Frequency-dependent muscarinic receptor modulation of acetylcholine and dopamine release from rabbit striatum

The effects of muscarinic receptor activation on the electrically evoked release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) or [3H]ACh were investigated in rabbit striatal slices. Release was measured in the presence of 10 microM hemicholinium and 1 microM sulpiride to block choline uptake and prevent the effects of released DA on DA receptors modulating release. Stimulation (120 pulses, 20 mA, 2 msec) at 0.3, 3 and 10 Hz produced (3H or 14C) ACh release that sharply declined with increasing stimulation frequency. A flat frequency-release curve was obtained for DA. Oxotremorine (OXO), a direct muscarinic agonist (1-100 microM), produced a concentration-dependent inhibition of ACh release, inversely related to stimulation frequency, at a fixed number of pulses (120). When the number of pulses was modified to produce similar amounts of ACh release (20 pulses at 0.1 Hz, 39 pulses at 0.3 Hz, 120 pulses at 3 Hz and 350 pulses at 10 Hz), much greater inhibition of ACh release by OXO (0.3 and 3 microM) was obtained with lower frequencies and lower number of pulses. Physostigmine, an acetylcholinesterase inhibitor, decreased ACh release with an inverse relationship to stimulation frequency. Atropine (1 microM), a selective muscarinic antagonist, enhanced the release of ACh more at 10 Hz than at 0.3 and 3 Hz and completely antagonized the effects of OXO (10 microM) and physostigmine (1 microM) at all three stimulation frequencies. OXO (3 and 10 microM) enhanced DA release at 3 Hz. Physostigmine (1 microM) and atropine (1 microM) had no effect on DA release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of [3H]dopamine release from guinea pig striatum by NMDA receptor/channel activators and inhibitors.

Excitatory amino acids, that interact with the N-methyl-D-aspartate (NMDA) receptor stimulate release of [3H]dopamine [3H]DA) from the striatum of the guinea pig and rat in a concentration-dependent manner. DA release was measured in the presence of domperidone and nomifensine to avoid complications associated with autoreceptor alteration of and reuptake of released DA. This release is inhibited by magnesium. Therefore, all experiments were performed in the absence of this ion. The competitive NMDA antagonists D-(-)2-amino-5-phosphonopentanoic acid and 3-[(+-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid and the noncompetitive antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine and phencyclidine also inhibit NMDA-stimulated release. Glycine enhances NMDA-stimulated release and can release [3H]DA in the absence of added NMDA. Release stimulated by glycine alone is not affected by 3-[(+-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonic acid. Conversely, if the glycine antagonist 3-amino-1-hydroxy-2-pyrrolidone or 6-cyano-7-nitroquinoxaline-2,3-dione is included, NMDA elicits less release of [3H]DA. This inhibition can be overcome by increasing the concentration of glycine. The kappa-selective opioid agonist trans-(+-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzene-acetamide is also capable of inhibiting the NMDA-stimulated release of [3H]DA from guinea pig and rat striatum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nicotinic receptor modulation of dopamine transporter function in rat striatum and medial prefrontal cortex

Nicotine activates nicotinic acetylcholine receptors (nAChRs) on dopamine (DA) terminals to evoke DA release, which subsequently is taken back up into the terminal via the DA transporter (DAT). nAChRs may modulate DAT function thereby contributing to the regulation of synaptic DA concentrations. The present study determined the dose-response for nicotine (0.1-0.8 mg/kg, s.c.) to modulate DA clearance in striatum and medial prefrontal cortex (mPFC) using in vivo voltammetry in urethane anesthetized rats and determined if this effect was mediated by nAChRs. Exogenous DA (200 microM) was pressure-ejected at 5-min intervals until reproducible baseline signals were obtained. Subsequently, nicotine or saline was administered, and DA pressure ejection continued at 5-min intervals for 60 min. In both striatum and mPFC, signal amplitude decreased by approximately 20% across the 60-min session in saline-injected rats. A monophasic dose-response curve was found in striatum, with a maximal 50% decrease in signal amplitude after 0.8 mg/kg. In contrast, a U-shaped dose-response curve was found in mPFC, with a maximal 50% decrease in signal amplitude after 0.4 mg/kg. Onset of nicotine response occurred 10 to 15 min after injection in both brain regions; however, the amount of time before maximal response was 45 and 30 min in striatum and mPFC, respectively. Mecamylamine (1.5 mg/kg) completely inhibited the nicotine-induced (0.8 and 0.4 mg/kg) decrease in signal amplitude in striatum and mPFC, respectively, indicating mediation by nAChRs. Thus, nicotine enhances DA clearance in striatum and mPFC in a mecamylamine-sensitive manner, indicating that nAChRs modulate DAT function in these brain regions.     

Increased dopamine receptor signaling and dopamine receptor-G protein coupling in denervated striatum

Chronic interruption of the nigrostriatal dopaminergic pathway leads to sensitized dopaminergic responses in striatum. We attempted to explore the mechanism(s) underlying this dopaminergic supersensitivity by assessing dopamine receptor signaling and receptor-G protein coupling in unilateral 6-hydroxydopamine-lesioned rats. Dopamine-stimulated adenylyl cyclase activity as well as dopamine-activated guanosine 5'-O-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding and [(3)H]palmitate incorporation by Galpha proteins were enhanced in tissues obtained from denervated striata without apparent changes in Galpha protein levels. Moreover, high-affinity binding sites of the D(1) dopamine receptor increased in lesioned compared with control striata without altering the expression level of the receptor. These denervation-mediated changes appear to correlate with the increase in D(1) dopamine receptor binding sites that co-immunoprecipitated with Galphas(olf)/q(11) proteins. In contrast, the total number of D(2) receptor binding sites was increased, yielding an increase in absolute number of high-affinity sites without significant changes in the proportion of high-affinity sites. Stimulation of the D(2) dopamine receptor enhanced coupling to Galphai protein; this was increased in the striata lesioned. The results provide an important molecular mechanism by which dopamine receptor-regulated signaling is enhanced following denervation of dopaminergic input to striatum. Although D(1) dopamine receptor supersensitivity appears to be mediated by enhanced coupling of the receptor to its G proteins, sensitization in the D(2) dopamine receptor system is mediated by increased D(2) receptor density and enhanced D(2) receptor-Gi protein coupling.     

Comparison of effects of haloperidol administration on amphetamine-stimulated dopamine release in the rat medial prefrontal cortex and dorsal striatum

Research has shown that there are important neurochemical differences between the mesocortical and mesostriatal dopamine systems. The work reported in this paper has sought to compare the regulation of dopamine release in the medial prefrontal cortex and the anterior caudate-putamen. In vivo microdialysis was used to recover dialysate fluid for subsequent assay for dopamine concentrations. The responses to D2 antagonist (haloperidol) administration, which has been shown to increase impulse-dependent dopamine release, were compared. Results demonstrated a diminished effect of systemic haloperidol administration on dopamine efflux in the prefrontal cortex. The responses to systemic administration of a nonimpulse-dependent, transporter-mediated, dopamine releaser (d-amphetamine) were also contrasted. Results again demonstrated a diminished pharmacological effect in the cortex. The potential interaction of stimulation of these two types of dopamine release was examined by coadministration of these compounds. Haloperidol pretreatment dramatically potentiated the dopamine-releasing effect of amphetamine administration. This effect was observed in both the cortex and the striatum. Subsequent work demonstrated that this effect of haloperidol was mediated by D2-like receptors in the prefrontal cortex. These results are discussed in relation to other neurochemical and neuroanatomical studies demonstrating sparse densities of dopamine transporter sites and dopamine D2 receptors in the cortex compared with the striatum. They demonstrate a functional correlate to the recently reported, largely extrasynaptic localization of dopamine transporter sites in the prefrontal cortex. Furthermore, they demonstrate the existence of cortical D2-like autoreceptors that may normally be "silent" under basal conditions.     

Kappa opioid receptor activity in spontaneously hypertensive rats

Earlier studies from this laboratory had indicated that there is a selective increase in the density of brain kappa opioid receptors labeled with [3H]ethylketocyclazocine in spontaneously hypertensive (SHR) rats in comparison to normotensive Wistar-Kyoto rats. The binding of a mu-ligand, [3H]naltrexone, and a delta-ligand, [3H]Tyr-D-Ser-Gly-Phe-Leu-Thr, to brain membranes of hypertensive and normotensive rats did not differ. The present studies were undertaken to determine further the role of kappa opioid receptors in hypertension. The binding of [3H]ethylketocyclazocine to brain membranes of hypertensive rats was much greater than those of normotensive rats. The density of kappa receptors was significantly higher in hypothalamic membranes of hypertensive rats as compared to normotensive rats. In order to determine the functional significance of the increased density of brain kappa opioid receptors in SHR rats, the effect of the kappa receptor agonists, tifluadom, U-50,488H and bremazocine, on two known actions associated with kappa receptors, namely analgesia and diuresis, were determined in SHR and normotensive rats. All three kappa agonists produced dose-dependent analgesia as measured by the tail-flick test. The intensity of the analgesic responses at each dose of the drugs in SHR rats was much greater than in normotensive Wistar-Kyoto rats. The kappa drugs also produced dose-dependent diuretic effects when the rats were loaded with 5% saline intragastrically. The increases in the volumes of urine produced by kappa drugs were much greater in SHR rats in comparison to normotensive rats. The basal tail-flick reaction time or urinary output in the two strains did not differ.(ABSTRACT TRUNCATED AT 250 WORDS)     

Paraquat exposure reduces nicotinic receptor-evoked dopamine release in monkey striatum

Paraquat, an herbicide widely used in the agricultural industry, has been associated with lung, liver, and kidney toxicity in humans. In addition, it is linked to an increased risk of Parkinson's disease. For this reason, we had previously investigated the effects of paraquat in mice and showed that it influenced striatal nicotinic receptor (nAChR) expression but not nAChR-mediated dopaminergic function. Because nonhuman primates are evolutionarily closer to humans and may better model the effects of pesticide exposure in man, we examined the effects of paraquat on striatal nAChR function and expression in monkeys. Monkeys were administered saline or paraquat once weekly for 6 weeks, after which nAChR levels and receptor-evoked [(3)H]dopamine ([(3)H]DA) release were measured in the striatum. The functional studies showed that paraquat exposure attenuated dopamine (DA) release evoked by alpha3/alpha6beta2(*) (nAChR that is composed of the alpha3 or alpha6 subunits, and beta2; the asterisk indicates the possible presence of additional subunits) nAChRs, a subtype present only on striatal dopaminergic terminals, with no decline in release mediated by alpha4beta2(*) (nAChR containing alpha4 and beta2 subunits, but not alpha3 or alpha6) nAChRs, present on both DA terminals and striatal neurons. Paraquat treatment decreased alpha4beta2(*) but not alpha3/alpha6beta2(*) nAChR expression. The differential effects of paraquat on nAChR expression and receptor-evoked [(3)H]DA release emphasize the importance of evaluating changes in functional measures. The finding that paraquat treatment has a negative impact on striatal nAChR-mediated dopaminergic activity in monkeys but not mice indicates the need for determining the effects of pesticides in higher species.     

In vivo dopamine clearance rate in rat striatum: regulation by extracellular dopamine concentration and dopamine transporter inhibitors

Dopamine transporter (DAT) inhibitors are expected to decrease dopamine (DA) clearance from the extracellular space of the brain. However, mazindol and cocaine have been reported to "anomalously" increase DA clearance rate. To better understand in vivo DAT activity both in the absence and presence of DAT inhibitors, clearance of exogenously applied DA was measured in dorsal striata of urethane-anesthetized rats using high-speed chronoamperometry. As higher amounts of DA were ejected, DA signal amplitudes, but not time courses, increased. Clearance rates increased until near maximal rates of 0.3 to 0.5 microM/s were attained. Provided baseline clearance rates were relatively low (< 0.1 microM/s), local application of either nomifensine or cocaine markedly increased exogenous DA signal amplitudes and time courses. Relative to the low baseline group, locally applied nomifensine decreased clearance rate when baseline clearance was high ( approximately 0.4 microM/s). However, even when baseline clearance rates were high, systemic injection of nomifensine, mazindol, GBR 12909, or benztropine increased DA signal amplitudes to a greater extent than time courses, consistent with the observed increases in clearance rates. In contrast, despite low baseline clearance rates, systemic injection of cocaine, WIN 35,428, or d-amphetamine preferentially increased DA signal time course, consistent with the observed decreases in clearance rates. Our results emphasize that as extracellular DA concentrations increase, DAT velocity increases to a maximum, partially explaining the ability of DAT inhibitors to increase DA clearance rates. However, by itself, kinetic activation is not sufficient to explain the ability of certain systemically administered DAT inhibitors to anomalously increase DA clearance.     

Cholecystokinin release from the rat caudate-putamen, cortex and hippocampus is increased by activation of the D1 dopamine receptor.

Studies were undertaken to examine the role of dopamine in the regulation of cholecystokinin (CCK) release in slices of the rat caudate-putamen (CP), a region where both neuroactive substances are abundant yet not extensively colocalized. It was found that dopamine, acting through a D1 dopamine receptor, is a major factor regulating CCK release in this tissue. Haloperidol and the selective D1 antagonist, SKF 83566, but not the D2 antagonist, sulpiride, inhibited potassium-evoked release from CP slices. Apomorphine enhanced both basal and potassium-evoked release, an effect which was reversed by (+/-)SKF 83566, but not by sulpiride. The D1 agonist, (+)SKF 38393, displayed agonist and antagonist activity in this system, whereas the D2 agonist, quinpirole, had no effect. Depletion of endogenous dopamine by pretreatment of animals with alpha-methyl-para-tyrosine decreased potassium-induced release by 45%, and in these animals, haloperidol no longer inhibited release. These dopaminergic agents altered CCK release in cortex and hippocampus in a similar fashion, although those tissues were less sensitive to their effects. These results suggest that dopamine, acting through D1 receptors, increases CCK release in CP, cortex and hippocampus. The results of previous studies showing that phorbol esters increase CCK release and that occupation of D1 receptors increases phosphoinositol turnover make it possible to hypothesize that dopamine, acting through D1 receptors, is increasing CCK release by increasing phosphoinositol turnover. Further studies will be required to verify this hypothesis and to determine whether dopamine is acting directly or indirectly to modulate CCK release.     

Presynaptic regulation of cardiac sympathetic function in guinea pigs

Cold stress (4 degrees C) induces a pressor response and variable increases in an index of sympathetic neural function, the rate constant of norepinephrine turnover, kNE. In heart, presynaptic cholinergic muscarinic and alpha-2 adrenergic influences may contribute to regional variation in responses of kNE to cold stress. Animals were pretreated with vehicle, a muscarinic cholinergic antagonist, quinuclidinyl benzilate (QNB), an alpha-2 adrenergic antagonist, yohimbine (YOH) or combined QNB + YOH. An increase in kNE was determined from incorporation of radiolabeled tyrosine into norepinephrine in a control period at 24 degrees C and again at 4 degrees C. The increment in kNE factored by the increment in blood pressure indicated the extent of increased sympathetic function in each cardiac region. In sino-atrial node, sympathetic function was increased significantly (P less than .05) by QNB + YOH compared to other treatments, suggesting that both cholinergic and alpha-2 adrenergic presynaptic influences were important. In contrast, in right and left ventricles, YOH or QNB + YOH, but not QNB alone, increased sympathetic function significantly, suggesting that only alpha-2 adrenergic influences were important. These data support the concept that presynaptic regulation of cardiac sympathetic function differs in sino-atrial node and ventricles of guinea pigs during activation with cold stress.     

Steroids modulate N-methyl-D-aspartate-stimulated [3H] dopamine release from rat striatum via sigma receptors

Steroids have been proposed as endogenous ligands at sigma receptors. In the current study, we examined the ability of steroids to regulate N-methyl-d-aspartate (NMDA)-stimulated [3H]dopamine release from slices of rat striatal tissue. We found that both progesterone and pregnenolone inhibit [3H]dopamine release in a concentration-dependent manner similarly to prototypical agonists, such as (+)-pentazocine. The inhibition seen by both progesterone and pregnenolone exhibits IC50 values consistent with reported Ki values for these steroids obtained in binding studies, and was fully reversed by both the sigma1 antagonist 1-(cyclopropylmethyl)-4-2'-4"flurophenyl)-2'oxoethyl)piperidine HBr (DuP734) and the sigma2 antagonist 1'-[4-[1-(4-fluorophenyl)-1-H-indol-3-yl]-1-butyl]spiro[iso-benzofuran-1(3H), 4'piperidine] (Lu28-179). Lastly, to determine whether a protein kinase C (PKC) signaling system might be involved in the inhibition of NMDA-stimulated [3H]dopamine release, we tested the PKCbeta-selective inhibitor 5,21:12,17-dimetheno-18H-dibenzo[i,o]pyrrolo[3,4 - 1][1,8]diacyclohexadecine-18,20(19H)-dione,8-[(dimethylamino)methyl]-6,7,8,9,10,11-hexahydro-monomethanesulfonate (9Cl) (LY379196) against both progesterone and pregnenolone. We found that LY379196 at 30 nM reversed the inhibition of release by both progesterone and pregnenolone. These findings support steroids as candidates for endogenous ligands at sigma receptors.     

The increased release of prostaglandin E2 by Kupffer cells from burned guinea pigs

Prostaglandin E2 (PGE2) is a very important immunosuppressive substance that is synthesized and released by all macrophages including Kupffer cells (KCs). In this study the changes of PGE2 released by KCs were evaluated in a burned guinea pig model. Prostaglandin E2 was released by KCs from burned guinea pigs at a consistently and significantly high level when stimulated with endotoxin. On postburn day 8, KCs that were cocultured with hepatocytes released significantly less PGE2. There was also a significant decrease in antibody-dependent cell-mediated cytotoxicity on postburn day 1. Kupffer cells can mediate immune suppression in burn injury by a prolonged increased production of PGE2 that has immunosuppressive effects on other cells and by a decreased cytotoxic effect soon after burn injury.     

Methylphenidate-induced increases in vesicular dopamine sequestration and dopamine release in the striatum: the role of muscarinic and dopamine D2 receptors

Methylphenidate (MPD) administration alters the subcellular distribution of vesicular monoamine transporter-2 (VMAT-2)-containing vesicles in rat striatum. This report reveals previously undescribed pharmacological features of MPD by elucidating its receptor-mediated effects on VMAT-2-containing vesicles that cofractionate with synaptosomal membranes after osmotic lysis (referred to herein as membrane-associated vesicles) and on striatal dopamine (DA) release. MPD administration increased DA transport into, and decreased the VMAT-2 immunoreactivity of, the membrane-associated vesicle subcellular fraction. These effects were mimicked by the D2 receptor agonist quinpirole and blocked by the D2 receptor antagonist eticlopride. Both MPD and quinpirole increased vesicular DA content. However, MPD increased, whereas quinpirole decreased, K(+)-stimulated DA release from striatal suspensions. Like MPD, the muscarinic receptor agonist, oxotremorine, increased K(+)-stimulated DA release. Both eticlopride and the muscarinic receptor antagonist scopolamine blocked MPD-induced increases in K(+)-stimulated DA release, whereas the N-methyl-d-aspartate receptor antagonist (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) was without effect. This suggests that D2 receptors mediate both the MPD-induced redistribution of vesicles away from synaptosomal membranes and the MPD-induced up-regulation of vesicles remaining at the membrane. This results in a redistribution of DA within the striatum from the cytoplasm into vesicles, leading to increased DA release. However, D2 receptor activation alone is not sufficient to mediate the MPD-induced increases in striatal DA release because muscarinic receptor activation is also required. These novel findings provide insight into the mechanism of action of MPD, regulation of DA sequestration/release, and treatment of disorders affecting DA disposition, including attention-deficit hyperactivity disorder, substance abuse, and Parkinson's disease.     

N-methyl-D-aspartate increases acetylcholine release from rat striatum and cortex: its effect is augmented by choline.

We examined the effects of N-methyl-D-aspartate (NMDA), a glutamate agonist, and of glutamate itself, on acetylcholine (ACh) release from superfused rat striatal slices. In a Mg(++)-free medium, NMDA (32-1000 microM) as well as glutamate (1 mM) increased basal ACh release by 35 to 100% (all indicated differences, P less than .05), without altering tissue ACh or choline contents. This augmentation was blocked by Mg++ (1.2 mM) or by MK-801 (10 microM). Electrical stimulation (15 Hz, 75 mA) increased ACh release 9-fold (from 400 to 3660 pmol/mg of protein): this was enhanced (to 4850 pmol/mg of protein) by NMDA (100 microM). ACh levels in stimulated slices fell by 50 or 65% depending on the absence or presence of NMDA. The addition of choline (40 microM) increased ACh release both basally (570 pmol/mg of protein) and with electrical stimulation (6900 pmol/mg of protein). In stimulated slices choline acted synergistically with NMDA, raising ACh release to 10,520 pmol/mg of protein. The presence of choline also blocked the fall in tissue ACh. No treatment affected tissue phospholipid or protein levels. NMDA (32-320 microM) also augmented basal ACh release from cortical but not hippocampal slices. Choline efflux from striatal and cortical (but not hippocampal) slices decreased by 34 to 50% in Mg(++)-free medium. These data indicate that NMDA-like drugs may be useful, particularly in combination with choline, to enhance striatal and cortical cholinergic activity. ACh release from rat hippocampus apparently is not affected by NMDA receptors.     

Effect of alpha-methyldopa on dopamine synthesis and release in rat striatum in vitro.

The effect of alpha-methyldopa and alpha-methyldopamine (alpha-MDA) on the rate of hydroxylation of radioactive tyrosine was studied in striatal slices from rat brain. This was done by measuring the formation of 3-H-H2O as well as the accumulation of 3-H-dopamine (3-H-DA) from L-3, 5-3-H-tyrosine. alpha-Methyldopa, at tissue concentrations similar to those found in vivo after systemic administration, produced a decrease in both 3-H-H2O and 3-H-DA. The marked decrease (91thyldopa injection, also inhibited 3-H-H2O formation. The inhibitory effect of alpha-methyldopa on 3-H-H2O formation was not reduced by the addition of brocresine, which prevents the formation of alpha-MDA. The effects of alpha-methyldopa and alpha-MDA on the release of 3-H-DA that had been taken up into brain slices, was also studied. Although alpha-methyldopa, 1000 muM, did not increase the release of 3H-DA from tissue, alpha-MDA did. However, the latter was more potent in inhibiting 3-H-H2O formation from 3-H-tyrosine than in releasing 3-H-DA. These results, as well as the close similarity between the percent reduction of 3-H-H2O formation and tissue 3-H-DA levels, suggest that alpha-methyldopa decreases tissue levels of dopamine by inhibiting tyrosine hydroxylase activity in DA neurons.     

Effects of delta and mu opiopeptides on the turnover and release of dopamine in rat striatum

Two mu and two delta opiopeptides were administered intracisternally and morphine was administered systemically to rats. The level of dopamine (DA) and its catabolites, homovanillic acid, dihydroxyphenylacetic acid and 3-methoxytramine were measured by high-pressure liquid chromatography with electrical detector in rat striatum to determine: 1) whether opioids alter the release of DA from striatal neuron (which would be indicated by changes in the level of 3-methoxytramine, the extraneuronal catabolite) and 2) whether delta or mu ligands have a greater effect on DA turnover. We found that the levels of 3-methoxytramine did not rise in response to the administration of any opiopeptide or morphine. However, mu opiopeptides produced a small but significant decrease in these levels, indicating that there was no increase, but instead a slight decrease in DA release. The delta opiopeptides produced larger increases in homovanillic acid and dihydroxyphenylacetic acid than the mu ligands, indicating that delta ligands are more effective on an equidose basis in increasing the turnover of striatal DA. The opiopeptides were also tested for pharmacological activity at the same dose (3 micrograms/rat). All four peptides were effective in reducing locomotor activity and producing analgesia. One peptide, Tyr-d-Ala-Gly-N-Mephe-Met-O-ol, also produced catalepsy. There was no segregation of these two behavioral responses according to ligand specificity. Morphine acted like a delta ligand in affecting DA turnover.     

The antitussive activity of delta-opioid receptor stimulation in guinea pigs

In this study, the activity of the delta-opioid receptor subtype-selective agonist, SB 227122, was investigated in a guinea pig model of citric acid-induced cough. Parenteral administration of selective agonists of the delta-opioid receptor (SB 227122), mu-opioid receptor (codeine and hydrocodone), and kappa-opioid receptor (BRL 52974) produced dose-related inhibition of citric acid-induced cough with ED(50) values of 7.3, 5.2, 5.1, and 5.3 mg/kg, respectively. The nonselective opioid receptor antagonist, naloxone (3 mg/kg, i.m.), attenuated the antitussive effects of codeine or SB 227122, indicating that the antitussive activity of both compounds is opioid receptor-mediated. The delta-receptor antagonist, SB 244525 (10 mg/kg, i.p.), inhibited the antitussive effect of SB 227122 (20 mg/kg, i.p.). In contrast, combined pretreatment with beta-funaltrexamine (mu-receptor antagonist; 20 mg/kg, s.c.) and norbinaltorphimine (kappa-receptor antagonist; 20 mg/kg, s.c.), at doses that inhibited the antitussive activity of mu- and kappa-receptor agonists, respectively, was without effect on the antitussive response of SB 227122 (20 mg/kg, i.p.). The sigma-receptor antagonist rimcazole (3 mg/kg, i.p.) inhibited the antitussive effect of dextromethorphan (30 mg/kg, i.p.), a sigma-receptor agonist, but not that of SB 227122. These studies provide compelling evidence that the antitussive effects of SB 227122 in this guinea pig cough model are mediated by agonist activity at the delta-opioid receptor.