Differential effects of yohimbine, naloxone and 8-OH-DPAT on ejaculatory response in male dogs.

The aim of this study was to compare the effects of the alpha(2)-adrenergic-receptor antagonist yohimbine, the 5-HT(lA)-receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the opioid-receptor antagonist naloxone (all of which have been shown to stimulate male sexual arousal/motivation in rats) on sexual responses in male dogs. Sexual responses (i.e., ejaculation, penile erection and pelvic thrusting behavior) were elicited by manual penile stimulation. Systemic administration of yohimbine (0.03-1.0 mg/kg) produced a biphasic dose response curve for the amount of ejaculated semen collected during genital stimulation (for 5 min), whereas 8-OH-DPAT (0.03-0.3 mg/kg) dose-dependently decreased the amount of ejaculated semen. Thus, yohimbine increased the amount of ejaculated semen at lower doses (0.03-0.3 mg/kg), but decreased it at the highest dose (1.0 mg/kg). The highest dose of yohimbine (1.0 mg/kg) and 8-OH-DPAT (0.3 mg/kg) also produced a significant delay of onset in both ejaculation and penile erection latency (time from starting the stimulation to the first ejaculation and full erection), and a decrease in the incidence of pelvic thrusting behavior. In contrast, administration of naloxone (0.03-1.0 mg/kg) did not affect the sexual responses elicited by genital stimulation. These results indicate that yohimbine and 8-OH-DPAT, but not naloxone, affect sexual responses, particularly ejaculation, and that the drugs which stimulate the mechanisms regulating sexual arousal/motivation in male rats do not show identical effects for sexual function in male dogs. The present findings also confirm our previous observations that the ejaculatory capacity in dogs can be stimulated by lower doses of yohimbine, as evidenced by an increase in the amount of ejaculated semen.

Alpha 2-adrenoceptor antagonists: effects on ejaculation, penile erection and pelvic thrusting behavior in dogs.

We previously reported that systemic administration of yohimbine, an alpha2-adrenoceptor antagonist, exerts a biphasic effect (stimulating and suppressing) on ejaculation in dogs, when this function is analyzed using the amount of ejaculated semen in response to genital stimulation. To clarify the effect of alpha2-adrenoceptor blockade on male sexual function, we investigated the effects of four selective alpha2-adrenoceptor antagonists, rauwolscine, idazoxan, RX821002 and mydaglizole, on sexual responses (ejaculation, penile erection and pelvic thrusting behavior) elicited by manual penile stimulation in dogs. Rauwolscine (intraperitoneal, 30 min before the testing) caused a biphasic effect on ejaculation; the amount of ejaculated semen produced by the stimulation was significantly increased by the lower doses (0.1 and 0.3 mg/kg), whereas it was decreased by the higher doses (1.0 and 2.0 mg/kg). The higher doses of rauwolscine also markedly inhibited both penile erection and pelvic thrusting behavior. Idazoxan and RX821002, at doses of 0.1 and 0.3 mg/kg, caused a significant increase in the amount of ejaculated semen without affecting other sexual functions. RX821002 (2.0 mg/kg), but not idazoxan (2.0 mg/kg), moderately inhibited both penile erection and pelvic thrusting behavior. Mydaglizole, a peripherally acting alpha2-adrenoceptor antagonist, did not affect the sexual responses at any doses (0.1-4.0 mg/kg). In the ejaculatory declining test, all alpha2-adrenoceptor antagonists (0.1 mg/kg), except for mydaglizole, completely prevented the decrease in ejaculatory capacity produced by antecedent ejaculation. These results indicate that, though the range of the effective dose is narrow, the alpha2-adrenoceptor antagonists that can block the central alpha2-adrenoceptors have the stimulatory effects on ejaculatory function. The difference of the sexual effects may be based on the action except for the alpha2-adrenoceptor blockade.

Selective antagonism by naloxonazine of antinociception by Tyr-D-Arg-Phe-beta-Ala, a novel dermorphin analogue with high affinity at mu-opioid receptors.

To examine the role of mu-opioid receptor subtypes, we assessed the antinociceptive effect of H-Tyr-D-Arg-Phe-beta-Ala-OH (TAPA), an analogue of dermorphin N-terminal peptide in mice, using the tail-flick test. Intracerebroventricularly (i.c.v.) or intrathecally (i.t.) injected TAPA produced potent antinociception with tail-flick as a thermal noxious stimulus. The selective mu(1)-opioid receptor antagonist, naloxonazine (35 mg/kg, s.c.), or the selective mu-opioid receptor antagonist, beta-funaltrexamine, 24 h before testing antagonized the antinociceptive effect of i.t. or i.c.v. TAPA on the response to noxious stimuli. Pretreatment with beta-funaltrexamine completely antagonized the antinociception by both i.c.v. and i.t. administered TAPA and [D-Ala(2), Me-Phe(4), Gly(ol)(5)]enkephalin (DAMGO). Especially in the tail-flick test, pretreatment with naloxonazine produced a marked rightward displacement of the i.t. TAPA dose-response curve for antinociception. Though DAMGO is a highly selective mu-opioid receptor agonist, pretreatment with naloxonazine partially blocked the antinociceptive response to DAMGO after i.c.v., but not after i. t. injection. These results indicate that TAPA can act as a highly selective mu(1)-opioid receptor agonist (notable naloxonazine-sensitive receptor agonist) at not only the supraspinal level, but also the spinal level. These data also reveal different antinociceptive mechanisms for DAMGO and for TAPA.

Role of histamine H(1) receptor in pain perception: a study of the receptor gene knockout mice.

To study the participation of histamine H(1) receptors in pain perception, histamine H(1) receptor knockout mice were examined for pain threshold by means of three different kinds of nociceptive tasks. These included assays for thermal nociception (hot-plate, tail-flick, paw-withdrawal), mechanical nociception (tail-pressure), and chemical nociception (abdominal constriction, formalin test, capsaicin test) which evoked pain by the activation in nociceptive Adelta and C fibers. The mutant mice lacking histamine H(1) receptors showed significantly fewer nociceptive responses to the hot-plate, tail-flick, tail-pressure, paw-withdrawal, formalin, capsaicin, and abdominal constriction tests. Sensitivity to noxious stimuli in histamine H(1) receptor knockout mice significantly decreased when compared to wild-type mice. This data indicates that histamine plays an important role in both somatic and visceral pain perceptions through histamine H(1) receptors. The difference in the effect of histamine H(1) receptors antagonist, the active (D-) and inactive (L-) isomers of chlorpheniramine on ICR mice further substantiates the evidence of the role of histamine H(1) receptors on pain threshold.

[Inhibition of mitogen-activated protein kinase cascade by baicalein, a flavonoid of natural origin].

Mitogen-activated protein kinase (MAPK) is a family of serine/threonine kinase that appears to be a component common to signalling pathway initiated by a wide range of factors including hormones, differentiation factors and mitogens. Baicalein is a flavonoid derived from the root of Scutellaria baicaleins. From searching the inhibitors of prostaglandin synthesis in C6 rat glioma cells, we found that baicalein had a potent inhibitory activity of prostaglandin synthesis induced by either histamine or A23187, a Ca2+ ionophore. Baicalein inhibited histamine- and A23187-induced phosphorylation of MAPK in the cells, which was known to cause the phosphorylation of cytosolic phospholipase A2. Baicalein also inhibited the phosphorylation of MAPK kinase (MEK) induced by histamine or A23187 in the cells. In vitro kinase assay, baicalein inhibited the activities of raf but not MEK. These results imply that baicalein is a specific inhibitor of MAPK cascade, acting on phosphorylation of MEK by raf.

Analysis of inhibitory effects of scutellariae radix and baicalein on prostaglandin E2 production in rat C6 glioma cells.

Inhibitory mechanism of the water extract of Scutellariae Radix on prostaglandin E2 (PGE2) release was examined in C6 rat glioma cells. Scutellariae Radix reduced a Ca2+ ionophore A23187-induced PGE2 release by inhibition of arachidonic acid (AA) liberation. Sho-saiko-to and San'o-shashin-to, which contain Scutellariae Radix, also inhibited PGE2 release. A23187 caused phosphorylation of mitrogen-activated protein kinase (MAPK), resulting in activation of cytosolic phospholipase A2 (cPLA2). Scutellariae Radix and baicalein inhibited the phosphorylation of MAPK. Baicalein, but not baicalin, inhibited A23187-induced PGE2 release. These results suggest that baicalein in Scutellariae Radix reduces AA liberation through the inhibition of the MAPK-cPLA2 pathway.

A comparative study of Kakkon-to and Keishi-to on prostaglandin E(2) release from rabbit astrocytes.

The effects of Kakkon-to (Ge-Ge-Tang) and Keishi-to (Gui-Zhi-Tang), Kampo medicines, on prostaglandin E(2) (PGE(2)) release were examined in rabbit astrocytes, in order to clarify their pharmacological properties as antipyretics. Rabbit astrocytes released PGE(2) in response to bradykinin. Short term (10 min) treatment of the cells with Kakkon-to, Keishi-to or aspirin resulted in reduction of bradykinin-induced PGE(2) release. However, long term (18 h) treatment of the cells with Kakkon-to resulted in augmentation of bradykinin-induced PGE(2) release after washing out Kakkon-to in the culture medium. In contrast, long term treatment with Keishi-to or aspirin resulted in inhibition of bradykinin-induced PGE(2) release after washing out the medicine in the culture medium. These results suggest that 1 ) Kakkon-to and Keishi-to contain active substance(s) that inhibit prostaglandin synthesis, and 2) long term treatment of the cells with Kakkon-to elicits changes on the cellular responses to bradykinin, but Keishi-to as well as aspirin irreversibly blocks the enzymes involved in PGE(2) synthesis.