Unresponsiveness of mu-opioid receptor knockout mice to lipopolysaccharide-induced fever

Recently, we demonstrated that lipopolysaccharide (LPS)-induced fever could be suppressed by a selective mu-opioid receptor antagonist, indicating that the mu-opioid system is involved in the LPS fever. In the present study, to confirm the role of the mu-opioid system in the pathogenesis of LPS fever, we used mice lacking the mu-opioid receptor. In the wild type (WT), following intraperitoneal (i.p.) injection of 100 microg kg(-1) of LPS, body temperature (T(b)) increased approximately 1 degrees C and remained elevated during the 360-min recording period. In the mu-opioid receptor knockout (MOR-KO) mice, the administration of 100 microg kg(-1) i.p. of LPS did not induce fever during the recording period. Saline by itself, given i.p., did not alter the T(b), either in WT or MOR-KO. These results confirm that the mu-opioid system is involved in LPS-induced fever.     

Central effect of histamine and peripheral effect of histidine on the formalin-induced pain response in mice

1. The present study was designed to investigate the role of brain histamine in modulating pain transmission in mice. 2. In conscious mice implanted with an intracerebroventricular (i.c.v.) cannula, the effects of i.v.c. injections of normal saline (control) and low and high doses histamine (2 and 40 microg/mouse, respectively) were investigated on the duration of paw licking and biting induced by subcutaneous (s.c.) injection of formalin (20 microL; 5%) into the plantar surface of the left hindpaw. 3. To clarify the involvement of histidine in the pain response, the effects of intraperitoneal (i.p.) injections of low and high doses of histidine (50 and 1000 mg/kg, respectively) alone or before i.c.v. injection of histamine were also examined. 4. Intraplantar injection of formalin induced a biphasic pain response (first phase: 0-5 min after injection; second phase: 20-40 min after injection). 5. Histamine (2 microg/mouse, i.c.v.) had no effect on the first phase of the pain response, but suppressed the second phase. The higher dose of histamine (40 microg/mouse, i.c.v.) suppressed both phases of the pain response. 6. Histidine, at 50 mg/kg, i.p., had no effect on the pain response, but the higher dose (1000 mg/kg, i.p.) suppressed the both phases of the pain response. 7. Pretreatment with the low dose of histidine (50 mg/kg, i.p.) prior to administration of 2 microg/mouse, i.c.v., histamine did not change the antinociception induced by low-dose histamine. However, pretreatment with the high dose of histidine (1000 mg/kg, i.p.) prior to 2 microg/mouse, i.c.v., histamine produced antinociception that resembled that seen following administration of the high dose of either histidine or histamine. Pretreatment with the low dose of histidine (50 mg/kg, i.p.) prior to administration of 40 microg/mouse, i.c.v., histamine has no effect on the pain response following high-dose histamine. Pretreatment with 1000 mg/kg, i.p., histidine prior to administration of 40 microg/mouse, i.c.v., histamine strongly suppressed both phases of the formalin-induced pain response, particularly the second phase. 8. The results of the present study indicate that: (i) activation of brain histamine produces antinociception in the mouse formalin test; (ii) peripheral loading with a high dose of histidine (1000 mg/kg, i.p.) alone exerts the same effect as that seen following 40 microg/mouse, i.c.v., histamine; and (iii) pretreatment with a high dose of histidine potentiates central histamine-induced antinociception.     

The anti-pyretic mechanism of alminoprofen]

The anti-pyretic activity of alminoprofen (AP), a non-steroidal anti-inflammatory agent, and its mode of action were investigated in conscious febrile rabbits. A fever was evoked by i.v. injection of lipopolysaccharide (LPS), intracisternal (i.c.) injection of leukocytic pyrogen (LP) or i.c. injection of arachidonic acid (AA). The amount of PGE2 or AP in the cerebrospinal fluid (CSF) after i.v. LPS was estimated using an RIA or HPLC method. AP (3-30 mg/kg, p.o.) dose-dependently inhibited the LPS (0.5 micrograms/kg, i.v.)-induced fever; AP, ibuprofen, indomethacin and pranoprofen had ED50 values of 9.64, 26.45, 4.41 and 11.91 mg/kg, p.o., respectively. PGE2 in the CSF was markedly increased during the elevation of body temperature after i.v. LPS (0.5 microgram/kg). AP (30 mg/kg, p.o.) markedly inhibited the increase in PGE2 that was observed in the CSF during fever developed in response to i.v. LPS (0.5 micrograms/kg). The AP concentration in the CSF 2 hr after AP (30 mg/kg, p.o.) was 2.86 x 10(-6) (1.15-4.57 x 10(-6) M, a concentration too low to inhibit PG synthesis. A dose-dependent fever was observed after i.c. LP (1-8 unit) or AA (10-100 micrograms). AP (30 mg/kg, p.o.) shifted the dose-response curves for the i.c. LP-induced fever to the right, but did not have any effect on the i.c. AA-induced fever. These results suggest that AP has a relatively potent anti-pyretic activity, and its mechanism of action involves competition with LP at a site in the CNS, but does not involve an inhibition of cyclooxygenase at a central site, which has been considered as an anti-pyretic mechanism of nonsteroidal anti-inflammatory drugs.     

Characterization of lipopolysaccharide-induced emesis in conscious piglets: effects of cervical vagotomy, cyclooxygenase inhibitors and a 5-HT(3) receptor antagonist

The emetic response to intraperitoneal (i.p., 0.5, 2, 8 mg kg(-1)) and intravenous (i.v., 200 microg kg(-1)) administration of bacterial lipopolysaccharides (LPS) was characterized in conscious piglets observed for 4 h. The latencies and the incidence of the emetic response to LPS (i.p.) decreased and increased, respectively, in a dose-dependent manner. In 14 additional piglets, a bilateral vagotomy performed 4 h prior to LPS administration abolished the vomiting induced by i.p. LPS (2 mg kg(-1)), and decreased its incidence by 77% in the i.v. injected animals. Sham-operated animals (n=6) exhibited a similar emetic pattern to the controls injected intraperitoneally with LPS (2 mg kg(-1)). In 7 piglets, the administration of granisetron, a 5-HT(3) receptor antagonist (i.v., 2 mg kg(-1)), 30 min prior to the i.p. LPS injection (2 mg kg(-1)) failed to reduce significantly the emetic activity; whereas, in 6 animals, a combination of meloxicam (0.3 mg kg(-1)) and indomethacin (5 mg kg(-1)), two cyclooxygenase (COX) inhibitors, administered per os 1.5 h prior to the i.p. LPS (2 mg kg(-1)) abolished the emetic response to endotoxins. The present results show that the activation of the medullary "vomiting centre" in response to i.p. administration of LPS is mediated via vagal afferents and is likely to involve prostaglandins.     

Pyrogens enhance beta-endorphin release in hypothalamus and trigger fever that can be attenuated by buprenorphine

At first, we investigated whether both beta-endorphin release level in the hypothalamus and body temperature can be altered after intracerebroventricular (i.c.v.) injection of either lipopolysaccharide (LPS), interleukin-1beta (IL-1beta), or prostaglandin E(2) (PGE(2)) in rats. It was found that in the rat, i.c.v. administration of either LPS (0.5 microg in 10 microl), IL-1beta (10 ng in 10 microl), or PGE(2) (200 ng in 10 microl), in addition to producing fever, upregulated the immunoreactivity of beta-endorphin in the preoptic anterior hypothalamus of rat brain. Secondarily, we assessed whether the fever induced by either LPS, IL-1beta, or PGE(2) can be altered by pretreatment with buprenorphine (an opioid receptor antagonist). The results revealed that i.c.v. administration of buprenorphine (1 - 10 microg in 10 microl) alone had an insignificant effect on the body temperature. However, the fever induced by i.c.v. injection of either LPS, IL-1beta, or PGE(2) was significantly attenuated by pretreatment with i.c.v. injection of buprenorphine 1 h before the pyrogen injection in rats. The results suggest that pyrogens enhance beta-endorphin release in the hypothalamus and trigger fever which can be attenuated by buprenorphine, an opioid receptor antagonist.     

Effects of systemic administration of beta-casomorphin-5 on learning and memory in mice

The effects of systemic administration of bovine beta-casomorphin-5 (Tyr-Pro-Phe-Pro-Gly), a mu-opioid receptor agonist derived from milk beta-casein, on spontaneous alternation behavior in the Y-maze (spatial short-term memory) and step-down-type passive avoidance response (non-spatial long-term memory) were investigated in mice. Intraperitoneal (i.p.) administration of beta-casomorphin-5 (0.1-20 mg/kg) did not have a significant effect on either spontaneous alternation behavior or passive avoidance response. However, a low dose (1 mg/kg, i.p.) of beta-casomorphin-5 improved scopolamine (1 mg/kg, s.c.)-induced impairment of spontaneous alternation behavior and passive avoidance response. Pretreatment with intracerebroventricular injections of beta-funaltrexamine (a mu-opioid receptor antagonist, 0.1 microg/mouse) and naloxonazine (a mu(1)-opioid antagonist, 5 microg/mouse), which did not improve scopolamine-induced impairment, prevented the ameliorating effect of beta-casomorphin-5 on scopolamine-induced impairment of passive avoidance response. These results indicated that systemic administration of a low dose (1 mg/kg, i.p.) of beta-casomorphin-5 improves the disturbance of learning and memory resulting from cholinergic dysfunction through central mediation involving mu(1)-opioid receptors.     

Studies on the relationship between the endotoxin induced fever and the antipyretic effect of reserpine in rabbits (author's transl)]

It has been well documented that fever could be mediated with endogenous pyrogen released from reticuloendothelial system(RES) by administration of bacterial endotoxin(LPS) intravenously to rabbits. On the contrary, reserpine which has various pharmacological activities by depleting catecholamines decreases the normal body temperature as well as endotoxin induced fever. In this paper, we focussed our attention on the effect of reserpine on the production of endogenous pyrogen with relation to the antipyretic effect in endotoxin fever and obtained the following results: Endogenous pyrogen could be detected by intravenous administration of LPS(0.5 micrograms/kg) during the fever. However, endogenous pyrogen was undetectable with intracisternal administration of LPS(0.01 microgram/body) which provoked long-lasting fever. Reserpine (1 mg/kg, i.v.) decreased both body temperature induced by intracisternal administration of LPS(0.01 microgram/body) or intravenous administration of LPS(0.5 microgram/kg), however the degree was more extensive in cases of LPS-induced fever. Pretreatment of rabbits with reserpine (1 mg/kg, i.v.) suppressed the fever induced by an intravenous administration of LPS(0.5 microgram/kg), but did not suppress the release of endogenous pyrogen. These data suggest that endogenous pyrogen may not be an important factor in the pathogenesis of endotoxin fever.     

Possible antinociceptive mechanisms of opioid receptor antagonists in the mouse formalin test

It has been reported that opioid receptor antagonist can induce antinociception in several nociceptive tests. In the intraplantar formalin pain model, however, opioid antagonist-induced antinociception, as well as its underlying mechanism, has not been well characterized. Therefore, in the mouse formalin test, we attempted to characterize the site of action and the possible opioid receptor subtypes. We found that naltrexone (a nonselective opioid antagonist) injected intraperitoneally (i.p., 1-20 mg/kg), intrathecally (i.t., 0.1-10 microg) and intracerebroventricularly (i.c.v., 0.1-10 microg) phase. Administration of beta-funaltrexamine (beta-FNA, 10-40 mg/kg i.p., 1.25-5 microg it or i.c.v.), naltrindole (1-10 mg/kg i.p., 1.25-5 microg it or i.c.v.) and nor-binaltorphimine (nor-BNI, 1-10 mg/kg i.p., 10-40 microg it or i.c.v.), which are selective mu-, delta- and kappa-opioid antagonists, respectively, also produced antinociception during the second phase. Additionally, we examined the involvement of the descending monoaminergic systems in the naltrexone-induced antinociception in the formalin test. Pretreatment with 5,7-dihydroxytryptamine (5,7-DHT, a serotonergic neurotoxin, 20 microg i.t.), but not N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, a noradrenergic neurotoxin, 20 microg i.t.), reversed the naltrexone-induced antinociception during the second phase. Our results suggest that blockade of supraspinally or spinally located opioid receptors may play roles in the regulation of antinociception during the tonic painful stage. In addition, opioid receptors localized at the neuroterminal of the descending serotonergic, but not noradrenergic, inhibitory system in the spinal cord appear to be involved in opioid antagonist-induced antinociception during the second tonic phase of the formalin test.     

1,8-cineole protects against liver failure in an in-vivo murine model of endotoxemic shock

The effects of 1,8-cineole on D-galactosamine/lipopolysaccharide (GalN/LPS)-induced shock model of liver injury was investigated in mice. The co-administration of GalN (700 mg kg(-1), i.p.) and LPS (5 microg kg(-1), i.p.) greatly elevated serum concentrations of tumour necrosis factor-alpha (TNF-alpha), alanine aminotransferase and aspartate aminotransferase, and induced massive hepatic necrosis and lethality in 100% of control mice. Pretreatment with 1,8-cineole (400 mg kg(-1), p.o.) and dexamethasone (1 mg kg(-1), s.c.), 60 min before GalN/LPS, offered complete protection (100%) against the lethal shock and acute elevation in serum TNF-alpha and serum transaminases. Hepatic necrosis induced by GalN/LPS was also greatly reduced by both 1,8-cineole and dexamethasone treatment. The results indicate that 1,8-cineole protects mice against GalN/LPS-induced liver injury through the inhibition of TNF-alpha production, and suggest that 1,8-cineole may be a promising agent to combat septic-shock-associated pathologies.     

An NMDA receptor-dependent hydroxyl radical pathway in the rabbit hypothalamus may mediate lipopolysaccharide fever

The aim of this study was to investigate the effects of antioxidants (e.g. alpha-lipoic acid and N-acetyl-L-cysteine) as well as N-methyl-D-aspartate (NMDA) receptor antagonists (e.g. MK-801 and LY235959) on the changes of both core temperature and hypothalamic levels of 2,3-dihydroxybenzoic acid (2,3-DHBA) induced by systemic administration of lipopolysaccharide (LPS) in rabbits. The measurements of 2,3-DHBA were used as an index of the intrahypothalamic levels of hydroxyl radicals. Intravenous administration of LPS (2-10 microg/kg) elicited a biphasic febrile response, with the core temperature maxima at 80 and 200 min post-injection. Each core temperature rise was accompanied by a distinct wave of cellular concentrations of 2,3-DHBA in the hypothalamus. The rise in both the core temperature and hypothalamic 2,3-DHBA could be induced by direct injection of glutamate (100-400 microg in 10 microl/rabbit) into the cerebroventricular fluid system. Either the early or the late phase of fever rise and increased hypothalamic levels of 2,3-DHBA following systemic injection of LPS were significantly antagonized by pretreatment with injection of alpha-lipoic acid (5-60 mg/kg, i.v.), N-acetyl-L-cysteine (2-20 mg/kg, i.v.), MK-801 (0.1-1 mg/kg, i.m.), or LY235959 (0.1-1 mg/kg, i.v.) 1 h before LPS injection. The increased levels of prostaglandin E(2) in the hypothalamus induced by LPS could be suppressed by alpha-lipoic acid or N-acetyl-L-cysteine pretreatment. These findings suggest that an NMDA receptor-dependent hydroxyl radical pathway in the hypothalamus of rabbit brain may mediate both the early and late phases of the fever induced by LPS.     

Curcumin inhibits the increase of glutamate, hydroxyl radicals and PGE2 in the hypothalamus and reduces fever during LPS-induced systemic inflammation in rabbits

Evidence has accumulated to suggest that systemic administration of lipopolysaccharide (LPS), in addition to elevating tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and interleukin-6 (IL-6) as well as fever, induces overproduction of glutamate, hydroxyl radicals and prostaglandin E(2) (PGE(2)) in the rabbit's hypothalamus. Current study was attempted to assess whether Curcumin exerts its antipyresis by reducing circulating pro-inflammatory cytokines and hypothalamic glutamate, hydroxyl radicals and PGE(2) in rabbits. The microdialysis probes were stereotaxically and chronically implanted into the preoptic anterior hypothalamus of rabbit brain for determination of glutamate, hydroxyl radicals, and PGE(2) in situ. It was found that systemic administration of LPS (2 microg/kg) induced increased levels of both core temperature and hypothalamic levels of both glutamate and hydroxyl radicals accompanied by increased plasma levels of TNF-alpha, IL-1beta, and IL-6. The rise in both the core temperature and hypothalamic glutamate and hydroxyl radicals could also be induced by direct injection of TNF-alpha, IL-1beta, or IL-6 into the lateral ventricle of rabbit brain. Pretreatment with Curcumin (5-40 mg/kg, i.p.) 1 h before an i.v. dose of LPS significantly reduced the LPS-induced overproduction of circulating TNF-alpha, IL-1beta, and IL-6, and brain glutamate, PGE(2), and hydroxyl radicals. Both the febrile response and overproduction of both glutamate and hydroxyl radicals in the hypothalamus caused by central administration of TNF-alpha, IL-1beta, or IL-6 could be suppressed by Curcumin. These results indicate that systemic injection of Curcumin may exert its antipyresis by inhibiting the glutamate-hydroxyl radicals-PGE(2) pathways in the hypothalamus and circulating TNF-alpha, IL-1beta, and IL-6 accumulation during LPS fever.     

Effect of chronic and acute administration of fluoxetine and its additive effect with morphine on the behavioural response in the formalin test in rats

Serotonergic systems are involved in the central regulation of nociceptive sensitivity. Fluoxetine, a selective inhibitor of the reuptake of serotonin (5-hydroxytryptamine, 5-HT), was administered orally (0.16, 0.32, 0.8 mg kg(-1) daily for 7 days), intraperitoneally (0.04, 0.08, 0.16 mg kg(-1) day(-1) for 7 days and a single dose of 0.32 mg kg(-1)) and intracerebroventricularly (10 microg/rat) to rats and nociceptive sensitivity was evaluated using the formalin test (50 microL of 2.5% formalin injected subcutaneously). The effect of fluoxetine was also studied in the presence of 5,7-dihydroxytryptamine creatinine sulfate (5,7-DHT) and after co-administration with morphine. Oral (0.8 mg kg(-1)), intraperitoneal (0.16 and 0.32 mg kg(-1)) and intracerebroventricular (10 microg/rat) fluoxetine induced antinociception in the late phase of the formalin test. Furthermore, intrathecal administration of 5-HT (100 microg/rat) induced an analgesic effect. The analgesic effect of fluoxetine (0.16 and 0.32 mg kg(-1), i.p.) and 5-HT (100 microg/rat, i.t.) was abolished by pre-treatment with 5,7-DHT (100 microg/rat, i.t.). In addition, the analgesic effect of 5-HT (100 microg/rat, i.t.) was decreased by pre-treatment with naloxone (2 mg kg(-1), i.p.). Morphine (5 mg kg(-1), i.p.) induced analgesia that was increased by fluoxetine (0.32 mg kg(-1), i.p.). These results suggest that fluoxetine has an antinociceptive effect in tonic inflammatory pain through functional alteration of the serotonergic system and also potentiates the analgesic effect of morphine.     

Monophosphoryl lipid A, a derivative of bacterial lipopolysaccharide, fails to induce B1-receptor-dependent responses to (des-Arg9)-bradykinin in the rabbit in vivo

OBJECTIVE: The aim of this study was to evaluate whether monophosphoryl lipid A (MLA) was able to induce a hypotensive response to (des-Arg9)-bradykinin in the rabbit in vivo, by inducing B1-receptor synthesis. MATERIALS AND METHODS: Arterial pressure was measured after intra-arterial administration of B1- and B2-receptor agonists and antagonists in control rabbits and in rabbits pre-treated 24 h earlier with MLA (100 microg kg(-1) i.v.) or lipopolysaccharide (LPS) (10 microg kg(-1) i.v.). RESULTS: Intra-arterial bradykinin administration induced a similar dose-dependent hypotension in all groups (BK 0.25 microg kg(-1), 36 +/- 3 mm Hg, BK 1 microg kg(-1), -39 +/- 3 mm Hg, p < 0.05 vs. control conditions) that was significantly antagonised by intra-arterial HOE 140 (2 microg kg(-1)) (-5 +/- 2 mm Hg, p < 0.05). Intra-arterial (des-Arg9)-bradykinin induced a hypotensive response in the LPS-pre-treated group (DBK 1 microg kg(-1), -6 +/- 1 mm Hg, DBK 10 microg kg(-1), -10 +/- 1 mm Hg, p < 0.05 vs. control conditions) that was totally abolished by intra-arterial (des-Arg9, Leu8)-bradykinin (10 microg kg(-1) min(-1)) (+1 +/- 2 mm Hg, p < 0.05). In the control and MLA-pre-treated groups, (des-Arg9)-bradykinin had no effect. CONCLUSION: MLA pre-treatment did not induce a hypotensive response to (des-Arg9)-bradykinin. We conclude that, in contrast to LPS, MLA does not induce B1-receptor synthesis, 24 h after its administration in the rabbit. Thus, the cardioprotective effects of MLA do not appear to be related to the kinin pathway.     

Febrigenic signaling to the brain does not involve nitric oxide

1. The involvement of peripheral nitric oxide (NO) in febrigenic signaling to the brain has been proposed because peripherally administered NO synthase (NOS) inhibitors attenuate lipopolysaccharide (LPS)-induced fever in rodents. However, how the unstable molecule of NO can reach the brain to trigger fever is unclear. It is also unclear whether NOS inhibitors attenuate fever by blocking febrigenic signaling or, alternatively, by suppressing thermogenesis in brown fat. 2. Male Wistar rats were chronically implanted with jugular catheters; their colonic and tail skin temperatures (T(c) and T(sk)) were monitored. 3. Study 1 was designed to determine whether the relatively stable, physiologically relevant forms of NO, that is, S-nitrosoalbumin (SNA) and S-nitrosoglutathione (SNG), are pyrogenic and whether they enhance LPS fever. At a neutral ambient temperature (T(a)) of 31 degrees C, afebrile or LPS (1 microg kg(-1), i.v.)-treated rats were infused i.v. with SNA (0.34 or 4.1 micromol kg(-1); the controls received NaNO(2) and albumin) or SNG (10 or 60 micromol kg(-1); the controls received glutathione). T(c) of SNA- or SNG-treated rats never exceeded that of the controls. 4. In Study 2, we tested whether the known fever-attenuating effect of the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) at a subneutral T(a) (when fever is brought about by thermogenesis) also occurs at a neutral T(a) (when fever is brought about by skin vasoconstriction). At a subneutral T(a) of 24 degrees C, L-NAME (2.5 mg kg(-1), i.v.) attenuated LPS (10 microg kg(-1), i.v.) fever, presumably by inhibiting thermogenesis. At 31 degrees C, L-NAME enhanced LPS fever by augmenting skin vasoconstriction (T(sk) fall). 5. In summary, both SNA and SNG had no pyrogenic effect of their own and failed to enhance LPS fever; peripheral L-NAME attenuated only fever brought about by increased thermogenesis. It is concluded that NO is uninvolved in febrigenic signaling to the brain.     

Role of potassium channels in the antinociception induced by agonists of alpha2-adrenoceptors

1. The effect of the administration of pertussis toxin (PTX) as well as modulators of different subtypes of K+ channels on the antinociception induced by clonidine and guanabenz was evaluated in the mouse hot plate test. 2. Pretreatment with pertussis toxin (0.25 microg per mouse i.c.v.) 7 days before the hot-plate test, prevented the antinociception induced by both clonidine (0.08-0.2 mg kg(-1), s.c.) and guanabenz (0.1-0.5 mg kg(-1), s.c.). 3. The administration of the K(ATP) channel openers minoxidil (10 microg per mouse, i.c.v.), pinacidil (25 microg per mouse, i.c.v.) and diazoxide (100 mg kg(-1), p.o.) potentiated the antinociception produced by clonidine and guanabenz whereas the K(ATP) channel blocker gliquidone (6 microg per mouse, i.c.v.) prevented the alpha2 adrenoceptor agonist-induced analgesia. 4. Pretreatment with an antisense oligonucleotide (aODN) to mKv1.1, a voltage-gated K+ channel, at the dose of 2.0 nmol per single i.c.v. injection, prevented the antinociception induced by both clonidine and guanabenz in comparison with degenerate oligonucleotide (dODN)-treated mice. 5. The administration of the Ca2+-gated K+ channel blocker apamin (0.5-2.0 ng per mouse, i.c.v.) never modified clonidine and guanabenz analgesia. 6. At the highest effective doses, none of the drugs used modified animals' gross behaviour nor impaired motor coordination, as revealed by the rota-rod test. 7. The present data demonstrate that both K(ATP) and mKv1.1 K+ channels represent an important step in the transduction mechanism underlying central antinociception induced by activation of alpha2 adrenoceptors.     

Synthesis,beta-adrenergic blocking activity and beta-receptor binding affinities of 1-substituted-3-(2-isopropyl-5-methyl-phenoxy)-propan-2-ol oxalates

The compounds 1-isopropylamino-3-(2-isopropyl-5-methyl-phenoxy)-propan-2-ol oxalate (5) and 1-tert-butylamino-3-(2-isopropyl-5-methyl-phenoxy)-propan-2-ol oxalate (6) were synthesized from thymol (1), a naturally occurring agent in Thymus vulgaris L. Pharmacological evaluation of 5 and 6 were carried out using mouse ECG and isolated rat uterus models. Pretreatment of 5 (100 microg/kg, i.v.) and 6 (50 microg/kg, i.v.) antagonized isoprenaline (2 microg/kg, i.v.) induced tachycardia, similar to that of atenolol (CAS 29122-68-7, 20 microg/kg, i.v.) pretreatment in mouse ECG experiments as measured by R-R interval. Pretreatment of 5 and 6 blocked isoprenaline and adrenaline induced relaxation of isolated rat uterus (unprimed). Also the compounds 5 and 6 were subjected to in vitro beta1- and beta2-adrenergic receptor binding assay using turkey erythrocyte membrane (beta1) and lung homogenate of rats (beta2). Both 5 and 6 showed beta-adrenergic receptor affinity comparable with that of propranolol (propranolol hydrochloride, CAS 318-98-9) with out selectivity to any one beta-adrenergic receptor. These results suggest that both the compounds possess non-selective beta-adrenergic blocking activity, with the tert-butyl derivative 6 being more active than the isopropyl derivative 5.     

The effect of intraperitoneal administration of leptin on short-term food intake in rats

The effects of intraperitoneal (i.p.) injection of leptin (1, 5, and 10 mug/kg) were investigated on the food consumption during a 60 min test meal in 21 h fasted rats. All doses of leptin produced significant reductions in cumulative food intake during the first 15 min and 30 min (at least, P<0.05) after administration. Similarly, i.p., but not subcutaneous (s.c.), administration of leptin (25 microg/kg) reduced food intake in 21 h fasted rats. Leptin (10 and 25 microg/kg, i.p.) did not reduce water intake in 16 h water-deprived rats, nor did leptin (25 microg/kg) produce aversion in a two-bottle conditioned taste aversion test indicating that the hypophagic effect of leptin is (i) behaviourally specific for food and not water intake, and (ii) not due to drug-induced malaise. Moreover, leptin (10 and 25 microg/kg, i.p.) did not significantly alter food intake in non-deprived rats when measured at 30 min intervals over a period of 24 h. Chemical vagotomy with capsaicin abolished the inhibitory effects of leptin (25 microg/kg, i.p) on food intake in fasted rats and suggest that the hypophagic effect is dependent on intact vagal afferent nerves. Furthermore, the hypophagia induced by leptin (10 microg/kg, i.p.) in fasted rats was not attenuated by systemic administration of the peripherally acting cholecystokinin(1) receptor antagonist, 2-naphthalenesulphanyl-L-aspartyl-2-(phenethyl) amide (2-NAP; 2 mg/kg, i.p.), indicating that the suppressant effects of leptin on food consumption are not secondary to the release of endogenous peripheral cholecystokinin.     

Sildenafil,a phosphodiesterase-5 inhibitor,enhances the antinociceptive effect of morphine

Various evidence has demonstrated a role of the nitric oxide (NO)/cGMP signaling pathway in the processing of nociception. The exact role of phosphodiesterase-5 (PDE-5) via the NO/cGMP pathway is not fully understood in pain response. The aim of the present study was to investigate the possible peripheral interaction between a PDE-5 inhibitor (sildenafil) and morphine. Carrageenan-induced hyperalgesia in rats and the acetic-acid-induced writhing test in mice were used as animal models. Local administration of sildenafil (50-200 microg/paw, i.pl.) exhibited a dose-dependent antinociceptive effect against the paw pressure test. Sildenafil also demonstrated an antinociceptive effect (1-10 mg/kg, i.p.) against in the writhing test. Co-administration of sildenafil (100 microg/paw, i.pl. and 2 mg/kg, i.p.) significantly enhanced the antinociceptive effect of morphine (2 microg/ paw, i.pl. and 2 mg/kg, i.p respectively). The antinociception produced by the drugs alone or combined was due to a local action, as its administration in the contralateral paws was ineffective. Pretreatment with N(G)-nitro-L-arginine methyl ester (an NO synthesis inhibitor), methylene blue (gunalyl cyclase inhibitor) or naloxone (opioid receptor antagonist) blocked the effect of a sildenafil-morphine combination in both tests. The results suggest that opioid receptor (NO and cGMP) mechanisms are involved in the combined antinociceptive effect. Further, sildenafil produced antinociception per se and increased the response of morphine, probably through the inhibition of cGMP degradation.     

Effect of agmatine on locus coeruleus neuron activity: possible involvement of nitric oxide

1. To investigate whether agmatine (the proposed endogenous ligand for imidazoline receptors) controls locus coeruleus neuron activity and to elucidate its mechanism of action, we used single-unit extracellular recording techniques in anaesthetized rats. 2. Agmatine (10, 20 and 40 microg, i.c.v.) increased in a dose-related manner the firing rate of locus coeruleus neurons (maximal increase: 95 +/- 13% at 40 microg). 3. I(1)-imidazoline receptor ligands stimulate locus coeruleus neuron activity through an indirect mechanism originated in the paragigantocellularis nucleus via excitatory amino acids. However, neither electrolytic lesions of the paragigantocellularis nucleus nor pretreatment with the excitatory amino acid antagonist kynurenic acid (1 micromol, i.c.v.) modified agmatine effect (10 microg, i.c.v.). 4. After agmatine administration (20 microg, i.c.v.), dose-response curves for the effect of clonidine (0.625 - 10 microg kg(-1) i.v.) or morphine (0.3 - 4.8 mg kg(-1) i.v.) on locus coeruleus neurons were not different from those obtained in the control groups. 5. Pretreatment with the nitric oxide synthase inhibitors N(omega)-nitro-L-arginine (10 microg, i.c.v.) or N(omega)-nitro-L-arginine methyl ester (100 microg, i.c.v.) but not with the less active stereoisomer N(omega)-nitro-D-arginine methyl ester (100 microg, i.c.v.) completely blocked agmatine effect (10 and 40 microg, i.c.v.). 6. Similarly, when agmatine (20 pmoles) was applied into the locus coeruleus there was an increase that was blocked by N(omega)-nitro-L-arginine methyl ester (100 microg, i.c.v.) in the firing rate of the locus coeruleus neurons (maximal increase 53 +/- 11% and 14 +/- 10% before and after nitric oxide synthase inhibition, respectively). 7. This study demonstrates that agmatine stimulates the firing rate of locus coeruleus neurons via a nitric oxide synthase-dependent mechanism located in this nucleus.     

Modification of formalin-induced nociception by different histamine receptor agonists and antagonists.

The present study evaluated the effects of different histamine receptor agonists and antagonists on the nociceptive response in the mouse formalin test. Intracerebroventricular (20-40 microg/mouse i.c.v.) or subcutaneous (1-10 mg/kg s.c.) injection of HTMT (H(1) receptor agonist) elicited a dose-related hyperalgesia in the early and late phases. Conversely, intraperitoneal (20 and 30 mg/kg i.p.) injection of dexchlorpheniramine (H(1) receptor antagonist) was antinociceptive in both phases. At a dose ineffective per se, dexchlorpheniramine (10 mg/kg i.p.) antagonized the hyperalgesia induced by HTMT (40 mug/mouse i.c.v. or 10 mg/kg s.c.). Dimaprit (H(2) receptor agonist, 30 mg/kg i.p.) and ranitidine (H(2) receptor antagonist, 20 and 40 mg/kg i.p.) reduced the nociceptive responses in the early and late phases. No significant change in the antinociceptive activity was found following the combination of dimaprit (30 mg/kg i.p.) with ranitidine (10 mg/kg i.p.). The antinociceptive effect of dimaprit (30 mg/kg i.p.) was prevented by naloxone (5 mg/kg i.p.) in the early phase or by imetit (H(3) receptor agonist, 25 mg/kg i.p.) in both early and late phases. The histamine H(3) receptor agonist imetit was hyperalgesic following i.p. administration of 50 mg/kg. Imetit-induced hyperalgesia was completely prevented by treatment with a dose ineffective per se of thioperamide (H(3) receptor antagonist, 5 mg/kg i.p.). The results suggest that histamine H(1) and H(3) receptor activations increase sensitivity to nociceptive stimulus in the formalin test.