The dopamine D(1) receptor agonist SKF-82958 serves as a discriminative stimulus in the rat

Effects of dopamine D1 receptor agonists in rats trained to discriminate dihydrexidine

RATIONALE

The full D1 receptor agonist dihydrexidine (DHX) [(+/-)-trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine hydrochloride] is under clinical development (DAR-100) for Parkinson's disease and schizophrenia. Despite the clinical development of DHX, very little is known about its discriminative stimulus properties in rats. To more fully characterize the discriminative stimulus properties of DHX, we trained rats to discriminate DHX (3 mg/kg, i.p.) from vehicle.

METHODS

Substitution tests in rats discriminating DHX (3 mg/kg, i.p.) from vehicle were performed with structurally distinct D1 receptor agonists with both partial and full intrinsic efficacy. In addition, the peripherally restricted D1 agonist, fenoldopam, was evaluated.

RESULTS

SKF 75670A, ABT-431, dinapsoline, SKF 81297, and SKF 82958 all fully substituted in a dose-dependent manner. The rank order of potency for substitution was SKF 82958<ABT-431<SKF 75670A<or=dinapsoline<SKF 81297<DHX. Fenoldopam (10 and 30 mg/kg) did not substitute and was without effect on rates of responding.

CONCLUSIONS

DHX produces prominent dopamine D1 receptor agonist effects in vivo and is likely to produce subjective effects in humans similar to other D1 receptor agonists.

D2 but not D1 dopamine receptor stimulation augments brain signaling involving arachidonic acid in unanesthetized rats

RATIONALE AND OBJECTIVES

Signal transduction involving the activation of phospholipase A2 (PLA2) to release arachidonic acid (AA) from membrane phospholipids, when coupled to dopamine D1- and D2-type receptors, can be imaged in rats having a chronic unilateral lesion of the substantia nigra. It is not known, however, if the signaling responses occur in the absence of a lesion. To determine this, we used our in vivo fatty acid method to measure signaling in response to D1 and D2 receptor agonists given acutely to unanesthetized rats.

METHODS

[1-(14)C]AA was injected intravenously in unanesthetized rats, and incorporation coefficients k* for AA (brain radioactivity/integrated plasma radioactivity) were measured using quantitative autoradiography in 61 brain regions. The animals were administered i.v. the D2 receptor agonist, quinpirole (1 mg kg(-1), i.v.), the D1 receptor agonist SKF-38393 (5 mg kg(-1), i.v.), or vehicle/saline.

RESULTS

Quinpirole increased k* significantly in multiple brain regions rich in D2-type receptors, whereas SKF-38393 did not change k* significantly in any of the 61 regions examined.

CONCLUSIONS

In the intact rat brain, D2 but not D1 receptors are coupled to the activation of PLA2 and the release of AA.

Effects of dopamine D1 receptor full agonists in rats trained to discriminate SKF 38393

Although the dopaminergic pharmacology of the D1 receptor full agonists, dinapsoline, dihydrexidine and the prodrug ABT-431 have been studied, no information is available on the ability of these agonists to substitute for the D1 agonist SKF 38393 in rats trained to discriminate this compound from vehicle. The present study was designed to characterize the potential D1 discriminative stimulus effects of these compounds. The selective dopamine D1-receptor agonists dihydrexidine [(+/-)-trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a] phenanthridine hydrochloride], ABT-431 [(-)-trans-9,10-diacetyloxy-2-propyl-4,5,5a,6,7,11b-hexahydro-3-thia-5-azacyclopent-1-ena[c]phenanthrene hydrochloride], the diacetyl prodrug derivative of A-86929, and dinapsoline [9-dihydroxy-2,3,7,11b-tetrahydro-1H-naph[1,2,3-de]isoquinoline] were studied in rats trained to discriminate racemic SKF 38393 [(+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol], a selective D1 receptor partial agonist from vehicle. All of the agonists substituted fully for the discriminative stimulus effects of SKF 38393. The rank order of potency for substitution was ABT-431 > dinapsoline > dihydrexidine > SKF 38393. The D1 receptor antagonist, SCH 23390, blocked the discriminative stimulus effects of SKF 38393. The D3/D2-receptor agonist PD 128,907 [S(+)-(4aR,10bR)-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]-benzopyrano[4,3-b]-1,4-oxazin-9-ol] did not substitute up to doses that produced profound rate-suppressant effects. Thus, consistent with their D1 receptor pharmacology, the full D1-receptor agonists substituted completely for the discriminative stimulus of SKF 38393.

Comparison of the discriminative-stimulus effects of SKF 38393 with those of other dopamine receptor agonists

The dopamine D(1)-like receptor agonists have traditionally been defined molecularly by their efficacy in stimulating adenylyl cyclase. However, evidence correlating the effectiveness of these drugs in behavioral assays and their effectiveness biochemically has not been forthcoming. The present study compared the discriminative-stimulus effects of the D(1)-like partial agonist SKF 38393 with several other D(1)-like agonists, an indirect agonist, cocaine, and a D(2)-like agonist, quinpirole. Rats were trained under a fixed-ratio 30-response schedule to discriminate SKF 38393 (5.6 mg/kg) from vehicle. Under this schedule, 30 consecutive responses on one of two keys were reinforced with food presentation after a pre-session injection of 5.6 mg/kg SKF 38393, and 30 consecutive responses on the alternative key were reinforced after saline injection. When daily performances were stable, substitution patterns for several compounds were assessed during test sessions in which 30 consecutive responses on either key were reinforced. Quinpirole and cocaine each produced saline-appropriate responding. In contrast, the D(1)-like agonists, SKF 75670 and SKF 77434, fully substituted for SKF 38393. Curiously, SKF 82958, which is considered a full agonist based on adenylyl cyclase assays, was less effective in substituting for SKF 38393 (maximum drug-appropriate responding 66%) than was the partial agonist SKF 75670. The present results suggest that second messenger effects other than stimulation of adenylyl cyclase may play an important role in the behavioral effects of dopamine D(1)-like agonists.

Intra-prefrontal cortex injections of SCH 23390 influence nucleus accumbens dopamine levels 24 h post-infusion

The dopaminergic pathway from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) is well known to be involved in the reinforcing properties of many drugs of abuse. The medial prefrontal cortex (mPFC) has been shown to exhibit significant influence over activity in this pathway, and has also been implicated in drug abuse. The present experiment investigated the ability of D1 activity in the mPFC to influence accumbal dopamine levels. NAcc dopamine (DA) was monitored before, immediately after, and 24 h following mPFC infusion of a D1 agonist (SKF 38393), D1 antagonist (SCH 23390), or a vehicle solution. Immediately following infusion of dopaminergic agents or vehicle, no significant changes in accumbal DA were observed. However, 24 h following infusion of the antagonist but not the agonist, significant elevations of accumbal DA were observed. Since elevated NAcc DA was only observed 24 h after treatment, these results provide evidence that long-term neural adaptations can be induced by transient neuropharmacological treatment.