The brain adenylate cyclase system of the tobacco hornworm, Manduca sexta

Pharmacological characterization of dopamine receptors in the corpus allatum of Manduca sexta larvae

Dopamine receptors previously identified in corpora allata (CA) of Manduca sexta last instars on the basis of dopamine effects on JH (juvenile hormone)/JH acid biosynthesis and cyclic AMP (cAMP) accumulation, were characterized pharmacologically. For this study, a broad spectrum of agonists or antagonists of D1, D2, D3 or D4 dopamine receptors, together with the dopamine metabolite N-acetyl-dopamine, other neurotransmitters and their agonists/antagonists, were tested for their effects on gland activity and cAMP production. The lack of effect of other neurotransmitters supports the specificity of the effect of dopamine and the dopamine specificity of the receptors. Only the D2 receptor antagonist spiperone had a potent effect on JH biosynthesis and cAMP formation by CA taken on day 0 of the last stadium, when dopamine stimulates both activities and thus appears to be acting via a D1-like receptor. Several other D2 receptor antagonists, and D1, D2/D1 and D4,3/D2 receptor antagonists were less effective. Thus, the D1-like receptor of the Manduca CA appears to be distinct pharmacologically from vertebrate D1 receptors. By contrast, a number of D2 agonists/antagonists had a significant effect on JH acid biosynthesis and cAMP production by the CA from day 6 of the last stadium, when dopamine inhibits both activities and thus appears to be acting via a D2-like receptor. Certain D1-specific agonists/antagonists were equally effective. The Manduca D2-like receptor therefore bears some pharmacological resemblance to vertebrate D2 receptors. N-acetyl dopamine acted as a dopamine agonist with day 6 CA, the first identified function for an N-acetylated biogenic amine in insects. Dopamine was found to have the same differential affect on the formation of cAMP in homogenates of day 0 and day 6 brains as it did with CA, and in the same concentration range. Dopamine receptor agonists/antagonists affecting cAMP formation by day 0 and day 6 CA homogenates had similar effects with brain homogenates. By contrast, dopamine only stimulated cAMP formation by homogenates of day 0 and day 6 abdominal or ventral nerve cord. These results suggest that D1- and D2-like dopamine receptors of Manduca are regionally as well as temporally localized.

Dopaminergic control of corpora allata activity in the larval tobacco hornworm, Manduca sexta

The corpora allata (CA) of insects are innervated by axons of non-neurosecretory cerebral neurons, and of the various known neurotransmitters in the brain of the tobacco hornworm, Manduca sexta, only dopamine is detected in the CA by electrochemical detection HPLC. This neurotransmitter stimulates the biosynthetic activity of the CA in vitro for the first 2 days of the last larval stadium, but inhibits CA from day 3 through day 6, the beginning of the prepupal period. Stimulation of JH synthesis has previously been linked with an increase in the production of cyclic AMP (cAMP) in the CA, and dopamine stimulates the adenylyl cyclase system of CA from larvae early in the fifth stadium, while on day 6, its effect is inhibitory. These results suggest: (1) the existence in the CA of both D1- and D2-like dopamine receptors, which in vertebrates stimulate and inhibit, respectively, adenylyl cyclase; and (2) the developmental control of their expression. A potent D1 agonist, (+/-)-SKF 82958-HBr, did not stimulate JH biosynthesis by day 0 CA as expected, but appeared to inhibit it at a concentration of 10(-5)M. Thus the apparent D1-like receptor in Manduca CA may be pharmacologically distinct from vertebrate D1 receptors. The existence of D2-like receptors is supported by the finding that a vertebrate D2 receptor agonist, (+/-) PPHT-HCl, and an antagonist, eticlopride, have the predicted effects on JH acid biosynthesis and cAMP production by day 6 Manduca CA. However, the D1 agonist also significantly reduces JH acid biosynthesis and cAMP production, indicating that while the Manduca D2-like receptor is pharmacologically similar to the vertebrate D2, it shares some characteristics with D1 receptors. The developmental regulation of these receptors by ecdysteroids is suggested by the fact that when day 0 larvae are treated in vivo with exogenous ecdysone:20-hydroxyecdysone, the biosynthetic activity of the CA in vitro 24 h later is no longer stimulated by dopamine.

Corpora allata of the larval tobacco hornworm contain a calcium/calmodulin-sensitive adenylyl cyclase

An assay was developed with which to study basic characteristics of an adenylyl cyclase in the corpora allata (CA) of the tobacco hornworm, Manduca sexta. The assay used glands collected and frozen at -80 degrees C, to circumvent the problem of tissue availability. With this protocol for storage of tissue, less than 25% of the enzyme activity in fresh tissue was lost. Substances such as sodium fluoride (NaF) and Gpp(NH)p (a non-hydrolyzable GTP analog), which typically stimulate the adenylyl cyclases in other insect tissues, increased enzyme activity several-fold. There was a progressive decrease in the capacity of the CA adenylyl cyclase to be stimulated by NaF during the fifth stadium, suggesting a possible developmental change in the capacity of the associated G protein to be stimulated by NaF. The calcium/calmodulin (CaM) dependence of adenylyl cyclase activity was also investigated. The results demonstrated that addition of up to 10(-4) M calcium to assays of enzyme activity in whole gland homogenates of both larval (day 0) and prepupal (day 6) CA resulted in only a slight increase in the activity of the enzyme over basal rates in the presence of the calcium chelator EGTA. However, addition of as little as 5 microM CaM in the presence of 10(-4) to 10(-3) M calcium increased adenylyl cyclase activity three-to five-fold. A similar stimulation was obtained with washed membrane preparations of day 0 and day 6 glands, but required a substantially higher concentration of CaM. Results demonstrated that the CA possess a calcium/CaM-dependent adenylyl cyclase from day 0 through day 6.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of putative dopamine receptors in neurites outgrowing from locust central nervous system explants using anti-idiotypic dopamine antibodies

Organotypic cultures established from the third thoracic ganglion of locust embryo have been used to investigate dopamine receptors. In this in vitro system, neurites emerge directly from the explants and form a dense network around the explants, presenting cell surface freely exposed for experimental labelling. Polyclonal anti-idiotypic antibodies raised in rabbits to antibodies against dopamine conjugate, and previously found to bind to dopamine receptors, have been used to investigate putative dopamine receptors in these neurites. Immunocytochemical detection by light microscopy employing immunofluorescence labelling, was correlated with electron microscopy, using peroxidase staining. In addition to a location for dopamine receptors on the neurite surface, intracellular binding sites were also found in neurites. This internal labelling might represent an intracellular pool of dopamine receptor precursors. The labelling was specific in that it was not present when the anti-idiotypic dopamine antibodies were replaced with non-immune serum or when preincubation with conjugated dopamine preceded incubation with anti-idiotypic dopamine antibodies.

Occurrence, quaternary structure and function of G protein subunits in an insect endocrine gland

The occurrence, structure and function of the alpha and beta subunits of GTP-binding proteins (G proteins) were investigated in the Manduca sexta prothoracic gland, a tissue which possesses a hormonally regulated adenylate cyclase. Subunit-specific antibodies were utilized in immunoblotting studies of tissue from Manduca prothoracic glands, brain, eyes and antennae, and compared to the substrates present in the heads of Drosophila, as well as in a mammalian cell line. All Manduca tissues examined showed putative G beta subunits of 37 and 38 kDa, an unidentified alpha subunit of 41 kDa, in addition to an eye specific alpha subunit of 42 kDa. Manduca tissues also produced putative Gs alpha subunits of 48 and 51 kDa which were coupled to prothoracic gland adenylate cyclase as demonstrated by immunoprecipitation. Prothoracic gland G proteins have a definite and limited quaternary structure, consistent with a heterotrimeric model, as demonstrated by crosslinking of prothoracic gland membrane preparations followed by immunoblotting. These studies also yielded data on relative titers of alpha subunits, and suggest that Gs alpha is present in lower amounts than other alpha subunits. The G protein subunits studied in the prothoracic gland appear strikingly similar in molecular weight, function and structure to their mammalian counterparts.

A calmodulin-sensitive adenylate cyclase in the prothoracic glands of the tobacco hornworm, Manduca sexta

The Ca2+/calmodulin (CaM) dependence of adenylate cyclase activity in Manduca sexta prothoracic glands was investigated. Membrane fractions from two developmental stages were used, day 3 of the last larval instar and day 0 of the pupal stage, both of which respond to the neuropeptide prothoracicotropic hormone (PTTH) with increased cAMP production dependent on extracellular Ca2+. The data revealed that both larval and pupal prothoracic gland membrane fractions have a Ca2+/CaM-dependent adenylate cyclase which is inhibited by CaM antagonists and EGTA. The larval adenylate cyclase shows a multiphasic response to Ca2+/CaM, with a 2-fold stimulation between 0.02 and 0.01 microM, a further increase in adenylate cyclase activity at concentrations greater than 2 microM and a potentiation of NaF-stimulated activity at doses greater than 0.1 microM Ca2+/CaM. Pupal prothoracic gland membrane fractions exhibit only the second phase of stimulation. Stimulation by the GTP analogs GTP-gamma-S and Gpp(NH)p is dependent on CaM in larval, but not in pupal membrane fractions, suggesting a role for CaM in Gs protein-mediated regulation of adenylate cyclase. However, adenylate cyclase activity in glands from both stages is dependent on CaM, supporting our initial premise that Ca2+ is required for cAMP synthesis in the prothoracic glands.

The evolutionary origin of eukaryotic transmembrane signal transduction

1. A comparison was made of transmembrane signal transduction mechanisms in different eukaryotes and prokaryotes. 2. Much attention was given to eukaryotic microbes and their signal transduction mechanisms, since these organisms are intermediate in complexity between animals, plants and bacteria. 3. Signal transduction mechanisms in eukaryotic microbes, however, do not appear to be intermediate between those in animals, plants and bacteria, but show features characteristic of the higher eukaryotes. 4. These similarities include the regulation of receptor function, adenylate cyclase activity, the presence of a phosphatidylinositol cycle and of GTP-binding regulatory proteins. 5. It is proposed that the signal transduction systems known to operate in present-day eukaryotes evolved in the earliest eukaryotic cells.

Pharmacological characterisation of the dopamine-sensitive adenylate cyclase in cockroach brain: evidence for a distinct dopamine receptor

Dopamine increases cyclic AMP production in crude membrane preparations of cockroach brain with plateaus in cyclic AMP production occurring between 1-10 microM and at 10 mM. Maximal production of cyclic AMP is 2.25 fold greater than that of control values. Octopamine also increases cyclic AMP production with a Ka of 1.4 microM and maximal production 3.5 fold greater than that of control. 5-Hydroxytryptamine does not increase cyclic AMP production. The effects of octopamine and dopamine are fully additive. The vertebrate dopamine agonists ADTN and epinine stimulate the dopamine-sensitive adenylate cyclase (AC) with Ka values of 4.5 and 0.6 microM respectively and with maximal effectiveness 1.7 fold greater than that of control. The selective D2-dopamine agonist LY-171555 stimulates cyclic AMP production to a similar extent with a Ka of 50 microM. Other dopamine agonists (apomorphine, SKF-82526, SKF-38393) have no stimulatory effects. The octopamine-sensitive AC is inhibited by a variety of antagonists known to affect octopamine and dopamine receptors, with the following order of potency: mianserin greater than phentolamine greater than cyproheptadine greater than piflutixol greater than cis-flupentixol greater than SCH-23390 greater than (+)-butaclamol greater than SKF-83566 greater than SCH-23388 greater than sulpiride greater than spiperone greater than haloperidol. The dopamine-sensitive AC is inhibited by the same compounds with the following order of potency: piflutixol greater than cis-flupentixol greater than (+)-butaclamol greater than spiperone greater than or equal to SCH-23390 greater than cyproheptadine greater than SKF-83566 greater than SCH 23388 greater than mianserin greater than phentolamine greater than sulpiride greater than haloperidol. With the exception of mianserin, 3H-piflutixol is displaced from brain membranes by dopamine antagonists with an order of potency similar to that observed for the inhibition of dopamine-sensitive AC. The results indicate that the octopamine- and dopamine-sensitive AC in cockroach brain can be distinguished pharmacologically and the dopamine receptors coupled to AC have pharmacological characteristics distinct from vertebrate D1- and D2-dopamine receptors.

Regulation of crab Y-organ steroidogenesis in vitro: evidence that ecdysteroid production increases through activation of cAMP-phosphodiesterase by calcium-calmodulin

In decapod crustaceans steroidogenic glands (Y-organs) produce the molting hormone, ecdysone. A putative neuropeptide, molt-inhibiting hormone (MIH), released from eyestalk neurosecretory cells, directly regulates Y-organs by suppressing steroidogenesis; the effect is mediated by an increase in cAMP. We explored calcium-cAMP interactions in the regulation of Y-organs in vitro of the crab, Cancer antennarius. Basal ecdysteroid production was enhanced by extracellular calcium (EC). MIH suppression did not require EC but its action was blocked by high EC. The inhibitors of Ca2+ flux, lanthanum and ruthenium red, mimicked and enhanced MIH action. Calcium ionophore A23187 raised basal steroidogenesis dose-dependently (10(-6) to 10(-4) M) and with time course (effect evident after 2 h) similar to that of suppression by MIH. Low EC enhanced the suppressive effects on steroidogenesis of forskolin and dibutyryl cyclic AMP ((Bu)2cAMP) but not of MIH, lysine vasopressin (LVP), or 3-isobutyl-1-methyl-xanthine (IBMX); basal Y-organ cAMP levels were elevated by low EC and reduced by A23187. A23187 reduced the steroidogenic-suppressive effects of MIH, LVP, forskolin and (Bu)2cAMP but not of IBMX; rises in cAMP induced by MIH, LVP, and forskolin but not by IBMX were blunted by A23187. These findings suggested a stimulatory action of calcium on phosphodiesterase (PDE). The calmodulin (CM) inhibitor trifluoperazine (TFP; 10(-5) to 10(-4) M) reduced basal and A23187-stimulated steroidogenesis, enhanced the inhibitory effects of MIH and (Bu)2cAMP on ecdysteroid production, enhanced the stimulatory effects of MIH and forskolin on cAMP, and blocked the inhibition of cAMP by A23187. Y-organ PDE activity was enhanced by increasing free Ca2+ (10(-7) to 10(-5) M) and inhibited by TFP (10(-5) to 10(-4) M). Adenylate cyclase activity of Y-organ cell particulate fraction was unaffected by Ca2+ or TFP. Calcium stimulates steroidogenesis, apparently by activating a calcium-CM-dependent cAMP-PDE: the action is counter to the cAMP-mediated MIH-inhibitory system. Ca2+ fluxes were measured with dispersed Y-organ cells, in the presence and absence of agents that alter cAMP levels. The ionophore A23187, but not MIH or forskolin, increased 45Ca2+ entry by 45% over untreated control cells. Efflux from 45Ca2+-preloaded cells was increased 30% by MIH and forskolin, but not A23187. These data, together with those further above, suggest that MIH suppresses steroidogenesis in part by fostering Ca2+ depletion, and that the effect is mediated by cAMP.

Involvement of cyclic AMP-dependent protein kinase in prothoracicotropic hormone-stimulated ecdysone synthesis

Prothoracicotropic hormone (PTTH) is a brain neuropeptide that stimulates the prothoracic glands to synthesize ecdysone, an event that leads to insect molting. Both cyclic AMP (cAMP) and calcium have been implicated in PTTH action, with current evidence favoring cAMP as the messenger directly regulating ecdysone synthesis. To further define the role of cAMP in PTTH action, the activity of cAMP-dependent protein kinase (cAMP-PK) was examined in prothoracic glands from two developmental stages of the tobacco hornworm, Manduca sexta (day 3 fifth instar larvae and day 0 pupae). Prothoracic glands at each of these stages of development possess two forms of cAMP-PK which resemble the vertebrate type I and type II isozymes, with the latter being the predominant form (greater than 90%). Marked developmental differences exist in the degree of activation of soluble cAMP-PK following in vitro exposure of the prothoracic glands to PTTH. In larval glands, soluble cAMP-PK is activated within 3-10 min of initial exposure to doses of PTTH that stimulate ecdysone synthesis. By contrast, activation of soluble cAMP-PK in pupal glands occurs only when PTTH is administered in the presence of a phosphodiesterase inhibitor. Developmental differences in the activation of cAMP-PK by PTTH were qualitatively identical to previously observed differences in PTTH-stimulated accumulation of intracellular cAMP. The results suggest an involvement of soluble cAMP-PK in the response of day 3 fifth instar larval prothoracic glands to PTTH, but indicate a difference in the nature, intracellular location, or time course of activation, of hormone-sensitive protein kinase in day 0 pupal glands.