Degradation of methyl and ethyl mercury by singlet oxygen generated from sea water exposed to sunlight or ultraviolet light

Photodegradation of methyl mercury (MeHg) and ethyl Hg (EtHg) in sea water was studied by sunlight or ultraviolet (UV) light exposure, and by determining inorganic Hg produced by degradation. Sea water containing 1 M MeHg or EtHg was exposed to sunlight or UV light. N-Acetyl-l-cysteine was added to the solution for preventing Hg loss during the light exposure. MeHg and EtHg in sea water were degraded by sunlight (>280 nm), UV light A (320–400 nm) and UV light B (280–320 nm), though the amounts of inorganic Hg produced from MeHg were 1/6th to 1/12th those from EtHg. Inorganic Hg production was greater with increasing concentration of sea water. Degradation of MeHg and EtHg by the UV light A exposure was inhibited by singlet oxygen (¹O₂) trappers such as NaN₃, 1,4-diazabicyclo[2,2,2]octane, histidine, methionine and 2,5-dimethylfuran. On the other hand, inhibitors or scavengers of Superoxide anion, hydrogen peroxide or hydroxyl radical did not inhibit the photodegradation of alkyl Hg. These results suggested that (¹O₂) generated from sea water exposed to sunlight, UV light A or UV light B was the reactive oxygen species mainly responsible for the degradation of MeHg and EtHg.     

Quantitative Evaluation of Capacity-Limited Hepatobiliary Transport Based on Hepatocellular Diffusion Model by MULTI(FEM)

The dose-dependency of hepatic uptake and hepatobiliary transport of a drug was evaluated by means of a nonlinear least square program incorporating the finite element method, MULTI(FEM). A perfusion experiment using isolated rat livers following a pulse input (i.e., under non-steady-state conditions) was performed at three dose levels of cefpiramide as a model drug. The hepatic extraction ratio (E_H) of cefpiramide decreased with an increase in dose, which demonstrates that the hepatic uptake is capacity-limited. The outflow time-profiles from the liver were represented by a two-compartment dispersion model with central Michaelis–Menten elimination, and the maximal elimination rate per central compartment volume (V_max) and the Michaelis constant (K_m) were estimated to be 1420 g/ml/min and 235 g/ml, respectively. The biliary mean transit time increased slightly with an increase in dose. The hepatocellular diffusion model under non-steady-state conditions considering nonlinear transport across the bile canalicular membrane was adopted to evaluate dose-dependency in the biliary excretion of cefpiramide. The maximal penetration velocity across the bile canalicular membrane per liver and the affinity constant of penetration across the bile canalicular membrane were estimated to be 40.1 g/min and 123 g, respectively. Considering that the volume of a rat liver (A_H.L) is approximately 10 ml, the Michaelis constant of penetration (k _m ^bmc ), which is an apparent parameter, was estimated to be approximately 12.3 g/ml. In conclusion, MULTI(FEM) is useful for evaluation of capacity-limited local disposition.     

Pharmacokinetic and biopharmaceutic profile of chlordiazepoxide HCl in healthy subjects: Single-dose studies by the intravenous,intramuscular, and oral routes

Single 30- mg doses of chlordiazepoxide HCl were administered to six healthy human subjects by the intravenous, oral, and intramuscular routes. Plasma concentration- time curves following intravenous administration were satisfactorily described by a biexponential equation consistent with a two-compartment open model system. Mean values of half-lives for the so-called distribution and elimination phases were 0.252 and 9.39 hr, respectively. The mean values for the volume of the central compartment (V ₁) and volume of distribution were 18.0 and 30.9% of body weight, respectively. Following oral administration, the drug was rapidly and completely absorbed. Absorption was first order (t_1/227 min), and three of the six subjects showed a discernible lag time of approximately 20 min. Drug absorption following intramuscular administration was comparatively slow. A two- compartment muscle model comprised of precipitated and solubilized drug in the muscle was found to satisfactorily characterize the absorption process following administration by this route.     

The subjective,behavioral and cognitive effects of subanesthetic concentrations of isoflurane and nitrous oxide in healthy volunteers

A prospective, crossover, double-blind trial was conducted in nine healthy volunteers in which the subjective, psychomotor and memory effects of isoflurane (0.0, 0.3 and 0.6%) and nitrous oxide (N₂O) (0, 20 and 40%) were examined. Dependent measures included visual analog scales and a standardized drug effects inventory (subjective effects), reaction time and eye-hand coordination (e.g., psychomotor performance), and immediate and delayed free recall (memory). There were some similarities in subjective effects between the two inhaled drugs (e.g., increased ratings of drunk and spaced out), but isoflurane had effects which N₂O did not have. Isoflurane but not N₂O increased visual analog scale ratings of confused, sedated, and carefree, and decreased ratings of in control of thoughts and in control of body. An odor was detected with isoflurane and it was disliked. Psychomotor performance was more grossly impaired during isoflurane inhalation than during N₂O inhalation. Psychomotor recovery from both agents was rapid and complete so that 5 min after the inhalation period had ceased, performance had returned to baseline levels. Both isoflurane and nitrous oxide impaired immediate and delayed free recall. The feasibility of using isoflurane in conscious sedation procedures is discussed.     

Methyl mercury reduces voltage-activated currents of rat dorsal root ganglion neurons

Methyl mercury (MeHg) is a widespread toxicant with major actions on the nervous system. Since the function of neurons depends on voltage gated ion channels, we examined the effects of micromolar concentrations of methyl mercury on voltage-activated calcium, potassium and sodium channel currents of cultured rat dorsal root ganglion (DRG) neurons. The cells, which were obtained from 2–4 day old rat pups, were whole-cell patch-clamped. Currents were separated by selective intra-and extracellular solutions as well as specific depolarizing voltage steps. We did not distinguish between different calcium, potassium or sodium channel subtypes.All three types of voltage-activated currents were irreversibly reduced by McHg in a concentration dependent manner. Voltage-activated calcium and potassium channel currents were more sensitive to MeHg (Calcium: IC₅₀ = 2.6±0.4 M; Potassium: IC₅₀ = 2.2±0.3 M) than voltage-activated sodium channels (IC₅₀ = 12.3±2.0 M). The Hill coefficients for the reduction of the currents were calculated as 1 for calcium and potassium channel currents and as 1.7 for sodium currents. In the cases of the voltage-activated calcium and sodium channel currents the reduction was clearly use dependent. Higher concentrations of McHg ( 5 M) resulted in a biphasic change in the holding membrane current at the potential of –80 mV in 25% of the cases.     

Effects of mercury bichloride on mouse kidney polyribosome structure and function

Polyribosome sedimentation pattern and their in vitro protein synthetic ability were investigated in kidneys of mice treated with a single injection of HgCl₂. Mercury bichloride, after 1 h, evokes polyribosome disaggregation, the extent of which is logarithmically correlated with the dose in the range of 2.5–20 moles/kg. With the dose of 2.5 moles/kg the effect occurs after 1 h, it is maximal between 1 h and 3 h. After 6 h polyribosomes are reaggregated. Cyclohehimide pretreatment does not prevent the HgCl₂ induced disaggregation of kidney polyribosomes. The cell-free system derived from kidneys of HgCl₂ treated mice (10 moles/kg, 1 h) has a decreased protein synthetic ability. Both, in livers of mice treated with 20 moles/kg HgCl₂ and in isolated rat's reticulocytes incubated with 20 M HgCl₂ during 1 h there were no apparent changes in the polyribosome sedimentation patterns.This work was supported by a grant from the Research Fund of SR Croatia and by a Wellcome Trust grant, and we are grateful to both these organisations     

Methoxamine enhances the release of endogenous noradrenaline from rabbit ear artery: possible involvement of ATP

Summary The effect of methoxamine, an ₁-adrenoceptor agonist, on the electrically-evoked release of endogenous noradrenaline was examined in the isolated rabbit ear artery. Noradrenaline was quantified by high performance liquid chromatography-electrochemical detection. The release of adenine nucleotides and nucleosides by methoxamine was examined using high performance liquid chromatography-fluorescence detection.The release of noradrenaline evoked by electrical field stimulation (EFS) at 4 Hz was reduced by tetrodotoxin 0.3 mol/l and clonidine 1 mol/l by approximately 80% and 50%, respectively. On the other hand, methoxamine at 10 but not 1 mol/l enhanced the release of noradrenaline to approximately twice the control, and the enhancement was prevented by prazosin 1 mol/l. The facilitatory action of methoxamine was also abolished after desensitization of P₂-purinoceptors by ,-methylene ATP 30 mol/l as well as by the presumed P₂-purinoceptor antagonist suramin given at 10 mol/l. Exogenous ATP 10 mol/l significantly enhanced the EFS-evoked release of noradrenaline, and the enhancement was abolished by ,-methylene ATP and suramin. None of the drugs changed the spontaneous outflow of noradrenaline. These results indicate that endogenous ATP, acting at prejunctional purinoceptors, may participate in the facilitatory effect of methoxamine. Indeed, methoxamine 10 mol/l significantly enhanced the spontaneous outflow of ATP and, less so, ADP. The methoxamine evoked release of ATP and ADP was antagonized by prazosin 1 mol/l.It is concluded that methoxamine releases endogenous ATP from postjunctional sites which then, via prejunctional purinoceptors, facilitates action potential-evoked release of noradrenaline in rabbit ear artery.Supported by grants from the Mita Research Foundation, Matsue, Japan and Kanae Research Foundation, Osaka, JapanCorrespondence to K. Takeuchi at the above address     

Infusion of γ2-MSH produce a conditioned taste aversion in morphine-dependent rats

A two flavour, unbiased, taste preference conditioning procedure was used to test for possible motivating effects of ₂-MSH. Three training trials failed to produce any significant effect with doses ranging from 2.4 to 40 g/ ICV infusion in drug-naive, non-operated or placebo-implanted rats. However, in rats made dependent by SC implantation of a morphine pellet 4 days earlier 15 g ₂-MSH/infusion produced a taste aversion that was comparable to that produced by infusion of a low dose of the competitive opioid receptor antagonist naloxone (0.32 g). The findings confirm with a conditioning procedure and with opiate-dependent animals the naloxone-like effects of ₂-MSH. They also suggest that this endogenously-located peptide may acquire an aversive property as a result of chronic morphine treatment.     

P2-purinoceptor antagonists discriminate three contraction-mediating receptors for ATP in rat vas deferens

The sites of action at which ATP elicits contraction of the rat vas deferens were studied by means of the P₂-purinoceptor antagonists pyridoxalphosphate-6-azophenyl-2,4-disulfonic acid (PPADS), suramin and reactive blue 2.Increasing concentrations of PPADS (up to 1 mM), suramin (up to 1 mM) and reactive blue 2 (up to 320 M) reduced and eventually abolished contractions elicited by the P_2X-purinoceptor-selective agonist ,-methylene ATP 3 M with IC₅₀ values of 2.1, 10.1 and 27.0 M, respectively. In contrast, PPADS and suramin caused only a partial inhibition of contractions elicited by ATP 1 mM, maximal reduction by about 40%, IC₅₀ values 1.3 and 5.0 M, respectively; reactive blue 2 did not change ATP-induced contractions. In tissues exposed to PPADS 320 M throughout, increasing concentrations of reactive blue 2 or suramin decreased contractions elicited by ATP 1 MM, IC₅₀ values 2.6 and 14.5 M, respectively. In tissues exposed to suramin 320 M throughout, increasing concentrations of PPADS decreased contractions elicited by ATP 1 mM, IC₅₀ 37.9 M, whereas reactive blue 2 slightly enhanced these contractions. In tissues exposed to reactive blue 2 100 M throughout, increasing concentrations of PPADS reduced contractions elicited by ATP 1 MM, IC₅₀ 26.6 M, whereas suramin caused no change. Pre-exposure to ,-methylene ATP 1 M to desensitize P_2X-purinoceptors reduced the response to ATP 1 mM by 91% in otherwise untreated tissues, but did not reduce the response to ATP 1 mM in tissues exposed throughout to PPADS 320 M, suramin 320 M or reactive blue 2 100 M. Neither PPADS nor suramin nor reactive blue 2 altered contractions elicited by KCl 35 mM. The P₁-purinoceptor antagonist 8-(p-sulfophenyl) theophylline 100 M did not change contractions elicited by ,-methylene ATP 3 M or ATP 1 mM.It is concluded that ATP 1 mM elicits contraction of the rat vas deferens through three sites: P_2X-purinoceptors which are blocked by PPADS, suramin and reactive blue 2; P_2Y-purinoceptors blocked by reactive blue 2 and suramin but resistant to PPADS; and non-P_2X-non-P_2Y-purinoceptors blocked by PPADS but resistant to inhibition by suramin and reactive blue 2. Correspondence to: R. Bültmann at the above address     

Effects of various structurally related beta-adrenoceptor blocking agents on maximum upstroke velocity of action potential in guinea-pig papillary muscles

Summary The effects of indenolol (7.0–100 mol/l), befunolol (15.3–250 mol/l), metoprolol (58.4–1000 mol/l), Kö707 (7.3–100 mol/l), D-25 (3.7–100 mol/l) and Kö1313 (73.9–1000 mol/l) on action potentials were investigated in isolated guinea-pig papillary muscles. All these aryloxyisopropylaminopropanol derivatives produced a concentration-dependent reduction of _max at a basic rate of 1 Hz. The reduction was less prominent when interstimulus intervals were prolonged. The time course of recovery of _max during diastole was studied by assessing _max in premature responses at 0.25, 0.1 and 0.027 Hz and in responses after interrupting driving stimuli of 1 Hz. The recovery process was approximated by a single exponential function. The results, together with those reported previously (Sada and Ban 1980, 1981), revealed: (1) a significant correlation between potencies of these derivatives for depression of _max at 0.027, 0.25, 1 and 5 Hz and their log n-octanol/water partition coefficients (log P); (2) The time constants of recovery were relatively concentration independent and correlated significantly with the molecular weights. The results suggest that the potency of these drugs for the depression of _max was mainly determined by log P, being modified secondarily by the time constants of recovery which are intimately associated with the molecular weights.     

Retrospective study on the reliability of an “approximate LD50” determined with a small number of animals

Based on 364 LD₅₀ determinations in mice and rats after intravenous and oral administration of drugs, the reliability of an approximate LD₅₀ was retrospectively tested.The difference between approximate LD₅₀ and LD₅₀ is — independent of species and route of administration — not greater than ± 20% of the LD₅₀ in 90% of the cases.Four to five doses — uniformly distributed over the dose-mortality range can suffice in reliably determining the approximate LD₅₀.The probability is 10% that an approximate LD₅₀ and LD₅₀ are significantly different from each other.152 parallel studies on male and female animals show that the LD₅₀ or approximate LD₅₀ must not be determined for both sexes. It is sufficient to test a dose near the LD₅₀ in the opposite sex. A 50–75% reduction of expenditure in animal material is possible in most of LD₅₀ determinations.     

Both β1- and β2-adrenoceptors mediate catecholamine-evoked arrhythmias in isolated human right atrium

Summary The involvement of ₁- and ₂-adrenoceptors in catecholamine-evoked arrhythmias was investigated in isolated human right atrial appendages obtained from 22 patients chronically treated with blockers (usually ₁-selective) and 9 patients not treated with blockers. A simple experimental model that assesses the incidence of arrhythmic contractions as a function of heart rate (pacing) is introduced. ₁-adrenoceptors were activated by (–)-noradrenaline during ₂-adrenoceptor blockade with 50 nmol/l ICI 118551. ₂-adrenoceptors were activated by (–)-adrenaline during ₁-adrenoceptor blockade with 300 nmol/l CGP 20712A. Both (–)noradrenaline and (–)-adrenaline caused arrhythmic contractions whose incidence was greater at low than at high pacing rates. CGP 20712A (300 nmol/l) blocked the (–)-noradrenaline-evoked contractions in 1/1 atrial strip from 1/1 patient not treated with a blocker and 17/17 atrial strips from 15/15 patients chronically treated with blockers. ICI 118551 (50 nmol/l) blocked the (–)-adrenaline-evoked contractions in 3/4 atrial strips from 3/4 patients not treated with blockers and 17/20 atrial strips from 15/18 patients chronically treated with blockers. The incidence of arrhythmic contractions evoked by both (–)-noradrenaline and (–)-adrenaline was higher in chronically blocked patients than in non blocked patients. We conclude that both ₁- and ₂-adrenoceptors mediate atrial arrhythmias and that the generation of these arrhythmias is facilitated by chronic ₁-adrenoceptor blockade. Correspondence to: A. J. Kaumann at the Clinical Pharmacology Unit, University of Cambridge, as above     

The influence of hyperthyroidism on β-adrenoceptor-mediated relaxation of isolated small mesenteric arteries

We investigated the influence of hyperthyroidism on relaxant responses of small mesenteric resistance arteries to -adrenoceptor agonists and to compounds stimulating the corresponding second-messenger system. Hyperthyroidism was induced by feeding rats for 28 days with 5 mg/kg L-thyroxine (T4)-containing rat chow. This treatment produced a stable hyperthyroid state, as indicated by several biochemical/metabolic and haemodynamic parameters. Preparations of small mesenteric arteries were mounted in an isometric wire myograph. Subsequently, concentration-effect curves were determined for isoproterenol, noradrenaline and salbutamol as well as for forskolin, dibutyryl-cAMP and theophylline. We also determined concentration-effect curves to the -adrenoceptor agonists in the presence of ICI 118,551 and CGP 20712A (i.e., in the presence of a selective ₂- and ₁-adrenoceptor antagonist, respectively). Apparent pA₂-values were calculated to determine which -adrenoceptor subtype causes vasodilation. These experiments indicate that -adrenoceptor-mediated vasodilation involves both ₁- and ₂-adrenoceptors in mesenteric resistance vessels of both hyperthyroid and control rats. In our experiments hyperthyroidism has a sensitizing influence on vascular responses induced by the -adrenoceptor agonist isoproterenol and the selective ₂-adrenoceptor agonist salbutamol. Sensitization to isoproterenol was abolished in the presence of ICI 118,551, whereas it was emphasized in the presence of CGP 20712A. Although this was not fully supported by the results obtained with noradrenaline, these results indicate that the sensitization to -adrenoceptor agonists is probably limited to the ₂-adrenoceptor/G-protein complex and not associated with alterations of the corresponding second messenger system.     

Regulation of adenylate cyclase activity in hamster adipocytes

Summary In ghosts of hamster adipocytes, the regulation of adenylate cyclase (ATP: pyrophosphate lyase, cyclizing; EC 4.6.1.1) activity by prostaglandins, -adrenergic agonists and nicotinic acid was studied. These three classes of antilipolytic agents caused adenylate cyclase inhibition without an apparent lag phase. Maximal inhibitions observed ranged between about 45% (by -adrenergic agonists) and 60% (by prostaglandins and nicotinic acid). The order of potency for the inhibitory prostaglandins (PG) was PGE₁ PGE₂>PGF₂PGI₂>PGD₂>6-keto PGF₁. The IC₅₀ values obtained were about 0.007, 0.06, 0.3 and 1 M for PGE₁, PGF₂, PGD₂ and 6-keto PGF₁, respectively. -Adrenergic agonists, studied in the presence of the -adrenergic blocking agent, propranolol (30 M), inhibited the fat cell enzyme with the order of potency (1)-adrenaline > (1)--methylnoradrenaline (1)-noradrenaline > clonidine tetryzoline > (1)-phenylephrine. The IC₅₀ values obtained for (1)-adrenaline and (1)-noradrenaline were about 3 and 10 M, respectively. The inhibitory effect of (1)-adrenaline was blocked by the -adrenergic antagonists with the potency order yohimbine phentolamine > prazosin. These findings suggest that an ₂ of receptors is involved in this catecholamine-induced inhibition. Nicotinic acid (10 M) reduced adenylate cyclase activity by about 60% with half-maximal effectiveness at about 0.6 M. The nicotinic acid derivatives, nicotinamide, -pyridylcarbinol and NAD (up to 100 M), had no effect on enzyme activity.Inhibition of the hamster adipocyte adenylate cyclase by the antilipolytic agents required the presence of both GTP, which reduced basal activity by about 80% at 10 M, and sodium ions, which specifically activated the GTP-affected from of the enzyme. Inhibition was also observed in the presence of ACTH, which in a GTP-dependent manner increased adenylate cyclase activity. Pretreatment of the enzyme preparation with NaF (10 mM) partially reduced the inhibitory effect, and preactivation with the stable GTP analogue, guanylyl 5-imidodiphosphate (100 M), abolished the adenylate cyclase inhibition by the antilipolytic agents.Abbreviations PG prostaglandin - GMP-P(NH)P guanylyl 5-imidodiphosphate Some of the data were presented in abstract form (Aktories et al., 1979a)     

Light,vitamin D and psychiatry

This is a review and a prospectus of effects of vitamin D on the brain. Effects of sunlight and equivalent artificial light on physiological and behavioral processes are probably mediated, in large part, through the skin-vitamin D-endocrine system. Experimental evidence from our laboratory reveals sites of action and concomitant direct effects of 1,25(OH)₂ vitamin D₃ (soltriol) on brain, spinal cord, pituitary and other endocrine tissues. This appears relevant for the activation and modulation of mental and endocrine processes, particularly related to seasonal and daily biorhythms. Effects of sunlight and corresponding artificial light are likely to be mediated through direct actions of soltriol on brain and endocrine tissues that are independent of its effect on calcium levels. Those direct actions are receptor mediated and appear to be dose related as they depend on intensity of light and length of exposure, considering light (photons) as a drug. A role for soltriol, the steroid hormone of sunlight, in the etiology and helioor phototherapy of affective disorders with cyclic seasonal onset (seasonal affective disorder) is discussed and the significance of research in the new frontier of vitamin D and brain relationships is noted.The term soltriol is used for the steroid hormone of sunlight, 1,25(OH)₂ vitamin D₃, in correspondence to other steroid hormones, such as estradiol and cortisol. Vitamin D is a misnomer, related to its historic discovery as an ingestible vital extract, before being recognized as a steroid hormone     

Studies on the chemical constituents from the roots of <Emphasis Type="Italic">Platycodon grandiflorum</Emphasis>

Three known monodesmosidic saponins: 3-O--d-glucopyranosyl-2,3,16,23,24-pentahydroxyolean-12-ene-28-oic acid, 3-O--d-glucopyranosyl polygalacic acid, and 3-O--d-glucopyranosyl-(13)--d-glucopyranosyl polygalacic acid; and two known nonsaponin compounds: a mixed compound of n-tetracosanoic acid (lignoceric acid), n-hexacosanoic acid (cerotic acid), and n-octacosanoic acid, and -monopalmitin; were isolated for the first time from the root of Platycodon grandiflorum A. DC. together with another seven known compounds: platycoside G₁ (3-O--d-glucopyranosyl-(16)--d-glucopyranosyl-(16)--d-glucopyranosyl-2,3,16,23,24-pentahydroxyolean-12-ene-28-oic acid 28-O--d-xylopyranosyl-(14)--l-rhamnopyranosyl-(12)--l-arabinopyranoside), deapio-platycodin D, Polygalacin D, deapio-platycodin D₃, platycoside A, -spinasterol, and -spinasteryl-3-O--d-glucopyranoside. Alkaline hydrolysis of platycoside G₁ afforded a new monodesmosidic prosaponin: 3-O--d-glucopyranosyl-(16)--d-glucopyranosyl-(16)--d-glucopyranosyl-2,3,16,23,24-pentahydroxyolean-12-ene-28-oic acid. Their chemical structures were elucidated on the basis of their spectral data and chemical evidence.     

Differential modulation by muscimol and baclofen on antinociception induced by morphine, β-endorphin,d-Pen2,5-enkephalin and U50,488H administered intracerebroventricularly in the mouse

The present study was designed to investigate the modulatory effects of stimulation of GABA_A and GABA_B receptors at supraspinal sites on antinociception induced by supraspinally administered -, -, -, and -opioid receptor agonists. The effects of the GABA_A and GABA_B receptor agonists, muscimol and baclofen respectively, on the antinociception induced by morphine (a -receptor agonist), -endorphin (an -receptor agonist), D-Pen^2,5-enkephalin (DPDPE, a -receptor agonist) and U50,488H ({trans-3,4-di-chloroN-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeocetamide}; a -receptor agonist) injected intracerebroventricularly (i.c.v.) were studied. The anti-nociception was assayed using the tail-flick and hot-plate tests. Muscimol at doses of 25–200 ng, administered i.c.v. alone did not affect the latencies of tail-flick and hot-plate thresholds, but attenuated dose-dependently the inhibition of the tail-flick and hot-plate responses induced by i.c.v. administered morphine (2 g), -endorphin (1 g), DPDPE (10 g), and U50,488H (60 g). Baclofen (1.25–10 ng) administered i.c.v. alone did not affect the latencies of the tail-flick and hot-plate responses, but attenuated dose-dependently the inhibition of the tail-flick and hot-plate responses induced by -endorphin and U50,488H, without affecting morphine-or DPDPE-induced responses. Our results indicate that activation of GABA_A receptors at the supraspinal sites by i.c.v. injection of muscimol antagonizes antinociception induced by supraspinally administered -, -, -, and -opioid receptor agonists. On the other hand, activation of GABA_B receptors at supraspinal sites by i.c.v. baclofen antagonizes antinociception induced by i.c.v. administered - and -opioid agonists, but not - or -opioid agonists.     

Investigation of the 5-hydroxytryptamine receptor mediating the “transient” short-circuit current response in guinea-pig ileal mucosa

5-Hydroxytryptamine (5-HT) stimulated an increase in short-circuit current (I_sc) in guinea-pig isolated ileal mucosa over a wide concentration range (0.1 nM-0.1 mM). The concentration-response relationship was biphasic, consisting of a high potency phase (0.1 nM–1 M) and a low potency phase (3–10 M). Stimulation of Isc observed at the high potency phase tended to be sustained while responses at the low potency phase (3–10 M) contained two components, an initial transient response followed by a maintained response. Both the high potency phase (maximum stimulation 30 A cm^–2) and the low potency phase (maximum stimulation 80 A cm ^–2) 5-HT response were antagonized by tetrodotoxin (TTX, 0.3 M) and atropine (1 M). However, another low potency (3 M-0.1 mM, maximum stimulation 30 A cm^–2) component of the 5-HT response was revealed in the presence of TTX or atropine.In the presence of methysergide (1 M), the concentration-response relationship of 5-HT was still biphasic and tropisetron (0.1 and 10 M) antagonized both phases of the 5-HT response. In the presence of methysergide, the high potency phase 5-HT response was mimicked by 5-methoxytryptamine (5-MeOT) and the selective 5-HT₄ agonist SC-53116 but not by BIMU 8. The potent 5-HT₄ antagonist GR 113808 antagonized the response to 5-MeOT in a surmountable manner with an affinity estimate of 9.6 ± 0.3 (n = 4). The 5-MeOT stimulated increase in I_sc was also antagonized in an unsurmountable manner by granisetron (1 M).In the presence of methysergide, desensitization of 5-HT₃ receptors with 2-methyl-5-hydroxytryptamine (10 M) abolished both phases of the 5-HT response. Under the same condition, desensitization of 5-HT₄ receptors with 5-MeOT (10 M) abolished only the high potency 5-HT response and dextrally shifted the low potency 5-HT response.These data show that neuronal and non-neuronal 5-HT receptors are involved in the regulation of secretion in ileal mucosa. We propose the presence of a neuronal 5-HT₄ receptor located upstream of the well characterized neuronal 5-HT₃ receptors to be responsible for the high potency 5-HT response. A schematic model is proposed to explain our findings and the relationship between this 5-HT₄ receptor and other 5-HT receptor subtypes regulating secretion that have been described in the literature.     

Cardiac ventricular β2-adrenoceptors in guinea-pigs and rats are localized on the coronary endothelium

Summary In mammalian heart tissue ₂ are known to coexist with ₁. In the present study, evidence that ₂ in guinea-pig and rat ventricles are primarily localized on the coronary endothelium is provided by competition binding studies with the subtype-selective -adrenoceptor antagonists ICI 89.406 (₁) and ICI 118.551 (₂) on four different plasma membrane preparations. (1) Following density gradient centrifugation of cardiac ventricular microsomes from rats or guinea-pigs, endothelial plasma membranes migrated at slightly higher density than the sarcolemmal membranes, as verified by endothelial (angiotensin converting enzyme) and sarcolemmal markers (adenylate cyclase, [³H] ouabain binding). At the activity peak of angiotensin converting enzyme, the relative amount of ₂ in guinea-pigs and rats was 25% and 65%, respectively. (2) On sarcolemmal membranes corresponding to the activity peak of adenylate, cyclase, -adrenoceptors consisted of the ₁ exclusively (guinea-pig), or to at least 90% (rat). (3) Cultures of coronary endothelial cells derived from guinea-pigs revealed only ₂. (4) Isolated guinea-pig cardiomyocytes contained only ₁, a finding recently established in rat myocytes as well.