P2X3 and P2X2/3 receptors are highly localized on peripheral and central processes of sensory afferent nerves, and activation of these channels contributes to the pronociceptive effects of ATP. A-317491 is a novel non-nucleotide antagonist of P2X3 and P2X2/3 receptor activation. A-317491 potently blocked recombinant human and rat P2X3 and P2X2/3 receptor-mediated calcium flux (Ki ‫؍‬ 22-92 nM) and was highly selective (IC50 >10 M) over other P2 receptors and other neurotransmitter receptors, ion channels, and enzymes. A-317491 also blocked native P2X3 and P2X2/3 receptors in rat dorsal root ganglion neurons. Blockade of P2X3 containing channels was stereospecific because the R-enantiomer (A-317344) of A-317491 was significantly less active at P2X3 and P2X2/3 receptors. A-317491 dosedependently (ED50 ‫؍‬ 30 mol͞kg s.c.) reduced complete Freund's adjuvant-induced thermal hyperalgesia in the rat. A-317491 was most potent (ED50 ‫؍‬ 10 -15 mol͞kg s.c.) in attenuating both thermal hyperalgesia and mechanical allodynia after chronic nerve constriction injury. The R-enantiomer, A-317344, was inactive in these chronic pain models. Although active in chronic pain models, A-317491 was ineffective (ED 50 >100 mol͞kg s.c.) in reducing nociception in animal models of acute pain, postoperative pain, and visceral pain. The present data indicate that a potent and selective antagonist of P2X 3 and P2X2/3 receptors effectively reduces both nerve injury and chronic inflammatory nociception, but P2X 3 and P2X2/3 receptor activation may not be a major mediator of acute, acute inflammatory, or visceral pain.T he cloning and characterization of the P2X 3 receptor, a specific ATP-sensitive ligand-gated ion channel that is selectively localized on peripheral and central processes of sensory afferent neurons (1-3), has generated much interest in the role of this receptor in nociceptive signaling (4). The discovery of the P2X 3 receptor has provided a putative mechanism for previous reports that ATP, released from sensory nerves (5), produces fast excitatory potentials in dorsal root ganglion (DRG) neurons (6). These actions appear to be physiologically relevant because iontophoretic application of ATP to human skin elicits pain (7) and exogenously applied ATP enhances pain sensations in a human blister base model (8).The P2X 3 receptor is natively expressed as a functional homomer and as a heteromultimeric combination with the P2X 2 (P2X 2/3 ) receptor (1, 2, 9). Both P2X 3 -containing channels are expressed on a high proportion of isolectin IB4-positive neurons in DRG (3, 10). These receptors share similar pharmacological profiles (11), but differ in their acute desensitization kinetics (10, 12). Immunohistochemical studies have shown that P2X 3 receptor expression is up-regulated in DRG neurons and ipsilateral spinal cord after chronic constriction injury (CCI) of the sciatic nerve (13). Additionally, CCI results in a specific ectopic sensitivity to ATP that is not observed on contralateral (uninjured) nerves (14).Recently, the phenotyp...

Blockade of the renin-angiotensin system was studied in male Sprague-Dawley rats during long-term inhibition of nitric oxide synthase. Nitro-L-arginine-methyl ester (L-NAME) was placed in the drinking water for 4 weeks (-100 nig/kg per day). Separate groups of rats were coadministered the angiotensin II antagonist A-81988 in the drinking water ranging from approximately 0.001 to 1 ing/kg per day. Control groups received only tap water or A-81988 alone. Each week, rats were placed in metabolic cages, and tail-cuff blood pressures and blood samples were taken. L-NAME produced a sustained elevation in tail-cuff pressure that was completely prevented by A-81988. No changes in creatinine clearance, sodium excretion, plasma creatinine concentration, or blood urea nitrogen were observed. Food and water intakes were identical in all groups. Water excretion was significantly increased in L-NAME-treated animals regardless of additional inhibitor treatment, suggesting a possible role for nitric oxide synthase in the control of water excretion; this effect was independent of blood pressure. Although less potent than A-81988, the angiotensin II antagonist losartan and the angiotensin converting enzyme inhibitor enalapril also blocked L-NAME-induced hypertension. In a separate series of experiments, rats were not given A-81988 until 2 weeks after hypertension had fully developed in L-NAME-treated rats. Within 1 week of treatment with the angiotensin II antagonist, tail-cuff pressure returned to normal. We conclude from these studies that long-term inhibition of endogenous nitric oxide production produces an angiotensin II-dependent form of hypertension. {Hypertension 1993;21:660-666) KEY WORDS • endothelium-derived relaxing factor • nitro-L-arginine-methyl ester • hypertension, essential • angiotensin converting enzyme inhibitors • renin-angiotensin system • kidney • rat studies • renal function T he endothelium-derived relaxing factor nitric oxide (NO) appears to play a significant role in the regulation of renal and systemic hemodynamics. One line of evidence that supports such a role is the observation of increased vascular resistance and blood pressure after acute inhibition of NO synthase, the enzyme responsible for production of NO and L-citrulline from L-arginine and oxygen.1 Recently, it has been reported that long-term inhibition of NO synthase will produce a sustained hypertension in otherwise normotensive rats or dogs.2 -4 This prolonged hypertension was associated with a mild degree of renal failure, as evidenced by decreases in glomerular filtration rate (GFR), proteinuria, and glomerular sclerotic injury. -4In addition to the enzyme present in vascular endothelium (type III), several isoforms of NO synthase have been identified in a variety of tissues, including the brain (type I).5 " 8 Interestingly, type I NO synthase is present in regions of the hypothalamus thought to be important in fluid-volume regulation, which suggests a role for this isoform in the central nervous

Apomorphine, a nonselective dopamine receptor agonist, facilitates penile erection and is effective in patients suffering from erectile dysfunction. The specific dopamine receptor subtype(s) responsible for its erectogenic effect is not known. Here we report that the dopamine D4 receptor plays a role in the regulation of penile function. ABT-724 is a selective dopamine D 4 receptor agonist that activates human dopamine D 4 receptors with an EC50 of 12.4 nM and 61% efficacy, with no effect on dopamine D 1, D2, D3, or D5 receptors. ABT-724 dose-dependently facilitates penile erection when given s.c. to conscious rats, an effect that is blocked by haloperidol and clozapine but not by domperidone. A proerectile effect is observed after intracerebroventricular but not intrathecal administration, suggesting a supraspinal site of action. s.c. injections of ABT-724 increase intracavernosal pressure in awake freely moving rats. In the presence of sildenafil, a potentiation of the proerectile effect of ABT-724 is observed in conscious rats. The ability of ABT-724 to facilitate penile erection together with the favorable side-effect profile indicates that ABT-724 could be useful for the treatment of erectile dysfunction. P enile erection is the result of a complex series of integrated neuronal and vascular events that leads to accumulation of blood in the penis to achieve rigidity. The coordination of several neural events is required to release endogenous mediators (i.e., nitric oxide) at the level of the penile smooth muscle to induce relaxation, whereas a disruption of this series of events can lead to erectile dysfunction (ED) (1, 2). Traditionally, drugs used for the treatment of ED have had a peripheral site of action, including phosphodiesterase (PDE)-5 inhibitors like sildenafil, but the recent demonstration that apomorphine can facilitate penile erection in ED patients has introduced a new approach to treatment, because apomorphine acts on central dopaminergic systems (3,4). Dopamine is the main catecholamine in the CNS and is involved in a variety of physiological functions, including sexual behavior, cognition, motor coordination, cardiovascular control, reward, hormonal regulation; abnormalities in dopaminergic neurotransmission have been implicated in Parkinson's disease, schizophrenia, attention-deficit disorder, depression, and drug abuse, among other disorders (5, 6). The regional distribution of the receptor subtypes, the recent generation of knock-out animals, and the availability of a large number of dopaminergic agents have aided researchers in clarifying the biological role of the dopamine receptors. Dopamine receptors in mammalian tissues have been classified as D 1 -like (D 1 and D 5 ) and D 2 -like (D 2 , D 3 , and D 4 ) based on their binding properties and their ability to activate or inhibit forskolin-induced adenylate cyclase activity, a classification supported by the cloning of the different subtypes during the last decade (7,8).Within the D 2 -like family, the D 2 receptor is highly expressed...