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B M Cox

Publications and source records attributed to B M Cox.

At least 19 recordsLinked to original sources
PubMed1992

Inhibition of dopamine uptake by cocaine and nicotine: tolerance to chronic treatments.

Chronic administration of cocaine (50 mg/kg/day for 7 days, s.c. via an osmotic minipump) produced tolerance to inhibition of [3H]dopamine uptake by cocaine in rat striatum but did not produce cross-tolerance to inhibition of [3H]dopamine uptake by nicotine. Chronic nicotinic treatment (6 mg/kg/day for 7 days, s.c. via an osmotic minipump), however, produced tolerance to the inhibition of [3H]dopamine uptake by nicotine and cross-tolerance to inhibition of uptake by cocaine in rat striatum. Nicotine did not inhibit uptake of [3H]dopamine in the nucleus accumbens of saline-treated animals. In this tissue, both cocaine and nicotine treatments produced tolerance to cocaine, as in striatum. Unlike the effects on [3H]dopamine uptake, chronic cocaine infusion did not have any effect on the ability of cocaine to inhibit [3H]serotonin uptake in either brain region.

Animals↗
PubMed1992

Inhibition of [3H]dopamine and [3H]serotonin uptake by cocaine: comparison between chopped tissue slices and synaptosomes.

Cocaine inhibits both [3H]dopamine and [3H]serotonin uptake in rat striatum and nucleus accumbens. In a chopped tissue slice preparation, the inhibition curve for [3H]dopamine uptake is biphasic, suggesting two components of uptake, whereas the curve for [3H]serotonin uptake is steep and apparently monophasic. In synaptosomal preparations, both curves are monophasic. Monensin, a sodium ionophore, inhibits uptake but does not change the shape of the cocaine inhibition curve in synaptosomes, suggesting that the biphasic inhibition curves in slices are not likely due to differential sodium gradients across the slices. In tissue slices, only the component which is more sensitive to inhibition by cocaine and related drugs is inhibitable by nicotine. This suggests that the two components of dopamine uptake in tissue slices may be differentially regulated.

Animals↗
PubMed1992

Serotonin-stimulated release of [3H]dopamine via reversal of the dopamine transporter in rat striatum and nucleus accumbens: a comparison with release elicited by potassium, N-methyl-D-aspartic acid, glutamic acid and D-amphetamine.

Release of preloaded radiolabeled dopamine ([3H]DA) elicited by several agents from terminal fields of mesolimbic and nigrostriatal projections in rats was compared. Several similarities between the two areas were observed. For example, potassium, which stimulates release both directly, through altering the potential across the membrane of the dopaminergic neuron, as well as indirectly, presumably by releasing endogenous excitatory neurotransmitters, exhibited some similarities to release stimulated by L-glutamate and N-methyl-D-aspartic acid. These included sensitivity to tetrodotoxin (TTX), Mg++ and Ca++. In contrast, release of [3H]DA stimulated by serotonin (5-HT), like that stimulated by D-amphetamine, depended upon a functional dopamine transport system and was less sensitive to TTX, Mg++ and Ca++. 5-HT-stimulated [3H]DA release in striatum (STR) and nucleus accumbens (NACC) was not modified by antagonists at 5-HT2 or 5-HT3 receptors. Differences were observed in release of [3H]DA from STR and NACC. Elevated potassium (20 mM) released about twice as much [3H]DA from NACC as it did from STR. 5-HT was also able to release more [3H]DA from NACC than from STR. Conversely, D-amphetamine released more [3H]DA from STR than from NACC. TTX increased release stimulated by potassium in STR, but decreased release stimulated by potassium in NACC. These observations suggest that receptor- and non-receptor-mediated mechanisms may contribute to regulation of [3H]DA release in mesolimbic and nigrostriatal areas of the brain. It is possible that endogenous 5-HT in STR or NACC acts as a local regulator of DA release acting via a transport-dependent mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Amphetamine↗
PubMed1991

Comparison of dopamine uptake and release in vitro in sheep and rat striatum.

Cocaine inhibits tritium-labeled dopamine ([3H]DA) uptake in rat (IC50 approximately 400 nM) and sheep (IC50 approximately 1 microM) striatum. GBR 12909, a selective DA uptake inhibitor, potently inhibits [3H]DA uptake in rat (IC50 less than 10 nM), but is less effective (only 60% of the uptake is inhibited at a concentration of 10 microM) and less potent (IC50 approximately 300 nM) in sheep. [3H]DA release from slices of rat or sheep striatum is stimulated by potassium (15-50 mM). In the presence of nomifensine (10 microM), cocaine (10 microM) had no effect on potassium-stimulated [3H]DA release in either species. [3H]DA release is increased by N-methyl-D-aspartate (NMDA) (10-1000 microM) in rat striatum but NMDA did not stimulate [3H]DA release in sheep striatum. These findings suggest that NMDA receptors either are absent from or do not regulate release of preloaded [3H]DA in sheep striatum.

Animals↗
PubMed1991

Nicotine indirectly inhibits [3H]dopamine uptake at concentrations that do not directly promote [3H]dopamine release in rat striatum.

The effects of both (-)- and (+)-nicotine isomers were examined on in vitro uptake and release of [3H]dopamine in rat striatum. Both isomers inhibited uptake of [3H]dopamine in chopped tissue at concentrations well below those necessary for promoting release of preloaded [3H]dopamine. (-)-Nicotine was more potent than (+)-nicotine both at inhibiting uptake and at promoting release. Unlike other dopamine uptake inhibitors, however, nicotine inhibited only 50% of the total uptake. In the presence of 1 nM nicotine, the residual [3H]dopamine uptake was less sensitive to inhibition by cocaine than uptake in the absence of nicotine. Nicotine did not compete against the binding of [3H]GBR 12935, a selective dopamine uptake inhibitor. The nicotinic receptor agonists carbachol and 1,1-dimethyl-4-phenylpiperazinium iodide also inhibited uptake, whereas the nicotinic antagonists chlorisondamine and mecamylamine blocked nicotine's effect. Thus, the effect of nicotine on dopamine uptake appears to be mediated by a receptor similar to the nicotinic acetylcholine receptor. These receptors do not seem to be on the terminals that are accumulating dopamine, however, since tetrodotoxin prevented the effect of nicotine on [3H]dopamine uptake and nicotine had no effect on uptake in a synaptosomal preparation.

Animals↗
PubMed1990

Daily cocaine treatment produces a persistent reduction of [3H]dopamine uptake in vitro in rat nucleus accumbens but not in striatum.

Daily administration of cocaine (15 mg/kg i.p. x 3 days) led to a decrease in the total amount of 15 nM [3H]dopamine uptake in rat nucleus accumbens with no change in uptake in the striatum when tested 24 h after the last injection. There was an increase in the Km for dopamine uptake in the nucleus accumbens of cocaine-treated rats, with no change in the Vmax. Furthermore, cocaine was a more potent inhibitor of [3H]dopamine uptake in vitro in the nucleus accumbens of rats treated with cocaine than in those which had received saline.

Animals↗
PubMed1990

Comparison of the effects of cocaine and other inhibitors of dopamine uptake in rat striatum, nucleus accumbens, olfactory tubercle, and medial prefrontal cortex.

It is thought that inhibition of dopamine reuptake into neurons may play a major role in the mechanisms by which cocaine produces its reinforcing effects. The striatum, while rich in dopamine terminals, is not implicated in drug reinforcement, whereas the mesolimbic dopamine pathway appears to play a primary role. It is therefore possible that the properties and drug sensitivities of the dopamine uptake systems in the nigrostriatal, mesolimbic, and mesocortical tracts differ. The effects of cocaine, GBR 12909, amfonelic acid, and methylphenidate on dopamine uptake in the striatum, nucleus accumbens, olfactory tubercle, and medial prefrontal cortex were examined. Over 80% of the dopamine uptake in each of the 4 regions was sodium-dependent and exhibited Km values of approximately 100 nM. Cocaine, GBR 12909, amfonelic acid, and methylphenidate each biphasically inhibited uptake in the striatum, nucleus accumbens and olfactory tubercle with GBR 12909 and amfonelic acid being approximately 50-fold more potent than cocaine or methylphenidate. In the medial prefrontal cortex, cocaine and GBR 12909 could inhibit only about 40% of the [3H]dopamine uptake. There are similarities in the properties and drug sensitivities of the dopamine uptake systems in brain areas which are implicated in drug reinforcement and those which are not.

Animals↗
PubMed1990

Characterization of binding of [3H]GBR 12935 (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)-piperazine) to membranes and to solubilized membrane extracts from terminal field regions of mesolimbic, mesocortical and nigrostriatal dopamine pathways.

The binding characteristics of [3H]GBR 12935 (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine), a selective dopmaine uptake inhibitor, were examined in intact membrane preparations and solubilized extracts of terminal field regions of dopamine pathways in the brain of the rats. There were many similarities in the properties of binding sites for [3H]GBR 12935 in the striatum, nucleus accumbens and olfactory tubercle. The binding of [3H]GBR 12935 was saturable and the affinity constants were not significantly different between regions of the brain. The binding of [3H]GBR 12935 was inhibited by amfonelic acid, GBR 12909, mazindol, methylphenidate and cocaine, with comparable affinities in each region of the brain and with the same order of potency in both preparations. Furthermore, the rank order of potencies for inhibiting the binding of [3H]GBR 12935 was the same as for inhibiting the uptake of [3H]dopamine in these regions of the brain. There did appear to be some degree of heterogeneity of binding sites for [3H]GBR 12935 in each of these regions of the brain, as both amfonelic acid and mazindol were best fitted by two-site models. Whether this apparent heterogeneity was due to the existence of two distinct binding sites or to two components of a single site is unclear. It did not, however, appear to be due to binding to uptake sites for norepinephrine or serotonin, as neither nisoxetine nor fluoxetine, selective inhibitors of the uptake of norepinephrine and serotonin, respectively, inhibited the binding of [3H]GBR 12935, at concentrations which inhibit the uptake of norepinephrine or serotonin.

Animals↗
PubMed1990

Distribution and characterization of VIP-related peptides in the rat spinal cord.

The possible existence in the rat spinal cord of a peptide related to VIP, VIP(22-28), has been evaluated. VIP contains paired basic aminoacid residues at which posttranslational cleavage of these peptides might occur. The lumbo-sacral region of rat spinal cord had the most VIP(22-28)-like immunoreactivity (ir-VIP(22-28]. Chromatographic analysis of spinal extracts showed that ir-VIP(22-28) consisted of two major peaks, one eluting as authentic VIP(1-28) and the other as VIP(22-28). HPLC confirmed these results, revealing the presence of intact VIP(1-28) and two or more less hydrophobic peptides, one of which corresponded to authentic VIP(22-28). The other two components found have not yet been identified. Further studies are necessary to provide information on the biological significance of VIP(22-28).

Animals↗
PubMed1989

Effects of pertussis toxin on opioid regulation of catecholamine release from rat and guinea pig brain slices.

Opioid agonists selective for mu-, delta-, and kappa-receptors are all capable of regulating the stimulated release of noradrenaline from three terminal fields (cortex, hippocampus, and cerebellum) of the noradrenergic projections from locus coeruleus in the guinea pig brain. Intracerebroventricular injections of pertussis toxin abolished the ability of a mu-selective agonist and of a delta-selective agonist to inhibit stimulated noradrenaline release, but left unaffected the concentration-related inhibition of NE release by a kappa agonist. Thus, mu- and delta-receptors have been shown to be coupled to their effector system in these noradrenergic neurons via guanyl nucleotide binding proteins (G proteins) which are sensitive to pertussis toxin, while kappa-receptors in the same neurons appear to be coupled through a different mechanism which is significantly less sensitive to pertussis toxin. In contrast to opioid receptor regulation of noradrenaline release in guinea pig hippocampus, mu-, but not delta- or kappa-agonists are capable of regulation of stimulated noradrenaline release from rat hippocampus and cortex, and kappa-, but not mu- or delta-agonists are capable of inhibiting the stimulated release of dopamine from rat striatum and cortex. Pertussis toxin injections significantly attenuated mu-agonist inhibition of noradrenaline release, but had no effect on the ability of a kappa-selective agonist to regulated dopamine release, confirming the insensitivity of the kappa-receptor-effector coupling system to pertussis toxin.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗
PubMed1989

Morphine-induced desensitization and down-regulation at mu-receptors in 7315C pituitary tumor cells.

Pituitary 7315c tumor cells maintained in culture were treated with varying concentrations of morphine from 10 nM to 300 microM, for periods of five or forty-eight hours. The ability of the mu-opioid receptor agonist, DAMGO, to inhibit forskolin-stimulated adenylyl cyclase in washed membrane preparations from the treated cells was compared with its activity in membranes from cells incubated in the absence of added morphine. In the same membrane preparations, the number and affinity of mu-opioid receptors was estimated by measurements of [3H]diprenorphine binding. After 5 hr of treatment with morphine concentrations of 100 nM or higher, a significant reduction in inhibition of adenylyl cyclase by DAMGO was observed. Little further loss of agonist activity was observed when the incubations were extended to 48 hr. After 5 hr of morphine treatment, there was no change in either the number of receptors, or their affinity for [3H]diprenorphine. However, after 48 hr of morphine treatment, greater than 25% reductions in receptor number were apparent with morphine pretreatment concentrations of 10 microM or higher. These results suggest that opioid tolerance in this system is primarily associated with a reduced ability of agonist-occupied receptor to activate the effector system. Receptor down-regulation was not necessary for loss of agonist response, although a reduction in receptor number occurred after exposure to high concentrations of morphine for periods longer than 5 hr.

Adenylyl Cyclases↗
PubMed1989

Selective opioid antagonist effects on opioid-induced inhibition of release of norepinephrine in guinea pig cortex.

Opioid agonists with selectivity for mu, delta and kappa-receptors have each been shown to inhibit the K+-stimulated release of [3H]norepinephrine (NE) from slices of guinea pig cortex maintained in vitro. In order to provide further evidence that each of these types of opioid receptor can regulate the release of NE in this tissue, experiments with receptor-type selective opioid antagonists have been conducted. In initial experiments, the selectivity of the antagonists for specific types of opioid receptors in the cortex of the guinea pig in an incubation medium of the same composition as that used for release studies was confirmed. The delta-receptor selective antagonist, ICI 174,864, prevented the inhibitory actions of the delta-selective agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE), but had little effect on the inhibitory actions of the mu-selective agonist, Tyr-D-Ala-Gly-MePhe-Gly-ol (DAMGO), or the kappa-selective agonist, U-50,488H. In contrast, the kappa-selective antagonist, nor-binaltorphimine (nor-BNI) prevented the inhibitory actions of U-50,488H, but had little effect on the inhibitory actions of DPDPE or DAMGO. The greater potency of the partially mu-selective antagonist, naloxone, in reversing the effects of DAMGO relative to those of DPDPE or U-50,488H was confirmed. These results support the conclusion that mu- delta- and kappa-opioid receptors each exert a negative regulatory effect on the stimulated release of NE in the cortex of the guinea pig.

Animals↗
PubMed1989

Selective changes in mu opioid receptor properties induced by chronic morphine exposure.

Chronic infusion of morphine to guinea pigs produced selective changes in mu agonist binding properties in cerebrocortical membrane preparations. Employing the mu-selective opioid agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO) in direct binding studies and in competition of labeled antagonist binding, we found that the major changes were a decrease in the number of sites with high affinity for agonist, a small reduction in total receptor number, and a loss in the ability of guanosine 5'-[gamma-thio]triphosphate to regulate binding. A fraction of high-affinity mu receptors appeared to retain their high affinity for agonist and their sensitivity to guanine nucleotide analogue after the induction of morphine tolerance, possibly because the morphine concentrations achieved in brain were insufficient to uncouple all mu receptors from associated guanine nucleotide-binding regulatory proteins. Some membrane preparations were treated with pertussis toxin, which has been shown to functionally uncouple mu opioid receptors from their effector systems. In these preparations, a single agonist-affinity state of the receptor was observed. The apparent dissociation constant for this affinity state in pertussis toxin-treated membranes was similar to the lower-affinity state observed in preparations from morphine-tolerant animals. In contrast to the changes observed at mu opioid binding sites, no significant changes in agonist affinity or binding density were observed for selective delta or kappa agonists, consistent with the development of selective tolerance at mu receptors.

Animals↗
PubMed1988

Multiple agonist-affinity states of opioid receptors: regulation of binding by guanyl nucleotides in guinea pig cortical, NG108-15, and 7315c cell membranes.

Multiple affinity states of opioid receptors of the mu and delta types have been identified in membranes prepared from cells which bear only one type of opioid receptor (mu receptors in 7315c cells, delta receptors in NG 108-15 cells), and in guinea pig cortical membranes where both types of receptors were present in the membrane preparations. States of mu and delta receptors which have agonist affinities too low to be identified by radiolabeled agonist have been measured indirectly by agonist competition for sites labeled by radioactive antagonist. Using analogues of guanyl nucleotides, we have examined the competition of the mu and delta agonists DAGO and DSLET against [3H]DIP or [3H]NAL binding to opioid receptors and identified several agonist affinity states. In the absence of added nucleotide, competition of DSLET for [3H]DIP binding to delta opioid receptors revealed the presence of two binding sites with differing apparent agonist affinities. Addition of GDP beta S produced a steep monophasic curve which was best fit by a one-site model. In contrast, in the presence of added GTP or GTP gamma S, two affinity states were again apparent for DSLET competition at the delta receptor. The competition curve with GTP was shifted to the right relative to that produced in the absence of added guanyl nucleotide, indicating the presence of a lower apparent affinity state than any observed under other treatment conditions. DAGO competed against [3H]DIP or [3H]NAL binding to mu receptors over a wide concentration range in the absence of added guanyl nucleotide, consistent with the occupation by this ligand of more than one agonist affinity state of the mu receptor. However, when GDP beta S was added to the incubation mixture, only a single binding site was identified. Two mu receptor affinity states were again observed in the presence of added GTP or GTP gamma S. One of these had significantly lower apparent affinity than those states detected in the absence of added nucleotide or with GDP beta S. Pertussis toxin treatment resulted in a monophasic agonist competition curve which was best fitted by a single-site model in both 7315c and NG108-15 cell membranes. Addition of 100 microM GTP did not affect the agonist Kapp or Bmax after pertussis toxin treatment, suggesting that sites labeled under these conditions were not functionally associated with a G protein. In general, the effects of guanyl nucleotides were qualitatively similar at mu and delta receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Effects of chronic morphine exposure on opioid inhibition of adenylyl cyclase in 7315c cell membranes: a useful model for the study of tolerance at mu opioid receptors.

The effects of prolonged morphine exposure on the mu opioid receptor in 7315c pituitary tumor cell membranes have been examined. Since a low concentration of naloxone reversed the inhibition of forskolin-stimulated adenylyl cyclase induced by the mu-selective agonist, Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO), and by high concentrations of [D-Pen2-D-Pen5]enkephalin (DPDPE), we suggest that these cells contain a homogeneous population of mu opioid receptors coupled to adenylyl cyclase via a guanyl nucleotide-binding protein. Studies measuring the ability of [D-Ala2-D-Leu5]enkephalin (DADLE), an opioid agonist, to inhibit adenylyl cyclase in cells that had been exposed to 100 microM morphine for varying periods of time, indicated that the agonist no longer inhibited enzyme activity after 5 hr of morphine exposure. Measurements of 3H-antagonist binding in membranes from cells exposed to morphine demonstrated a decreased receptor density after 24 hr of 100 microM morphine exposure with no change in the antagonist affinity. Computer analysis indicated a 20% decrease in the number of mu receptors labeled after 24 hr of morphine exposure and a 60% decrease after 72 hr of exposure. Computer analysis of agonist competition against 3H-antagonist binding confirmed the existence of one binding site with an affinity intermediate between the high and low apparent affinity states observed in membranes from untreated cells. Addition of 10 microM GTP gamma S did not affect the agonist affinity or receptor density in membranes from morphine-treated cells, suggesting that the receptors were uncoupled from G proteins, as observed in 7315c cell membranes that have been treated with pertussis toxin. Thus chronic morphine treatment induced a rapid loss of opioid mu receptor-mediated inhibition of adenylyl cyclase (desensitization), and a more slowly developing reduction in receptor number. The desensitization was accompanied by a loss of guanyl nucleotide regulation of agonist affinity. These findings are comparable to results reported for the delta opioid receptor and the beta-adrenergic receptor upon prolonged agonist exposure.

Adenylyl Cyclase Inhibitors↗
PubMed1988

Kappa receptor regulation of dopamine release from striatum and cortex of rats and guinea pigs.

The effects of opioid agonists with selectivity for kappa, mu and delta types of opioid receptors on the K+-stimulated release of [3H]dopamine (DA) from striatum and cortex of rat and guinea pig loaded previously with the monoamine have been studied. The kappa agonist U50488H did not affect base-line release of [3H]DA measured in 5 mM K+, but produced a dose-dependent inhibition of the release of [3H]DA stimulated by 20 mM K+ from slices of striatum in rat and guinea pig, with an IC50 of about 0.5 nM in each case. In contrast, the mu-selective agonist, Tyr-D-Ala-Gly-(Me)Phe-Gly-ol, and the delta-selective agonist, [D-Pen2-D-Pen5]enkephalin, did not inhibit stimulated release from the slice preparations at concentrations up to 1 microM. The inhibitory effects of U50488H were antagonized by naloxone, and the potent and selective kappa antagonist, nor-binaltorphimine (nor-BNI). Similar results were obtained when release of [3H]DA from rat and guinea pig cortex slices was examined. In guinea pig cortex, the maximum inhibition of DA release induced by U50488H was 80% of control-stimulated fractional release. In rat cortex and in striatum of both species the maximum release was about 40% of control fractional release. Thus, in the guinea pig, the mesocortical dopaminergic pathway appears more sensitive to the inhibitory effects of U50488H than the nigrostriatal system. The effects of the opioids on the K+ (12.5 mM)-stimulated release of [3H]DA from guinea pig striatal synaptosomes also were determined.(ABSTRACT TRUNCATED AT 250 WORDS)

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗