Effects of dopamine on phosphoinositide hydrolysis in slices of rat striatum and cortex

Dopamine D1 receptor signaling: does GαQ-phospholipase C actually play a role?

Despite numerous studies showing therapeutic potential, no central dopamine D1 receptor ligand has ever been approved, because of potential limitations, such as hypotension, seizures, and tolerance. Functional selectivity has been widely recognized as providing a potential mechanism to develop novel therapeutics from existing targets, and a highly biased, functionally selective D1 ligand might overcome some of the past limitations. SKF-83959 [6-chloro-3-methyl-1-(m-tolyl)-2,3,4,5-tetrahydro-1H-benzo[d]azepine-7,8-diol] is reported to be a highly biased D1 ligand, having full agonism at D1-mediated activation of phospholipase C (PLC) signaling (via GαQ) and antagonism at D1-mediated adenylate cyclase signaling (via GαOLF/S). For this reason, numerous studies have used this compound to elucidate the physiologic role of D1-PLC signaling, including a novel molecular mechanism (GαQ-PLC activation via D1-D2 heterodimers). There is, however, contradictory literature that suggests that SKF-83959 is actually a partial agonist at both D1-mediated adenylate cyclase and β-arrestin recruitment. Moreover, the D1-mediated PLC stimulation has also been questioned. This Minireview examines 30 years of relevant literature and proposes that the data strongly favor alternate hypotheses: first, that SKF-83959 is a typical D1 partial agonist; and second, that the reported activation of PLC by SKF-83959 and related benzazepines likely is due to off-target effects, not actions at D1 receptors. If these hypotheses are supported by future studies, it would suggest that caution should be used regarding the role of PLC and downstream pathways in D1 signaling.

SKF-83959 is not a highly-biased functionally selective D1 dopamine receptor ligand with activity at phospholipase C

SKF-83959 [6-chloro-7,8-dihydroxy-3-methyl-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine] is reported to be a functionally selective dopamine D1 receptor ligand with high bias for D1-mediated phospholipase C (PLC) versus D1-coupled adenylate cyclase signaling. This signaling bias is proposed to explain behavioral activity in both rat and primate Parkinson's disease models, and a D1-D2 heterodimer has been proposed as the underlying mechanism. We have conducted an in-depth pharmacological characterization of this compound in dopamine D1 and D2 receptors in both rat brain and heterologous systems expressing human D1 or D2 receptors. Contrary to common assumptions, SKF-83959 is similar to the classical, well-characterized partial agonist SKF38393 in all systems. It is a partial agonist (not an antagonist) at adenylate cyclase in vitro and ex vivo, and is a partial agonist in D1-mediated β-arrestin recruitment. Contrary to earlier reports, it does not have D1-mediated effects on PLC signaling in heterologous systems. Because drug metabolites can also contribute, its 3-N-demethylated analog also was synthesized and tested. As expected from the known structure-activity relationships of the benzazepines, this compound also had high affinity for the D1 receptor and somewhat higher intrinsic activity than the parent ligand, and also might contribute to in vivo effects of SKF-83959. Together, these data demonstrate that SKF-83959 is not a highly-biased functionally selective D1 ligand, and that its reported behavioral data can be explained solely by its partial D1 agonism in canonical signaling pathway(s). Mechanisms that have been proposed based on the purported signaling novelty of SKF-83959 at PLC should be reconsidered.

Pharmacology of signaling induced by dopamine D(1)-like receptor activation

Dopamine D(1)-like receptors consisting of D(1) and D(5) subtypes are intimately implicated in dopaminergic regulation of fundamental neurophysiologic processes such as mood, motivation, cognitive function, and motor activity. Upon stimulation, D(1)-like receptors initiate signal transduction cascades that are mediated through adenylyl cyclase or phosphoinositide metabolism, with subsequent enhancement of multiple downstream kinase cascades. The latter actions propagate and further amplify the receptor signals, thus predisposing D(1)-like receptors to multifaceted interactions with various other mediators and receptor systems. The adenylyl cyclase response to dopamine or selective D(1)-like receptor agonists is reliably associated with the D(1) subtype, while emerging evidence indicates that the phosphoinositide responses in native brain tissues may be preferentially mediated through stimulation of the D(5) receptor. Besides classic coupling of each receptor subtype to specific G proteins, additional biophysical models are advanced in attempts to account for differential subcellular distribution, heteromolecular oligomerization, and activity-dependent selectivity of the receptors. It is expected that significant advances in understanding of dopamine neurobiology will emerge from current and anticipated studies directed at uncovering the molecular mechanisms of D(5) coupling to phosphoinositide signaling, the structural features that might enhance pharmacological selectivity for D(5) versus D(1) subtypes, the mechanism by which dopamine may modulate phosphoinositide synthesis, the contributions of the various responsive signal mediators to D(1) or D(5) interactions with D(2)-like receptors, and the spectrum of dopaminergic functions that may be attributed to each receptor subtype and signaling pathway.

Alterations of inositol phosphate turnover in striatum of aged rats

Age-related inositol phosphate turnover in the rat central nervous system was investigated. Higher phospholipase-C activity and drastically higher (almost 2.5-fold) inositol 1,4,5-trisphosphate concentration in the corpus striatum (caudate-putamen) of extremely old (approximately 40 months) female Wistar rats in comparison to the young adult (approximately 3.5 months) rats were observed. Dopamine seems to slightly inhibit total inositol phosphate formation and this effect was antagonized by (-)-sulpiride.

Selective dopaminergic mechanism of dopamine and SKF38393 stimulation of inositol phosphate formation in rat brain

We have previously reported that dopamine and the D1 receptor-selective agonist, SKF38393, stimulate the formation of inositol phosphates in rat brain slices (Undie and Friedman, 1990, J. Pharmacol. Exp. Ther. 253, 987). The present experiments were conducted to determine if actions at alpha-adrenoceptors or at serotonergic sites may contribute to, or interact with, the observed stimulation of phosphoinositide hydrolysis by dopamine receptor agonists. Rat striatal slices prelabeled with [3H]inositol were treated with up to 500 microM dopamine, norepinephrine, serotonin (5-HT), or the dopamine D1 receptor agonist, SKF38393, and accumulated inositol phosphates determined. The action of norepinephrine was dose-dependently blocked by the selective alpha 1-adrenoceptor antagonist, prazosin, but not by SCH23390. The actions of dopamine and SKF38393 were dose-dependently blocked by the dopamine D1 receptor antagonist, SCH23390, but not by prazosin. The effects of 5-HT were blocked by the nonselective 5-HT antagonist, methiotepin, the selective 5-HT2 antagonist, ketanserin, the mixed 5-HT2/5-HT1C antagonist, mianserin, and, with much less potency, by the selective 5-HT1C antagonist, mesulergine. On the contrary, the serotonin receptor antagonists did not block the response to SKF38393, and there was no dose-dependent blockade of the 5-HT response by SCH23390. These observations indicate that the actions of dopamine and SKF38393 in stimulating inositol phosphate formation are selectively mediated through a D1-like dopamine receptor.

Induction of D2 dopamine receptor mRNA synthesis in a 6-hydroxydopamine parkinsonian rat model

By injecting 6-hydroxydopamine into the pars compacta of the substantia nigra, we produced a hemiparkinsonian rat model in which there is almost complete destruction of the dopaminergic nigrostriatal pathway but sparing of the dopaminergic mesolimbic system. The lesion has been characterized by several criteria: a rotational behavior in response to apomorphine, a complete loss of tyrosine hydroxylase immunoreactivity in the lesioned substantia nigra, a near total depletion of dopamine and metabolites in the striatum ipsilateral to the lesion, and a supersensitivity of the dopamine D2 receptors in the ipsilateral striatum. Dopaminergic striatal deafferentation was accompanied by an increase of D2 receptor mRNA synthesis in the striatum ipsilateral to the lesion, suggesting that the increased D2 receptor density observed after the lesion is due to an increase of the synthesis of receptor molecules. This synthesis appears to be regulated at the transcriptional level.

Activation of the 5-HT1C receptor expressed in Xenopus oocytes by the benzazepines SCH 23390 and SKF 38393

1. A cloned 5-HT1C receptor expressed in Xenopus laevis oocytes was used to characterize the action of four dopamine D1-selective benzazepines at the 5-HT1C receptor. Additionally, the apparent binding of the D1-selective benzazepines to 5-HT1C receptors was measured in the choroid plexus of the pig. 2. In voltage-clamped oocytes expressing the cloned 5-HT1C receptor, 5-hydroxytryptamine (5-HT) elicited a characteristic inward current response with an EC50 of 13 nM. SCH 23390 acted as a stereoselective agonist (or partial agonist) with an EC50 of about 550 nM. SKF 38393 (1 microM-1 mM), SKF 77434 (100 microM), and SKF 82958 (100 microM) also acted as agonists (or partial agonists) at the cloned 5-HT1C receptor. SKF 38393 was not stereoselective at the 5-HT1C receptor. 3. The response to SCH 23390 activated slowly and, although the response contained many oscillations characteristic of the activation of the phosphatidylinositol signal transduction system, SCH 23390 rarely elicited the rapid spike-like response seen routinely in response to 5-HT. However, the responses to SKF 38393, SKF 77434, and SKF 82958 were identical in appearance to the response to 5-HT, except that the responses to the benzazepines were smaller. These comparisons were made by applying both a benzazepine and 5-HT to each individual oocyte expressing the cloned 5-HT1C receptor. 4. Consistent with the responses measured in oocytes, SCH 23390 bound stereoselectively to 5-HT1C receptors in the choroid plexus of the pig (Ki = 6.3 nM), and SKF 38393 bound non-stereoselectively with lower affinity (Ki = 2.0-2.2 microM).5. It is concluded that while these benzazepines demonstrate selectivity for the dopamine D1 receptor, they also can act as agonists or partial agonists at the 5-HT1c receptor in situ and as expressed in Xenopus oocytes. The oocyte expression system is useful for studies of the functional pharmacology of these 5-HTic receptors. Information about the pharmacological actions and variations in stereoselectivity among dopamine and 5-HT receptors should be of interest in modelling the interactions of ligands with these G-protein coupled receptors, and in the testing of such models through receptor mutagenesis.