Effects of dopaminergic drugs on the concentrations of PGE2 and PGF2alpha in various brain regions

It has long been shown by Biggio and Guidotti that multisynaptic nigro-cerebellar pathway of dopaminergic origin can control cerebellar cyclic guanosinmonophosphate (cGMP) content, a good index of the activity of Purkinje cells. In this line, it has been reported that haloperidol and sulpiride, significantly decrease cerebellar cGMP content while opposite changes are observed with apomorphine. In an attempt to establish whether other cerebellar cGMP-related parameters may be influenced by dopamine drugs. Authors have investigated the effects of haloperidol, sulpiride and apomorphine on cerebellar PGE2 and PGF2alpha. Results obtained indicate that haloperidol and sulpiride significantly reduce cerebellar PGE2 and PGF2alpha content while opposite changes are induced by apomorphine. Similar results have been observed in substantia nigra but not in other brain regions, such as corpus striatum and medial basal hypothalamus. The possibility that the observed changes in cerebellar PG-content may result from the modulation of striatal dopamine receptors is discussed.     

Effect of chronic treatment with neuroleptics on the content of 3',5'-cyclic guanosine monophosphate in cerebellar cortex of rats.

Chronic treatment with haloperidol is associated with complete tolerance to the decreasing effect of the neuroleptic on cerebellar cGMP content, vice versa chronic haloperidol causes hypersensitivity to the enhancing effect of apomorphine on cerebellar cGMP. Thus, the administration of 0.5 mg/Kg of haloperidol decreases cerebellar cGMP by 80% in control rats but fails to alter this nucleotide in rats chronically treated with haloperidol (0.5 mg/Kg twice daily for 20 days). A dose of 0.5 mg/Kg of apomorphine enhances cGMP by approximately 25 and 60 percent in control rats and in rats chronically treated with haloperidol, respectively. The results suggest that: a) There is a functional link between striatum and cerebellum; b) Cerebellar cGMP is a sensitive index of the state of activation of striatal dopamine receptors.     

NEUROCHEMICAL PARAMETERS IN THE HYPERRESPONSIVE PHASE AFTER A SINGLE DOSE OF NEUROLEPTICS TO MICE

Abstract— Four days after a single dose of teflutixol (5 mg/kg i.p.), at which time mice are superresponsive to dopamine agonists, e.g. apomorphine, the specific binding of [³H]haloperidol, [³H]cis (Z)-flupenthixol, [³H]apomorphine, [³H]dopamine, [³H]propylbenzilylcholine mustard and [³H]GABA to striatal membranes in vitro is equal to that of saline-treated mice. Specific binding of [³H]haloperidol is also unchanged 3 days following a single dose of fluphenazine (5mg/kg i.p.) and 2 days following haloperidol (5 mg/kg i.p.), but slightly decreased 3 days following cis(Z)-flupenthixol (5 mg/kg i.p.).
The possibility that remaining neuroleptic or active metabolites could obscure a slight increase in dopamine receptor binding was rejected, since remaining amounts of [³H]teflutixol in the final binding assay 4 days after intraperitoneal injection of [³H]teflutixol (5 mg/kg) were too small to influence the binding of [³H]haloperidol in vitro .
It is concluded that the pharmacological superresponsiveness and the decrease in dopamine synthesis and release seen after the initial receptor blockade following a single dose of neuroleptic drugs in mice are nor accompanied by changes in dopamine, muscarine or GABAergic receptor characteristics in corpus striatum. The possibility that changes occur in a small number of functional operative dopamine receptors cannot be excluded, however.     

Effects of dopaminergic agonists and antagonists on cerebellar guanosine-3',5'-monophosphate (cGMP).

Apomorphine was found to cause an increase in cerebellar cGMP content. Bromocriptine, at a dose that caused stereotypies, neither elevated cGMP, nor blocked the apomorphine- induced rise in cGMP. The apomorphine-induced rise in cGMP was effectively blocked by haloperidol and some other neuroleptics, but not by sulpiride. These actions of the neuroleptics correlated with their ability to displace ³H-spiroperidol from striatal membranes, suggesting that dopamine receptor interactions were important in the cGMP changes noted. Based on the observation that haloperidol antagonized the increase induced by restraint, it is suggested that dopaminergic systems are involved in the reaction to stress.     

Activation of D1 and D2 Dopamine Receptors Inhibits Protein Kinase C Activity in Striatal Synaptoneurosomes

Abstract: The effects of D₁ and D₂ dopamine ligands on protein kinase C (PKC) activity were examined in synaptoneurosomes. Incubation with D₁ agonists (SKF 38393, fenodopam), in the presence of calcium, decreased the soluble and increased the particulate PKC activity. These effects were reversed by SCH 23390, which by itself had the opposite effect of increasing the soluble and decreasing the particulate PKC activity. In contrast, incubation with the D₂ agonists [LY 171555, (+)-3-(3-hydroxyphenyl)- N - n -propylpiperidine, RU 24213] increased the soluble and decreased the particulate PKC activity. These effects were reversed by sulpiride. (−)-3-(3-Hydroxyphenyl)- N - n -propylpiperidine had a D₂ antagonist profile. Apomorphine showed a biphasic dose-response change; i.e., it decreased particulate PKC activity at the D₂ receptor at low concentrations (0.1 µ M ) and increased it at the D₁ receptor at higher concentrations (10 µ M ). Pretreatment with tetrodotoxin or omission of calcium in the incubation medium did not alter the responses of the D₂ agonists, but it reversed the changes in PKC activity induced by the D₁ agonists and converted the biphasic response of apomorphine to a monophasic inhibition. These results indicate that (1) D₁ and D₂ dopamine receptors are negatively coupled to PKC and (2) the increase in particulate PKC activity seen with the D₁ drugs in the presence of calcium is mediated indirectly via a transneuronal effect.     

Dopaminergic Regulation of Cannabinoid Receptor mRNA Levels in the Rat Caudate-Plitamen: An In Situ Hybridization Study

Abstract: By quantitative in situ hybridization, we examined in vivo in the rat caudate-putamen the effects on levels of cannabinoid receptor mRNA of an interruption of dopamine neurotransmission for up to 1 month, by either 6-hydroxydopamine lesioning of the medial forebrain bundle or dopamine receptor blockade. We found, in a first set of experiments, that unilateral 6-hydroxydopamine dopa-minergic deafferentation of the striatum (characterized by a contralateral turning behavior in response to apomor-phine, the almost complete disappearance of the tyrosine hydroxylase hybridization signal in the substantia nigra, and an increase of preproenkephalin A mRNA level in the striatum) was associated with significantly increased (45%) cannabinoid receptor mRNA levels in the homolateral caudate-putamen. In a second set of experiments, treatments with the dopamine D₁ receptor antagonist SCH-23390, haloperidol, and the D₂ receptor antagonist sulpiride induced significantly higher cannabinoid receptor mRNA levels (respectively, 67, 34, and 27%) in the caudate-putamen. These observations suggest for the first time that, in vivo, cannabinoid receptor gene expression in the caudate-putamen is under the negative control of dopamine receptor-mediated events.     

Chronic Dopamine D2 Receptor Activation Does Not Affect Survival and Differentiation of Cultured Dopaminergic Neurons: Morphological and Neurochemical Observations

Abstract: Primary cultures of rat ventral mesencephalon were used to elucidate the role of chronic stimulation of dopamine (DA) D₂ autoreceptors in the development of fetal dopaminergic neurons in vitro. Cultured dopaminergic neurons, as visualized by tyrosine hydroxylase immunocytochemistry, became more differentiated in the course of cultivation time and exhibited specific high-affinity uptake for [³H]DA. In rat striatal tissue, activation of D₂ receptors has been shown to inhibit the release of DA. Previously accumulated [³H]DA was released from the cultures upon depolarization in a Ca²⁺-dependent manner. K⁺-evoked [³H]DA release could be inhibited by the selective D₂ receptor agonists LY 171555 and N0437 in a concentration-dependent manner. The inhibitory effects of LY 171555 and N0437 were antagonized by the selective DA D₂ receptor antagonist sulpiride. These observations are indicative for the expression of functional D₂ receptors in the cultures. Daily treatment of these cultures for 7 days with LY 171555 or sulpiride did not lead to any change in protein content, the number of tyrosine hydroxylase-immunoreactive neurons, or the uptake capacity for [³_H]DA. Our data demonstrate that chronic stimulation of DA D₂ receptors does not impair survival or differentiation of cultured fetal dopaminergic neurons.     

D2 Receptors May Modulate the Function of the Striatal Transporter for Dopamine: Kinetic Evidence from Studies In Vitro and In Vivo

Abstract: Recently it was hypothesized by others that the D₂dopamine receptor can regulate the uptake of dopamine. However, the evidence in support of this hypothesis, although compelling, was not based on observations related to direct measures of the kinetic activity of the transporter itself. Here kinetic evidence in support of this hypothesis is shown. The apparent time-resolved initial velocity of the transport of 1.0 μ M dopamine into striatal suspensions, measured using rotating disk electrode voltammetry, was found to increase in the presence of the D₂ receptor agonist, quinpirole, at 100 μ M . This effect was reversed by sulpiride. In separate studies it was shown that acute and chronic treatments with haloperidol at 0.5 mg/kg, i.p., reduced the reuptake transport of dopamine in vivo following intrastriatal stimulation of its release by K⁺. Thus, it appears that D₂ receptors may influence the functioning of the striatal transporter for dopamine. These results are consistent with a model in which presynaptically released dopamine may feed back onto the function of its transporter to increase the velocity of the clearance of synaptic dopamine following an action potential, suggesting the existence of a mechanism, in addition to release and synthesis modulation, for fine-tuning dopaminergic chemical signaling.     

Effects of (−)-Sulpiride on Dopamine Release in Striatum of Developing Rats: Degree of Depolarization Influences Responsiveness

Abstract: The purpose of this study was to determine the effects of localized delivery of the D₂ antagonist (−)-sulpiride (via microdialysis) on spontaneous and evoked dopamine release in the neostriatum of urethane-anesthetized rats 5, 10, 15, 21, and 70 days of age. Sulpiride increased spontaneous dopamine release approximately threefold relative to baseline measures, and this effect decreased with maturation. The relationship between sulpiride- and potassium-evoked release was complex; sulpiride increased evoked dopamine outflow at 5, 10, and 15 days of age. At 21 and 70 days of age, however, the effects of sulpiride were inversely related to the degree of stimulation with potassium. Furthermore, the D₂ agonist quinpirole (100 µ M ) reversed the effects of sulpiride (10 µ M ), suggesting receptor mediation. These experiments demonstrate that the maturational decline in the efficacy and potency of D₂ antagonism appears to be related to the degree of stimulation at the nerve terminal.     

Specific Receptor-Mediated Inhibition of Cyclic AMP Synthesis by Dopamine in a Neuroblastoma × Brain Hybrid Cell Line NCB-20

Abstract: Dopamine and dopamine receptor agonists were found to inhibit adenylate cyclase activity dose-de-β ndently in a neuroblastoma × Chinese hamster brain explant hybrid cell line NCB-20. Apomorphine (with an IC₅₀ value of 10 n M ) was the most effective inhibitor, followed by 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydro-naphthaline (ADTN), dopamine, and N -dipropyldopa-mine. The inhibition was potently reversed by sulpiride, butaclamol, and flupenthixol in a stereospecific manner, but was unaffected by yohimbine, except at high concentrations. Clonidine also inhibited adenylate cyclase activity in these cells and this was reversed by the α₂-adrenoreceptor antagonist yohimbine, but not by sulpiride. [ d -Ala², d -Leu⁵]Enkephalin inhibited adenylate cyclase activity in NCB-20 cells at nanomolar concentrations; this was reversed by naloxone. All three inhibitory neurotransmitters were able to reverse the stimulation of cyclic AMP synthesis by serotonin or prostaglandin E₁The dopamine receptor that modulates cyclic AMP synthesis in NCB-20 cells is pharmacologically quite distinct from a high-affinity spiperone binding site identified in these cells, but shows the pharmacologic specificity of the D₂ receptor previously described in mammalian brain.     

Dopamine Agonist-Mediated Inhibition of Acetylcholine Release in Rat Striatum Is Modified by Thyroid Hormone Status

Abstract: K⁺-evoked acetyl[³H]choline ([³H]ACh) release was inhibited in a concentration-dependent manner by apomorphine and the D₂ agonist quinpirole in striatal slices prepared from euthyroid and hypothyroid rats. However, there was a significant increase in the maximum inhibition observed with both agonists in the hypothyroid compared with the euthyroid group, which paralleled the increased D₂ agonist sensitivity reported for stereotyped behavior. The D₂ antagonist raclopride decreased, and the D, antagonist SCH 23390 increased, the inhibition of [³H]ACh release by apomorphine, confirming an inhibitory role for D₂ receptors and an opposing role for D₁ receptors. Because there is no difference in D₁ or D₂ receptor concentration between the euthyroid and hypothyroid groups, it is suggested that thyroid hormone modulation of D₂ receptor sensitivity affects a receptor-mediated event. Following intrastriatal injection of pertussis toxin (PTX), apomorphine no longer inhibited [³H]ACh release. In fact, increased [³H]- ACh release was observed, an effect reduced by SCH 23390, providing evidence that D₁ receptors enhance [³H]- ACh release, and confirming that a PTX-sensitive G protein mediates the D₂ response. As it has been reported that thyroid hormones modulate G protein expression, this mechanism may underlie their effect on dopamine agonist- mediated inhibition of ACh.     

Benzodiazepine Antagonist Ro 15–1788: Binding Characteristics and Interaction with Drug-Induced Changes in Dopamine Turnover and Cerebellar cGMP Levels

Abstract: The recently discovered benzodiazepine antagonist Ro 15-1788 was characterized in binding studies, and its potency and selectivity were determined in vivo by interaction with drug-induced changes in dopamine turnover and cerebellar cGMP level. Ro 15-1788 reduced [³H]flunitrazepam binding in the brain in vivo with a potency similar to that of diazepam and effectively inhibited [³H]diazepam binding in vitro (IC₅₀= 2.3 ± 0.6 nmol/liter). [³H]Ro 15-1788 bound to tissue fractions of rat cerebral cortex with an apparent dissociation constant ( K _D) of 1.0 ± 0.1 nmol/liter. The in vitro potency of various benzodiazepines in displacing [³H]Ro 15-1788 from its binding site was of the same rank order as found previously in [³H]diazepam binding. Autoradiograms of [³H]Ro 15-1788 binding in sections of rat cerebellum showed the same distribution of radioactivity as with [³H]flunitrazepam. The attenuating effect of diazepam on the chlorpromazine- or stress-induced elevation of homovanillic acid in rat brain was antagonized by Ro 15-1788. Among a series of compounds which either decreased or increased the rat cerebellar cGMP level, only the effect of benzodiazepine receptor ligands (diazepam, zopiclone, CL 218 872) was antagonized by Ro 15-1788. Thus, Ro 15-1788 is a selective benzodiazepine antagonist acting at the level of the benzodiazepine receptor in the central nervous system. Peripheral benzodiazepine binding sites in kidney and schistosomes were not affected by Ro 15-1788.     

Comparative Effects of Chronic Haloperidol and Sulpiride Treatment on Nigral and Striatal GABA Content

Nigral and striatal GABA contents were assayed in male rats treated chronically with haloperidol or sulpiride, two dopamine-receptor blocking agents that have different neuropharmacological spectra in regard to their biochemical, behavioural, and clinical properties. No great difference was observed between the chronic effects of haloperidol and sulpiride on nigral and striatal GABA content. However, low doses (30 μg/kg, intraperitoneally) of the dopamine-receptor agonist apomorphine, injected 12 h after the discontinuation of chronic haloperidol or chronic sulpiride treatment, induced opposite changes in nigral GABA levels suggesting the existence of a different “status” of the dopamine receptors during the 12 h-period following the withdrawal of haloperidol or sulpiride.     

Serotonin 5-HT1D and 5-HT1A Receptors Respectively Mediate Inhibition of Glutamate Release and Inhibition of Cyclic GMP Production in Rat Cerebellum In Vitro

Abstract: The K⁺-evoked overflow of endogenous glutamate from cerebellar synaptosomes was inhibited by serotonin [5-hydroxytryptamine (5-HT); pD₂ = 8.95], 8-hydroxy-2-(di- n -propylamino)tetralin (8-OH-DPAT; pD₂ = 7.35), and sumatriptan (pD₂ = 8.43). These inhibitions were prevented by the selective 5-HT_1D receptor antagonist N -[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)(1,1-biphenyl)-4-carboxamide (GR-127935). The three agonists tested also inhibited the cyclic GMP (cGMP) response provoked in slices by K⁺ depolarization; pD₂ values were 9.37 (5-HT), 9.00 (8-OH-DPAT), and 8.39 (sumatriptan). When cGMP formation was elevated by directly activating glutamate receptors with NMDA or α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), the inhibition of the cGMP responses displayed the following pattern: 5-HT (pD₂ values of 8.68 and 8.72 against NMDA and AMPA, respectively); 8-OH-DPAT (respective pD₂ values of 9.15 and 9.00); sumatriptan (0.1 µ M ) was ineffective. The 5-HT_1A receptor antagonist ( S )-(+) N-tert -butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl]-2-phenylpropionamide dihydrochloride [(+)-WAY 100135] did not prevent the inhibition of glutamate release by 5-HT but blocked the inhibition by 8-OH-DPAT of the NMDA/AMPA-evoked cGMP responses. It is suggested that presynaptic 5-HT_1D receptors mediate inhibition directly of glutamate release and indirectly of the cGMP responses to the released glutamate; on the other hand, activation of (postsynaptic) 5-HT_1A receptors causes inhibition of the cGMP responses linked to stimulation of NMDA/AMPA receptors.     

The effect of drugs which alter GABA-ergic function on cerebellar guanosine-3',5'-monophosphate content.

The actions of several classes of drugs, thought to be involved with gamma-amino-butyric acid (GABA) mechanisms, have been examined for effects on cerebellar cGMP content. Picrotoxin and TRH increased, while ethanol and diazepam decreased, cerebellar cGMP. Doses of gamma-hydroxybutyrate (GHB) and baclofen caused no significant effect at doses that caused behavioral changes. These cGMP actions were contrasted with those induced by the dopaminergic agents, apomorphine and haloperidol, which respectively, raised and lowered cerebellar cGMP. Apomorphine-induced increases in cGMP were blocked by haloperidol, but not by GHB or baclofen given eight min before sacrifice. However, baclofen given one hour before sacrifice caused effects similar to those of haloperidol. These results are discussed in terms of dopaminergic-GABAergic interactions.     

Dopaminergic Regulation of Septohippocampal Cholinergic Neurons

Abstract: The extent to which acetylcholine (ACh) release in the hippocampus is regulated by dopaminergic mechanisms was assessed using in vivo microdialysis in freely moving rats. Systemic administration of the dopamine (DA) receptor agonist apomorphine (1.0 mg/kg) or the specific D₁ agonist CY 208–243 (1.0 mg/kg) increased microdialysate concentrations of ACh in the hippocampus. The D₂ receptor agonist quinpirole (0.5 mg/kg) produced a small but statistically significant decrease in hippocampal ACh release. d -Amphetamine (2.0 mg/kg) increased ACh release, an effect that was blocked by the D₁ receptor antagonist SCH 23390 (0.3 mg/kg) but not by the D₂ antagonist raclopride (1.0 mg/kg). These findings suggest that endogenous DA stimulates septo-hippocampal cholinergic neurons primarily via actions at D₁ receptors. In addition, these results are similar to previous findings regarding the dopaminergic regulation of cortical ACh release, and suggest that the anatomical continuum formed by basal forebrain cholinergic neurons that project to the cortex and hippocampus acts as a functional unit, at least with respect to its regulation by DA.     

Effects of Amphetamine on Carrier-Mediated and Electrically Stimulated Dopamine Release in Slices of Rat Caudate Putamen and Nucleus Accumbens

Abstract: The effects of (+)-amphetamine on carrier-mediated and electrically stimulated dopamine release were investigated using fast cyclic voltammetry in rat brain slices incorporating the nucleus accumbens, and in the caudate putamen. In the caudate putamen, dopamine release either increased with increasing frequency of local electrical stimulation (hot spots) or did not increase significantly (cold spots); dopamine release increased with increasing frequency of electrical stimulation in the nucleus accumbens. Local pressure application of (+)-amphetamine from a micropipette caused dopamine efflux at all sites examined, and this was not affected by sulpiride, indicating that efflux of dopamine caused by (+)-amphetamine is not regulated by dopamine D₂ autoreceptors. (+)-Amphetamine reduced single-pulse electrically stimulated dopamine release at all sites; sulpiride reversed this decrease, indicating that endogenous dopamine released by (+)-amphetamine activates dopamine D₂ autoreceptors. In nucleus accumbens and hot spots, (+)-amphetamine did not affect 20-pulse 50-Hz-stimulated dopamine release, whereas in cold spots it potentiated 20-pulse 50-Hz-stimulated dopamine release. We conclude that (+)-amphetamine modifies electrically stimulated dopamine release by uptake inhibition or by indirect activation of D₂ autoreceptors; the precise mechanism is determined by site and duration of electrical stimulation.     

Inhibition of Adenylyl Cyclase Activity by a Homogeneous Population of Dopamine Receptors: Selective Blockade by Antisera Directed Against Gi1 and/or Gi2

Abstract: The 7315c pituitary tumor cell expresses a homogeneous population of dopamine receptors that are functionally similar to brain dopamine D₂ receptors. [³H]-Sulpiride binding to 7315c cell homogenates was specific and saturable, and K _i values for compounds to compete for these sites were highly correlated with values for the same compounds at D₂ receptors in brain. Dopamine maximally inhibited ∼65% of forskolin-stimulated cyclase activity in cell membranes. Some D₂ agonists had lower efficacies, suggesting that some compounds are partial agonists at this receptor. Removal of GTP from the assay buffer or pretreatment of the tissue with pertussis toxin abolished the inhibition of adenylyl cyclase by dopamine. Immunodetection of most of the known Gα subunits revealed that G_i1, G_i2, G_i3, G_o, G_q, and G_s are present in the 7315c membrane. Pretreatment with the AS antibody (which recognizes the C-terminal regions of Gα_i1 and Gα_i2) significantly attenuated the inhibition of adenylyl cyclase activity by dopamine, whereas antibodies to C-terminal regions of the other Gα subunits had no effect. These findings suggest that the dopamine D₂ receptor regulates cyclase inhibition predominantly via G_i1 and/or G_i2 and that the 7315c tumor cells provide a useful model for studying naturally expressed dopamine D₂ receptors in the absence of other dopamine receptor subtypes.     

Characterization of the Functional Activity of Dopamine Ligands at Human Recombinant Dopamine D4 Receptors

Abstract: The human D₄ dopamine receptor has been expressed in Sf9 insect cells where it appears to couple to endogenous G proteins. Increased guanine nucleotide exchange to G proteins is a reflection of receptor activation and can be followed using a [³⁵S]GTPγS binding assay. By measuring D₄ receptor stimulation of [³⁵S]-GTPγS binding we have been able to characterize several dopaminergic compounds for their functional activity at this receptor. In Sf9 cells expressing the D₄ receptor, dopamine, quinpirole, and dp -2-aminodihydroxy-1,2,3,4-tetrahydronaphthalene were all full agonists, whereas (−)-apomorphine appeared to be a partial agonist. No increase in [³⁵S]GTPγS binding was observed for noninfected cells or cells infected with an unrelated sequence. The quinpirole-stimulated [³⁵S]GTPγS binding could be inhibited by the antagonists clozapine, eticlopride, and haloperidol, and a Schild analysis of these data showed that all three compounds were acting as competitive antagonists of D₄ receptors. The rank order of affinities derived from the Schild analysis correlated with that obtained from [³H]spiperone competition binding assays. In conclusion, we have shown that, using this assay system, it is possible to investigate functionally the pharmacology of a recombinant G protein-coupled receptor in the absence of any information regarding the eventual second messenger pathways involved.     

Dopamine Receptor Stimulation Decreases Cytosolic γ Protein Kinase C Immunoreactivity in Rat Hippocampal Slices: Evidence for Increased Ca2+-Dependent Proteolysis

Abstract: The effect of dopamine (DA) receptor stimulation on the distribution of γ protein kinase C (γPKC) in hippocampal slices was assessed. Nanomolar concentrations of DA decreased cytosolic γPKC (56%) without altering membrane γPKC levels, resulting in decreased total γPKC immunoreactivity. The maximal decrease in cytosolic γPKC occurred at 20 min of incubation and was significantly blocked by the D₁ DA antagonist SCH 23390 (10⁻⁶ M ) but not by the D₂ antagonist sulpiride (10⁻⁵ M ). The D₁ agonists SKF 38393 and A 77636 mimicked the effect of DA with similar responses produced at 10 µ M and 1 n M , respectively. The D₂ agonist quinpirole had no effect on γPKC immunoreactivity, thus indicating that this dopaminergic response is mediated through a D₁-like receptor. DA had no effect on α, δ, or ζPKC isozyme immunoreactivity in the same hippocampal preparations. The DA-induced decrease in cytosolic γPKC immunoreactivity was blocked by the Ca²⁺-dependent protease inhibitor N -acetyl-Leu-Leu-norleucinal (100 µ M ) and by the inorganic Ca²⁺ channel blocker Co²⁺. The data suggest that DA stimulates a D₁-like DA receptor, which increases the influx of Ca²⁺ and activates the Ca²⁺-dependent proteolysis of γPKC.