagonists would also decreased exploratory behavior; and (iii) the decrease in exploratory activity found after AP7 administration into the DMH does not involve any gross locomotor impairment. Male Wistar rats (n=5-16/group) with cannulas aimed at the DMH were submitted to the following behavioral tests: EPM, Vogel, catalepsy and rota-rod. Diazepam (3 mg/kg) and haloperidol (2.5 mg/kg) were used as positive controls in the Vogel, rota-rod and catalepsy tests. AP7 failed to modify the number of punished licks in the Vogel test. It also did not induce any change on the rota-rod and catalepsy tests. Diazepam increased the number of punished licks and reduced the latency to fall in the rota-rod. Both 7-chlorokynurenic acid (4-8 nmol), an antagonist of the glycine competitive site in the NMDA receptor and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo-[f]-quinoxaline-7-sulphonamide (NBQX, 1-10 nmol), a non-NMDA receptor antagonist, decreased the total distance moved in the EPM. The former compound also decreased open arm exploration at the dose of 4 nmol. The results suggest that the antagonism of ionotropic glutamate receptors in the DMH does not induce anxiolytic effects in the EPM or Vogel tests, but decreases exploratory behavior in a new environment.

Medial amygdala modulation of predator odor-induced unconditioned fear in the rat

This study examined the participation of the medial amygdala (MeA) in unconditioned fear. Rats received ibotenic acid lesions in the MeA or central amygdala (CeA) prior to cat-odor exposure. MeA-lesioned rats exhibited a significant reduction in freezing duration and made frequent contact with a cloth containing cat odor. In contrast, CeA lesions had no significant effects on unconditioned fear. The freezing reduction produced by MeA lesions was not due to a performance deficit because MeA-lesioned rats, unlike CeA-lesioned rats, were capable of freezing in postshock test intervals. Furthermore, MeA lesions did not alter olfactory function and general locomotor activity. Results demonstrate that the MeA plays a major role in modulating predator odor-induced unconditioned fear.

Role of nucleus tractus solitarius 5-HT3 receptors in the defense reaction-induced inhibition of the aortic baroreflex in rats

Different stressful conditions elicit a typical behavior called the defense reaction. Our aim was to determine whether 5-HT3 receptors in the nucleus tractus solitarius (NTS) are involved in 1) the inhibition of the baroreflex bradycardia and 2) the rise in blood pressure, which are known to occur during the defense reaction. In urethane-anesthetized rats, the defense reaction was elicited by electrical stimulation of the dorsomedial nucleus of the hypothalamus (DMH) or the dorsal part of the periaqueductal gray (dPAG). Direct electrical stimulation of the aortic depressor nerve was used to trigger the typical baroreflex responses. Aortic stimulation at high (100-150 microA) and low (50-90 microA) intensity produced a decrease in heart rate of -39 to -44% (relative to baseline, Group 1 responses, n = 113) and -19 to -24% (Group 2 responses, n = 43), respectively. In spontaneously breathing rats, Group 1 and Group 2 bradycardiac responses were inhibited during DMH (-75 +/- 4% and -96 +/- 4%, n = 38 and n = 11, respectively), as well as dPAG (-81 +/- 3% and -95 +/- 4%, n = 36 and n = 10, respectively) stimulation. The aortic baroreflex bradycardia was hardly affected by DMH or dPAG stimulation when bicuculline (5 pmol), a specific GABAA receptor antagonist, had previously been microinjected into the NTS. Likewise, NTS microinjections of granisetron, a specific 5-HT3 receptor antagonist, prevented, in a dose-dependent manner, the baroreflex bradycardia inhibition. In addition, intra-NTS granisetron did not affect the rise in blood pressure induced by either site stimulation. These data show that 5-HT3 receptors in the NTS are involved in the GABAergic inhibition of the aortic baroreflex bradycardia, but not in the rise in blood pressure, occurring during the defense reaction elicited by DMH or dPAG stimulation.

Representations of motivational drives in mesial cortex, medial thalamus, hypothalamus and midbrain

We propose that neural representations of motivational drives, including sexual desire, hunger, thirst, fear, power-dominance, the motivational aspect of pain, the need for sleep, and nurturance, are represented in four areas in the brain. These are located in the medial hypothalamic/preoptic area, the periaqueductal gray matter (PAG) in the midbrain/pons, the midline and intralaminar thalamic nuclei, and in the anterior part of the mesial cortex, including the medial prefrontal and anterior cingulate areas. We attempt to determine the locations of each of these representations within the hypothalamus/preoptic area, periaqueductal gray and cortex, based on the available literature on activation of brain structures by stimuli that evoke these forms of motivation, on the effects of electrical and chemical stimulation and lesions of candidate structures, and on hodological data. We discuss the hierarchical organization of the representations for a given drive, outputs from these representations to premotor structures in the medulla, caudate-putamen, and cortex, and their contributions to involuntary, learned-sequential (operant) and voluntary behaviors.

The medial pain system: neural representations of the motivational aspect of pain

In this article, we propose that the pathways mediating the motivational aspect of pain originate in laminae VII and VIII of the spinal cord, and in the deep layers of the spinal trigeminal complex, and projections from these areas reach three central structures where pain motivation is represented, the ventrolateral quadrant of the periaqueductal gray, posterior hypothalamic nucleus, and intralaminar thalamic nuclei. A final representation of the motivational aspect of pain is located within the anterior cingulate cortex, and this representation receives inputs from the intralaminar nuclei. Outputs from these representations reach premotor structures located in the medulla, striatum, and cingulate premotor cortex. We discuss pathways and structures that provide inputs to these representations, including those involved in producing involuntary (innate) and instrumental responses which occur in response to the recognition of stimuli associated with footshock and other nociceptive stimuli.

Fear and power-dominance drive motivation: neural representations and pathways mediating sensory and mnemonic inputs, and outputs to premotor structures

Based on the available literature on activation of brain structures by fear- and anger-inducing stimuli, on the effects of electrical and chemical stimulation and lesions of candidate structures, and on connectional data, we propose that both the fear and power-dominance drives are represented in four distinct locations: the medial hypothalamus, lateral/dorsolateral periaqueductal gray, midline thalamic nuclei, and medial prefrontal cortex. The hypothalamic fear representation is located in the dorsomedial and posterior hypothalamic nuclei, the midbrain representation in the caudal part of the lateral/dorsolateral periaqueductal gray, the thalamic representation primarily in parts of the paraventricular and reuniens thalamic nuclei, and the cortical representation in prelimbic cortex. The hypothalamic power-dominance representation is located in the anterior hypothalamic nucleus, dorsomedial aspect of the ventromedial nucleus, and in adjacent parts of the medial preoptic area. The corresponding midbrain representation occurs in rostral part of the lateral/dorsolateral periaqueductal gray, and the thalamic representation in parts of the paraventricular, parataenial, and reuniens thalamic nuclei. We discuss sensory/mnemonic inputs to these representations, and outputs to premotor structures in the medulla, caudate-putamen, and cortex, and their differential contributions to involuntary, learned sequential, and voluntary motor acts. We examine potential contributions of neuronal activities in these representations to the subjective awareness of fear and anger.

The medial hypothalamic defensive system: hodological organization and functional implications

The hypothalamus is a relatively small division of the vertebrate forebrain that plays especially important roles in neural mechanisms assuring homeostasis, defense, and reproduction. Previous studies from our laboratory have suggested a distinct circuit in the medial hypothalamic zone as critically involved in the organization of innate defensive behavior. Thus, after exposure to a natural predator known to elicit innate defensive responses, increased Fos levels in the medial zone of the hypothalamus have been found restricted to the anterior hypothalamic nucleus, dorsomedial part of the ventromedial nucleus, and dorsal premammillary nucleus (PMd). Previous anatomical studies have shown that these Fos-responsive cell groups in the medial hypothalamus are interconnected in a distinct neural system, in which the PMd appears to be a critical element for the expression of defensive responses elicited by the presence of a predator. The purpose of this review is to provide an overview of what is currently known about the functional and hodological organization of this hypothalamic circuit subserving defensive responses.

Evidence that the medial amygdala projects to the anterior/ventromedial hypothalamic nuclei to inhibit maternal behavior in rats

The maternal behaviors shown by a rat that has given birth are not shown by a virgin female rat when she is first presented with young. This absence of maternal behavior in virgins has been attributed to the activity of a neural circuit that inhibits maternal behavior in nulliparae. The medial amygdala and regions of the medial hypothalamus such as the anterior and ventromedial hypothalamic nuclei have previously been shown to inhibit maternal behavior, in that lesions to these regions promote maternal responding. Furthermore, we have recently shown that these and other regions, such as the principal bed nucleus of the stria terminalis, the ventral lateral septum, and the dorsal premammillary nucleus, show higher pup-induced Fos-immunoreactivity in non-maternal rats exposed to pups than during the performance of maternal behavior, indicating that they too could be involved in preventing maternal responsiveness. The current study tested whether the medial amygdala projects to the anterior/ventromedial hypothalamic nuclei in a neural circuit that inhibits maternal behavior, as well as to see what other brain regions could participate in this circuit. Bilateral excitotoxic lesions of the medial amygdala, or of the anterior/ventromedial hypothalamic nuclei, promoted maternal behavior. Unilateral medial amygdala lesions caused a reduction of pup-induced Fos-immunoreactivity in the anterior/ventromedial hypothalamic nuclei in non-maternal rats ipsilateral to the lesion, as well as in the principal bed nucleus of the stria terminalis, ventral lateral septum, and dorsal premammillary nucleus. Finally, unilateral medial amygdala lesions paired with contralateral anterior/ventromedial hypothalamic nuclei lesions promoted maternal behavior, although ipsilateral lesion placements were also effective.Together, these results indicate that the medial amygdala projects to the anterior/ventromedial hypothalamic nuclei in a neural circuit that inhibits maternal behavior, and that the principal bed nucleus of the stria terminalis, ventral lateral septum, and dorsal premammillary nucleus could also be involved in this circuit.

Tracing from the dorsal premammillary nucleus prosencephalic systems involved in the organization of innate fear responses

The dorsal premammillary nucleus (PMd) is thought to play a critical role in the expression of fear responses to environmental threats. We have previously reported that, during an encounter with a predator, the PMd presents an impressive increase in Fos levels and cell body-specific chemical lesions therein virtually eliminated the expression of escape and freezing responses. Therefore, the PMd may be viewed as a strategic starting point to delineate prosencephalic circuits seemingly critical for the organization of innate fear responses. In the present review, we provide a comprehensive examination of the neural circuits putatively involved in influencing this hypothalamic site, and supplement this analysis with recent observations from our laboratory on the expression of Fos protein in the central nervous system of rats exposed to a live predator.

Effects of excitatory amino acids and nitric oxide on flight behavior elicited from the dorsolateral periaqueductal gray

Microinjection of excitatory amino acids (EAA) into the dorsolateral periaqueductal gray (dlPAG) induces flight reactions while EAA antagonists show anxiolytic effects. Part of the effects mediated by NMDA receptors may involve an increase in nitric oxide (NO) production. We showed that nitric oxide synthase (NOS) inhibitors injected into the dlPAG induced anxiolytic effects. Conversely, SIN-1, a NO donor, produced orientated flight reactions that resemble stimulation of the medial hypothalamus. This compound also produced extensive Fos-like immunoreactivity in this region and in other areas related to defensive reactions such as the medial amygdala and cingulate cortex. Since part of the effects of NO involves increases in guanylate cyclase levels, we found that intra-dlPAG injection of 8-Br-cGMP induced a brief flight reaction followed by increased locomotion. In another experiment, we showed that single or repeated restraint stress produced an increased expression of neuronal NOS in the dlPAG and other areas related to defense, as measured by in situ hybridization, diaphorase histochemistry and immunocytochemistry. Together, these data suggest that NO may participate in the modulation of defensive responses in the dlPAG.

Differential expression of Fos protein in the rat brain induced by performance of avoidance or escape in the elevated T-maze

Analysis of Fos protein expression was used to map brain areas activated by exposure of male Wistar rats to the elevated T-maze, an animal model of anxiety where tasks of inhibitory avoidance or one-way escape can be separately performed. The apparatus consists of three elevated arms--one enclosed and two open. In the inhibitory avoidance task--considered to represent learned fear--the time taken by rats to leave from the enclosed arm in three consecutive trials is measured. One-way escape task is measured by recording the time taken by animals to withdraw from the open arm and is thought to reflect innate fear. Control animals were placed three times at the end of the transversal arm of a T-maze composed of three enclosed arms and withdrawal latencies from this arm was similarly measured. Performance of avoidance task increased Fos-like immunoreactivity in the medial amygdaloid nucleus, in the anterior hypothalamic nucleus and in the median raphe nucleus. In contrast, performance of escape task enhanced Fos-like immunoreactivity in the basolateral amygdaloid nucleus and in the dorsal periaqueductal gray matter of the mesencephalon. Both behavioural tasks promoted an increase in Fos-like immunoreactivity in the paraventricular nucleus of the thalamus and in the dorsomedial hypothalamic nucleus. Therefore, the obtained results indicate that different sets of brain structures were, respectively, activated by inhibitory avoidance and one-way escape. This evidence supports the original hypothesis that two types of fear/anxiety are generated in the elevated T-maze.

GABAergic and glutamatergic modulation of exploratory behavior in the dorsomedial hypothalamus

Systemic injection of glutamate NMDA receptor antagonists or drugs that facilitate GABA(A)-mediated neurotransmission produces anxiolytic effects. The dorsomedial hypothalamic (DMH) region is proposed to be a possible site of action of these drugs. The objective of the present study was to investigate if facilitation of GABA(A)-mediated neurotransmission or blockade of NMDA receptors in the DMH would produce anxiolytic effects in the elevated plus-maze (EPM). Seven days after surgery, male Wistar rats with unilateral cannulas in the DMH were submitted to the behavioral studies. Results showed that midazolam, a benzodiazepine anxiolytic (30-60 nmol/0.3 microl), produced a dose-dependent increase in open arm exploration without changing the number of enclosed arm entries, indicating an anxiolytic effect. This effect was antagonized by previous treatment with flumazenil, a benzodiazepine receptor antagonist (60 nmol/0.3 microl). Flumazenil alone had an anxiogenic effect, decreasing exploration of the open arms of the EPM. 2-Amino-7-phosphonoheptanoic acid (AP7), an NMDA receptor antagonist (0.2-2 nmol/0.3 microl), did not modify open arm exploration but decreased general exploratory activity. These results indicate that benzodiazepine receptors located in the DMH could modulate anxiety. Interference with NMDA receptor-mediated neurotransmission in this region, however, seems to change general exploratory activity rather than anxiety.

Afferents to the central nucleus of the amygdala and functional subdivisions of the periaqueductal gray: neuroanatomical substrates for affective behavior

Evidence suggests the periaqueductal gray (PAG) is involved in the integration of behavioral and autonomic components of affective behavior. Our laboratory has shown that electrical stimulation of the ventrolateral periaqueductal gray (vl PAG) versus the dorsolateral periaqueductal gray (dl PAG), in the rabbit, elicits two distinct behavioral/cardiorespiratory response patterns. Furthermore, evidence suggests that the amygdaloid central nucleus (ACe) may influence cardiovascular activity during emotional states. The purpose of this study was to delineate the topography and determine the origin of forebrain projections to the PAG and the ACe, as well as commonalties and differences in the pattern of afferents. Examination of common afferents may lend insights into their function as components of a forebrain system regulating autonomic activity during emotional states. Separate retrograde tracers were injected into functional subdivisions of the PAG and the ACe in rabbits. PAG injections led to neuronal labeling in numerous cortical regions including the ipsilateral medial prefrontal and insular cortices. Additionally, bilateral labeling was observed in several hypothalamic nuclei including the paraventricular nucleus, the dorsomedial nucleus and the ventromedial nucleus as well as the region lateral to the descending column of the fornix. Sparse labeling was also seen in various basal forebrain regions, thalamic nuclei and amygdaloid nuclei. Many of these regions were also labeled following injections in the ACe. Although double-labeled cells were never observed, afferents to the ACe were often proximal to PAG afferents. Implications of these findings are discussed in terms of two functionally distinct behavioral/cardiovascular response patterns.

Rodent anxiety and kindling of the central amygdala and nucleus basalis

We studied lasting behavioral effects of kindling of three parts of the central nucleus of the amygdala and the anterior nucleus basalis in the right hemisphere of male Wistar rats. Kindling lastingly changed two measures of anxiety in the elevated plus-maze. The nature of the change depended on the location of the kindled focus. Kindling of the posterior central nucleus decreased both open-arm exploration and frequency of risk assessment in the elevated plus-maze 1 week after the fourth stage 5 seizure. Kindling of the middle parts of the central nucleus was without behavioral effects. Kindling of the anterior central nucleus and the anterior nucleus basalis increased risk assessment, which was interpreted as an anxiolytic effect. Changes in risk assessment produced by kindling of the central nucleus were dependent on open-arm avoidance, whereas the effects of nucleus basalis kindling were independent of open-arm avoidance. Analysis of covariance and factor analysis support the view that control of risk assessment is by circuitry, which is independent of that which controls open-arm avoidance. Moreover, part of this circuitry appears to involve the anterior nucleus basalis. Changes in plus-maze behavior were independent of changes in exploration or activity in either the plus-maze or hole board. These findings add to a growing body of evidence that suggests that subtle differences in location of a kindled focus within the rat amygdala lead to different behavioral outcomes.

Using c-Fos immunocytochemistry to identify forebrain regions that may inhibit maternal behavior in rats

Evidence indicates there is a neural system that inhibits maternal behavior in virgin rats. It has been suggested that pregnancy hormones promote the onset of maternal behavior by reducing the behavioral influence of this system. The authors used c-Fos immunocytochemistry to identify brain regions more activated by pup exposure in nonmaternal rats than in maternal rats. Previous experiments indicated that some of these regions, such as the posterodorsal medial amygdala and several medial hypothalamic sites, inhibit maternal behavior. For others, such as the ventral lateral septum, dorsal premammillary nucleus, and principal bed nucleus of the stria terminalis, this is the first indication that they could also inhibit maternal responding. These regions have previously been implicated in promoting defensive behaviors, consistent with the finding that nonmaternal rats actively avoid pups. These findings suggest the existence of a neural circuit through which pup exposure could promote defensive responses in virgin rats, and how pregnancy hormones could reduce such activity to stimulate maternal behavior.

Behavioral and c-fos expression changes induced by nitric oxide donors microinjected into the dorsal periaqueductal gray

The dorsolateral periaqueductal gray matter (DLPAG) is a key structure in behavioral and autonomic expression of defensive reactions. Inhibitors of nitric oxide (NO) synthase microinjected into the DLPAG are anxiolytic. The purpose of the present study was to evaluate the behavioral effects produced by administration of two NO donors, SIN-1 or DEA/NO, into the DLPAG. We also employed the detection of Fos-like immunoreactivity (FLI) to reveal brain areas activated by SIN-1 administration. Male Wistar rats (n = 7-11/group) received intra-DLPAG injection of SIN-1 (150 or 300 nmol), DEA/NO (150, 300, or 600 nmol), or saline (0.5 microl), and their behavior was observed in an open arena during 15 min. For the FLI assay, the animals (n = 3-5/group) were sacrificed 2.25 h after the drug. In a second experiment, rats (n = 5-7/group) received a first intra-DLPAG injection of saline or methylene blue (MB, 30 nmol), an NO antagonist, followed by saline or SIN-1 (300 nmol). SIN-1 (300 nmol) induced a flight response characterized by coordinated running and oriented jumps with escape attempts. Similar but short-lasting changes were seen after the administration of DEA/NO. FLI was dose-dependently induced by SIN-1 in several regions related to defensive reactions, including the periaqueductal gray, hypothalamic nuclei, medial amygdala, and cingulate cortex (analysis of variance, p < 0.05). A greater number of neurons showing FLI was found ipsilateral to the microinjection site. The drug effect was greater at this side in the bed nucleus of the stria terminalis, paraventricular and lateral hypothalamus, cingulate cortex, septohipoccampal nucleus, and horizontal limb of the diagonal band. The increase in SIN-1-induced FLI was attenuated by MB pretreatment in most regions. These results suggest that NO may participate in the modulation of defensive responses in the DLPAG.

Neural plasticity, neuropeptides and anxiety in animals--implications for understanding and treating affective disorder following traumatic stress in humans

Exposure of rats to cats (predator stress) lastingly increases rodent anxiety-like behavior (ALB) in the elevated plus-maze. Previous work shows that lasting changes in ALB following predator stress depend on NMDA and CCKB receptors. In this paper we describe the effects of differing degrees of predator exposure on behavior. Effects depend on the behavioral measure. In general, exposure to predator odor is less provocative of lasting change in ALB than is unprotected exposure to a cat. In addition, we examine the development of effects of unprotected predator exposure over time. Lasting effects on ALB begin at 30 min to 1 h after predator stress and persist for at least 3 weeks. We also report a complex pattern of effects of predator stress on neuroendocrine and stress peptide (bombesin, CRF and AVP) levels in a variety of brain areas. Not surprisingly, predator exposure increases plasma levels of corticosterone and ACTH. Central changes in peptide content in the hypothalamo-pituitary axis, related hypothalamic nuclei, limbic and brain stem areas are also noted. Finally, path analysis demonstrates a replicable relationship between cat behavior, rat defensive behavior and degree of increase in ALB one week later. It is proposed that behavioral changes following predator stress may model anxiety associated with PTSD.

NMDA receptors mediate lasting increases in anxiety-like behavior produced by the stress of predator exposure--implications for anxiety associated with posttraumatic stress disorder

It has been proposed that NMDA-dependent long-term potentiation (LTP) of limbic system circuits controlling defensive behavior underlies stressor-induced lasting increases in anxiety-like behavior (ALB). Findings in cats given the stress-inducing beta-carboline, FG-7142, support this hypothesis. An animal model of lasting affective change following traumatic stress has recently been developed. In this model, lasting increases in anxiety-like behavior (ALB) assessed in the elevated plus maze are produced by a single 5-min exposure of a rat to a cat. Rats become more anxious in the plus maze for up to 3 weeks after the exposure. The present study demonstrates that blockade of NMDA receptors in rats with MK-801, AP7, or CPP, given 30 min prior to exposure to a cat, prevents the increase in ALB assessed 1 week later. MK-801 or AP7, given 30 min after exposure to a cat, do not prevent the increase in ALB seen 1 week later, however. MK-801, but not CPP or AP7, promotes approaches to cats during exposure. This "fearlessness" may reflect some anxiolytic action of MK-801. Approach to cats following injection of MK-801 was eliminated by prior injection of Prazosin. Prazosin did not interfere with the block of increases in ALB following cat exposure, however. These findings are consistent with the view that NMDA receptors are involved in initiation, but not maintenance of neural changes mediating lasting increases in anxiety following severe stress. The significance of these findings for PTSD are discussed.

Suprapontine control of respiration

Despite focus on brainstem areas in central respiratory control, regions rostral to the medulla and pons are now recognized as being important in modulating respiratory outflow during various physiological states. The focus of this review is to highlight the role that suprapontine areas of the mammalian brain play in ventilatory control mechanisms. New imaging techniques have become invaluable in confirming and broadening our understanding of the manner in which the cerebral cortex of humans contributes to respiratory control during volitional breathing. In the diencephalon, the integration of respiratory output in relation to changes in homeostasis occurs in the caudal hypothalamic region of mammals. Most importantly, neurons in this region are strongly sensitive to perturbations in oxygen tension which modulates their level of excitation. In addition, the caudal hypothalamus is a major site for 'central command', or the parallel activation of locomotion and respiration. Furthermore, midbrain regions such as the periaqueductal gray and mesencephalic locomotor region function in similar fashion as the caudal hypothalamus with regard to locomotion and more especially the defense reaction. Together these suprapontine regions exert a strong modulation upon the basic respiratory drive generated in the brainstem.

Evidence that NMDA-dependent limbic neural plasticity in the right hemisphere mediates pharmacological stressor (FG-7142)-induced lasting increases in anxiety-like behavior. Study 1--Role of NMDA receptors in efferent transmission from the cat amygdala

The anxiogenic beta-carboline, FG-7142, produces intense anxiety in humans and anxiety-like behavior in animals. FG-7142 also mimics the effects of exogenous stressors. In cats, FG-7142 lastingly changes defensive and aggressive behavior. Long-term potentiation (LTP) of neural transmission between limbic structures known to modulate feline defensive response to threat accompany behavioral changes. A series of three reports describes experiments designed to test the hypothesis that behavioral changes depend upon an N-methyl-D-aspartate (NMDA) receptor-based LTP of efferent transmission from the amygdala. This first study characterizes the dose and time effects of injection of the NMDA receptor blocker 7-amino-phosphono-heptanoic acid (AP7) on efferent transmission from the cat amygdala to the ventromedial hypothalamus (VMH). Effects of doses of 0.5-10mg/kg (i.v.) of AP7 on potentials evoked in the VMH by single pulse stimulation of the basal amygdala were examined. In order to localize the action of the drug, concurrent measurements were taken of potentials evoked in the VMH by stimulation of the efferent fibers from the amygdala to the VMH (ventral amygdalofugal pathway, VAF). There was a dose-dependent reduction in the amygdalo-VMH evoked potential. The greatest reduction occurred at 5 mg/kg. Effects peaked at 10 min, and persisted for at least 1 h after injection. In contrast, AP7 increased the VAF-VMH-evoked potential at 10 min after injection, with a maximal increase at 5mg/kg. The data suggest that NMDA receptors intrinsic to the amygdala modulate excitatory efferent transmission from amygdala to VMH in the cat. It is speculated that a glutamatergic projection to gamma-aminobutyric acid tonic inhibitory systems in the VMH accounts for the VAF-VMH results.