Somatostatin pretreatment desensitizes somatostatin receptors linked to adenylate cyclase and facilitates the stimulation of cyclic adenosine 3':5'-monophosphate accumulation in anterior pituitary tumor cells

Somatostatin-14 (SRIF) inhibits both hormone- and forskolin-stimulated cyclic adenosine 3':5'-monophosphate (cyclic AMP) formation in tumor cells of the mouse anterior pituitary (AtT-20/D16-16). However, long-term pretreatment of cells with SRIF modifies the responsiveness of this system in two ways: The response of adenylate cyclase to stimulatory agents is enhanced, whereas the ability of SRIF to inhibit stimulated cyclic AMP formation is reduced. The supersensitive adenylate cyclase response and the SRIF desensitization were dependent on the concentration and duration of SRIF pretreatment. Enhancement of forskolin-stimulated cyclic AMP formation occurred within 4 hr, whereas that of corticotropin-releasing-factor-, (-)-isoproterenol-, and vasoactive intestinal peptide-induced cyclic AMP accumulation required 16 hr of pretreatment. The elevated responses to each of these stimulants were due to increases in their maximal ability to stimulate cyclic AMP formation. Cycloheximide treatment blocked the enhanced cyclic AMP response induced by SRIF pretreatment, suggesting a requirement for protein synthesis. In membrane preparations, SRIF pretreatment facilitated activation of adenylate cyclase by forskolin, sodium fluoride, and guanosine 5'-(beta,tau-imido)-triphosphate without affecting basal activity. These results suggest that desensitization of an inhibitory input to adenylate cyclase is accompanied by a supersensitivity of adenylate cyclase to stimulatory agents through a process requiring protein synthesis.     

Guanosine-5'-O-thiodiphosphate functions as a partial agonist for the receptor-independent stimulation of neural adenylate cyclase

GTP-binding proteins (G proteins) have been implicated as mediators of several aspects of neuronal signal transduction including ion channels, phosphatidyl inositol turnover and the stimulation or inhibition of adenylate cyclase. Several investigators have employed the stable guanosine diphosphate (GDP) analog, guanosine 5'-O-thiodiphosphate (GDP beta S) to block putative G protein-mediated processes. Although GDP beta S is assumed to block G protein function, some investigators have reported partial activation of G protein-mediated processes by this compound. In this study we demonstrate that GDP beta S functions as a partial agonist for the adenylate cyclase system. In rat cerebral cortex membranes, GDP beta S activates adenylate cyclase with an EC50 similar to the hydrolysis resistant GTP analog, guanylylimidodiphosphate (GppNHp), but to a far lower extent. Further, GDP beta S antagonizes the activation of adenylate cyclase by high doses of GppNHp or GTP gamma S (another stable GTP analog) but potentiates adenylate cyclase activation by low doses of these nucleotides. High doses of GDP beta S provoke, only partially, exchange of nucleotides among G proteins, as measured by the transfer of the photoaffinity GTP analog, azidoanilido-GTP, between the inhibitory and stimulatory GTP-binding proteins. In the presence of the beta-adrenergic agonist, isoproterenol, GDP beta S fails to support stimulation of C6 glioma membrane adenylate cyclase and inhibits GppNHp- or GTP gamma S-mediated stimulation of that enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta-adrenergic stimulation of protein (arginine) methyltransferase activity in cultured cerebral cells from embryonic mice

Several adrenergic effectors and neurotransmitters were tested as potential regulators of myelin basic protein (MBP) and histone methyltransferase activities. Both enzymes were specifically activated by beta-adrenergic agonists in a stereospecific manner. Cyclic AMP (but not AMP) stimulated the enzymes to the same extent as did the beta-adrenergic agonist, (-) isoproterenol. The studies suggest that beta-adrenergic agonists stimulate adenylate cyclase thereby causing an increased production of cyclic AMP which stimulates the methyltransferases. Cycloheximide addition to the reaction mixture did not affect the stimulation due to cyclic AMP, indicating that new protein synthesis is not involved in the cyclic AMP stimulation of the methyltransferases. Thyroid hormone (T3) has been shown to stimulate MBP methyltransferase [Amur et al, 1984] and could exert its stimulatory effect through beta-adrenergic-dependent systems. But the beta-adrenergic antagonist, propranolol, did not block the stimulation by T3, suggesting that the effect of T3 is not mediated through beta-adrenergic-dependent systems. Thus, the methylation of MBP seems to be regulated both by T3 and by neurotransmitters and/or hormones mediating their effects through cyclic AMP production, whereas the methylation of histones seems to be regulated only by the latter.     

Peptidergic modulation of G-protein coupled cyclic-AMP accumulation in the rat caudate nucleus.

Somatostatin, substance P, and vasoactive intestinal polypeptide were incubated in an adenylate cyclase assay with a particulate fraction of caudate-putamen tissue of the rat in order to examine the effect of the neuropeptides on G-protein coupled adenylate cyclase in vitro. Somatostatin induced an enhancement of cyclic AMP formation in presence of guanine nucleotides and cholera toxin but inhibited pertussis toxin and forskolin enzyme stimulation. Pertussis toxin and cholera toxin also depressed forskolin-induced stimulation as described previously. Somatostatin was able to antagonize these inhibitory effects of both toxins. On the contrary, substance P reduced GTP and cholera toxin stimulated striatal adenylate cyclase, without affecting forskolin activation. In our preparation, VIP did not influence basal adenylate cyclase activity or the stimulation by guanine nucleotides, cholera toxin, and pertussis toxin. VIP potently inhibited the enhancement of cyclic AMP formation by forskolin and completely antagonized the inhibitory effect of cholera toxin on forskolin activation. These results suggest that neuromodulatory effects of somatostatin, substance P, and VIP are mediated by the inhibitory as well as stimulatory guanine nucleotide proteins G-i and G-s coupled to an adenylate cyclase system.     

Isolated neuronal growth cones from developing rat forebrain possess adenylate cyclase activity which can be augmented by various receptor agonists

Isolated neuronal growth cones from neonatal rat forebrain were found to contain a high specific activity of adenylate cyclase (61 pmol cyclic AMP/min/mg protein) compared to the pelleted starting homogenate (5 pmol cyclic AMP/min/mg protein). Forskolin at 10(-4) M increased adenylate cyclase activity in both the pelleted homogenate and growth cone fraction by 70 and 217 pmol cyclic AMP/min/mg protein, respectively, over basal levels. The incremental effect of forskolin was 3-fold greater in the growth cone fraction than in the pelleted homogenate. However, relative to basal levels in each of the two fractions, forskolin increased adenylate cyclase activity in the growth cone fraction by only approx. 5-fold compared to 15-fold in the pelleted homogenate. Dopamine (10(-4) M), vasoactive intestinal polypeptide (10(-6) M) and isoproterenol (10(-5) M) also augmented adenylate cyclase activity in the two fractions. In the growth cone fraction, dopamine and vasoactive intestinal polypeptide produced a stimulation over basal levels by approx. 20 pmol cyclic AMP/min/mg protein while isoproterenol produced a stimulation of approx. 10 pmol cAMP/min/mg protein. The incremental effects of these receptor agonists in the growth cone fraction are approx. 5-fold greater than in the pelleted homogenate. The dopamine-sensitive adenylate cyclase activity in the growth cone fraction could be blocked by the compound SCH23390, a selective D1 receptor antagonist. At saturating concentrations, all combinations of dopamine, vasoactive intestinal polypeptide and isoproterenol were found to be completely additive on adenylate cyclase activity in the growth cone fraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

α2-Adrenergic receptors mediate inhibition of cyclic AMP production in neurons in primary culture

The actions of adrenergic agents on the intracellular production of cyclic adenosine monophosphate (AMP) was examined in intact cortical and striatal neurons in primary culture, generated from the fetal mouse brain. Exposure of striatal neurons to the β-adrenergic agonist isoproterenol (10 μM) resulted in a 5-fold increase in intraneuronal cyclic AMP; norepinephrine (100 μM), alone or in combination with isoproterenol, produced only a 3-fold increase in cyclic AMP levels. However, in the presence of yohimbine (10 μM), cyclic AMP productions due to norepinephrine or isoproterenol plus norepinephrine were identical to isoproterenol alone. When striatal or cortical neurons were exposed to pertussis toxin (100 ng/ml) overnight, there was no detectable difference between isoproterenol- and norepinephrine-stimulated cyclic AMP production. These data suggest thatα₂-adrenergic receptors mediate the attenuation of cyclic AMP production in neurons and do so via the inhibitory guanine nucleotide regulatory protein of adenylate cyclase.     

Cyclic AMP-mediated potentiation of muscarinic hyperpolarization in neuroblastoma cells

Adenosine, 2-chloroadenosine and prostaglandin E₁ which are known to increase cyclic AMP in neuroblastoma cells potentiated the acetylcholine-induced muscarinic hyperpolarization of the cells without changing the resting membrane potential. The potentiation caused by 2-chloroadenosine was further augmented by Ro 20-1724, a phosphodiesterase inhibitor. A direct intracellular pressure application of cyclic AMP potentiated the muscarinic hyperpolarization without changing the resting membrane potential. Morphine which inhibits adenylate cyclase antagonized 2-chloroadenosine-induced potentiation of the muscarinic hyperpolarization. These results suggest that changes in cyclic AMP level modulate the muscarinic response of neuroblastoma cells.     

Catecholaminergic traits of chick sympathetic neurons may be differentially regulated by a cGMP-dependent pathway.

The purine metabolites inosine and adenosine selectively increase the catecholamine, but not the acetylcholine production in cultured chick superior cervical ganglion neurons via an as yet unknown intracellular pathway. In order to elucidate some of the molecular events involved in this differential regulation of neurotransmitter production by purines, the SCG neurons were cultured in the presence of cyclic nucleotide analogs and activators of adenylate and guanylate cyclase. Neither 8-bromo-cyclic AMP (8-Br-cAMP), 8-bromo-cyclic GMP (8-Br-cGMP), or forskolin, an activator of adenylate cyclase, could mimic the effect of inosine, i.e. differentially increase catecholamine production. Sodium nitroprusside, an activator of guanylate cyclase, however, has a strong potentiating action on the effect of inosine. The noradrenergic properties of chick sympathetic neurons may thus be differentially modulated by a cGMP-dependent pathway.     

The cyclic AMP second messenger system in man: the effects of heredity, hormones, drugs, aluminum, age and disease on signal amplification

The intracellular effects of a number of hormonal signals are mediated by the cyclic AMP second messenger system in man and the ubiquitous distribution of hormone-stimulated adenylate cyclase suggests the importance of this enzyme complex in normal aging and pathophysiological states. Various vectors including heredity, endogenous catecholamines, steroid hormones, and drugs affect the activity of hormone-stimulated adenylate cyclase in man. The effect of heredity was studied using lymphocytes obtained from monozygotic twin pairs and age and sex-matched sib pairs. Only for forskolin-stimulated activity is a significant proportion of individual variance attributable to heredity, suggesting the relative stability of the catalytic subunit. Beta-adrenergic and prostaglandin E-1 activity are "state" characteristics and their activities are controlled by environmental parameters. A significant reduction in isoproterenol-stimulated cyclic AMP accumulation between the menses and luteal phase of the menstrual cycle is observed in lymphocytes obtained from 11 female subjects. The lowest level of beta-adrenergic receptor activity is associated with the highest levels of progesterone and estradiol hormone levels in blood. Lithium at therapeutic concentrations markedly inhibits adenylate cyclase activity in platelet membranes. Moreover, marked individual differences are observed in sensitivity to lithium as determined by Dixon plot derived Ki values for 9 normal, healthy subjects. Human adenylate cyclase obtained from platelets and lymphocytes is activated by micromolar amounts of aluminum in the presence of NaF. Irreversible activation of adenylate cyclase by aluminum is suggested as a possible mechanism of this metal's neurotoxicity. The biochemical basis for the age-associated decline in beta-adrenergic responsiveness in man is discussed. Several investigations suggest a deficit at two levels in the adenylate cyclase complex: an impaired coupling of the receptor/N protein subunits and an additional lesion distal to the receptor at the level of N/C coupling. Perfusion studies with salbutamol suggest that the decline in beta-adrenergic sensitivity is general and not restricted to lymphocytes. Possible abnormalities in cyclic AMP signal amplification and recognition in various disease states is discussed. Increased prostaglandin E-1-stimulated cyclic AMP accumulation is observed in lymphocytes obtained from patients with Alzheimer's disease compared to age-matched controls and correlated with severity of the disease state.(ABSTRACT TRUNCATED AT 400 WORDS)     

Adenylate cyclase and the search for new compounds with the clinical profile of lithium

It is possible to evaluate the beta-adrenergic receptor-adenylate cyclase complex in the human periphery by measuring the plasma cyclic AMP rise after adrenergic agonists. A clinical trial of the beta 2 adrenergic agonist salbutamol in depression provided an opportunity to test whether adrenergic receptor subsensitivity does occur during clinical antidepressant treatment. After 1 and 3 weeks of oral salbutamol treatment, depression scores declined significantly in 11 depressed patients, while the plasma cyclic AMP response to i.v. salbutamol declined over 60%. The results support the concept that receptor sensitivity changes occur during human antidepressant therapy. Data are presented that Li, too, markedly reduces activity of beta-adrenergic adenylate cyclase in humans. The effect was evaluated by studying the effect of Li at therapeutic serum concentrations on the plasma cyclic AMP response to subcutaneous epinephrine. The Li effect is specific, since the plasma cyclic AMP response to glucagon is not inhibited. In rat cortical slices Li inhibition of noradrenaline-induced cyclic AMP accumulation is clearly demonstrable only at concentrations close to 2 mM Li. However, fresh human brain slices from edges of surgically-removed tumors show Li inhibition at 1 mM Li concentrations. These results imply that in brain as well as periphery, human noradrenergic adenylate cyclase is inhibited by therapeutic concentrations of Li. Demeclocyclin, a tetracycline-derived antibiotic, was found to inhibit noradrenaline-sensitive adenylate cyclase in rat cortical slices and to inhibit amphetamine-induced hyperactivity in rats in an open field. Clinical trials should search for new compounds with the clinical profile of Li.     

The mechanisms of action of lithium. II. Effects on adenylate cyclase activity and beta-adrenergic receptor binding in normal subjects.

As part of a study of the effects of lithium carbonate on neurochemical function in man, platelet and lymphocyte adenylate cyclase activity and lymphocyte beta-adrenergic receptor binding characteristics were determined before and after 2 weeks of lithium treatment in 10 normal volunteers. Lithium had differential effects on platelet and lymphocyte adenylate cyclase activity. In platelets, basal and stimulated (guanyl imidodiphosphate [Gpp[NH]p] or cesium fluoride) adenylate cyclase activity was significantly augmented by lithium treatment. By contrast, in lymphocytes, Gpp(NH)p- and cesium fluoride-stimulated adenylate cyclase activity was unaffected, while basal activity was decreased modestly after lithium. These results are consistent with preclinical studies that suggest that lithium's effects on adenylate cyclase activity are specific with respect to tissue and brain region and that lithium may interfere with guanine nucleotide binding (G) protein function. Lithium treatment significantly increased the ratio of low- to high-affinity dissociation constants for agonist displacement of antagonist binding to lymphocyte beta-adrenergic receptors (thought to reflect coupling between the beta-adrenergic receptor and stimulatory G protein). Lithium had significant effects on measures associated with signal transduction that might be contrasted to its more subtle effects on neuronal function (norepinephrine release) and neuroendocrine systems (responses to serotoninergic challenge) in these same subjects (reported in a companion article). Lithium's primary site of action may be on signal transduction mechanisms. These effects subsequently may be manifested in changes in neurotransmitter function that may be important to lithium's mood-stabilizing actions.     

Receptors, adenylate cyclase, depression, and lithium

Although numerous studies have suggested that depression may be associated with a reduction in synaptic noradrenaline in the brain, direct beta-adrenergic receptor agonists have not been tested in the treatment of depression until recently. Moreover, newer theories of antidepressant action suggest that a reduction in beta-adrenergic receptor sensitivity is a better correlate of antidepressant treatment than noradrenaline turnover changes. It is possible to evaluate the beta-adrenergic receptor-adenylate cyclase complex in the human periphery by measuring the plasma cyclic AMP rise after adrenergic agonists. A clinical trial of the beta-2 adrenergic agonist salbutamol in depression provided an opportunity to test whether adrenergic receptor subsensitivity does occur during clinical antidepressant treatment. Plasma cyclic AMP before treatment with salbutamol rose 26% in response to salbutamol 0.25 mg iv. After 1 and 3 weeks of oral salbutamol treatment, depression scores declined significantly in 11 depressed patients, while the plasma cyclic AMP response to iv salbutamol declined over 60%. The beta-adrenergic adenylate cyclase remained subsensitive 4 days after cessation of salbutamol therapy. The results support the concept that receptor sensitivity changes occur during human antidepressant therapy. Data are presented that Li, too, markedly reduces activity of beta-adrenergic adenylate cyclase in humans. The effect was evaluated by studying the effect of Li at therapeutic serum concentrations on the plasma cyclic AMP response to subcutaneous epinephrine. The Li effect is specific, since the plasma cyclic AMP response to glucagon is not inhibited. The plasma cyclic GMP response to subcutaneous epinephrine, suggested as a model for presynaptic alpha-noradrenergic mechanisms, is also partially inhibited by Li therapy. Since cyclic AMP and cyclic GMP may be viewed as balancing substances, their interaction may provide a mechanism for Li's dual clinical effects in mania and depression. It is important that in vivo techniques be developed for evaluating receptor changes. The plasma cyclic AMP response to adrenergic stimulation provides an in vivo measure of receptor function that can be useful in studying drug effects during the clinical treatment of humans.     

Orexins/Hypocretins Acting at G<Subscript>i</Subscript> Protein-Coupled OX<Subscript>2</Subscript> Receptors Inhibit Cyclic AMP Synthesis in the Primary Neuronal Cultures

Orexins A and B are newly discovered neuropeptides with pleiotropic activity. They signal through two G protein-coupled receptors: OX₁ and OX₂. In this study, we examined the expression of orexin receptors and effects of the receptors’ activation on cyclic AMP formation in the primary neuronal cell cultures from rat cerebral cortex. Both types of orexin receptors were expressed in rat cortical neurons; the level of OX₂R was markedly higher compared to OX₁R. Orexin A (an agonist of OX₁R and OX₂R) and [Ala¹¹-D-Leu¹⁵]orexin B (a selective agonist of OX₂R) did not affect basal cyclic AMP formation in the primary neuronal cell cultures. Both peptides (0.001–1 μM) inhibited, in a concentration-dependent manner and IC₅₀ values in low nanomolar range, the increase in the nucleotide production evoked by forskolin (1 μM; a direct activator of adenylyl cyclase), pituitary adenylate cyclase-activating polypeptide (PACAP27; 0.1 μM), and vasoactive intestinal peptide (VIP; 3 μM). Effects of orexin A on forskolin-, PACAP27-, and VIP-stimulated cyclic AMP synthesis were blocked by TCS OX2 29 (a selective antagonist of OX₂R), and unaffected by SB 408124 (a selective antagonist of OX₁R). Pretreatment of neuronal cell cultures with pertussis toxin (PTX) abolished the inhibitory action of orexin A on forskolin- and PACAP-stimulated cyclic AMP accumulation. It is suggested that in cultured rat cortical neurons orexins, acting at OX₂ receptors coupled to PTX-sensitive G_i protein, inhibit cyclic AMP synthesis.     

Muscarinic-dopaminergic synergism on retinal cyclic AMP formation

The cholinergic agonist, carbachol, produces a small increase in cyclic AMP concentration in the isolated rat retina, and markedly potentiates dopamine-stimulated cyclic AMP formation. This effect of carbachol is mediated through a muscarinic receptor, is calcium-independent, and is not due to inhibition of phosphodiesterase activity. Activation of miscarinic receptors may potentiate dopaminergic responses in the retina by enhancing coupling of the dopamine receptor to adenylate cyclase.     

Somatostatin inhibition of anterior pituitary adenylate cyclase activity: different sensitivity between male and female rats

Somatostatin (SRIF) is a potent inhibitor of growth hormone (GH) secretion. Although cyclic AMP (cAMP) has been suggested as intracellular mediator of SRIF action, a complete characterization of its effect and the different sensitivity between male and female animals, has not yet been carried out. In this study SRIF inhibited basal and GH-releasing factor (GRF) stimulated anterior pituitary adenylate cyclase activity with a greater effectiveness in male than in female glands. Similarly SRIF reduction of forskolin-stimulated anterior pituitary adenylate cyclase activity, was more pronounced in male than in female animals. By using pertussis toxin (PTX), which uncouples inhibitory receptors from adenylate cyclase catalytic subunit, SRIF inhibition of both basal and forskolin-stimulated adenylate cyclase activity was nearly abolished. These results show that anterior pituitary SRIF receptors are coupled in an inhibitory fashion with adenylate cyclase enzyme, and that male rat adenohypophyses are more responsive to SRIF inhibition.     

17-beta Oestradiol Pretreatment of Mouse Striatal Neurons in Culture Enhances the Responses of Adenylate Cyclase Sensitive to Biogenic Amines

Embryonic striatal neurons from the mouse grown in primary culture (6 day old culture) were used in order to investigate the effects of 17-beta oestradiol (17-beta E2) on biogenic amine-sensitive adenylate cyclases. Pretreatment (28 h) of intact cells with 17-beta E2 (10-9 M) enhanced cyclic AMP production induced by either dopamine, isoproterenol, serotonin, or 2-chloro-adenosine (maximal effective concentrations). These effects of 17-beta E2 on biogenic amine-sensitive adenylate cyclases occurred after several hours (8 h at least) and were seen in most cases with a concentration as low as 10-11 M (EC50: 10-10 M). They were additive with those induced by phenol red (5.6 microg/l) and chemically specific since 17alpha-oestradiol, 2(OH)17-beta E2, progesterone, and dexamethasone were without effect. In addition, they were not seen in cells which had been pretreated (30 h) with cycloheximide or alpha-amanitin, suggesting an involvement of de novo protein synthesis. Since 17-beta E2 did not influence cyclic AMP production induced by either forskolin or manganese ions, the stimulatory effects of 17-beta E2 pretreatment on biogenic amine-sensitive adenylate cyclases were not linked to an increase in the amount of enzyme catalytic units. 17-beta E2 pretreatment enhanced twofold the number of beta-adrenergic receptors (as estimated by the specific binding of (125I)iodocyanopindolol) but did not, in contrast, affect either the number or the affinity of dopaminergic receptors (as estimated by (125I)SCH 23982 binding). Therefore, the enhancing effects of 17-beta E2 pretreatment on biogenic amine-sensitive adenylate cyclases could be related either to an increased number of coupled receptors or to modifications of the adenylate cyclase transducing system (occurring probably at the G-protein level) or to a combination of the two.     

Development of beta-adrenergic receptors and the in vitro accumulation of cyclic AMP in the chick spinal cord

To clarify the functional development of the descending monoaminergic input to the chick spinal cord we have studied the ontogeny of beta-adrenergic receptors by measuring the specific binding the tritiated dihydroalprenolol (DHA). In addition, we examined the ability of isoproterenol to stimulate the accumulation of cyclic AMP in slices of developing chick spinal cord. Results show that the chick spinal cord contains a high density of beta-adrenergic receptors that are apparently linked to adenylate cyclase. During development, both the density of beta-receptors, as determined by the specific binding of DHA, and the response of tissue slices to isoproterenol underwent marked changes. beta-Adrenergic receptors (approximately 4 fmol/mg tissue) were first detected on the fourteenth day in ovo. Receptor density increased to approximately 20 fmol/mg by day 20. Between day 20 and the time of hatching, a sharp increase in receptor density, to approximately 50 fmol/mg, was seen. The density of receptors remained high until the second day after hatching, fell off to approximately 30 fmol/mg by the fourth day, and remained relatively unchanged through day 30. The response of spinal cord slices to isoproterenol showed a similar pattern of development with the peak response (7-fold increase in levels of cyclic AMP) occurring at or near the time of hatching. During the period between day 18 in ovo and the time of hatching, when both the response of tissue slices to isoproterenol and the density of beta-receptors increased markedly, the activity of phosphodiesterase did not change. Therefore, the pronounced changes in adrenergic responsiveness that occurred near the time of hatching appear to be related primarily to changes in the density of beta-adrenergic receptors coupled to adenylate cyclase. Such developmental changes in the density of beta-adrenergic receptors and adrenergic responsiveness may play an important role in determining the functional state of the descending monoaminergic systems in the chick spinal cord.     

Catecholamines and cyclic nucleotides in the modulation of superior cervical ganglia

Four hypotheses have been advanced to explain the relationships between the cholinergic presynaptic afferents, principal ganglionic neurons, and small, intensely fluorescent (SIF) cells in synaptic transmission in the mammalian superior cervical ganglion (SCG). The first hypothesis involves the role of the dopaminergic SIF cell and cyclic AMP in the modulation of ganglionic transmission through the generation of a slow inhibitory postsynaptic potential (s-IPSP). The second concerns the generation of a slow excitatory postsynaptic potential (s-EPSP), and the role of dopamine and cyclic AMP in potentiating it. The third postulates that a presynaptic α-adrenergic receptor is responsible for inhibiting f-EPSP generation. A fourth hypothesis concerns the localization of the β-adrenergic receptor—adenylate cyclase complex in SCG. This paper discusses the evidence for each hypothesis, with special emphasis on species variations in the modulation of ganglionic transmission by catecholamines, adrenergic receptors, and cyclic AMP.     

Lithium does not prevent agonist-induced subsensitivity of human adenylate cyclase

In a previous study 11 depressed patients were treated with salbutamol, a beta-2 adrenergic agonist, and beta-2 adrenergic receptor sensitivity was evaluated by measuring the plasma cyclic AMP rise after an iv dose of salbutamol. Salbutamol treatment induced subsensitivity of the beta-adrenergic adenylate cyclase with a time course paralleling the antidepressant effects. In the present study nine patients who were depressed despite treatment with lithium were treated with salbutamol plus lithium. Subsensitivity of the beta-adrenergic adenylate cyclase developed in the presence of lithium to the same degree as in patients treated with salbutamol alone. These results represent the first human study of the theory that lithium stabilizes receptor sensitivity changes. Lithium's failure to prevent subsensitivity agrees with reports that lithium fails to prevent impramine-induced subsensitivity of beta-adrenergic receptors in rat cortex. Lithium stabilization of receptor sensitivity of beta-adrenergic receptors in rat cortex. Lithium stabilization of receptor sensitivity would therefore appear to be unidirectional, preventing supersensitivity but not subsensitivity.     

Neurite outgrowth in dorsal root ganglia induced by islet neogenesis-associated protein peptide involves protein kinase A activation

Islet neogenesis-associated protein (INGAP) peptide is a candidate therapeutic for diabetes and corrects sensory dysfunction in experimental diabetes in mice. In this study, we investigated the mechanism of action by which INGAP peptide promotes neurite outgrowth in sensory neurons of the dorsal root ganglia. Treatment of dorsal root ganglia primary dispersed cultures with INGAP peptide led to the displacement of fluorescently labeled forskolin from adenylate cyclase, the cyclic AMP-generating enzyme that has been implicated in neuritogenesis. The addition of forskolin or dibutyryl cyclic AMP enhanced the effects of INGAP peptide on neurite outgrowth in dorsal root ganglia explant cultures. Furthermore, pharmacological inhibition of adenylate cyclase with SQ22,536 or of protein kinase A with H89 or KT5720 significantly reduced the neurite-promoting effects of INGAP peptide. These results suggest that INGAP peptide-induced neurite outgrowth in the dorsal root ganglia partially involves cyclic AMP-dependent activation of protein kinase A.