Interactions Between Vasoactive Intestinal Peptide and Dopamine in the Rabbit Retina: Stimulation of a Common Adenylate Cyclase

Although 3,4-dihydroxyphenylethylamine (dopamine, DA) and vasoactive intestinal peptide (VIP) have been reported to stimulate adenylate cyclase activity in the rabbit retina, possible interactions between VIP-sensitive and DA-sensitive adenylate cyclase systems have not been previously investigated. To elucidate the interactions between these two putative transmitter-stimulated cyclase systems, the effects of VIP, DA, and VIP + DA on the conversion of [alpha-32P]ATP to [32P]cyclic AMP in rabbit retinal homogenates were measured. VIP stimulated adenylate cyclase activity in a biphasic manner, suggesting that two classes of VIP receptors may be involved in the induction of cyclic AMP formation. DA was less potent than VIP, and stimulated cyclase activity with a monophasic dose-response curve. When assayed together, these stimulations were partially nonadditive, implying the existence of a common adenylate cyclase pool that may be stimulated by both putative neurotransmitters. The dopaminergic antagonist (+)-butaclamol completely blocked dopaminergic stimulation, but had no significant effect on VIP-induced stimulation, indicating that VIP interacts with specific VIP receptor sites, which are distinct from the dopaminergic receptor sites. Furthermore, the specific D-2 dopaminergic receptor agonist LY141865 demonstrated no inhibitory effect on adenylate cyclase activity, suggesting that the interaction between the VIP- and DA-sensitive adenylate cyclase systems does not result from a D-2 receptor-mediated cyclase inhibition in the rabbit retina. Finally, at maximally effective concentrations, DA and VIP were less potent than fluoride or forskolin in the stimulation of cyclic AMP formation, suggesting that adenylate cyclase pools that are not sensitive to DA and VIP may also be present in this retina.(ABSTRACT TRUNCATED AT 250 WORDS)     

Epinephrine and norepinephrine stimulation of adenylate cyclase in bovine retina homogenate: Evidence for interaction with the dopamine D1 receptor

Dopamine, epinephrine and norepinephrine provoke dose-dependent stimulation of adenylate cyclase activity in bovine retina homogenates. The stimulatory effect of all three compounds is inhibited by the D1 antagonist SCH 23390 and, in a stereoselective manner by the cisisomer of flupenthixol. The D2 antagonist haloperidol and alpha- and beta- adrenergic antagonists failed to block the catecholamine stimulation. These results evidence that, in bovine retina, not only dopamine but also epinephrine and norepinephrine interact with dopamine D1 receptors that are stimulatory coupled to an adenylate cyclase system.     

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.     

Role of Phospholipids in Coupling of Adenosine and Dopamine Receptors to Striatal Adenylate Cyclase

Treatment of striatal washed particles with phospholipase A(2) or C abolished the activation of adenylate cyclase by dopamine but not by N(16)-phenylisopropyl adenosine (PIA). The inhibition of dopamine-sensitive cyclase was dependent on Ca2+ and increased with time and phospholipase concentration. F(-)-sensitive cyclase was not affected by phospholipase A(2) treatment, but was enhanced by phospholipase C treatment. Phospholipase D did not affect basal, PIA, dopamine, or F(-)-sensitive cyclase activities. The observed effects of phospholipase A(2) were not due to either the detergent effect of lysophospholipids or to contaminating proteases. Dopamine-sensitive cyclase, inactivated by pretreatment with phospholipase A(2), was restored by asolectin (a soybean mixed phospholipid), phosphatidylcholine, phosphatidylethanolamine, or phosphatidylserine, but not by phosphatidylinositol. Phosphatidylserine and phosphatidylcholine were equipotent in restoring dopamine-sensitive activity. Lubrol-PX, a nonionic detergent, abolished completely the dopamine-sensitive cyclase activity, whereas PIA-sensitive activity was slightly inhibited. In contrast, digitonin inhibited dopamine- and PIA-sensitive cyclase activity in a parallel fashion. Lubrol-PX released some adenylate cyclase into a 16,000 x g supernatant fraction that was stimulated by PIA but not by dopamine. Removal of most of the free detergent by Bio-bead SM 2 enhanced stimulation by PIA but did not restore sensitive cyclase. The data suggest that the requirement for phospholipids for the coupling of dopamine and adenosine receptors to the striatal adenylate cyclase may be different and that the adenosine receptors may be more tightly coupled to the enzyme than are dopamine receptors.     

Serotonin sensitive adenylate cyclase activity in monkey anterior limbic cortex: antagonism by molindone and other antipsychotic drugs.

Serotonin (5-HT) sensitive adenylate cyclase in monkey anterior limbic cortex homogenates was further characterized and the effects of antipsychotic drugs and 5-HT anatagonists investigated. Differences in time course for stimulation by agonists and in responsiveness to receptor anatagonists of 5-HT-and dopamine (DA)-stimulated activities, were observed. Also there was an additivity of 5-HT and DA at maximally effective concentrations. Classical 5-HT antagonists blocked the 5-HT sensitive adenylate cyclase with a rank order of potency: methiothepin > cyproheptadine > methysergide. These 5-HT antagonists also effectively inhibited DA sensitive adenylate cyclase. Most antipsychotic drugs tested antagonized 5-HT stimulated activity although these drugs exhibited greater efficacies in blocking DA stimulated activity. Exceptions were molindone which failed to antagonize DA sensitive adenylate cyclase but effectively blocked 5-HT sensitive cyclase and pipamperone which was inactive in both cyclase systems. Haloperidol was a more selective antagonist of the DA sensitive adenylate cyclase than were the other antipsychotic drugs tested.     

Receptors coupled to adenylate cyclase in isolated rabbit retina

Intact pieces or homogenates of rabbit retina were exposed to various established or putative retinal neurotransmitters for the study of receptors (or receptor-subtypes) linked to the production of cAMP. Since a dopamine-sensitive adenylate cyclase has been previously characterized in the retina of several species, the novel D₁-agonist SKF 38393 was applied under similar experimental conditions. This compound was found to be more potent (although less efficacious) than dopamine, confirming the existence of a population of D₁-receptors. On the other hand, the novel D₁-antagonist SCH 23390 was able to inhibit the stimulating effects of dopamine and of SKF 38393 in a concentration-dependent manner. Attempts to detect D₂-receptors (negatively coupled with adenylate cyclase) were not conclusive, when a selective D₂-agonist (quinpirole) was applied in the absence or presence of a D₂-antagonist (sulpiride).

A stimulation of cAMP production (mediated by A₂-receptors) was also detected in response to adenosine or adenosine-analogs, which was blocked by IBMX in a concentration-dependent manner. The effects of adenosine were potentiated in the presence of dipyridamole, an adenosine uptake inhibitor. Compared to the effects of dopamine and adenosine, the stimulation induced by VIP, a retinal neuropeptide, was found to be much more pronounced.

These results indicate that retinal cAMP can be generated by three different neurotransmitters in an independent way via the stimulation of specific receptors.     

Inhibition of pituitary adenylate cyclase by atrial natriuretic factor

The effect of synthetic rat atrial natriuretic factor (ANF) on adenylate cyclase activity was studied in rat anterior and posterior pituitary homogenates. ANF (Arg 101-Tyr 126) inhibited adenylate cyclase activity in anterior and posterior pituitary homogenates in a concentration dependent manner. The maximum inhibitions observed were 42% in anterior pituitary with an apparent Ki of 10(-10) M, and 25% with an apparent Ki of 10(-11) M in posterior pituitary. Corticotropin-releasing factor (CRF), vasoactive intestinal peptide (VIP) and prostaglandins (PGE1) stimulated adenylate cyclase to various degrees in anterior pituitary homogenates and ANF inhibited the stimulatory effect of all these hormones. In addition ANF was also able to inhibit the stimulation exerted by NaF and forskolin which activate adenylate cyclase by receptor independent mechanism. Similarly, the stimulatory effects of N-Ethylcarboxamide adenosine (NECA), NaF and forskolin on adenylate cyclase in posterior pituitary homogenates were also inhibited by ANF. This is the first study demonstrating the inhibitory effect of ANF on pituitary adenylate cyclase.     

Regulation of Calmodulin- and Dopamine-Stimulated Adenylate Cyclase Activities by Light in Bovine Retina

Abstract: Neural retina from most species contains 3,4-dihydroxyphenylethylamine (dopamine) receptors coupled to stimulation of adenylate cyclase activity. It has been demonstrated that release of dopamine from its neurons and subsequent occupation of dopamine receptors is increased by light. In this study, we have shown that adenylate cyclase activity in bovine retina is highly responsive to the endogenous Ca²⁺-binding protein, cal-modulin, and that calmodulin can increase dopamine-sen-sitive adenylate cyclase activity in bovine retina. We further demonstrate that both dopamine- and calmodulin-stimulated adenylate cyclase activities can be regulated by alterations in light. Bovine retinas were dissected from the eye under a low-intensity red safety light, defined as dark conditions, and incubated for 20 min in an oxygenated Krebs Henseleit buffer under either dark or light conditions. The retinas were then homogenized and adenylate cyclase activity measured in a paniculate fraction washed to deplete it of endogenous Ca²⁺ and calmodulin. Activation of adenylate cyclase activity by calmodulin, dopamine, and the nonhydrolyzable GTP analog, gua-nosine-5′-(β,γ-imido)triphosphate (GppNHp), was significantly (60%) greater in paniculate fractions from retinas that had been incubated under dark conditions as compared to those incubated under light conditions. Basal, Mn²⁺-, and GTP-stimulated adenylate cyclase activities were not altered by changes in lighting conditions. Calmodulin could increase the maximum stimulation of adenylate cyclase by dopamine in retinas incubated under either dark or light conditions, but the degree of its effect was greater in retinas incubated under light conditions. Activation of adenylate cyclase by calmodulin, dopamine, and GppNHp in paniculate fractions from retinas incubated under light conditions was indistinguishable from the activation obtained when retinas were incubated in the dark in the presence of exogenous dopamine. These results suggest that an increased release of dopamine occurs in light. The decreased response of adenylate cyclase to exogenous dopamine can then be explained by a subsequent down-regulation of dopamine receptor activity. The down-regulation of dopamine receptor activity can also regulate activation of adenylate cyclase by GppNHp and calmodulin. The results suggest that dopamine, calmodulin, and GppNHp are modulators of a common component of adenylate cyclase activity, and this component is regulated by light.     

Inhibition by S-adenosyl-L-homocysteine of dopamine dependent adenylate cyclase in rat retina

S-adenosyl-L-homocysteine (S-AH), a potent inhibitor of biological transmethylation, decreased the response of rat retina adenylate cyclase to dopamine and to 2-amino-6, 7-dihydroxytetrahydronaphtalene (ADTN). This effect appeared for 10^?7M of S-adenosyl-L-homocysteine and was linear for concentration ranging to 10^?4M. S-adenosyl-L-homocysteine did not decrease the cyclic AMP accumulation with sodium fluoride, a non specific adenylate cyclase activator. On the other hand, the incorporation of methyl group was reduced in rat retina homogenates by S-adenosyl-L-homocysteine. These findings suggest that the activity of the dopamine dependent adenylate cyclase is linked to a methylation process.     

Adenylate Cyclases in Vertebrate Retina: Enzymatic Characteristics in Normal and Dystrophic Mouse Retina

Adenylate cyclase activity and the effects of various activators and inhibitors of this enzyme were measured in retinas from normal mice (C57BL/6J) and congenic animals with photoreceptor dystrophy. In normal retina, approximately 250 microM-ATP was required for half-maximal stimulation of the enzyme. Activity was supported by Mg2+ and Mn2+, but Ca2+ was ineffective. The enzyme was inhibited by EGTA and stimulated by 5'-guanylylimidodiphosphate (GPP(NH)P), dopamine, and NaF. The stimulatory effects of GPP(NH)P and dopamine were greater in the presence of EGTA. Examination of microdissected normal retinas revealed that the inner (neural) retina had adenylate cyclase activity four times that of the photoreceptor cell layers, and that EGTA inhibited activity in the inner retina, but had no effect in the outer retina. In dystrophic retinas basal enzyme activity was 60% higher than that in normal retina. The enzyme in this tissue was stimulated by EGTA, GPP(NH)P, and dopamine, and their effects were additive. These results indicate that adenylate cyclase activity in vertebrate retina is under complex regulation by substrate, divalent cations, guanine nucleotides, dopamine, and perhaps calmodulin. In addition, the data demonstrate that adenylate cyclase is not evenly distributed in the retina and that it is regulated differently in the inner and outer retina. Finally, the present results indicate that regulation of this enzyme in dystrophic retina may be qualitatively and quantitatively different from that in normal retina.     

6,7-Dihydroxy-2-dimethylaminotetralin (TL-99) stimulates postjunctional D-1 and D-2 dopamine receptors

6,7-Dihydroxy-2-dimethylaminotetralin (TL-99) mimicks the ability of dopamine either to enhance adenylate cyclase activity in homogenates of goldfish retina or to inhibit adenylate cyclase activity in homogenates of the intermediate lobe (IL) of the rat pituitary gland previously treated with cholera toxin. Both the dopamine stimulated adenylate cyclase activity in the fish retina and the dopamine inhibited adenylate cyclase activity in the rat IL are associated with cells postjunctional to the dopaminergic neurons innervating these tissues. Therefore, the present data do not support the contention that TL-99 is a selective presynaptic dopamine receptor agonist.     

Dopamine-sensitive adenylate cyclase and cAMP phosphodiesterase in substantia nigra and corpus striatum of rat brain.

A dopamine-sensitive adenylate cyclase has been identified in substantia nigra of the rat brain. Low concentrations of dopamine markedly increased the accumulation of cyclic AMP while l-norepinephrine and isoproterenol had little effect at concentrations up to 100 μM. The phenothiazine, trifluoperazine, is a potent inhibitor of the s. nigral adenylate cyclase (I.C.₅₀ of 0.1 μM), while the adrenergic receptor blocking agents propranolol and phentolamine are ineffective. Basal activity of striatal adenylate cyclase and cAMP phosphodiesterase is approximately 3-fold higher than that found in substantia nigra.     

Adenylate Cyclase Activity in the Superior Cervical Ganglion of the Rat

Abstract: Adenylate cyclase activity in cell-free homogenates of the rat superior cervical ganglion (SCG) was assayed under a variety of experimental conditions. Adenylate cyclase activity was decreased by approximately one-half when 1 m M EGTA was included in the homogenization buffer and assay mixture, indicating the presence of a Ca²⁺-sensitive adenylate cyclase in the ganglion. In the presence of EGTA, basal adenylate cyclase activity in homogenates of the SCG was 12.9 ± 0.6 pmol cyclic AMP/ganglion/10 min. Enzyme activity was stimulated three- to fourfold by 10 m M NaF or 10 m M MnCl₂, Both GTP and its nonhydrolyzable analog guanylylimidodiphosphate (GppNHp) stimulated adenylate cyclase in a concentration-dependent manner over the range of 0.1–10.0 μ M . Stimulation by GppNHp was five to six times greater than that produced by GTP at all concentrations tested. Decentralization of the ganglion had no effect on basal or stimulated adenylate cyclase activity. Receptor-linked stimulation of adenylate cyclase was not obtained with any of the following: isoproterenol, epi-nephrine, histamine, dopamine, prostaglandin E₂, or va-soactive intestinal peptide. Thus the receptor-linked regulation of adenylate cyclase activity appears to be lost in homogenates of the ganglion.     

Activation of adenylate cyclase from rat striatum and tuberculum olfactorium by adenosine

Adenosine caused a dose-dependent stimulation of adenylate cyclase in homogenates from rat striatum and tuberculum olfactorium (200 and 300% stimulation by 100 muM adenosine). The effect of adenosine was not antagonized by haloperidol. Subcellular fractionation suggested that adenosine stimulates a different adenylate cyclase than dopamine. Basal adenylate cyclase activity in freshly prepared homogenates was reduced by dialysis and by the addition of adenosine deaminase. Basal adenylate cyclase activity was enchanced by papaverine and dipyridamole, but reduced by theophylline and isobutylmethylxanthine. The results are compatible with the opinion that endogenous adenosine is capable of activating adenylate cyclase in these areas of the rat brain.     

A dopamine sensitive adenylate cyclase in the salivary glands of Amblyomma americanum (L.)

1. Enzyme activity, basal or dopamine-stimulated (10 microM), was linear with time to 25 min and with protein concentration to 0.8 mg protein/ml of final assay volume. Activity was maximal between pH 7.0 and 7.5. 2. Mg2+ maximally stimulated basal or dopamine-sensitive adenylate cyclase activity at about 4 mM. 3. Adenylate cyclase had a Km of 0.042 mM for ATP and maximum velocities for basal and dopamine-stimulated activity of 107 and 179 pmol cyclic AMP formed/mg protein per min, respectively. 4. Half-maximal stimulation of the enzyme occurred at about 4.2 x 10(-7) M dopamine with the threshold being less than 10(-9) M. Dopamine increased the Vmax but had no effect on the Km of ATP. 5. Eighty-five to 90% of the adenylate cyclase activity was found in the particulate fraction. 6. Calcium ion produced a marked inhibition of adenylate cyclase activity above 0.04 mM and half-maximal inhibition occurred near 0.1-0.2 mM.     

Photolytic studies on the carbon monoxide complex of sulphaemoglobin.

Salivary-gland homogenates contain 5-hydroxytryptamine-stimulated adenylate cyclase. Half-maximal stimulation was obtained with 0.1 microM-5-hydroxytryptamine in the presence of added guanine nucleotides. Gramine antagonized the stimulation of cyclase caused by 5-hydroxytryptamine. In the presence of hormone, guanosine 5'-[gamma-thio]triphosphate produced a marked activation of adenylate cyclase activity. Stimulation of adenylate cyclase by forskolin or fluoride did not require the addition of guanine nucleotides or hormone. In the presence of EGTA, Ca2+ produced a biphasic activation of cyclase activity. Ca2+ at 1-100 microM increased activity, whereas 2000 microM-Ca2+ inhibited cyclase activity. The neuroleptic drugs trifluoperazine and chlorpromazine non-specifically inhibited adenylate cyclase activity even in the absence of Ca2+. The cyclic AMP phosphodiesterase activity in homogenates was not affected by Ca2+ or exogenous calmodulin. This enzyme was also inhibited by trifluoperazine in the absence of Ca2+. These results indicate that Ca2+ elevates adenylate cyclase activity, but had no effect on cyclic AMP phosphodiesterase of salivary-gland homogenates.     

Functional corticotropin-releasing factor (CRF) receptors in mouse spleen: evidence from adenylate cyclase studies

Radioligand binding studies have previously identified a high affinity, magnesium-dependent, guanine nucleotide-sensitive binding site for corticotropin-releasing factor (CRF) in mouse spleen. In order to determine the functional nature of these CRF binding sites, we examined the effects of CRF on adenylate cyclase activity in mouse spleen homogenates. The stimulation of adenylate cyclase activity was dependent on time, tissue protein concentration, and guanine nucleotides. CRF-stimulated adenylate cyclase activity was evident in the presence of guanosine-5'-triphosphate (GTP) and its precursor guanosine-5'-diphosphate (GDP) but was not detected in the presence of the hydrolysis-resistant GTP analogs, guanyl-5'-imidodiphosphate [Gpp(NH)p] and guanosine-5'-gamma-thiotriphosphate (GTP-gamma-S). The rank order of potency for CRF analogs and fragments in stimulating adenylate cyclase activity was comparable to their affinities for CRF binding sites in mouse spleen homogenates. The putative receptor antagonist, alpha helical ovine CRF(9-41), did not stimulate adenylate cyclase activity but did attenuate the stimulation by various concentrations of rat/human CRF. In summary, these data demonstrate the functional nature of CRF receptors in mouse spleen as evidenced by CRF stimulation of cAMP production and suggest that this peptide may play a physiological role in regulating immune function.     

Effects of Vasoactive Intestinal Peptide and Other Peptides on Cyclic AMP Accumulation in Intact Pieces and Isolated Horizontal Cells of the Teleost Retina

The effects of vasoactive intestinal peptide (VIP) and several other peptides have been examined on cyclic AMP accumulation in intact pieces and isolated horizontal cells of the teleost (carp) retina. VIP was the most effective peptide examined, inducing a dose-related response, and an approximately fivefold increase in cyclic AMP production when used at a concentration of 10 microM. Porcine histidine isoleucine-containing peptide and secretin, peptides structurally related to VIP, also stimulated cyclic AMP accumulation, but at concentrations of 10 microM induced responses which were only approximately 40% and 10%, respectively, of the response observed with 10 microM VIP. In contrast, several other peptides, including glucagon, neurotensin, somatostatin, luteinizing hormone-releasing hormone, alpha-melanocyte-stimulating hormone, cholecystokinin octapeptide26-33, gastrin-releasing peptide, thyrotropin-releasing hormone, and VIP10-28 were totally inactive. The response to 10 microM VIP was not antagonized by several dopamine antagonists, indicating the presence of a population of specific VIP receptors coupled to adenylate cyclase, distinct from the population of dopamine receptors coupled to adenylate cyclase also known to be present in this tissue. Finally, experiments involving the use of fractions of isolated horizontal cells indicate that these neurons possess a population of VIP receptors coupled to cyclic AMP production which would appear to share a common pool of adenylate cyclase with a population of similarly coupled dopamine receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Basal, dopamine- and octopamine-stimulated adenylate cyclase activity in the brain of the moth, M amestra configurat A, during its metamorphosis.

—The specific activities of basal, dopamine- and octopamine-stimulated adenylate cyclase in the brain of the moth, Mamestra configurata, increased by an order of magnitude or more during the transition from pupal to adult brain. Increases in adenylate cyclase activity outpaced increases in brain weight and protein content by a wide margin, especially during the last 10 days of metamorphosis when most of the change in enzyme activity occurred. The greatly enhanced adenylate cyclase activity appears to reflect increased demands placed on the adult brain as an integrative centre, especially in the integration of sensory input from the eyes and antennae. Growth of the optic lobes during brain development contributed a substantial proportion of basal (33%), dopamine-sensitive (21%) and octopamine-sensitive (37%) adenylate cyclase activity to the adult brain. High levels of octopamine and the presence of active octopamine-sensitive adenylate cyclase suggest a cyclic AMP-mediated physiological function for octopamine in the optic lobes of the insect brain, probably as a neurotransmitter or neuromodulator.     

Regulation of Adenosine-Sensitive Adenylate Cyclase from Rat Brain Striatum

An adenosine-sensitive adenylate cyclase has been characterized from rat brain striatum. In whole homogenates as well as in particulate fractions, N⁶-phenylisopropyl adenosine (PIA), 2-chloroadenosine, and adenosine N′-oxide were equipotent in stimulating adenylate cyclase. Although GTP inhibited basal as well as PIA-stimulated activity of whole homogenates, the enzyme showed an absolute dependency on GTP for stimulation by PIA, dopamine, epinephrine, and norepinephrine in a particulate fraction derived from discontinuous sucrose gradient centrifugation. Adenosine exerts two effects on this adenylate cyclase, stimulation at low concentrations and inhibition at high concentrations, suggesting the presence of two adenosine binding sites. The stimulation of adenylate cyclase by PIA was dependent on the concentration of Mg^2-. The degree of stimulation by PIA was greater at a low concentration of Mg²⁺, which suggests that stimulation by PIA was accompanied by increasing the apparent affinity for Mg²⁺. Activation of adenylate cyclase by PIA was blocked by theophylline or 3-isobutyl- 1-methylxanthine (IBMX). The pH optimum for basal or (PIA + GTP)-stimulated activities was broad, with a peak between 8.5 and 9.5. In the presence of GTP, stimulation by an optimal concentration of PIA was additive, with maximal stimulation by the catecholamines. Phospholipase A₂ treatment at a concentration of 1 U/ml for 5 min completely abolished the stimulatory effect of dopamine, whereas PIA-stimulated activity remained unaltered. These data suggest that rat brain striatum either has a single adenylate cyclase, which is stimulated by catecholamines and adenosine by distinct mechanisms, or has different cyclase populations, stimulated by either adenosine or catecholamines.