Evidence for the existence of alpha- and beta-adrenoceptors on cultured glial cells--an electrophysiological study

Noradrenaline-mediated facilitation of inhibitory synaptic transmission in the dorsal horn of the rat spinal cord involves interlaminar communications

In the dorsal horn of the spinal cord (DH), noradrenaline (NA) is released by axons originating from the locus coeruleus and induces spinal analgesia, the mechanisms of which are poorly understood. Here, the effects of NA on synaptic transmission in the deep laminae (III-V) of the DH were characterized. It was shown that exogenously applied, as well as endogenously released, NA facilitated inhibitory [γ-aminobutyric acid (GABA)ergic and glycinergic] synaptic transmission in laminae III-IV of the DH by activating α1-, α2- and β-adrenoceptors (ARs). In contrast, NA had no effect on excitatory (glutamatergic) synaptic transmission. Physical interruption of communications between deep and more superficial laminae (by a mechanical transection between laminae IV and V) totally blocked the effects of α2-AR agonists and strongly reduced the effects of α1-AR agonists on inhibitory synaptic transmission in laminae III-IV without directly impairing synaptic release of GABA or glycine from neurons. Short-term pretreatment of intact spinal cord slices with the glial cell metabolism inhibitor fluorocitrate or pharmacological blockade of ionotropic glutamate and ATP receptors mimicked the consequences of a mechanical transection between laminae IV and V. Taken together, the current results indicate that the facilitation of inhibitory synaptic transmission in laminae III-IV of the DH by NA requires functional interlaminar connections between deep and more superficial laminae, and might strongly depend on glia to neuron interactions. These interlaminar connections and glia to neuron interactions could represent interesting targets for analgesic strategies.

P2X7 receptors in neurohypophysial terminals: evidence for their role in arginine-vasopressin secretion

Arginine-vasopressin (AVP) plays a major role in maintaining cardiovascular function and related pathologies. The mechanism involved in its release into the circulation is complex and highly regulated. Recent work has implicated the purinergic receptor, P2X7R, in a role for catecholamine-enhanced AVP release in the rat hypothalamic-neurohypophysial (NH) system. However, the site of P2X7R action in this endocrine system, and whether or not it directly mediates release in secretory neurons have not been determined. We hypothesized that the P2X7R is expressed and mediates AVP release in NH terminals. P2X7R function was first examined by patch-clamp recordings in isolated NH terminals. Results revealed that subpopulations of isolated terminals displayed either high ATP-sensitivity or low ATP-sensitivity, the latter of which was characteristic of the rat P2X7R. Additional recordings showed that terminals showing sensitivity to the P2X7R-selective agonist, BzATP, were further inhibited by P2X7R selective antagonists, AZ10606120 and brilliant blue-G. In confocal micrographs from tissue sections and isolated terminals of the NH P2X7R-immunoreactivity was found to be localized in plasma membranes. Lastly, the role of P2X7R on AVP release was tested. Our results showed that BzATP evoked sustained AVP release in NH terminals, which was inhibited by AZ10606120. Taken together, our data lead us to conclude that the P2X7R is expressed in NH terminals and corroborates its role in AVP secretion.

Gene expression deficits in pontine locus coeruleus astrocytes in men with major depressive disorder

BACKGROUND

Norepinephrine and glutamate are among several neurotransmitters implicated in the neuropathology of major depressive disorder (MDD). Glia deficits have also been demonstrated in people with MDD, and glia are critical modulators of central glutamatergic transmission. We studied glia in men with MDD in the region of the brain (locus coeruleus; LC) where noradrenergic neuronal cell bodies reside and receive glutamatergic input.

METHODS

The expression of 3 glutamate-related genes (SLC1A3, SLC1A2, GLUL) concentrated in glia and a glia gene (GFAP) were measured in postmortem tissues from men with MDD and from paired psychiatrically healthy controls. Initial gene expression analysis of RNA isolated from homogenized tissue (n = 9-10 pairs) containing the LC were followed by detailed analysis of gene expressions in astrocytes and oligodendrocytes (n = 6-7 pairs) laser captured from the LC region. We assessed protein changes in GFAP using immunohistochemistry and immunoblotting (n = 7-14 pairs).

RESULTS

Astrocytes, but not oligodendrocytes, demonstrated robust reductions in the expression of SLC1A3 and SLC1A2, whereas GLUL expression was unchanged. GFAP expression was lower in astrocytes, and we confirmed reduced GFAP protein in the LC using immunostaining methods.

LIMITATIONS

Reduced expression of protein products of SLC1A3 and SLC1A2 could not be confirmed because of insufficient amounts of LC tissue for these assays. Whether gene expression abnormalities were associated with only MDD and not with suicide could not be confirmed because most of the decedents who had MDD died by suicide.

CONCLUSION

Major depressive disorder is associated with unhealthy astrocytes in the noradrenergic LC, characterized here by a reduction in astrocyte glutamate transporter expression. These findings suggest that increased glutamatergic activity in the LC occurs in men with MDD.

Noradrenaline activation of neurotrophic pathways protects against neuronal amyloid toxicity

Degeneration of locus coeruleus (LC) noradrenergic forebrain projection neurons is an early feature of Alzheimer's disease. The physiological consequences of this phenomenon are unclear, but observations correlating LC neuron loss with increased Alzheimer's disease pathology in LC projection sites suggest that noradrenaline (NA) is neuroprotective. To investigate this hypothesis, we determined that NA protected both hNT human neuronal cultures and rat primary hippocampal neurons from amyloid-beta (Abeta(1-42) and Abeta(25-35)) toxicity. The noradrenergic co-transmitter galanin was also effective at preventing Abeta-induced cell death. NA inhibited Abeta(25-35)-mediated increases in intracellular reactive oxygen species, mitochondrial membrane depolarization, and caspase activation in hNT neurons. NA exerted its neuroprotective effects in these cells by stimulating canonical beta(1) and beta(2) adrenergic receptor signaling pathways involving the activation of cAMP response element binding protein and the induction of endogenous nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). Treatment with functional blocking antibodies for either NGF or BDNF blocked NA's protective actions against Abeta(1-42) and Abeta(25-35) toxicity in primary hippocampal and hNT neurons, respectively. Taken together, these data suggest that the neuroprotective effects of noradrenergic LC afferents result from stimulating neurotrophic NGF and BDNF autocrine or paracrine loops via beta adrenoceptor activation of the cAMP response element binding protein pathway.

Distribution of adrenergic receptors in the enteric nervous system of the guinea pig, mouse, and rat

Adrenergic receptors in the enteric nervous system (ENS) are important in control of the gastrointestinal tract. Here we describe the distribution of adrenergic receptors in the ENS of the ileum and colon of the guinea pig, rat, and mouse by using single- and double-labelling immunohistochemistry. In the myenteric plexus (MP) of the rat and mouse, alpha2a-adrenergic receptors (alpha2a-AR) were widely distributed on neurons and enteric glial cells. alpha2a-AR mainly colocalized with calretinin in the MP, whereas submucosal alpha2a-AR neurons colocalized with vasoactive intestinal polypeptide (VIP), neuropeptide Y, and calretinin in both species. In the guinea pig ileum, we observed widespread alpha2a-AR immunoreactivity on nerve fibers in the MP and on VIP neurons in the submucosal plexus (SMP). We observed extensive beta1-adrenergic receptor (beta1-AR) expression on neurons and nerve fibers in both the MP and the SMP of all species. Similarly, the beta2-adrenergic receptor (beta2-AR) was expressed on neurons and nerve fibers in the SMP of all species, as well as in the MP of the mouse. In the MP, beta1- and beta2-AR immunoreactivity was localized to several neuronal populations, including calretinin and nitrergic neurons. In the SMP of the guinea pig, beta1- and beta2-AR mainly colocalized with VIP, whereas, in the rat and mouse, beta1- and beta2-AR were distributed among the VIP and calretinin populations. Adrenergic receptors were widely localized on specific neuronal populations in all species studied. The role of glial alpha2a-AR is unknown. These results suggest that sympathetic innervation of the ENS is directed toward both enteric neurons and enteric glia.

Activation of beta-adrenoceptors opens calcium-activated potassium channels in astroglial cells

In the present study, effects of the alpha(2)- and beta-adrenoceptor agonists clonidine and isoproterenol on astrocytes in astroglial/neuronal cocultures from rat cerebral cortex were evaluated. The calcium- and potassium-sensitive dyes fura-2 and potassium-binding benzofuran isophtalate (PBFI) were used to study alterations in intracellular concentrations of calcium ([Ca(2+)](i)) and potassium ([K(+)](i)), respectively, while the perforated patch clamp technique was used to analyze transmembrane currents. Exposure to isoproterenol or clonidine elicited an immediate increase in [Ca(2+)](i) that was totally abolished in calcium-free extracellular media. Isoproterenol also decreased [K(+)](i), but clonidine did not. The reduction in [K(+)](i) was inhibited in Ca(2+)-free media. As evaluated with the perforated patch technique, isoproterenol (10(-6)-10(-4) M) induced a slowly developing and long lasting outward current that also was totally abolished in calcium-free buffer. This current was blocked by external tetraethylammonium (TEA, 10 mM) and charybdotoxin (ChTX, 10 nM), but was not affected by apamin (50 nM). The current-to-voltage (I-V) relationships for the isoproterenol-induced currents showed a markedly negative reversal potential, -96 mV+/-7, (mean+/-S.D., n=5). These results suggest that the stimulation of astroglial beta-adrenoceptors by isoproterenol opens calcium-activated potassium channels (K((Ca))). Preincubation with forskolin significantly increased the isoproterenol-induced currents compared with controls, indicating that the opening of astroglial K((Ca)) channels after beta-adrenergic stimulation not only depends on [Ca(2+)](i) but also synergistically involves the cAMP transduction system to which beta-adrenoceptors are known to be positively coupled.

Effects of mild protein prenatal malnutrition and subsequent postnatal nutritional rehabilitation on noradrenaline release and neuronal density in the rat occipital cortex

There is evidence that severe malnutrition started during gestation and continued through lactation affects adversely the morphologic development of the neocortex leading to increased neuronal cell packing density and decreased dendritic branching. Nevertheless, the effect of purely mild protein prenatal malnutrition on neocortical development remains rather unexplored. This study evaluates the effects of mild protein prenatal malnutrition (8% casein diet, calorically compensated by carbohydrates) and subsequent postnatal nutritional rehabilitation (25% casein diet) on: (i) the potassium-induced release of [(3)H]-noradrenaline (NA) in occipital cortex slices obtained from 1- and 22-day-old pups; and (ii) the packing density of neurons in lateral, dorso-lateral and dorsal regions of the occipital cortex of 22-day-old pups by using the optical dissector method. The experiments were performed in rats normally fed during gestation and lactation (G(+)L(+)), malnourished during gestation but rehabilitated during lactation (G(-)L(+)) and malnourished during gestation and lactation (G(-)L(-)). At day 1 of age, no significant differences in body and brain weights were observed between prenatally well-nourished and malnourished pups. Nevertheless, at this early age, pups born from mothers submitted to the 8% casein diet had significantly higher cortical net percent NA release than pups born from mothers receiving the 25% casein diet. At weaning (22 days of age) G(-)L(+) rats had, compared to the G(+)L(+) group, similar body weight, brain weight and [(3)H]-NA release values, but significantly higher neuron density scores in the lateral region of the occipital cortex. In contrast, at 22 days of age, G(-)L(-) rats exhibited, compared to G(+)L(+) animals, significant deficits in body and brain weights as well as significant increases in cortical net percent NA release together with enhanced packing density of neurons in the lateral, dorso-lateral and dorsal regions of the occipital cortex. Moreover, in G(-)L(-) animals was not found the laterodorsal histogenetic gradient of neuronal cell packing density observed in G(+)L(+)rats. Results suggest that mild prenatal malnutrition per se is able to induce deleterious effects on cortical neuronal density, in spite of nutritional rehabilitation during lactation, through a mechanism involving central NA hyperactivity during gestation. Prosecution of malnutrition after birth magnifies both neurochemical and morphometric disorders.

Basolateral amygdala noradrenergic influences on memory storage are mediated by an interaction between beta- and alpha1-adrenoceptors

Extensive evidence indicates that norepinephrine modulates memory storage through an activation of beta-adrenoceptors in the basolateral nucleus of the amygdala (BLA). Recent findings suggest that the effects of beta-adrenergic activation on memory storage are influenced by alpha1-adrenoceptor stimulation. Pharmacological findings indicate that activation of postsynaptic alpha1-adrenoceptors potentiates beta-adrenoceptor-mediated activation of cAMP formation. The present study examined whether inactivation of alpha1-adrenoceptors in the BLA would alter the dose-response effects on memory storage of intra-BLA infusions of a beta-adrenoceptor agonist, as well as that of a synthetic cAMP analog. Male Sprague Dawley rats received bilateral microinfusions into the BLA of either the beta-adrenoceptor agonist clenbuterol (3-3000 pmol in 0.2 microliter) or 8-bromoadenosine 3':5'-cyclic monophosphate (8-bromo-cAMP) (0.2-7 nmol in 0.2 microliter) alone or together with the alpha1-adrenoceptor antagonist prazosin (0.2 nmol) immediately after training in an inhibitory avoidance task. Retention was tested 48 hr later. Clenbuterol induced a dose-dependent enhancement of retention, and prazosin attenuated the dose-response effects of clenbuterol. Posttraining intra-BLA infusions of 8-bromo-cAMP also induced a dose-dependent enhancement of retention latencies. However, concurrent infusion of prazosin did not alter the dose-response effects of 8-bromo-cAMP. These findings are consistent with the view that alpha1-adrenoceptors affect memory storage by modulating beta-adrenoceptor activation in the BLA. Moreover, these findings are consistent with those of pharmacological studies indicating that beta-adrenoceptors modulate memory storage by a direct coupling to adenylate cyclase, whereas alpha1-receptors act indirectly by influencing the beta-adrenoceptor-mediated influence on cAMP formation.

Modulation of mechanically induced calcium waves in hippocampal astroglial cells. Inhibitory effects of alpha 1-adrenergic stimulation

The effects of different adrenoceptor agonists were investigated on mechanically induced Ca2+ waves in astroglial cells in astroglial-neuronal mixed cultures from rat hippocampus. In the initial part of the study some properties of the waves were characterized. The results show that the initiation of the Ca2+ waves was not critically dependent on extracellular Ca2+ but both the calcium signal and the propagation area of the calcium wave were significantly reduced when the experiments were performed in Ca2+-free buffer. In addition, using the phospholipase C (PLC) inhibitor U-73122 (1 microM) and the gap junction uncoupler octanol (1 mM), the results showed that the Ca2+ wave propagation required PLC activation and functional gap junctions. Further, the data also showed that the protein kinase C (PKC) activator phorbol-12-myristate-13-acetate (PMA 150 nM) reduced the spreading of the waves. The adrenoceptor agonists isoproterenol (iso; beta), phenylephrine (phe; alpha1) and clonidine (clon; alpha2) were evaluated for their short-term (<30 s) effects on the wave propagation. The propagation area was persistently decreased 1, 3 and 5 min after removal of phe. No effects were observed after incubation with iso or clon. Furthermore, using U-73122 or PMA together with phe, shortly incubated, the experiments showed that PLC was a central regulator in the initial phase of the initiation procedure of wave propagation. However, under these conditions PKC was shown not to be involved. Instead it appeared that PKC exerted its inhibitory action on the Ca2+ waves in a latter phase, after prolonged phe exposure. Taken together, the results show that the propagation of Ca2+ waves between astroglial cells in primary cultures can be inhibited/regulated in two principally different ways which involve a pronounced time component. The results also further point out the adrenergic signaling system as an important mediator of dynamic neuron-astroglial information exchange.

Ultrastructural evidence for combined action of noradrenaline and vasoactive intestinal polypeptide upon neurons, astrocytes, and blood vessels of the rat cerebral cortex

The intracortical organization of the noradrenaline (NA) and vasoactive intestinal polypeptide (VIP) systems provides ample opportunity for functional convergence, and accumulated evidence indicates that NA and VIP share certain cellular actions upon both neuronal and nonneuronal cortical elements. In the present study, a double immunolabeling method was combined with a silver-gold intensification procedure to examine the ultrastructural relationships of the NA coeruleocortical afferents and the intrinsic VIP neurons with three main constituents of the cortex: neurons, astrocytes, and blood vessels. Electron microscopy of singly or doubly labeled material indicated that NA and VIP boutons are engaged in a variety of anatomical relationships with both neuronal and nonneuronal elements. Dendritic shafts and perikarya of nonpyramidal neurons, some of which are VIP positive, receive combined NA and VIP synapses. A significant number of cortical microvessels are in intimate contact with NA or VIP profiles. NA axons often form perivascular loops, and VIP dendritic shafts of large diameter are frequently observed to bend around the vessel circumference. Serial section examination demonstrates that some NA boutons are directly apposed to the capillary wall at sites of glial end-feet discontinuities, whereas VIP boutons contact astrocytic sleeves of capillaries but never cross the perivascular astroglial barrier. Some VIP dendrites containing coated vesicles make intimate contact with the capillary basal lamina. Astrocytic perikarya, mainly in the supragranular layers, are also directly apposed to NA and/or VIP elements. These complex anatomical relationships provide a structural basis for the known interactions between NA and VIP in the control of cortical metabolism and function.

Photoperiodically driven changes in Fos expression within the basal tuberal hypothalamus and median eminence of Japanese quail

The rapid photoperiodic response in Japanese quail is so precise that it allows neural analyses of how photoperiodic information is transduced into an endocrine response. After transfer from short [SD; 6L:18D (6:18 hr light/dark cycle)] to long (LD; 20L:4D) days, luteinizing hormone (LH) first rises 20 hr after dawn. Using Fos immunocytochemistry, we examined the basal tuberal hypothalamus (BtH) to determine the relationship between brain cell activation and the first endocrine changes. Two separate cell populations within the BtH expressed Fos-like immunoreactivity (FLI) by hour 18 of the first LD. Importantly, this activation occurred before the LH rise. Median eminence activation appeared within glial cells, whereas activated infundibular nucleus cells were neuronal, providing support to the view that gonadotropin-releasing hormone (GnRH) release can be controlled at the terminals by glia. The FLI induction parallels LH changes, suggesting that gene expression may be involved in events preceding photostimulation and is the earliest photoperiodically stimulated physiological change yet reported. Additional experiments provided further support for this hypothesis. First, photoperiodically induced activation is not a result peculiar to castrates because intact birds displayed similar results. Second, the critical length of 14 hr of light had to be exceeded to cause both BtH activation and a LH rise 30 hr from dawn. Finally, valuable evidence of the response specificity was provided by using a unique property of the quail photoperiodic clock in which exposure to 10L:26D, but not 10L:14D, causes photoinduction. The 36 hr paradigm increased both plasma LH and BtH activation.

Adrenoceptor-induced changes of intracellular K+ and Ca2+ in astrocytes and neurons in rat cortical primary cultures

The calcium- and potassium sensitive fluorescent dyes fura-2 and K+-binding benzofuran isophtalate (PBFI) were used to detect changes in [Ca2+]i and [K+]i in type 1 astrocytes and neurons in mixed astroglial/neuronal rat cortical primary cultures after adrenoceptor stimulation. Noradrenalin (NA), phenylephrine (phe; alpha1-agonist), clonidine (clon; alpha2-agonist) and isoproterenol (iso; beta-agonist) were used. All agonists were able to increase [Ca2+]i and decrease [K+]i in the astrocytes with the exception of clon, which could not induce potassium responses. In the neurons, NA and phe evoked calcium transients while clon and iso did not. NA and clon were able to elicit reductions in [K+]i but no responses were seen after phe or iso stimulation. In neurons, the NA-evoked reductions in [K+]i always appeared immediately and gradually (after 30-50 s) returned to baseline even in the presence of the agonists. On the other hand, in the astrocytes, the NA-induced reductions in [K+]i appeared with some latency and always persisted at the lower level in the presence of the agonists. In addition, external tetraethylammonium (TEA) could severely reduce the NA-induced K+ responses in the astrocytes. The results indicate a clear heterogeneity regarding both adrenoceptor expression and response characteristics between astroglial cells and neurons.

Characterization of beta-adrenergic receptors of freshly isolated astrocytes and neurons from rat brain

The binding and characteristics of rat brain beta-adrenergic receptors (beta-AR) isolated from astrocytes and neurons were investigated. Equilibrium binding experiments demonstrated that beta-AR were more concentrated on astrocytes than on neurons isolated from forebrain, cerebral cortex and cerebellum. Inhibition experiments revealed that beta 1-AR and beta 2-AR were present in the two cell types. Isoproterenol revealed two interchangeable states of high and low affinity binding to both beta 1- and beta 2-AR in neurons. The high affinity binding sites were sensitive to guanylylimidodiphosphate (GppNHp). Similar results were found with other beta-AR agonists but not with salbutamol and salmeterol which recognized both affinity states of the neuronal beta 2-AR but only the low affinity state of beta 1-AR. In astrocytes only the low affinity state of beta-AR was observed.

Ultrastructural relationships between noradrenergic nerve fibers and non-neuronal elements in the rat cerebral cortex

Pharmacological and biochemical data suggest that noradrenaline (NA)-containing fibers not only regulate the activity of cortical neurons but also influence the functional state of non-neuronal elements. In the present study, immunocytochemistry with an antiserum against NA, followed by silver-gold intensification of the immunoreaction end-product, was employed to examine the ultrastructural relationships between the NA fiber system and the intraparenchymal blood vessels, oligodendrocytes, and astrocytes in the rat visual cortex. Electron microscopy revealed a large number of fine varicose NA fibers to be in intimate contact with cortical capillaries. Examination of single thin sections showed that NA boutons were usually separated from the capillary wall by a fine astroglial sleeve. However, serial section analysis revealed that the continuity of the astrocytic end feet was interrupted at sites, resulting in direct apposition of the perivascular NA fibers to the capillary basal lamina. Noradrenergic fibers were found to contact both types of macroglial cells. Single or clustered oligodendrocytes in intimate contact with NA fibers were observed throughout the cortical depth. Individual contacts could be followed in more than six successive thin sections, and oligodendrocyte plasma membrane frequently exhibited a light thickening at the sites of the NA fiber apposition. NA fiber-astroglial relationships were largely encountered in supragranular layers. In these layers, astrocytic cell bodies were characteristically outlined by fine varicose NA fibers. However, no plasma membrane differentiations were observed at the sites of intimate NA fiber apposition. The present ultrastructural findings provide the anatomical substrate for the control exerted by the NA fiber system over cortical microvasculature and macroglia.

Glial alpha 2-receptors probably inhibit the high-affinity uptake of noradrenaline into astrocytes in the rat brain in vivo

The effect of alpha 2-receptor blockage on the extraneuronal turnover of noradrenaline (NA) has been studied in the intact rat brain. Tropolone and yohimbine, along with reserpine or desmethylimipramine, were given 30 min after intracerebroventricular injection of [7-3H]NA, i.e. after the tracer had been stored or inactivated. Tropolone given alone did not change the fractions of 3H-activity recovered as [3H]NA from hypothalamus, septum, striatum and pons-medulla, but in the presence of yohimbine improved the [3H]NA recovery in all areas except pons-medulla. The maximum effect was seen in the hypothalamus of reserpine-treated rats. Since the alpha 2-autoreceptors were blocked, the increased [3H]NA recovery does not reflect a down-regulated neuronal NA turnover. Instead it seems to show that a fraction greater than normal of neuronally released NA had been taken up into astrocytes and remained unmetabolized if catechol-O-methyltransferase was inactive. It is assumed that yohimbine enabled the protective tropolone effect by blocking astrocytic alpha 2-receptors that otherwise, either by itself or by antagonizing beta-receptor-induced hyperpolarization or cAMP formation, had impaired parameters that stimulate the high-affinity NA Uptake 1 of astrocytes (e.g. membrane potential, Na+,K(+)-ATPase) or control the gap junction permeability in the glial syncytium.

Receptors on astrocytes--what possible functions?

Receptors for transmitters, as varied as those expressed by neurons, have been described on primary astrocyte cultures prepared from new-born rats and mice. A variety of functional effects and considerable cell-to-cell and regional heterogeneity have been observed for such receptors in vitro. The various systems available for studying the presence and properties of receptors on astrocytes in situ, and the results from these studies, are discussed. Much fewer studies using these more difficult systems have been done. So far, some resemblances and differences between in situ and in vitro work have been observed. More of these in situ studies, to supplement the ongoing in vitro work, are needed to enable us to determine unequivocally which receptors are present on astrocytes, and their functions in vivo. If there is cell-to-cell and CNS regional heterogeneity in vivo comparable to that seen in vitro, these analyses will be very complex. To illustrate the importance and variety of receptor-linked functions, a number of suggestions are made in this commentary, based on current proposals for the roles of astrocytes. However, it is argued that we need to have a more complete understanding of astrocyte functions in vivo, before we can really understand the functional significance of astrocyte receptors.

Leucine5-enkephalin afferents to midbrain dopaminergic neurons: light and electron microscopic examination

The relationship between leucine5-enkephalin-containing nerve terminals and midbrain dopaminergic neurons was studied in the adult rat by light and electron microscopy. For light microscopy, alternate midbrain sections were immunostained with rabbit polyclonal antibodies against leucine5-enkephalin and tyrosine hydroxylase, by means of the peroxidase antiperoxidase technique. Leucine5-enkephalin stained fibers and terminals were observed with varying density in the retrorubral field (dopaminergic nucleus A8 region), substantia nigra pars compacta (dopaminergic nucleus A9 region), and ventral tegmental area and related nuclei (dopaminergic nucleus A10 region). For electron microscopy, midbrain sections were immunostained with a mouse monoclonal antibody against leucine5-enkephalin and a rabbit polyclonal antibody against tyrosine hydroxylase, by means of the peroxidase antiperoxidase technique and silver-intensified colloidal gold reactions, respectively. The nucleus A10 area was examined at the electron microscopic level, and there were a) both symmetric (75%) and asymmetric (25%) synapses made between leucine5-enkephalin axon terminals and dopaminergic dendrites, and also synaptic contacts with unlabeled dendrites; b) leucine5-enkephalin synaptic contacts with dopaminergic dendrites that were covered with astrocytic membranes; and c) leucine5-enkephalin appositions with unlabeled nerve terminals that made synaptic contacts with dopaminergic dendrites, suggestive of axo-axonic connections. These findings provide the structural basis for both direct and indirect control of A10 dopaminergic neurons by enkephalin-containing nerve terminals.

Interleukin-6 production by astrocytes: induction by the neurotransmitter norepinephrine

Astrocytes contribute to the immunocompetence of the central nervous system (CNS) via their expression of class II major histocompatibility complex (MHC) antigens and the production of inflammatory cytokines such as interleukin-1 beta (IL-1 beta), tumor necrosis factor alpha (TNF-alpha) and interleukin-6 (IL-6). Of these cytokines, IL-6 is of particular interest because one of its many immune and inflammatory actions is the promotion of immunoglobulin synthesis, and it is thought that IL-6 expression within the brain exacerbates autoimmune diseases of the CNS, which are marked by local immunoglobulin production. Several stimuli induce astrocyte IL-6 expression, including such inducible endogenous factors as IL-1 beta and TNF-alpha. We have investigated the possibility that a constitutively present endogenous factor, the neurotransmitter norepinephrine (NE), can induce astrocyte IL-6 production. We report that NE induces both IL-6 mRNA and protein in primary neonatal rat astrocytes, with optimal induction at 10 microM. IL-6 protein induction by NE is comparable to that seen with IL-1 beta or TNF-alpha, and NE synergizes with these cytokines for a ten-fold enhanced effect. In contrast to astrocytes, microglia are relatively unresponsive to NE, IL-1 beta and TNF-alpha for IL-6 production. Experiments with the beta-adrenergic receptor agonist isoproterenol, and alpha and beta-adrenergic receptor antagonists (propranolol, phentolamine, atenolol, and yohimbine) indicate that beta 2 and alpha 1-adrenergic receptors are involved in NE induction of astrocyte IL-6 expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for nonsynaptic serotonergic and noradrenergic innervation of the rat dorsal horn and possible involvement of neuron-glia interactions

We investigated the synaptic incidence of the contacts established by serotonergic and noradrenergic descending fibers in the dorsal horn of the rat spinal cord. Serial electron microscopic sections were performed. Synapses were scarce. The majority of serotonergic and noradrenergic varicosities (more than 60%) are characterized by nonsynaptic contacts. Numerous glial profiles, and particularly astrocytic profiles, were observed in apposition with serotonergic and noradrenergic varicosities. The proportion of astroglia was higher around serotonergic and noradrenergic varicosities devoid of synaptic specialization. The length of the contact between immunoreactive nonsynaptic varicosities and astrocytes was twice as long as that between synaptic varicosities and astrocytes. Thus, the modulation of sensitive messages by serotonin and noradrenaline through pauci-synaptic varicosities in the dorsal horn of the spinal cord could be an example of the concept of "volume transmission" [Fuxe and Agnati (1991) Volume Transmission in the Brain: Novel Mechanisms for Neural Transmission, Advances in Neuroscience, Vol. 1, pp. 1-9.] in the central nervous system. Analysis of the microenvironment of serotonergic and noradrenergic varicosities led us to make the hypothesis that glial cells, particularly astrocytes, could play some role in volume transmission.

The evidence for astrocytes as a target for central noradrenergic activity: expression of adrenergic receptors

Our recognition and understanding of adrenergic receptor expression by astrocytes and their cultured counterparts, astroglia, has occurred primarily over the past 2 decades. The advances in our knowledge have come about largely through the advent of new techniques with which to study neurotransmitter receptors, coupled with improvements in our ability to isolate, purify, and identify this central nervous system (CNS) cell type. The development of pharmacological tools such as second messenger assays, iodinated ligands, autoradiography, and intracellular electrophysiological recordings, paralleled that of cultured clonal cells lines of glial origin, purified astroglial primary cultures, isolations of astrocytes from adult tissues, and immunocytochemical staining for the astrocyte-specific glial fibrillary acidic protein (GFAP). As these techniques were combined and applied to the study of astrocyte pharmacology, our understanding of adrenergic receptor expression by these cells deepened. This review is an account of how these events have shaped our understanding of astrocytic adrenergic receptor expression.

Astroglia in Alzheimer's disease

Amyloid deposits are characteristic of Alzheimer's Disease (AD) and there is growing evidence that amyloid may play an important role in the genesis of this neurodegenerative disease. This review discusses data which suggests that reactive astrocytes and microglia may be a necessary concomitant with amyloid to produce the neuropathology which manifests as AD. Several hypotheses and supporting data for mechanisms by which reactive astrocytes may mediate this neuropathology are presented. These include the possibility that amyloid induces excitotoxicity by interferring with astrocytic glutamate uptake, the possibility that amyloid has this effect via an action on a tachykinin-related receptor and the possibility that proteoglycans released by astrocytes may facilitate the deposition of amyloid plaques. Both symptomatic treatment to enhance cognitive function and treatment to stop the progression of AD are needed. It is hoped that answers to some of the unique questions raised here may provide new insight into the etiology and treatment of AD.

Expression of adrenergic receptors in individual astrocytes and motor neurons isolated from the adult rat brain

Attempts to show the distribution of adrenergic receptors (ARs) in autoradiographs of a brainstem motor nucleus following elimination of motor neurons yielded the unexpected result of an increase in beta-AR density. This increase was related to the gliosis accompanying the motor neuron degeneration. To determine the cells on which the AR subtypes were located, we dissociated cells from various regions of the adult rat brain and subsequently identified astrocytes by glial fibrillary acidic protein (GFAP) immunofluorescence. Slides containing the astrocytes were prepared for autoradiography using the nonselective beta ligand 125I-iodocyanopindolol (125ICYP) or the alpha 1 ligand 125IBE 2254 (125I-HEAT). The addition of the selective beta 1 blocker betaxolol or the beta 2 blocker ICI 118.551 to the incubation medium to displace 125ICYP binding was used to determine the binding of beta-AR subtypes. The great majority (greater than 88%) of isolated astrocytes sampled from the trigeminal motor nucleus, cerebral cortex, striatum, and cerebellum showed beta-AR binding. Astrocytes from the first three regions had similar average densities of beta-ARs, whereas the density in cerebellar astrocytes was 2- to 3-fold greater. The beta 2-AR subtype was proportionally greater than the beta 1 subtype in each region. Reactive astrocytes isolated from the trigeminal motor nucleus after degeneration of motor neurons showed a beta-AR density nearly 2-fold greater than resting astrocytes from the same region, with the beta 1 subtype showing the greater proportional increase. There was no beta-AR binding on trigeminal motor neurons. Astrocytes also showed a significant level of alpha 1-AR binding. No differences in alpha 1-AR binding were found in normal astrocytes isolated from the different regions, nor was there an increase in reactive astrocytes. In contrast, trigeminal motor neurons had an alpha 1-AR density nearly 10 times greater than astrocytes. In terms of the NE modulation of synaptic responses in motor neurons, the distribution of ARs would permit NE to act indirectly through alpha 1 and beta receptors on astrocytes and directly through alpha 1 receptors on motor neurons.

A role for glial cells in activity-dependent central nervous plasticity? Review and hypothesis

Activity-dependent plasticity relies on changes in neuronal transmission that are controlled by coincidence or noncoincidence of presynaptic and postsynaptic activity. These changes may rely on modulation of neural transmission or on structural changes in neuronal circuitry. The present overview summarizes experimental data that support the involvement of glial cells in central nervous activity-dependent plasticity. A role for glial cells in plastic changes of synaptic transmission may be based on modulation of transmitter uptake or on regulation of the extracellular ion composition. Both mechanisms can be initiated via neuronal-glial information transfer by potassium ions, transmitters, or other diffusible factor originating from active neurons. In addition, the importance of changes in neuronal circuitry in many model systems of activity-dependent plasticity is summarized. Structural changes in neuronal connectivity can be influenced or mediated by glial cells via release of growth or growth permissive factors on neuronal activation, and by active displacement and subsequent elimination of axonal boutons. A unifying hypothesis that integrates these possibilities into a model of activity-dependent plasticity is proposed. In this model glial cells interact with neurons to establish plastic changes; while glial cells have a global effect on plasticity, neuronal mechanisms underlie the induction and local specificity of the plastic change. The proposed hypothesis not only explains conventional findings on activity-dependent plastic changes, but offers an intriguing possibility to explain several paradoxical findings from studies on CNS plasticity that are not yet fully understood. Although the accumulated data seem to support the proposed role for glial cells in plasticity, it has to be emphasized that several steps in the proposed cascades of events require further detailed investigation, and several "missing links" have to be addressed by experimental work. Because of the increasing evidence for glial heterogeneity (for review see Wilkin et al., 1990) it seems to be of great importance to relate findings on glial populations to the developmental stage and topographical origin of the studied cells. The present overview is intended to serve as a guideline for future studies and to expand the view of "neuro" physiologists interested in activity-dependent plasticity. Key questions that have to be addressed relate to the mechanisms of release of growth and growth-permissive factors from glial cells and neuronal-glial information transfer. It is said that every complex problem has a simple, logical, wrong solution. Future studies will reveal the contribution of the proposed simple and logical solution to the understanding of central nervous plasticity.

Alpha- and beta-adrenoceptor regulation of cyclic AMP accumulation in cultured rat astrocytes. A comparison of primary protoplasmic and mixed fibrous/protoplasmic astroglial cultures

The effect of noradrenaline and isoprenaline on cyclic AMP accumulation has been investigated in primary rat astrocytes which contain either (a) protoplasmic astrocytes alone or (b) both fibrous and protoplasmic astrocytes. Isoprenaline and noradrenaline stimulated cyclic AMP formation in both astrocyte culture preparations. Combinations of noradrenaline (1 microM) and isoprenaline (1 microM) produced a cyclic AMP response which was 58% and 26% of that produced by isoprenaline alone in protoplasmic and mixed fibrous/protoplasmic cultures, respectively. In both preparations this inhibitory effect of noradrenaline was antagonized by the alpha 2-adrenoceptor antagonist yohimbine (1 microM). A striking feature of the concentration-response curve for isoprenaline (EC50 = 0.8 microM) in mixed fibrous/protoplasmic cultures was that the cyclic AMP response decreased sharply at concentrations above 1 microM. This phenomenon was not seen in cultures containing protoplasmic astroglia alone. The fall in the isoprenaline concentration-response curve was not observed in the presence of the alpha-adrenoceptor antagonist phentolamine (1 microM), the dihydropyridine calcium antagonist isradipine (10 microM), the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.1 mM) or in nominally calcium-free medium. The effect of phentolamine was mimicked by the alpha 1-adrenoceptor antagonist prazosin (1 microM) but not by the alpha 2-antagonist yohimbine (1 microM). In conclusion, the data from this study suggest that two different populations of astrocytes in in vitro culture are able to raise intracellular cyclic AMP levels via beta-adrenoceptor activation and that there are differences in the extent of alpha-adrenoceptor (both alpha 1- and alpha 2-) mediated inhibition of cyclic AMP accumulation between the two primary astroglial cell preparations.

Multiple adrenergic receptor subtypes controlling cyclic AMP formation: comparison of brain slices and primary neuronal and glial cultures

The adrenergic receptor subtypes involved in cyclic AMP responses to norepinephrine (NE) were compared between slices of rat cerebral cortex and primary neuronal and glial cultures from rat brain. In neuronal cultures, NE and the beta-adrenergic receptor agonist isoproterenol (ISO) caused similar increases in cyclic AMP, which were not altered by the alpha-adrenergic receptor antagonist phentolamine. In glial cultures, NE caused a much smaller cyclic AMP response than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists (phentolamine greater than yohimbine greater than prazosin). alpha 2-Adrenergic receptor agonists partially inhibited the ISO response in glial cultures to a level similar to that observed with NE alone (clonidine = UK 14,304 greater than NE greater than 6-fluoro-NE greater than epinephrine). In slices from cerebral cortex, NE caused a much larger increase in cyclic AMP than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists with a different order of potency (prazosin greater than phentolamine greater than yohimbine). alpha 1-Adrenergic receptor agonists potentiated the response to ISO to a level similar to that observed with NE alone (epinephrine = NE greater than phenylephrine greater than 6-fluoro-NE greater than methoxamine). In all three tissue preparations, large responses to both alpha 1-receptor activation (increases in inositol phosphate accumulation) and alpha 2-receptor activation (decreases in forskolin-stimulated cyclic AMP accumulation) were observed. These data indicate that all of the major adrenergic receptor subtypes (beta, alpha 1, alpha 2) are present in each tissue preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Action of baclofen, GABA and antagonists on the membrane potential of cultured astrocytes of rat spinal cord

The action of gamma-aminobutyric acid A (GABAA) and B (GABAB)-agonists has been studied on the membrane potential of astrocytes in explant cultures of rat spinal cord by means of intracellular microelectrode recordings. The GABAB-agonists (-)-baclofen and CGP 27 492 (3-aminopropyl phosphonous acid; 10(-6) to 10(-4) M) caused a hyperpolarization of the majority of astrocytes studied. On approximately 25% of the cells, the compounds had no effect. The hyperpolarization by baclofen (10(-4) M) was reversibly antagonized by the GABAB-antagonist 5-hydroxysaclofen (10(-4) M). GABA and the GABAA-agonist muscimol (10(-4) and 10(-3) M) depolarized approximately two thirds of the glial cells tested, whereas the remaining third remained unaffected. The GABAA-antagonist bicuculline (10(-4) and 10(-3) M) only reduced the depolarization by GABA (10(-4) M) but did not completely block it. On half of the cells tested, the depolarization by GABA was not affected by bicuculline, suggesting that the glial GABAA-receptor is different from the neuronal GABAA-receptor. Our electrophysiological investigations together with recent autoradiographic binding studies strongly suggest the existence of GABAB-receptors on astrocytes whereas there is less evidence for GABAA-sites on these cells.

Alpha 1-adrenergic receptors in the brain: characterization in astrocytic glial cultures and comparison with neuronal cultures

Binding of [125I]HEAT to membranes prepared from primary cultures of astrocytic glial cells was time-dependent and 70-85% specific. Various adrenergic agonists and antagonists competed for [125I]HEAT binding according to the potencies of prazosin greater than, yohimbine greater than or equal to, clonidine, norepinephrine (NE), and propranolol. Scatchard analysis showed the Bmax of 209 fmol/mg protein and a Kd of 184 pM for [125I]HEAT binding by astrocytic glial membranes. Pretreatment of astrocytes with NE resulted in a dose-dependent downregulation of [125I]HEAT binding sites with a maximal response observed after 8 h at 100 microM NE. Removal of NE from cultures after pretreatment resulted in a time- and protein synthesis-dependent recovery of binding sites to control levels within 120 h. Incubation of astrocytic glial cultures with NE stimulated phosphoinositide (PI) hydrolysis in a time- and dose-dependent manner with a maximal stimulation of 2-fold observed in 60 min by 100 microM NE. Clonidine expressed differential effects on alpha 1-adrenergic receptors of the neuronal and astrocytic glial cultures. Pretreatment with 10 microM clonidine caused a 40% decrease in the Bmax of [125I]HEAT binding without influencing the Kd value in neuronal cultures. This downregulatory effect of clonidine was associated with a reduction in the ability of NE to stimulate PI hydrolysis in clonidine pretreated cells. In contrast to neuronal cultures, clonidine neither downregulated [125I]HEAT binding sites nor stimulated PI hydrolysis in glial cultures.

Beta-adrenergic receptor regulation, through cyclic AMP, of nerve growth factor expression in rat cortical and cerebellar astrocytes

1. Type 1 astrocytes prepared from 3-day rat cortex and cerebellum express the 1.3-kb nerve growth factor (NGF) mRNA and synthesize and release beta-NGF. 2. Isoproterenol (IP), a beta-adrenergic agonist, stimulates NGF mRNA content in cortical astrocytes; this increase is blocked by the beta-adrenergic antagonist propranolol but not the alpha-antagonist phenoxybenzamine. The EC50 for the effect of IP is 5 nM. 3. IP increases astrocyte cyclic AMP as does forskolin, which directly activates adenylate cyclase and also increases NGF mRNA content. Cerebellar astrocytes contain about one-third as much NGF mRNA, which can also be increased by forskolin and cyclic AMP. 4. These results suggest that CNS astrocytes can serve as a source of NGF and that the NGF gene is one of the class of cyclic AMP regulated genes.

Evidence for GABAB-receptors on cultured astrocytes of rat CNS: autoradiographic binding studies

The cellular localization of GABA-binding sites was studied in explant cultures of rat cerebellum, brain stem and spinal cord by means of autoradiography. Labelling of GABAB-sites was done with 3H(-)baclofen or 3H-GABA in presence of unlabelled bicuculline. Binding sites for these radio-ligands were found on many neurones and on a large number of astrocytes. Labelling of glial cells was usually weaker than that of neurones. Combining autoradiography with staining with anti-glial fibrillary acidic protein (GFAP) revealed that the glial cells labelled with 3H-baclofen or 3H-GABA were GFAP-positive. In contrast, when GABAA-sites were localized using 3H-GABA in presence of unlabelled baclofen, the GABAA-agonists 3H-muscimol and 3H-THIP, or the antagonist 3H-(+)-bicuculline, binding only occurred to neurones but not to astrocytes. Immunohistochemical investigations with the monoclonal antibody (bd-17) against the GABAA/benzodiazepine/chloride channel complex revealed that neurones were specifically stained whereas glial cells were immunonegative. From our observations it is suggested that astrocytes possess GABAB-receptors but there is little evidence for the existence of GABAA-sites on glial elements.

Norepinephrine-evoked calcium transients in cultured cerebral type 1 astroglia

We have used the CA++ indicator dye fura-2 AM and computerized imaging systems to investigate adrenergic regulation of intracellular calcium in cultured cerebral type 1 astroglia. We have found that norepinephrine (NE) and other adrenergic agonists stimulate increases in intracellular calcium in over 80% of type 1 astroglia tested. A wide range in effective NE concentrations was seen. With sufficient agonist concentrations the calcium response was biphasic, exhibiting an initial sharp peak followed by a sustained calcium elevation. This secondary component was sensitive to reductions in extracellular calcium concentrations and dependent on the continued presence of agonist. Pharmacological studies indicated that astroglial calcium responses were mediated by alpha 1- and alpha 2-adrenergic receptors. At times these two receptor subtypes appeared to underlie calcium responses by the same cells, whereas other cells only responded to stimulation of one or the other subtypes of alpha-adrenergic receptor. Finally, we have also observed spontaneous and agonist-evoked oscillations in astroglial calcium levels. The major findings of these studies indicate that 1) astroglial cells respond to alpha-adrenergic receptor stimulation with increased intracellular calcium, 2) these responses can be mediated by alpha 1-and/or alpha 2-adrenergic receptors, and 3) subpopulations of cerebral type 1 astroglia exist with respect to alpha-adrenergic receptor expression.

Alpha-receptor and cholinergic receptor-linked changes in cytosolic Ca2+ and membrane potential in primary rat astrocytes

Both phenylephrine and carbachol caused a sustained increase in Ca2+ influx and intracellular free Ca2+ of primary astrocytes as measured with 45Ca2+ and fura-2. The responses to phenylephrine and carbachol were additive, suggesting that they use different releasable pools of Ca2+. If extracellular Ca2+ was removed by EGTA only a transient rise in cytosolic Ca2+ was seen upon application of the agonists. Both compounds caused depolarization of the astrocyte membrane as determined with the optical probe 3,3-diethylthiadicarboxyamineiodide. Activation of protein kinase C with 12-tetradecanoylphorbol myristate acetate (TPA) or the diacylglycerol analogue dioctanoylglycerol (DiC8) also depolarized the cells. A prior activation of protein kinase C with TPA or DiC8 abolished the depolarizing effect of phenylephrine suggesting that they act through the same mediators. If the cells were made ideally permeable to K+ with the ionophore valinomycin, or the K+ channels had been blocked with Ba2+, neither TPA nor phenylephrine had any significant effect on the membrane potential. Neither TPA nor phenylephrine had any effect on the 86Rb+ equilibrium potential across the cell membrane. The results suggest that the depolarizing effect of these substances could be through a blocking of K+ channels.

Is Alzheimer's disease an anterograde degeneration, originating in the brainstem, and disrupting metabolic and functional interactions between neurons and glial cells?

A novel hypothesis is suggested for the pathogenesis of Alzheimer's disease, i.e. that a degeneration of adrenergic neurons in locus coeruleus and/or of serotonergic neurons in the raphe nuclei leads to impairment in metabolic and functional interactions between neurons and astrocytes (in the cerebral cortex and hippocampus as well as in nucleus basalis magnocellularis), and that a resulting deficient supply of substrates and failing energy metabolism in both neurons and astrocytes causes neuronal cell death in these areas and thus interference with additional transmitter systems. The hypothesis is based on (1) the topographical distribution of ascending pathways from locus coeruleus and the raphe nuclei; (2) the peculiar termination of many of these fibres in varicosities, from which released transmitter molecules reach their targets by diffusion, rather than in genuine synapses, suggesting a partly non-neuronal target; (3) the effects of locus coeruleus lesions in experimental animals; (4) the emergence of new knowledge in cellular neurobiology, indicating profound metabolic and functional interactions between neurons and astrocytes; and (5) the effects of adrenergic and serotonergic agonists upon metabolism and function in rodent astrocytes and neurons. These compounds influence energy metabolism, membrane transport of potassium and production of growth factors in astrocytes, and glutamate release from glutamatergic neurons. They thus influence essential metabolic interactions between neurons and astrocytes, as well as neuronal-astrocytic interactions in potassium homeostasis at the cellular level. Obviously, neither the individual findings alone, nor their combination into a conceptual framework, prove the correctness of the hypothesis. However, they do provide a basis for further experimental work, using postmortem brain tissue from Alzheimer's patients and lesion studies in rodents, which can confirm or refute the hypothesis.

Pharmacological identification of the alpha-adrenergic receptor type which inhibits the beta-adrenergic activated adenylate cyclase system in cultured astrocytes

The adrenergic agonist norepinephrine can exert its influence on cell function by activating both alpha- and beta-adrenergic receptors. In astrocytes, the alpha-adrenergic receptor activity of norepinephrine is known to inhibit the cyclic AMP response elicited by its action at beta-adrenergic receptors. Pharmacological studies were conducted to identify the subtype of alpha-adrenergic receptor which mediates this inhibitory action. The alpha 2-adrenergic antagonist yohimbine potentiated the cyclic AMP response elicited by norepinephrine, whereas the alpha 1-adrenergic antagonist prazosin did not affect the response. The alpha 2-adrenergic agonist clonidine inhibited the cyclic AMP response elicited by the beta-adrenergic agonist isoproterenol and this inhibition could be blocked by yohimbine but not by prazosin. In contrast, the alpha 1-adrenergic agonist phenylephrine did not inhibit the cyclic AMP response to isoproterenol. These studies indicate that the inhibitory action of norepinephrine is mediated by its action at alpha 2-adrenergic receptors.

Astrocytes from forebrain, cerebellum, and spinal cord differ in their responses to vasoactive intestinal peptide

Astrocytes from cortex, cerebellum, and spinal cord responded to isoproterenol and vasoactive intestinal peptide (VIP) with increases in intracellular cyclic AMP levels. The response to VIP was as great as that to isoproterenol in cortical astrocytes (180-fold and 185-fold, respectively), and the effect of VIP in combination with isoproterenol was partially additive. Spinal cord astrocytes also responded to VIP and isoproterenol with equal potency (seven- to ninefold and eight- to 13-fold, respectively), but the level of response was much smaller than in cortex. Spinal cord astrocytes were synergistic in their response to VIP and isoproterenol. The response to VIP was lowest in cerebellar astrocytes (only threefold), and no additivity was observed when VIP was added together with isoproterenol. A small response to alpha-melanocyte stimulating hormone (alpha-MSH) was also observed in cortex and cerebellum, but not in spinal cord. Somatostatin inhibited the response to isoproterenol in cortex and cerebellum, but had no effect in spinal cord. The results from the above study show that astrocytes obtained from these three regions of the rat CNS express quite different responses to VIP and alpha-MSH and further point to possible astrocyte heterogeneity.

Synaptic inputs and electrical coupling among magnocellular neuroendocrine cells

This paper first briefly reviews the evidence for synaptic and nonsynaptic plasticity among the neurons and glia of the magnocellular hypothalamo-neurohypophysial system. Emphasis is placed upon the importance of the roles played by astrocytes in the remodeling of the magnocellular nuclei under various conditions of increased hormone demand. Evidence is then reviewed from more recent studies showing that there is electrical coupling among magnocellular neurons, and that this coupling shows plasticity similar to that shown for other characteristics of the system (e.g., chemical synapses, dendritic bundling etc.). Further, evidence is presented that extent of electrical coupling can be modified not only by manipulating the physiological state of the animal (such as lactation), but also by electrical stimulation of newly described olfactory afferent inputs to the cells of the supraoptic nucleus. The possible functional significance of these findings is discussed in relation to the behavior of nursing rats.

Electrophysiological properties of glial cells: comparison of brain slices with primary cultures

Intracellular recordings were obtained from glial cells in the CA1 region of rat hippocampal slices to compare their electrophysiological properties with the previously reported properties of glial cells in primary tissue culture. The average resting potential was -77 mV and the average input resistance was 3.2 M omega. Barium (10 mM) depolarized glial cells in brain slices and increased input resistance, but barium action potentials which have been observed in primary cultures, were not observed in brain slices. gamma-aminobutyric acid (GABA) and glutamate depolarized glial cells. Spontaneous oscillations of membrane potential were observed occasionally. The mechanism underlying these responses are unknown as yet.

Autoradiographic studies on the uptake of adenosine and on binding of adenosine analogues in neurons and astrocytes of cultured rat cerebellum and spinal cord

The cellular localization of the uptake of [3H]adenosine and of binding of labelled adenosine analogues was studied in explant cultures of rat cerebellum and spinal cord by means of autoradiography. [3H]Adenosine was taken up by many neurons and astrocytes in both cerebellar and spinal cord cultures. The uptake of adenosine was inhibited in the absence of sodium or at 0 degrees C, suggesting an active transport mechanism. In both types of cultures, a great number of neurons showed binding sites for the A1-receptor agonist [3H]R-N6-phenylisopropyladenosine and for the mixed A1/A2-agonist [3H]N(ethyl)carboxamidoadenosine. Binding sites for both radioligands were also found on astrocytes, suggesting that these cells have receptors for the purinergic neurotransmitter adenosine. This suggestion is further supported by recent electrophysiological studies from our laboratory demonstrating that adenosine and its analogues produce hyperpolarizations of astrocytes which are blocked by the adenosine antagonist theophylline.

Norepinephrine inhibits gamma-interferon-induced major histocompatibility class II (Ia) antigen expression on cultured astrocytes via beta-2-adrenergic signal transduction mechanisms

The astrocyte is now recognized as a facultative immunocompetent antigen-presenting cell that can initiate intracerebral immune responses. However, despite the presence of activated T lymphocytes and their associated lymphokines within the central nervous system, there is a paucity in the expression of the major histocompatibility (MHC) antigens on normal neural tissue. These membrane-localized glycoproteins are required for the process of antigen presentation and, therefore, for the initiation of immune responses. To date, little is understood regarding the nature of inhibitory mechanisms that might be responsible for maintaining the brain as an immunoprivileged site. In this study we found that norepinephrine, a major brain transmitter, significantly inhibited gamma interferon-induced MHC class II antigen expression on astrocytes derived from neonatal Lewis rats. We show that this inhibition can be attenuated by the addition of a beta-adrenergic antagonist, propranolol, but not by the addition of a beta 1-selective antagonist, atenolol, or by an alpha-adrenergic antagonist, phentolamine. Furthermore, it was found that a similar inhibition could be achieved by the addition of either dibutyryl-cAMP or dipyridimole, a phosphodiesterase inhibitor. Therefore, it seems that norepinephrine-mediated inhibition of MHC class II antigen expression on astrocytes works through beta 2-adrenergic signal transduction pathways. Taken together, these in vitro results suggest that the brain contains inhibitory factors that may play a pivotal role in the regulation of intracerebral immune responses by modulating the expression of MHC antigens on astrocytes.

Are astroglial cells involved in morphine tolerance?

Morphine gives rise to a cascade of events in the nervous system affecting, among others, neurotransmitter metabolism. Tolerance develops for various effects shortly after administration of the drug. Also, physical dependence develops and can be demonstrated by precipitation of withdrawal reactions. Biochemical events in nervous tissue have been extensively studied during morphine treatment. This overview will focus upon brain protein metabolism since macromolecular events might be of importance for development of long-term effects, such as tolerance and physical dependence. Both dose- and time-dependent changes in brain protein synthesis and the syntheses of specific proteins have been demonstrated after morphine treatment, although methodological considerations are important. Different experimental models (animal and tissue culture models) are presented. It might be interesting to note that astroglial protein synthesis and the secretion of proteins to the extracellular medium are both changed after morphine treatment, these having been evaluated in astroglial enriched primary cultures and in brain tissue slices. The possibility is suggested that proteins released from astroglial cells participate in the communication with other cells, including via synaptic regions, and that such communication might of significance in modifying the synaptic membranes during morphine intoxication.

Norepinephrine inhibits gamma-interferon-induced MHC class II (Ia) antigen expression on cultured brain astrocytes

Recent evidence that astrocytes can be induced to express the class II major histocompatability (MHC) antigens suggests that these cells may be involved in the development of intracerebral immune responses. The principal inducer of MHC class II antigen (Ia) expression is a soluble lymphokine, gamma-interferon (gamma-IFN). Normally astrocytes do not express significant levels of Ia antigens despite the fact that agents such as gamma-IFN may be present in the central nervous system (CNS). Here we report that a major neurotransmitter, norepinephrine (NE), inhibits, in a dose-response fashion, the ability of gamma-IFN to induce Ia antigen expression on cultured astrocytes derived from newborn BALB/c mice. This finding may indicate that the brain contains inhibitory modulators that serve to prevent the up-regulation of intracerebral immune responsiveness.

Norepinephrine and cyclic adenosine 3':5'-cyclic monophosphate enhance a nifedipine-sensitive calcium current in cultured rat astrocytes

We employed two microelectrode current-clamp and voltage-clamp methods to examine the modulation of Ca++ channels by norepinephrine and cyclic AMP (cAMP) in cultured astrocytes from the rat cerebral cortex. Currents owing to Ca++ channels were maximized by replacing Ca++ with Ba++ in the extracellular solution and pharmacologically blocking K+ and Na+ currents. In current-clamp experiments, we observed that norepinephrine, isoproterenol (an agonist of beta-receptors for norepinephrine), or dibutyryl cAMP (dbcAMP, a membrane permeant analogue of cAMP) induced or enhanced slow Ba++-dependent action potentials in the cells. In voltage-clamp experiments, we confirmed that the slow action potentials were generated by a voltage-activated and Ba++-dependent inward current. This current was mediated by channels that resembled L-type calcium channels (cf. McCleskey et al., Journal of Experimental Biology 124:177-190, 1986) in their voltage-activation range, slow inactivation, and sensitivity to blockage by Co++, Cd++, and nifedipine. DbcAMP, or isoproterenol, enhanced the Ba++ current. Modulation of Ca++ channel function in glial cells could have functional implications.

Pharmacological properties of the norepinephrine-induced depolarization of astrocytes in primary culture: evidence for the involvement of an alpha 1-adrenergic receptor

The membrane potentials of astrocytes in primary cultures prepared from neonatal rat cerebral cortices were depolarized by (-)-norepinephrine. The average first response to 10(-5) M (-)-norepinephrine was 24 mV from an average resting potential of -68 mV, and the average for the second response was 14 mV. Thus this process showed marked desensitization. The response was attributed to an activation of an alpha 1-receptor since it was about 1000 times more sensitive to inhibition by prazosin than to yohimbine or idazoxan. In addition, depolarization was seen to the application of 10(-5) M phenylephrine.

The role of astrocytes in hepatic encephalopathy

The Alzheimer type II astrocyte change is the distinctive morphologic alteration in brain of humans and experimental animals succumbing to hepatic encephalopathy (HE). Whether this change is a primary event in the pathogenesis of HE or whether it is secondary to injury of some other component(s) of the CNS has not been clarified. Studies in a rat model of HE have revealed early reactive changes in astrocytes characterized by cytoplasmic hypertrophy. During the later phases, degenerative changes ensue corresponding to the Alzheimer type II change observed by light microscopy. In view of the role of astrocytes in ammonia detoxification and the importance of ammonia in the pathogenesis of HE, we have suggested that the initial astrocytic changes are the morphological correlates of ammonia detoxification. We have speculated that the later degenerative alterations could lead to failure by astrocytes to carry out key functions (e.g., neurotransmitter uptake, ion regulation, and the like) and contribute the development of the encephalopathy. Recently, the potential involvement of astrocytes in HE has been further investigated, using primary astrocyte cultures. Exposure of cultures to ammonia at clinically relevant concentrations has shown morphologic changes closely resembling those observed in experimental HE in vivo. These deleterious effects can partly be prevented by raising cyclic AMP levels in cells. Other potential toxins (octanoic acid, phenol) have shown pathologic changes as well. Although some alterations were common to all three, they each possessed distinctive pathological effects. A synergistic interaction has also been demonstrated with these toxins. Functional studies of ammonia-treated astrocytes have shown the following: With low doses or short-term exposure, the uptakes of K+, glutamate, and GABA remained unchanged or slightly increased, whereas with higher doses or longer treatment, those activities diminished. A fall in ATP values occurred with prolonged ammonia treatment. Preliminary findings have shown no significant derangements in the beta-adrenergic receptor, except for a slight decrease in receptor affinity. However, cyclic AMP production was diminished following stimulation with isoproterenol. A slight rise in the number of benzodiazepine receptors was found. These studies indicate that profound changes occur in astrocytes following exposure to ammonia and other putative toxins. It is proposed that toxins and factors involved in the precipitation of HE do so by affecting astroglial properties. Derangements in such properties may lead to glial dysfunction (primary gliopathy), resulting in an encephalopathic state.