Involvement of adrenoceptors in the angiotensin II-induced expression of inducible transcription factors in the rat forebrain and hypothalamus

Adrenoceptors in brain: cellular gene expression and effects on astrocytic metabolism and [Ca(2+)]i

Recent in vivo studies have established astrocytes as a major target for locus coeruleus activation (Bekar et al., 2008), renewing interest in cell culture studies on noradrenergic effects on astrocytes in primary cultures and calling for additional information about the expression of adrenoceptor subtypes on different types of brain cells. In the present communication, mRNA expression of alpha(1)-, alpha(2)- and beta-adrenergic receptors and their subtypes was determined in freshly isolated, cell marker-defined populations of astrocytes, NG2-positive cells, microglia, endothelial cells, and Thy1-positive neurons (mainly glutamatergic projection neurons) in murine cerebral cortex. Immediately after dissection of frontal, parietal and occipital cortex of 10-12-week-old transgenic mice, which combined each cell-type marker with a specific fluorescent signal, the tissue was digested, triturated and centrifuged, yielding a solution of dissociated cells of all types, which were separated by fluorescence-activated cell sorting (FACS). mRNA expression in each cell fraction was determined by microarray analysis. alpha(1A)-Receptors were unequivocally expressed in astrocytes and NG2-positive cells, but absent in other cell types, and alpha(1B)-receptors were not expressed in any cell population. Among alpha(2)-receptors only alpha(2A)-receptors were expressed, unequivocally in astrocytes and NG-positive cells, tentatively in microglia and questionably in Thy1-positive neurons and endothelial cells. beta(1)-Receptors were unequivocally expressed in astrocytes, tentatively in microglia, and questionably in neurons and endothelial cells, whereas beta(2)-adrenergic receptors showed tentative expression in neurons and astrocytes and unequivocal expression in other cell types. This distribution was supported by immunochemical data and its relevance established by previous studies in well-differentiated primary cultures of mouse astrocytes, showing that stimulation of alpha(2)-adrenoceptors increases glycogen formation and oxidative metabolism, the latter by a mechanism depending on intramitochondrial Ca(2+), whereas alpha(1)-adrenoceptor stimulation enhances glutamate uptake, and beta-adrenoceptor activation causes glycogenolysis and increased Na(+), K(+)-ATPase activity. The Ca(2+)- and cAMP-mediated association between energy-consuming and energy-yielding processes is emphasized.

Regulation of c-fos, c-jun and c-myc Gene Expression by Angiotensin II in Primary Cultured Rat Astrocytes: Role of ERK1/2 MAP Kinases

We have previously shown that angiotensin II (Ang II) stimulates astrocyte growth through activation of ERK1/2 mitogen activated protein (MAP) kinases. In the current study, we determined whether Ang II stimulates the expression of c-fos, c-jun and c-myc in brainstem astrocyte cultures. Reverse transcriptase-PCR analysis showed c-fos, c-jun, and c-myc mRNAs were induced by Ang II. The EC50 values for Ang II stimulation of c-fos, c-jun and c-myc were 1.3, 1.68 and 1.4 nM, respectively. Ang II (100 nM) induced peak stimulation for all genes by 45 min followed by a gradual decline. Inhibition of ERK1/2 by PD98059 attenuated Ang II-induced c-fos and c-myc mRNA expression (by 75% and 100%, respectively) but was ineffective in preventing Ang II induction of c-jun. These studies show for the first time in brainstem astrocytes that Ang II induces the expression of c-fos, c-myc and c-jun, and showed that ERK1/2 mediate Ang II stimulation of c-fos and c-myc. These data implicate the ERK1/2 MAP kinase pathway as a divergent point in controlling Ang II stimulation of immediate early response genes in the central nervous system.

Heat acclimation and cross-tolerance against novel stressors: genomic–physiological linkage

Heat acclimation (AC) is a "within lifetime" reversible phenotypic adaptation, enhancing thermotolerance and heat endurance via a transition to "efficient" cellular performance when acclimatory homeostasis is reached. An inseparable outcome of AC is the development of cross-tolerance (C-T) against novel stressors. This chapter focuses on central plasticity and the molecular-physiological linkage of acclimatory and C-T responses. A drop in temperature thresholds (T-Tsh) for activation of heat-dissipation mechanisms and an elevated T-Tsh for thermal injury development imply autonomic nervous system (ANS) and cytoprotective network involvement in these processes. During acclimation, the changes in T-Tsh for heat dissipation are biphasic. Initially T-Tsh drops, signifying the early autonomic response, and is associated with perturbed peripheral effector cellular performance. Pre-acclimation values return when acclimatory homeostasis is achieved. The changes in the ANS suggest that acclimatory plasticity involves molecular and cellular changes. These changes are manifested by the activation of central peripheral molecular networks and post-translational modifications. Sympathetic induction of elevated HSP 72 reservoirs, with faster heat shock response, is only one example of this. The global genomic response, detected using gene-chips and cluster analyses imply upregulation of genes encoding ion channels, pumps, and transporters (markers for neuronal excitability) in the hypothalamus at the onset of AC and down regulation of metabotrophic genes upon long term AC. Peripherally, the transcriptional program indicates a two-tier defense strategy. The immediate transient response is associated with the maintenance of DNA and cellular integrity. The sustained response correlates with long-lasting cytoprotective-signaling networks. C-T is recorded against cerebral hypoxia, hyperoxia, and traumatic brain injury. Using the highly developed ischemic/reperfused heart model as a baseline, it is evident that C-T stems via protective shared pathways developed with AC. These comprise constitutive elevation of HIF 1alpha and associated target pathways, HSPs, anti-apoptosis, and antioxidative pathways. Collectively the master regulators of AC and C-T are still enigmatic; however, cutting-edge investigative techniques, using a broad molecular approach, challenge current ideas, and the data accumulated will pinpoint novel pathways and provide new perspectives.

Heat acclimation affects the neuromodulatory role of AngII and nitric oxide during combined heat and hypohydration stress

We studied the effect of heat acclimation on the neuromodulatory role of angiotensin (AngII) and nitric oxide during combined heat (39 degrees C) and hypohydration (water deprivation, -10% body weight) stress. Rats were divided into control (C), short (2d-STHA) or long (30d-LTHA) acclimation (34 degrees C) groups. AngII, 7-nitroindazole (7NI)-nNOS blocker, or both were centrally administered (5 mul, bolus) under light chloroform anesthesia prior to each experimental paradigms: (1) In vivo: measurements of skin-vasodilatation (VTsh) and salivation-cooling (STsh) thresholds, and heat endurance in conscious heat/hypohydrated stressed rats; (2) expression of AT(1) and AT(2) AngII receptors and nNOS were measured in the hypothalamus (Western blot); (3) transcript levels of the coding genes were measured using real-time PCR. A synthesis of the results shows a biphasic acclimatory profile of VTsh, STsh, and transcript levels of all studied genes, with transient up/down-regulatory changes on STHA. AngII affected the physiological integrative outcome primarily during euhydration, although AT membranal changes (except in LTHA) were confined to hypohydration. 7NI had an impact during hypohydration. Evidence is provided that AngII and 7NI modulate thermoregulation primarily via AT(1) and AT(2) receptors, with predominance of AT(2) signaling following LTHA and/or hypohydration, opposing a drop in AT(1)-mediated thresholds. The final shaping of AngII signaling depends on cross-talk between nNOS and AngII receptors at both molecular and protein levels. Hypohydration induces transcriptional responses but desensitizes AngII receptors signaling, attenuating their effect on VTsh and STsh, and abolishing the beneficial thermoregulatory effects achieved by heat acclimation. nNOS, AngII receptor-independent pathway is also implicated.

Differential regulation of central vasopressin receptors in transgenic rats with low brain angiotensinogen

The consequences of permanent alteration to the brain renin-angiotensin system (RAS) on central vasopressinergic system was studied in transgenic rats with low brain angiotensinogen [TGR(ASrAOGEN)]. Levels of vasopressin (AVP) and V1a receptor mRNAs were measured by ribonuclease protection assay (RPA) and AVP by radioimmunoassay (RIA). AVP (100 pmol/50 nl) was microinjected into the nucleus tractus solitarii (NTS) of urethane-anesthetized TGR(ASrAOGEN) and Sprague-Dawley (SD) rats and the mean arterial pressure (MAP) and heart rate (HR) baroreflex induced by phenylephrine were evaluated. AVP but not its mRNA levels were significantly lower in the hypothalamus and hypophysis of TGR(ASrAOGEN) rats. Brainstem V1a mRNA levels were significantly higher in TGR(ASrAOGEN) in comparison to SD rats (5.2+/-0.4% vs. 3.3+/-0.2% of beta-actin mRNA, P<0.05). In contrast, the hypothalamic V1a mRNA levels in TGR(ASrAOGEN) were not different from those found in SD rats. AVP microinjections induced a greater decrease in MAP in TGR(ASrAOGEN) in comparison with SD rats (-19.9+/-5.2 vs. -7.5+/-0.7 mm Hg, P<0.01). The significantly higher baroreflex sensitivity observed in TGR compared to that of SD rats was normalized after AVP microinjection. The increased brainstem V1a mRNA levels and sensitivity to AVP in TGR(ASrAOGEN) rats indicates a functional upregulation of AVP receptors in the NTS. The fact that the hypothalamic V1a mRNA levels are not altered indicates that these receptors are differentially regulated in different brain regions. This study demonstrates that a permanent deficit in brain angiotensinogen synthesis can alter the functionality of central vasopressinergic system.

Nitric oxide and angiotensin II: neuromodulation of thermoregulation during combined heat and hypohydration stress

We investigated the central role of nitric oxide and AngII on thermoregulation in rats (Rattus norvegicus, Sabra strain,) undergoing heat-stress in euhydration or hypohydration (water deprivation, -10% b.wgt). Experimental rats received AngII (100 pm), 7-nitroindazole-an antagonist of neuronal nitric oxide synthase (7NI-100 nm), or AngII+7NI in a 5-microl bolus intracerebroventricularly (i.c.v.) under light chloroform anesthesia; untreated control rats received saline or DMSO (5%). We used three experimental paradigms: (1) heat defense responses [salivation (STsh), vasodilatation (VTsh) temperature thresholds and heat-endurance] in conscious, heat-stressed (39 degrees C) rats; (2) Western immunoblotting to detect AngII AT(1) and AT(2) receptors and nNOS protein expression; (3) real-time PCR to measure gene transcripts. In the in vivo experiment, 7NI decreased thermoregulatory thresholds, namely, NO had a reciprocal effect that was more pronounced during hypohydration (e.g. euhydration: STsh: -0.7+/-0.01 degrees C, hypohydration: -0.9+/-0.18 degrees C, p<0.05). AngII decreased STsh by 0.9+/-0.18 degrees C (p<0.05) upon euhydration but increased it in hypohydration (+1.7+/-0.28 degrees C, p<0.05). A novel finding was the involvement of AT(2) receptors in thermoregulation, which was more pronounced upon hypohydration. The response to NO was mediated via AT(1) and AT(2) receptors signaling, as well as independently. A synthesis of the results from all experimental paradigms suggests (1) a dominant influence (decrease) of NO on AT(1) receptors, thereby changing AT(1)/AT(2) receptor ratio and their signaling pathway; primarily upon hypohydration; (2) an influence of AngII (increase) on receptor density, more pronounced during hypohydration, at both gene transcription and translation levels; and (3) an effect of AngII on nNOS protein levels, implying a mutual effect of AngII and NO.

Transcription factors and drugs in the brain

In mammalian cells, protein de novo synthesis is mainly regulated at the stage of gene transcription by RNA polymerase II in the nucleus. Transcription factors are proteins that bind to the specific nucleotide sequences at promoter or enhancer regions on target genes to control the transcription of mRNA from genomic DNA. In this article, we have outlined the signal responsiveness of different transcription factors to particular drugs in the brain. Nuclear transcription factors rapidly respond to a variety of extracellular signals carried by neurotransmitters, hormones and autacoids as a third messenger in frequent situations. Translated proteins are responsible for a number of physiological and pathological events for a long period in the brain. We have also discussed possible involvement of transcription factors in molecular mechanisms underlying development of tolerance and dependence to drugs following acute and chronic administration.