The effect of opioid drugs on the release of dopamine and 5-hydroxytryptamine from rat striatum following activation of nicotinic-cholinergic receptors

Effects of tryptophan depletion on acute smoking abstinence symptoms and the acute smoking response

Given the putative role of serotonin in the modulation of smoking withdrawal and the central actions of nicotine, this study examined the affective and neuroelectric correlates of smoking abstinence and cigarette smoking following depletion of the serotonin precursor, tryptophan. In a randomized, double-blind two session (tryptophan depletion [TD] vs. nondepletion), placebo-controlled design, spectrally analyzed electroencephalogram (EEG), self-ratings of withdrawal symptoms and mood states were assessed in 18 male cigarette smokers before smoking abstinence, 5 h postsmoking abstinence and again following sham smoking and the smoking of one cigarette. Compared to a nutritionally balanced amino acid (AA) mixture containing tryptophan (i.e., placebo mixture), oral ingestion of a similar mixture devoid of tryptophan resulted in a 70% reduction of plasma tryptophan but failed to alter the appearance or reversal (by acute cigarette smoking) of withdrawal symptoms, negative mood states and increased slow wave EEG in male smokers deprived of cigarettes. These results, although not supporting a role for the serotonergic system in acute smoking and early smoking abstinence symptoms, are discussed in relation to the neuropharmacology of smoking behavior and suggestions for future work.

Opioid modulation of ventral pallidal inputs

While the ventral pallidum (VP) is known to be important in relaying information between the nucleus accumbens and target structures, it has become clear that substantial information processing occurs within the VP. We evaluated the possibility that opioid modulation of other transmitters contained in VP afferents is involved in this process. Initially, we demonstrated that opioids hyperpolarized VP neurons in vitro and suppressed spontaneous firing in vivo. The ability of opioids to modulate other transmitters was determined using microiontophoretically applied ligands and extracellular recordings of VP neurons from chloral hydrate-anesthetized rats. With neurons that responded to iontophoresed opioid agonists, the ejection current was reduced to a level that was below that necessary to alter spontaneous firing. This "subthreshold" current was used to determine the ability of mu opioid receptor (microR) agonists to alter VP responses to endogenous (released by electrical activation of afferents) and exogenous (iontophoretically applied) transmitters. microR agonists decreased the variability and enhanced the acuity (e.g., "signal-to-noise" relationship) of VP responses to activation of glutamatergic inputs from the prefrontal cortex and amygdala. By contrast, microR agonists attenuated both the slow excitatory responses to substance P and GABA-induced inhibitions that resulted from activating the nucleus accumbens. Subthreshold opioids also attenuated inhibitory responses to stimulating midbrain dopaminergic cells. These results suggest that a consequence of opioid transmission in the VP is to negate the influence of some afferents (e.g., midbrain dopamine and accumbal GABA and substance P) while selectively potentiating the efficacy of others (e.g., cortical and amygdaloid glutamate). Interpreted in the context of opiate abuse, microR opioids in the VP may serve to diminish the influence of reinforcement (ventral tegmental area and nucleus accumbens) in the transduction of cognition (prefrontal cortex) and affect (amygdala) into behavior. This may contribute to drug craving that occurs even in the absence of reward.

Characterization of the decrease of extracellular striatal dopamine induced by intrastriatal morphine administration

The effect of intrastriatally-administered morphine on striatal dopamine (DA) release was studied in freely moving rats. Morphine (1, 10 or 100 microM) was given into the striatum by reversed microdialysis, and concentrations of DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were simultaneously measured from the striatal dialysates. Intrastriatally-administered morphine significantly and dose-dependently decreased the extracellular concentration of DA, the concentrations of the acidic DA metabolites were only slightly decreased. The effect of morphine was antagonized by naltrexone (2.25 mg kg(-1), s.c.). Pretreatment with a preferential kappa-opioid receptor antagonist, MR2266 [(-)-5,9 alpha-diethyl-2-(3-furylmethyl)-2'-hydroxy-6,7-benzomorphane; 1 mg kg(-1), s.c.], had no effect on the decrease of extracellular DA evoked by intrastriatal morphine (100 microM). Intrastriatal administration of the selective micro-opioid receptor agonist [D-Ala2,MePhe4,Gly-ol5] enkephalin (DAMGO; 1 microM), significantly decreased the extracellular concentration of DA in the striatum. When the rats were given morphine repeatedly in increasing doses (10-25 mg kg(-1), s.c.) twice daily for 7 days and withdrawn for 48 h, the decrease of extracellular DA induced by morphine (100 microM) was significantly less than that seen in saline-treated controls. Our results show that besides the well-known stimulatory effect there is a local inhibitory component in the action of morphine on striatal DA release in the terminal regions of nigrostriatal DA neurones. Tolerance develops to this inhibitory effect during repeated morphine treatment. Furthermore, our results suggest that the effect of intrastriatally-administered morphine is mediated by the micro-opioid receptors.

Effects of nicotine and mecamylamine on rat dorsal raphe neurons

This study investigates the hypothesis that serotonin mediates certain nicotine effects, such as mood improvement and the suppression of the ponto-geniculo-occipital spikes of rapid eye movement sleep. The influence of nicotine (10-300 microM) on the firing rate of dorsal raphe neurons and on serotonin release was therefore, studied in rat midbrain slices. Nicotine increased the firing rate, 10-90%, in 67.5% recorded neurons and decreased it, 8-100%, in the remaining 32.5%. Serotonin release increased 2-7 times after nicotine administration, regardless of firing frequency, but the absolute value of serotonin release was 3 times higher during the decreases than during the increases in firing rate. Mecamylamine (1-20 microM) transiently stimulated the dorsal raphe neurons and competitively antagonized the nicotine-induced serotonin release. The results support the working hypothesis and additionally show that mecamylamine also stimulates dorsal raphe neurons.

Effects of naloxone on amphetamine induced striatal dopamine release in vivo: a microdialysis study

The opiate antagonist naloxone (NX) alters amphetamine (AMPH) induced behaviors including locomotor activity, rearing and stereotypy. However, the exact nature of the NX induced alteration of AMPH induced behaviors is controversial, with some studies using high (5-40 mg/kg) doses of NX reporting an inhibition, and others using low (< or = 1-2 mg/kg) doses observing a potentiation. As these behaviors are mediated by AMPH induced dopamine (DA) release, the effect of NX on the latter was examined by microdialysis in an effort to resolve the controversy. Saline and NX pretreated groups subsequently administered AMPH were compared in vivo across nine separate 10 min intervals as well as by grouped intervals. NX alone (0.8 mg/kg) and saline exerted statistically equivalent effects on striatal DA release with the exception of the fifth interval, where a small but significant increase was seen after NX. On the other hand, the same dose of NX significantly enhanced AMPH induced striatal DA release relative to saline pretreated animals during each of four separate intervals, from 30 to 70 minutes following AMPH (1.5 mg/kg), and across all nine intervals combined. NX pretreatment (0.8 mg/kg) followed by a higher dose of AMPH (3.0 mg/kg) produced a significantly greater cumulative effect on DA release than saline pretreatment over the last six combined intervals (30-90 min) and over two grouped intervals (30-50 min and 40-60 min inclusive). However, a comparison of single rather than paired or grouped intervals revealed no significant differences. Previous studies have also examined the effect of NX on AMPH induced striatal DA release using in vivo microdialysis. However, the doses used were invariably high (5 mg/kg) and the results on striatal DA release always inhibitory. The present results suggest that NX potentiates AMPH induced striatal DA release when lower doses of NX are used. These results combined with those of previous studies also suggest that NX exerts a biphasic effect on AMPH induced DA release, with lower doses potentiating release and higher doses inhibiting release. This is close agreement with behavioral observations and may be due to the effect of low versus high doses of NX on intraterminal calcium influx.

Delta, mu and kappa opioid receptor agonists inhibit dopamine overflow in rat neostriatal slices

The actions of opioid receptor agonists on stimulus evoked dopamine overflow in rat neostriatal slices were investigated using fast cyclic voltammetry. Activation of delta and mu receptors reversibly depressed striatal dopamine efflux induced by intrastriatal stimulation. The inhibitory effect of DADLE (D-Ala2, D-Leu5-enkephalin, delta/mu agonist), DPDPE (D-Pen2,5-enkephalin, delta selective) and DALDA (D-Arg2, Lys4-dermorphin-(1,4)-amide, mu selective), respectively, were concentration dependent and could be blocked by application of receptor subtype selective antagonists. At a concentration of 1 microM, the kappa receptor agonist U-50488H inhibited dopamine overflow. This effect could be partially antagonized by kappa receptor selective antagonists. Prior application of virtually ineffective concentrations (< or = 0.1 microM) of the kappa agonist reduced the efficacy of 1 microM U-50488H suggesting a desensitization of the receptor. Since the stimulus induced dopamine overflow in striatal slices can be attributed solely to the release of dopamine from presynaptic terminals, these experiments demonstrate that delta, mu and kappa opioid receptors exert an inhibitory control on striatal dopamine release via a presynaptic mechanism.

Opioid modulation of amphetamine-stimulated dopamine release and concentration in rat striatal slices

The effects of morphine and naltrexone on amphetamine-stimulated release and total concentration of dopamine (DA) from rat striatal slices in vitro were examined in this study. Adult male Sprague-Dawley rats were sacrificed and the striata were dissected, sliced, and then incubated in buffer solution at 37 degrees C with amphetamine in the presence or absence of various concentrations of morphine, naltrexone (or both), or the dihydroxyphenylalanine (DOPA) decarboxylase inhibitor (NSD-1015). The concentrations of DOPA, DA, and dihydroxyphenylacetic acid (DOPAC) in the tissue slices and buffer media were measured by HPLC/EC. Amphetamine enhanced DA release and also increased total DA concentrations. However, neither morphine nor naltrexone alone altered DA concentration in the media or tissue slices relative to control (no drug added). Moreover, neither morphine nor naltrexone at 1, 10, or 100 microM altered amphetamine-stimulated DA release. However, morphine (1 or 10 microM) inhibited the amphetamine-stimulated increase in total concentration of DA. This effect of morphine was blocked by naltrexone. NSD-1015 alone or in combination with morphine did not alter amphetamine-stimulated DA release, but significantly reduced DA concentration in striatal slices. NSD-1015 alone also increased DOPA accumulation in both media and tissue slices, and this effect was inhibited by the addition of morphine. These results indicate that morphine inhibits the amphetamine-stimulated increase in total DA content, but not the amphetamine-stimulated release of DA.

Effects of systemic nicotine on serotonin release in rat brain

We used in vivo microdialysis to examine the acute effects of systemically administered nicotine (0.8-8.0 mg/kg, s.c.) on extracellular levels of serotonin (5-HT) in the frontal cortex of awake rats and animals anesthetized with chloralose/urethane. In anesthetized animals, 5-HT efflux was elevated during the initial 15 min after nicotine administration (2-8 mg/kg), but then returned to baseline values. All of the effective nicotine doses also lowered and then raised blood pressure in these animals. However, other drugs which raised (methoxamine, 0.07 mg/kg, i.v.) or lowered (mecamylamine, 5 mg/kg, i.p.) blood pressure without directly activating nicotinic receptors failed to alter 5-HT release. Moreover, pretreatment with a centrally active dose of mecamylamine, a known nicotinic antagonist, blocked the effects of nicotine (4 mg/kg) on 5-HT release. For studies on awake rats the perfusion fluid also contained fluoxetine, since basal 5-HT levels were barely detectable without this uptake blocker. In such animals, 1.6 mg/kg of nicotine significantly increased 5-HT release, an effect apparent in the initial 20 min after treatment and persisting for at least 2 h. These observations demonstrate that systemically administered nicotine increases frontocortical 5-HT release, that this effect is independent of the cardiovascular responses to the drug, and that it probably results from the activation of previously described nicotinic receptors on raphe neurons. The present findings are consistent with the hypothesis that the appetitive and mood disturbances associated with nicotine withdrawal may be mediated by diminished serotoninergic transmission.

Chronic treatment with chlorpromazine, thioridazine or haloperidol increases striatal enkephalins and their release from rat brain

The aim of this paper is to study the effect of 1, 2 or 3 months' administration of chlorpromazine (CPZ), thioridazine (TDZ) (2 or 6 mg/kg) or haloperidol (HAL) (0.25 or 1 mg/kg) IP on the level of leu- and met-enkephalin (ENK) in striatum. A dose- and time-dependent increase of striatal ENK level was observed after chronic administration of the neuroleptics (NL), but 8 days after withdrawal of chronically administered NL striatal ENK was decreased. Apomorphine pretreatment significantly attenuated the elevation in ENK produced by chronic injections of NL. In perfusion fluid obtained from the lateral ventricle of animals treated 1 month with HAL a dose-dependent increase of ENK levels was observed, which was augmented by potassium ions. It is concluded that: 1) Chronic administration of neuroleptic drugs that block dopamine receptors increases the level and the release of striatal enkephalins; 2) The results support the hypothesis that activation of dopaminergic neurons tonically inhibits the synthesis of enkephalins in the striatum.

Effects of orchidectomy on nigro-striatal dopaminergic function: behavioral and physiological evidence

Abstract In the present experiments, we examined the effect of castration upon two indices of nigro-striatal dopaminergic function in the male rat. In Experiment I, differences in spontaneous locomotor behavioral activity between intact and castrated male rats were examined. The total distance traveled, horizontal activity and mean revolutions of castrated male rats were significantly greater than that of intact males. No significant differences between intact and castrated males were obtained for vertical activity. In Experiment II, the spontaneous in vitro dopamine release from the corpus striatum of intact and castrated rats as sampled during the light-phase (1500 h) and dark-phase (2400 h) of the photoperiod was examined. At both time periods, the spontaneous in vitro dopamine release of castrated males was significantly greater than that of intact males. Both intact and castrated males showed statistically significant increases in dopamine release at the 2400 h compared to the 1500 h time period. To examine if testicular hormones were responsible for these castration induced changes in dopamine release, in Experiment III we treated castrated male rats with testosterone propionate. Administration of testosterone propionate (0.1 mg/day x 5 days) significantly reduced in vitro dopamine release compared to untreated or castrated male rats receiving vehicle treatment. These results demonstrate that testicular hormones, most likely testosterone, have a markedly suppressive effect upon the nigro-striatal dopaminergic system as evidenced from changes in spontaneous behavioral activity and in vitro dopamine release.

Chronic treatment with naltrexone enhances morphine-stimulated dopamine neurotransmission: neurochemical and behavioral evidence

Rats were implanted for 10 days with a slow-release pellet of naltrexone or were given sham surgery. At one of various different intervals during or after implantation of the pellet, the synthesis of dopamine (DA) was assessed in the nigrostriatal and mesolimbic systems. The results indicated that naltrexone alone was without effect on the synthesis of DA. However, one day after removal of the pellet, naltrexone-treated animals displayed an enhanced response to the DA-stimulatory action of morphine (15 mg/kg) in both the nigrostriatal and mesolimbic systems. This change was accompanied by an increase in specific binding of the mu-specific radioligand [3H]DAGO in whole brain and by an increase in the depressant action of morphine on locomotor activity. In contrast, at 10 days after removal of the pellet, naltrexone was without effect on morphine-induced changes in the synthesis of DA and locomotor activity, thus indicating that the supersensitivity to morphine was transient. These results support the idea that opioids modulate DAergic neurotransmission in the nigrostriatal and mesolimbic pathways and that this modulatory role may underlie opiate-induced changes in locomotor behavior.

Effects of thalamic lesion on the bilateral regulation of serotoninergic transmission in rat basal ganglia

Unilateral kainic acid lesion of the rat centromedian-parafascicular complex (CM-PF) of the thalamus induced a decrease in the 5-hydroxyindole acetic acid/5-hydroxytryptamine ratio both in ipsi and contralateral striatum and substantia nigra, and an increase in both ipsi and contralateral frontal cerebral cortex. No change in apparent serotonin turnover was detected in anterior raphe nuclei. Serotonin synthesis, estimated by measuring 5-hydroxytryptophan accumulation after injection of a decarboxylase inhibitor, was not affected by the CM-PF lesion. The possible pathways involved in the control of serotonin transmission by CM-PF are discussed.

The importance of striatal interneurons in age-related effects upon potassium- and amphetamine-stimulated dopamine release

The ability of superfused corpus striatal tissue fragments to release endogenous dopamine (DA) in response to potassium (K+ 30 mM) stimulation was significantly attenuated in 18- to 24-month- compared to 2- to 4-month-old male rats. These age-related effects on K+ stimulation were completely abolished with the addition of tetrodotoxin (1 microM) to the superfusion medium. Moreover, no difference in stimulated DA release was obtained between these two age groups following amphetamine stimulation (10 microM).

Intermittent infusion of progesterone potentiates whereas continuous infusion reduces amphetamine-stimulated dopamine release from ovariectomized estrogen-primed rat striatal fragments superfused in vitro

In the present experiment we examined the effect of a direct in vitro infusion of progesterone upon spontaneous and amphetamine-stimulated in vitro dopamine (DA) release and post-superfusion DA tissue concentration of corpus striatum tissue fragments from ovariectomized and estrogen-treated female rats. An intermittent infusion of progesterone at a dose of 2 ng/ml produced a significant increase in amphetamine-stimulated DA release and post-superfusion DA tissue concentration compared to similar superfusions infused with medium alone or cholesterol (2 ng/ml). Higher (50 ng/ml) or lower (0.2 ng/ml) doses of progesterone were ineffective and a continuous infusion of progesterone at an identical total concentration to that of the intermittent 2 ng/ml dose inhibited both amphetamine-stimulated DA release and post-superfusion DA tissue concentration. With the exception of 5 alpha DHP (dihydroxyprogesterone) intermittent infusions of various metabolites, a synthetic progestin (R5020) at 2 ng/ml and estradiol at both 0.2 ng/ml and 2 ng/ml failed to modify significantly the amphetamine-stimulated DA response. However, pregnanolone, 5 alpha DHP, R5020 at 2 ng/ml and estradiol at 0.2 ng/ml increased post-superfusion DA tissue concentration to levels comparable to that of progesterone. These results demonstrate that in vitro progesterone can directly alter the amphetamine-stimulated DA release from dopaminergic terminals of corpus striatal tissue fragments. This effect appears quite specific for progesterone as well as for a specific dose and mode of infusion of this gonadal steroid. Moreover, progesterone can exert opposite effects upon the amphetamine-evoked DA release from the corpus striatum as a function of its mode of infusion suggesting a means by which one hormone can differentially alter central nervous system function.

Daily changes in in vitro spontaneous dopamine efflux from the corpus striatum of male rats

In the present experiment we examined the spontaneous in vitro dopamine (DA) efflux from superfused corpus striatum (CS) of male rats at 3-hr intervals throughout a 24-hr photoperiod (lights on 0500-1900 hr). Maximal mean efflux, and post-superfusion DA concentrations were obtained at 0600 hr. With the exception of 0600 hr, mean efflux was lower during the light compared to the dark phase. Interestingly, the direction of the efflux profiles also varied as a function of time demonstrating increasing, decreasing and relatively stable profiles over the superfusion period. These changes in overall mean efflux, post-superfusion tissue concentration and efflux rate profile direction indicate that circadian processes play a complex role upon the synthesis/release process of DA from the nigrostriatal dopaminergic system that is revealed under the dynamic conditions of in vitro superfusion of isolated CS fragments.

Further evidence for a role of delta-opiate receptors in the presynaptic regulation of newly synthesized dopamine release

The effects of the specific delta-agonist of opiate receptors, DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), the specific mu-agonist DAGO (Tyr-D-Ala-Gly-(Me)Phe-Gly-ol) and of kelatorphan (N-((2R)-3-(hydroxyaminocarbonyl-2-benzyl-1-oxopropyl)-L-alanine), a potent inhibitor of the enkephalin-degrading enzymes, on the spontaneous release of [3H]dopamine ([3H]DA) synthesized from [3H]tyrosine were examined in rat striatal slices. DTLET (10(-7) M, 10(-6) M) and kelatorphan (5 X 10(-6) M) enhanced markedly the release of newly synthesized [3H]DA, while DAGO (10(-6) M) was inactive. The stimulatory effects of DTLET (10(-7) M) and kelatorphan (5 X 10(-6) M) were prevented in the presence of naloxone (3 X 10(-6) M; 10(-4) M respectively) or ICI 154,129 (10(-5) M), a selective antagonist of delta-opiate receptors. While DTLET (10(-7) M) stimulated the 30 mM potassium-evoked release of newly synthesized [3H]DA, it did not affect the potassium-evoked release of [3H]DA previously synthesized in tissues. A higher concentration of DTLET (10(-6) M) was required in the latter case. In contrast to the release observed with striatal slices, DTLET (10(-7) M), 10(-6) M) or DAGO (10(-6) M) did not affect the spontaneous release of newly synthesized [3H]DA from nucleus accumbens slices. In addition, DTLET (10(-6) M) was without effect on the potassium-evoked release of newly synthesized [3H]DA in this structure. The present results confirmed that delta-opiate receptors are involved in the presynaptic regulation of [3H]DA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine and the action of opiates: a reevaluation of the dopamine hypothesis of schizophrenia. With special consideration of the role of endogenous opioids in the pathogenesis of schizophrenia

It is suggested that the antipsychotic efficacy of opioids in patients suffering from schizophrenia may result from an interaction of opioids with the dopaminergic system. The modulatory effect of opioids on dopaminergic functions has already been demonstrated in basic experiments: Anatomical and biochemical data reveal an interaction between opioid receptors and dopamine (DA) actions on dopaminergic nerve terminals, cell bodies, and afferent nerve endings. Endogenous enkephalin levels correlate well with the endogenous dopamine content in various brain areas. Systemic or iontophoretic administration of morphine alters the spontaneous activity of ventral tegmental dopaminergic neurons. Morphine and enkephalin effectively enhance pituitary prolactin release, whereas dopamine inhibits it. Opioid agonists effectively alter DA release, DA reuptake, and DA metabolism in the striatum and substantia nigra. In reverse, chronic neuroleptic treatment enhances the synthesis and release of pituitary beta-endorphin. Opioids affect contralateral rotation elicited by dopamine agonists in animals with unilateral lesions of the nigrostriatal pathway. Phencyclidine, a psychotropic drug that shares certain pharmacological characteristics with the putative sigma-opioid receptor ligand SKF 10,047, indirectly mimics the effects of dopamine agonists on prolactin release, release of acetylcholine, etc. It is suggested that an imbalance of opiate-DA interaction might be involved in the pathogenesis of schizophrenia. Consequently, clinical studies on the effects of opioids on psychotic symptoms should also examine opioid influence on dopaminergic functions in these patients.

The effect of morphine on monoamine release and content in guinea-pig brain slices

The effect of morphine on the efflux of (3H) monoamines as well as the endogenous monoamine contents in electrically stimulated brain slices was investigated. Only at a concentration at high as 30 microM did the drug reduce the tritium efflux and counteracted the monoamine depletion caused by prolonged electrical stimulation. This effect was antagonized by Naloxone 10 microM. Besides the good agreement between the two methods used to evaluate drug effects the discrepancy between morphine concentrations active on the neurosecretory process and those effective in the whole animal is stressed. The opioids may act in vivo either by modulating the firing rate of the monaminergic neurons or by affecting other related neuronal pools.