A radioisotopic method for measuring the formation of adenosine 3',5'-cyclic monophosphate in incubated slices of brain

Alpha 2-adrenoceptors mediate inhibition of cyclic AMP production in the spinal cord after stimulation of cyclic AMP with forskolin but not after stimulation with capsaicin or vasoactive intestinal peptide

In slices obtained from the ventral and the dorsal guinea pig spinal cord both forskolin and vasoactive intestinal peptide (VIP) caused a dose-dependent stimulation of the production of cyclic AMP. By contrast capsaicin stimulated cyclic AMP formation only in the dorsal cord; no effect was observed in the ventral cord. The alpha 2-adrenergic agonist UK-14,304 dose-dependently inhibited the production of cyclic AMP in both the dorsal and ventral aspects of the cord when the formation of cyclic AMP had been stimulated with 3 microM forskolin, the maximal inhibition amounting to 25-32%. Also the basal (i.e., unstimulated) production of cyclic AMP was inhibited, the inhibition amounting to about 16-18%. However, after stimulation of cyclic AMP formation in the dorsal cord with capsaicin, UK-14,304 was virtually ineffective in inhibiting the accumulation of cyclic AMP. Also, when the formation of cyclic AMP was stimulated with VIP, UK-14,304 was virtually ineffective in inhibiting the formation of cyclic AMP both in the ventral and the dorsal parts of the cord. When cyclic AMP production had been stimulated with forskolin the ability of UK-14,304 to inhibit the formation of cyclic AMP was not attenuated by capsaicin, either in the ventral or in the dorsal cord. The results are discussed with the notion that cyclic AMP inhibitory spinal cord alpha 2-adrenoceptors are located on cells accessible to stimulation of cyclic AMP with forskolin but not with capsaicin or VIP.

Glucocorticoid administration increases receptor-mediated and forskolin-stimulated cyclic AMP accumulation in rat brain cerebral cortical slices

Experiments were undertaken to examine the influence of dexamethasone administration on adrenergic receptor-mediated cAMP accumulation in rat brain cerebral cortical slices. While the initial response to continuous treatment with dexamethasone or ACTH was a decrease in the alpha-adrenergic receptor-mediated augmentation of isoproterenol-stimulated cAMP production, more prolonged administration of these substances also increased beta-adrenergic receptor-stimulated cAMP accumulation. Dose-response studies revealed a dose-dependent increase in beta-adrenergic receptor-stimulated cAMP production. Prolonged administration of either dexamethasone or ACTH also increased 2-chloradenosine-, vasoactive intestinal peptide (VIP)-, and forskolin-stimulated cAMP production in the brain slice. These results, together with previous findings, suggest that glucocorticoids have at least two effects on the brain neurotransmitter receptor-coupled cAMP generating system which may be important for maintaining homeostasis during times of stress.

Inhibition of adenylate cyclase in bovine ciliary process and rabbit iris ciliary body to alpha 2-adrenergic agonists

Alpha 2-adrenergic inhibition of adenylate cyclase in bovine ciliary processes and rabbit iris ciliary body (ICB) was studied. With bovine ciliary process membrane, it was found that cAMP production in the presence of 1 microM isoproterenol was increased with increasing NaCl concentrations from 0 to 200 mM. Clonidine, an alpha 2-adrenergic agonist, produced a NaCl concentration-dependent inhibition of cAMP production in the presence of isoproterenol with a maximum inhibition at 200 mM NaCl (P less than 0.05). NaCl concentrations had no effect on basal adenylate cyclase activities and activity in the presence of clonidine alone. The alpha 2-adrenergic agonists, lofexidine, clonidine and p-amino-clonidine were tested for their ability to inhibit isoproterenol-stimulated adenylate cyclase in bovine ciliary process membrane in the presence of 200 mM NaCl. Their dose-response curves showed that they had similar IC50's but the maximum inhibition differed among these agonists. Clonidine was found to be a partial agonists producing 55% of the inhibition obtained with lofexidine. The specificity of inhibition of isoproterenol-stimulated adenylate cyclase by alpha 2-agonists and blockade by pertussis toxin was examined by adenine labelling in rabbit ICB. The results demonstrate that alpha 2-adrenergic receptors exert specific inhibitory effects on adenylate cyclase activity in rabbit ICB, which are mediated by an inhibition guanine nucleotide regulatory protein, Gi.

Effects of lectins and tunicamycin on cAMP response to parathyroid hormone

Carbohydrate moieties of cell surface glycoproteins with an external orientation play a role in hormone recognition and/or transmembrane signal transmission. We have examined the effect of various lectins, which interact with specific cell surface glycosyl residues, and of tunicamycin, an antibiotic that inhibits glycosylation of proteins, on the adenosine 3',5'-cyclic monophosphate (cAMP) response to parathyroid hormone (PTH) in confluent cultured osteoblast-like rat osteosarcoma cells (UMR-106) and opossum kidney cells (OK cells). Incubation of both cell lines with wheat germ lectin (WGL), but not with concanavalin A, succinylated wheat germ, ricin, or soybean lectins, markedly reduced the PTH-induced cAMP production, whereas the stimulation obtained with forskolin, a compound that acts directly on the adenylate cyclase enzyme, was not affected. In contrast, tunicamycin did not cause any decrease in the cAMP response to PTH. These results indicate that the masking of sialic acid residue by WGL considerably blunted PTH-stimulated cAMP production in cultured osteoblast-like and kidney cells. An 80% inhibition of glycosylation of cell surface proteins did not appear to affect the response to PTH. Thus the functional role of this carbohydrate moiety in the PTH receptor remains to be determined.

Characterization of dopamine and beta-adrenergic receptors linked to cyclic AMP formation in intact cells of the clone D384 derived from a human astrocytoma

3,4-Dihydroxyphenylethylamine (dopamine) and beta-adrenergic receptor agonists and antagonists were assessed for their effects on cyclic AMP accumulation in human astrocytoma derived clone D384 cells. Dopamine, SKF 38393, and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene increased cyclic AMP content with Ka values of 2.0, 0.2, and 1.6 microM. The D1-selective antagonists SCH 23390 (Ki, 1.2 nM) and SKF 83566 (Ki, 0.8 nM) were over 5,000-fold more potent than the D2-selective antagonist domperidone (Ki, 6.7 microM) at inhibiting dopamine stimulation of cyclic AMP formation. SCH 23388 (Ki, 560 nM; the S-enantiomer of SCH 23390) was 400-fold less potent than SCH 23390. Isoprenaline, adrenaline, salbutamol, and noradrenaline increased cyclic AMP content with Ka values of 0.13, 0.12, 0.22, and 7.60 microM. The beta 2-selective antagonist ICI 118,551 (Ki,0.8 nM) was almost 8,000-fold more potent than the beta 1-selective antagonist practolol (Ki, 5.9 microM) at inhibiting isoprenaline stimulated cyclic AMP accumulation. These results demonstrate that D384 cells express D1-dopamine and beta 2-adrenergic receptors linked to adenylate cyclase. Furthermore, the dopamine receptor expressed by D384 cells exhibits a pharmacological profile typical of a mammalian striatal D1-receptor and therefore the use of this clone represents another approach to studying central D1-receptors.

The influence of electroshock on adrenoceptor function in rat brain cerebral cortex: selectivity for the alpha-adrenoceptor site

The present study was undertaken to examine the effect of electroshock on adrenoceptor-mediated cAMP and inositol phosphate accumulation in rat brain cerebral cortical slices. Under the conditions of these experiments, isoproterenol-induced cAMP accumulation was unaltered by electroshock, although there was a significant reduction in the norepinephrine- and isoproterenol + 6-fluoronorepinephrine-stimulated responses. No change in alpha-adrenoceptor-mediated inositol phosphate accumulation was noted. The results indicate that electroshock selectively modifies an alpha-adrenoceptor system in brain that differs from that associated with inositol phosphate accumulation.

Cultured mesenteric vascular smooth muscle cells express dopamine DA1-receptors

Incubation of cultured mesenteric vascular smooth muscle cells with dopamine, in the presence of propranolol, caused an increase in cyclic AMP formation in a concentration-dependent manner (Ka apparent 6.8 +/- 0.5 microM). This effect of dopamine was inhibited by the DA1-receptor antagonist SCH 23390 (Ki = 1 nM). These results suggest that cultured mesenteric vascular smooth muscle cells express DA1-receptors linked to adenylate cyclase.

Evidence for pharmacologically distinct subsets of GABAB receptors

Activation of GABAB receptors augments neurotransmitter-stimulated cyclic AMP accumulation while inhibiting forskolin-mediated second messenger production. Previous studies have revealed that GABAB receptors are associated with a pertussis toxin sensitive G protein, such as Gi. While such a linkage is consistent with the finding that GABAB receptor activation inhibits forskolin-mediated second messenger accumulation, it fails to explain how GABAB agonists are capable of augmenting receptor-mediated cyclic AMP production. The present experiments were undertaken to explore the possible existence of pharmacologically distinct GABAB receptors in an attempt to explain this apparent discrepancy. For the study, a variety of agents were examined for their ability to inhibit GABAB binding to brain membranes and to modify isoproterenol- or forskolin-stimulated second messenger production in rat brain slices. Of the compounds studied, only 3-aminopropylphosphonic acid and 4-aminobutylphosphonic acid were found to inhibit GABAB binding. However, 4-aminobutylphosphonic acid failed to influence either isoproterenol- or forskolin-stimulated cyclic AMP production. On the other hand, while 3-aminopropylphosphonic acid also failed to affect isoproterenol-stimulated second messenger accumulation, it inhibited the forskolin-mediated response. Given this finding, and the fact that some of the agents tested are known to influence GABAB receptor function in other systems, the results indicate a multiplicity of pharmacologically distinct GABAB receptor recognition sites. This discovery paves the way for the development of more selective GABAB receptor agonists and antagonists possessing different therapeutic potentials.

Inhibition of cyclic AMP production by alpha 2-adrenoceptor stimulation in the guinea-pig spinal cord slices

In spinal cord slices isolated from guinea-pig and preincubated with 3H-adenine, 0.3-30 microM forskolin induced a dose-dependent increase in the content of 3H-cAMP, the maximal increase being about 8-fold. The selective alpha 2-adrenergic agonist UK-14,304 (10 microM) reduced both the basal and the forskolin stimulated levels of 3H-cAMP by 18-32%. Dose response curves of the effect of UK-14,304 on cAMP production in the spinal cord slices, stimulated with 3 microM forskolin, showed an IC50 of 37 nM and a maximally inhibitory effect of 27%. A number of other alpha 2-adrenergic agonist (clonidine, guanfacine, B-HT 920 and B-HT 933) also inhibited the forskolin stimulated 3H-cAMP production; clonidine and guanfacine being almost equipotent with UK-14,304, but their maximal inhibitory effects being only about 6-7%. B-HT 920 and B-HT 933 were less potent and their maximal inhibitory effects about 16-21%. The dose response curve of UK-14,304 on inhibition of forskolin stimulated cAMP production was shifted almost 50-fold to the right by 0.3 microM yohimbine. Prazosin (0.3 microM) did not affect the UK-14,304 dose response curve. It is concluded that alpha 2-adrenoceptor stimulation mediates inhibition of cAMP production in the guinea-pig spinal cord.

Forskolin effects on the voltage-gated K+ conductance of human T cells

Forskolin, a direct activator of adenylate cyclase, modifies the voltage-dependent K+ conductance of quiescent human peripheral blood T lymphocytes. In the presence of greater than 20 microM forskolin, the average voltage-gated current in whole-cell patch clamp is significantly decreased. The voltage dependence and kinetics of activation are not changed from untreated control cells. However, inactivation becomes biphasic. Much of the current inactivates very quickly (complete in 10 ms), and the remaining outward current inactivates more slowly with a time constant closer to that of control cells. To determine whether this effect is mediated by a rise in intracellular cAMP, cells were preincubated and subsequently voltage-clamped in the presence of other agents that raise the cAMP levels in T cells (isoproterenol plus a phosphodiesterase inhibitor, or dibutyryl cAMP) with no effect on the K+ conductance. Similarly, cells put in whole-cell patch clamp with cAMP, GTP, ATP, and theophylline added to the electrode filling solution showed no change in K+ current. Because other procedures that raise cAMP did not duplicate the effect of forskolin, we investigated the effect of 1,9-dideoxyforskolin, an analogue of forskolin that does not stimulate adenylate cyclase in human lymphocytes. This drug induced changes in the whole-cell K+ conductance identical to those observed with forskolin. Both forskolin and dideoxyforskolin inhibit mitogen-induced proliferation of lymphocytes. Because inhibition of proliferation occurs in the presence of known K+ channel blockers, these results suggest that forskolin has an effect on T cell mitogenesis that is mediated by inhibition of K+ conductance and is independent of cAMP.

Augmentation of neurotransmitter receptor-stimulated cyclic AMP accumulation in rat brain: differentiation between the effects of baclofen and phorbol esters

The augmentation of isoproterenol or vasoactive intestinal peptide (VIP)-stimulated cyclic AMP accumulation in rat brain slices by the GABAB agonist baclofen was compared to that mediated by tumor-promoting phorbol esters. The protein kinase C inhibitor H7 and desensitization of protein kinase C reduced the cyclic AMP augmenting effect of the phorbol ester, but not baclofen. Incubation of brain slices in the presence of both baclofen and a phorbol ester amplified the cyclic AMP response to isoproterenol or VIP to a greater degree than that found with either baclofen or the phorbol ester alone, with the increased augmentation appearing to be additive. These findings indicate that although stimulation of GABAB receptors or protein kinase C activation by phorbol esters have similar effects on transmitter-stimulated cyclic AMP production in brain, these augmenting actions appear to be independently mediated.

Prostanoid inhibition of canine parietal cells: mediation by the inhibitory guanosine triphosphate-binding protein of adenylate cyclase

We have investigated the mechanisms underlying prostaglandin inhibition of histamine-stimulated parietal cell function. Enzyme-dispersed canine parietal cells were enriched by elutriation. The accumulation of the weak base [14C]aminopyrine was used as an index of parietal cell function and cyclic adenosine monophosphate content was measured by radioimmunoassay. Step density gradients of the elutriator-enriched parietal cell fractions indicated that parietal cells accounted for the histamine stimulation of cyclic adenosine monophosphate production and inhibition by the prostaglandin E analogue Enprostil. Pertussis toxin adenosine diphosphate-ribosylates a subunit with a molecular weight of 41,000, thereby inactivating the inhibitory guanine nucleotide-binding protein of adenylate cyclase. Pertussis toxin treatment of parietal cells in overnight suspension culture was used to determine if inhibitory guanosine triphosphate-binding protein mediated prostanoid inhibition. In control cultured cells, prostaglandin E2 and Enprostil markedly inhibited forskolin- and histamine-stimulated aminopyrine accumulation. In parietal cells treated with pertussis toxin (300 ng/ml) for 18 h, stimulation of parietal cell function by histamine, isobutylmethylxanthine, and forskolin was unaltered compared with control cells, whereas prostaglandin E2 and Enprostil inhibition was markedly reduced. In pertussis toxin-treated cells, histamine-stimulated cyclic adenosine monophosphate generation was unaltered, whereas Enprostil inhibition of histamine-stimulated cyclic adenosine monophosphate production was markedly reduced. Pertussis toxin treatment of membranes from control, but not from pertussis toxin-treated, cells induced the [32P]adenosine diphosphate-ribosylation of a membrane protein with a molecular weight of 41,000, presumably the alpha-subunit of inhibitory guanosine triphosphate-binding protein. We conclude that prostanoids inhibit parietal cell function by receptor-mediated interaction with the inhibitory guanine nucleotide-binding protein of adenylate cyclase.

Factor derived from human lung carcinoma associated with hypercalcemia mimics the effects of parathyroid hormone on phosphate transport in cultured renal epithelia

A decrease in renal tubular reabsorption of inorganic phosphate (Pi) can be observed in hypercalcemia of malignancy. In the present study we investigated the effect of serum-free conditioned medium (CM) from cells, derived from a lung carcinoma (BEN) of a hypercalcemic patient, and of PTH on cyclic AMP (cAMP) production and sodium-dependent Pi transport (NaPiT) in epithelia of two renal cell lines. In opossum kidney cells (OK), PTH is known to enhance cAMP production and inhibit NaPiT; in contrast, in LLC-PK1 cells, PTH has no effect on NaPiT since this kidney cell line is devoid of PTH receptors. In OK cells, BEN CM induced a three- to fourfold increase of cAMP production, which was blunted by the PTH inhibitors bPTH(3-34) and bPTH(7-34). NaPiT, as assessed by measuring the initial rate of Pi uptake, was inhibited in a dose-dependent manner by BEN CM, with an effect maximal between 1h30 and 6 hr of incubation (40 +/- 4% and 47 +/- 4%, respectively), corresponding to the effect produced by 1-3 nM bPTH(1-34). The Na-dependent transport of a glucose analog was affected neither by BEN CM nor by PTH. In LLC-PK1 cells, neither BEN CM nor PTH altered cAMP production nor NaPiT after 1h30 of incubation. At 6 hr, BEN CM caused a slight decrease in NaPiT. In conclusion, these results constitute the first evidence of a direct and selective inhibition by tumor-derived factor(s) of NaPiT in cultured renal epithelia. Most of the renal NaPiT inhibitory activity produced by the lung tumor required the presence of a PTH receptor-adenylate cyclase system.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y receptor-effector coupling mechanisms in cultured vascular smooth muscle cells

Primary cultures of rabbit pulmonary artery (RPA) vascular smooth muscle (VSM) were utilized to determine the coupling of neuropeptide Y (NPY) receptors to several effector systems in VSM. NPY inhibited forskolin-stimulated adenylate cyclase by 65%, with an EC50 of 0.3 nM. However, NPY did not stimulate phosphoinositide (PI) hydrolysis or the elevation of cytosolic calcium, (Ca+2)i, in cultured RPA-VSM cells, nor did it potentiate norepinephrine-induced PI hydrolysis or elevation of (Ca+2)i. These results suggest that NPY-induced vasocontraction is not mediated by PI hydrolysis or the modulation of (Ca+2)i.

Influence of rolipram on the cyclic 3',5'-adenosine monophosphate response to histamine and adenosine in slices of guinea-pig cerebral cortex

The effect of the phosphodiesterase (PDE) inhibitor rolipram on the cyclic AMP responses to adenosine, histamine and combinations of these two agonists, was examined in [3H]adenine-labelled slices of guinea-pig cerebral cortex. Constant levels of [3H]-cyclic AMP were achieved within 10 min of agonist addition, both in the presence and absence of rolipram (0.1 mM). Histamine (1 mM) produced an 8-fold increase in [3H]-cyclic AMP (compared with basal) which was increased 7-fold by rolipram. The responses to adenosine (0.1 mM) and adenosine and histamine in combination were larger than that to histamine alone (46-fold or more compared with basal) but the potentiation by rolipram was much smaller (2.5-fold or less). With both agonists the effect of rolipram was dose-dependent, the steady state [3H]-cyclic AMP levels increasing 1-2-fold for a 10-fold increase in rolipram concentration. Removal of the histamine or adenosine stimulus once steady state had been reached resulted in a rapid fall in [3H]-cyclic AMP levels with a half time of less than 5 min. Rolipram (0.1 mM) did not significantly alter the initial rates of fall in [3H]-cyclic AMP levels but increased the time taken for them to return to basal levels. The findings of higher steady state levels of cyclic AMP in the presence of rolipram, together with an almost unaltered rate of cyclic AMP turnover, are consistent with an interaction of rolipram with PDE which is overcome by an increase in cyclic AMP concentration. However, the relatively smaller effects of rolipram on the higher steady levels of cyclic AMP produced by adenosine and the rather shallow dose-dependence of the PDE inhibitor on the responses to both agonists are inconsistent with a simple competitive inhibition of total PDE activity in responding cells. The results can be explained, however, by the involvement of different forms of PDE, with the rolipram-sensitive, calcium-independent form dominating at low cyclic AMP levels and the rolipram-insensitive, calcium-dependent form becoming more important when cyclic AMP levels are higher.

Differential effects of prostaglandins and isoproterenol on cAMP content and Na, K pump activity in rat submandibular acini

The beta-adrenergic agonist isoproterenol and prostaglandins E1 and E2 (but not F2 alpha) increased the cAMP content of rat submandibular acini in vitro, but only isoproterenol enhanced ouabain-sensitive 86Rb (K) uptake. These findings suggest that cAMP is not involved in the activation of the Na, K pump in salivary cells.

In vivo or in vitro exposure to imipramine reduces alpha 2-adrenoceptor-mediated inhibition of cyclic AMP production in rat brain cerebral cortical slices

The effect of in vivo or in vitro exposure to imipramine on alpha 2-adrenoceptor-mediated inhibition of cAMP production in rat brain slices was examined. Chronic administration (10 mg/kg once daily for 14 or 21 days) of imipramine or in vitro exposure (100 microM, 60 min) to the antidepressant significantly reduced the ability of UK-14,304, an alpha 2-adrenoceptor agonist, to inhibit forskolin-stimulated cAMP accumulation in cerebral cortical slices. This reduction was due to a decrease in the maximal response to the alpha 2-adrenergic agonist rather than to a decrease in its potency. Besides yielding a rapid and direct method for studying the effect of imipramine on brain receptors in vitro, these findings provide biochemical evidence in support of the notion that this drug, and perhaps other antidepressants, modifies alpha 2-adrenoceptor function in the central nervous system.

Supersensitive beta-adrenergic receptors are down-regulated in rat brain by mianserin administration

Experiments were undertaken to determine whether alterations in rat brain monoaminergic tone influence the manner in which mianserin administration modifies beta-adrenergic receptor function. Whereas the chronic administration of mianserin had little effect on isoproterenol-stimulated cAMP accumulation in control cerebral cortical slices, treatment with this antidepressant prevented or reversed beta-adrenergic receptor super-sensitivity resulting from reserpine or 6-hydroxydopamine administration. In contrast, 5,7 dihydroxytryptamine treatment had no effect on beta-adrenergic receptor activity, or on the ability of mianserin to modify isoproterenol-stimulated cAMP accumulation. The results indicate that antidepressant-induced alterations in beta-adrenergic receptor activity may, in some cases, be a function of receptor sensitivity at the time of drug administration.

Alpha 2-adrenergic receptor-mediated sensitization of forskolin-stimulated cyclic AMP production

Preincubation of HT29 human colonic adenocarcinoma cells with alpha 2-adrenergic agonists resulted in a 10- to 20-fold increase in forskolin-stimulated cyclic AMP production as compared to cells preincubated without agonist. Similar results were obtained using either a [3H]adenine prelabeling assay or a cyclic AMP radioimmunoassay to measure cyclic AMP levels. This phenomenon, which is termed sensitization, is alpha 2-adrenergic receptor-mediated and rapid in onset and reversal. Yohimbine, an alpha 2-adrenergic receptor-selective antagonist, blocked norepinephrine-induced sensitization, whereas prazosin (alpha 1-adrenergic) and sotalol (beta-adrenergic) did not. The time for half-maximal sensitization was 5 min and the half-time for reversal was 10 min. Only a 2-fold sensitization of cyclic AMP production stimulated by vasoactive intestinal peptide was observed, indicating that sensitization is relatively selective for forskolin. Sensitization reflects an increased production of cyclic AMP and not a decreased degradation of cyclic AMP, since incubation with a phosphodiesterase inhibitor and forskolin did not mimic sensitization. Increasing the levels of cyclic AMP during the preincubation (using a phosphodiesterase inhibitor) had no effect on sensitization, indicating that sensitization is not caused by decreased cyclic AMP levels during the preincubation. This rapid and dramatic sensitization of forskolin-stimulated cyclic AMP production is a previously unreported effect that can be added to the growing list of alpha 2-adrenergic responses that are not mediated by a decrease in cyclic AMP.

Antidepressant administration has a differential effect on rat brain alpha 2-adrenoceptor sensitivity to agonists and antagonists

The aim of this study was to characterize the action of antidepressants on rat brain alpha 2-adrenoceptor function by examining their effect on the yohimbine-sensitive component of the cAMP response to adrenoceptor agonists. Using a prelabeling technique for measuring cAMP accumulation, it was found that chronic (2 weeks) administration of antidepressants increased the potency of yohimbine, an alpha 2-adrenoceptor antagonist, to inhibit norepinephrine-stimulated cAMP accumulation in rat brain cerebral cortical slices. In contrast, antidepressant treatment decreased the ability of the alpha-adrenoceptor agonist 6-fluoronorepinephrine to augment isoproterenol-stimulated cAMP accumulation. The results suggest that antidepressants alter rat brain alpha 2-adrenergic function by shifting the receptor to antagonist-preferring state. Such an effect may contribute to the change in noradrenoceptor responsiveness that is characteristic of antidepressant administration.

Effects of synaptic activity on the metabolism and release of purines in the rat superior cervical ganglion

The release of radioactive metabolites from isolated rat superior cervical ganglia was measured under various conditions following preloading with 3H-adenosine. The 3H label was recovered primarily in the adenosine metabolites, ATP, ADP, AMP, IMP, and inosine, rather than in adenosine itself. Increased release was evoked by preganglionic stimulation or by exposure to a high-K+ medium, whereas in a low-Ca2+-high-Mg2+ medium, both spontaneous release and evoked release of most metabolites were inhibited. Exposure of the ganglion to an atmosphere of N2 also increased the release of most labeled metabolites, but this release was not substantially affected by a low-Ca2+ medium. The fluorescent derivatives of the endogenous adenine-containing compounds present in the ganglion were prepared from homogenates and separated by high-performance liquid chromatography (HPLC). By the end of the testing period (6 hr), levels of ATP in the isolated ganglia had dropped to 10-20% of the initial values, while levels of ADP, AMP, and adenosine increased. There was little difference in these values between nonstimulated ganglia and those exposed to N2 or to a high-K+ medium.

Properties of a 3H-adenine prelabeled vesicular preparation from guinea pig cerebral cortex. Optimization of incubation conditions for basal- and histamine-stimulated 3H-cyclic AMP levels

The effects of several variables related to incubation conditions were studied on both the basal and the histamine-stimulated 3H-cyclic AMP levels in a 3H-adenine prelabeled vesicular preparation of guinea pig cerebral cortex. Varying the preincubation time (i.e., the duration of incubation before labeling) caused pronounced differences in the postlabeling time course of 3H-cyclic AMP levels. A preincubation time of 45 min was found to minimize time-related postlabeling declines in 3H-cyclic AMP basal activity. Labeling of the homogenate for an entire experiment in one vessel ("bulk labeling") also contributed to time-related declines in activity; these changes were minimized by labeling aliquots of homogenate individually. Rapid sample mixing during incubation was found to be necessary to ensure adequate suspension of the preparation and to achieve a stable basal activity. Such mixing altered the pH of the samples; maintenance of the pH at 7.4 required a decrease in the bicarbonate concentration and/or use of supplementary buffers. Several parameters related to histamine concentration-response curves were found to be affected by various combinations of buffers and labeling methods. Individual labeling in Krebs-ringer-bicarbonate (15 mM) buffer gave the most stable and reproducible responses to histamine. This method permits detailed pharmacological characterization of transmitter-stimulated cyclic AMP changes in a cell-free system that retains many characteristics of brain slices.

The vitamin C treatment of allergy and the normally unprimed state of antibodies

I previously described that bowel tolerance (the amount that almost causes diarrhea) to oral ascorbic acid, increases in a person somewhat proportionally to the "toxicity" of his disease. Ascorbic acid ameliorates symptoms and sometimes cures certain diseases at high threshold levels near bowel tolerance. High concentrations of ascorbate cause the redox potential of the redox couple (ascorbate/dehydroascorbate, AA/DHA) to become reducing in diseased tissues. Allergic and sensitivity reactions are frequently ameliorated and sometimes completely blocked by massive doses of ascorbate. I now hypothesize that one mechanism in blocking of allergic symptoms is the reducing of the disulfide bonds between the chains in antibody molecules making their bonding antigen impossible. I further hypothesize that antibodies seek to match antigens only in areas where stray free radicals or a relatively oxidizing redox potential exists. The redox state of normal, healthy tissue does not allow for the bonding of antibodies to antigen. When antioxidant, free radical scavenging systems are overwhelmed, inflammatory, hypersensitivity, and "autoimmune" conditions may result.

Phorbol esters enhance neurotransmitter-stimulated cyclic AMP production in rat brain slices

The effect of phorbol esters on cyclic AMP production in rat CNS tissue was examined. Using a prelabeling technique for measuring cyclic AMP accumulation in brain slices, it was found that phorbol 12-myristate, 13-acetate (PMA) enhanced the cyclic AMP response to forskolin and a variety of neurotransmitter receptor stimulants while having no effect on second messenger accumulation itself. A short (15-min) preincubation period with PMA was required to obtain maximal enhancement, whereas the augmentation was lessened by prolonged exposure (3 h) to the phorbol. The response to PMA was concentration dependent (EC50 = 1 microM) and regionally selective, being most apparent in forebrain, and was not influenced by removal of extracellular calcium or by inhibition of phosphodiesterase or phospholipase A2. Only those phorbols known to stimulate protein kinase C augmented the accumulation of cyclic AMP. Moreover, the membrane substrates phosphorylated by endogenous C kinase and by a partially purified preparation of this enzyme were similar. The results suggest that phorbol esters, by activating protein kinase C, modify the cyclic AMP response to brain neurotransmitter receptor stimulation in brain by influencing a component of the adenylate cyclase system beyond the transmitter recognition site.

A procedure for measuring alpha 2-adrenergic receptor-mediated inhibition of cyclic AMP accumulation in rat brain slices

The alpha 2-adrenergic receptor regulation of cyclic adenosine monophosphate (cAMP) accumulation in rat brain slices was examined. using a prelabeling technique for measuring second messenger production. The mixed alpha-adrenergic agonist 6-fluoronorepinephrine, as well as the more selective alpha 2-agonists clonidine and UK-14,304, caused a concentration-dependent inhibition of forskolin-stimulated cAMP accumulation in cerebral cortical slices, whereas phenylephrine, a selective alpha 1-adrenergic agonist, had no inhibitory effect in this system. Moreover, alpha 2-adrenergic receptor antagonists were more potent than alpha 1-adrenergic antagonists in blocking the inhibitory response to UK-14,304. Neither alpha 1- nor alpha 2-adrenergic agonists displayed any inhibitory effect when cAMP accumulation was stimulated by isoproterenol, vasoactive intestinal peptide or 2-chloroadenosine. The results provide further evidence that some alpha 2-adrenergic receptors are negatively coupled to adenylate cyclase in brain, and yield a procedure for studying this phenomenon in intact central nervous system tissue.

Characterization of alpha 1-adrenoceptors which increase cyclic AMP accumulation in rat cerebral cortex

The pharmacological properties of the alpha-adrenoceptors which increase cyclic AMP accumulation were studied in slices of rat cerebral cortex after inactivation of beta-adrenoceptors with bromoacetylalprenololmenthane. Norepinephrine increased basal cyclic AMP accumulation 2-fold, and potentiated the effect of adenosine 5-fold. The Ki and EC50 values for antagonists and agonists for both the basal and potentiated responses were generally similar to those for alpha 1-adrenoceptor-stimulated inositol phosphate accumulation in the same preparation. However, significant differences in the potencies of all agonists, and the antagonists phentolamine and BE2254 were observed between the basal and potentiated cyclic AMP responses. The differences in agonist potencies did not appear to be due to the existence of a receptor reserve. Norepinephrine, epinephrine, alpha-methylnorepinephrine and 6-fluoronorepinephrine were full agonists, while methoxamine and phenylephrine were partial agonists in both systems. The results suggest that norepinephrine increases cyclic AMP accumulation in rat cerebral cortex through alpha 1-adrenoceptors similar to those increasing phosphatidylinositol metabolism in the same tissue.

Noradrenaline modulates calcium channels in avian dorsal root ganglion cells through tight receptor-channel coupling

Averaged ensemble Ba currents were recorded from tissue cultured embryonic chick dorsal root ganglion (d.r.g.) cells using the cell-attached patch-clamp technique. Noradrenaline (NA) applied to extrapatch membrane had no clear consistent effect on drug-free patch currents. This finding supports a previous suggestion that second messengers may not be involved in NA-mediated decreases in Ca currents in sensory neurones (Forscher & Oxford, 1985). Cell-attached patch currents sometimes increased slowly after extrapatch application of NA, but were not reversibly decreased by drug treatment. Large patch currents were used to trigger cellular action potentials. NA reversibly decreased action potential duration as reflected in extracellularly recorded patch action currents. Simultaneously recorded inward patch currents were not affected. D.r.g. cell adenylate cyclase activity was assayed. NA did not affect intracellular cyclic AMP levels at concentrations which cause 30-70% decreases in gCa in dialysed cells (Forscher & Oxford, 1985). Treatment with forskolin (50 microM) or isoprenaline (10 microM) resulted in 60- and 2-fold increases respectively in adenylate cyclase activity over basal levels. These results suggest that NA decreases Ca currents by direct NA interactions with the Ca channel or a molecule tightly coupled to channel function in d.r.g. cells.

A fragment of vasoactive intestinal peptide, VIP(10-28), is an antagonist of VIP in the colon carcinoma cell line, HT29

The 19 amino acid carboxyl terminus fragment of vasoactive intestinal peptide (VIP), VIP(10-28), inhibits [125I]VIP binding in intact HT29 colonic adenocarcinoma cells and in membranes from these cells. However, VIP(10-28) alone has no effect on adenylate cyclase activity (membranes) or cyclic AMP synthesis (intact cells) in HT29 cells although VIP receptor agonists are markedly stimulatory. The indicated antagonist character of VIP(10-28) was confirmed by rightward parallel shifts of VIP dose response curves in the presence of VIP(10-28) in adenylate cyclase and cyclic AMP synthesis experiments. The equilibrium dissociation constant values for VIP(10-28) from these experiments agree with values from inhibition binding studies. The lack of effect of VIP(10-28) on forskolin dose response curves in HT29 adenylate cyclase assays indicates the specificity of the VIP(10-28) antagonism, thus suggesting that VIP(10-28) may be a useful tool in studying VIP receptor regulation and other aspects of the mechanisms of VIP action. The potential regulatory role of a proteolytically generated fragment of VIP acting antagonistically at VIP receptors is discussed.

An examination of the involvement of phospholipases A2 and C in the alpha-adrenergic and gamma-aminobutyric acid receptor modulation of cyclic AMP accumulation in rat brain slices

Experiments were undertaken to define the role of two calcium-associated enzyme systems in modulating transmitter-stimulated production of cyclic nucleotides in rat brain. Cyclic AMP (cAMP) accumulation was examined in cerebral cortical slices using a prelabeling technique. The enhancement of isoproterenol-stimulated cAMP production by alpha-adrenergic and gamma-aminobutyric acid-B (GABAB) agonists was reduced by exposing the tissue to EGTA, a chelator of divalent cations, or quinacrine, a nonselective inhibitor of phospholipase A2. Likewise, chronic (2 weeks) administration of corticosterone decreased the alpha-adrenergic and GABAB receptor modulation of second messenger production. Neither cyclooxygenase nor lipoxygenase inhibitors selectively influenced the facilitating response of alpha-adrenergic and GABAB agonists. Other experiments revealed that although norepinephrine and 6-fluoronorepinephrine stimulated inositol phosphate (IP) production in cerebral cortical slices with potencies equal to those displayed in the cyclic nucleotide assay, selective alpha 1-adrenergic agonists were less efficacious on IP formation and were without effect in the cAMP assay. Conversely, a selective alpha 2-adrenergic receptor agonist facilitated the cAMP response to a beta-adrenergic agonist without affecting IP formation. The rank orders of potency of a series of alpha-adrenergic antagonists suggest that IP accumulation is mediated solely by alpha 1-adrenergic receptors, whereas the augmentation of cAMP accumulation is regulated by a mixed population of alpha-adrenergic sites. The results suggest that the alpha-adrenergic and GABAB receptor-mediated enhancement of isoproterenol-stimulated cAMP formation appears to be more closely associated with phospholipase A2 than phospholipase C and may be mediated by arachidonate or some other fatty acid.

Phorbol esters down-regulate protein kinase C in rat brain cerebral cortical slices

The effect of phorbol esters on cyclic AMP production in rat cerebral cortical slices was studied using a prelabelling technique to measure cyclic nucleotide accumulation. Cholera toxin-stimulated cyclic AMP accumulation was enhanced approximately 2-fold by phorbol 12-myristate, 13-acetate (PMA) which alone had no effect on cyclic AMP production. The augmentation by PMA was maximal within the first hour of incubation, decreasing progressively thereafter. Protein kinase C activity was decreased 80-90% during a 3 hr exposure to PMA, as was 3H-phorbol 12,13-dibutyrate binding. Both phosphatidyl serine and arachidonic acid were found to enhance protein kinase C activity in a concentration-dependent manner, an effect that was attenuated by prolonged incubation of the brain tissue with PMA. The results indicate that exposure of brain slices to phorbol esters causes a down-regulation of rat brain protein kinase C, and that this modification corresponds with a decrease in the ability of PMA to augment cyclic AMP production, suggesting a functional relationship between the two systems in rat brain.

An investigation of the low intrinsic activity of adenosine and its analogs at low affinity (A2) adenosine receptors in rat cerebral cortex

The potencies and intrinsic activities of adenosine analogs for stimulating cyclic AMP accumulation in slices of rat cerebral cortex were examined. 5'-N-Ethylcarboxamidoadenosine (NECA) caused the greatest increase in cyclic AMP accumulation (19.2-fold). 2-Chloroadenosine (2-CAD) induced a similar increase, but adenosine and six other analogs caused much smaller increases. All agonists tested had similar potencies in activating this response. Inhibition of adenosine uptake with 10 microM dipyridamole did not affect the maximal response to any agonist, although the potency of adenosine was increased approximately threefold. Each analog was also able to block partially the stimulation of cyclic AMP accumulation caused by NECA. Levels of cyclic AMP accumulation in the presence of NECA plus another analog were similar to those observed when the analog alone was present, as expected for partial agonists. Furthermore, the EC50 value for R-(-)-N6(2-phenylisopropyl)adenosine in increasing cyclic AMP accumulation was similar to the KI value for inhibiting the response to NECA. The EC50 value for adenosine was substantially higher than the KI value for inhibiting the response to NECA; however, in the presence of dipyridamole, the two values were more closely correlated. The response to NECA was blocked by 8-phenyltheophylline, 1,3-diethyl-8-phenylxanthine, and 8-p-sulfophenyltheophylline, with KI values from 1 to 10 microM. The results suggest that adenosine analogs stimulate cyclic AMP accumulation in cerebral cortex through low-affinity receptors, but that some analogs only partially activate these receptors. Adenosine itself may also be a partial agonist, or its actions may be obscured by simultaneous activation of another receptor.

Potentiation of the antiplatelet action of adenosine in whole blood by dipyridamole or dilazep and the cAMP phosphodiesterase inhibitor, RA 233

Adenosine (Ado, 10-50 microM), a potent inhibitor of ADP-induced human platelet aggregation in platelet-rich plasma (PRP), does not inhibit aggregation in whole blood. However, the Ado analogs, 2-fluoroadenosine, 2-chloroadenosine and 5'-N-ethylcarboxamidoadenosine (NECA) which are resistant to deamination (2-fluoroadenosine) or deamination and phosphorylation (2-chloroadenosine and NECA), inhibit aggregation in whole blood with IC50 values of 12 microM, 2.3 microM and 0.26 microM, respectively. The inhibitory effect of NECA (200 nM) is potentiated by the platelet cAMP phosphodiesterase (PDE) inhibitor RA 233 (5 microM). Inhibition of the erythrocytic nucleoside transport system by dilazep (1 microM) or dipyridamole (10 microM), or blockade of Ado metabolism by 2'-deoxycoformycin (5 microM) plus 5-iodotubercidin (10 microM), evokes the antiaggregatory action of Ado in whole blood (IC50 congruent to 2 microM). RA 233 (5 microM) potentiates Ado-mediated inhibition about 10-fold when nucleoside transport or Ado metabolism is blocked. Ado (10 microM or 200 nM) is rapidly metabolized within 1 min in whole blood. When nucleoside transport is inhibited by dilazep or dipyridamole, or when Ado metabolism is blocked by 2'-deoxycoformycin and 5-iodotubercidin, 50-60% of the Ado remains in the plasma after 5 min. These results show that the failure of Ado to inhibit platelet aggregation in whole blood results from its rapid uptake and metabolism by erythrocytes. More importantly, these data emphasize the key role of nucleoside transport inhibition in the antiplatelet actions of dipyridamole and dilazep. In addition, superior therapeutic results may be obtained from the combination of blockade of nucleoside transport system with inhibition of platelet cAMP PDE.

REM sleep deprivation induces a decrease in norepinephrine-stimulated 3H-cyclic AMP accumulation in slices from rat brain

Beta adrenergic sites in rat brain are reduced after repeated treatment with antidepressant drugs, with REM sleep deprivation (REMSd) having the same effect. This paper reports the effects of REMSd in the production of 3H-cyclic AMP in frontal cortical slices by NE challenge. Data presented in this paper report a marked decrease in 3H-cyclic AMP synthesis after REMSd, which is in accordance with previous results showing adrenergic receptor down-regulation following REMSd. Results are discussed in view of possible interaction with dopaminergic systems and depression management.

Reduced hormone-stimulated adenylate cyclase activity in NIH-3T3 cells expressing the EJ human bladder ras oncogene

Recent studies have shown that the 21-kilodalton protein (p21) Ha-ras gene product shares sequence homology with and may exhibit biochemical properties similar to the mammalian guanine nucleotide-binding proteins. These data suggested that one of the biochemical functions of p21 in the vertebrate cell may be to regulate adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1]. We determined both in intact NIH-3T3 murine cells and in membranes isolated from these cells that the hormone-stimulated adenylate cyclase activity of cells expressing the EJ human bladder carcinoma oncogene (EJ-ras) is significantly reduced compared with control cells. Thus, the levels of cAMP measured in the EJ-ras-transformed cells by radioimmunoassay are reduced 78% and 93% after prostaglandin and isoproterenol stimulation, respectively, compared with the levels in control cells. Treatment of the EJ-ras-transformed cells with pertussis toxin or cholera toxin did not correct the alterations in adenylate cyclase activity. Cells expressing the normal human Ha-ras gene displayed intermediate levels of adenylate cyclase hormone sensitivity; these levels of adenylate cyclase activity were greater than those in the EJ-ras-transformed cells but lower than in control cells. Hormone-stimulated adenylate cyclase activities in cells transfected with Rous sarcoma virus DNA were similar to those in control cells. These data support the hypothesis that both the normal and mutated Ha-ras p21s are related to guanine nucleotide-binding proteins.

Circadian rhythms in catecholamine metabolites and cyclic nucleotide production

Circadian rhythms in noradrenergic (NE) and dopaminergic (DA) metabolites and in cyclic nucleotide production were measured in discrete regions of rat brain. A circadian rhythm was found in the concentration of the NE metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), in the hippocampus. No MHPG rhythm was found in frontal, cingulate, parietal, piriform, insular or temporal cortex, or in hypothalamus. Circadian rhythms in the concentration of the NE metabolite, 3,4-dihydroxyphenylglycol (DHPG), occurred in occipital and parietal cortex and hypothalamus, with no rhythm observable in temporal or insular cortex, hippocampus, pons-medulla or cerebellum. The 24-hr mean concentration of MHPG varied 3.5-fold, highest in cingulate and lowest in parietal, temporal and occipital cortex. The 24-hr mean concentration of DHPG varied 6-fold, highest in hypothalamus and lowest in parietal cortex. Circadian rhythms in the concentration of the DA metabolite, homovanillic acid (HVA), were found in olfactory tubercle, amygdala and caudate-putamen, but not in nucleus accumbens. A circadian rhythm in the concentration of the DA metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), occurred in nucleus accumbens, but not in olfactory tubercle or caudate-putamen. The mean 24-hr concentration of HVA was highest in caudate-putamen, intermediate in nucleus accumbens, and lowest in olfactory tubercle and amygdala. The mean 24-hr concentration of DOPAC was highest in nucleus accumbens and lower in olfactory tubercle and caudate-putamen. Circadian rhythms were found in the concentration of cyclic GMP (cGMP) in all regions measured except parietal cortex. The mean 24-hr concentration varied 128-fold, highest in nucleus accumbens, frontal poles, and hypothalamus and lowest in cingulate cortex. Circadian rhythms in cyclic AMP (cAMP) concentration were found in piriform, temporal, occipital, cingulate, and parietal cortex, amygdala and nucleus accumbens. No rhythms were found in frontal or insular cortex, hypothalamus, hippocampus, caudate-putamen or olfactory tubercle. The 24-hr mean cAMP concentration varied 4-fold, highest in parietal cortex and lowest in caudate-putamen and amygdala. Norepinephrine metabolites and dopamine metabolites were rhythmic in few regions. It is, therefore, unlikely that the rhythmicity measured in adrenergic receptors is, in general, a response to rhythmic changes in adrenergic transmitter release. The putative second messenger response systems, especially cGMP, were more often rhythmic. The rhythms in cGMP are parallel in form and region to those in the alpha 1-adrenergic receptor and may act as 2nd messenger for that receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Synergistic interaction between alpha- and beta-adrenergic receptors in rat brain slices: possible site for antidepressant drug action

Experiments were undertaken to examine the characteristics of the adrenergic receptor-coupled cAMP system in rat brain slices. It was found that the potentiation of isoproterenol-stimulated cAMP accumulation by 6-fluoronorepinephrine, an alpha-adrenergic agonist, is largely dependent upon the degree of beta-receptor occupancy, with prazosin-sensitive alpha-adrenergic receptors contributing less to this interaction. Chronic administration of a variety of antidepressants decreased the potentiating interaction between 6-fluoronorepinephrine and isoproterenol even under conditions where there were no obvious effects on the alpha- or beta- adrenergic components themselves. Chronic administration of imipramine had no effect on the interaction between 6-fluoronorepinephrine and adenosine, suggesting that the drug selectively modifies the coupling between the alpha- and beta-adrenergic systems. The results suggest that antidepressants influence the coupling between 6-fluoronorepinephrine and isoproterenol receptors independent of any effect on the individual recognition sites.

[3H]adenosine uptake and release from synaptosomes. Alterations by barbiturates

The effects of barbiturates on adenosine movements across the synaptic plasma membrane have been investigated using rodent whole brain synaptosomes. The hypothesis tested was that some of the depressant actions of these drugs may be mediated through interference with an endogenous adenosine system. Adenosine uptake was studied using synaptosomes prepared from Swiss-Webster mice. After preincubation at 37 degrees, [3H]adenosine was added to the synaptosomes in the presence or absence of pentobarbital, methohexital, phenobarbital, or 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB) at various concentrations and times. All four compounds significantly inhibited [3H]adenosine uptake at concentrations of 100-300 microM. Pentobarbital did not affect the distribution of synaptosomal adenosine metabolites. Release of [3H]adenosine was studied using the P2 pellet from male CD-1 mice. Addition of 50 mM KCl caused an enhancement of 3H-efflux mainly due to increased release of adenosine and inosine. This effect was abolished in the presence of 250 microM ethylene glycol bis(beta-aminoethyl-ether)-N,N2-tetraacetic acid (EGTA). Pentobarbital, 0.3 mM, caused a significant increase in the net potassium-induced release of [3H]adenosine. These results suggest that some of the depressant effects of barbiturates may be due to inhibition of adenosine reuptake and enhancement of release resulting in elevated synaptic adenosine levels.

Trimipramine, a tricyclic antidepressant exerting atypical actions on the central noradrenergic system

Trimipramine is a tricyclic antidepressant which has only weak effects on noradrenergic systems. Its mechanism of action is not understood, but its clinical effectiveness has been proven over a period of 20 years. In the present investigation, trimipramine was shown to have no effect on noradrenaline (NA)-stimulated adenylate cyclase activity after either acute or once daily application for 28 days. There was no change in the KD or Bmax of [3H]DHA binding, demonstrating a lack of effect on beta-adrenergic receptors. However, it did exert effects on the cortical noradrenergic projection of the locus coeruleus and these effects were opposite to those described for typical antidepressants. In acute experiments, systemic injections of trimipramine potently activated locus coeruleus neurons and produced a reduction in the depressant action of noradrenaline administered iontophoretically to neurons in the cingulate cortex. After 4 weeks' treatment with a daily injection of 30 mg/kg i.p. trimipramine, cingulate cortical neurons became supersensitive to the action of iontophoretically applied noradrenaline. Although it is unknown whether these effects are related to the therapeutic efficacy of trimipramine, the results demonstrate that the down-regulation of central noradrenergic sensitivity in the rat is not a prerequisite for clinical efficacy of antidepressants in man.

Modulation of beta-adrenergic response in rat brain astrocytes by serum and hormones

Purified astrocyte cultures from neonatal rat cerebrum respond to isoproterenol, a beta-adrenergic agonist, with a transient rise in cAMP production. This astroglial property was regulated by serum, a chemically defined medium (serum-free medium plus hydrocortisone, putrescine, prostaglandin F2 alpha, insulin, and fibroblast growth factor) and epidermal growth factor. Compared to astrocytes grown in serum-supplemented medium, astrocytes grown in the chemically defined medium were nonresponsive to isoproterenol stimulation, and this difference did not appear to be due to selection of a subpopulation of cells by either medium. The data suggest that a decreased beta-adrenergic receptor number and an increased degradation of cAMP may account for the reduced response to beta-adrenergic stimulation. The nonresponsive state of astrocytes in the defined medium was reversible when the medium was replaced with serum-supplemented medium. An active substance(s) in serum was responsible for restoring the responsiveness of astrocytes. Each of the five components of the defined medium had little effect by itself; however, together they acted synergistically to desensitize astrocytes to beta-adrenergic stimulation. On the other hand, epidermal growth factor, a potent mitogen for astrocytes, was very competent by itself in reducing the cAMP response of astrocytes to beta-adrenergic stimulation. Thus purified astrocytes grown in the chemically defined medium appear to be a good model for the study of hormonal interactions and of serum factors which may modulate the beta-adrenergic response.

The role of cyclic AMP in the regulation of ecdysone synthesis

The involvement of cAMP in the steroidogenic action of the cerebral neuropeptide, prothoracicotropic hormone (PTTH), was examined in the tobacco hornworm, Manduca sexta. PTTH stimulates ecdysone synthesis by paired organs in the thorax, the prothoracic glands. The steroidogenic effect of PTTH on prothoracic glands of day 3 fifth instar larvae and day 0 pupae is mimicked in vitro by agents that act by increasing the intracellular levels of cAMP (1-methyl-3-isobutylxanthine, cAMP analogs and forskolin). In addition, partially purified big PTTH (Mr 28500 daltons) stimulates the formation of cAMP in both stages of glands in a rapid, dose-dependent manner. However, a significant accumulation of cAMP in response to PTTH occurs only in larval prothoracic glands. In pupal glands, effects of the neuropeptide on cAMP synthesis are seen only in the presence of a phosphodiesterase inhibitor. The results indicate that cAMP is involved as a second messenger in PTTH action, but that the developmental status of the prothoracic glands influences the degree to which cAMP accumulates in response to the neurohormone.

Organic calcium channel blockers enhance [3H]purine release from rat brain cortical synaptosomes

The release of [3H]purines was investigated in a crude mitochondrial fraction (P2 fraction) from rat brain cortex pre-loaded with [3H]adenosine for 30 sec at 37 degrees C in vitro. Potassium, veratridine and glutamate were used as depolarizing agents to evoke the release of [3H]purines. Ca2+ removal, the addition of EGTA, and treatment with organic or inorganic Ca2+ antagonists did not inhibit [3H]purine release in this preparation. On the other hand, Ca2+ removal and the addition of EGTA greatly enhanced 3H-purine release induced by glutamate. D-600 and diltiazem enhanced K+-evoked [3H]purine release, and nifedipine increased veratridine evoked [3H]purine release indicating that either these Ca2+ antagonists have different sites of action, or that K+ and veratridine may release [3H]purine from different metabolic pools. Organic Ca2+ antagonists failed to enhance the [3H]purine release evoked by glutamate, further supporting the notion that various depolarizing agents may release [3H]purines from different cellular compartments.

GABAB receptors and norepinephrine-stimulated cAMP production in rat brain cortex

The ability of GABAergic compounds to influence cAMP accumulation in rat brain cortex was examined. It was found that GABAB receptor agonists such as GABA, baclofen, and kojic amine potentiate the cAMP response in cerebral cortex during exposure to norepinephrine. Isoguvacine and THIP, selective GABAA receptor agonists, did not demonstrate this effect. The response to baclofen was stereoselective, with virtually all activity residing in the (-)isomer. Bicuculline methiodide had no effect on the agonist-induced potentiation and the rank order of potency for GABAB agonists to potentiate the cAMP response is identical to their rank order of potency in the GABAB binding assay. These data suggest that GABAB receptors are capable of influencing the brain cyclic nucleotide system.