Catecholamine-stimulated cyclic GMP accumulation in the rat pineal: apparent presynaptic site of action

Pathologic role of nitrergic neurotransmission in mood disorders

Mood disorders are chronic, recurrent mental diseases that affect millions of individuals worldwide. Although over the past 40 years the biogenic amine models have provided meaningful links with the clinical phenomena of, and the pharmacological treatments currently employed in, mood disorders, there is still a need to examine the contribution of other systems to the neurobiology and treatment of mood disorders. This article reviews the current literature describing the potential role of nitric oxide (NO) signaling in the pathophysiology and thereby the treatment of mood disorders. The hypothesis has arisen from several observations including (i) altered NO levels in patients with mood disorders; (ii) antidepressant effects of NO signaling blockers in both clinical and pre-clinical studies; (iii) interaction between conventional antidepressants/mood stabilizers and NO signaling modulators in several biochemical and behavioral studies; (iv) biochemical and physiological evidence of interaction between monoaminergic (serotonin, noradrenaline, and dopamine) system and NO signaling; (v) interaction between neurotrophic factors and NO signaling in mood regulation and neuroprotection; and finally (vi) a crucial role for NO signaling in the inflammatory processes involved in pathophysiology of mood disorders. These accumulating lines of evidence have provided a new insight into novel approaches for the treatment of mood disorders.

Neural regulation of dark-induced abundance of arylalkylamine N-acetyltransferase (AANAT) and melatonin in the carp (Catla catla) pineal: an in vitro study

In all the vertebrates, synthesis of melatonin and its rhythm-generating enzyme arylalkylamine N-acetyltransferase (AANAT) reaches its peak in the pineal during the night in a daily light-dark cycle, but the role of different neuronal signals in their regulation were unknown for any fish. Hence, the authors used specific agonist and antagonists of receptors for different neuronal signals and regulators of intracellular calcium (Ca(2+)) and adenosine 3',5'-cyclic monophosphate (cAMP) in vitro to study their effects on the abundance of AANAT and titer of melatonin in the carp (Catla catla) pineal. Western blot analysis followed by quantitative analysis of respective immunoblot data for AANAT protein, radioimmunoassay of melatonin, and spectrophotometric analysis of Ca(2+) in the pineal revealed stimulatory effects of both adrenergic (α(1) and β(1)) and dopaminergic (D(1)) agonists and cholinergic (both nicotinic and muscarinic) antagonists, inhibition by both adrenergic and dopaminergic antagonists and cholinergic agonists, but independent of the influence of any agonists or antagonists of α(2)-adrenergic receptors. Band intensity of AANAT and concentration of melatonin in the pineal were also enhanced by the intracellular calcium-releasing agent, activators of both calcium channel and adenylate cyclase, and phophodiesterase inhibitor, but suppressed by inhibitor of calcium channel and adenylate cyclase as well as activator of phophodiesterase. Moreover, an inhibitory effect of light on the pineal AANAT and melatonin was blocked by both cAMP and proteasomal proteolysis inhibitor MG132. Collectively, these data suggest that dark-induced abundance of AANAT and melatonin synthesis in the carp pineal are a multineuronal function, in which both adrenergic (α(1) and β(1), but not α(2)) and dopaminergic signals are stimulatory, whereas cholinergic signals are inhibitory. This study also provides indications, though arguably not conclusive evidence, that in either case the neuronal mechanisms follow a signal-transduction pathway in which Ca(2+) and cAMP may act as the intracellular messengers. It also appears that proteasomal proteolysis is a conserved event in the regulation of AANAT activity in vertebrates.

The NMDA receptor/nitric oxide pathway: a target for the therapeutic and toxic effects of lithium

Although lithium has largely met its initial promise as the first drug discovered in the modern era of psychopharmacology, to date no definitive mechanism for its effects has been established. It has been proposed that lithium exerts its therapeutic effects by interfering with signal transduction through G-protein-coupled receptor (GPCR) pathways or direct inhibition of specific targets in signaling systems, including inositol monophosphatase and glycogen synthase kinase-3 (GSK-3). Recently, increasing evidence has suggested that N-methyl-D-aspartate receptor (NMDAR)/nitric oxide (NO) signaling could mediate some lithium-induced responses in the brain and peripheral tissues. However, the probable role of the NMDAR/NO system in the action of lithium has not been fully elucidated. In this review, we discuss biochemical, preclinical/behavioral and physiological evidence that implicates NMDAR/NO signaling in the therapeutic effect of lithium. NMDAR/NO signaling could also explain some of side effects of lithium.

Neuronal regulation of photo-induced pineal photoreceptor proteins in carp Catla catla

In the present in vitro study on the pineal in carp Catla catla, specific agonist and antagonists of receptors for different neuronal signals and regulators of intra-cellular Ca(++) and cAMP were used to gather basic information on the neuronal signal transduction cascade mechanisms in the photo-induced expression of rod-like opsin and alpha-transducin-like proteins in any fish pineal. Western-blot analysis followed by quantitative analysis of respective immunoblot data for both the proteins revealed that photo-induced expression of each protein was stimulated by cholinergic (both nicotinic and muscarinic) agonists and a dopaminergic antagonist, inhibited by both cholinergic antagonists and a dopaminergic agonist, but not affected by any agonists or antagonists of adrenergic (alpha(1), alpha(2) and beta(1)) receptors. Moreover, expression of each protein was stimulated by voltage gated L type calcium channel blocker, adenylate cyclase inhibitor and phosphodiesterase activator; but suppressed by the activators of both calcium channel and adenylate cyclase, and by phosphodiesterase inhibitor. Collectively, we report for the first time that both cholinergic and dopaminergic signals play an important, though antagonistic, role in the photo-induced expression of photoreceptor proteins in the fish pineal through activation of a signal transduction pathway in which both calcium and cAMP may act as the intracellular messengers.

Can melatonin regulate the expression of prohormone convertase 1 and 2 genes via monomeric and dimeric forms of RZR/ROR nuclear receptor, and can melatonin influence the processes of embryogenesis or carcinogenesis by disturbing the proportion of cAMP and cGMP concentrations? Theoretic model of controlled apoptosis

The presented model of controlled apoptosis has been based on the assumption that correct information exchange between an organism as a whole, and each of its cells is conditioned by mutual proportions of cAMP and cGMP concentrations (CcAMP, CcGMP), according to the formula CcAMP x CcGMP = 'a' (constant). The regulation of balance of these 'second messengers' in a cell and an extracellular space would depend on the mutual proportions of concentrations of Melatonin and monomers of Melanin. These indoloderived compounds could be the activators of the transcription factors i.e. RZR and NFkappa-B, regulating the expression of Prohormone Convertase (PC) gen and Nitric Oxide Synthase (NOS) gen, respectively. Additionally, maternal Melatonin and Nitric Oxide (NO), being able to pass through trophoblast or placenta freely, would play decisive role in the synchronization of embryogenesis and intrauterine development of the fetus. In case of an embryo or a fetus, the result of CcAMP and CcGMP multiplication, different from the proper constant 'a'-value, would mean occurrence of disorders in the structure and functioning of the cellular tensegrity system and, in consequence, disturbances in the intercellular information exchange. It would lead to deviation in cellular metabolism, oriented cell movement, uncontrolled apoptosis, and as a consequence, would lead to the development of fetal defects. In case of a child or an adult, a sudden occurrence and prolongation of such disturbances in CcAMP-CcGMP proportions would induce a process of apoptosis of normal cells and an initiation of a cancerogenesis. On the other hand, the recovery of equilibrium in the information exchange system would initiate apoptosis of neoplastic cells, and simultaneously, proliferation of connective tissue cells. According to the presented hypothesis, a decrease in CcAMP and destabilization of the CcAMP-CcGMP balance in an embryo or a fetus would result from relatively excessive amounts of maternal Melatonin (monomers) in fetal circulation, while a decrease of CcAMP and destabilization of the CcAMP-CcGMP balance in a child or an adult would be a consequence of relatively insufficient amounts of Melatonin (dimers) in an organism. It seems possible, that determination of both CcAMP and CcGMP would enable an early detection of high risk of developmental defects occurrence in an embryo or a fetus and neoplastic processes in a child or an adult. This method might also be considerably useful in monitoring a safe substitutional hormonotherapy.

Photoneural regulation of rat pineal nitric oxide synthase

We report here a photoneural regulation of nitric oxide synthase (NOS) activity in the rat pineal gland. In the absence of the adrenergic stimulation following constant light exposure (LL) or denervation, pineal NOS activity is markedly reduced. A maximal drop is measured after 8 days in LL. When rats are housed back in normal light:dark (LD) conditions (12:12), pineal NOS activity returns to normal after 4 days. A partial decrease in pineal NOS activity is also observed when rats are placed for 8 days in LD 18:6 or shorter dark phases, indicating that pineal NOS activity reflects the length of the dark phase. Because it is known that norepinephrine (NE) is released at night from the nerve endings in the pineal gland and this release is blocked by exposure to light, our data suggest that NOS is controlled by adrenergic mechanisms. Our observation may also explain the lack of cyclic GMP response to NE observed in animals housed in constant light.

Norepinephrine induces Ca2+ release from intracellular stores in rat pinealocytes

In rat pinealocytes, an increase in intracellular Ca2+ concentration ([Ca2+]i) due to Ca2+ influx in response to norepinephrine (NE) is a well recognized event involved in regulating several metabolic functions. Since NE also stimulates the metabolism of phosphatidyl inositols in rat pineal gland, it is conceivable that Ca2+ release from intracellular stores also contributes to the NE-induced increase in [Ca2+]i. In this communication, we report that in rat pinealocytes loaded with fura-2, a Ca2+ indicator, NE induced a transient increase in [Ca2+]i that preceded the known Ca2+ influx. This novel [Ca2+]i response to NE was detected in pinealocytes bathed with Ca2+-free saline and prevented by TMB-8, a blocker of Ca2+ release from intracellular stores, supporting the notion that the transient NE-induced Ca2+ response was due to Ca2+ release from intracellular stores. In addition, after an extended exposure to NE a new addition of this neurotransmitter did not elicit the phasic Ca2+ response, and application of increasing amounts of NE induced a Ca2+ response that was progressively smaller, suggesting desensitization. Thus, NE is proposed to increase [Ca2+]i in rat pinealocytes by two mechanisms: (1) phasic release from intracellular stores and (2) tonic influx through a mechanism activated by larger applications of NE than required to evoke the phasic release.

Differential involvement of the arachidonic acid cascade on the alpha 1-adrenergic potentiation of vasoactive intestinal peptide- versus beta-adrenergic-stimulated cyclic AMP and cyclic GMP accumulation in rat pinealocytes

In the rat pineal gland, alpha 1-adrenergic agonists, which stimulate arachidonic acid release, also potentiate vasoactive intestinal peptide (VIP)- or beta-adrenergic-stimulated cyclic AMP (cAMP) and cyclic GMP (cGMP) accumulation. In this study, the possible involvement of the arachidonic acid pathway in the potentiation mechanism was examined in dispersed rat pinealocytes using two inhibitors of the arachidonic acid cascade, indomethacin and nordihydroguaiaretic acid. These two inhibitors appeared to have differential effects on the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP responses. Whereas nordihydroguaiaretic acid was effective in suppressing both the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP responses, indomethacin inhibited selectively the VIP-mediated cAMP and cGMP responses. The role of arachidonic acid metabolites was further determined using several prostaglandins--A2, I2, E2, and F2 alpha--and leukotrienes--B4, C4, and D4. Of the seven compounds tested, prostaglandins E2 and F2 alpha stimulated basal cAMP but not cGMP accumulation. The prostaglandin E2- and F2 alpha-stimulated cAMP responses were additive to those stimulated by VIP or beta-adrenergic receptors. The other five compounds had no effects on basal or VIP- or beta-adrenergic-stimulated cAMP or cGMP accumulation. Taken together, these findings indicate that the arachidonic acid cascade is likely involved in the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP accumulation. However, the specific arachidonic acid metabolite involved in the potentiation mechanisms of VIP- versus beta-adrenergic-stimulated cyclic nucleotide responses may be different.

Melatonin biosynthesis in the mammalian pineal gland

Rhythmic production of melatonin by the mammalian pineal occurs in response to noradrenergic stimulation which produces a cascade of biochemical events within the pinealocyte. In the rat, massive changes in NAT activity result from an increase in intracellular c-AMP levels produced by a synergistic interaction whereby an alpha 1 activation amplifies beta-adrenergic stimulation. The intracellular events mediating this effect are described. A major aspect of the temporal control of melatonin production is the programmed down-regulation of responses to noradrenergic stimulation once the initial surge of c-AMP is produced. Noradrenergic activation of the gland also influences other enzymic functions, including tryptophan hydroxylase and HIOMT activities, and produces a dramatic increase in intracellular c-GMP levels. Other neurotransmitters and neuropeptides, e.g. VIP, may also influence pineal function and comparisons are made between the rat, the subject of the bulk of experimental studies, and other species.

Differential responses of rat pineal thyroxine type II 5'-deiodinase and N-acetyltransferase activities to either light exposure, isoproterenol, phenylephrine, or propranolol

1. Compared to pineal N-acetyl transferase (NAT) activity, which exhibited a dramatic drop following acute light exposure at night, nocturnal rat pineal thyroxine type II 5'-deiodinase (5'-D) activity was minimally influenced by the same light exposure. The injection of cycloheximide, a potent inhibitor of protein synthesis, although it did curtail the rise in NAT activity for at least 2 hr, did not elicit decreases in the activities of either 5'-D or NAT enzymes. Propranolol, a beta-adrenergic blocker, either delayed the continued nocturnal rise in 5'-D activity when injected at 0000 hr or slightly enhanced the fall in 5'-D activity when injected at 0200 hr. These results suggest that interruption of the synthesis of proteins is responsible for the slow deterioration of 5'-D activity induced by either light or propranolol. 2. The slight fall in 5'-D activity induced by light at night was prevented by isoproterenol; phenylephrine, however, did not prevent the fall and the effect of isoproterenol + phenylephrine was similar to that obtained with isoproterenol alone. On the other hand, the light-inhibited NAT activity recovered after the injection of isoproterenol; phenylephrine did not elicit any effect, but the injection of both isoproterenol and phenylephrine simultaneously caused a greater NAT response than that induced by isoproterenol alone. 3. When injected during the day, phenylephrine had no effect on either pineal 5'-D or NAT activities; however, the injection of either isoproterenol alone or isoproterenol + phenylephrine elicited 5-fold and 10-fold increases in nocturnal, light-suppressed 5'-D and NAT activities, respectively. During the day, phenylephrine did not potentiate the effects of isoproterenol on NAT activity as it did at night. When the effects of isoproterenol on the 5'-D activity were compared to rats exposed to light during the day and at night, the activity of 5'-D reached a higher level at night than during the day.

Cyclic nucleotide metabolism in pineal homogenates

Adenylate cyclase (AC) in pineal particulate fractions from rabbit, rat, cow, and the vole Microtus montanus was stimulated by L-norepinephrine (NE) and L-isoproterenol (ISO). NE stimulation of rabbit and bovine pineal AC was biphasic, with a plateau between 0.01 microM and 1.0 microM and additional stimulation by NE above 1.0 microM. Stimulation by different ISO concentrations gave a typical hyperbolic curve, and optimal stimulation by ISO exceeded that by NE. Melatonin decreased ISO and NE stimulation of AC 10-20%. Although, alpha-adrenergic agonists increase beta-agonist-mediated adenosine-3',5'-cyclic monophosphate (cyclic AMP) accumulation in intact pinealocytes, similar amplification of AC stimulation was not seen with broken-cell preparations. Most (60-70%) pineal guanylate cyclase (GC) was recovered in supernatant fractions after centrifugation of homogenates at 110,000 x g; this soluble GC was unaffected by potential agonists. Low concentrations (0.01-1 nM) of NE, ISO, and phenylephrine (PE) stimulated GC in impure and purified membrane fractions, but each inhibited at concentrations above 10 microM. All concentrations of ISO and NE inhibited GC in the presence of the alpha-agonist PE. Melatonin alone did not affect particulate GC, but L-ISO stimulation was not seen in the presence of equivalent concentrations of melatonin. The in vitro data are consistent with both alpha- and beta-receptor regulation of cyclic nucleotide metabolism in pinealocytes. Endogenous NE may differentially regulate cyclic AMP and guanosine-3',5'-cyclic monophosphate (cyclic GMP) in pineal; low NE concentrations that stimulate GC have only a slight effect on AC, but higher NE concentrations that inhibit GC maximally stimulate AC. Particulate GC and AC also were resolved by equilibrium centrifugation, to give several discrete peaks of enzyme activity. The results support the existence of several forms of AC and GC, which have different responses to adrenergic agonists.

See-saw signal processing in pinealocytes involves reciprocal changes in the alpha 1-adrenergic component of the cyclic GMP response and the beta-adrenergic component of the cyclic AMP response

Pineal cyclic AMP and cyclic GMP are regulated by norepinephrine (NE) acting through alpha 1- and beta-adrenoceptors. beta-Adrenergic stimulation appears to be an absolute requirement and alpha 1-adrenergic activation amplifies beta-adrenergic stimulation of the cyclic AMP response 10-fold and the cyclic GMP response 100-fold, respectively. Chronic deprivation of adrenergic stimulation, due to exposure to constant light (LL) or by surgical denervation, enhances the cyclic AMP response and diminishes the cyclic GMP response as compared to control animals in a 10:14 light/dark (LD) cycle. This phenomenon is termed see-saw signal processing. In the current study we find these changes do not reflect shifts in the time course or Ka of these responses. Dose-response studies indicate the beta-adrenergic component of cyclic AMP stimulation is enhanced and the alpha 1-adrenergic component of cyclic GMP stimulation is diminished in LL pinealocytes. Several observations indicate these changes may reflect alterations in Ca2+-sensitive postreceptor mechanisms.

Changed sensitivity of alpha 2-adrenoceptors mediating a decrease in protein kinase inhibitor activity in the brain of vasopressin-hypertensive rats

Clonidine produced an increase of cGMP content and a decrease of the endogenous type II inhibitor of protein kinase in rat hypothalamic slices. When administered to rats, the effect of clonidine on type II inhibitor activity in the hypothalamus and brain-stem depended on the dose. Low doses (10-50 micrograms X kg-1 i.p.) produced an increase, probably by stimulating presynaptic alpha 2-adrenoceptors, whereas large doses (200-1000 micrograms X kg-1 i.p.) produced a decrease of type II inhibitor activity by stimulating postsynaptic receptors. The development of vasopressin hypertension was associated with a gradual reduction of the response of the type II inhibitor to low and high doses of clonidine. In vasopressin-hypertensive rats neither small nor large doses of clonidine were able to induce changes in type II inhibitor activity suggesting subsensitivity of pre- and postsynaptic alpha 2-adrenoceptors. However, clonidine appeared to be equally effective in blocking electrically stimulated [3H]noradrenaline release from hypothalamic slices of vasopressin-hypertensive and control, normotensive rats. Reduced reactivity of postsynaptic alpha 2-adrenoceptors seems to be of great importance since treatment of vasopressin-hypertensive rats with 6-hydroxydopamine resulted in a decrease of blood pressure and reappearance of the sensitivity of postsynaptic alpha 2-adrenoceptors to clonidine.

Denervation supersensitivity of the rat pineal to norepinephrine-stimulated [3H]inositide turnover revealed by lithium and a convenient procedure

A convenient procedure for the assay of myo-[2-3H(N)]inositol ([3H]inositol) metabolites in cells or small amounts of tissue was developed. The procedure is a composite of modifications of published methods. After preincubation with [3H]inositol, rat pineal glands were disrupted in an acidified organic solvent mixture. Lipids were separated from the hydrophilic products and precursor using Sephadex G-25 columns and further analyzed by TLC. Hydrophilic products were further analyzed by anion-exchange column chromatography using Dowex AG1-X8 (formate form). In the presence of lithium, increases in inositol phosphates consequent to stimulation of the glands by norepinephrine were apparent within 10 min. The response in denervated glands was considerably greater than in intact pineals.

Activation of protein kinase C potentiates isoprenaline-induced cyclic AMP accumulation in rat pinealocytes

The pineal gland has proven to be an excellent model for the study of adrenergic control systems. Noradrenaline, released from sympathetic nerve terminals in the pineal gland, regulates a large nocturnal increase in melatonin synthesis by stimulating the activity of arylalkylamine N-acetyltransferase (NAT, EC 2.3.1.87) 30-70-fold. An essential step in both the induction and maintenance of high NAT activity is an increase in intracellular cyclic AMP. Noradrenaline acts via beta-adrenoceptors to increase pineal cyclic AMP by activating adenylate cyclase, and the activation of pineal alpha 1-adrenoceptors potentiates beta-adrenergic stimulation not only of NAT but of both cyclic AMP and cyclic GMP. Here we describe investigations designed to test whether alpha 1-adrenergic potentiation of beta-adrenergic stimulation of pineal cyclic AMP involves protein kinase C. Our results suggest that kinase activation is involved and the data provide the first demonstration of a synergistic interaction between Ca2+-phospholipid-dependent protein kinase (protein kinase C) and neurotransmitter-dependent stimulation of cyclic AMP.

Melatonin: a coordinating signal for mammalian reproduction?

There is a daily rhythm in the production of the pineal hormone melatonin in all mammalian species. Production is stimulated by darkness and inhibited by light. This provides a signal reflecting the changing environmental lighting cycle. In seasonally breeding mammals that use changes in the photoperiod to time their reproductive cycles, temporal signals to the reproductive system are controlled by the daily rhythm in melatonin production.

Interdependent effects of the ionophore A23187 and serum on the serotonin N-acetyltransferase activity of cultured chick pineal glands

Marked effects of the ionophore A23187 on the cycle of N-acetyltransferase (NAT) activity in cultured chick pineal glands were observed under three conditions of illumination. However, the effects were qualitatively and quantitatively dependent on the batch of fetal calf serum used in the medium and time of explanation into culture. Ionophore increased the level of NAT activity remaining in glands exposed prematurely to light regardless of the serum used. The ionophore suppressed the "spike" in cyclic GMP content of glands cultured in the dark, and extended the period of maximum cyclic GMP content of glands under diurnal illumination.

Hamster and rat pineal gland beta-adrenoceptor characterization with iodocyanopindolol and the effect of decreased catecholamine synthesis on the receptor

Rat and hamster pineal glands were used in binding studies to characterize their beta-adrenoceptors with a new specific antagonist ligand, iodocyanopindolol. The receptors were saturable, and the ligand was selective and demonstrated stereospecificity for both species. The rat pineal had a 20-fold greater density of beta-adrenoceptors, while the affinity was one-third that of the hamster pineal. utilizing this radioligand, we examined the effects of decreased sympathetic input to the pineal on beta-adrenergic receptors in both species. Decreased noradrenergic input to the pineal gland of the hamster was accomplished by superior cervical ganglionectomy, or by exposing the animals to continuous light for 36 hours. Parallel studies were conducted with hamster pineal gland in which catecholamine synthesis was measured. The results indicate that a selective decrease in catecholamine synthesis in the hamster pineal does not change the beta-adrenoceptor density or affinity. In contrast, a concomitant increase in beta-adrenoceptor density but not affinity occurs in the rat pineal gland after similar decreased sympathetic input.

Electrical stimulation of sympathetic nerves increases the concentration of cyclic AMP in rat pineal gland

Electrical stimulation of the superior cervical ganglia causes a rapid increase in the concentration of cyclic AMP in the pineal gland of rats. This effect is dependent upon the frequency, voltage, and duration of the stimulus and is markedly potentiated by pretreating the animals with desmethylimipramine. The increase in cyclic AMP is blocked by prior treatment of the rats with reserpine, bretylium, or propanolol but not with phentolamine. These results provide direct evidence that electrical stimulation of sympathetic nerves increases cyclic AMP in a target organ through the release of norepinephrine from presynaptic terminals acting on postsynaptic beta-adrenergic receptors.

Seesaw signal processing in pineal cells: homologous sensitization of adrenergic stimulation of cyclic GMP accompanies homologous desensitization of beta-adrenergic stimulation of cyclic AMP

Studies of the adrenergic regulation of cyclic GMP in the pineal gland show that (-)-norepinephrine stimulates cyclic GMP primarily in pineal cells, rather than in nerve endings as previously thought. The response exhibits the interesting and unusual characteristic of homologous sensitization: It is maintained by neural stimulation and disappears gradually as a consequence of depressed neural stimulation, due to denervation or decentralization of the superior cervical ganglia or to constant light. The response is restored in intact animals that had been in a constant-light environment when they are returned to a normal light cycle and in ganglionectomized animals by norepinephrine treatment. These findings are especially interesting because the pineal adrenergic--cyclic AMP stimulus--response system exhibits homologous desensitization. The occurrence of homologous sensitization of a cyclic GMP response and desensitization of a cyclic AMP response, which we term seesaw signal processing, in the same tissue or cell has intriguing implications. It provides a mechanism through which the qualitative nature of a multicomponent response can be modified. Such a mechanism could play a role in signal processing by neural or neuroendocrine tissues that release two or more extracellular messages.

The beta-adrenergic receptor and its mode of coupling to adenylate cyclase

The article first includes a discussion on the classification of catecholamine receptors followed by a discussion on the binding studies of beta-receptors and their affinity labeling. Next a brief discussion on the solubilization and the current attempts to purify the receptor is presented. A large section is then devoted to the mode of coupling between beta-receptors and cyclase where much space is devoted to the role of GTP and of the membrane matrix. The review ends with a discussion on beta-receptor desensitization, supersensitivity, and the "spare receptor" concept.

Cyclic GMP System in epidermis: I. Effect of ischemia

When keratome-sliced pig epidermis was floated on Hank's balanced salt solution, we observed a rapid decrease in the intracellular level of cyclic GMP. A portion of the lost cyclic GMP was detected in the incubation medium. When the epidermis was kept in air at room temperature, the cyclic GMP level also decreased rapidly but to a lesser degree. Incubating the epidermal slice at 37 degrees C in Hank's balanced salt solution with the addition of 3-isobutyl-1-methyl xanthine (IBMX) prevented the decrease. Also, after the cyclic GMP level had fallen, it could be raised to be the in vitro level by the addition of IBMX. Increased amounts of cyclic GMP were detectable in the medium in this case. These data indicate that the decrease in cyclic GMP in ischemic epidermis is due to sudden activation of epidermal cyclic GMP-phosphodiesterase and also in part due to leakage of cyclic GMP extracellularly. In contrast to the rapid decline in the cyclic GMP level, ischemia caused a rapid and transient increase in epidermal cyclic AMP. This confirms previous data by ourselves and by others (Br J Dermatol 92: 249-254, 1975; J Invest Dermatol 68:125-127, 1977). These "ischemic effects" must be avoided in order to measure the "in vivo level" of cyclic nucleotides in epidermis.

The influence of 8-Br 3', 5'-cyclic nucleotide analogs and of inhibitors of 3', 5'-cyclic nucleotide phosphodiesterase, on noradrenaline secretion and neuromuscular transmission in guinea-pig vas deferens

The effects of 3', 5'-cyclic nucleotide phosphodiesterase (PDE) inhibitors and of 8-Br 3', 5'-cyclic nucleotide analogs on nerve-muscle transmission were studied in the guinea-pig vas deferens preincubated with 3H-noradrenaline. 8-Br cyclic AMP and the PDE inhibitors 3-isobutyl-1-methylxanthine (IBMX) and 3-propionyl-4-hydrazinopyrazolopyridine (SQ 20006) enhanced the secretion of 3H-NA evoked by transmural nerve stimulation. 8-Br cyclic GMP was without effect in this respect. The muscle contraction evoked by transmural nerve stimulation, high potassium or by application of exogenous noradrenaline was depressed by IBMX and SQ 2006. The contraction evoked by transmural nerve stimulation was enhanced by 8-Br cyclic AMP and depressed by 8-Br cyclic GMP. These findings suggest differential involvement of 3', 5'-adenosine- and guanosine-cyclic nucleotides in excitation-secretion-coupling in the noradrenergic sympathetic nerves, and in excitation-contraction-coupling in the smooth muscle, of guinea-pig vas deferens.

Synaptic potentials in sympathetic ganglia: are they mediated by cyclic nucleotides?

The hypothesis that cyclic nucleotides are intracellular second messengers mediating the generation of synaptic potentials was studied in the sympathetic ganglia of the bullfrog. Synaptic potentials and the effect of administering cyclic nucleotides and agents which affect cyclic nucleotide metabolism were recorded by the sucrose gap technique. The administration of adenosine 3',5'-monophosphate (cyclic AMP), guanosine 3',5'-monophosphate (cyclic GMP), or several of their derivatives produced little or no change in membrane potential. Prostaglandin E1 did not block the generation of postsynaptic potentials. Theophylline produced membrane effects that were different from those associated with postsynaptic potential generation; it also reduced the slow excitatory postsynaptic potential (EPSP) and potentiated the slow inhibitory postsynaptic potential (IPSP). The administration of papaverine, however, reduced both the slow EPSP and the slow IPSP. Although synaptic stimulation increases both cyclic GMP and cyclic AMP in these neurons, these results raise the possibility that these cyclic nucleotides may have functionla roles other than mediation of synaptic potentials.

Phosphorylated proteins as physiological effectors

A variety of neurotransmitters, hormones, and other regulatory agents affect the phosphorylation of specific proteins in their target tissues. The types of stimuli that share this common effect on protein phosphorylation include numerous substances that do not act through cyclic AMP. These and other observations suggest that many different classes of regulatory substances achieve certain of their biological effects by altering the phosphorylation of specific proteins.

Efflux of cyclic nucleotides from rat pineal: release of guanosine 3',5'-monophosphate from sympathetic nerve endings

Potassium and norepinephrine stimulate the efflux of adenosine 3',5'-monophosphate (cyclic AMP) and guanosine 3',5'-monophosphate (cyclic GMP) from intact pineal glands. The postsynaptic beta-adrenergic receptor mediates the efflux of cyclic AMP. In contrast, the efflux of cyclic GMP requires calcium and intact nerve endings. It appears that sympathetic nerve endings may release cyclic GMP into the synaptic space.