Treatment with antidepressants and down regulation of beta-adrenergic receptors

The effect of chronic and acute treatment with several antidepressant drugs and electroconvulsive shock (ECS) on the responsiveness of norepinephrine (NE) receptor coupled adenylate cyclase and beta-adrenergic receptor binding sites in rat brain has been studied by several groups of investigators. It has been consistently reported that chronic administration (2 to 4 weeks of treatment) of antidepressant drugs or ECS causes a decreased accumulation of NE stimulated cyclic AMP accumulation in rat brain. It has also been reported that chronic treatment with antidepressant drugs or ECS causes decreased binding of [³H]dihydroalprenolol (DHA) or [¹²⁵I]hydroxybenzylpindolol (I-HYP) (ligands used for studies of beta-adrenergic receptors) in rat brain. However, such effects are not observed after acute or single administration of these drugs or ECS. Since there appears to be a close correlation between the time course of the effects of these drugs on NE receptor coupled adenylate cyclase and beta-adrenergic receptor binding, and since most of the antidepressant drugs tested thus far produce such effects, it has been suggested that down regulation or decreased responsiveness of beta-adrenergic receptors may be related to the therapeutic effects manifested by antidepressant drugs. These observations offer the possibility that determination of beta-adrenergic receptor responsiveness after chronic treatment with drugs may prove useful in the preclinical screening of potential antidepressant drugs. In this paper, studies related to down regulation of beta-adrenergic receptors, including our own, are reviewed.     

Beta-adrenergic regulation of Sertoli cell adenylyl cyclase: desensitization by homologous hormone

Incubation of Sertoli cell-enriched cultures with D,L-isoproterenol caused a time- and concentration-dependent, homologous desensitization of isoproterenol-responsive adenyl cyclase, whereas the response to FSH was unaffected. Half-maximal desensitization was achieved within 1 h of preincubation, after which a more gradual loss of response was observed. Preincubation of Sertoli cells for 24 h with increasing concentrations of D,L-isoproterenol demonstrated that the concentration required to obtain half-maximal densensitization was approximately 10-fold lower than the Km for activation of adenylyl cyclase. The function of the guanine nucleotide regulatory component (N-component) of the adenylyl cyclase complex in hormonally desensitized Sertoli cells, as evaluated by activation of adenylyl cyclase by GTP, GMPP(NH)P, fluoride and Mg2+, was not affected by the hormone pretreatment. Preincubation of Sertoli cells with a high concentration of dbcAMP (10(-3) M) for 24 h was associated with a 45% reduction in adenylyl cyclase activation by both FSH and isoproterenol. Also in this case fluoride- and GTP-stimulated adenylyl cyclase activities were normal. However, the effects of dibutyryl cyclic AMP occurred much more slowly than agonist-induced desensitization, indicating that cAMP may not be the primary mediator of homologous desensitization of Sertoli cell adenylyl cyclase by isoproterenol.     

Neuronal, glial and meningeal localizations of neurotransmitter-sensitive adenylate cyclases in cerebral cortex of mice

The neurotransmitter-sensitive adenylate cyclases, respectively present in the dissociated cells of new-born mouse cerebral cortex (containing both neuronal and glial cells) and in a homogeneous population of glial cells, were compared. The dissociated cells from the cerebral cortex of new-born mice were found to contain Ca2+-, dopamine-, serotonin- and purinergic-sensitive adenylate cyclases. The dopaminergic receptor involved was extensively characterized and was similar to that described in adult animals. Beta-adrenergic-sensitive adenylate cyclase was present but was poorly active. After 3 weeks in culture, the neurons disappeared and a homogeneous population of glial cells was obtained (96% of the cells synthetized glial fibrillary acidic protein). These glial cells contained a highly potent beta-adrenergic-sensitive adenylate cyclase, and adenosine- adn Ca2+-sensitive enzymes. Ca2+ stimulation of the adenylate cyclase was due to the presence of calmodulin. We suggested that the dopaminergic- and serotoninergic-sensitive adenylate cyclases which disappeared during culture are probably localized in neuronal cells. The presence of Ca2+-, adenosine- and beta-adrenergic-sensitive adenylate cyclases in glial cells does not exclude their presence in neuronal cells. For comparison, the same experiments were conducted on meningeal layers of new-born mice and on meningeal cells in culture. They both contained beta-adrenergic- and purinergic-sensitive adenylate cyclases.     

Forskolin stimulates the conversion of tyrosine to dopamine in catecholaminergic neural tissue

Forskolin stimulates the conversion of tyrosine to dopamine in slices and synaptosomes from rat striatum, in synaptosomes from rat hypothalamus, and in slices from bovine retina. In striatal synaptosomes, there is an approximately 80% stimulation of dopamine formation from tyrosine, but no effect with DOPA as substrate, suggesting that the effect is on tyrosine hydroxylase. Stimulation is saturable with a half-maximal effect at under 1 μM forskolin. Forskolin stimulation is additive with, and hence independent of, activation due to KCl, but not that due to dibutyryl-cyclic AMP. Stimulation of dopamine synthesis by forskolin may provide an approach for elucidating the regulation of the adenylate cyclase system associated with catecholaminergic nerve endings.     

LH/hCG Responsive Adenylyl cyclase in Rat Testis and Testes from Testicular Feminized Male (Tfm) Rats and Mice: Desensitization by Homologous Hormone

Testes of testicular feminized (tfm) rats and mice, as well as of normal male rats contain an LH/hCG responsive adenylyl cyclase. Basal, as well as hCG stimulated activities were higher in tfm rats and mice than in normal rats. The presence of an LH/hCG responsive adenylyl cyclase in the testis of tfm rats and mice shows that the greatly elevated LH levels present in males having this syndrome, giving 80–90% reduction in LH//hCG receptors, do not cause an uncoupling of the remaining receptors from the adenylyl cyclase. It also shows that androgens are not essential for coupling of the LH/hCG receptors to the adenylyl cyclase.
Injection of 200 IU of hCG into adult normal rats and tfm rats caused, after 48 h, a complete loss of LH/hCG stimulated adenylyl cyclase, whereas the FSH responsive adenylyl cyclase in both animal preparations was maintained. Desensitization of the LH responsive adenylyl cyclase by hCG in normal rats, confirms previous studies showing lack of hCG stimulated cyclic-AMP secretion after a comparable dose of hCG in vivo. Similarly, hCG (50 IU) caused a transient loss of LH/hCG responsive adenylyl cyclase in tfm mice, with a complete disappearance of response after 24 h. At 48 and 72 h after injection of hCG the response gradually returned to normal. The fact that hCG caused a complete desensitization of the LH/hCG responsive adenylyl cyclase in both tfm rats and mice, proves that androgen receptor mediated events are not involved in hCG desensitization of the adenylyl cyclase in Leydig cells.     

Cellular localization of the Mn2+-dependent adenylyl cyclase (AC) in rat testis

In the present study we have examined the exact cellular localization of the Mn²⁺-dependent adenylyl cyclase (AC) in the rat testis. Several pieces of evidence indicate that this testis specific enzyme is exclusively localized to haploid cells in the testis: firstly, the appearance of the Mn²⁺-dependent adenylyl cyclase coincides in time with the occurrence of haploid cells (1C) in the rat testis. Secondly, testes of tfm rats and mice, in which little or no haploid cells are found, exhibit negligible Mn²⁺-dependent AC activity. Finally, separation of various testicular cells by BSA gradient sedimentation (Staput fractionation), revealed that this enzyme activity follows the location of haploid cells; the highest specific AC activities being found in the fractions containing elongated spermatids. Some activity found in the primary spermatocyte fraction (4C cells) can to a large extent be explained by the contamination of haploid cells in this fraction.     

Light and electron microscopic demonstration of guanylate cyclase in rat brain

In the endothelial cells of capillaries in the rat CNS, we have observed abundant, circumferentially oriented, smooth, membrane-bound profiles, found just beneath, and parallel to, the abluminal plasmalemma. These structures are seen particularly well in tissue exposed to potassium ferricyanide-reduced OsO4. We studied these structures cytochemically, using glucose-6-phosphatase as a marker for endoplasmic reticulum, and acid phosphatase as a marker for the Golgi-associated endoplasmic reticulum containing lysosomal enzymes (GERL). We found that they contained glucose-6-phosphate hydrolyzing activity but did not contain acid phosphatase activity. Comparable structures were not seen in the continuous capillaries of skeletal muscle. Based on their morphology and content of glucose-6-phosphate hydrolyzing activity, we conclude that these structures are uniquely oriented smooth endoplasmic reticulum, which is much more abundant in capillaries of the CNS than in other continuous capillaries. The function of this distinctive feature of the CNS capillary is not known.     

Inhibition and superactivation of the calcium-stimulated isoforms of adenylyl cyclase

It was shown previously that chronic exposure to opiate agonists increases adenylyl cyclase (AC) activity, a phenomenon termed AC superactivation (or supersensitization). More recently, we showed that acute Gi/o- coupled receptor activation inhibits the activity of several AC isozymes, including Ca2+/calmodulin-stimulated AC-I and -VIII, whereas chronic receptor activation induces their superactivation. Here, we report that both acute Mu-opioid receptor-induced inhibition and chronic induced superactivation of AC-I and -VIII are pertussis toxin sensitive. In addition, we show that proteins that interfere with the activity of Gbetagamma subunits (Gbetagamma scavengers) strongly attenuate the acute inhibition of AC-I and -VIII and the superactivation of AC-I, and abolish the superactivation of AC-VIII. Based on these results, we suggest that Gbetagamma is involved in the acute inhibition and chronic agonist-induced superactivation of AC types I and VIII.     

Role of phosphatidylinositol-3-kinase and protein kinase Cζ in the adenylate cyclase signal mechanism of action of relaxin in muscle tissues of rats and mollusks

We showed that phosphatidylinositol-3-kinase and protein kinase C are involved in the adenylate cyclase signal mechanism of relaxin action. A selective inhibitor of phosphatidylinositol-3-kinase wortmannin blocked the stimulatory effect of relaxin on adenylate cyclase in rat skeletal muscles and Anodonta cygnea smooth muscles. Antibodies against protein kinase C abolished the relaxin-induced stimulation of adenylate cyclase in rat muscles, but not in mollusk muscles. Our results indicate that phosphatidylinositol-3-kinase and protein kinase C play a role in the adenylate cyclase signal mechanism of relaxin action.Translated from Byulleten Eksperimentalnoi Biologii i Meditsiny, Vol. 138, No. 10, pp. 420–423, October, 2004