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

Inactivation of the paraventricular nucleus attenuates the cardiogenic sympathetic afferent reflex in the spontaneously hypertensive rat

BACKGROUND

Myocardial ischemia causes the release of bradykinin, which stimulates cardiac afferents, causing sympathetic excitation and chest pain. Glutamatergic activation of the paraventricular hypothalamic nucleus (PVN) in the spontaneously hypertensive rat (SHR) drives elevated basal sympathetic activity. Thus, we tested the hypothesis that inactivation of the PVN attenuates the elevated reflex response to epicardial bradykinin in the SHR and that ionotropic PVN glutamate receptors mediate the elevated reflex.

METHODS

We recorded the arterial pressure and renal sympathetic nerve activity (RSNA) response to epicardial bradykinin application in anesthetized SHR and Wistar Kyoto (WKY) rats before and after PVN microinjection of GABA A agonist muscimol or ionotropic glutamate receptor antagonist kynurenic acid.

RESULTS

Muscimol significantly decreased the arterial pressure response to bradykinin from 180.4 ± 5.8 to 119.5 ± 6.9 mmHg in the SHR and from 111.8 ± 7.0 to 84.2 ± 8.3 mmHg in the WKY and the RSNA response from 186.2 ± 7.1 to 142.7 ± 7.3% of baseline in the SHR and from 201.0 ± 11.5 to 160.2 ± 9.3% of baseline in the WKY. Kynurenic acid significantly decreased the arterial pressure response in the SHR from 164.5 ± 5.0 to 126.2 ± 7.7 mmHg and the RSNA response from 189.9 ± 13.7to 168.5 ± 12.7% of baseline but had no effect in the WKY.

CONCLUSION

These results suggest that tonic PVN activity is critical for the full manifestation of the CSAR in both the WKY and SHR. Glutamatergic PVN activity contributes to the augmented CSAR observed in the SHR.

BDNF shifts excitatory-inhibitory balance in the paraventricular nucleus of the hypothalamus to elevate blood pressure

Presympathetic neurons in the paraventricular nucleus of the hypothalamus (PVN) play a key role in cardiovascular regulation. We have previously shown that brain-derived neurotrophic factor (BDNF), acting in the PVN, increases sympathetic activity and blood pressure and serves as a key regulator of stress-induced hypertensive responses. BDNF is known to alter glutamatergic and GABA-ergic signaling broadly in the central nervous system, but whether BDNF has similar actions in the PVN remains to be investigated. Here, we tested the hypothesis that increased BDNF expression in the PVN elevates blood pressure by enhancing N-methyl-d-aspartate (NMDA) receptor (NMDAR)- and inhibiting GABA_A receptor (GABA_AR)-mediated signaling. Sprague-Dawley rats received bilateral PVN injections of AAV2 viral vectors expressing green fluorescent protein (GFP) or BDNF. Three weeks later, cardiovascular responses to PVN injections of NMDAR and GABA_AR agonists and antagonists were recorded under α-chloralose-urethane anesthesia. In addition, expressions of excitatory and inhibitory signaling components in the PVN were assessed using immunofluorescence. Our results showed that NMDAR inhibition led to a greater decrease in blood pressure in the BDNF vs. GFP group, while GABA_AR inhibition led to greater increases in blood pressure in the GFP group compared to BDNF. Conversely, GABA_AR activation decreased blood pressure significantly more in GFP vs. BDNF rats. In addition, immunoreactivity of NMDAR1 was upregulated, while GABA_AR-α1 and K⁺/Cl^- cotransporter 2 were downregulated by BDNF overexpression in the PVN. In summary, our findings indicate that hypertensive actions of BDNF within the PVN are mediated, at least in part, by augmented NMDAR and reduced GABA_AR signaling.NEW & NOTEWORTHY We have shown that BDNF, acting in the PVN, elevates blood pressure in part by augmenting NMDA receptor-mediated excitatory input and by diminishing GABA_A receptor-mediated inhibitory input to PVN neurons. In addition, we demonstrate that elevated BDNF expression in the PVN upregulates NMDA receptor immunoreactivity and downregulates GABA_A receptor as well as KCC2 transporter immunoreactivity.

Hypothalamic aging and hormones

It is the heterogeneous changes of hypothalamic functions that determine the chronological sequence of aging in mammals. Recently, it was hypothesized by Cai the decrease in slow-wave sleep (SWS) resulting from skin aging as responsible for the degeneration of hypothalamic suprachiasmatic nucleus (SCN). It was soon hypothesized by the European people in television that the increase in body fat as responsible for the degeneration of male preoptic sexually dimorphic nucleus (SDN-POA), via the aromatase converting testosterone to estradiol as proposed by Cohen. It is the hypothalamic paraventricular nucleus (PVN) that remains unchanged in neuron number during aging for psychological stress. In this chapter, it is briefly reviewed more manifestations of hypothalamic related mammalian aging processes, including (1) the aging of ovary by lipid, estradiol and hypothalamus; (2) the aging of muscle, stomach, intestine, thymus, and the later aging of brain, regulated by growth hormone/insulin-like growth factor 1(GH/IGF1); (3) the cardiovascular hypertension from PVN activation, the bone and other peripheral aging by psychological stress, and that of kidney by vasopressin. It is classified these aging processes by the primary regulation from one of the three hypothalamic nuclei, although still necessary to investigate and supplement their secondary regulation by the hypothalamic nuclei in future. It is the hypothalamic structural changes that shift the functional balance among these three hypothalamic systems toward aging.

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.

Role of the prefrontal lobe in young normotensives with a family history of hypertension and hypertensives

Accumulating evidence has demonstrated a significant relationship between prefrontal lobe and hypertension. Elevated blood pressure is usually associated with a prefrontal hemodynamic abnormality. However, the detailed process is still unclear. In this study, we designed a startle protocol and tested the response of the cerebral cortex and cardiovascular system in young normotensive subjects with a family history of hypertension (FH+). Additionally, the cold forehead test (CFT) was performed in hypertensive subjects. In total, 40 young normotensive subjects (21 with FH+ and 19 without a family history of hypertension (FH-)) and 49 middle-aged subjects (21 normotensives (NT) and 28 hypertensives (HT)) were recruited. Our results showed that the magnitude of startle-evoked alpha oscillation at the parasympathetic-related prefrontal cortex (FP1 and FP2) in the FH+ group was significantly smaller than in the FH- group. Acute bradycardia (RRI increase) was observed in FH- subjects but disappeared in the FH+ group. The coupling between instant cardiac acute response (increased RRI) and prefrontal arousal (magnitude of evoked oscillation) was significantly weakened in the FH+ group compared with the FH- group. Furthermore, the decrease in HR induced by parasympathetic outflow during CFT was absent in HT subjects. Hence, we concluded that the impairment of parasympathetic outflow derived from the prefrontal lobe occurs in both healthy young offspring of hypertensive and hypertensive patients.

Mechanisms Responsible for Genetic Hypertension in Schlager BPH/2 Mice

It has been 45 years since Gunther Schlager used a cross breeding program in mice to develop inbred strains with high, normal, and low blood pressure (BPH/2, BPN/3, and BPL/1 respectively). Thus, it is timely to gather together the studies that have characterized and explored the mechanisms associated with the hypertension to take stock of exactly what is known and what remains to be determined. Growing evidence supports the notion that the mechanism of hypertension in BPH/2 mice is predominantly neurogenic with some of the early studies showing aberrant brain noradrenaline levels in BPH/2 compared with BPN/3. Analysis of the adrenal gland using microarray suggested an association with the activity of the sympathetic nervous system. Indeed, in support of this, there is a larger depressor response to ganglion blockade, which reduced blood pressure in BPH/2 mice to the same level as BPN/3 mice. Greater renal tyrosine hydroxylase staining and greater renal noradrenaline levels in BPH/2 mice suggest sympathetic hyperinnervation of the kidney. Renal denervation markedly reduced the blood pressure in BPH/2 but not BPN/3 mice, confirming the importance of renal sympathetic nervous activity contributing to the hypertension. Further, there is an important contribution to the hypertension from miR-181a and renal renin in this strain. BPH/2 mice also display greater neuronal activity of amygdalo-hypothalamic cardiovascular regulatory regions. Lesions of the medial nucleus of the amygdala reduced the hypertension in BPH/2 mice and abolished the strain difference in the effect of ganglion blockade, suggesting a sympathetic mechanism. Further studies suggest that aberrant GABAergic inhibition may play a role since BPH/2 mice have low GABA_A receptor δ, α4 and β2 subunit mRNA expression in the hypothalamus, which are predominantly involved in promoting tonic neuronal inhibition. Allopregnanolone, an allosteric modulator of GABA_A receptors, which increase the expression of these subunits in the amygdala and hypothalamus, is shown to reduce the hypertension and sympathetic nervous system contribution in BPH/2 mice. Thus far, evidence suggests that BPH/2 mice have aberrant GABAergic inhibition, which drives neuronal overactivity within amygdalo-hypothalamic brain regions. This overactivity is responsible for the greater sympathetic contribution to the hypertension in BPH/2 mice, thus making this an ideal model of neurogenic hypertension.

Diazepam as an oral hypnotic increases nocturnal blood pressure in the elderly

BACKGROUND

No study has evaluated the cardiovascular effects of diazepam in elderly subjects that assume diazepam to induce sleep.

PURPOSE

The present study was carried out in order to evaluate the effects of chronic administration of diazepam as hypnotic drug on blood pressure (BP) and heart rate (HR) in healthy elderly subjects.

PATIENTS AND METHODS

Healthy, elderly subjects, aged 65-74 years, were treated with diazepam 5 mg or placebo-both administered once a day in the evening-for 4 weeks in two cross-over periods, each separated by a 2-week placebo period, according to a randomized, double-blind, cross-over design. At the end of each study period, clinical as well as 24-h ambulatory BP and HR were evaluated.

RESULTS

A total of 25 subjects were included in the analysis. At the end of a 4-week diazepam treatment, clinical as well 24-h BP and HR mean values were not significantly affected. Analysis of sub-periods showed that during night-time, systolic BP (SBP) values under diazepam were 7.6% higher than under placebo, with a mean difference of 7.9 mmHg (p < 0.01), diastolic BP (DBP) values were 5.8% higher, with a mean difference of 3.7 mmHg (p < 0.05 vs placebo) and HR values were 6.6% higher with a mean difference of 4.2 b/min (p < 0.05). The HR increase observed with diazepam persisted during the morning hours, whereas during the afternoon and evening hours SBP, DBP and HR values were similar in the two treatment groups.

CONCLUSIONS

In elderly subjects chronic assumption of diazepam as hypnotic agent produced an increase in BP, in particular SBP, during night-time and of HR during night-time and morning hours. These effects, which probably depend on a diazepam-mediated increase in sympathetic drive and decrease in vagal tone, might be of clinical relevance due to the role of increased BP and HR as independent predictors of cardiovascular morbidity and mortality.

MEMRI reveals altered activity in brain regions associated with anxiety, locomotion, and cardiovascular reactivity on the elevated plus maze in the WKY vs SHR rats

Individuals with anxiety/depression often have exaggerated cardiovascular responses to stressful stimuli and a comorbidity with hypertension. Alternatively, individuals with hypertension can be more anxious. In the present study cardiovascular changes were evaluated during behavioral testing of anxious behavior on the elevated plus maze (EPM) in the spontaneously hypertensive rat (SHR), a rodent model of neurogenic hypertension, and compared to the response of the more anxious, but normotensive, Wistar-Kyoto rat (WKY). Manganese-enhanced magnetic resonance imaging (MEMRI) was used to identify regional differences in baseline brain activity. Parallel to indicators of elevated behavioral anxiety on the EPM, WKYs had a greater increase in blood pressure but not heart rate when compared to the SHR while on the EPM. Associated with differences in anxiety-related behavior and autonomic responses, we observed increased baseline activity in the amygdala, central gray, habenula and interpeduncular nucleus with MEMRI of the WKY compared to the SHR. Conversely, elevated baseline brain activity was found in regions associated with blood pressure control and system arousal, including the hypothalamus, locus coeruleus and pedunculopontine tegmental nucleus, in the SHR vs WKY, in-line with increased resting blood pressure and increased mobility in this strain. Lastly, reduced activity in hippocampal regions was identified in the SHR compared to the WKY and may be associated with cognitive impairment previously reported in the SHR. Thus, autonomic reactivity may be a true measure of stress in rodent models of anxiety and MEMRI presents a powerful technique to uncover novel brain mechanisms of blood pressure control.

Impaired Hypothalamic Regulation of Sympathetic Outflow in Primary Hypertension

The hypothalamic paraventricular nucleus (PVN) is a crucial region involved in maintaining homeostasis through the regulation of cardiovascular, neuroendocrine, and other functions. The PVN provides a dominant source of excitatory drive to the sympathetic outflow through innervation of the brainstem and spinal cord in hypertension. We discuss current findings on the role of the PVN in the regulation of sympathetic output in both normotensive and hypertensive conditions. The PVN seems to play a major role in generating the elevated sympathetic vasomotor activity that is characteristic of multiple forms of hypertension, including primary hypertension in humans. Recent studies in the spontaneously hypertensive rat model have revealed an imbalance of inhibitory and excitatory synaptic inputs to PVN pre-sympathetic neurons as indicated by impaired inhibitory and enhanced excitatory synaptic inputs in hypertension. This imbalance of inhibitory and excitatory synaptic inputs in the PVN forms the basis for elevated sympathetic outflow in hypertension. In this review, we discuss the disruption of balance between glutamatergic and GABAergic inputs and the associated cellular and molecular alterations as mechanisms underlying the hyperactivity of PVN pre-sympathetic neurons in hypertension.

Regulation of sympathetic vasomotor activity by the hypothalamic paraventricular nucleus in normotensive and hypertensive states

The hypothalamic paraventricular nucleus (PVN) is a unique and important brain region involved in the control of cardiovascular, neuroendocrine, and other physiological functions pertinent to homeostasis. The PVN is a major source of excitatory drive to the spinal sympathetic outflow via both direct and indirect projections. In this review, we discuss the role of the PVN in the regulation of sympathetic output in normal physiological conditions and in hypertension. In normal healthy animals, the PVN presympathetic neurons do not appear to have a major role in sustaining resting sympathetic vasomotor activity or in regulating sympathetic responses to short-term homeostatic challenges such as acute hypotension or hypoxia. Their role is, however, much more significant during longer-term challenges, such as sustained water deprivation, chronic intermittent hypoxia, and pregnancy. The PVN also appears to have a major role in generating the increased sympathetic vasomotor activity that is characteristic of multiple forms of hypertension. Recent studies in the spontaneously hypertensive rat model have shown that impaired inhibitory and enhanced excitatory synaptic inputs to PVN presympathetic neurons are the basis for the heightened sympathetic outflow in hypertension. We discuss the molecular mechanisms underlying the presynaptic and postsynaptic alterations in GABAergic and glutamatergic inputs to PVN presympathetic neurons in hypertension. In addition, we discuss the ability of exercise training to correct sympathetic hyperactivity by restoring blood-brain barrier integrity, reducing angiotensin II availability, and decreasing oxidative stress and inflammation in the PVN.

Modulation of cardiorespiratory function mediated by the paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) coordinates autonomic and neuroendocrine systems to maintain homeostasis and to respond to stress. Neuroanatomic and neurophysiologic experiments have provided insight into the mechanisms by which the PVN acts. The PVN projects directly to the spinal cord and brainstem and, specifically, to sites that control cardio-respiratory function: the intermediolateral cell columns and phrenic motor nuclei in the spinal cord and rostral ventrolateral medulla (RVLM) and the rostral nuclei in the ventral respiratory column (rVRC) in the brainstem. Activation of the PVN increases ventilation (both tidal volume and frequency) and blood pressure (both heart rate and sympathetic nerve activity). Excitatory and inhibitory neurotransmitters including glutamate and GABA converge in the PVN to influence its neuronal activity. However, a tonic GABAergic input to the PVN directly modulates excitatory outflow from the PVN. Further, even within the PVN, microinjection of GABA(A) receptor blockers increases glutamate release suggesting an indirect mechanism by which GABA control contributes to PVN functions. PVN activity alters blood pressure and ventilation during various stresses, such as maternal separation, chronic intermittent hypoxia (CIH), dehydration and hemorrhage. Among the several PVN neurotransmitters and neurohormones, vasopressin and oxytocin modulate ventilation and blood pressure. Here, we review our data indicating that increases in vasopressin and vasopressin type 1A (V(1A)) receptor signalling in the RVLM and rVRC are mechanisms increasing blood pressure and ventilation after exposure to CIH. That blockade of V(1A) receptors in the medulla normalizes baseline blood pressure as well as blunts PVN-evoked blood pressure and ventilatory responses in CIH-conditioned animals indicate the role of vasopressin in cardiorespiratory control. In summary, morphological and functional studies have found that the PVN integrates sensory input and projects to the sympathetic and respiratory control systems with descending projections to the medulla and spinal cord.

Increased vasopressin transmission from the paraventricular nucleus to the rostral medulla augments cardiorespiratory outflow in chronic intermittent hypoxia-conditioned rats

A co-morbidity of sleep apnoea is hypertension associated with elevated sympathetic nerve activity (SNA) which may result from conditioning to chronic intermittent hypoxia (CIH). Our hypothesis is that SNA depends on input to the rostral ventrolateral medulla (RVLM) from neurons in the paraventricular nucleus (PVN) that release arginine vasopressin (AVP) and specifically, that increased SNA evoked by CIH depends on this excitatory input. In two sets of neuroanatomical experiments, we determined if AVP neurons project from the PVN to the RVLM and if arginine vasopressin (V(1A)) receptor expression increases in the RVLM after CIH conditioning (8 h per day for 10 days). In the first set, cholera toxin beta subunit (CT-beta) was microinjected into the RVLM to retrogradely label the PVN neurons. Immunohistochemical staining demonstrated that 14.6% of CT-beta-labelled PVN neurons were double-labelled with AVP. In the second set, sections of the medulla were immunolabelled for V(1A) receptors, and the V(1A) receptor-expressing cell count was significantly greater in the RVLM (P < 0.01) and in the neighbouring rostral ventral respiratory column (rVRC) from CIH- than from room air (RA)-conditioned rats. In a series of physiological experiments, we determined if blocking V(1A) receptors in the medulla would normalize blood pressure in CIH-conditioned animals and attenuate its response to disinhibition of PVN. Blood pressure (BP), heart rate (HR), diaphragm (D(EMG)) and genioglossus muscle (GG(EMG)) activity were recorded in anaesthetized, ventilated and vagotomized rats. The PVN was disinhibited by microinjecting a GABA(A) receptor antagonist, bicuculline (BIC, 0.1 nmol), before and after blocking V(1A) receptors within the RVLM and rVRC with SR49059 (0.2 nmol). In RA-conditioned rats, disinhibition of the PVN increased BP, HR, minute D(EMG) and GG(EMG) activity and these increases were attenuated after blocking V(1A) receptors. In CIH-conditioned rats, a significantly greater dose of blocker (0.4 nmol) was required to blunt these physiological responses (P < 0.05). Further, this dose normalized the baseline BP. In summary, AVP released by a subset of PVN neurons modulates cardiorespiratory output via V(1A) receptors in the RVLM and rVRC, and increased SNA in CIH-conditioned animals depends on up-regulation of V(1A) receptors in the RVLM.

Deficits in substance P mRNA levels in the CeA are inversely associated with alcohol-motivated responding

In the present study, in vitro and in vivo studies were conducted to determine the relationship between innate substance P (SP) levels and alcohol-motivated behavior in alcohol-preferring (P) and nonpreferring (NP) rat lines. In Experiment 1, in situ hybridization and quantitative autoradiography were used to detect and measure SP mRNA levels in discrete brain loci of the P and NP rats. The results indicated significantly lower SP mRNA levels in the central nucleus of the amygdala (CeA) of P compared with those of NP rats. Experiment 2 evaluated the effects of SP, microinfused into the CeA, on alcohol (10%, v/v) and sucrose (2%, w/v) motivated responding in the P rat. The results revealed that, when infused into the CeA (1-8 microg), SP reduced alcohol responding by 48-85% of control levels, with no effects on sucrose responding. Neuroanatomical control infusions (1-8 microg) into the caudate putamen (CPu) also failed to significantly alter alcohol- or sucrose-motivated behaviors. Given the selective reductions on alcohol (compared to sucrose) responding by direct intracranial infusion of SP, the data suggest that deficits in SP signaling within the CeA (an anxiety regulating locus) are inversely associated with alcohol-motivated behaviors. Activation of SP receptors in the CeA may reduce anxiety-like behavior in the P rat and contribute to reductions on alcohol responding. The SP system may be a suitable target for the development of drugs to reduce alcohol-drinking behavior in humans.

The paraventricular nucleus of the hypothalamus - a potential target for integrative treatment of autonomic dysfunction

BACKGROUND

The paraventricular nucleus of the hypothalamus (PVN) has emerged as one of the most important autonomic control centers in the brain, with neurons playing essential roles in controlling stress, metabolism, growth, reproduction, immune and other more traditional autonomic functions (gastrointestinal, renal and cardiovascular).

OBJECTIVES

Traditionally the PVN was viewed as a nucleus in which afferent inputs from other regions were faithfully translated into changes in single specific outputs, whether neuroendocrine or autonomic. Here we present data which suggest that the PVN plays significant and essential roles in integrating multiple sources of afferent input and sculpting an integrated autonomic output by concurrently modifying the excitability of multiple output pathways. In addition, we highlight recent work that suggests that dysfunction of such intranuclear integrative circuitry contributes to the pathology of conditions such as hypertension and congestive heart failure.

CONCLUSIONS

This review highlights data showing that individual afferent inputs (subfornical organ), signaling molecules (orexins, adiponectin), and interneurons (glutamate/GABA), all have the potential to influence (and thus coordinate) multiple PVN output pathways. We also highlight recent studies showing that modifications in this integrated circuitry may play significant roles in the pathology of diseases such as congestive heart failure and hypertension.

Plasticity of pre- and postsynaptic GABAB receptor function in the paraventricular nucleus in spontaneously hypertensive rats

GABA(B) receptor function is upregulated in the paraventricular nucleus (PVN) of the hypothalamus in spontaneously hypertensive rats (SHR), but it is unclear whether this upregulation occurs pre- or postsynaptically. We therefore determined pre- and postsynaptic GABA(B) receptor function in retrogradely labeled spinally projecting PVN neurons using whole cell patch-clamp recording in brain slices in SHR and Wistar-Kyoto (WKY) rats. Bath application of the GABA(B) receptor agonist baclofen significantly decreased the spontaneous firing activity of labeled PVN neurons in both SHR and WKY rats. However, the magnitude of reduction in the firing rate was significantly greater in SHR than in WKY rats. Furthermore, baclofen produced larger membrane hyperpolarization and outward currents in labeled PVN neurons in SHR than in WKY rats. The baclofen-induced current was abolished by either including G protein inhibitor GDPbetaS in the pipette solution or bath application of the GABA(B) receptor antagonist in both SHR and WKY rats. Blocking N-methyl-d-aspartic acid receptors had no significant effect on baclofen-elicited outward currents in SHR. In addition, baclofen caused significantly greater inhibition of glutamatergic excitatory postsynaptic currents (EPSCs) in labeled PVN neurons in brain slices from SHR than WKY rats. By contrast, baclofen produced significantly less inhibition of GABAergic inhibitory postsynaptic currents (IPSCs) in labeled PVN neurons in SHR than in WKY rats. Although microinjection of the GABA(B) antagonist into the PVN increases sympathetic vasomotor tone in SHR, the GABA(B) antagonist did not affect EPSCs and IPSCs of the PVN neurons in vitro. These findings suggest that postsynaptic GABA(B) receptor function is upregulated in PVN presympathetic neurons in SHR. Whereas presynaptic GABA(B) receptor control of glutamatergic synaptic inputs is enhanced, presynaptic GABA(B) receptor control of GABAergic inputs in the PVN is attenuated in SHR. Changes in both pre- and postsynaptic GABA(B) receptors in the PVN may contribute to the control of sympathetic outflow in hypertension.

c-fos gene expression is increased in the paraventricular hypothalamic nucleus of Sprague-Dawley rats with visceral pain induced by acetic acid without detectable changes of corticotrophin-releasing factor mRNA: a quantitative approach with an image analysis system

This study is the first of its kind to demonstrate that c-Fos immunoreactivity (ir) together with c-fos mRNA in their immediately adjacent tissue sections of a discrete brain region can be reliably measured. The c-fos gene expression in the paraventricular hypothalamic nucleus (PVN) of Sprague-Dawley rats for an animal model for visceral or somatovisceral pain induced by 2% acetic acid (AA) was used in this study. Specifically, c-fos mRNA signals were measured by quantitative autoradiography after in situ hybridization using c-fos oligodeoxynucleotide probe, and c-Fos-ir signals were represented by c-Fos immunostaining, as detected using c-Fos antibody in a regular immunohistochemistry. Signals from both c-Fos-ir and c-fos mRNA in the PVN were measured from their immediately adjacent cryostat sections. For the measurement of c-Fos-ir, it was carried out by reading 10 rectangles (1,000 microm(2)/rectangle) on each PVN section with c-Fos immunostaining. Specific signals were obtained from subtracting the nonspecific background signal from the total signals using a computer-assisted image analysis system. Results indicated that the AA treatment induced a significant increase of both c-Fos-ir and c-fos mRNA in the PVN. Interestingly, there was no increase of corticotrophin-releasing factor (CRF) mRNA expression in the PVN and central nucleus of the amygdala of Sprague-Dawley rats subjected to the AA treatment. In summary, this study has demonstrated that c-Fos-ir in the PVN with an anatomical resolution can be semiquantitatively measured after immunohistochemistry using an image analysis system, and that increased c-fos mRNA in the PVN 1 hr after the AA treatment is associated with no changes of the CRF mRNA expression.

Role of γ-Aminobutyric Acid (GABA)Aand GABABReceptors in Paraventricular Nucleus in Control of Sympathetic Vasomotor Tone in Hypertension

The paraventricular nucleus (PVN) of the hypothalamus is involved in tonic regulation of sympathetic outflow. Impaired GABAergic control of PVN neurons may contribute to the elevated sympathetic drive in hypertension. In this study, we examined the function of GABA(A) and GABA(B) receptors in the PVN in control of sympathetic nerve activity and arterial blood pressure (ABP) in normotensive and hypertensive rats. Lumbar sympathetic activity (LSNA) and ABP were recorded from anesthetized spontaneously hypertensive rats (SHRs), Sprague-Dawley (SD) rats, and Wistar-Kyoto (WKY) rats. Bilateral microinjection of bicuculline (0.01-0.15 nmol), a GABA(A) receptor antagonist, into the PVN increased LSNA and ABP in normotensive WKY and SD rats in a dose-dependent manner. This response was significantly attenuated in SHRs. Furthermore, the decrease in LSNA and ABP induced by a GABA(A) receptor agonist, muscimol (0.05-1.5 nmol), in the PVN was significantly less in SHRs than in normotensive controls. In contrast, microinjection of the GABA(B) receptor agonist baclofen (0.3-4.5 nmol) into the PVN decreased LSNA and ABP in SHRs. However, in WKY and SD rats, baclofen only decreased LSNA and ABP at the highest dose tested. In addition, blockade of GABA(B) receptors in the PVN with CGP52432 (3-[[(3,4-dichlorophenyl)methyl]amino]propyl]diethoxymethyl)phosphinic acid) (0.15-3.0 nmol) dose-dependently increased LSNA and ABP in SHRs but not in normotensive controls. Collectively, this study provides new evidence that GABA(A) receptor function is attenuated, whereas the function of GABA(B) receptors is enhanced, in the PVN of SHRs.

Spontaneously hypertensive, Wistar Kyoto and Sprague-Dawley rats differ in their use of place and response strategies in the water radial arm maze

This study further characterises the use of mnemonic systems in the spontaneously hypertensive rat (SHR), which is frequently used as a rodent model of attention deficit hyperactivity disorder. The objective of this study was to assess the preference of male SHR, Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats for a place or response strategy when trained on an ambiguous T-maze task, and also to examine whether all strains acquired information about both strategies during ambiguous training, regardless of their preferred strategy. In the first experiment, SHR and WKY showed a preference for a response strategy on the ambiguous T-maze task; in contrast, SD displayed a preference for a place strategy. In the second experiment, all strains demonstrated that they learned information about both the response and place strategies during ambiguous training. However, on a conditioned place preference test SHR did not display as strong a preference for the place arm as WKY and SD. This finding supports previous research in a conditioned cue preference test, in which SHR did not display a preference for the cue associated with the platform. These observations that the strains differ with respect to behavioural strategy in a learning task suggest that they differ in the underlying neural circuitry that serves goal-directed behaviour, and are consistent with SHR having deficits associated with the nucleus accumbens.

Plasticity of GABAergic control of hypothalamic presympathetic neurons in hypertension

Increased sympathetic outflow contributes to the pathogenesis of hypertension. However, the mechanisms of increased sympathetic drive in hypertension remain unclear. We examined the tonic GABAergic inhibition in control of the excitability of paraventricular (PVN) presympathetic neurons in spontaneously hypertensive rats (SHR) and normotensive controls, including Sprague-Dawley (SD) and Wistar-Kyoto (WKY) rats. Whole cell patch-clamp recordings were performed on retrogradely labeled PVN neurons projecting to the rostral ventrolateral medulla (RVLM) in brain slices. The basal firing rate of PVN neurons was significantly decreased in 13-wk-old SD and WKY rats but increased in 13-wk-old SHR, compared with their respective 6-wk-old controls. The GABA(A) antagonist bicuculline consistently increased the firing of PVN neurons in normotensive controls. Surprisingly, bicuculline either decreased the firing or had no effect in 59.3% of labeled cells in 13-wk-old SHR. In contrast, the GABA(B) antagonist CGP-55845 had no effect on the firing of PVN neurons in normotensive controls but significantly increased the firing of 75% of cells studied in 13-wk-old SHR. Furthermore, the evoked GABA(A) current decreased significantly in labeled PVN neurons of 13-wk-old SHR compared with that in normotensive controls. Both the frequency and amplitude of GABAergic spontaneously inhibitory postsynaptic currents were also reduced in 13-wk-old SHR. This study demonstrates an unexpected functional change in GABA(A) and GABA(B) receptors in regulation of the firing activity of PVN-RVLM neurons in SHR. This change in GABA(A) receptor function and GABAergic inputs to PVN output neurons may contribute to increased sympathetic outflow in hypertension.

A comparative autoradiographic study of the density of [3H]SR95531, [3H]MK-801 and [3H]cGMP binding in the locus coeruleus and central pontine grey of spontaneously hypertensive and Wistar-Kyoto rats

The Spontaneously Hypertensive rat (SHR) has been previously shown to have a host of neurochemical differences compared with their normotensive counterpart, the Wistar-Kyoto (WKY) rat. Using quantitative receptor autoradiography, the density of GABA(A) and NMDA receptors and [3H]cGMP binding within the locus coeruleus (LC) and central pontine grey (CGPn) were compared in the SHR and WKY rat using the radioligands [3H]SR95531, [3H]MK-801 and [3H]cGMP respectively. It was found that [3H]SR95531 binding was significantly greater in both the LC and CGPn of the SHR compared with the WKY rat (unpaired t test; P < 0.05). Greater binding densities of [3H]MK-801 and [3H]cGMP were also observed in the LC of the SHR compared with the WKY rat; however, no differences in the binding density of these two ligands were observed in the CGPn. It is suggested that these neurochemical differences within the LC of the SHR may relate to phenotypic differences between SHR and WKY rats that have previously been reported.

D5 dopamine receptor knockout mice and hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. All of the five dopamine receptor genes (D1, D2, D3, D4, and D5) expressed in mammals and some of their regulators are in loci linked to hypertension in humans and in rodents. Under normal conditions, D1-like receptors (D1 and D5) inhibit sodium transport in the kidney and the intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats, and humans with essential hypertension, the D1-like receptor-mediated inhibition of sodium transport is impaired because of an uncoupling of the D1-like receptor from its G protein/effector complex. The uncoupling is genetic, and receptor-, organ-, and nephron segment-specific. In human essential hypertension, the uncoupling of the D1 receptor from its G protein/effector complex is caused by an agonist-independent serine phosphorylation/desensitization by constitutively active variants of the G protein-coupled receptor kinase type 4. The D5 receptor is also important in blood pressure regulation. Disruption of the D5 or the D1 receptor gene in mice increases blood pressure. However, unlike the D1 receptor, the hypertension in D5 receptor null mice is caused by increased activity of the sympathetic nervous system, apparently due to activation of oxytocin, V1 vasopressin, and non-N-methyl D-aspartate receptors in the central nervous system. The cause of the activation of these receptors remains to be determined.

Regulation of synaptic inputs to paraventricular-spinal output neurons by alpha2 adrenergic receptors

Neurons in the paraventricular nucleus (PVN) that project to the brain stem and spinal cord are important for autonomic regulation. The excitability of preautonomic PVN neurons is controlled by the noradrenergic input from the brain stem. In this study, we determined the role of alpha(2) adrenergic receptors in the regulation of excitatory and inhibitory synaptic inputs to spinally projecting PVN neurons. Excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) were recorded using whole cell voltage-clamp techniques on PVN neurons labeled by a retrograde fluorescence tracer injected into the thoracic spinal cord of rats. Bath application of 5-20 muM clonidine, an alpha(2) receptor agonist, significantly reduced the amplitude of evoked GABAergic IPSCs in a dose-dependent manner. Also, 10 microM clonidine significantly decreased the frequency (from 2.68 +/- 0.41 to 1.22 +/- 0.40 Hz) but not the amplitude of miniature IPSCs (mIPSCs), and this effect was blocked by the alpha(2) receptor antagonist yohimbine. Furthermore, clonidine increased the paired-pulse ratio of evoked IPSCs from 1.25 +/- 0.05 to 1.61 +/- 0.08 (P < 0.05). On the other hand, clonidine had little effect on evoked glutamatergic EPSCs, mEPSCs, and the paired-pulse ratio of evoked EPSCs in most labeled cells examined. Additionally, immunofluorescence labeling revealed that the alpha(2A) receptor and GABA immunoreactivities were co-localized in close apposition to labeled PVN neurons. Collectively, these data suggest that stimulation of alpha(2) adrenergic receptors primarily attenuates GABAergic inputs to PVN output neurons to the spinal cord. The presynaptic alpha(2) receptors function as heteroreceptors to modulate synaptic GABA release and contribute to the hypothalamic regulation of sympathetic outflow.

Effect of intravenous angiotensin II infusion on responses to hypothalamic PVN injection of bicuculline

The hypothalamic paraventricular nucleus (PVN) plays an important role in the sympathoexcitatory response to elevated plasma angiotensin II (Ang II). However, the mechanism by which Ang II influences sympathetic activity is not fully understood. In this study, we tested the hypothesis that GABA(gamma-aminobutyric acid)-ergic function in the PVN is reduced by peripheral infusion of Ang II. To accomplish this, rats received either intravenous Ang II (12 ng/kg per minute) or vehicle (D5W) for 7 days, and renal sympathetic nerve activity (SNA), mean arterial pressure (MAP), and heart rate (HR) responses were recorded after unilateral PVN microinjection of the GABA-A receptor antagonist bicuculline methiodide (BMI, 0.1 nmol). Results indicate that in contrast to a significant increase in renal SNA, MAP, and HR observed in vehicle-infused rats (P<0.05), BMI injection into the PVN of Ang II-infused animals was without effect on all recorded variables. In a separate groups of animals, ganglionic blockade produced a significantly greater fall in MAP (P<0.01) in Ang II-infused rats than in vehicle-infused control rats, indicating that the contribution of SNA to the maintenance of blood pressure was elevated in the Ang II-infused group. Overall, these data indicate that cardiovascular and sympathoexcitatory responses to acute GABA-A receptor antagonism in the PVN are significantly blunted in rats after 7 days of intravenous infusion of Ang II. We conclude that an Ang II-induced reduction in GABAergic inhibition within the PVN may contribute to elevated SNA observed in this study.

Noradrenaline excites and inhibits GABAergic transmission in parvocellular neurons of rat hypothalamic paraventricular nucleus

Noradrenaline (NA) is a major neurotransmitter that regulates many neuroendocrine and sympathetic autonomic functions of the hypothalamic paraventricular nucleus (PVN). Previously NA has been shown to increase the frequency of excitatory synaptic activity of parvocellular neurons within the PVN, but little is known about its effects on inhibitory synaptic activity. In this work, we studied the effects of NA (1-100 microM) on the spontaneous inhibitory synaptic currents (sIPSC) of type II PVN neurons in brain slices of the rat using the whole cell patch-clamp technique. Spontaneous IPSCs were observed from most type II neurons (n = 121) identified by their anatomical location within the PVN and their electrophysiological properties. Bath application of NA (100 microM) increased sIPSC frequency by 256% in 59% of the neurons. This effect was blocked by prazosin (2-20 microM), the alpha(1)-adrenoceptor antagonist and mimicked by phenylephrine (10-100 microM), the alpha(1)-adrenoceptor agonist. However, in 33% of the neurons, NA decreased sIPSC frequency by 54%, and this effect was blocked by yohimbine (2-20 microM), the alpha(2)-adrenoceptor antagonist and mimicked by clonidine (50 microM), the alpha(2)-adrenoceptor agonist. The Na(+) channel blocker, tetrodotoxin (0.1 microM) blocked the alpha(1)-adrenoceptor-mediated effect, but not the alpha(2)-adreonoceptor-mediated one. Both of the stimulatory and inhibitory effects of NA on sIPSC frequency were observed in individual neurons when tested with NA alone, or both phenylephrine and clonidine. Furthermore, in most neurons that showed the stimulatory effects, the inhibitory effects of NA were unmasked after blocking the stimulatory effects by prazosin or tetrodotoxin. These data indicate that tonic GABAergic inputs to the majority of type II PVN neurons are under a dual noradrenergic modulation, the increase in sIPSC frequency via somatic or dendritic alpha(1)-adrenoceptors and the decrease in sIPSC frequency via axonal terminal alpha(2)-adrenoceptors on the presynaptic GABAergic neurons.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Effects of six antihypertensive drugs on blood pressure and hypothalamic GABA content in spontaneously hypertensive rats

In order to investigate the effects of antihypertensive drugs on blood pressure and gamma-amino butyric acid (GABA) content in the hypothalamus and the possible relationship between blood pressure decrease and GABA content changes, blood pressure and GABA contents after chronic (20 weeks) treatments of nitrendipine, atenolol, captopril, hydrochlorothiazide, dihydralazine and prazosin were studied in spontaneously-hypertensive rats (SHR). The acute and subacute (1 week) effects of nitrendipine on GABA contents was also observed in SHR. It was found that 20 week treatments with six different antihypertensive agents produced a decrease in systolic blood pressure and an increase in GABA content. The blood pressure level was significantly correlated with GABA content in the hypothalamus, but not with that in the cortex. Acute treatment with a single dose of nitrendipine, did not alter GABA content. Bicuculline, a GABA receptor antagonist, did not attenuate the hypotensive effect of nitrendipine. In conclusion, chronic treatments by different antihypertensive agents produced an increase of hypothalamic GABA content and a decrease of blood pressure. The increase of GABA content induced by nitrendipine seems likely to be secondary to blood pressure decrease.

gamma-Aminobutyric acid (GABA)--A function and binding in the paraventricular nucleus of the hypothalamus in chronic renal-wrap hypertension

The goal of this study was to determine whether gamma-aminobutyric acid (GABA)ergic transmission and GABA binding are altered in chronic renal-wrap hypertension. Three groups of hypertensive and sham-operated rats were prepared for separate protocols. Four weeks later, the animals were prepared with femoral artery catheters for the measurement of mean arterial pressure. In all groups, blood pressure was significantly higher in the renal-wrapped animals. In the first study, bilateral microinjection of the GABA-A antagonist, bicuculline (50 pmol/site), into the paraventricular nucleus of the hypothalamus (PVN) caused a greater increase in arterial pressure (21.9+/-1.4 versus 16.7+/-1.8 mm Hg, P<0.05) and heart rate (135+/-15 versus 98+/-12 bpm, P=0.064) in hypertensive rats. [(3)H]Flunitrazepam was used to measure binding to the GABA-A receptor. Magnocellular neurons and the adjacent medial parvicellular neurons had more intense binding compared with the remainder of the PVN. B(max) was greater for the higher density binding area; the K(d) value was less in the high-density region. There were no differences in these parameters between normotensive and hypertensive animals. Competitive reverse transcription-polymerase chain reaction was used to measure the expression of mRNA for the alpha(1) subunit of the GABA-A receptor. No difference was observed in the mRNA between renal-wrapped and sham-operated rats. In summary, inhibition of GABA-A receptors in the PVN is augmented in the chronic phase of hypertension and is unrelated to a change in the expression of the number or affinity to the receptor. These findings suggest that the greater GABAergic activity is the result of an increase in GABA release in the PVN in chronic renal-wrap hypertension.

Acute and chronic restraint stress: effects on [125I]-galanin binding in normotensive and hypertensive rat brain

The neuropeptide galanin (GAL) has been implicated in the neural response to a number of stressors including restraint; however, the effect of restraint stress on GAL receptor density in the central nervous system (CNS) has not been investigated. Normotensive (Wistar-Kyoto; WKY) and hypertensive (spontaneously hypertensive; SHR) rats were subjected to a daily 60-min restraint stress paradigm for 0 (control), 1, 3, 5 or 10 consecutive days, and the density of [125I]-GAL binding sites following exposure to restraint was compared between strains using quantitative autoradiography. Significant differences in basal (no stress) levels of GAL receptor density between WKY and SHR were detected in regions such as the central nucleus of the amygdala (Ce) and ventromedial hypothalamus (VMH) (P<0.05). In WKY, restraint stress (1 day) induced significant decreases in GAL receptor density in forebrain regions such as the Ce (-41%) and medial nucleus of the amygdala (-41%) (P<0.05). Chronic restraint (10 days) did not induce significant decreases in these nuclei in WKY, indicating that forebrain neurons containing GAL receptors in WKY possessed a functional ability to adapt to repeated restraint. In addition, restraint stress induced significant decreases in GAL receptor density in SHR in regions such as the lateral parabrachial nucleus (-43%; 5 days of restraint) and hypoglossal nucleus ( approximately -18% for entire restraint period) (P<0.05). In conclusion, restraint stress resulted in region- and strain-specific alterations in GAL receptor density, some of which may contribute to the altered stress response previously observed in hypertensive rats. The results clearly support the hypothesis that neuropeptides such as GAL are an integral component of the neural response to psychological stress, although the functional significance of the changes in GAL receptor density described in this study awaits elucidation.

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.

Reduced GABA inhibition of sympathetic function in renal-wrapped hypertensive rats

Animals with bilateral cannulas in the paraventricular nucleus were made hypertensive by a one-kidney, figure eight renal wrap procedure or sham operated. Femoral artery and vein catheters were inserted for arterial pressure measurement and plasma catecholamine determination. After recovery and 4 days after hypertension surgery, bicuculline methiodide or muscimol was microinjected into the paraventricular nucleus. In some rats, nitroprusside was infused intravenously to reflexly stimulate the sympathetic nervous system. In control rats, bicuculline increased blood pressure, heart rate, and plasma norepinephrine and epinephrine concentrations. In contrast, in hypertensive rats blood pressure did not change while the heart rate response was maintained. Plasma norepinephrine and epinephrine responses were reduced 75 and 68%, respectively. Muscimol injections decreased arterial pressure in the hypertensive rats. Heart rate responses to nitroprusside were similar in the two groups of rats, while the plasma catecholamine responses were attenuated in the hypertensive animals. These data suggest that GABA function in the paraventricular nucleus is reduced in renal wrap hypertension.

Arterial nerve growth factor (NGF) mRNA, protein, and vascular smooth muscle cell NGF secretion in hypertensive and hyperactive rats

Elevated levels of nerve growth factor (NGF) protein and NGF mRNA have been reported in the vessels of spontaneously hypertensive rats (SHR: hypertensive, hyperactive) compared to Wistar-Kyoto (WKY) rats. Elevated NGF may be involved in the development of hypertension in SHRs. We examined vascular NGF mRNA and protein content and the regulation of NGF secretion by vascular smooth muscle cells (VSMCs) from two inbred strains (WKHT: hypertensive; WKHA: hyperactive) derived from SHRs and WKYs. Our goal was to determine if receptor-mediated defects in NGF regulation play a role in increased secretion of VSMC NGF from hypertensive animals. Tissue NGF mRNA content was determined by competitive, quantitative RT-PCR. Tissue NGF and NGF content in cultured VSMC-conditioned medium was quantified using a two-site ELISA. Tail artery NGF mRNA was elevated in WKHTs compared to WKHAs. Tissue NGF protein was elevated in WKHT aorta, mesenteric, and tail artery compared to WKHAs. Pharmacologically induced increases in NGF output were blocked with inhibition of transcription or protein synthesis. Basal NGF secretion by WKHT VSMCs was significantly higher than WKHAs. The observed increases in VSMC NGF output in SHRs over WKYs in response to beta-adrenergic agents are not preserved in the WKHT:WKHA comparison. Protein kinase C-dependent increases in SHR VSMC NGF appear in both WKHTs and WKHAs. In contrast, elevated NGF levels due to disturbances in alpha-adrenergic, peptidergic, and purinergic control of NGF output are features common to both genetic models of hypertension (SHR and WKHT). These results suggest that the defect in smooth muscle NGF metabolism observed in SHRs cosegregates with a hypertensive rather than a hyperactive phenotype. Moreover, altered receptor-mediated regulation (alpha-adrenergic, peptidergic, and purinergic) of VSMC NGF production may contribute to elevated vascular tissue NGF, suggesting a mechanism leading to the high levels of NGF associated with hypertension in SHRs and WKHTs.

Pharmacology and function of imidazole 4-acetic acid in brain

1. Imidazole 4-acetic acid (IMA) is a naturally occurring metabolite in brain, although it is unclear what biochemical pathways are involved in its biosynthesis and breakdown. Some evidence, however, suggests that IMA is an oxidation product of histamine. 2. The compound has pronounced neuropharmacological properties, many of which are consistent with an activation of GABA(A) receptors. Indeed, IMA is able to displace [3H]GABA from GABA(A) sites in a potent manner. 3. IMA displays definite partial agonist characteristics as an enhancer of benzodiazepine binding to the GABA(A) receptor complex in membrane preparations. In addition, it has an affinity for GABA(C) receptors, where it seems to act as an antagonist, and perhaps as a weak partial agonist. A third recognition site for IMA in brain is the I1-imidazoline receptor. 4. Parenteral administration to experimental animals leads to a sleep-like state which can often be accompanied by seizures. In addition, central application of IMA has been associated with a dose-related reduction in arterial pressure and sympathetic nervous discharge. 5. No specific receptor site or uptake system for IMA has yet been discovered, adding uncertainty to its role in central nervous system function. Yet the possibility cannot be overlooked that IMA plays a role in regulating blood pressure.

Quantitative Autoradiography on [(35)S]TBPS Binding Sites of Gamma- Aminobutyric Acid(A) Receptors in Discrete Brain Regions of High- Alcohol-Drinking and Low-Alcohol- Drinking Rats Selectively Bred forHigh- and Low-Alcohol Preference

It has been documented that ethanol can potentiate brain gamma-aminobutyric acid (GABA)ergic function, and there is a close link between the GABA(A) receptor complex and effects of ethanol, including reinforcement of alcohol which is a fundamental element of alcohol preference. However, it is unknown in what discrete brain regions GABA(A) receptors might be associated with alcohol preference. In the present study, [(35)S]t-butylbicyclophosphorothionate ([(35)S]TBPS) was used to localize GABA(A) receptors in high-alcohol-drinking (HAD) rats and low-alcohol-drinking (LAD) rats which were selectively bred for high and low alcohol preference, respectively. Initial qualitative observations indicated that [(35)S]TBPS binding sites were abundant in many brain areas including the cerebral cortex, hypothalamus and amygdala of HAD and LAD rats. Furthermore, the quantitative autoradiographic analysis revealed fewer [(35)S]TBPS binding sites of GABA(A) receptors in the amygdaloid complex, central medial thalamic nucleus, lateral hypothalamic nucleus and anterior hypothalamic nucleus of HAD rats than LAD rats. Collectively, this study has indicated that HAD rats selectively bred for high alcohol preference possess lower [(35)S]TBPS binding in the brain. Since lower TBPS binding has been proposed to reflect enhanced GABAergic function, as evidenced in rats with seizure or under alcohol withdrawal, the results from the present study suggest that HAD rats might have an enhanced GABAergic function. It is thus likely that enhanced GABAergic function in the brain might be related to high alcohol preference which is characteristic in HAD rats. In addition, the present result showing no difference of [(35)S]TBPS binding in the nucleus accumbens is also in agreement with a notion that [(35)S]TBPS binding may represent only a small spectrum of the GABA(A) receptor complex which is constituted of a sophisticated subunit combination whose functional compositions are still unknown. In conclusion, the present study supports the working hypothesis that GABA(A) receptors are involved in alcohol preference in HAD rats.

Downregulation of corticotropin-releasing factor mRNA, but not vasopressin mRNA, in the paraventricular hypothalamic nucleus of rats following nutritional stress

Stress can cause disturbance of homeostasis to result in illness. Stress can also induce various gene expression in different neuronal systems. For example, nutritional stress induced by acute food deprivation upregulates corticotropin-releasing factor (CRF) mRNA, whereas osmotic stress increases vasopressin (VP) mRNA. However, it is unknown if nutritional stress induced by chronic food deprivation has synergistic effects on CRF and VP mRNAs. We have used in situ hybridization in conjunction with quantitative autoradiography to demonstrate that nutritional stress induced by a 4-day food deprivation results in a body-weight loss with a significant decrease of CRF mRNAs, but not VP mRNAs in the paraventricular hypothalamic nucleus (PVN) of Sprague-Dawley rats. The present study has thus indicated that a chronic nutritional stress does not have synergistic effects on CRF and VP mRNAs. The decrease of CRF mRNAs is obviously related to the body-weight loss induced by food deprivation. This study thus supports a notion that the CRF, but not VP, neurons in the PVN play an important role in their neuroadaptation associated with body weight loss. Thus, it is conceivable that downregulated CRF neurons in the hypothalamus could be involved in pathogenesis of human eating disorder with severe weight loss, whereas upregulated CRF neurons could be associated with an opposite form of the eating disorder that causes obesity.

Genetic variation in immediate and delayed postictal refractory period in rats

We determined postictal refractoriness in Sprague-Dawley rats by comparing lengths of two suprathreshold ECS seizures given 15 s to 24 h apart. A bimodal (immediate and delayed) decrease in seizure duration was found, suggesting ECS alters mechanisms of seizure termination. Since adenosine is implicated in seizure termination, we determined immediate (30 s) and delayed (24 h) postictal ECS refractoriness in Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats which vary in adenosine properties and initial ECS seizure length. At 30 s, the decrease in seizures did not differ between WKY (-44%) and SHR (-36%) rats. At 24 h, SHR rats showed no change while the WKY rats showed a 20% decrease in seizure length (P < 0.01). These two strains also differed in the ability of the adenosine antagonist caffeine (50 mg/kg, i.p.) to prolong ECS seizures (no change for WKY, +13% for SHR, P < 0.001). The results suggest immediate and delayed postictal refractoriness are subject to genetic variation and may depend on central adenosine mechanisms.

Calcitonin gene-related peptide (CGRP) content and CGRP receptor binding sites in discrete forebrain regions of alcohol-preferring vs. -nonpreferring rats, and high alcohol-drinking vs. low alcohol-drinking rats

This study showed that alcohol-preferring (P) rats and high alcohol-drinking (HAD) rats possess fewer calcitonin gene-related peptide (CGRP) receptor binding sites than their respective controls in the central amygdaloid nucleus (CeA) which is known to be related to anxiety. Since P and HAD rats are selectively bred for high alcohol preference, and alcohol can produce anxiolytic effect, one can postulate that P and HAD rats preferentially drink alcohol in order to obtain its anxiolytic effect. This study supports a hypothesis that deficit of CGRP receptors in the CeA of P and HAD rats may contribute to alcohol preference.