Differential effects of selective and non-selective neuroleptics on intracellular and extracellular cyclic AMP accumulation in rat striatal slices

Concurrent modulation of extracellular levels of noradrenaline and cAMP during stress and by anxiogenic- or anxiolytic-like neuropeptides in the prefrontal cortex of awake rats

The purpose of this study was to examine the effects of stress and the role of locally infused anxiogenic-like neuropeptides galanin, CCK-8, vasopressin, substance P and neurokinin A, and anxiolytic-like peptides NPY, nociceptin/orphanin FQ, somatostatin and neurotensin, on modulation of noradrenaline (NA) and cAMP efflux monitored simultaneously by microdialysis in the medial prefronatal cortex of awake rats. Concentrations of cAMP were determined by a newly developed method based on derivatization of cAMP with 2-chloroacetaldehyde followed by HPLC with fluorescence detection. Local infusion of forskolin (10 and 30 μM) dose-dependently increased the cAMP levels to 417% and 1050% of the control group, respectively. Similarly, local infusion of NA (10 μM) increased the cAMP to the peak level of 168%. A 5-min tail pinch and a 10-min swim stress rapidly increased the NA and cAMP levels to 167% and 203% (NA) and 141% and 161% (cAMP), respectively. Infusion of galanin and CCK-8 (0.5 nmol, and 1.5 nmol/0.5 μl) dose-dependently increased NA to the peak levels of 191% and 179% and cAMP levels to 174% and 166%, respectively. The peak levels following infusions of vasopressin, substance P and neurokinin A were 91%, 135% and 86% for NA and 131%, 83% and 76% for cAMP, respectively. Infusions of anxiolytic-like peptides at highest concentrations significantly increased (NPY, 136%) or decreased (nociceptin, 71%; somatostatin, 86%) the NA levels, whereas neurotensin had no effect. The cAMP levels decreased to 86% (NPY, neurotensin), 78% (nociceptin), somatostatin infusion was without effect. The present findings confirmed a close correlation between the stress-induced increases in prefrontal cortical NA and cAMP levels, as well as, concurrent changes in NA and cAMP levels following infusions of galanin and CCK-8 (increased levels) and nociceptin/orphanin FQ (decreased levels). Infusions of other neuropeptides showed a more complex pattern of NA and cAMP responses.

A specific CRH antagonist attenuates ACTH-stimulated cortisol secretion in ovine adrenocortical cells

Corticotropin releasing hormone (CRH) has been detected in the adrenal gland of many species and may be involved in regulation of glucocorticoid secretion. In cultured human fetal adrenal definitive/transitional zone cells, CRH upregulates the adrenocorticotropic hormone (ACTH) receptor and steroidogenic enzymes and is blocked by the selective CRH type 1 receptor (CRH(1)) antagonist, antalarmin. Based on these findings and evidence that antalarmin infusion into sheep suppressed prepartum increases in cortisol, we hypothesized that antalarmin would influence adrenal cortisol secretion. Antalarmin strongly attenuated ACTH and forskolin (FSK)-stimulated cortisol and cyclic adenosine monophosphate (cAMP) release from cultured ovine adrenocortical cells but did not prevent ACTH binding to cells or ACTH-induced proliferation in adult cells. Corticotropin releasing hormone was minimally effective as a secretagogue but increased the cortisol response to subsequent ACTH. These results suggest that antalarmin attenuates ACTH-induced cortisol secretion from cultured ovine adrenal cortical cells at a site distal to the ACTH receptor. Although CRH may modulate the secretory response to ACTH, it is probably not a direct cortisol secretagogue in the sheep.

Inhibition of the striatum-enriched phosphodiesterase PDE10A: A novel approach to the treatment of psychosis

Phosphodiesterase 10A (PDE10A) is a recently identified cyclic nucleotide phosphodiesterase expressed primarily in dopaminoreceptive medium spiny neurons of the striatum. We report that papaverine is a potent, specific inhibitor of PDE10A and use this compound to explore the role of PDE10A in regulating striatal function. Papaverine administration produces an increase in striatal tissue levels of cGMP and an increase in extracellular cAMP measured by microdialysis. These cyclic nucleotide changes are accompanied by increases in the phosphorylation of CREB and ERK, downstream markers of neuronal activation. In rats, papaverine potentiates haloperidol-induced catalepsy, consistent with the hypothesis that inhibition of PDE10A can increase striatal output and prompting a further evaluation of papaverine in models predictive of antipsychotic activity. Papaverine is found to inhibit conditioned avoidance responding in rats and mice and to inhibit PCP- and amphetamine-stimulated locomotor activity in rats. The effects of papaverine on striatal cGMP and CREB and ERK phosphorylation, as well as on conditioned avoidance responding, were absent in PDE10A knockout mice, indicating that the effects of the compound are the result of PDE10A inhibition. These results indicate that PDE10A regulates the activation of striatal medium spiny neurons through effects on cAMP- and cGMP-dependent signaling cascades. Furthermore, the present results demonstrate that papaverine has efficacy in behavioral models predictive of antipsychotic activity. Thus, inhibition of PDE10A may represent a novel approach to the treatment of psychosis.

Ontogeny and effects of thyroid hormone on beta1-adrenergic receptor mRNA expression in ovine fetal kidney cortex

OBJECTIVE

Previous studies indicate that thyroidectomy (TX) decreases renin gene expression in ovine fetal renal cortex in late gestation. Fetal ovine renin-containing renocortical cells become increasingly responsive to beta-adrenergic stimulation as gestation proceeds. Increases in plasma thyroid hormone concentrations parallel this change, suggesting that there is a positive developmental relationship between the two. To examine this hypothesis, we determined the ontogeny of beta1-adrenergic receptor (beta1R) mRNA expression, and the effect of thyroid hormone on in vivo and in vitro expression in fetal sheep.

METHODS

Renocortical tissue was obtained from naive, TX, and sham-operated fetuses to determine beta1R mRNA levels. Renin-containing renocortical cells from TX or sham fetuses were treated with isoproterenol (Iso) or forskolin (FSK) for analysis of cellular cyclic adenosine monophosphate (cAMP) levels. Renocortical cells from naive fetuses were treated with triiodothyronine (T3) to assess cellular beta1R mRNA levels. Fetal plasma thyroxine (T4) level was determined.

RESULTS

Renocortical beta1R mRNA expression increased significantly between 100 and 140 days' gestational age (dGA), while TX attenuated this increase (P <.01). Renocortical cellular cAMP levels were higher in sham compared to TX fetuses following incubation with Iso or FSK (P <.05). Cells incubated with T3 exhibited significantly increased beta1R mRNA expression (P <.05).

CONCLUSION

The data suggest that thyroid hormone may be involved in modulating ovine fetal renocortical beta1R gene expression during development. We speculate that the increased beta1R mRNA expression in renal cortical cells as development progresses may mediate the increases in renin gene response to beta-adrenergic stimulation in late gestation.

Involvement of cyclic AMP-dependent and -independent mechanisms in the relaxation of rat detrusor muscle via beta-adrenoceptors

We investigated the cAMP-dependent and -independent mechanisms of relaxation via beta-adrenoceptor in rat detrusor muscle with and without pre-contraction. A microdialysis technique was used to measure detrusor tension and cAMP level on the same detrusor tissue. In non-contracted tissue, isoproterenol, clenbuterol (beta2-adrenoceptor agonist) and FR165101, ((8S)-8-{[(2R)-2-(3-chlorophenyl)-2-hydroxyethyl]amino}-6,7,8,9-tetrahydro-5H-benzocyclohepten-2-yl)oxy]acetic acid hydrochloride (beta3-adrenoceptor agonist) relaxed detrusor muscle and cAMP levels also increased in a concentration dependent manner. SQ22536 (adenylyl cyclase inhibitor) markedly suppressed relaxation, suggesting that beta-adrenoceptor-mediated relaxation may be attributed mainly to cAMP-dependent mechanism. In high K+ pre-contracted tissue, although relaxation advanced in a concentration dependent manner, cAMP production reached a plateau at concentrations of more than 10(-7) M. SQ22536 had only a small inhibitory effect. However, large-conductance, Ca2+-activated K+ (BK(Ca)) channel inhibitors, charybdotoxin and iberiotoxin markedly suppressed relaxation. These results suggest that in addition to cAMP-dependent pathway, BK(Ca) channels are involved in the beta-adrenoceptor agonists-induced relaxation in pre-contracted detrusor muscle.

Functional and autoradiographic characterization of dopamine D2-like receptors in the guinea pig heart

Dopamine receptors include the D1- (D1 and D5 subtypes) and D2-like (D2, D3, and D4 subtypes) families. D1-like receptors are positively and D2-like receptors negatively coupled to the adenylyl cyclase. Dopamine D2-like (D4 subtype) receptors have been identified in human and rat hearts. However the presence of D2 and D3 receptor subtypes is unclear. Furthermore, their role in cardiac functions is unknown. By autoradiographic studies of guinea pig hearts, we identified D3 and D4 receptors, using the selective radioligands [3H]-7-OH-DPAT and [3H]emonapride (YM-09151-2 plus raclopride). Western blot analysis confirmed D3 and D4 receptors in the right and left ventricle of the same species. Selective agonists of D3 and D4 receptors (+/-)-7-OH-DPAT and PD 168 077 (10(-9) to 10(-5) M, respectively) induced a significant negative chronotropic and inotropic effect in the isolated guinea pig heart preparation. Negative inotropic effect induced by PD 168 077 was associated with an inhibition in cyclase activity. No changes in cyclase activity were found with (+/-)-7-OH-DPAT. The aim of this study is to support the presence of D3 and D4 receptors in the heart. Although our results suggest that D3 and D4 receptors are functionally active in the heart, we need additional information with an antagonist and an agonist of improved potency and selectivity to understand the respective roles of D3 and D4 receptors in the cardiac functions.

Prevention of methamphetamine-induced behavioral sensitization in rats by a cyclic AMP phosphodiesterase inhibitor, rolipram

Effects of an interaction between rolipram, a cyclic adenosine 3', 5'-monophosphate (cyclic AMP) phosphodiesterase inhibitor, and methamphetamine on the development of behavioral sensitization were observed in rats. In vivo microdialysis showed that a single dose of 4 mg/kg methamphetamine (i.p.) significantly increased striatal dopamine levels while coadministration with 4 mg/kg rolipram (i.p.) did not affect these levels. Also, methamphetamine alone did not alter striatal cyclic AMP levels but coadministration with rolipram and rolipram alone significantly increased these levels. The administration of 4 mg/kg methamphetamine (i.p.) once a day for 5 days significantly enhanced hyperlocomotion and rearing induced by a 2-mg/kg methamphetamine challenge (i.p.) after a 1-week withdrawal period, compared with controls or coadministration with 4 mg/kg rolipram (i.p.). Striatal dopamine levels, detected by in vivo microdialysis, were increased following the challenge but were comparable between the groups. These findings suggest that rolipram prevents methamphetamine-induced behavioral sensitization by increasing cyclic AMP levels while not affecting dopamine-releasing processes.

A common mechanism mediates long-term changes in synaptic transmission after chronic cocaine and morphine

The mesolimbic system is known to play a role in self-administration of opioids and psychostimulants. Although morphine and cocaine act by separate cellular mechanisms initially, the present study describes a common change in synaptic regulation of dopamine cells in the ventral tegmental area 1 week after termination of chronic treatment with either drug. Normally, D1 receptor activation augmented the amplitude of a gamma-aminobutyric acid type B (GABA(B)) inhibitory postsynaptic potential (IPSP), but in drug-experienced animals, D1 receptor activation caused an inhibition of the GABA(B) IPSP. The inhibition was blocked by adenosine A1 receptor antagonists and by agents that disrupted the metabolism of cAMP. This long-lasting dopamine-adenosine interaction may be one mechanism involved in dopamine-mediated craving and relapse to drug-seeking behaviors.

In vivo evidence that nonneuronal beta-adrenoceptors as well as dopamine receptors contribute to cyclic AMP efflux in rat striatum

We applied in vivo microdialysis to assess the effects of dopaminergic and beta-adrenergic receptor stimulation on cyclic AMP efflux in rat striatum under chloral hydrate anesthesia. Dopamine (up to 1 mM) infused for 20 min through the probe did not increase cyclic AMP, whereas both the selective dopamine D1 agonist SKF 38393 and D2 antagonist sulpiride produced modest increases. It is interesting that the beta-adrenoceptor agonist isoproterenol produced a marked increase (204.7% of basal level at 1 mM) which was antagonized by the beta-adrenoceptor antagonist propranolol. Pretreatment with a glial selective metabolic inhibitor, fluorocitrate (1 mM), by a 5-h infusion through the probe attenuated basal cyclic AMP efflux by 30.3% and significantly blocked the response to isoproterenol. By contrast, striatal injection of a neurotoxin, kainic acid (2.5 micrograms), 2 days before the dialysis experiment did not affect basal cyclic AMP or the response to isoproterenol, but blocked the response to SKF 38393. These data demonstrate the beta-adrenoceptors as well as dopamine receptors contribute to cyclic AMP efflux in rat striatum in vivo. They also suggest that basal and beta-adrenoceptor-stimulated cyclic AMP efflux are substantially dependent on intact glial cells.

Serotonin 5-HT1A receptor activation increases cyclic AMP formation in the rat hippocampus in vivo

In vivo microdialysis was used to examine the efflux of cyclic AMP (cAMP) into the extracellular fluid of the ventral hippocampus in the freely moving rat. The changes in extracellular cAMP concentration were monitored in response to forskolin and the serotonin 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The basal level of hippocampal extracellular cAMP was 2.3 +/- 0.2 pmol/ml (n = 6), after a 3-h postsurgery stabilisation period. Perfusion of forskolin (100 microM) through the probe for 30 min significantly increased the efflux of cAMP, which returned to baseline levels within 90 min. 8-OH-DPAT (0.3 mg/kg s.c.) also significantly increased cAMP efflux, whereas a similar volume of saline had no effect. Desensitisation of the 8-OH-DPAT-induced increase in cAMP efflux was observed following a second administration of 8-OH-DPAT after a 4-h interval. Administration of 8-OH-DPAT did not alter the efflux of cAMP when forskolin was perfused through the probe. Pretreatment with WAY 100135 [N-tert-butyl 3-4-(2-methoxyphenyl)piperazine-1-yl-2-phenylpropanamide dihydrochloride] (5 mg/kg s.c.), a specific 5-HT1A receptor antagonist, prevented the 8-OH-DPAT-induced increase in cAMP efflux. The data indicate that the 8-OH-DPAT-induced increase in cAMP efflux in vivo is mediated by a 5-HT1A receptor.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Ischemia-induced changes in extracellular levels of striatal cyclic AMP: role of dopamine neurotransmission

Dopamine has been demonstrated to be involved in the development of ischemic neuronal damage in the striatum. This detrimental effect of dopamine may involve activation of second messenger systems, such as the cyclic AMP (cAMP) cascade, which may enhance the susceptibility of striatal neurons to ischemia. In the present study, we have evaluated the relationship between ischemia-induced changes in cAMP and dopamine neurotransmission. Microdialysis probes were implanted in both striata, and a D1 antagonist (SCH-23390, 100 microM) was administered through one probe and modified Ringer's solution through the other. After a stabilization period, rats (n = 6) were subjected to 20 min of ischemia by two-vessel occlusion plus hypotension. Extracellular samples were collected from both striata, before, during, and after ischemia, and analyzed for cAMP by radioimmunoassay. Ischemia induced a significant increase in extracellular cAMP (means +/- SE, fmol/microliter; baseline: 4.35 +/- 1.1, ischemia: 12.2 +/- 1.98), which was also observed at 4 h of recirculation (mean level of 8.45 +/- 1.14). Treatment with the D1 antagonist significantly inhibited the rise in extracellular cAMP during ischemia and recirculation. These results indicate that an ischemia-induced surge in dopamine and activation of D1 receptors are involved in the generation of cAMP during ischemia and recirculation. Because activation of the adenylate cyclase cascade may modulate the effects of glutamate, generation of cAMP through this pathway may play a role in facilitating the injurious effects of dopamine during ischemia.

Studies on the cellular localization of biochemical responses to catecholamines in the brain

Studies were undertaken to determine the cellular localization of the cyclic adenosine monophosphate (cAMP) response of various forebrain regions to beta-adrenoceptor stimulation. Using brain slices, it was found that the gliotoxin, fluorocitrate (FC), which blocks metabolism selectively in glial cells, virtually abolished the cAMP response to beta-receptor stimulation whereas the neurotoxin, kainic acid (KA), was without effect. FC was confirmed by electrophysiological recording to be selective for glial cells in the brain slices. Similar results were found for these agents on in vivo brain cAMP responses to beta-receptor stimulation using a new microdialysis technique to measure in vivo responses. It is concluded that the cAMP response to beta-adrenoceptor stimulation in various regions of the forebrain occurs predominantly in glia. To determine if this could be correlated with a second biochemical response to beta-receptor stimulation, preliminary studies were undertaken on the localization of the immediate early gene, c-fos, produced in the brain after in vivo stimulation of beta-receptors. It was found that unlike the cAMP responses the c-fos response to beta-receptor stimulation occurs predominantly in neurons. The possible relationship of these two responses is discussed.

Acute reserpine treatment induces down regulation of D-1 dopamine receptor associated adenylyl cyclase activity in rat striatum

Behavioural studies suggest a functional interaction between D-1 and D-2 systems in normal rat striatum to alter motor behaviour and which is disrupted by dopamine depletion induced by acute reserpine treatment. Consequently, we have investigated the effect of acute reserpine treatment on the biochemical interaction between D-1 and D-2 receptors present in rat striatal slices. Twenty-four hours following the administration of reserpine (5 mg/kg i.p.), striatal dopamine content was depleted by more than 73%; the density (B(max)) of D-1 receptor sites measured by the in vitro binding of [3H]SCH 23390 to striatal membranes was increased while the binding of [3H]spiperone to D-2 receptor sites was unaltered. Reserpine treatment had no effect on the affinity (Kd) of [3H]SCH 23390 or [3H]spiperone for D-1 and D-2 sites. Basal levels of cyclic AMP accumulation in striatal slices prepared from reserpine-treated rats were lower than those observed in control slices. In striatal slices prepared from normal rats, dopamine (10-320 microM) and the D-1 agonist SKF 38393 (0.1-3.2 microM) induced concentration-dependent increases in cyclic AMP accumulation. The D-1 antagonist SCH 23390 (10 microM) abolished the accumulation of cyclic AMP produced by dopamine or SKF 38393. The D-2 antagonist (+/-)-sulpiride (50 microM) enhanced the response to dopamine (10-320 microM) while the D-2 agonist quinpirole (10 microM) abolished the response to SKF 38393 (0.1-3.2 microM). However, 24 hr after reserpine treatment the ability of dopamine (10-320 microM) and SKF 38393 (0.1-3.2 microM) to elicit an increase in cyclic AMP accumulation was markedly reduced in striatal slices. SCH 23390 (10 microM) did not enhance the trend for an increase in cyclic AMP accumulation produced by dopamine. Also, quinpirole (10 microM) did not affect the response to SKF 38393 (0.1-3.2 microM) in striatal slices from reserpine pretreated rats. The data confirm the positive linkage between D-1 receptors and adenylyl cyclase and the inhibitory coupling to D-2 sites in striatal slices from normal, rats. Acute reserpine treatment appears to cause an uncoupling of D-1 receptors associated with adenylyl cyclase.

Dissociation of the striatal D-2 dopamine receptor from adenylyl cyclase following 6-hydroxydopamine-induced denervation

Intracellular cyclic AMP accumulation following exposure to dopamine (DA) agonists and and antagonists was measured in striatal slices from rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal pathway and which showed contralateral circling to apomorphine. Both DA (10-320 microM) and the D-1 agonist SKF 38393 (0.1-32 microM) increased cyclic AMP accumulation in striatal slices from the lesioned and intact hemispheres. The EC50 for DA to increase cyclic AMP accumulation in slices was greater in the 6-OHDA-lesioned striata compared to the intact striatum, but the EC50 for SKF 38393 was not affected. The D-1 antagonist SCH 23390 (10 microM) completely inhibited the ability of DA and SKF 38393 to increase cyclic AMP accumulation in striatal slices from both denervated and intact sides of the brain. In slices from the intact hemisphere the increase in DA-induced cyclic AMP accumulation was enhanced by the D-2 antagonist (+/-)-sulpiride (50 microM) but (+/-)-sulpiride had no effect on the DA response in slices from the lesioned side. Similarly, the ability of SKF 38393 to enhance cyclic AMP accumulation was blocked by the D-2 agonist quinpirole (10 microM) in striatal slices from the intact hemisphere but not in tissue from the lesioned side. The density of striatal D-1 and D-2 receptors assessed by [3H]SCH 23390 and [3H]spiperone binding did not differ between the hemispheres although there was an increase in the affinity of D-1 receptors for [3H]SCH 23390 in the lesioned striatum. After striatal deafferentiation there appears to be an uncoupling of the "inhibitory" D-2 receptor from the D-1 receptor-associated adenylyl cyclase.

Dynamics of cyclic-AMP efflux in relation to alpha-MSH secretion from melanotrope cells of Xenopus laevis

An important factor in regulating secretion from endocrine cells is the cytoplasmic concentration of cyclic-AMP. Many regulatory substances are known to either stimulate or inhibit the production of this second messenger through activation of their receptors. In the present study, we have monitored changes in cyclic-AMP efflux from melanotrope cells of Xenopus laevis in response to established neurochemical regulators of alpha-MSH secretion. In vitro superfusion of neurointermediate lobes allows for a dynamic recording of cyclic-AMP production in relation to hormone secretion. Unlike alpha-MSH secretion, the efflux of cyclic-AMP was not dependent on the concentration of extracellular calcium, indicating that hormone release and cyclic-AMP efflux are mediated by different mechanisms. The phosphodiesterase inhibitor IBMX and the adenylate cyclase activator forskolin stimulated cyclic-AMP efflux, but had no stimulatory effect on alpha-MSH release. This indicates that an increase in cyclic-AMP production in melanotrope cells is not necessarily accompanied by an increase in the rate of alpha-MSH release. Corticotropin-releasing factor stimulated cyclic-AMP efflux with dynamics similar to that induced by the amphibian peptide sauvagine. Dopamine and the GABAB receptor agonist baclofen both inhibited cyclic-AMP efflux and alpha-MSH release, with similar dynamics of inhibition and similar dose-response relationships. It is proposed that an inhibition of cyclic-AMP efflux is coupled to an inhibition of alpha-MSH secretion.

A potential mechanism for copper amplification of prostaglandin E2 action: attenuation of prostaglandin E2-induced efflux of cyclic AMP from median eminence explants

It is known that prostaglandin E2 (PGE2) stimulation of LHRH release from the median eminence-arcuate nucleus (MEA) is mediated by the cAMP pathway, and a short pretreatment with copper markedly amplifies this release process. Because stimulation of cAMP accumulation is accompanied by cAMP efflux in many tissues, we considered the possibility that attenuation of cAMP efflux is one mechanism by which copper can enhance PGE2 action. When rat MEA explants were incubated in vitro, PGE2 induced a rapid (less than 2.5 min) and sustained (15 min) increase in cAMP efflux, the degree of which was a function of [PGE2]: by 5 min exposure to 10 microM PGEs2, efflux was 8-fold greater than the control (no PGE2) and it accounted for 12.4% of the total (tissue + medium) cAMP. Unlike the dramatic increase in cAMP efflux, PGE2 induced a moderate increase in cAMP content (49%) and in the incorporation of [3H] adenine into [3H] cAMP (78%); this increase was transient: it was evident after a 2.5 but not after a 5 min period of PGE2 exposure. Copper pretreatment did not alter this PGE2-induced increase in tissue cAMP content. In contrast, copper markedly inhibited (by 49%-66%) PGE2-induced cAMP efflux and this inhibition was noted regardless of the length of PGE2 exposure and PGE2 concentration. There was no evidence for hydrolysis of [3H]3',5'-cAMP included in the medium during the incubation with PGE2 with and without copper pretreatment. In summary, copper attenuated PGE2-induced cAMP efflux from MEA explants and this attenuation is not a consequence of a reduction in the availability of intracellular cAMP nor of hydrolysis of cAMP extracellularly.

Further evidence for a glial localization of rat cortical beta-adrenoceptors: studies of in vivo cyclic AMP responses to catecholamines

The present experiments were designed to clarify the cellular localization of postsynaptic beta-receptors in the rat cortex by studying the cellular source and pharmacological characteristics of in vivo cAMP responses to catecholamines. The method used to study in vivo cAMP responses in the brain involved microdialysis both to deliver catecholamines to cerebral tissue and to sample cAMP released in response to local beta-receptor activation. It was found that selective blockade of the metabolism of glial cells by fluorocitrate infusion produced a virtually complete (90%) inhibition of the cortical cAMP response to norepinephrine (NE). Selective damage of neurons by kainic acid infusion had little effect on the response. Pharmacological experiments showed that the response was selectively antagonized by a beta 1-receptor blocker which also selectively antagonized the cAMP response to NE in brain slices known to be localized in glial cells. These results support the hypothesis that beta-adrenoceptors of the rat cortex are predominantly localized on glial cells and therefore strongly suggest that these cells are an important target of the locus coeruleus noradrenergic system.

Lithium effects on noradrenergic-linked adenylate cyclase activity in intact rat brain: an in vivo microdialysis study

The effects of chronic lithium treatment on adenylate cyclase activity in intact rat brain were examined using in vivo microdialysis. Basal extracellular cyclic adenosine monophosphate (AMP) increased in a dose-dependent manner after norepinephrine was added to the perfusate. Chronic lithium treatment increased basal brain extracellular fluid cyclic AMP levels, while decreasing the magnitude of the cyclic AMP response to stimulation with 100 microM norepinephrine.

In vivo measurement of extracellular cyclic AMP in the brain: use in studies of beta-adrenoceptor function in nonanesthetized rats

Microdialysis measurement of extracellular cyclic AMP (cAMP) in the cerebral cortex of conscious rats was evaluated as a method for assessing central beta-adrenoceptor function in vivo. Extracellular levels of the nucleotide were found to average 3 pmol/ml under baseline conditions. Local infusion of the beta-agonists norepinephrine (NE) and isoproterenol produced rapid (3 min) and marked (three- to sevenfold) dose-dependent increases in extracellular cAMP, which were potentiated by the phosphodiesterase inhibitor rolipram, and blocked by the beta-antagonist timolol. Responses to both catecholamines underwent rapid desensitization (6-9 min) and recovered within several hours. Time-course studies revealed that the baseline cAMP level underwent a gradual increase and then a decrease over the course of a single 8-h run, and peaked at 24 h postimplantation. Responses to NE were stable for the first 24 h after implantation, then increased at 48 and 120 h. The causes of the latter changes may include reactions to novelty, local inflammatory responses, and/or reactions of adjacent glial cells to implantation. Overall, the results indicate that the microdialysis-cAMP method can be extended to nonanesthetized animals and may be a useful tool for studying neurotransmission at central adenylate cyclase-coupled membrane receptors during various behavioral states.

Activation of postsynaptic striatal dopamine receptors, monitored by efflux of cAMP in vivo

Intracerebral dialysis was used to monitor the change of extracellular concentration of striatal cAMP in rats anaesthetised with chloral hydrate. Forskolin (1-10 microM), an activator of adenylate cyclase, caused a concentration-dependent increase in efflux of cAMP, which was decreased by (+)PHNO (10 microM), an effect probably mediated by D2 sites, since (-)-sulpiride, a D2 receptor antagonist prevented these effects. Dopamine (1-100 microM) also increased the efflux of cAMP but only when the activity of monoamine oxidase and reuptake of dopamine were concomitantly blocked. The D1 receptor agonist SKF 38393 (1-100 microM) caused a concentration-dependent increase in efflux of cAMP, which was blocked by the D1 receptor antagonist SCH 23390 (1-100 microM), but was unaffected by the D2 receptor antagonist sulpiride (10 microM) or by depletion of the concentration of striatal dopamine after pretreatment with 6-hydroxydopamine. Taken together, these results indicate that intracerebral dialysis may be used to monitor the interaction of drugs with post-synaptic dopamine receptors in vivo.

Selective effects of [D-Ser2(O-t-butyl),Leu5]enkephalyl-Thr6 and [D-Ser2(O-t-butyl),Leu5]enkephalyl-Thr6 (O-t-butyl), two new enkephalin analogues, on neurotransmitter release and adenylate cyclase in rat brain slices

The selectivity and potency of two new enkephalin-derived delta-opioid receptor agonists, DSTBULET ([D-Ser2(O-t-butyl),Leu5]enkephalyl-Thr6) and BUBU ([D-Ser2(O-t-butyl),Leu5]enkephalyl-Thr6(O-t-butyl] were determined with functional tests in vitro of mu-, delta- and kappa-opioid receptor activation in the rat brain. Both peptides concentration dependently (1 nM-1 microM) inhibited the release of radiolabeled acetylcholine (ACh) from striatal slices (pD2 7.6-7.9), an effect exclusively mediated by delta-opioid receptor activation. Fentanyl isothiocyanate (FIT), an irreversible delta-antagonist, completely blocked the inhibitory effects of DSTBULET and BUBU. Up to a concentration of 1 microM, the peptides did not affect striatal [3H]dopamine (DA) release nor cortical [3H]noradrenaline (NA) release, processes which are known to be inhibited by opioids activating kappa and mu-receptors, respectively. Furthermore, both DSTBULET and BUBU caused a strong inhibition (pD2 8.2-8.3) of D-1 dopamine receptor-stimulated cyclic AMP efflux from striatal slices, an effect known to be mediated by mu- and/or delta-opioid receptor activation. However, the peptides were without effect when D-1 and D-2 dopamine receptors were stimulated simultaneously, a situation in which only mu-agonists are able to inhibit the resulting cAMP efflux. In conclusion, DSTBULET and BUBU appear to display a high selectivity and potency toward functional delta-opioid receptors in the brain.

Dopamine D2 receptors exert tonic regulation over discrete neurotensin systems of the rat brain

Blockade of dopamine D2 receptors with either the selective antagonist, sulpiride, or the non-selective antagonist, haloperidol, induces 2- to 3-fold increases in the content of neurotensin-like immunoreactivity in the striatum and the nucleus accumbens of the rat brain. Quantitatively similar increases were also observed (a) in the striatum following selective degeneration of more than 85% of the nigrostriatal dopamine pathway with 6-hydroxydopamine and (b) in both the striatum and the nucleus accumbens after non-selective depletion of brain dopamine using reserpine plus alpha-methyl-p-tyrosine. Interestingly, treatment of animals with sulpiride or haloperidol, following the depletion of dopamine by either 6-hydroxydopamine or reserpine plus alpha-methyl-p-tyrosine, did not add to the elevation in neurotensin content of either structure caused by the dopamine depletion alone. These data suggest that an intact dopamine system is required for the neuroleptics to exert their effects on individual neurotensin systems. In addition, the same mechanism appears to underlie the responses of the neurotensin pathways to treatments with the neuroleptics or dopamine-depleting drugs. A likely explanation for the effects of neuroleptics and dopamine-depleting drugs is that they eliminate tonic activity on D2 receptors by basally released dopamine in the striatum and the nucleus accumbens. Supportive evidence for this hypothesis is that concurrent administration of the D2 receptor agonist, LY 171555, with reserpine, completely blocked the effects of reserpine-induced dopamine depletion on neurotensin systems of the striatum and the nucleus accumbens.

One year treatment with haloperidol or clozapine fails to alter neostriatal D1- and D2-dopamine receptor sensitivity in the rat

Rats were treated continuously with either haloperidol (HAL), clozapine (CLOZ) or tap water for one year. There were no differences between age-matched control and antipsychotic drug (APD) treated groups regarding the effects of the D1-agonist (+)-SKF 38393 or the D2-agonist quinpirole on striatal cAMP content. However, the combination of SKF (10 microM) and quinpirole (1 microM) produced a marked synergistic response in HAL-treated animals as compared to controls. Our data fail to support the hypothesis that APD produce their neurological side effects by inducing D2-receptor hypersensitivity in the basal ganglia. However, the results do suggest that chronic APD treatment alters the interaction between D1- and D2-neostriatal receptors.

Mu-, delta- and kappa-opioid receptor-mediated inhibition of neurotransmitter release and adenylate cyclase activity in rat brain slices: studies with fentanyl isothiocyanate

We investigated the effects of [D-Ala2,D-Leu5]enkephalin (DADLE). [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO), [D-Pen2,D-Pen5]enkephalin (DPDPE) (0.01-1 microM) and bremazocine (0.001-0.3 microM) on the electrically evoked release of radiolabelled neurotransmitters and on the dopamine (DA)-stimulated cyclic AMP efflux from superfused rat brain slices. The differential inhibitory effects of these agonists on the evoked neurotransmitter release indicate that the opioid receptors mediating presynaptic inhibition of [3H]noradrenaline (NA, cortex), [14C]acetylcholine (ACh, striatum) and [3H]DA (striatum) release represent mu, delta and kappa receptors, respectively. In agreement with this classification, preincubation (60 min) of the slices with the delta-opioid receptor-selective irreversible ligand, fentanyl isothiocyanate (FIT, 0.01-1 microM), antagonized the inhibitory effects of DADLE and DPDPE on striatal [14C]ACh release only. On the other hand, the D-1 DA receptor-stimulated cyclic AMP efflux from striatal slices appeared to be inhibited by activation of mu as well as of delta receptors. In this case, the reversible mu antagonist, naloxone (0.1 microM), fully antagonized the inhibitory effect of the mu agonist, DAGO, without changing the effect of the delta agonist DPDPE but was ineffective as an antagonist in slices pretreated with FIT (1 microM). The inhibitory effect of DAGO on the electrically evoked [3H]NA release was antagonized by naloxone whether the receptors were irreversibly blocked by FIT or not. These data not only further support the existence of independent presynaptic mu-, delta- and kappa-opioid receptors in rat brain but also evidence strongly that mu and delta receptors mediating the inhibition of DA-sensitive adenylate cyclase could share a common binding site (for naloxone and FIT) and, therefore, may represent constituents of a functional opioid receptor complex.

Dopamine receptor stimulation does not affect phosphoinositide hydrolysis in slices of rat striatum

The effect of dopamine receptor stimulation on the accumulation of labelled inositol phosphates in rat striatal slices under basal and stimulated conditions was examined following preincubation with [3H]inositol. Incubation of striatal slices with the selective D-1 agonist SKF 38393 or the selective D-2 agonist LY 171555 for 5 or 30 min did not affect the basal accumulation of labelled inositol mono-, bis-, tris-, and tetrakisphosphate. Resolution by HPLC of inositol trisphosphate into inositol-1,3,4-tris-phosphate and inositol-1,4,5-trisphosphate isomers revealed that under basal conditions dopamine did not influence the accumulation of inositol-1,4,5-trisphosphate. Depolarisation evoked by KCl, or addition of the muscarinic receptor agonist carbachol, produced a marked increase in the accumulation of labelled inositol phosphates in both the presence and absence of lithium. Addition of dopamine did not reduce the ability of KCl or carbachol to increase inositol phospholipid hydrolysis. In the presence of lithium, dopamine (100 microM) enhanced KCl-stimulated inositol phospholipid hydrolysis, but this effect appears to be mediated by alpha 1 adrenoceptors because it was blocked by prazosin. SKF 38393 (10 microM) or LY 171555 (10 microM) also did not affect carbachol-stimulated inositol phospholipid hydrolysis. These data, in contrast to recent reports, suggest that striatal dopamine receptors do not appear to be linked to inositol phospholipid hydrolysis.

Use of microdialysis to measure brain noradrenergic receptor function in vivo

The present studies were undertaken to explore the use of intracerebral microdialysis to measure brain adrenergic receptor activity in vivo using the efflux of cyclic adenosine monophosphate (cAMP) in the extracellular fluid as an index of receptor function. An initial study with brain slices showed that extracellular levels of cAMP were highly correlated with intracellular levels after noradrenergic receptor activation. Detectable levels of extracellular cAMP were obtained from microdialysis probes implanted in the frontal cortex. Stable levels (3.5 +/- 2.7 pmol/ml) were obtained between 1.5 and 7 h after implantation. Perfusion of probes with NE (10(-5) to 10(-3) M) led to dose-dependent increases in cAMP efflux. The response to NE was blocked by infusion of the beta-antagonist, timolol, indicating that it reflected beta-receptor activation. Similar responses were obtained at 2 and 24 h after implantation indicating that the responses were not affected by acute traumatic effects of implantation. The findings show that the microdialysis technique can be successfully applied to the in vivo study of central adrenoceptor function. This suggests that the in vivo measurement of second messengers by microdialysis will provide a valuable new neurochemical and neuropharmacological technique.

Quantitative radioimmunocytochemical evidence that haloperidol and SCH 23390 induce opposite changes in substance P levels of rat substantia nigra

Chronic blockade of the dopamine (DA) D2 receptor by repeated systemic administration of the butyrophenone neuroleptic, haloperidol (HAL), is known to lead to a decrease in levels of the neuroactive peptide, substance P (SP), in the rat striatum and substantia nigra (SN). Using a high-resolution, quantitative radioimmunocytochemistry (RIC) technique, we have shown the HAL-induced decrease in rat nigral SP to be both dose- and time-dependent. In addition, chronic administration of the highly selective D2 antagonist, S(-)-sulpiride, also decreased nigral SP. Following blockade of the dopamine D1 receptor by chronic administration of the selective D1 antagonist, SCH 23390, we found, in contrast, that levels of SP in SN were increased in a dose- and time-dependent fashion. The magnitude of the maximum SCH 23390-induced elevation (20-30%) of nigral SP was approximately equal to that of the maximum HAL-induced decrease. The opposite response of nigral SP levels to repeated injections of a D1 or D2 antagonist suggests that the two DA receptor subtypes exert tonic, opposing, modulatory influences on the SP content of the striatonigral pathway.

Excitatory and inhibitory action of dopamine on hippocampal neurons in vitro. Involvement of D2 and D1 receptors

The study examined the effects of dopaminergic agonists on the extracellularly recorded spontaneous activity of CA1 neurons in hippocampal slices of the rat. Dopamine evoked excitation or inhibition of the neuronal firing rate, the first reaction being more sensitive to the substance. Having used selective dopaminergic agonists (pergolide and 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepines) and selective antagonists (haloperidol, spiperone, sulpiride and (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3- benzazepine-7-ol), it was concluded that the excitation evoked by dopamine was due to activation of D2 dopamine receptors, while the inhibition was the result of D1 receptor activation. The effects of the dopamine agonists on the firing of CA1 neurons were long-lasting, which suggests a modulating role of dopamine in the hippocampus.

Endogenous dopamine functionally activates D-1 and D-2 receptors in striatum

Rat striatal slices incubated with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine at 1 mM were exposed to different concentrations (1-100 microM) of the catecholamine-releasing drug amphetamine. This produced both a concentration-dependent release of endogenous dopamine and accumulation of cyclic AMP in the slices. The cyclic AMP accumulation due to amphetamine was greatly increased when slices were coincubated with the selective dopamine D-2 antagonist (-)-sulpiride (30 microM), but the amphetamine-induced release of dopamine from the slices was the same in the presence or absence of (-)-sulpiride. Pretreatment of animals with reserpine (5 mg/kg s.c., 18 h before death) and in vitro incubation with alpha-methyl-p-tyrosine (50 microM for 90 min), respectively, reduced the ability of amphetamine (1-100 microM) [in the presence of 30 microM (-)-sulpiride] to induce release of dopamine and to elevate cyclic AMP accumulation in striatal slices. A similar reduction in amphetamine-induced dopamine release and cyclic AMP accumulation in striatal slices was observed 7 days following unilateral 6-OHDA lesions of the medial forebrain bundle of rats. These results suggest that amphetamine induces release of endogenous dopamine from the terminals of nigrostriatal dopamine neurones. Released dopamine is then able functionally and concomitantly to activate D-1 and D-2 receptors, seen as stimulation and inhibition of cyclic AMP accumulation, respectively.

Opioid and D-2 receptor mediated inhibition of forskolin-stimulated cyclic AMP efflux from rat neostriatal slices

Opioid and D-2 receptor agonists inhibit adenylate cyclase activity in neostriatal slices and homogenates. In the present study we used cyclic AMP efflux from rat neostriatal tissue as a parameter to estimate the effects of these drugs on cyclic AMP formation. Both the mu-opioid receptor agonist morphine and the D-2 dopamine receptor agonist LY 171555 were able to inhibit the forskolin-stimulated cyclic AMP efflux. The effects of morphine and LY 171555 could be reversed by naloxone and sulpiride, respectively. These data indicate that measurements of cyclic AMP efflux from brain slices is an accurate reflection of the effects of receptor stimulation on adenylate cyclase activity.

Neurochemical and behavioural evidence that dopamine D-2 receptors in striatum couple to the Ni regulatory protein and inhibition of cyclic AMP accumulation

The effects of intrastriatal pertussis toxin, which inactivates the regulatory protein Ni, were investigated in various models of striatal D-2 receptor function in the rat. Using a multiple injection technique unilateral intrastriatal injections of pertussis toxin induced, after a lag phase, ipsilateral postural asymmetries which intensified upon peripheral administration of apomorphine. Injections of pertussis toxin also partially reduced the ability of a selective D-2 agonist, RU 24926 [N-n-propyl di-beta (3-hydroxyphenyl)-ethylamine], to inhibit cyclic AMP accumulation due to a selective D-1 agonist SKF 38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine), whilst also reducing the affinity of dopamine for D-2 binding sites in striatal membranes from animals given prior intrastriatal injections of the toxin. Pertussis toxin also increased striatal dopamine metabolism, seen as a reduction in the dopamine: DOPAC (3,4-Dihydroxyphenylacetic acid) ratio, similar to that seen following intrastriatal injections of the selective D-2 antagonist (+/-)-sulpiride. These results suggest that pertussis toxin has dopamine D-2 antagonist-like properties in the rat striatum, consistent with the idea that striatal D-2 function may rely, in part at least, upon the regulatory protein Ni and adenylate cyclase inhibition.

D1 dopamine receptor--the search for a function: a critical evaluation of the D1/D2 dopamine receptor classification and its functional implications

The present review focuses on the hypothesized D1/D2 dopamine (DA) receptor classification, originally based on the form of receptor coupling to adenylate cyclase activity. The pharmacological effects of compounds exhibiting putative selective agonist or antagonist profiles at those DA receptors positively coupled to adenylate cyclase activity (D1 DA receptors) are extensively reviewed. Comparisons are made with the effects of putative selective D2 DA receptor agonists and antagonists, and on the basis of this work, the DA receptor classification is critically evaluated. A variety of biochemical, behavioral, and electrophysiological evidence is presented which supports the view that D1 and D2 DA receptors can interact in both an opposing and synergistic fashion. Particular attention is focused on the possibility that D1 receptor stimulation is required to enable the expression of certain D2 receptor-mediated effects, and the functional consequences of this form of interaction are considered. A hypothetical model is presented which considers how both the opposing and enabling forms of interaction between D1 and D2 DA receptors can control behavioral expression. Finally, the clinical relevance of this work is discussed and the potential use of selective D1 receptor agonists and antagonists in the treatment of psychotic states and Parkinson's disease is considered.

Specific inhibition of dopamine D-1-mediated cyclic AMP formation by dopamine D-2, muscarinic cholinergic, and opiate receptor stimulation in rat striatal slices

The ability of different receptors to mediate inhibition of cyclic AMP accumulation due to a variety of agonists was examined in rat striatal slices. In the presence of 1 mM 3-isobutyl-1-methylxanthine, dopamine D-2, muscarinic cholinergic, and opiate receptor stimulation by RU 24926, carbachol, and morphine (all at 10(-8)-10(-5) M), respectively, inhibited the increase in cyclic AMP accumulation in slices of rat striatum due to dopamine D-1 receptor stimulation by 1 microM SKF 38393. In contrast, these inhibitory agents were unable to reduce the ability of a number of other agonists, including isoprenaline, prostaglandin E1, 2-chloroadenosine, vasoactive intestinal polypeptide, and cholera toxin, to increase cyclic AMP levels in striatal slices. These results suggest that in rat striatum either dopamine D-2, muscarinic cholinergic, and opiate receptors are only functionally linked to dopamine D-1 receptors or that the D-1 and D-2 receptors linked to adenylate cyclase lie on the cells, distinct from other receptors capable of elevating striatal cyclic AMP levels.