Central amygdaloid nucleus lesion attenuates exaggerated hemodynamic responses to noise stress in the spontaneously hypertensive rat

Mitochondrial Dysfunction and Changes in High-Energy Compounds in Different Cellular Models Associated to Hypoxia: Implication to Schizophrenia

Schizophrenia (SZ) is a multifactorial mental disorder, which has been associated with a number of environmental factors, such as hypoxia. Considering that numerous neural mechanisms depends on energetic supply (ATP synthesis), the maintenance of mitochondrial metabolism is essential to keep cellular balance and survival. Therefore, in the present work, we evaluated functional parameters related to mitochondrial function, namely calcium levels, mitochondrial membrane potential, redox homeostasis, high-energy compounds levels and oxygen consumption, in astrocytes from control (Wistar) and Spontaneously Hypertensive Rats (SHR) animals exposed both to chemical and gaseous hypoxia. We show that astrocytes after hypoxia presented depolarized mitochondria, disturbances in Ca²⁺ handling, destabilization in redox system and alterations in ATP, ADP, Pyruvate and Lactate levels, in addition to modification in NAD⁺/NADH ratio, and Nfe2l2 and Nrf1 expression. Interestingly, intrauterine hypoxia also induced augmentation in mitochondrial biogenesis and content. Altogether, our data suggest that hypoxia can induce mitochondrial deregulation and a decrease in energy metabolism in the most prevalent cell type in the brain, astrocytes. Since SHR are also considered an animal model of SZ, our results can likewise be related to their phenotypic alterations and, therefore, our work also allow an increase in the knowledge of this burdensome disorder.

Essential Role of Ovarian Hormones in Susceptibility to the Consequences of Witnessing Social Defeat in Female Rats

BACKGROUND

Women are at greater risk than men of developing depression and comorbid disorders such as cardiovascular disease. This enhanced risk begins at puberty and ends following menopause, suggesting a role for ovarian hormones in this sensitivity. Here we used a model of psychosocial witness stress in female rats to determine the stress-induced neurobiological adaptations that underlie stress susceptibility in an ovarian hormone-dependent manner.

METHODS

Intact or ovariectomized (OVX) female rats were exposed to five daily 15-minute witness-stress exposures. Witness-stress-evoked burying, behavioral despair, and anhedonia were measured. Cardiovascular telemetry was combined with plasma measurements of inflammation, epinephrine, and corticosterone as indices of cardiovascular dysfunction. Finally, levels of interleukin-1β and corticotropin-releasing factor were assessed in the central amygdala.

RESULTS

Witness stress produced anxiety-like burying, depressive-like anhedonia, and behavioral despair selectively in intact female rats, which was associated with enhanced sympathetic responses during stress, including increased blood pressure, heart rate, and arrhythmias. Moreover, intact female rats exhibited increases in 12-hour resting systolic pressure and heart rate and reductions in heart rate variability. Notably, OVX female rats remained resilient. Moreover, intact, but not OVX, female rats exposed to witness stress exhibited a sensitized cytokine and epinephrine response to stress and distinct increases in levels of corticotropin-releasing factor and interleukin-1β in the central amygdala.

CONCLUSIONS

Together these data suggest that ovarian hormones play a critical role in the behavioral, inflammatory, and cardiovascular susceptibility to social stress in female rats and reveal putative systems that are sensitized to stress in an ovarian hormone-dependent manner.

Central mechanisms regulating coordinated cardiovascular and respiratory function during stress and arousal

Actual or potentially threatening stimuli in the external environment (i.e., psychological stressors) trigger highly coordinated defensive behavioral responses that are accompanied by appropriate autonomic and respiratory changes. As discussed in this review, several brain regions and pathways have major roles in subserving the cardiovascular and respiratory responses to threatening stimuli, which may vary from relatively mild acute arousing stimuli to more prolonged life-threatening stimuli. One key region is the dorsomedial hypothalamus, which receives inputs from the cortex, amygdala, and other forebrain regions and which is critical for generating autonomic, respiratory, and neuroendocrine responses to psychological stressors. Recent studies suggest that the dorsomedial hypothalamus also receives an input from the dorsolateral column in the midbrain periaqueductal gray, which is another key region involved in the integration of stress-evoked cardiorespiratory responses. In addition, it has recently been shown that neurons in the midbrain colliculi can generate highly synchronized autonomic, respiratory, and somatomotor responses to visual, auditory, and somatosensory inputs. These collicular neurons may be part of a subcortical defense system that also includes the basal ganglia and which is well adapted to responding to threats that require an immediate stereotyped response that does not involve the cortex. The basal ganglia/colliculi system is phylogenetically ancient. In contrast, the defense system that includes the dorsomedial hypothalamus and cortex evolved at a later time, and appears to be better adapted to generating appropriate responses to more sustained threatening stimuli that involve cognitive appraisal.

α1 and α2-adrenoceptors in the medial amygdaloid nucleus modulate differently the cardiovascular responses to restraint stress in rats

Medial amygdaloid nucleus (MeA) neurotransmission has an inhibitory influence on cardiovascular responses in rats submitted to restraint, which are characterized by both elevated blood pressure (BP) and intense heart rate (HR) increase. In the present study, we investigated the involvement of MeA adrenoceptors in the modulation of cardiovascular responses that are observed during an acute restraint. Male Wistar rats received bilateral microinjections of the selective α1-adrenoceptor antagonist WB4101 (10, 15, and 20 nmol/100 nL) or the selective α2-adrenoceptor antagonist RX821002 (10, 15, and 20 nmol/nL) into the MeA, before the exposure to acute restraint. The injection of WB4101 reduced the restraint-evoked tachycardia. In contrast, the injection of RX821002 increased the tachycardia. Both drugs had no influence on BP increases observed during the acute restraint. Our findings indicate that α1 and α2-adrenoceptors in the MeA play different roles in the modulation of the HR increase evoked by restraint stress in rats. Results suggest that α1-adrenoceptors and α2-adrenoceptors mediate the MeA-related facilitatory and inhibitory influences on restraint-related HR responses, respectively.

Neuroanatomical substrates involved in cannabinoid modulation of defensive responses

Administration of Cannabis sativa derivatives causes anxiolytic or anxiogenic effects in humans and laboratory animals, depending on the specific compound and dosage used. In agreement with these findings, several studies in the last decade have indicated that the endocannabinoid system modulates neuronal activity in areas involved in defensive responses. The mechanisms of these effects, however, are still not clear. The present review summarizes recent data suggesting that they involve modulation of glutamate and GABA-mediated neurotransmission in brain sites such as the medial prefrontal cortex, amygdaloid complex, bed nucleus of the stria terminalis, hippocampus and dorsal periaqueductal gray. Moreover, we also discuss results indicating that, in these regions, the endocannabinoid system could be particularly engaged by highly stressful situations.

Cardiovascular responses to microinjection of noradrenaline into the medial amygdaloid nucleus of conscious rats result from α₂-receptor activation and vasopressin release

The medial amygdaloid nucleus (MeA) is involved in the modulation of physiological and behavioral processes, as well as regulation of the autonomic nervous system. Moreover, MeA electrical stimulation evokes cardiovascular responses. Thus, as noradrenergic receptors are present in this structure, the present study tested the effects of local noradrenaline (NA) microinjection into the MeA on cardiovascular responses in conscious rats. Moreover, we describe the types of adrenoceptor involved and the peripheral mechanisms involved in the cardiovascular responses. Increasing doses of NA (3, 9, 27 or 45 nmol/100 nL) microinjected into the MeA of conscious rats caused dose-related pressor and bradycardic responses. The NA cardiovascular effects were abolished by local pretreatment of the MeA with 10 nmol/100 nL of the specific α₂-receptor antagonist RX821002, but were not affected by local pretreatment with 10 nmol/100 nL of the specific α₁-receptor antagonist WB4101. The magnitude of pressor response evoked by NA microinjected into the MeA was potentiated by intravenous pretreatment with the ganglion blocker pentolinium (5 mg/kg), and blocked by intravenous pretreatment with the selective V₁-vasopressin antagonist dTyr(CH₂)₅ (Me)AVP (50 μg/kg). In conclusion, our results show that microinjection of NA into the MeA of conscious rats activates local α₂-adrenoceptors, evoking pressor and bradycardic responses, which are mediated by vasopressin release.

Stress-induced changes in c-Fos and corticotropin releasing hormone immunoreactivity in the amygdala of the spontaneously hypertensive rat

The present study was undertaken to test the hypothesis that dysregulation of the amygdala contributes to the exaggerated autonomic response to stress in an animal model of essential hypertension. Spontaneously hypertensive (SHR) and normotensive Wistar male rats were chronically instrumented and exposed to 20 min of either air jet stress (AJS) or air noise alone (CON). AJS induced a significant increase in both heart rate and arterial pressure that was greater in the SHR. AJS induced a significant increase in c-Fos-like immunoreactivity (FLI) throughout the caudal-rostral extent of the basolateral, medial, and central (CEA) subnuclei of the amygdala. Differences in FLI between strains were localized to the rostral CEA and the SHR expressed significantly less FLI. AJS also induced a significant increase in the number of corticotrophin releasing hormone (CRH) positive neurons in the CEA. Differences between strains were localized to the caudal CEA and the number of CRH-positive cells was significantly greater in the SHR. The stress-induced increase in CRH labeling in caudal CEA of the SHR was coupled to a greater increase in FLI in the rostral locus coeruleus (LC) of the SHR versus the Wistar. AJS also induced significant increases in FLI in several hypothalamus subnuclei, but no strain-related differences were identified. These results suggest for the first time that dysregulation of CRH-positive cells in the caudal CEA and reduced excitation and/or exaggerated inhibition of rostral CEA neurons may contribute to the exaggerated cardiovascular response to stress in the SHR, possibly through descending modulation of the rostral LC.

Endocannabinoid system and fear conditioning

The endocannabinoid system has been proposed to modulate neuronal functions involved in distinct types of defensive reactions, possibly counteracting the harmful consequences of stressful stimuli. However, the precise brain sites for this action remain to be further explored. This chapter summarizes the data about the role of the endocannabinoid system in the processing of conditioned fear as well as the potential neural subtract for its actions.

A review of neuroimaging studies of stressor-evoked blood pressure reactivity: emerging evidence for a brain-body pathway to coronary heart disease risk

An individual's tendency to show exaggerated or otherwise dysregulated cardiovascular reactions to acute stressors has long been associated with increased risk for clinical and preclinical endpoints of coronary heart disease (CHD). However, the 'brain-body' pathways that link stressor-evoked cardiovascular reactions to CHD risk remain uncertain. This review summarizes emerging neuroimaging research indicating that individual differences in stressor-evoked blood pressure reactivity (a particular form of cardiovascular reactivity) are associated with activation patterns in corticolimbic brain areas that are jointly involved in processing stressors and regulating the cardiovascular system. As supported empirically by activation likelihood estimates derived from a meta-analysis, these corticolimbic areas include divisions of the cingulate cortex, insula, and amygdala--as well as networked cortical and subcortical areas involved in mobilizing hemodynamic and metabolic support for stress-related behavioral responding. Contextually, the research reviewed here illustrates how behavioral medicine and health neuroscience methods can be integrated to help characterize the 'brain-body' pathways that mechanistically link stressful experiences with CHD risk.

Role of the bed nucleus of the stria terminalis in the cardiovascular responses to acute restraint stress in rats

The aim of this work was to test the hypothesis that the bed nucleus of the stria terminalis (BST) and noradrenergic neurotransmission therein mediate cardiovascular responses to acute restraint stress in rats. Bilateral microinjection of the non-specific synaptic blocker CoCl(2) (0.1 nmol/100 nl) into the BST enhanced the heart rate (HR) increase associated with acute restraint without affecting the blood pressure increase, indicating that synapses within the BST influence restraint-evoked HR changes. BST pretreatment with the selective alpha(1)-adrenoceptor antagonist WB4101 (15 nmol/100 nl) caused similar effects to cobalt, indicating that local noradrenergic neurotransmission mediates the BST inhibitory influence on restraint-related HR responses. BST treatment with equimolar doses of the alpha(2)-adrenoceptor antagonist RX821002 or the beta-adrenoceptor antagonist propranolol did not affect restraint-related cardiovascular responses, reinforcing the inference that alpha(1)-adrenoceptors mediate the BST-related inhibitory influence on HR responses. Microinjection of WB4101 into the BST of rats pretreated intravenously with the anticholinergic drug homatropine methyl bromide (0.2 mg/kg) did not affect restraint-related cardiovascular responses, indicating that the inhibitory influence of the BST on the restraint-evoked HR increase could be related to an increase in parasympathetic activity. Thus, our results suggest an inhibitory influence of the BST on the HR increase evoked by restraint stress, and that this is mediated by local alpha(1)-adrenoceptors. The results also indicate that such an inhibitory influence is a result of parasympathetic activation.

Individual differences in stressor-evoked blood pressure reactivity vary with activation, volume, and functional connectivity of the amygdala

Individuals who exhibit exaggerated blood pressure reactions to psychological stressors are at risk for hypertension, ventricular hypertrophy, and premature atherosclerosis; however, the neural systems mediating exaggerated blood pressure reactivity and associated cardiovascular risk in humans remain poorly defined. Animal models indicate that the amygdala orchestrates stressor-evoked blood pressure reactions via reciprocal signaling with corticolimbic and brainstem cardiovascular-regulatory circuits. Based on these models, we used a multimodal neuroimaging approach to determine whether human individual differences in stressor-evoked blood pressure reactivity vary with amygdala activation, gray matter volume, and functional connectivity with corticolimbic and brainstem areas implicated in stressor processing and cardiovascular regulation. We monitored mean arterial pressure (MAP) and concurrent functional magnetic resonance imaging BOLD signal changes in healthy young individuals while they completed a Stroop color-word stressor task, validated previously in epidemiological studies of cardiovascular risk. Individuals exhibiting greater stressor-evoked MAP reactivity showed (1) greater amygdala activation, (2) lower amygdala gray matter volume, and (3) stronger positive functional connectivity between the amygdala and perigenual anterior cingulate cortex and brainstem pons. Individual differences in amygdala activation, gray matter volume, and functional connectivity with corticolimbic and brainstem circuits may partly underpin cardiovascular disease risk by impacting stressor-evoked blood pressure reactivity.

The expression of contextual fear conditioning involves activation of an NMDA receptor-nitric oxide pathway in the medial prefrontal cortex

The ventral portion of medial prefrontal cortex (vMPFC) is involved in contextual fear-conditioning expression in rats. In the present study, we investigated the role of local N-methyl-D-aspartic acid (NMDA) glutamate receptors and nitric oxide (NO) in vMPFC on the behavioral (freezing) and cardiovascular (increase of arterial pressure and heart rate) responses of rats exposed to a context fear conditioning. The results showed that both freezing and cardiovascular responses to contextual fear conditioning were reduced by bilateral administration of NMDA receptor antagonist LY235959 (4 nmol/200 nL) into the vMPFC before reexposition to conditioned chamber. Bilateral inhibition of neuronal NO synthase (nNOS) by local vMPFC administration of the Nω-propyl-L-arginine (N-propyl, 0.04 nmol/200 nL) or the NO scavenger carboxy-PTIO (1 nmol/200 nL) caused similar results, inhibiting the fear responses. We also investigated the effects of inhibiting glutamate- and NO-mediated neurotransmission in the vMPFC at the time of aversive context exposure on reexposure to the same context. It was observed that the 1st exposure results in a significant attenuation of the fear responses on reexposure in vehicle-treated animals, which was not modified by the drugs. The present results suggest that a vMPFC NMDA–NO pathway may play an important role on expression of contextual fear conditioning.

Intra-amygdala injection of GABAA agonist, muscimol, reduces tachycardia and modifies cardiac sympatho-vagal balance during restraint stress in rats

At present, little is known about the brain origin of stress-induced cardiac sympathetic drive responsible for stress-induced tachycardia. Our aim was to determine the effect of bilateral microinjections of the GABA(A) receptor agonist, muscimol, into the amygdaloid complex on both the heart rate and cardiac autonomic activity during restraint stress. Experiments were performed in male Sprague-Dawley rats (n=9), with pre-implanted electrocardiographic electrodes. Heart rate increased sharply after the onset of the restraint and reached a peak 1-2 min later (from 344+/-6-440+/-20 BPM). Subsequently, heart rate began to fall, and during the next 10-15 min approached the steady-state level of 384+/-11. After vehicle, mean heart rate during each of three 10-min restraint epochs was significantly higher compared with the pre-restraint level. After muscimol, mean heart rate was significantly elevated only during the first 10 min of restraint. There was no difference in the early peak tachycardia between both conditions. Muscimol substantially accelerated the fall of the HR from the peak to the steady-state level, and thus the area under the curve value for muscimol (503+/-162 BPM x min) was significantly smaller than that for vehicle (1221+/-231 BPM x min); P<0.05. After vehicle, the high-frequency spectral power of the heart rate decreased and the low-frequency power increased during the restraint, resulting in a significant rise of the low frequency/high frequency ratio from 1.2+/-0.2-2.8+/-0.6 (n=9, P<0.05). Muscimol suppressed these stress-induced effects. We conclude that inhibition of the amygdala neurons abolishes the sustained component of tachycardia during the restraint, has no effect on the early tachycardic component, and prevents stress-induced alterations in the heart rate variability indices.

Role of limbic peptidergic circuits in regulation of arterial pressure, relevant to development of essential hypertension

It is generally accepted that the essential hypertension (EH) is caused by interactions among congenital gene, multiple pathogenetic pressor factors, and disorder of physiologic depressor factors. The central nervous system may play a key role in the development of EH. The underlying mechanisms, however, are not well understood. Studies show that peptidergic transmitters in the limbic forebrain are involved in long-term regulation of arterial pressure and in the pathogenesis of EH. In the limbic forebrain there are peptidergic pressor and depressor circuits. The former includes corticotropin releasing factor-, substance P-, and angiotensin II-circuits; and the latter includes beta-endorphin- and atrial natriuretic peptide-circuits. These circuits extensively interconnect and interact with each other. The altered functions of them may be the pathogenesis of EH. In this review, we focus on the roles of limbic peptidergic circuits in regulation of arterial pressure, relevant to the neurogenetic mechanisms in developing EH.

Involvement of medial prefrontal cortex neurons in behavioral and cardiovascular responses to contextual fear conditioning

To explore the ventral medial prefrontal cortex (vMPFC) involvement in behavioral and autonomic fear-conditioned responses to context, vMPFC synaptic transmission was temporarily inhibited by bilateral microinjections of 200 nL of the nonselective synapse blocker CoCl(2) (1 mM). Behavioral activity (freezing, motor activity and rearing) as well as evoked cardiovascular responses (arterial pressure and heart rate) was analyzed. Rats were pre-exposed to the footshock chamber (context) and shock stimulus was used unconditioned stimulus. During re-exposure to context, conditioned rats spent 80% of the session in freezing while non-conditioned rats (no shock group) spent less than 15% of the session time in freezing. Conditioned rats had significantly lower activity scores than non-conditioned animals. Exposure to context increased mean arterial pressure (MAP) and heart rate (HR) of both groups. MAP and HR of the conditioned animals were markedly increased and remained at a high and stable level, whereas MAP and HR increases in non-conditioned animals were less pronounced and declined during the session. CoCl(2) microinjected in the vMPFC significantly reduced freezing and attenuated MAP and HR increase of the conditioned group. Cobalt-induced vMPFC inhibition also significantly reduced MAP and HR increase observed in non-conditioned animals, without any behavioral changes. The effect of vMPFC acute ablation on MAP and HR did not seem to be specific to the fear response because they were also evident in non-conditioned animals. The results indicate that vMPFC integrity is crucial for expression of fear-conditioned responses to context, such as freezing and cardiovascular changes, suggesting that fear-conditioned responses to context involve cortical processing prior to amygdalar output. They also indicate a cardiovascular response observed during re-exposure of non-conditioned rats to the context is completely dependent on vMPFC integrity.

Effects of cannabidiol and diazepam on behavioral and cardiovascular responses induced by contextual conditioned fear in rats

Cannabidiol (CBD) is a non-psychotomimetic compound from Cannabis sativa that induces anxiolytic-like effects similar to diazepam in animal models of innate aversive behavior. However, the effects of CBD contextual conditioned fear have not been studied. Therefore, the aim of this work was to compare the behavioral and cardiovascular effects of CBD and diazepam, a prototype anxiolytic, in animals submitted to a contextual conditioned fear paradigm. Male Wistar rats were submitted to a 10min conditioning session (six footshocks, 2.5 mA, 3s, delivered at pseudo-random intervals). The behavioral and cardiovascular responses to the context were measured 24h later in a 10 min test session. Diazepam (2.5 mg/kg), FG-7142 (8 mg/kg), a benzodiazepine inverse agonist, or CBD (10 mg/kg) were administered i.p. before the test session. Conditioned rats submitted to the aversive context exhibited more freezing behavior and a larger increase in blood pressure and heart rate as compared to non-conditioned animals. These effects were attenuated by CBD and diazepam in the conditioned animals. These drugs did not have any effect in non-conditioned rats. FG-7142 treatment failed to change the behavioral and cardiovascular responses to the aversive context. In conclusion, the results suggest that CBD has anxiolytic-like properties similar to those of diazepam in a rat model of conditioned fear to context.

Chronic cold stress alters prefrontal cortical modulation of amygdala neuronal activity in rats

BACKGROUND

Recent studies suggest that long-term exposure to stress can sensitize animals to subsequent novel or acute stressors. Stressors affect amygdala activity, and the prefrontal cortex has been implicated in the regulation of responses to stress. Little is known, however, about how the physiology of amygdala neurons is altered by chronic stressors or the role of the prefrontal cortex in these changes.

METHODS

We used in vivo extracellular recordings from neurons in the rat central and basolateral amygdala nuclei to examine the effects of chronic stress on the basal firing and responses of amygdala neurons to a novel stressor. Additionally, prefrontal cortical afferents were severed to examine its role in the modulation of the response to stressors.

RESULTS

Chronic exposure to cold enhanced the sensitivity of central amygdala neurons to footshock. A portion of this may be due to enhanced basolateral amygdala output. Furthermore, prefrontal cortical regulation of this response is weakened by chronic stress.

CONCLUSIONS

The physiology of the amygdala is altered by chronic stress. Furthermore, the prefrontal cortical regulation of these responses may be weakened after chronic stress. This is a potential biological substrate for abnormal affect upon chronic stress and its effect on affective disorders.

Pressor and tachycardic responses evoked by microinjections of l-glutamate into the medial prefrontal cortex of unanaesthetized rats

The ventral medial prefrontal cortex (vMPFC) is involved in central cardiovascular control. In the present study, we studied the cardiovascular effects of injections of L-glutamate into the vMPFC of unanaesthetized rats and the mechanisms of these effects. Male Wistar rats were used and L-glutamate was microinjected in the vMPFC in a final volume of 200 nL. Microinjections of L-glutamate (9, 27, 81, 150 or 300 nmol) caused long-lasting, dose-related pressor and tachycardic responses in unanaesthetized rats. No differences were observed among cardiovascular responses when L-glutamate was injected into the three sub-areas that comprise the vMPFC, namely the prelimbic, the infralimbic and the dorsal peduncular cortices. No responses were observed when the dose of 81 nmol of L-glutamate was microinjected into surrounding structures such as the cingulate cortex area 1, the corpus callosum and the tenia tecta, indicating a predominant action on the vMPFC. The cardiovascular response to L-glutamate into the vMPFC was blocked by intravenous pretreatment with the ganglion blocker pentolinium (10 mg/kg, i.v.) or the beta1-adrenoceptor antagonist atenolol (1.5 mg/kg, i.v.), supporting the involvement of the cardiac sympathetic nervous system in the response to L-glutamate. Pretreatment with the muscarinic antagonist homatropine methyl bromide (1 mg/kg, i.v.) reduced the latency to the onset of the pressor and tachycardic responses to L-glutamate injected into the vMPFC without significant effects on response duration or maximum effect. We conclude that stimulation of the vMPFC with L-glutamate caused pressor and tachycardic responses in unanaesthetized rats, responses which were dependent on cardiac sympathetic nerve activation and were potentiated by blockade of peripheral muscarinic receptors.

ROLE OF THE CENTRAL NUCLEUS OF THE AMYGDALA IN THE CONTROL OF BLOOD PRESSURE: DESCENDING PATHWAYS TO MEDULLARY CARDIOVASCULAR NUCLEI

1. One of the key areas that links psychologically induced stress with the blood pressure-regulatory system is the central nucleus of the amygdala (CeA). This is an integratory forebrain nucleus that receives input from higher centres in the forebrain and has extensive connections with the hypothalamus and the medulla oblongata, areas involved in the regulation of the cardiovascular reflexes. 2. Based on studies using electrical or chemical stimulation or electrolytic lesions of the CeA, it has become clear that the CeA plays an important role in the regulation of blood pressure in response to stressful or fearful stimuli. 3. Two important medullary areas known to receive projections from the CeA are the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM). The NTS is the site of the first synapse for afferent fibres originating from baroreceptors, chemoreceptors and the heart, whereas the RVLM contains neurons that maintain resting blood pressure and sympathetic nerve activity via projections to sympathetic preganglionic neurons in the intermediolateral cell column of the thoracolumbar spinal cord. 4. Electron microscopic studies using combined anterograde tracing and pre- and post-embedding immunogold labelling have shown that the pathways originating from the CeA to the NTS are inhibitory and may use GABA as a neurotransmitter. The results of these studies suggest that blood pressure changes produced by activation of the CeA may be mediated by attenuation of baroreceptor reflexes through a GABAergic mechanism at the level of the NTS. 5. Neuronal tract tracing combined with neurofunctional studies using the Fos protein as a marker of activated neurons indicate that the CeA projects directly to baroreceptive neurons in the NTS and RVLM that are activated by changes in blood pressure. 6. In conclusion, studies that have examined the efferent pathways of the CeA suggest that CeA neurons with projections to medullary baroreceptive neurons may play a vital role in the reflex changes in sympathetic nerve activity that are involved in blood pressure regulation in response to stress or anxiety.

Does the amygdala modulate adaptation to repeated stress?

Exposure of the rat to restraint results in activation of the hypothalamic-pituitary-adrenal (HPA) axis, a characteristic pattern of c-fos expression in the brain and increased cardiovascular function. These responses adapt with repeated exposure of an individual to the same stress. Corticosterone secretion habituates, and c-fos mRNA expression in the paraventricular nucleus of the hypothalamus (PVN) decreases. The increased expression of corticotropin releasing hormone mRNA in the PVN also becomes less prominent, whereas vasopressin mRNA progressively increases. The neural mechanisms responsible for this adaptation remain obscure. Because of its role in conditioned learning, we have hypothesised that the amygdala might be involved in this adaptive process. Here we show that large neurotoxic lesions of the amygdala in male rats do not prevent acute stress activation of the HPA axis following 30 min restraint, whilst more discrete lesions of the central nucleus actually exacerbate the acute response. Rats with large amygdala lesions demonstrate delayed habituation of corticosterone and c-fos to repeated restraint, an affect not apparent with central nucleus lesions. Furthermore we show that neither type of lesion significantly reduced tachycardiac responses to single or repeated restraint as measured by telemetry. We conclude that the amygdala and the central nucleus are not necessary for HPA and cardiovascular activation in response to stress (though the central nucleus may modulate it), and that adaptation to repeated stress is only modestly dependent upon the amygdala.

Subfornical organ-angiotensin II pressor system takes part in pressor response of emotional circuit

It has been proved that there are the subfornical organ (SFO)-nucleus paraventricularis (NPV)-rostral ventrolateral medulla (RVL) angiotension II (AngII) pressor system and the central amygdaloid nucleus (AC)-lateral hypothalamus/perifornical region (LH/PF) emotional pressor system in the brain. Because the LH/PF contains abundant AngII ergic neurons projecting to the SFO, the purpose of the present study was to examine whether the (SFO-NPV-RVL) AngII pressor system takes part in the AC-pressor response via AngII ergic neurons in the LH/PF. The results showed that (1) L-glutamate microinjection into the AC or LH/PF induced pressor responses. (2) Both the AC- and LH/PF-pressor responses could be reversed by preinjection of [Sar(1), Thr(8)]-angiotensin II (an antagonist of AngII) into either the SFO, NPV or RVL. Taken together with our previous findings that the projections of the CRF-ergic and SP-ergic neurons in the AC could activate the LH/PF, the above findings prove that: besides several known mechanisms of the brain AngII inducing pressor response, the (SFO-NPV-RVL) AngII pressor system also takes part in the AC-emotional pressor response via AngII ergic projections from the LH/PF to the SFO, which may be the neurophysiological basis of the brain AngII playing an important role in developing hypertension of the SHRs.

Differential impact of audiogenic stressors on Lewis and Fischer rats: behavioral, neurochemical, and endocrine variations

Exposure to intense noise can trigger a cascade of neuroendocrine events reminiscent of a stress response, including activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Using male Fischer and Lewis rats, which exhibit differences in their corticosterone response to stressors, this investigation assessed effects of acute noise exposure on neurochemical and neuroendocrine responses. In response to the noise exposure, Fischer rats displayed greater plasma adrenocorticotropin-releasing hormone (ACTH) and corticosterone responses than their Lewis counterparts. However, both strains responded with similar increases of plasma prolactin, suggesting that strain differences in the HPA response were not likely because of differences in noise perception. Post-mortem analyses revealed that noise exposure induced strain-dependent variations of corticotropin-releasing hormone (CRH) across several brain regions. These effects were evident irrespective of whether the rats were noise exposed in a familiar (home cage) or unfamiliar environment. In vivo, dynamic assessment of immunoreactive (ir)-CRH at the pituitary gland revealed that noise exposure elicited an immediate rise in ir-CRH among Fischer rats, relative to the delayed response in Lewis rats. Similarly, the rise in local interstitial corticosterone was more rapid and pronounced in Fischer rats. In contrast to these differences, ir-CRH released at the central nucleus of the amygdala (CeA) was gradual and protracted following noise exposure in both strains. Behaviorally, the Fischer rats displayed an active stress response, whereas the Lewis strain adopted freezing as a defensive style. The role of CRH in the genesis of the overall strain-dependent response to stressors is discussed.

The human amygdala and the emotional evaluation of sensory stimuli

A wealth of animal data implicates the amygdala in aspects of emotional processing. In recent years, functional neuroimaging and neuropsychological studies have begun to refine our understanding of the functions of the amygdala in humans. This literature offers insights into the types of stimuli that engage the amygdala and the functional consequences that result from this engagement. Specific conclusions and hypotheses include: (1) the amygdala activates during exposure to aversive stimuli from multiple sensory modalities; (2) the amygdala responds to positively valenced stimuli, but these responses are less consistent than those induced by aversive stimuli; (3) amygdala responses are modulated by the arousal level, hedonic strength or current motivational value of stimuli; (4) amygdala responses are subject to rapid habituation; (5) the temporal characteristics of amygdala responses vary across stimulus categories and subject populations; (6) emotionally valenced stimuli need not reach conscious awareness to engage amygdala processing; (7) conscious hedonic appraisals do not require amygdala activation; (8) activation of the amygdala is associated with modulation of motor readiness, autonomic functions, and cognitive processes including attention and memory; (9) amygdala activations do not conform to traditional models of the lateralization of emotion; and (10) the extent and laterality of amygdala activations are related to factors including psychiatric status, gender and personality. The strengths and weakness of these hypotheses and conclusions are discussed with reference to the animal literature.

The neural correlates of aversive auditory stimulation

Previous neuroimaging studies indicate that the human amygdala activates during exposure to aversive visual, olfactory and gustatory stimuli. To examine amygdala responses to aversive auditory stimuli, we exposed healthy human subjects to unpleasant sounds while regional cerebral blood flow (rCBF) was assayed with O-15 PET. Eight subjects, all of whom described themselves as reactive to aversive sounds, participated in the study. Relative to white noise, the aversive sounds produced significant rCBF increases in the lateral amygdala/claustrum region. Significant activations also localized to the dorsal brainstem, medial temporal pole, basal forebrain (nucleus accumbens), insula, right auditory association cortices, putamen, thalamus and cerebellum. These data indicate that the amygdala responds to aversive auditory stimuli in a manner similar to its response to unpleasant stimuli in other sensory modalities. The data further highlight a widely distributed network of cortical and subcortical areas activated during exposure to aversive sounds.

Beta-endorphin- and GABA-mediated depressor effect of specific electroacupuncture surpasses pressor response of emotional circuit

It has been proved that input of specific electroacupuncture (EA) can activate beta-endorphin(beta-EP)ergic and noradrenergic neurons projecting to the rostral ventrolateral medulla (RVL), the latter acting upon the RVL-GABAergic interneurons, thereby produce depressor effect. The present study further shows that: (1) The EA depressor effect is strong enough to surpass the pressor response of the AC (nucleus amygdaloideus centralis)-emotional circuit, (2) both beta-endorphin (beta-EP) and GABA in the RVL mediate the EA antagonistic effect, (3) the EA effect does not take place in the AC and paraventricular nucleus (two key nuclei besides the RVL, which also have beta-EPergic input) in the emotional circuit.

Cardiovascular and behavioural responses to psychological stress in spontaneously hypertensive rats: effect of treatment with DSP-4

We used a model of psychological stress combining exposure to an open-field novel environment, radio-telemetric measurement of blood pressure and heart rate, and behavioural tracking analysis of behavioural parameters. All rats showed significant increases in blood pressure and heart rate for the duration of open-field exposure, with spontaneously hypertensive rats (SHR) showing markedly greater pressor responses and tachycardia when compared to either Wistar-Kyoto (WKY) or Sprague-Dawley rats (SD rats). Behavioural responses in the open-field were unrelated to the magnitude of cardiovascular responses. Open-field exposure on 4 consecutive days induced similar pressor responses and tachycardia on each day. By contrast, behavioural responses were reduced from the second day of open-field exposure. Treatment of SHR and WKY rats with DSP-4, to deplete central noradrenaline levels, did not affect cardiovascular responses in SHR, whereas WKY rats showed a trend towards inhibition. However, in WKY rats, but not SHR, DSP-4 treatment caused a marked reduction in behavioural activity in the open-field. In conclusion, these data show that: (1) SHR display marked cardiovascular hyperreactivity to psychological open-field stress when compared to two normotensive rat strains; (2) unlike behavioural responses, cardiovascular stress responses do not habituate upon repeated stress exposure; and (3) noradrenergic projections from the locus coeruleus do not appear to play a major role in cardiovascular stress responses in SHR or WKY rats, although they may be involved in behavioural responses in WKY rats.

Behavioral, neurochemical and endocrinological characterization of the early social isolation syndrome

Rearing rats in isolation has been shown to be a relevant paradigm for studying early life stress and understanding the genesis of depression and related affective disorders. Recent studies from our laboratory point to the relevance of studying the social isolation syndrome as a function of home caging conditions. Accordingly, the present series of experiments assessed the contribution of each condition to the expression of the prepulse inhibition of the acoustic startle, food hoarding and spontaneous locomotor activity. In addition, ex vivo neurochemical changes in the brains of isolated and grouped rats reared either in sawdust-lined or in grid-floor cages were determined by measuring dopamine and serotonin as well as their major metabolites in a "psychosis circuit" that includes mainly the hippocampus and selected hippocampal efferent pathways projecting towards the anterior cingulate and infralimbic cortices, nucleus accumbens, dorsolateral caudate nucleus, amygdala and entorhinal cortex. The results of the present study demonstrate that rearing rats in isolation (i) produces a syndrome of generalized locomotor hyperactivity; (ii) increases the startle response; (iii) impairs prepulse inhibition; (iv) tends to increase food hoarding behavior; (v) increases basal dopamine turnover in the amygdaloid complex; (vi) decreases basal dopamine turnover in the infralimbic part of the medial prefrontal cortex; and (vii) decreases basal turnover of serotonin in the nucleus accumbens. In the entorhinal cortex, dopamine neurotransmission seemed to be more sensitive to the caging conditions since a decreased basal turnover of dopamine was observed in grid-reared animals. Plasma corticosterone levels were also increased in grid-reared animals compared with rats reared in sawdust cages. Finally, isolates reared on grids showed a significant positive correlation between plasma corticosterone levels and dopamine in the left nucleus accumbens.Altogether, these results support the contention that there is a link between social isolation, attention deficit, spontaneous locomotor hyperactivity and reduced dopamine turnover in the medial prefrontal cortex. Furthermore, our data demonstrate that rearing rats in grid-floor cages represents a form of chronic mild stress associated with increased corticosterone levels, decreased basal turnover of entorhinal dopamine and increased dopamine activity in the left nucleus accumbens. Finally, a significant and selective decrease in the basal turnover of serotonin in the nucleus accumbens of isolated rats may be linked to the isolation-induced locomotor hyperactivity.

Cardiovascular responses to NMDA injected into nuclei of hypothalamus or amygdala in conscious rats

The nuclei of hypothalamus and amygdala have been shown to be involved in the central cardiovascular homeostasis. Recent studies suggest that glutamate-containing neurons have an important role in the regulation of the central cardiovascular function. In this study, we demonstrate the roles of the central nucleus of the amygdala and the paraventricular nucleus of the amygdala and the paraventricular nucleus or the dorsomedial nucleus of the hypothalamus in N-methyl-D-aspartate (NMDA) induced blood pressure and heart rate changes in conscious Sprague-Dawley rats. Intracerebroventricular or parenchymal injections of NMDA evoke increases in arterial pressure. The NMDA-induced elevations in blood pressure are more prominent when NMDA is administered into the dorsomedial nucleus of the hypothalamus. Microinjections of NMDA into the dorsomedial hypothalamus exert significant heart rate increases, whereas NMDA when administered into the paraventricular nucleus of the hypothalamus or into the central nucleus of the amygdala has no significant effect on the heart rate. The dorsomedial nucleus of the hypothalamus is found to be the most effective site in this respect. The present study provides strong evidence for the tonic glutamatergic influence on blood pressure and heart rate via NMDA receptors located within the dorsomedial nucleus and to a lesser extent via those located within the paraventricular nucleus of the hypothalamus.

Release of glutamate and GABA in the amygdala of conscious rats by acute stress and baroreceptor activation: differences between SHR and WKY rats

To reveal the functional importance of amino acid neurotransmission in the amygdala (AMY) of conscious spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats, the in vivo release of glutamate (GLU) and GABA in this brain structure was studied using the push-pull superfusion technique. Basal GLU and GABA release rates in the AMY were comparable in SHR and WKY rats, although arterial blood pressure (BP) in SHR (152+/-6 mmHg) was higher than in WKY rats (102+/-4 mmHg). Neuronal depolarization by superfusion with veratridine enhanced the release of GLU and GABA to a similar extent in both rat strains. On the other hand, exposure to noise stress (95 dB) for 3 min led to a tetrodotoxin-sensitive increase in GLU release in the AMY of SHR, but not WKY rats. The concurrent pressor response to noise was enhanced in SHR as compared to WKY rats. A rise in BP induced by intravenous infusion of phenylephrine for 9 min had no effect on amino acid release in the AMY of both strains. The data suggest an exaggerated stress response of glutamatergic neurons in the AMY of SHR as compared with WKY rats, which might be of significance for the strain differences in the cardiovascular and behavioural responses to stress. The results also show that, in both rat strains, glutamatergic and GABAergic neurons in the AMY are not modulated by baroreceptor activation. Moreover, hypertension in adult SHR does not seem to be linked to a disturbed synaptic regulation of glutamatergic or GABAergic transmission in the AMY.

Corticotropin releasing factor and substance P mediate the nucleus amygdaloideus centralis-nucleus ventromedialis-nucleus dorsomedialis pressor system

Prolonged emotional stress is an important factor in the development of neurogenic hypertension, but its mechanism is still unclear. The purpose of the present study is to analyze the possible neural basis of hypertension induced by prolonged emotional stress. In the brain many nuclei are involved in emotional reaction, stress or defense response; among them the nucleus amygdaloideus centralis (AC) is the most important one which widely connects with other nuclei controlling emotion and stress, such as nucleus ventromedialis (NVM), nucleus dorsomedialis (NDM), nucleus paraventricularis (NPV) etc. These nuclei contain corticotropin releasing factor (CRF)- and substance P (SP)-immunoreactive cell bodies, nerve terminals and corresponding receptors. Our previous and present studies showed that microinjection of CRF or SP into these nuclei induced pressor responses. These data imply that excitation of the AC can activate many nuclei controlling emotion and stress via CRF and SP, and excessive activities of these nuclei may be the neural basis of hypertension induced by prolonged emotional stress. The present study revealed that (1) the AC pressor response to glutamate (Glu) could be reduced by preinjection of CRF antagonist (alpha-Helical CRF[9-41] or SP antagonist ([D-Pro(2), D-Phe(7), D-Trp(9)]-substance P) into bilateral NVM, (2) the NVM pressor response to Glu were decreased by pretreatment of the NDM with CRF- or SP-antagonist, (3) the AC-, NVM- or NDM-pressor responses were all attenuated by preinjection of CRF- or SP-antagonist into bilateral NPV or rostral ventrolateral medulla (RVL). The results indicate that excitation of the AC can indirectly activate the NPV and RVL to evoke pressor response via the NVM-NDM, CRF and SP are transmitters in each connection of this pathway; this is one component of the mechanism underlying the AC pressor response. Taken together with the findings of our previous studies, it provides neurophysiological basis for the above-mentioned implications.

Corticotropic-releasing hormone and serotonin interact in the human brainstem: behavioral implications

The objective of this human post mortem study was to determine whether neurons which synthesize corticotropic-releasing hormone and serotonin form circuits implicated in the pathophysiology of major depression and suicide. For the first time, a sensitive, dual immunocytochemical procedure was used to identify circuits formed by corticotropic-releasing hormone-synthesizing and serotonergic cell groups. Corticotropic-releasing hormone-immunoreactive varicose fibers and puncta with morphological characteristics of terminals were labeled in the midline raphe, periventricular gray and pontine parabrachial complex, on single-labeled tissues processed immunocytochemically with a rabbit antibody to rat/human corticotropic-releasing hormone. Presumptive synaptic interactions with monoaminergic neurons were demonstrated with dual labeling techniques. Corticotropic-releasing hormone-immunoreactive terminals apposed neuronal somata and primary dendrites of serotonergic neurons in the pontine raphe. Serotonergic neurons were immunolabeled with a mouse antibody to phenylalanine hydroxylase, an enzyme with substantial sequence homology to tryptophan hydroxylase. Interactions in the lateral parabrachial nucleus were suggested by precise overlap of corticotropic-releasing hormone and serotonergic terminal fields. Corticotropic-releasing hormone projections were confirmed to noradrenergic neurons containing neuromelanin in the locus ceruleus. Maps of corticotropic-releasing hormone fiber trajectories suggest that these pathways may derive from the forebrain and, locally, from the human homologue of Barrington's nucleus--a neurochemically specialized division of the laterodorsal tegmental complex. Chemosensory functions were predicted by novel evidence for corticotropic-releasing hormone- and monoaminergic neurovascular and subependymal fiber plexuses. In conclusion, corticotropic-releasing hormone may influence the activity of two major monoaminergic cell systems implicated in the stress-diathesis model of mental illness, through neural and humoral mechanisms.

Neuroendocrine responses to an emotional stressor: evidence for involvement of the medial but not the central amygdala

The amygdala plays a pivotal role in the generation of appropriate responses to emotional stimuli. In the case of emotional stressors, these responses include activation of the hypothalamic-pituitary-adrenal (HPA) axis. This effect is generally held to depend upon the central nucleus of the amygdala, but recent evidence suggests a role for the medial nucleus. In the present study, c-fos expression, amygdala lesion and retrograde tracing experiments were performed on adult rats in order to re-evaluate the role of the central as opposed to the medial amygdala in generating neuroendocrine responses to an emotional stressor. Brief restraint (15 min) was used as a representative emotional stressor and was found to elicit c-fos expression much more strongly in the medial than central nucleus of the amygdala; relatively few Fos-positive cells were seen in other amygdala nuclei. Subsequent experiments showed that ibotenic acid lesions of the medial amygdala, but not the central amygdala, greatly reduced restraint-induced activation of cells of the medial paraventricular nucleus, the site of the tuberoinfundibular corticotropin-releasing factor cells that constitute the apex of the HPA axis. Medial amygdala lesions also reduced the activation of supraoptic and paraventricular nucleus oxytocinergic neurosecretory cells that commonly accompanies stress-induced HPA axis activation in rodents. To assess whether the role of the medial amygdala in the control of neuroendocrine cell responses to emotional stress might involve a direct projection to such cells, retrograde tracing of amygdala projections to the paraventricular nucleus was performed in combination with Fos immunolabelling. This showed that although some medial amygdala cells activated by exposure to an emotional stressor project directly to the paraventricular nucleus, the number is very small. These findings provide the first direct evidence that it is the medial rather than the central amygdala that is critical to hypothalamic neuroendocrine cell responses during an emotional response, and also provide the first evidence that the amygdala governs oxytocin as well as HPA axis responses to an emotional stressor.

Cardiovascular effects of intracerebroventricular bicuculline in rats with absence seizures

Gamma-aminobutyric acid (GABA) plays an important role in both central cardiovascular homeostasis and pathogenesis of epileptic seizures. Previous studies have indicated a critical role of the amygdala in the spread of seizures from brainstem to forebrain and in the regulation of autonomic responses such as blood pressure and heart rate. The purpose of the present study was to examine blood pressure and heart rate effects of bicuculline, a GABA(A) antagonist, and the effect of lesions of the central or the basolateral nuclei of the amygdala on bicuculline-induced cardiovascular responses in conscious WAG/Rij rats with absence epilepsy. Intracerebroventricular administration of 0.3 and 0.5 nmol of bicuculline produces an increase in blood pressure and a slight bradycardia in non-epileptic Wistar rats. The blood pressure response to intracerebroventricular bicuculline is significantly potentiated in epileptic WAG/Rij rats. Bilateral lesions of the basolateral nucleus of the amygdala of WAG/Rij rats completely prevent the pressor response to 0.5 nmol of bicuculline, whereas unilateral lesion of the basolateral nucleus does not affect blood pressure changes in epileptic WAG/Rij rats. Additionally, the pressor effect of 0.5 nmol of bicuculline is not attenuated by bilateral electrolytic ablation of the central nucleus of the amygdala in WAG/Rij rats. Heart rate response to bicuculline is not significantly changed in the lesioned groups. These findings indicate (a) altered GABAergic function in blood pressure regulation; and (b) a critical role of the basolateral nucleus in GABA(A)-mediated blood pressure control in epileptic WAG/Rij rats.

Forebrain pathways mediating stress-induced hormone secretion

Exposure to hostile conditions initiates the secretion of several hormones, including corticosterone/cortisol, catecholamines, prolactin, oxytocin, and renin, as part of the survival mechanism. Such conditions are often referred to as "stressors" and can be divided into three categories: external conditions resulting in pain or discomfort, internal homeostatic disturbances, and learned or associative responses to the perception of impending endangerment, pain, or discomfort ("psychological stress"). The hormones released in response to stressors often are referred to as "stress hormones" and their secretion is regulated by neural circuits impinging on hypothalamic neurons that are the final output toward the pituitary gland and the kidneys. This review discusses the forebrain circuits that mediate the neuroendocrine responses to stressors and emphasizes those neuroendocrine systems that have previously received little attention as stress-sensitive hormones: renin, oxytocin, and prolactin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABAA, histamine, and serotonin receptors alter the neuroendocrine stress response. The effects of these drugs are discussed in relation to their effects on forebrain neural circuits that regulate stress hormone secretion. For psychological stressors such as conditioned fear, the neural circuits mediating neuroendocrine responses involve cortical activation of the basolateral amygdala, which in turn activates the central nucleus of the amygdala. The central amygdala then activates hypothalamic neurons directly, indirectly through the bed nucleus of the stria terminalis, and/or possibly via circuits involving brainstem serotonergic and catecholaminergic neurons. The renin response to psychological stress, in contrast to those of ACTH and prolactin, is not mediated by the bed nucleus of the stria terminalis and is not suppressed by benzodiazepine anxiolytics. Stressors that challenge cardiovascular homeostasis, such as hemorrhage, trigger a pattern of neuroendocrine responses that is similar to that observed in response to psychological stressors. These neuroendocrine responses are initiated by afferent signals from cardiovascular receptors which synapse in the medulla oblongata and are relayed either directly or indirectly to hypothalamic neurons controlling ACTH, prolactin, and oxytocin release. In contrast, forebrain pathways may not be essential for the renin response to hemorrhage. Thus current evidence indicates that although a diverse group of stressors initiate similar increases in ACTH, renin, prolactin, and oxytocin, the specific neural circuits and neurotransmitter systems involved in these responses differ for each neuroendocrine system and stressor category.

Role of corticotropin-releasing factor and substance P in pressor responses of nuclei controlling emotion and stress

The wide distribution of corticotropin-releasing factor (CRF) and substance P (SP)-immunoreactive cell bodies, nerve terminals and corresponding receptors in pressor nuclei controlling emotion and stress implies that CRF and SP may play important roles in pressor responses of these nuclei; hence CRF or SP was microinjected into these nuclei respectively in Wistar male rats anesthetized with urethane to test this possibility. Microinjection of CRF into nucleus amygdaloideus centralis, nucleus paraventricularis, nucleus ventromedialis, lateral hypothalamus-perifornical region, periaqueductal gray matter, nucleus parabrachialis, locus coeruleus or rostral ventrolateral medulla respectively could evoke pressor responses (but CRF injection into nucleus dorsomedialis could not elicit significant pressor responses). Injection of substance P into all the above nuclei could also elicit hypertensive responses of different magnitudes, whereas normal saline injection into these nuclei had no effect. These results indicate that both CRF and SP in the above mentioned nuclei may play important roles in hypertension induced by prolonged emotional stress.

Interaction of GABA and excitatory amino acids in the basolateral amygdala: role in cardiovascular regulation

Activation of the amygdala in rats produces cardiovascular changes that include increases in heart rate and arterial pressure as well as behavioral changes characteristic of emotional arousal. The objective of the present study was to examine the interaction of GABA and excitatory amino acid (EAA) receptors in the basolateral amygdala (BLA) in regulating cardiovascular function. Microinjection of the GABAA receptor antagonist bicuculline methiodide (BMI) or the E A A receptor agonists NMDA or AMPA into the same region of the BLA of conscious rats produced dose-related increases in heart rate and arterial pressure. Injection of the nonselective EAA receptor antagonist kynurenic acid into the BLA prevented or reversed the cardiovascular changes caused by local injection of BMI or the noncompetitive GABA antagonist picrotoxin. Conversely, local pretreatment with the glutamate reuptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid enhanced the effects of intra-amygdalar injection of BMI. The cardiovascular effects of BMI were also attenuated by injection of either the NMDA antagonist 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or the AMPA receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX). When these two EAA receptor antagonists were combined, their ability to suppress BMI-induced tachycardic and pressor responses was additive. These findings indicate that the cardiovascular effects caused by blockade of GABAergic inhibition in the BLA of the rat are dependent on activation of local NMDA and AMPA receptors.

Role of paraventricular and dorsomedial nuclei of the hypothalamus and central nucleus of the amygdala on muscimol-induced cardiovascular responses

Gamma-aminobutyric acid (GABA) plays an important role in the central control of cardiovascular functions. Previous evidence indicates that a tonically active GABAergic system exists in forebrain structures. The purpose of this study was to examine the role of the unilateral lesion of the central nucleus of amygdala, paraventricular or dorsomedial nuclei of the hypothalamus on muscimol-induced cardiovascular responses. Electrolytic ablation of nuclei was made by a monopolar isolated electrode under a stereotaxic instrument, 3-5 days before the experiments. Effects of intracerebroventricular injections of muscimol were investigated in intact, lesioned and sham-lesioned rats. On the day of the experiments, blood pressure and heart rate recordings were carried out in male Sprague-Dawley conscious rats. Muscimol produced decreases in arterial blood pressure and heart rate. The hypotensive effect of muscimol was completely inhibited in rats with dorsomedial nucleus lesions, whereas the bradycardic effect was partially prevented. The results indicate that the dorsomedial nucleus of the hypothalamus plays an important role on muscimol-induced blood pressure and heart rate responses.

Role of the AV3V region in the pressor responses induced by amygdala stimulation

The role of the anteroventral third ventricle (AV3V) region in the pressor responses to carbachol injected into the lateral cerebral ventricle (i.c.v.), the electrical stimulation of and carbachol-induced stimulation of, the central nucleus of the amygdala were investigated in conscious, unrestrained Sprague-Dawley rats. I.c.v. and intra-amygdalar carbachol caused a significant rise in blood pressure of 22.9 +/- 2.8 and 16.8 +/- 2.2 mmHg, respectively. Electrical stimulation (1 ms, 80 Hz, 50-300 microA, for 30 s) of the central nucleus of amygdala also produced intensity-dependent pressor effects. Electrolytic lesion of the AV3V region abolished the pressor responses induced by carbachol and by electrical amygdala stimulation. The heart rate changes were also significantly inhibited in the AV3V-lesioned rats. These results indicate that the integrity of the AV3V region is essential for the central cholinergic cardiovascular changes induced by central amygdaloid nucleus stimulation.

Carbachol-induced pressor responses and muscarinic M1 receptors in the central nucleus of amygdala in conscious rats

The type of muscarinic receptor in the central nucleus of the amygdala that mediates the carbachol-evoked pressor responses was investigated in conscious unrestraint Sprague-Dawley rats. Carbachol (100 ng) injected into the lateral cerebral ventricle caused a significant rise in blood pressure of 31.8+/-4.5 mmHg and a decrease in heart rate of 80.0+/-12.2 beats/min. Pirenzepine (10-75 nmol) injected into the central nucleus of the amygdala inhibited carbachol-induced pressor responses dose-dependently. The bradycardic response to carbachol was also inhibited by pirenzepine, but no dose-dependency was observed. Injection of pirenzepine into the basolateral amygdala at a dose (50 nmol) that inhibited carbachol-induced changes in mean arterial pressure and heart rate when injected into the central nucleus of the amygdala failed to exert any inhibition. Methoctramine at a dose of 50 nmol injected into both the central nucleus of the amygdala and the basolateral amygdala did not cause any significant alteration in the responses. These results indicate that muscarinic M1 receptors in the central nucleus of the amygdala are involved in cardiovascular regulation mediated by central cholinergic pathways.

Modulation of the pressor response elicited by carbachol and electrical stimulation of the amygdala by muscarinic antagonists in conscious rats

1. The nature of the muscarinic receptor involved in mediating cardiovascular changes caused by unilateral microinjection of carbachol (5 nmol) into, and electrical stimulation (200-300 microA) of, the amygdaloid complex was investigated in conscious, unrestrained female Sprague-Dawley rats. 2. Unilateral microinjection of carbachol (5 nmol; n = 6) and electrical stimulation (200-300 microA, 80 Hz, 30 s; n = 4) caused a significant rise in blood pressure of 21 +/- 4 mmHg and 25 +/- 5 mmHg, respectively. These changes were associated with no overall effect on heart rate. The effects of electrical stimulation were found to be repeatable. 3. Pretreatment i.c.v. with pirenzepine (5-20 mmol; n = 6-7 for each dose), dose-dependently inhibited the rise in blood pressure induced by carbachol, whereas AF-DX 116 (100 nmol; n = 6) failed to have any effect on the carbachol-induced pressure response. Neither antagonist alone had any effect on resting baseline variables. 4. Unilateral microinjections of atropine sulphate (1-100 nmol; n = 4-6 for each dose), pirenzepine (0.03-10 nmol; n = 4 for each dose) or AF-DX 116 (10-60 nmol; n = 4-5 for each dose), into the amygdala, dose-dependently inhibited the rise in blood pressure caused by electrical stimulation (200-300 microA). The ID50 values were 1.05, 0.23 and 39.5 nmol, respectively. Although pirenzepine seemed to be more potent than atropine, this difference was not significant. 5. It is concluded that the rise in blood pressure elicited by unilateral microinjection of carbachol into, or electrical stimulation of, the amygdaloid complex is mediated by M1-muscarinic receptors.

The role of amygdala and hypothalamus in GABAA antagonist bicuculline-induced cardiovascular responses in conscious rats

gamma-Aminobutyric acid (GABA) is known to play an important role in the central control of cardiovascular functions. GABAergic agonists and antagonists elicit blood pressure and heart rate changes when injected into the brain. It was demonstrated here that bicuculline methiodide (BMI), a GABAA antagonist, caused dose-dependent increases in both blood pressure and heart rate in conscious rats when injected intracerebroventricularly. The roles of the central nucleus of the amygdala (CeA), the paraventricular nucleus (PVN) and the dorsomedial nucleus (DMH) of the hypothalamus in BMI-induced blood pressure and heart rate changes were investigated in this study. The pressor effect of BMI was significantly attenuated by the electrolytic ablation of DMH and PVN, whereas it was only slightly, but insignificantly reduced by CeA lesions. The microinjection of BMI into the DMH and the PVN elicited significant pressor and tachycardic responses whereas only a slight increase was observed in rats injected BMI into the CeA. The BMI-induced increases in both blood pressure and heart rate were more prominent when given into the DMH. These results indicate that the DMH plays an important role in GABAergic control of cardiovascular functions. The PVN and CeA seem to have a minor part in this respect.

Lower GABAA receptor binding in the amygdala and hypothalamus of spontaneously hypertensive rats

The central GABAergic system is associated with normal blood pressure regulation, but the role of GABA receptors in genetic hypertension remains unclear. This study was conducted to investigate GABAA receptor binding in several brain regions of spontaneously hypertensive (SHR) rats during development of hypertension. GABAA receptor binding was labeled with [35S]TBPS and was assessed by quantitative autoradiography with the aid of a computer-assisted image analysis system. Densities of GABAA receptor binding sites were significantly lower in all hypothalamic and amygdaloid nuclei evaluated in 4-week-old SHR rats, when compared with their age-matched normotensive Wistar-Kyoto rats. At 12 weeks of age, GABAA receptor binding remained significantly lower in the central amygdaloid nucleus and paraventricular hypothalamic nucleus of SHR rats. Collectively, the results suggest that GABAA receptors in these nuclei are likely to be involved in the initiation and maintenance of hypertension. In conclusion, this study supports a notion that downregulation of GABAA receptor binding occurs in the hypothalamus and amygdala of SHR rats and may play a role in genetic hypertension.

Cardiovascular effects of centrally active cholinomimetics in conscious and anesthetized rats: the role of amygdala

Central cardiovascular effects of cholinergic agonists depend on the dose, site and mode of administration, species, and to the state of the animal. Intravenous injection of physostigmine and intracerebroventricular injection of carbachol produced pressor and tachycardic responses in urethane-anesthetized rats. Both agents also elicited pressor responses in conscious rats, but bradycardia occurred in the presence of anesthesia. Additionally, pressor responses to physostigmine, but not to carbachol, were significantly exaggerated by urethane anesthesia. These results demonstrate that anesthesia depresses cardiovascular reflexes and the inhibitory control mechanisms on acetylcholine release from the nerve endings involved in cardiovascular regulation. The role of the central nucleus of the amygdala (CNA) was also investigated in this study. The pressor effects of intracerebroventricular injection of carbachol were significantly attenuated by electrolytic ablation of the CNA, but heart rate changes were not altered both in anesthetized and conscious rats. These results indicate that the CNA plays a role in cholinergic control of blood pressure, but not in the regulation of heart rate.

Collateral axonal projections from ventrolateral medullary non-catecholaminergic neurons to central nucleus of the amygdala

Retrograde tract-tracing techniques were used to investigate whether catecholaminergic neurons in the ventrolateral medulla (VLM) send collateral axonal projections to both central nuclei of the amygdala (ACe) in the rat. Rhodamine-labelled latex microspheres or fluorogold (2%) were microinjected into the region of either the right or left ACe. After a survival period of 10-12 days, the rats were sacrificed and transverse sections of the brainstem were processed immunohistochemically for the identification of cell bodies containing the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH) or phenylethanolamine-N-methyltransferase (PNMT). Neuronal perikarya containing the retrogradely transported tracers were observed throughout the rostrocaudal extent of VLM, bilaterally. Approximately 10% of the retrogradely labelled neurons were observed to contain both retrograde tracers. The majority (79 +/- 6.8%) of these double labelled neurons were located within the caudal VLM and their number decreased rostrally. In addition, the proportion of double labelled neurons to single labelled neurons in VLM decreased rostrally; approximately 11% in the caudal VLM and 6% in the rostral VLM. Furthermore, approximately 21% of all VLM neurons that projected to ACe were found to be catecholaminergic: 75% of these were immunoreactive to TH and 25% to PNMT. However, no neurons were found in VLM that contained both retrograde tracers and immunoreactivity to TH or PNMT. These data demonstrate that axons originating from non-catecholaminergic neurons in VLM bifurcate to innervate ACe bilaterally. Although the function of these VLM neurons that project to both ACe is not known, they may be the anatomical substrate by which VLM neurons relay simultaneously autonomic and/or visceral sensory information to influence the activity of ACe.