Down-regulation of beta-adrenergic activity in astroglia by chronic treatment with an antidepressant drug

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

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

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

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

Chronic treatment of human astrocytoma cells with lithium, carbamazepine or valproic acid decreases inositol uptake at high inositol concentrations but increases it at low inositol concentrations

Inositol uptake was measured at concentrations of 25, 40 and 50 microM in human astrocytoma cell cultures treated for 1-3 weeks with pharmacologically relevant concentrations of LiCl, valproic acid or carbamazepine as well as in control cultures that had not been treated with any drug. After at least 2 weeks of treatment, each of these 3 conventional anti-bipolar drugs increased the uptake significantly at 25 microM inositol, had no effect at 40 microM, and decreased it at 50 microM inositol. Reduction of the drug concentrations by 50% abolished the stimulation of uptake at 25 microM inositol by lithium and valproic acid and reduced that by carbamazepine. These findings may contribute to an understanding of the mechanisms of action for anti-bipolar medication, and explain the controversy in the literature whether or not brain inositol is reduced after chronic administration of lithium.

Beta-adrenoceptors in the tree shrew brain. I. Distribution and characterization of [125I]iodocyanopindolol binding sites

1. The number and distribution pattern of beta-adrenergic receptors in the brain have been reported to be species specific. The aim of the present study was to describe binding of the beta-adrenoceptor ligand [125I]iodocyanopindolol in the brain of the tree shrew (Tupaia belangeri), a species which provides an appropriate model for studies of psychosocial stress and its consequences on central nervous processes. 2. 125I-Iodocyanopindolol (125ICYP) labeling revealed a high degree of nonspecific binding, which was due mainly to interactions of this ligand with serotonin binding sites. For a quantitative evaluation of beta 1- and beta 2-adrenoceptors, serotonin binding sites had to be blocked by 100 microM 5HT. 3. Binding of the radioligand to beta 1- and beta 2-adrenoceptors was characterized using the beta 1-specific antagonist CGP20712A and the beta 2-specific antagonist ICI118.551. beta 1-adrenoceptor binding is present in the whole brain, revealing low receptor numbers in most brain regions (up to 1.5 to 2.7 fmol/mg). A slight enrichment was observed in cortical areas (lateral orbital cortex: 4.0 +/- 0.7 fmol/mg) and in the cerebellar molecular layer (8.7 +/- 1.0 fmol/mg). 4. Competition experiments demonstrated high- and low-affinity binding sites with considerable variations in Ki values for CGP20712A, showing that various affinity states of beta 1-adrenoceptors are present in the brain (Ki: 0.61 nM to 67.1 microM). In the hippocampus, only low-affinity beta 1-adrenoceptors were detected (Ki: 1.3 +/- 0.2 microM). Since it is known that 125ICYP labels not only membrane bound but also internalized beta-adrenoceptors, it can be assumed that the large population of the low-affinity sites represents internalized receptors which may be abundant due to a high sequestration rate. 5. High numbers of beta 2-adrenoceptors are present in only a few brain structures of tree shrews (external layer of the olfactory bulb, 15.8 +/- 2.0 fmol/mg; claustrum, 19.3 +/- 1.5 fmol/mg; anteroventral thalamic nucleus, 19.4 +/- 1.5 fmol/mg; cerebellar molecular layer, 55.0 +/- 4.3 fmol/mg). Also for this class of beta-adrenoceptors, high- and low-affinity binding sites for the beta 2-selective antagonist ICI118.551 were observed, indicating that 125ICYP labels membrane bound and internalized beta 2-adrenoceptors. Only in the cerebellar molecular layer was a high percentage of high-affinity beta 2-adrenoceptors detected (Ki for ICI118.551 was 1.8 +/- 0.3 nM for 90% of the receptors). 6. In conclusion, beta 1- and beta 2-adrenoceptor binding can be localized and quantified by in vitro receptor autoradiography in the brains of tree shrews when serotonergic binding sites are blocked. Modulatory effects of long-term psychosocial conflict on the central nervous beta-adrenoceptor system in male tree shrews are described in the following paper.

Idazoxan down-regulates β-adrenoceptors on C6 glioma cells in vitro

Incubation of the C6 cells with 10 microM idazoxan (an alpha 2-adrenoceptor antagonist and putative antidepressant) for 5 days in vitro resulted in a 23% reduction of beta-adrenoceptor number and a 37% decrease in isoproterenol-induced cyclic AMP accumulation. In contrast, post-receptor stimulated cyclic AMP accumulation (by the use of forskolin or cholera toxin) was unaffected. The desensitization of the beta-adrenoceptor was accompanied by an increase in the KL/KH ratio for this receptor. Chronic in vitro treatment of C6 glioma cells with idazoxan did not significantly affect cholera or pertussis toxin catalyzed ribosylation of Gs and Gi/Go in these cells. Similarly, idazoxan did not alter either the basal levels of protein kinase C (PKC) alpha, or its cytoplasm to membrane translocation. These results suggest that idazoxan may have direct postsynaptic effects, the site of which may be at the level of receptor/G protein interaction.

Noradrenergic facilitation of motor neurons: localization of adrenergic receptors in neurons and nonneuronal cells in the trigeminal motor nucleus

Both alpha- and beta-adrenergic receptors (ARs) are involved in the facilitation of the monosynaptic jaw-closing reflex in the trigeminal motor nucleus (MoV) caused by norepinephrine (NE). The amplitude of muscle spindle afferent-evoked EPSPs in masseter motor neurons is 65% greater when noradrenergic axons to the motor nucleus are concomitantly activated and seems to be due to a presynaptic mechanism (Vornov, J. J., and J. Sutin. 1986. J. Neurosci. 6: 30-37). To determine the subtypes of ARs located on motor neurons and other cells, the cytotoxic lectin Ricin communis was injected into the masseter nerve of the trigeminal motor root to eliminate motor neurons in the masseter subnucleus of MoV. Autoradiography following incubation of tissue sections in the alpha 1 ligand 125IBE 2254 (125I-HEAT) or the nonselective beta ligand [125I]iodocyanopindolol (125ICYP) showed a decrease in alpha 1-AR binding related to the motor neuron degeneration and an increase in beta-AR binding associated with the glial reaction. To determine the extent to which glial proliferation was responsible for the increase in beta-ARs, cytosine arabinofuranoside (AraC) was administered to inhibit mitosis. Following AraC treatment, the total number of glial cells in the ricin-treated MoV was similar to that in normal MoV. Both beta-AR density and GFAP immunoreactivity remain increased, but to a lesser degree than following the ricin treatment alone. AraC also partially prevented the increase of immunolabeled or histochemically visualized microglia and capillary endothelial cells. The coincidence of the increases in beta-AR binding and GFAP in a region devoid of neurons argues that reactive astrocytes and other nonneuronal cells express beta-ARs in vivo. To determine whether the increase in astroglial beta-ARs was due to an up-regulation resulting from transynaptic degeneration of NE terminals, NE content was measured in MoV tissue punches, and NE terminals were visualized by immunocytochemical labeling of dopamine-beta-hydroxylase. NE content and NE terminal density remained unchanged following ricin-induced motor neuron degeneration.

Reduction in beta-adrenoceptor density in cultured rat glioma C6 cells after incubation with antidepressants is dependent upon the culturing conditions used

The hydrophilic beta-adrenoceptor ligand (-)-[3H]CGP-12177 binds to intact C6 cells with a high affinity (KD approximately 0.1 nM) and with a high degree of specificity. The binding was inhibited by DL-propranolol (Ki approximately 1 nM). Treatment of cells cultured in Dulbecco's modified Eagle medium (DMEM) without fetal calf serum for 4 days with desipramine reduced the (-)-[3H]CGP-12177 specific binding in a concentration-dependent manner, a reduction from 127 to 102 fmol/mg of protein being found at a ligand concentration of 1 nM after treatment with 10 microM desipramine. Lesser effects were seen after treatment for 1 day. A similar result was found with maprotiline, and reductions in specific binding were seen after 4 days of treatment with amitriptyline, iprindole, and citalopram. The reduction in binding-site density (measured per milligram of protein to compensate for variability in cell density per well), however, was paralleled in all cases by a reduction in the rate of cell proliferation. When C6 glioma cells were cultured in Ham's medium without fetal calf serum during the antidepressant treatment period, a higher specific binding was observed than for the DMEM-cultured cells, and 10 microM desipramine was without effect on either the (-)-[3H]CGP-12177 specific binding or cell proliferation. It is concluded that the effects of the antidepressants tested upon the density of (-)-[3H]CGP-12177 specific binding sites in intact C6 cells may be secondary to the toxicity of the compounds under the conditions used.

Effects of some antidepressant drugs on the activity of glial cell enzymes in culture

An oligodendrocyte-enriched culture was incubated with the antidepressant drugs, amitriptyline, desipramine, mianserin and citalopram, in a concentration of 3 X 10(-6) M for 24 h and 14 days. The activity of glycerol-3-phosphate dehydrogenase (G3PDH), lactate dehydrogenase (LDH), pyruvate carboxylase (PC) and the incorporation of [3H]thymidine was measured. An increase in both the incorporation of [3H]thymidine into DNA and the activities of G3PDH and PC was observed. However, LDH activity decreased after a 2-week incubation. The above results indicate that the antidepressant drugs change the metabolism of glial cells.

The antidepressant effect of beta-adrenoreceptor subsensitivity: a brief review and clinical implications

This paper reviews briefly the very common finding in chronic antidepressant use, of subsensitivity of the beta-adrenoreceptor-linked cyclic AMP system. This subsensitivity is observed with a number of different antidepressant treatments, including pharmacological, electrical and sleep deprivation. The subsensitivity requires intact noradrenergic and serotonergic systems, functionally linking the two neurotransmitters most often implicated in depression. Thyroid hormones and estrogens also cause subsensitivity, while the opposite effect is seen with Reserpine and Propranolol. A modified conditioning/sensitization model is proposed, implicating psychosocial stressors with a biological inability to down-regulate beta-adrenoreceptors.

Influence of opioids on beta-receptors down-regulation: studies in cultured C6 glioma cells

Opioids' modulation of beta-receptors' density and function has been investigated in a cultured cell line system. Rat C6 glioma cells do not have opioid receptors or, at least, the number of these receptors is very low, but cell exposure to desmethylimipramine (DMI) causes expression of functional opioid receptors as indicated by the increased [3H]DHM binding and by the acquired ability of opioids to inhibit ISO-stimulated cAMP accumulation. Cell exposure to DMI also causes beta-receptors' down-regulation as indicated by the decline in [3H]DHA binding coupled to a reduced ability of isoproterenol (ISO) to stimulate cAMP accumulation in intact cells. In the present paper we show that cell exposure to opioid agonists during DMI treatment counteracted DMI-induced beta-receptor loss. Similarly, opioid agonists added at the beginning of ISO exposure in DMI-pretreated cells, inhibited ISO-induced beta-receptor tachyphylaxis. These results suggest that opioids may exert a protective effect on beta-receptor function and this appears to be a common mechanism which is operant when overstimulation of beta-receptors takes place.

The influence of some antidepressant drugs on the nuclear volume of rat cingular cortex cells in culture

The changes in the volume of cell nuclei of the rat cingular cortex were investigated in culture after incubation with some antidepressant drugs. Two-week incubation of the cingular cortex culture with both tricyclic (desipramine, imipramine, amitryptyline) and non-tricyclic (mianserin) antidepressants in concentration of 3 X 10(-6) M resulted in a decrease of the volume of the cell nuclei. Because the size of the nucleus is regarded as a criterion of the cell metabolic activity, our results may point to a diminished activity of metabolic processes of the cells.

Effect of the tricyclic antidepressant desipramine on beta-adrenergic receptors in cultured rat glioma C6 cells

Rat glioma C6 cells, cultured in the presence of the tricyclic antidepressant desipramine, lost a significant number of beta-adrenergic receptors in a time- and dose-dependent manner. A similar loss was observed whether binding was determined on intact cells with the hydrophilic beta-adrenergic antagonist (+/-)-[3H]4-(3-tert-butylamino-2-hydroxypropoxyl)benzimidazole-2-o n HCl ([3H]CGP-12177) or on cell lysates with the more hydrophobic antagonists [125I]iodocyanopindolol or [3H]dihydroalprenolol. When stimulated with the agonist isoproterenol, desipramine-treated cells accumulated less cyclic AMP than control cells. The affinity of the beta-adrenergic receptors for either antagonist or agonist was unchanged after desipramine treatment. Desipramine interacted only weakly with the receptors and competed for [125I]iodocyanopindolol binding with a Ki of 30 microM. The presence in the culture medium of alprenolol or propranolol, potent beta-adrenergic antagonists, however, did not prevent the reduction in receptors by desipramine. Desipramine also caused a loss of beta-adrenergic receptors from cells maintained in serum-free medium and the cells themselves did not contain or secrete endogenous catecholamines. Although desipramine is a potent inhibitor of catecholamine uptake, it appears unlikely that the observed loss of beta-adrenergic receptors in rat glioma C6 cells exposed to the drug is due to an increase in extracellular catecholamine levels or to a direct interaction with the receptors.

Chronic exposure of human cells in culture to the tricyclic antidepressant desipramine reduces the number of beta-adrenoceptors

The effects of the antidepressant drug desipramine (DMI) on the density of beta-adrenoceptor sites were studied on intact cultured human cells: skin fibroblasts, lung fibroblasts and macrophages. Direct binding studies were performed with the radioligand 3H-CGP 12177, a hydrophilic beta-adrenergic antagonist. The confluent cell cultures were exposed to DMI and all three cell types showed a dose-dependent decrease in the number of beta-adrenergic binding sites. This receptor desensitisation was only seen after chronic exposure of the cells to DMI. The extent of desensitisation was comparable to that seen in brain following chronic treatment of rats with DMI. The affinity of the binding sites to the radioligand was not affected by the antidepressant drug action. From these results we suggest that the in vivo effect of antidepressant drugs on postsynaptic beta-adrenoceptor density, at least in part, reflects a primary drug action and not only an adaptive change to presynaptic events.

Long-term exposure of cortical cell cultures to clonazepam reduces benzodiazepine receptor binding

To determine if reduced drug efficacy after long-term exposure to clonazepam may be a consequence of benzodiazepine receptor alterations, cerebral cortical cell cultures were exposed to the drug (200 nM) for 14 days. Receptor binding was assayed on living cells in situ. After drug exposure, binding in experimental cultures differed markedly from controls with respect to total, specific, and clonazepam-displaceable (neuronal) benzodiazepine binding (60%, 53%, and 6% of control values, respectively) but recovered within 96 h of drug removal. RO5-4864-displaceable (nonneuronal) binding was modestly reduced at 0 time (72% of control), but returned to control values in 24 h. The differences in binding could be attributable to a relatively reduced affinity of the high-affinity binding site (Kd approximately 18 nM for controls and approximately 30 nM for drug-exposed cultures) but not to changes in the low-affinity binding site or to reduced numbers of receptors.

Desensitization of beta-receptors on primary astrocyte cultures by norepinephrine but not by tricyclic antidepressants

Primary astrocyte cultures from neonatal rat cerebral hemispheres were treated chronically for up to 3 weeks with the tricyclic antidepressants amitryptyline (AMT) or desipramine (DMI), or acutely with AMT and DMI added at the same time as the agonist, norepinephrine (NE). AMT and DMI were added at concentrations from 10(-9) to 10(-5) M. Both types of treatment did not decrease the increase in cyclic AMP (cAMP) content of these cells in response to a 10 min exposure to 10(-5) M NE. Chronic exposure to the antidepressants also did not affect stimulation of cAMP by isoproterenol (iso) in both rat and mouse primary astrocyte cultures. In contrast to the lack of effect of the tricyclic antidepressants pretreatment of the cultures with 10(-5) M NE resulted in total inhibition of the cAMP response after 2 h, with a 50% decrease occurring in about 45 min. This is similar to the agonist-induced desensitization of the beta-receptor-adenylate cyclase system seen in many other cells. This effect could, in part, be a direct response to increased intracellular cAMP since pretreatment with 0.25 and 1.0 mM N6-2'-O-dibutyryl cAMP (DBcAMP) also resulted in total inhibition of the cAMP response after 4 h. Receptor labelling experiments using [125I]cyanopindolol showed no decreases in apparent binding sites up to 3 h after exposure to 10(-5) M norepinephrine, suggesting that the rapid desensitization of the cAMP response was primarily due to an uncoupling of the receptor from the adenyl cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluoride-stimulated adenylate cyclase activity in rat brain following chronic treatment with psychotropic drugs

Chronic treatment with imipramine and reserpine increased fluoride-stimulated activity of adenylate cyclase in homogenates of cerebral cortex and "limbic" forebrain of the rat. Concomitant treatment with lithium counteracted this effect, while lithium alone had no effect on the activity of adenylate cyclase.

Adaptation to stress and brain noradrenergic receptors

The present paper reviews the effects of stress on noradrenergic receptor function in the brain. Most forms of stress thus far examined have been found to reduce either the magnitude of the cAMP response to stimulation by catecholamines (CAs) and/or the density of beta adrenergic receptors in the brain. These effects (a) generally occur in the cerebral cortex, (b) are more marked after chronic than acute stress, (c) may be the result of excessive release of norepinephrine (NE), ACTH or serotonin (5-HT) and (d) may occur in neurons glia or both. The function of these receptor alterations is not known but is presumed to be related in some manner to adaptation to chronic stress. A review of similar changes occurring in peripheral organs after repeated stress or CA injections reveals that subsensitivity of beta adrenergic receptors can be associated with either decreases or increases in CA-stimulated organ output. The latter findings caution against concluding that there is a decreased postsynaptic noradrenergic function after adaptation to chronic stress. Instead they suggest that it may be more appropriate to view stress-induced receptor subsensitivity as part of a more complex pattern of adaptive changes which includes alterations in the size, number, efficiency and output of CA effector cells.

Reduced benzodiazepine receptor binding in cerebral cortical cultures chronically exposed to diazepam

Fetal mouse cerebral cortex in culture was exposed chronically to diazepam, and benzodiazepine (BDZ) receptor activity was assayed on the intact cell. Results have indicated: (1) minimal evidence of generalized drug toxicity, determined by choline acetyltransferase activity, tetanus labeling, high-affinity uptakes of gamma-aminobutyric acid and beta-alanine, protein, and light microscopy; (2) a significant dose-dependent reduction (down-regulation) of BDZ receptor activity that occurs after chronic exposure to clinically relevant concentrations of the drug; (3) a disproportionate suppression of the neuronal portion of the receptor after both acute and chronic exposure; and (4) recovery patterns of the BDZ receptor that differ substantially after acute versus chronic treatment. The latter finding may partially explain the observed clinical manifestations of drug tolerance and reduced anticonvulsant efficacy after prolonged use of this class of drugs.

Primary astrocyte cultures--a key to astrocyte function

Morphological studies have established the ubiquitous nature of astrocytes in the CNS. Their processes surround capillaries and synapses, form the subpial and subependymal layers, and seemingly invest every neuronal surface not covered by other neuronal surfaces or oligodendroglial membranes. Although such interrelationships have long suggested that astrocytes may play many critical roles, there still remains relatively little experimental information on the functions and properties of these cells. About a decade ago it became evident that primary cultures from neonatal rodent brains can consist predominantly of normal astrocytes. Based on these findings there is now an increasing number of studies in which such primary cultures are being used to help unravel the continuing enigma of the properties and functions of astrocytes. Aspects of this work are reviewed in this article. Such work has already shown that astrocytes in primary culture exhibit the basic electrophysiological characteristics which had been the only functional property well established for these cells in situ. Further studies of the electrophysiological properties of these cells, which can be correlated with ion transport studies, are beginning to show that astrocytes may have more complex electrophysiological properties than had previously been supposed, as well as a number of important electrically silent ion fluxes. In addition, astrocytes in primary culture show uptake of and receptors for a number of transmitters, properties which have wide-ranging implications. Studies in culture also support work in vivo that astroglia may have an important role in neuronal development.

Acute and chronic effects of antidepressant drugs on beta-adrenergic function in astrocytes in primary cultures: an indication of glial involvement in affective disorders?

Acute and chronic effects of the antidepressant drugs tranylcypromine, a monoamine oxidase inhibitor, and amitriptyline, a monoamine uptake inhibitor, were studied on beta-adrenergic receptor function in mouse astrocytes in primary cultures. In clinically relevant concentrations, acute administration of either antidepressant drug had a direct inhibitory effect on the binding of the beta-adrenergic ligand dihydroalprenolol and on the isoproterenol-induced accumulation of cyclic AMP. However, in the absence of isoproterenol, these drugs enhanced the formation of cyclic AMP in the astrocytes. Chronic exposure to amitriptyline or tranylcypromine led to a decrease in isoproterenol-induced accumulation of cyclic AMP, and the time course for the development of this phenomenon was similar to that reported for whole brain in vivo. These findings suggest that these antidepressant drugs act as a partial agonists at beta-adrenergic receptors on astrocytes, and that the down-regulation of beta-adrenergic activity that occurs in vivo after chronic administration of antidepressant drugs may, to a large extent, take place in astrocytes and may result from the partial beta-agonist nature of the drugs.

Tricyclic antidepressant desipramine induces stereospecific opiate binding and lipid modifications in rat glioma C6 cells

Incubation for 48 hours of C6 glioma cell cultures with 10(-4)M tricyclic antidepressant desipramine gave rise to a quantitative increase of total lipids and to qualitative modifications of glycosphinegolipids involving detection by thin-layer chromatography of spots migrating according to cerebroside and sulfatide and presence of an abnormal ganglioside pattern. These lipid modifications were associated with the appearance of stereospecific binding of opiates (dihydromorphine) with a dissociation constant of 30-60 nM. These results favor an important role of lipids in opioid receptor function.

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

The action of adrenergic alpha- and beta-agonists and antagonists has been studied on the membrane potential and resistance of glial cells of cultured rat central nervous system. Noradrenaline and the alpha-adrenoceptor stimulating agents phenylephrine and clonidine (10(-7) to 10(-4)M) depolarized the glial membrane, whereas the beta-agonist isoprenaline caused a hyperpolarization at low concentrations (10(-7) and 10(-6)M). The effects of noradrenaline and phenylephrine were reversibly blocked by the alpha-antagonist phentolamine, whereas those of isoprenaline were antagonized by the beta-blocker atenolol. Atenolol did not affect the depolarization by noradrenaline. The glial depolarization induced by the alpha-agonists was not the consequence of a change in the extracellular K+-concentration unlike that produced by amino acid transmitters. The present results, together with those of biochemical and autoradiographic binding studies, suggest that alpha- and beta-adrenoceptors occur on glial cells and that the glial depolarization is mediated by alpha-receptors, whereas the hyperpolarization is due to activation of beta-receptors.

Evidence for the existence of alpha- and beta-adrenoceptors on neurones and glial cells of cultured rat central nervous system--an autoradiographic study

The cellular localization of the binding of radioactive noradrenaline and alpha- and beta-adrenoceptor antagonists was studied in organotypic cultures of rat cerebellum, brain stem and spinal cord using autoradiography. In cerebellar cultures, many neurones, which appeared to be Purkinje cells, were labelled by [3H]noradrenaline and by the beta-antagonists [3H]dihydroalprenolol and [3H]carazolol, whereas no binding of the alpha-antagonists [3H]prazosin and [3H]rauwolscine was detected. In cultures of spinal cord and brain stem, [3H]noradrenaline and the beta-antagonists were bound to many large neurones. Binding of [3H] alpha-antagonists was observed to a small number of brain stem and spinal neurones, the labelling being much weaker than that produced by the [3H] beta-antagonists. The antidepressant [3H]desmethylimipramine was bound to many neurones and glial cells in cerebellar, brain stem and spinal cord cultures. Glial cells also possessed binding sites for [3H]noradrenaline and alpha- and beta-adrenoceptor antagonists, findings that are consistent with recent electrophysiological observations which indicate the existence of alpha- and beta-adrenoceptors on cultured astrocytes.