Both Astrocytes and Neurons Contribute to the Potentiation Mediated by alpha1-Adrenoceptors of the beta-Adrenergic-Stimulated Cyclic AMP Production in Brain

Adrenaline induces calcium signal in astrocytes and vasoconstriction via activation of monoamine oxidase

Adrenaline or epinephrine is a hormone playing an important role in physiology. It is produced de-novo in the brain in very small amounts compared to other catecholamines, including noradrenaline. Although the effects of adrenaline on neurons have been extensively studied, much less is known about the action of this hormone on astrocytes. Here, we studied the effects of adrenaline on astrocytes in primary co-culture of neurons and astrocytes. Application of adrenaline induced calcium signal in both neurons and astrocytes, but only in neurons this effect was dependent on α- and β-receptor antagonists. The effects of adrenaline on astrocytes were less dependent on adrenoreceptors: the antagonist carvedilol had only moderate effect on the calcium signal and the agonist of adrenoreceptors methoxamine induced a signal only in small proportion of the cells. We found that adrenaline in astrocytes activates phospholipase C and subsequent release of calcium from the endoplasmic reticulum. Calcium signal in astrocytes is initiated by the metabolism of adrenaline by the monoamine oxidase (MAO), which activates reactive oxygen species production and induces lipid peroxidation. Inhibitor of MAO selegiline inhibited both adrenaline-induced calcium signal in astrocytes and the vasoconstriction that indicates an important role for monoamine oxidase in adrenaline-induced signalling and function.

Adrenergic stimulation of single rat astrocytes results in distinct temporal changes in intracellular Ca(2+) and cAMP-dependent PKA responses

During the arousal and startle response, locus coeruleus neurons, innervating practically all brain regions, release catecholamine noradrenaline, which reaches neural brain cells, including astrocytes. These glial cells respond to noradrenergic stimulation by simultaneous activation of the α- and β-adrenergic receptors (ARs) in the plasma membrane with increasing cytosolic levels of Ca(2+) and cAMP, respectively. AR-activation controls a myriad of processes in astrocytes including glucose metabolism, gliosignal vesicle homeostasis, gene transcription, cell morphology and antigen-presenting functions, all of which have distinct temporal characteristics. It is known from biochemical studies that Ca(2+) and cAMP signals in astrocytes can interact, however it is presently unclear whether the temporal properties of the two second messengers are time associated upon AR-activation. We used confocal microscopy to study AR agonist-induced intracellular changes in Ca(2+) and cAMP in single cultured cortical rat astrocytes by real-time monitoring of the Ca(2+) indicator Fluo4-AM and the fluorescence resonance energy transfer-based nanosensor A-kinase activity reporter 2 (AKAR2), which reports the activity of cAMP via its downstream effector protein kinase A (PKA). The results revealed that the activation of α1-ARs by phenylephrine triggers periodic (phasic) Ca(2+) oscillations within 10s, while the activation of β-ARs by isoprenaline leads to a ∼10-fold slower tonic rise to a plateau in cAMP/PKA activity devoid of oscillations. Thus the concomitant activation of α- and β-ARs triggers the Ca(2+) and cAMP second messenger systems in astrocytes with distinct temporal properties, which appears to be tailored to regulate downstream effectors in different time domains.

β-Adrenoceptor mRNA Levels Can Be Increased Via β-Adrenoceptor-independent Events

The drug (-)-oxaprotiline has been used as a tool to study the regulation of the β-adrenoceptor in rat C6 glioma cells.

Treatment with (-)-oxaprotiline for 30 min results in an increase in steady-state β-adrenoceptor mRNA levels. The effect is β-adrenoceptor-independent, is not additive or synergistic with isoprenaline treatment, and does not involve activation of adenylate cyclase.

The data show that (-)-oxaprotiline can affect β-adrenoceptor mRNA levels via a mechanism that bypasses the receptor, perhaps involving direct activation of protein kinase A.

Colocalization of neurotransmitter receptors on astrocytes in explant cultures of rat CNS

In recent years evidence has accumulated that astrocytes express functional receptors for a variety of neurotransmitters/neuromodulators. By means of electrophysiological and combined autoradiographic and immunohistochemical methods we have demonstrated the colocalization of cholinergic, adrenergic and peptidergic receptors on astrocytes in explant cultures from various regions of rat central nervous system. A great number of biochemical and electrophysiological studies from other laboratories have shown that most of the neurotransmitters exert their effects on second messenger systems and on Ca2+-activated K+-channels. Furthermore, certain neurotransmitters are involved in the regulation of energy metabolism by stimulating enzymatic breakdown of glycogen in astrocytes. It was suggested that there is a cross-talk between the various neurotransmitter receptors on the glial membrane and that these receptors act in a synergistic or antagonistic way. The coexistence of cholinergic and peptidergic receptors on astrocytes is of great interest since both neurotransmitter systems are involved in cognitive functions and are impaired in patients with Alzheimer's dementia. The question is therefore raised whether not only neurones but also astrocytes might be involved in neurodegenerative disorders such as Alzheimer's disease.

Alpha-1-noradrenergic neurotransmission, corticosterone, and behavioral depression

BACKGROUND

Impaired brain alpha-1 noradrenergic neurotransmission has been implicated in some of the symptoms of depressive illness but has been difficult to investigate experimentally because of the insensitivity of current animal models of depression. The present experiment addressed this problem by examining the effects of pharmacologic blockade and corticosteroid-induced desensitization of alpha-1 receptors on two newer, more sensitive models in mice: the inhibition of nest-leaving and the tail suspension tests (TST).

METHODS

Male mice were administered either prazosin, betaxolol, atipamezole, corticosterone, or repeated restraint stress prior to measurement of either nest-leaving or TST. General behavioral function was assessed in horizontal wire, swim, and latency to escape footshock tests.

RESULTS

Prazosin increased depressive behavior in the nest-leaving and TSTs, whereas corticosterone and restraint stress did so only in the more sensitive nest-leaving test. Betaxolol also reduced nest-leaving, suggestive of an alpha-1 beta-1 receptor synergy. The effects of these agents could not be attributed to hypotension, sedation, or general behavioral impairment.

CONCLUSIONS

The fact that a reduction in alpha-1 noradrenergic neurotransmission increases depressive behavior, coupled with the fact that this change can result from elevated corticosteroid secretion, provides further support for a role of this factor in depressive illness. As not all alpha-1 functions are reduced in depression, it is likely that only a subgroup or specific locality of alpha-1 receptors are affected.

Effects of noradrenaline on frequency tuning of auditory cortex neurons during wakefulness and slow-wave sleep

This study shows the effects of noradrenaline (NA) on receptive fields of auditory cortex neurons in awake animals; it is the first one to describe the effects of NA on neurons in sensory cortex, in different natural states of vigilance. The frequency receptive field of 250 auditory cortex neurons was determined before, during and after ionophoretic application of NA while recording the state of vigilance of unanaesthetized guinea-pigs. When NA significantly changed the spontaneous activity (85 out of 250 cells), the dominant effect was a decrease (61 out of 85 cells, 72%). When NA significantly changed the evoked activity (107 out of 250 cells), the dominant effect was also a decrease (84 out of 107 cells, 78%). During and after NA application, the signal-to-noise ratio (S/N, i.e. evoked/spontaneous activity) was unchanged, but the selectivity for pure-tone frequencies was enhanced. When the effects occurring in wakefulness and in slow-wave sleep (SWS) were compared, it appeared that the predominantly inhibitory effect of NA on spontaneous and evoked activity was present in both states. The S/N ratio was unchanged and the selectivity was increased in both states. However, during SWS, the percentage of cells inhibited by NA was lower, and the effects on the frequency selectivity were smaller than in wakefulness. In contrast, GABA produced similar inhibitory effects on spontaneous and on evoked activity during wakefulness and SWS. Comparisons with previous data obtained using the same protocol in urethane anaesthetized animals (Manunta & Edeline 1997) indicate that the effects of NA were qualitatively the same. Based on these results, we suggest that any hypothesis concerning the role of NA in cortical plasticity should take into account the fact that the predominantly inhibitory effects of NA lead to decrease the size of the receptive field.

The C-terminal third intracellular loop of the rat AT1A angiotensin receptor plays a key role in G protein coupling specificity and transduction of the mitogenic signal

To identify the role(s) of the third intracellular loop of the angiotensin II (AngII) type 1A (AT1A) receptor in G protein coupling specificity and receptor activation, several chimerae were constructed and characterized. The cDNA sequence encoding the C-terminal segment of the third intracellular loop of the AT1A receptor (residues 234-240) was replaced with the homologous regions of the alpha1B adrenergic (alpha1B-AR), the beta2 adrenergic (beta2-AR), and the AngII type 2 (AT2) receptors. These chimeric receptors were stably expressed in Chinese hamster ovary cells, and their pharmacological and functional properties were characterized, including AngII-induced inositol phosphate and cyclic AMP (cAMP) productions, [3H]thymidine incorporation into DNA, and internalization. The affinities of these chimeric receptors for [Sar1]AngII, [Sar1,Ile8]AngII, and losartan were essentially normal; however, the affinity of these mutants was increased by a factor of 10-40 for the AT2-specific ligand CGP42112A. The functional properties of the alpha1B-AR chimera were essentially identical to those of the wild type AT1A receptor. On the other hand, replacement with the beta2-AR segment produced a partial reduction of the inositol phosphate production, a measurable AngII-induced cAMP accumulation, a reduced internalization, and a total impairment to transduce the mitogenic effect of AngII. The AT2 chimera presented a normal internalization, but was inactive in all the other functional tests. In conclusion, the distal segment of the third intracellular loop of the rat AT1A receptor plays a pivotal role in coupling selectivity and receptor signaling via G protein(s) as well as in the activation of the specific signaling pathways involved in the mitogenic actions of AngII.

Roles of second-messenger systems and neuronal activity in the regulation of lordosis by neurotransmitters, neuropeptides, and estrogen: a review

Many neurotransmitters and neuropeptides can affect the rodent feminine sexual behavior, lordosis, when administered in the ventromedial hypothalamus (VMH), midbrain central gray (MCG), or other brain regions. A survey of the electrophysiological and biochemical actions of these neural agents revealed that there is a very consistent association between lordosis facilitation with both the activation of the phosphoinositide (PI) pathway and the excitation of VMH and MCG neurons. In contrast, lordosis inhibition is associated, less consistently, with alterations of the adenylate cyclase (AC) system and the inhibition of neuronal activity. The findings that lordosis could be facilitated by going beyond membrane receptors and directly activating the PI pathway, suggest that this second-messenger pathway is a common mediator for the lordosis-facilitating agents. Furthermore, as in the case of stimulating membrane receptors, direct activation of this common mediator also requires estrogen priming for lordosis facilitation. Therefore, it is likely that the PI pathway is modulated by estrogen in the permissive action of estrogen priming. Indeed, a literature review shows that estrogen can affect selective isozymes of key enzyme families of the PI pathway at various levels. Such selective modulations, at several levels, could easily alter the course of a PI cascade; thence, the eventual functional outcome. These findings prompt us to propose that estrogen enables lordosis to be facilitated by a selective modulation of the PI pathway.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

Mutation of Asp74 of the rat angiotensin II receptor confers changes in antagonist affinities and abolishes G-protein coupling

Aspartic acid in the second transmembrane domain is a highly conserved amino acid among the G protein-coupled receptors and is functionally important for agonist binding and G-protein coupling in beta 2-adrenergic and luteinizing hormone receptors. To determine whether this aspartic acid is also involved in the function of the rat vascular angiotensin II receptor subtype 1 (AT1a), Asp74 was replaced either by asparagine or by glutamic acid. When expressed in CHO cells, the two mutants and the wild-type receptor displayed similarly high affinities for the agonist [Sar1, Tyr(125I)4]angiotensin II [where Sar is sarcosine and Tyr(125I) is monoiodinated tyrosine] and the other agonists: ([Sar1]angiotensin II > angiotensin II > angiotensin III >> angiotensin I). However, the Asn74 mutant shows striking differences in its affinity for some antagonists when compared with the wild-type receptor: the affinity for DUP753 was decreased 10-fold, whereas it was increased 6-fold for [Sar1,Ala8]angiotensin II and 20-fold for CGP42112A. These pharmacological changes were associated with a major defect in transmembrane signaling, since angiotensin II was unable to stimulate inositol phosphate production and increase cytosolic Ca2+ concentration through the two mutated receptors, whereas a clear dose-dependent stimulation was observed in cells expressing the wild-type receptor. Angiotensin II was able to promote DNA synthesis through the wild type but not through the mutated receptors. These data indicate that the conserved Asp74 residue of the AT1a receptor is important for the binding of angiotensin II antagonists and is essential for the transmembrane signaling cascade.

Synergistic effects in the alpha 1- and beta 1-adrenergic regulations of intracellular calcium levels in striatal astrocytes

1. Using indo-1 as a calcium fluorescent probe, we have observed the following in striatal astrocytes in primary culture. 2. The stimulation of alpha-adrenoceptors induces a rapid rise in cytosolic calcium resulting from an internal calcium mobilization followed by an external calcium influx (4-min duration). 3. The stimulation of beta 1-adrenoceptors evokes only a slight internal calcium mobilization (90-sec duration). 4. The simultaneous stimulation of beta 1- and alpha 1-adrenoceptors induces a more prolonged calcium influx (10 min). The latter phenomenon could explain the calcium-dependent synergistic effects of alpha 1 and beta stimulation on cAMP production already described in the brain.