Effect of dopamine on activation of rat striatal adenylate cyclase by free Mg2+ and guanyl nucleotides

ADP-ribosylation by cholera toxin of membranes derived from brain modifies the interaction of adenylate cyclase with guanine nucleotides and NaF

We have developed a method to ADP-ribosylate the stimulatory guanine nucleotide-binding protein of adenylate cyclase (GS) in brain membranes by using cholera toxin. In particular, we used isonicotinic acid hydrazide and 3-acetylpyridine adenine dinucleotide to inhibit the potent NAD-glycohydrolase activity of brain membranes, and we used the detergent Triton X-100 (at 0.1%) to improve the accessibility of the toxin and guanine nucleotides used for supporting the ADP-ribosylation. This method reveals that GS is a very abundant protein in membranes derived from calf brain (approximately 30 pmol/mg of protein). In brain, GS exists in large excess over the previously reported amount of the adenylate cyclase catalytic subunit. The modification of GS with an ADP-ribosyl residue (a) elicits a four- to fivefold activation of adenylate cyclase by GTP, (b) increases the stabilization of adenylate cyclase by GTP, and (c) reduces adenylate cyclase activation by fluoride but does not change basal activity, activation by guanosine 5'-(beta, gamma-imido)triphosphate, or the sensitivity of adenylate cyclase to heat-induced denaturation. A correlation between ADP-ribosylation and the alterations in the activation of adenylate cyclase by guanine nucleotides and by fluoride is presented.

Transient states of adenylate cyclase in brain membranes

Basal activity of adenylate cyclase from the amygdala of sheep brain and the neostriatum of turkey brain decays in two phases at 37 degrees C. The first phase is rapid (t1/2 = 2.3 +/- 0.3 min) and results in the loss of 60-70% of basal activity. The second phase is slow (t1/2 approximately 100 min) during which time the catalytic units denature irreversibly. The GTP analogue guanosine-5' (beta-gamma imino) triphosphate (p[NH]ppG) prevents the rapid decay by stabilizing the enzyme at its initial level of activity and also reactivates the enzyme to initial levels during or immediately following the early phase, indicating that denaturation of neither the guanylnucleotide units nor the catalytic units causes the rapid decline in basal activity. Activation by p[NH]ppG is rapid at 37 degrees C, but the binding of p[NH]ppG to the guanylnucleotide subunit also occurs at nonactivatory temperatures. This is determined by the protection of catalytic units from thermal or N-ethylmaleimide inactivation after extensive washing. Thus, at 25 degrees C all of the catalytic units can be stabilized by saturating p[NH]ppG concentrations. At 0 degree C, 35% of the catalytic units can be stabilized by saturating p[NH]ppG concentrations within 30 s. The half-saturation constant for the binding of p[NH]ppG at 0 degree C is identical to that derived in an assay at 37 degrees C, or after an incubation of the membranes for 10 min at 45 degrees C, when the process of thermal denaturation is 80% complete (K1/2 approximately 3 +/- 2 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of a low Km GTPase activity in rat striatum by calmodulin

In rat striatum, the activation of adenylate cyclase by the endogenous Ca2+-binding protein, calmodulin, is additive with that of GTP but is not additive with that of the nonhydrolyzable GTP analog, guanosine-5'-(beta, gamma-imido)triphosphate (GppNHp). One possible mechanism for this difference could be an effect of calmodulin on GTPase activity which has been demonstrated to "turn-off" adenylate cyclase activity. We examined the effects of Ca2+ and calmodulin on GTPase activity in EGTA-washed rat striatal particulate fractions depleted of Ca2+ and calmodulin. Calmodulin inhibited GTP hydrolysis at concentrations of 10(-9)-10(-6) M but had no effect on the hydrolysis of 10(-5) and 10(-6) M GTP, suggesting that calmodulin inhibited a low Km GTPase activity. The inhibition of GTPase activity by calmodulin was Ca2+-dependent and was maximal at 0.12 microM free Ca2+. Maximal inhibition by calmodulin was 40% in the presence of 10(-7) M GTP. The IC50 for calmodulin was 100 nM. In five tissues tested, calmodulin inhibited GTP hydrolysis only in those tissues where it could also activate adenylate cyclase. Calmodulin could affect the activation of adenylate cyclase by GTP in the presence of 3,4-dihydroxyphenylethylamine (DA, dopamine). Calmodulin decreased by nearly 10-fold the concentration of GTP required to provide maximal stimulation of adenylate cyclase activity by DA in the striatal membranes. The characteristics of the effect of calmodulin on GTPase activity with respect to Ca2+ and calmodulin dependence and tissue specificity parallel those of the activation of adenylate cyclase by calmodulin, suggesting that the two activities are closely related.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple dopamine receptors and behavior

The therapeutic effects of dopamine (DA) agonists and DA antagonists used in the treatment of schizophrenia (antipsychotics, DA antagonists), Huntington's chorea (DA antagonists) and Parkinson's disease (antiparkinsonian agents, DA agonists) have been thought to result largely from actions on DA receptors located in the striatum (caudate nucleus and putamen). Many of the classical drugs used to treat these disorders are known to have a high incidence of extrapyramidal side effects (EPS). However, a number of drugs, the atypical antipsychotics and antiparkinsonian agents, have been developed which have a low incidence of EPS. It has been of enormous interest to researchers and clinicians alike to determine what characteristics of the atypical antipsychotics and antiparkinsonian agents are responsible for their unique behavioral profile. Because all of the antipsychotics and antiparkinsonian agents act on DA receptors, much attention has focused on potential differences in the interactions of the atypical agents with DA receptors. An hypothesis that has been raised, due to the knowledge that there are multiple subtypes of DA receptors located in the striatum, is that the atypical agents could have their therapeutic actions as a result of an interaction with one specific subtype of DA receptor. This review emphasizes two major points: (1) it is unlikely that the atypical antipsychotics and antiparkinsonian agents interact with only one subtype of DA receptor, or have their therapeutic actions only through that receptor; (2) other pharmacological characteristics of these agents are more critically involved in their unique behavioral effects. The applicability of animal models to assess the pharmacological and behavioral profiles of these agents is discussed, and the relevance to the clinical profiles of these agents is emphasized.

Factors affecting solubilized dopamine-sensitive adenylate cyclase

Bovine striatal adenylate cyclase was solubilized with sodium cholate and assayed for its responsiveness to a variety of agents. Magnesium ions (0-20 mM), guanylyl-imidodiphosphate (10-50 microM), and striatal lipids were effective in increasing enzyme activity. The adenylate cyclase could be stimulated by dopamine, and neuroleptic drugs inhibited the effect of dopamine (50 microM) with potencies that paralleled their clinical potencies. The IC50 values for spiroperidol, haloperidol and chlorpromazine were 0.2, 3 and 500 nM respectively.

The activity of dopamine-stimulated adenylate cyclase from rat brain stratum is modulated by temperature and the bilayer-fluidizing agent, benzyl alcohol

Benzyl alcohol achieved a marked activation of the adenylate cyclase activity in a partially purified membrane preparation from rat brain striata, although inhibition resulted at high concentrations. The degree of activation observed depended on the ligand used to stimulate the enzyme, with that observed in the presence of guanosine 5'-[beta,gamma-imido]triphosphate (p[NH]ppG) (5.8-fold)>dopamine+p[NH]ppG (5-fold)> GTP (3-fold)>dopamine+high GTP (2.25-fold)>dopamine (+low GTP)=basal (+low GTP) (1.7-fold). The differences in the concentration-dependence of both the activation and inhibition of dopamine-stimulated and basal activities of the enzyme meant that increasing benzyl alcohol concentrations caused a net elevation in the fold-stimulation of the basal activity by dopamine. Arrhenius plots of p[NH]ppG-, GTP-, fluoride-, dopamine-plus-high GTP- and dopamine-plus-p[NH]ppG-stimulated activities all exhibited a single break occurring at around 22 degrees C. This break point was decreased to around 13 degrees C when 50mm-benzyl alcohol was added to the assays. In the presence of dopamine (+low GTP), Arrhenius plots exhibited two distinct breaks, one at around 21 degrees C and the other at around 11 degrees C. When benzyl alcohol (50mm) was added to these assays of dopamine (+low GTP)-stimulated activity, a single break at around 14 degrees C was observed. For the basal activity the Arrhenius plot exhibited a single break at around 15 degrees C both in the presence and in the absence of 50mm-benzyl alcohol. It is suggested that the enzyme is activated by productive collisions between independent mobile entities and that the activity of the enzyme may be regulated by changes in membrane fluidity. The breaks in the Arrhenius plots of all of the ligand-stimulated activities, but not the basal activity, are attributed to lipid-phase separations occurring in either the inner or the outer halves of the bilayer.

Attenuation of catecholamine-coupled adenylate cyclase following surgical isolation of rat caudate

Six weeks following complete unilateral surgical isolation of the rat caudate nucleus, activation of adenylate cyclase was reduced in response to dopamine (DA), norepinephrine (NE), 5' -guanylyl-imidodiphosphate [Gpp(NH)p], DA + Gpp(NH)p, and NaF. The low Km form of cyclic AMP phosphodiesterase was elevated in the isolated side when compared to the intact caudate. No changes in activities of guanylate cyclase or in high Km cyclic AMP phosphodiesterase (with or without the calcium-dependent regulator protein, calmodulin or CDR) were observed between the control and isolated caudate. Histologically, the neural damage to the isolated caudate was principally confined to reduced numbers of dendritic spines of the remaining intrinsic caudate neurons.

Postmortem stability of dopamine-sensitive adenylate cyclase, guanylate cyclase, ATPase, and GTPase in rat striatum

The stability of dopamine-sensitive adenylate cyclase, guanylate cyclase, ATPase, and GTPase was measured in homogenates of rat striatal tissue frozen from 0 to 24 h postmortem. ATPase, GTPase, and Mg2+-dependent guanylate cyclase activities showed no significant change over this period. Mn2+-dependent guanylate cyclase activity was stable for 10 h postmortem. Basal and dopamine-stimulated adenylate cyclase activity decreased markedly during the first 5 h. However, when measured in washed membrane preparations, these adenylate cyclase activities remained stable for at least 10 h. Therefore, the postmortem loss of a soluble activator, such as GTP, may decrease the adenylate cyclase activity in homogenates. These results are not consistent with an earlier suggestion that there is a postmortem degradation of the enzyme itself. Other kinetic parameters of dopamine-sensitive adenylate cyclase can also be measured independently of postmortem changes. Thus, it is possible to investigate kinetic parameters of dopamine-sensitive adenylate cyclase, guanylate cyclase, ATPase, and GTPase in human brain obtained postmortem.

Catecholamine-sensitive guanylate cyclase from human caudate nucleus

Partial purification of soluble guanylate cyclase on DEAE-Sephacel yields two separate peaks of guanylate cyclase activity. After 10-fold purification of the soluble enzyme, guanylate cyclase is markedly inhibited by micromolar concentrations of dopamine (I50 = 0.2 microM). Dopamine inhibition is observed whether the reaction is conducted with Mn2+ or with Mg2+, under atmosphere or N2(g), and using enzyme from either peak from the DEAE-Sephacel column. Other catecholamines also inhibit partially purified guanylate cyclase with an order of potency at 1 microM of: dopamine = L-DOPA > norepinephrine = isoproterenol = adrenochrome > epinephrine. The structural requirements for inhibition are two free hydroxyl groups on the phenyl ring and an ethylamine side chain. Dopamine also inhibits the Triton X-100-solubilized microsomal guanylate cyclase after partial purification on DEAE-Sephacel. Neither chlorpromazine, propranolol, nor phentolamine at 20 microM effectively block the dopamine inhibition of partially purified soluble guanylate cyclase. Micromolar concentrations of the reducing agents dithiothreitol and glutathione also inhibit partially purified guanylate cyclase, but unlike these agents, catecholamines can inhibit whether added in the reduced or the oxidized forms. Inhibition of enzyme activity by micromolar concentrations of dopamine, adrenochrome, or dithiothreitol is rapidly reversed by dilution and the dopamine inhibition is competitive with MgGTP. Inhibition does not appear to involve covalent binding or to result from the ability of catecholamines to reduce the concentrations of oxygen or free radicals in solution.