Substituted Indazoles as Nav1.7 Blockers for the Treatment of Pain.

The genetic validation for the role of the Nav1.7 voltage-gated ion channel in pain signaling pathways makes it an appealing target for the potential development of new pain drugs. The utility of nonselective Nav blockers is often limited due to adverse cardiovascular and CNS side effects. We sought more selective Nav1.7 blockers with oral activity, improved selectivity, and good druglike properties. The work described herein focused on a series of 3- and 4-substituted indazoles. SAR studies of 3-substituted indazoles yielded analog 7 which demonstrated good in vitro and in vivo activity but poor rat pharmacokinetics. Optimization of 4-substituted indazoles yielded two compounds, 27 and 48, that exhibited good in vitro and in vivo activity with improved rat pharmacokinetic profiles. Both 27 and 48 demonstrated robust activity in the acute rat monoiodoacetate-induced osteoarthritis model of pain, and subchronic dosing of 48 showed a shift to a lower EC50 over 7 days.

Synthesis and evaluation of 2-amido-3-carboxamide thiophene CB2 receptor agonists for pain management.

SAR studies on a series of thiophene amide derivatives provided CB(2) receptor agonists. The activity of the compounds was characterized by radioligand binding determination, multiple functional assays, ADME, and pharmacokinetic studies. A representative compound with selectivity for CB(2) over CB(1) effectively produced analgesia in behavioral models of neuropathic, inflammatory, and postsurgical pain. Control experiments using a CB(2) antagonist demonstrated the efficacy in the pain models resulted from CB(2) agonism.

Central and peripheral sites of action for CB2 receptor mediated analgesic activity in chronic inflammatory and neuropathic pain models in rats.

BACKGROUND AND PURPOSE: Cannabinoid CB2 receptor activation by selective agonists has been shown to produce analgesic effects in preclinical models of inflammatory and neuropathic pain. However, mechanisms underlying CB2-mediated analgesic effects remain largely unknown. The present study was conducted to elucidate the CB2 receptor expression in 'pain relevant' tissues and the potential sites of action of CB2 agonism in rats. EXPERIMENTAL APPROACH: Expression of cannabinoid receptor mRNA was evaluated by quantitative RT-PCR in dorsal root ganglia (DRGs), spinal cords, paws and several brain regions of sham, chronic inflammatory pain (CFA) and neuropathic pain (spinal nerve ligation, SNL) rats. The sites of CB2 mediated antinociception were evaluated in vivo following intra-DRG, intrathecal (i.t.) or intraplantar (i.paw) administration of potent CB2-selective agonists A-836339 and AM1241. KEY RESULTS: CB2 receptor gene expression was significantly up-regulated in DRGs (SNL and CFA), spinal cords (SNL) or paws (CFA) ipsilateral to injury under inflammatory and neuropathic pain conditions. Systemic A-836339 and AM1241 produced dose-dependent efficacy in both inflammatory and neuropathic pain models. Local administration of CB2 agonists also produced significant analgesic effects in SNL (intra-DRG and i.t.) and CFA (intra-DRG) pain models. In contrast to A-836339, i.paw administration of AM-1241 dose-relatedly reversed the CFA-induced thermal hyperalgesia, suggesting that different mechanisms may be contributing to its in vivo properties. CONCLUSIONS AND IMPLICATIONS: These results demonstrate that both DRG and spinal cord are important sites contributing to CB2 receptor-mediated analgesia and that the changes in CB2 receptor expression play a crucial role for the sites of action in regulating pain perception.

Diaryldiamines with dual inhibition of the histamine H(3) receptor and the norepinephrine transporter and the efficacy of 4-(3-(methylamino)-1-phenylpropyl)-6-(2-(pyrrolidin-1-yl)ethoxy)naphthalen-1-ol in pain.

A series of compounds was designed as dual inhibitors of the H(3) receptor and the norepinephrine transporter. Compound 5 (rNET K(i) = 14 nM; rH(3)R K(i) = 37 nM) was found to be efficacious in a rat model of osteoarthritic pain.

Antinociceptive effects of histamine H3 receptor antagonist in the preclinical models of pain in rats and the involvement of central noradrenergic systems.

The histamine H(3) receptor is predominantly expressed in the central nervous system and plays a role in diverse physiological mechanisms. In the present study, the effects of GSK189254, a potent and selective H(3) antagonist, were characterized in preclinical pain models in rats. Systemic GSK189254 produced dose-dependent efficacy (ED(50)=0.77 mg/kg i.p.) in a rat model of monoiodoacetate (MIA) induced osteoarthritic (OA) pain as evaluated by hindlimb grip force. The role of H(3) receptors in regulating pain perception was further demonstrated using other structurally distinct H(3) antagonists. GSK189254 also displayed efficacy in a rat surrogate model indicative of central sensitization, namely phase 2 response of formalin-induced flinching, and attenuated tactile allodynia in the spinal nerve ligation model of neuropathic pain (ED(50)=1.5mg/kg i.p.). In addition, GSK189254 reversed persistent (CFA) (ED(50)=2.1mg/kg i.p,), whereas was ineffective in acute (carrageenan) inflammatory pain. When administered intrathecally (i.t.) to the lumbar spinal cord, GSK189254 produced robust effects in relieving the OA pain (ED(50)=0.0027 mg/kg i.t.). The systemic GSK189254 effect was completely reversed by the alpha-adrenergic receptor antagonist phentolamine (i.p. and i.t.) but not by the opioid receptor antagonist naloxone (i.p.). Furthermore, the i.t. GSK189254 effect was abolished when co-administered with phentolamine (i.t.). These results suggest that the spinal cord is an important site of action for H(3) antagonism and the effect can be associated with activation of the noradrenergic system. Our data also provide support that selective H(3) antagonists may represent a class of agents for the treatment of pain disorders.

H4 receptor antagonism exhibits anti-nociceptive effects in inflammatory and neuropathic pain models in rats.

The histamine H(4) receptor (H(4)R) is expressed primarily on cells involved in inflammation and immune responses. To determine the potential role of H(4)R in pain transmission, the effects of JNJ7777120, a potent and selective H(4) antagonist, were characterized in preclinical pain models. Administration of JNJ7777120 fully blocked neutrophil influx observed in a mouse zymosan-induced peritonitis model (ED(50)=17 mg/kg s.c., 95% CI=8.5-26) in a mast cell-dependent manner. JNJ7777120 potently reversed thermal hyperalgesia observed following intraplantar carrageenan injection of acute inflammatory pain (ED(50)=22 mg/kg i.p., 95% CI=10-35) in rats and significantly decreased the myeloperoxide activity in the carrageenan-injected paw. In contrast, no effects were produced by either H(1)R antagonist diphenhydramine, H(2)R antagonists ranitidine, or H(3)R antagonist ABT-239. JNJ7777120 also exhibited robust anti-nociceptive activity in persistent inflammatory (CFA) pain with an ED(50) of 29 mg/kg i.p. (95% CI=19-40) and effectively reversed monoiodoacetate (MIA)-induced osteoarthritic joint pain. This compound also produced dose-dependent anti-allodynic effects in the spinal nerve ligation (ED(50)=60 mg/kg) and sciatic nerve constriction injury (ED(50)=88 mg/kg) models of chronic neuropathic pain, as well as in a skin-incision model of acute post-operative pain (ED(50)=68 mg/kg). In addition, the analgesic effects of JNJ7777120 were maintained following repeated administration and were evident at the doses that did not cause neurologic deficits in rotarod test. Our results demonstrate that selective blockade of H(4) receptors in vivo produces significant anti-nociception in animal models of inflammatory and neuropathic pain.

Indol-3-ylcycloalkyl ketones: effects of N1 substituted indole side chain variations on CB(2) cannabinoid receptor activity.

Several 3-acylindoles with high affinity for the CB(2) cannabinoid receptor and selectivity over the CB(1) receptor have been prepared. A variety of 3-acyl substituents were investigated, and the tetramethylcyclopropyl group was found to lead to high affinity CB(2) agonists (5, 16). Substitution at the N1-indole position was then examined. A series of aminoalkylindoles was prepared and several substituted aminoethyl derivatives were active (23-27, 5) at the CB(2) receptor. A study of N1 nonaromatic side chain variants provided potent agonists at the CB(2) receptor (16, 35-41, 44-47, 49-54, and 57-58). Several polar side chains (alcohols, oxazolidinone) were well-tolerated for CB(2) receptor activity (41, 50), while others (amide, acid) led to weaker or inactive compounds (55 and 56). N1 aromatic side chains also afforded several high affinity CB(2) receptor agonists (61, 63, 65, and 69) but were generally less potent in an in vitro CB(2) functional assay than were nonaromatic side chain analogues.

Pharmacological modulation of movement-evoked pain in a rat model of osteoarthritis.

This study was conducted to characterize movement-induced pain in a rat model of knee joint osteoarthritis and validate this behavioral assessment by evaluating the effects of clinically used analgesic compounds. Unilateral intra-articular administration of a chondrocyte glycolytic inhibitor monoiodoacetate, was used to induce knee joint osteoarthritis in Sprague-Dawley rats. In this osteoarthritis model, histologically erosive disintegration of the articular surfaces of the ipsilateral joint are observed which closely mimic the clinical picture of osteoarthritis. Movement-induced pain behavior was measured using hind limb compressive grip force evaluation. The animals exhibited pain behaviors epitomized by a long-lasting decrement in bilateral compressive hind limb grip force following unilateral knee injury. The effects of clinically used reference analgesics were evaluated 20 days following i.a. injection of monoiodoacetate. Full analgesic activity was observed for tramadol, celecoxib and diclofenac; moderate effects for indomethacin, duloxetine and gabapentin but weak or no effects for acetaminophen, ibuprofen and lamotrigine. As morphine reduced grip force in naïve rats, its analgesic effects could not be accurately evaluated in this model. Finally, the effects of celecoxib were maintained following chronic dosing. The results indicate that this in vivo model utilizing a movement-induced pain behavior spawned by knee joint osteoarthritis may provide a valuable tool in examining the role of potential analgesic targets in osteoarthritic pain. As the model is clinically relevant, it will further enhance the mechanistic understanding of chronic arthritic joint pain and help in developing newer and better therapeutic strategies to manage osteoarthritis pain.

Discovery and biological evaluation of novel cyanoguanidine P2X(7) antagonists with analgesic activity in a rat model of neuropathic pain.

We disclose the design of a novel series of cyanoguanidines that are potent (IC(50) approximately 10-100 nM) and selective (> or = 100-fold) P2X(7) receptor antagonists against the other P2 receptor subtypes such as the P2Y(2), P2X(4), and P2X(3). We also found that these P2X(7) antagonists effectively reduced nociception in a rat model of neuropathic pain (Chung model). Particularly, analogue 53 proved to be effective in the Chung model, with an ED(50) of 38 micromol/kg after intraperitoneal administration. In addition compound 53 exhibited antiallodynic effects following oral administration and maintained its efficacy following repeated administration in the Chung model. These results suggest an important role of P2X(7) receptors in neuropathic pain and therefore a potential use of P2X(7) antagonists as novel therapeutic tools for the treatment of this type of pain.

Localization of histamine H4 receptors in the central nervous system of human and rat.

Existing data on the expression of H(4) histamine receptor in the CNS are conflicting and inconclusive. In this report, we present the results of experiments that were conducted in order to elucidate H(4) receptor expression and localization in the brain, spinal cord, and dorsal root ganglia (DRG). Here we show that transcripts of H(4) receptor are present in all analyzed regions of the human CNS, including spinal cord, hippocampus, cortex, thalamus and amygdala, with the highest levels of H(4) mRNA detected in the spinal cord. In rat, H(4) mRNA was detected in cortex, cerebellum, brainstem, amygdala, thalamus and striatum. Very low levels of H(4) mRNA were detected in hypothalamus, and no H(4) signal was detected in the rat hippocampus. Fairly low levels of H(4) mRNA were detected in examined peripheral tissues including spleen and liver. Interestingly, strong expression of H(4) mRNA was detected in the rat DRG and spinal cord. Immunohistochemical analysis revealed expression of H(4) receptors on neurons in the rat lumbar DRG and in the lumbar spinal cord. Our observations provide evidence of the H(4) presence in both human and rodent CNS and offer some insight into possible role of H(4) in itch and pain.

cis-4-(Piperazin-1-yl)-5,6,7a,8,9,10,11,11a-octahydrobenzofuro[2,3-h]quinazolin-2-amine (A-987306), a new histamine H4R antagonist that blocks pain responses against carrageenan-induced hyperalgesia.

cis-4-(Piperazin-1-yl)-5,6,7a,8,9,10,11,11a-octahydrobenzofuro[2,3-h]quinazolin-2-amine, 4 (A-987306) is a new histamine H(4) antagonist. The compound is potent in H(4) receptor binding assays (rat H(4), K(i) = 3.4 nM, human H(4) K(i) = 5.8 nM) and demonstrated potent functional antagonism in vitro at human, rat, and mouse H(4) receptors in cell-based FLIPR assays. Compound 4 also demonstrated H(4) antagonism in vivo in mice, blocking H(4)-agonist induced scratch responses, and showed anti-inflammatory activity in mice in a peritonitis model. Most interesting was the high potency and efficacy of this compound in blocking pain responses, where it showed an ED(50) of 42 mumol/kg (ip) in a rat post-carrageenan thermal hyperalgesia model of inflammatory pain.

Structure-activity studies on a series of a 2-aminopyrimidine-containing histamine H4 receptor ligands.

A series of 2-aminopyrimidines was synthesized as ligands of the histamine H4 receptor (H4R). Working in part from a pyrimidine hit that was identified in an HTS campaign, SAR studies were carried out to optimize the potency, which led to compound 3, 4- tert-butyl-6-(4-methylpiperazin-1-yl)pyrimidin-2-ylamine. We further studied this compound by systematically modifying the core pyrimidine moiety, the methylpiperazine at position 4, the NH2 at position 2, and positions 5 and 6 of the pyrimidine ring. The pyrimidine 6 position benefited the most from this optimization, especially in analogs in which the 6- tert-butyl was replaced with aromatic and secondary amine moieties. The highlight of the optimization campaign was compound 4, 4-[2-amino-6-(4-methylpiperazin-1-yl)pyrimidin-4-yl]benzonitrile, which was potent in vitro and was active as an anti-inflammatory agent in an animal model and had antinociceptive activity in a pain model, which supports the potential of H 4R antagonists in pain.

Rotationally constrained 2,4-diamino-5,6-disubstituted pyrimidines: a new class of histamine H4 receptor antagonists with improved druglikeness and in vivo efficacy in pain and inflammation models.

A new structural class of histamine H 4 receptor antagonists (6-14) was designed based on rotationally restricted 2,4-diaminopyrimidines. Series compounds showed potent and selective in vitro H 4 antagonism across multiple species, good CNS penetration, improved PK properties compared to reference H 4 antagonists, functional H 4 antagonism in cellular and in vivo pharmacological assays, and in vivo anti-inflammatory and antinociceptive efficacy. One compound, 10 (A-943931), combined the best features of the series in a single molecule and is an excellent tool compound to probe H 4 pharmacology. It is a potent H 4 antagonist in functional assays across species (FLIPR Ca (2+) flux, K b < 5.7 nM), has high (>190x) selectivity for H 4, and combines good PK in rats and mice (t 1/2 of 2.6 and 1.6 h, oral bioavailability of 37% and 90%) with anti-inflammatory activity (ED 50 = 37 micromol/kg, mouse) and efficacy in pain models (thermal hyperalgesia, ED 50 = 72 micromol/kg, rat).

Indol-3-yl-tetramethylcyclopropyl ketones: effects of indole ring substitution on CB2 cannabinoid receptor activity.

A series of potent indol-3-yl-tetramethylcyclopropyl ketones have been prepared as CB 2 cannabinoid receptor ligands. Two unsubstituted indoles ( 5, 32) were the starting points for an investigation of the effect of indole ring substitutions on CB 2 and CB 1 binding affinities and activity in a CB 2 in vitro functional assay. Indole ring substitutions had varying effects on CB 2 and CB 1 binding, but were generally detrimental to agonist activity. Substitution on the indole ring did lead to improved CB 2/CB 1 binding selectivity in some cases (i.e., 7- 9, 15- 20). All indoles with the morpholino-ethyl side chain ( 32- 43) exhibited weaker binding affinity and less agonist activity relative to that of their tetrahydropyranyl-methyl analogs ( 5- 31). Several agonists were active in the complete Freund's adjuvant model of chronic inflammatory thermal hyperalgesia ( 32, 15).

Discovery of 3-methyl-N-(1-oxy-3',4',5',6'-tetrahydro-2'H-[2,4'-bipyridine]-1'-ylmethyl)benzamide (ABT-670), an orally bioavailable dopamine D4 agonist for the treatment of erectile dysfunction.

The goal of this study was to identify a structurally distinct D(4)-selective agonist with superior oral bioavailability to our first-generation clinical candidate 1a (ABT-724) for the potential treatment of erectile dysfunction. Arylpiperazines such as (heteroarylmethyl)piperazine 1a, benzamide 2, and acetamides such as 3a,b exhibit poor oral bioavailability. Structure-activity relationship (SAR) studies with the arylpiperidine template provided potent partial agonists such as 4d and 5k that demonstrated no improvement in oral bioavailability. Further optimization with the (N-oxy-2-pyridinyl)piperidine template led to the discovery of compound 6b (ABT-670), which exhibited excellent oral bioavailability in rat, dog, and monkey (68%, 85%, and 91%, respectively) with comparable efficacy, safety, and tolerability to 1a. The N-oxy-2-pyridinyl moiety not only provided the structural motif required for agonist function but also reduced metabolism rates. The SAR study leading to the discovery of 6b is described herein.

1-aryl-3-(4-pyridine-2-ylpiperazin-1-yl)propan-1-one oximes as potent dopamine D4 receptor agonists for the treatment of erectile dysfunction.

A new series of dopamine D4 receptor agonists, 1-aryl-3-(4-pyridinepiperazin-1-yl)propanone oximes, was designed through the modification of known dopamine D4 receptor agonist PD 168077. Replacement of the amide group with a methylene-oxime moiety produced compounds with improved stability and efficacy. Structure-activity relationsips (SAR) of the aromatic ring linked to the N-4-piperazine ring confirmed the superiority of 2-pyridine as a core for D4 agonist activity. A two-methylene linker between the oxime group and the N-1-piperazine ring displayed the best profile. New dopamine D4 receptor agonists, exemplified by (E)-1-(4-chlorophenyl)-3-(4-pyridin-2-ylpiperazin-1-yl)propan-1-one O-methyloxime (59a) and (E)-1-(3-chloro-4-fluorophenyl)-3-(4-pyridin-2-ylpiperazin-1-yl)propan-1-one O-methyloxime (64a), exhibited favorable pharmacokinetic profiles and showed oral bioavailability in rat and dog. Subsequent evaluation of 59a in the rat penile erection model revealed in vivo activity, comparable in efficacy to apomorphine. Our results suggest that the oximes provide a novel structural linker for 4-arylpiperazine-based D4 agonists, possessing leadlike quality and with potential to develop a new class of potent and selective dopamine D4 receptor agonists.

A-412997 is a selective dopamine D4 receptor agonist in rats.

A-412997 (2-(3',4',5',6'-tetrahydro-2'H-[2,4'] bipyridinyl-1'-yl)-N-m-tolyl-acetamide) is a highly selective dopamine D4 receptor agonist that binds with high affinity to rat dopamine D4 and human dopamine D4.4 receptors (Ki=12.1 and 7.9 nM, respectively). In contrast to the dopamine D4 receptor agonists PD168077 and CP226269, A-412997 showed a better selectivity profile and no affinity <1000 nM for other dopamine receptors or any other proteins in a panel of seventy different receptors and channels. In functional assays using calcium flux, A-412997 was a potent full agonist at rat dopamine D4 receptors (28.4 nM, intrinsic activity=0.83) and did not activate rat dopamine D2L receptors, unlike CP226269. Dopamine D4 receptor selective agonists have been shown to induce penile erection in rats by central mechanisms. A-412997 induces penile erection in a conscious rat model (effective dose=0.1 micromol/kg, s.c.) with comparable efficacy as the nonselective D2-like agonist, apomorphine. When dosed systemically, A-412997 crossed the blood brain barrier rapidly and achieved significantly higher levels than PD168077. A-412997 is a highly selective dopamine D4 receptor agonist and a useful tool to understand the role of dopamine D4 receptors in rat models of central nervous system processes and disease.

Central oxytocinergic and dopaminergic mechanisms regulating penile erection in conscious rats.

A series of in vivo studies in a conscious rat model was conducted to investigate the role of oxytocinergic and dopaminergic neurotransmission in the central regulation of penile erection. Oxytocin, when administrated either intracerebroventricularly (i.c.v.) or intrathecally (i.t.) at the spinal levels of L4-L6, produced dose-related erectogenic effects with a maximum at 0.1 microg/rat i.c.v. or 0.03 microg/rat i.t. Oxytocin-evoked penile activity was attenuated by the inhibitory effect of the selective oxytocin antagonist vasotocin analog [Pmp-Tyr(Me)-Ile-Thr-Asn-Cys]-Pro-Orn-Tyr-NH2 (0.1-1 microg, i.c.v. or i.t.). Penile erection induced by oxytocin was blocked by the dopaminergic receptor antagonist clozapine (1-10 micromol/kg i.p.) in a dose-dependent manner. Conversely, oxytocin antagonist microinjected locally (i.c.v. or i.t.) significantly attenuated the pro-erectile effects of systemic (s.c.) apomorphine, a centrally acting erectogenic agent through dopaminergic receptors. Together, these data indicate a possible concomitant role between dopamine and oxytocin in mediating penile erection at both the spinal and supraspinal sites.

2-[4-(3,4-Dimethylphenyl)piperazin-1-ylmethyl]-1H benzoimidazole (A-381393), a selective dopamine D4 receptor antagonist.

2-[4-(3,4-Dimethylphenlyl)piperazin-1-ylmethyl]-1H benzoimidazole (A-381393) was identified as a potent dopamine D4 receptor antagonist with excellent receptor selectivity. [3H]-spiperone competition binding assays showed that A-381393 potently bound to membrane from cells expressing recombinant human dopamine D4.4 receptor (Ki=1.5 nM), which was 20-fold higher than that of clozapine (Ki=30.4 nM). A-381393 exhibited highly selective binding for the dopamine D4.4 receptor (>2700-fold) when compared to D1, D2, D3 and D5 dopamine receptors. Furthermore, in comparison to clozapine and L-745870, A-381393 exhibits better receptor selectivity, showing no affinity up to 10 microM for a panel of more than 70 receptors and channels, with the exception of moderate affinity for 5-HT2A (Ki=370 nM). A-381393 potently inhibited the functional activity of agonist-induced GTP-gamma-S binding assay and 1 microM dopamine induced-Ca2+ flux in human dopamine D4.4 receptor expressing cells, but not in human dopamine D2L or D3 receptor cells. In contrast to L-745870, A-381393 did not exhibit any significant intrinsic activity in a D4.4 receptor. In vivo, A-381393 has good brain penetration after subcutaneous administration. A-381393 inhibited penile erection induced by the selective D4 agonist PD168077 in conscious rats. Thus, A-381393 is a novel selective D4 antagonist that will enhance the ability to study dopamine D4 receptors both in vitro and in vivo.

Antidepressant-like effect of D(2/3) receptor-, but not D(4) receptor-activation in the rat forced swim test.

Dopamine plays a role in the pathophysiology of depression and therapeutic effects of antidepressants but the contribution of individual D(2)-like receptor subtypes (D(2), D(3), D(4)) to depression is not known. We present evidence that activation of D(2)/D(3), but not D(4) receptors, can affect the outcome in the rat forced swim test (FST). Nomifensine, a dopamine uptake inhibitor (7, 14, and 28 micromol/kg); quinpirole, a D(2)-like receptor and agonist (0.4, 1.0, and 2.0 micromol/kg); PD 12,8907, a preferential D(3) receptor agonist (0.17, 0.35, and 0.7 micromol/kg); PD 168077 (0.1, 0.3, and 1.0 micromol/kg) and CP 226269 (0.3, 1.0, and 3.0 micromol/kg), both selective D(4) receptor agonists, were administered s.c. 24, 5, and 0.5/1 h before testing. Nomifensine, quinpirole at all doses and PD 128907 at the highest dose decreased immobility time in FST. PD 168077 and CP 226269 had no effect on the model. To further clarify what type of dopamine receptors were involved in the anti-immobility effect of quinpirole, we tested different antagonists. Haloperidol, a D(2)-like receptor antagonist (0.27 micromol/kg), completely blocked the effect of quinpirole; A-437203 (LU-201640), a selective D(3) receptor antagonist (17.46 micromol/kg), showed a nonsignificant trend to attenuate the effect of the low dose of quinpirole, and L-745,870, a selective D(4) receptor antagonist (1.15 micromol/kg), had no effect. The pharmacological selectivity of the compounds tested suggests that the antidepressant-like effects of quinpirole are most likely mediated mainly by D(2) and to a lesser extent by D(3) but not D(4) receptors.