Synergistic interactions between alpha 1- and alpha 2-adrenergic receptors in activating 3H-inositol phosphate formation in primary glial cell cultures

Interaction between alpha(1)- and alpha(2)-adrenoreceptors contributes to enhanced constrictor effects of norepinephrine in mesenteric veins compared to arteries

Mesenteric veins are more sensitive than arteries to the constrictor effects of sympathetic nerve stimulation and alpha-adrenoceptor agonists. We tested the hypothesis that alpha(1)- and alpha(2)-adrenoceptors interact to enhance adrenergic reactivity of mesenteric veins. We studied neurogenic and agonist-induced constrictions of mesenteric veins and arteries in vitro. Norepinephrine concentration-response curves were left-shifted in veins compared to arteries. UK 14,304 (0.01-1 microM, alpha(2)-adrenoceptor receptor agonist) did not constrict arteries or veins but enhanced constrictions and Ca(2+) signals mediated by alpha(1)-adrenoceptor stimulation in veins. Yohimbine (alpha(2)-adrenoceptor receptor antagonist) and MK912 (alpha(2C)-adrenoceptor receptor antagonist), but not alpha(2A)- or alpha(2B)-adrenoceptor antagonists, produced rightward shifts in norepinephrine concentration-response curves in veins. Pharmacological studies revealed that alpha(1D)-adrenoceptors mediate venous constrictions. Norepinephrine responses in veins from alpha(2C)-adrenoceptor knock-out (KO) mice were not different from wild type veins. Yohimbine inhibited norepinephrine constrictions in alpha(2C)-adrenoceptor KO veins suggesting that there is upregulation of other alpha(2)-adrenoceptors in alpha(2C)-KO mice. These data indicate that alpha(1D)- and alpha(2C)-adrenoceptors interact in veins but not in arteries. This interaction enhances venous adrenergic reactivity. Mesenteric vein-specific alpha(2)-adrenoceptor linked Ca(2+) and perhaps other signaling pathways account for enhanced venous adrenergic reactivity.

Mechanisms of cross-talk between G-protein-coupled receptors resulting in enhanced release of intracellular Ca2+

Alteration in [Ca(2+)](i) (the intracellular concentration of Ca(2+)) is a key regulator of many cellular processes. To allow precise regulation of [Ca(2+)](i) and a diversity of signalling by this ion, cells possess many mechanisms by which they are able to control [Ca(2+)](i) both globally and at the subcellular level. Among these are many members of the superfamily of GPCRs (G-protein-coupled receptors), which are characterized by the presence of seven transmembrane domains. Typically, those receptors able to activate PLC (phospholipase C) enzymes cause release of Ca(2+) from intracellular stores and influence Ca(2+) entry across the plasma membrane. It has been well documented that Ca(2+) signalling by one type of GPCR can be influenced by stimulation of a different type of GPCR. Indeed, many studies have demonstrated heterologous desensitization between two different PLC-coupled GPCRs. This is not surprising, given our current understanding of negative-feedback regulation and the likely shared components of the signalling pathway. However, there are also many documented examples of interactions between GPCRs, often coupling preferentially to different signalling pathways, which result in a potentiation of Ca(2+) signalling. Such interactions have important implications for both the control of cell function and the interpretation of in vitro cell-based assays. However, there is currently no single mechanism that adequately accounts for all examples of this type of cross-talk. Indeed, many studies either have not addressed this issue or have been unable to determine the mechanism(s) involved. This review seeks to explore a range of possible mechanisms to convey their potential diversity and to provide a basis for further experimental investigation.

Enduring vulnerability to transient reinstatement of hemiplegia by prazosin after traumatic brain injury

A single dose of an alpha1-noradrenergic antagonist transiently reinstates hemiplegia after recovery from brain injury, which suggests that noradrenaline (NA) is required to maintain recovery. No systematic studies have determined the postinjury duration of this vulnerability. This study used a within-subject, dose-response design to determine whether prazosin (PRAZ), an alpha1-NA antagonist, or propranolol (PROP), a beta-NA antagonist, would continue to reinstate hemiplegia over time after recovery from weight-drop traumatic brain injury (TBI). PRAZ transiently reinstated hemiplegia as measured by beam walk (BW) score in a dose-dependent manner, with the same degree of symptom reinstatement at 1, 3, 6, and 12 months post-TBI. Between-animal variability in reinstatement of hemiplegia by PRAZ was predicted by severity of deficits in BW ability 24 h after TBI. In contrast, PRAZ did not reinstate tactile placing deficits at 1 month post-TBI suggesting a different mechanism of maintaining recovery for each task. Reinstatement of symptoms are not due to sedation. Only TBI rats receiving PRAZ, not high, sedating doses of PROP or saline (SAL), showed return of hemiplegia. These data indicate that vulnerability to transient reinstatement of hemiplegia on some tasks endures long after functional recovery from TBI.

Functional characterisation of alpha 1-adrenoceptor subtypes mediating noradrenaline-induced inositol phosphate formation in rat thalamus slices

In cross-chopped slices from rat thalamus and in the presence of 10 mM LiCl, noradrenaline stimulated the accumulation of [3H]inositol phosphates with [3H]inositol monophosphates ([3H]IP1) being the major product detected (86 +/- 2% of total [3H]inositol phosphates). Noradrenaline-induced [3H]IP1 accumulation was concentration-dependent and yielded and EC50 of 4.6 +/- 0.2 microM, maximum effect of 272 +/- 3% of basal formation and Hill coefficient (nH) of 1.6 +/- 0.1. The effect of 100 microM noradrenaline was inhibited by the alpha 1-adrenoceptor antagonists prazosin, (+)-niguldipine, 5-methylurapidil and WB-4101 (2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4-benzodioxane). The inhibition curve for prazosin best fit to a single-site model whereas curves for (+)-niguldipine, 5-methylurapidil and WB-4101 best fit to a two-site model. The putative alpha 1D-adrenoceptor-selective antagonist BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8- azaspiro[4.5]decane-7,9-dione) showed low potency and efficacy to inhibit the response to noradrenaline. Pre-treatment of the slices with chloroethylclonidine (100 microM; 30 min) decreased by 64 +/- 4% the maximum response. Noradrenaline-induced [3H]IP1 accumulation was significantly reduced by Ca2+ removal (by 64 +/- 2%) and by the Ca(2+)-channel blockers Ni2+, Co2+ and nimodipine (inhibition of 56 +/- 6%, 54 +/- 5% and 41 +/- 5%, respectively). Taken together these results indicate that noradrenaline-induced inositol phosphate formation in thalamus slices is mainly mediated by the activation of both alpha 1B and alpha 1A subtypes of alpha 1-adrenoceptors.

Cellular and subcellular distribution of alpha 2A-adrenergic receptors in the visual cortex of neonatal and adult rats

Activation of alpha 2-adrenergic receptors (alpha 2AR) in the cerebral cortex has been shown to modulate visually guided delayed response tasks as well as anxiety and depression. We used an antiserum directed specifically against the A subtype of alpha 2AR (alpha 2AAR) to determine the cell types and subcellular sites for noradrenergic reception mediated by this receptor in the adult and the developing rat visual cortices. Light microscopic examination of adult tissue revealed numerous labeled perikarya in layers II-VI, many of which appeared distinctly pyramidal. A few perikarya in layer I also were immunoreactive. In all layers, alpha 2AAR immunoreactivity (alpha 2AAR-ir) was present within proximal dendrites and fine processes. In neonatal tissue, there was an intense, distinct band of immunoreactivity spanning the layer composed of tightly packed immature cell bodies, i.e., the cortical plate. The band dissipated as this tier differentiated postnatally into the supragranular layers. Electron microscopy showed that the supragranular layers, which contain the highest density of noradrenergic fibers, also contain the highest areal density of labeled postsynaptic junctions beyond 2 weeks of age. Throughout the ages, the majority of immunoreactivity occurred at sites which, in single ultrathin sections, appeared to be nonjunctional sites of axons, dendrites, and in glial processes. Our observations indicate that (1) both pyramidal and nonpyramidal neurons are receptive to norepinephrine via alpha 2AAR, (2) alpha 2AAR synthesis is robust prior to synaptogenesis, and (3) alpha 2AAR operates both pre- and postsynaptically.

Adenosine receptor activation potentiates phosphoinositide hydrolysis and arachidonic acid release in DDT1-MF2 cells: putative interrelations

Studies were undertaken in an effort to discern possible mechanisms by which the A1 adenosine receptor agonist cyclopentyladenosine (CPA) enhances the norepinephrine-stimulated (NE-stimulated) hydrolysis of phosphoinositides in DDT1-MF2 cells. Measurements of arachidonic acid release revealed similar behaviours to those observed in measurements of phosphoinositide hydrolysis. In the presence of NE, both second messenger responses were potentiated by the addition of CPA, whereas in the absence of NE, CPA had little or no effect on either second messenger. The stimulation and potentiation of both second messenger responses were enhanced in the presence of extracellular calcium, and in each case these effects were persistent over time. For either second messenger system the stimulation by NE and the potentiation by CPA appeared to utilize separate mechanisms as evidenced by the fact that the potentiations by CPA were selectively antagonized by a cAMP analogue or by pertussis toxin, whereas the stimulations by NE were essentially unaffected by these agents. Inhibition of phospholipase A2 (PLA2) also blocked the potentiation of PLC by CPA, without affecting NE-stimulated phosphoinositide hydrolysis. Furthermore, in the presence of CPA, the exogenous administration of PLA2 was found to stimulate phosphoinositide hydrolysis in these cells. These data are consistent with a hypothesis whereby the apparent potentiation of NE-stimulated phosphoinositide hydrolysis by CPA is actually due to the stimulation by CPA of a second pathway of phospholipase C activity which is additive to that of NE. The activation of PLC and PLA2 by NE produces phospholipid products which may play a permissive role in the pathway coupling adenosine A1 receptors to these phospholipases. The formation of lysophosphatidic acid is suggested as one possible mediator of this permissive effect.

Subcellular calcium oscillators and calcium influx support agonist-induced calcium waves in cultured astrocytes

We have analysed Ca2+ waves induced by norepinephrine in rat cortical astrocytes in primary culture using fluorescent indicators fura-2 or fluo-3. The temporal pattern of the average [Ca2+]i responses were heterogeneous from cell to cell and most cells showed an oscillatory response at concentrations of agonist around EC50 (200 nM). Upon receptor activation, [Ca2+]i signals originated from a single cellular locus and propagated throughout the cell as a wave. Wave propagation was supported by specialized regenerative calcium release loci along the length of the cell. The periods of oscillations, amplitudes, and the rates of [Ca2+]i rise of these subcellular oscillators differ from each other. These intrinsic kinetic properties of the regenerative loci support local waves when stimulation is continued over long periods of time. The presence of local waves at specific, invariant cellular sites and their inherent kinetic properties provide for the unique and reproducible pattern of response seen in a given cell. We hypothesize that these loci are local specializations in the endoplasmic reticulum where the magnitude of the regenerative Ca2+ release is higher than other regions of the cell. Removal of extracellular Ca2+ or blockade of Ca2+ channels by inorganic cations (Cd2+ and Ni2+) during stimulation of adrenergic receptors alter the sustained plateau component of the [Ca2+]i response. In the absence of Ca2+ release, due to store depletion with thapsigargin, agonist occupation alone does not induce Ca2+ influx in astrocytes. This finding suggests that, under these conditions, receptor-operated Ca2+ entry is not operative. Furthermore, our experiments provide evidence for local Ca2+ oscillations in cells which can support both wave propagation as well as spatially discrete Ca2+ signalling.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Role of phosphoinositide hydrolysis in astrocyte volume regulation

Astrocytes exposed to hypoosmotic stress swell and subsequently reduce their size to almost their original volume, a phenomenon called regulatory volume decrease (RVD). We found that during hypoosmotic swelling there was a twofold increase in phosphatidylinositol (PI) hydrolysis. This increase was inhibited by the phospholipase C inhibitor, U-73122 (10 microM). Inhibition of PI hydrolysis resulted in blockage of RVD. We also examined whether agents that stimulate PI hydrolysis would enhance RVD. These agents significantly accelerated RVD. The rank order of potency was endothelin (20 nM) > or = norepinephrine (100 microM) > endothelin-3 (7 nM) > thrombin (1 U/ml) > or = ATP (500 microM) > bradykinin (20 microM) > or = carbachol (500 microM), as indicated by RVD rate constants. The extent of PI hydrolysis induced by these agents at the beginning of RVD exhibited a logarithmic relationship with the magnitude of RVD enhancement. Also, there was a linear relationship between the rate of PI hydrolysis and RVD rate constants. Our results suggest that stimulated PI hydrolysis is involved in the regulation of cell volume in astrocytes.

Involvement of pertussis toxin-sensitive and -insensitive mechanisms in alpha-adrenoceptor modulation of noradrenaline release from rat sympathetic neurones in tissue culture

1. Sympathetic neurones derived from superior cervical ganglia of neonatal rats and maintained in tissue culture were used to investigate the modulation of neurotransmitter release by presynaptic receptors. Three week old cultures of neurones were loaded with [3H]-noradrenaline to label endogenous neurotransmitter stores. Release of noradrenaline was evoked by depolarization with raised extracellular K+ in the presence of desipramine and corticosterone to prevent uptake of released catecholamine. 2. Potassium (55 mmol l-1) depolarization for 30 s caused more than a four fold increase in 3H overflow from basal levels but this increase was reduced by up to 40% in the presence of exogenous noradrenaline (1 mumol l-1). The inhibition by noradrenaline of depolarization-evoked overflow was blocked by the alpha 1/alpha 2-adrenoceptor antagonist, phentolamine. Phentolamine alone did not increase K(+)-evoked 3H overflow. 3. The alpha 2-adrenoceptor antagonist, yohimbine, produced a concentration-dependent block of the inhibition by noradrenaline of K(+)-evoked overflow, while the alpha 1-adrenoceptor antagonist, prazosin, was without effect at concentrations up to 0.1 mumol l-1. 4. The beta-adrenoceptor antagonist, propranolol, neither reduced K(+)-evoked overflow nor increased the degree of inhibition caused by the addition of 1 mumol l-1 noradrenaline. 5. The alpha 2-adrenoceptor agonist, clonidine (1 mumol l-1) was less effective than noradrenaline at inhibiting K(+)-evoked overflow, while the alpha 1-adrenoceptor agonist, phenylephrine (1 mumol l-1) had no significant effect. 6. The L-channel calcium blocker, nicardipine (1 mumol l-1) significantly inhibited 3H overflow evoked by K+. In the presence of L-channel block, however, noradrenaline still inhibited residual evoked overflow.7. In the presence or absence of nicardipine, pertussis toxin pretreatment (1 nmol 1-1) reduced, but did not prevent, the effect of noradrenaline (1 micromol 1-1). Pertussis toxin alone caused a significant enhancement of K+-evoked 3H overflow.8. The data indicate that on postganglionic neurones of cultured rat sympathetic ganglia there are alpha 2-adrenoceptors that modulate neurotransmitter release, but no functional beta-adrenoceptors that mediate an enhancement of transmitter release. The data suggest further that in this preparation the mechanism of alpha2-adrenoceptor modulation may involve pertussis toxin sensitive and insensitive G-proteins and effects on calcium channels other than L-type.

Lead increases inositol 1,4,5-trisphosphate levels but does not interfere with calcium transients in primary rat astrocytes

Alteration of receptor-mediated signal transduction pathways by inorganic lead (Pb) has been postulated to contribute to the neurotoxicity of this environmental toxicant, some of these effects involving astrocytes. As Pb is known to mimic Ca2+ in various biological systems or alter Ca(2+)-mediated cellular processes, we analyzed the effect of Pb exposure on alpha 1 receptor activated astrocytic phosphoinositide metabolism and Ca2+ responses in primary astrocyte cultures prepared from cerebral cortex of 1-day-old rats. Exposure to norepinephrine (NE; 10-100 microM) resulted in a significant increase in astrocytic inositol 1,4,5-trisphosphate levels, concomitant with an increase in intracellular Ca2+ levels. Fifteen minute exposure to Pb (10 microM lead acetate) significantly increased inositol 1,4,5-trisphosphate generation compared with controls, both in the presence and absence of NE. However, the inositol 1,4,5-trisphosphate-mediated Ca2+ transients following NE stimulation was unaltered in the presence of Pb (1-100 microM). NE-evoked intracellular Ca2+ responses, both in the presence and absence of extracellular Ca2+ did not differ between control and Pb-treated astrocytes. Additional studies failed to demonstrate the occurrence of Pb influx into astrocytes within the first 12 min of exposure such that Ca2+ responses would be directly affected. It therefore appears unlikely that astrotoxic effects of Pb are mediated via direct changes in intracellular Ca2+ transients.

Receptor subtype involved and mechanism of norepinephrine-induced stimulation of glutamate uptake into primary cultures of rat brain astrocytes

Glutamate uptake into rat brain astrocytes is potently stimulated by addition of norepinephrine (NE). This effect is mediated by alpha 1-adrenergic receptors expressed by these cells (Hansson and Rönnbäck: Life Sci 44:27, 1989; Brain Res 548:215, 1991). The present study was undertaken in order to identify the adrenergic receptor subtype involved, and to determine the sequence of events following receptor activation. NE increased glutamate uptake rates in a dose- and time-dependent manner (EC50 = 6 microM). Both, the selective alpha 1-receptor antagonist prazosin (IC50 = 2.5 microM) and the alpha 1b-adrenergic receptor subtype specific alkylating agent chloroethyl-clonidine (CEC, 100 microM) prevented NE (100 microM) evoked stimulation of glutamate uptake. Furthermore, omission of Ca2+ from the extracellular medium had no significant influence on NE-induced increase in glutamate uptake, indicating that the stimulatory effect is mediated by alpha 1b-adrenergic receptors. Treatment of cells with pertussis toxin (PTX) for 24 h or with 12-O-tetradecanoylphorbol-13-acetate (TPA) for 30-45 min prior to NE addition abolished the NE-mediated effect on glutamate uptake. Addition of TPA alone resulted in a rapid increase of glutamate uptake, which declined to control levels when TPA was applied 30 min prior to uptake initiation by glutamate. The increase in glutamate uptake elicited by TPA and NE added at the same time showed no additivity of the stimulatory effect resulting from treatment with each agent alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Dibutyryl cyclic AMP-induced morphological differentiation of rat brain astrocytes increases alpha 1-adrenoceptor induced phosphoinositide breakdown by a mechanism involving protein synthesis

Elevation of intracellular cAMP levels by treatment of cultured astrocytes with dibutyryl cyclic AMP (dBcAMP) resulted in a dose-dependent morphological transformation from a flat, polygonal phenotype into a stellate-like cell shape. This morphological differentiation was accompanied by an increase in maximal inositolphosphate (InsPn)-accumulation after stimulation of phosphoinositide (PI)-breakdown by norepinephrine (NE). Maximal enhancement of NE-induced PI-breakdown was observed after treatment of the cells with 0.15 mM dBcAMP for 7 days. While there was a clear effect of dBcAMP-induced differentiation on the maximal NE-induced PI-response, no effect on the dose-response relationship was detectable, resulting in similar EC50-values for astrocytes cultured either in the absence or presence of dBcAMP. The enhancement of NE-stimulated InsPn-formation was dependent on the duration of dBcAMP-treatment. More than a 6 h incubation time was needed to observe an increase in NE-induced PI-breakdown. Furthermore, the enhancing effect of dBcAMP could be prevented by inclusion of the protein-synthesis inhibitor cycloheximide and the blocker of mRNA-transcription actinomycin D. Both the alpha 1-adrenoceptor antagonists prazosin and WB 4101 potently inhibited NE-mediated PI-breakdown. Pretreatment of astrocytes with 100 microM CEC, an alpha 1B-adrenoceptor-specific, irreversible antagonist increased the EC50 values for NE-induced InsPn-accumulation in non-treated as well as in dBcAMP-treated cultures, indicating that both the alpha 1A- and alpha 1B-adrenoceptor subtypes were expressed under both culturing conditions. Reduction of extracellular Ca2+ or pretreatment of the cells with either 12-O-tetradecanoyl-phorbol-13-acetate (TPA), or pertussis toxin (PTX) resulted in a significant reduction of NE-stimulated InsPn formation. The effects of the tested effectors were similar under both culturing conditions indicating that the susceptibility of components of the signalling pathway via alpha 1-adrenoceptors to these modulators was not influenced by morphological differentiation. Different mechanistic aspects of dBcAMP-action on NE-mediated signal-transduction are discussed.

Comparison of calcium ionophore and receptor-activated inositol phosphate formation in primary glial cell cultures

The possible role of Ca2+ influx in alpha 1-adrenoceptor-stimulated [3H]inositol phosphate [( 3H]InsP) formation was examined in primary cultures of glial cells from 1-day-old rat brain. The Ca2+ ionophore A23187 caused a concentration- and time-dependent increase in [3H]InsP formation similar in magnitude to that caused by norepinephrine (NE). Responses to A23187 and NE were both completely dependent on extracellular Ca2+, with a similar concentration dependence. However, cadmium was more potent in blocking the response to A23187 than to NE. Lanthanum (1 mM) blocked the response to NE, although cobalt (5 mM) did not. The [3H]InsP response to A23187 was not additive with the response to NE or to the muscarinic agonist carbachol, although responses to NE and carbachol were addictive Both A23187 and ionomycin inhibited the additive stimulation caused by a combination of NE and carbachol, and this inhibition was potentiated by cadmium. Ionomycin stimulated [3H]InsP formation at concentrations lower than those inhibiting receptor-mediated responses, and this stimulation was not additive with responses to NE or carbachol. High-performance liquid chromatography separation showed similar patterns of [3H]InsPs formed in response to both Ca2+ ionophore and receptor agonists. These results raise the possibility that receptor-activated Ca2+ influx may be involved in stimulation of [3H]InsP formation in these cells.