Cocaine-induced increases in vesicular dopamine uptake: role of dopamine receptors

The vesicular monoamine transporter-2 is the sole transporter responsible for sequestration of monoamines, including dopamine (DA), into synaptic vesicles. Previous studies demonstrate that agents that inhibit DA transporter function, such as cocaine, increase vesicular [(3)H]DA uptake and binding of the ligand [(3)H]dihydrotetrabenazine ([(3)H]DHTBZ), as assessed in vesicles prepared from treated rats. The present studies examine the role of DA receptors in these cocaine-induced effects. Results demonstrate that administration of the D(2) DA receptor antagonist, eticlopride, but not the D(1) DA receptor antagonist, SCH23390, inhibited these cocaine-induced increases. Similar to the effects of cocaine, treatment with the D(2) agonist, quinpirole, increased both vesicular [(3)H]DA uptake and [(3)H]DHTBZ binding. In contrast, administration of the D(1) agonist, SKF81297, was without effect on vesicular [(3)H]DA uptake or [(3)H]DHTBZ binding. Finally, coadministration of quinpirole and cocaine did not further increase vesicular [(3)H]DA uptake or [(3)H]DHTBZ binding when compared with treatment with either agent alone. These data suggest that cocaine-induced increases in vesicular DA uptake and DHTBZ binding are mediated by a D(2) receptor-mediated pathway. Furthermore, results indicate that D(2) receptor activation, per se, is sufficient to increase vesicular DA uptake.     

Mitochondrial stress-induced dopamine efflux and neuronal damage by malonate involves the dopamine transporter

Endogenous striatal dopamine (DA) overflow has been associated with neuropathological conditions resulting from ischemia, psychostimulants, and metabolic inhibition. Malonate, a reversible inhibitor of succinate dehydrogenase, models the effects of energy impairment in neurodegenerative disorders. We have previously reported that the striatal DA efflux and damage to DA nerve terminals resulting from intrastriatal malonate infusions is prevented by prior DA depletion, suggesting that DA plays a role in the neuronal damage. We presently report that the malonate-induced DA efflux is partially mediated by reverse transport of DA from the cytosol to the extracellular space via the DA transporter (DAT). Pharmacological blockade of the DAT with a series of structurally different inhibitors [cocaine, mazindol, 1-(2-(bis(4-fluophenyl methoxy) ethyl)-4-(3-(4-fluorophenyl)-propyl)piperazine) dimethane sulfonate (GBR 13098) and methyl(-)-3beta-(p-fluorophenyl)-1alphaH,5alphaH-tropane-2beta-carboxylate1,5-naphthalene (Win 35,428)] attenuated malonate-induced DA overflow in vivo and protected mice against subsequent damage to DA nerve terminals. Consistent with these findings, the DAT inhibitors prevented malonate-induced damage to DA neurons in mesencephalic cultures and also protected against the loss of GABA neurons in this system. The DAT inhibitors did not modify malonate-induced formation of reactive oxygen species or lactate production, indicating that the DAT inhibitors neither exert antioxidant effects nor interfere with the actions of malonate. Taken together, these findings provide direct evidence that mitochondrial impairment and metabolic stress cause striatal DA efflux via the DAT and suggest that disruptions in DA homeostasis resulting from energy impairment may contribute to the pathogenesis of neurodegenerative diseases.     

Electrophysiological effects of cocaine in the mesoaccumbens dopamine system: repeated administration

Behavioral evidence indicates that the potent rewarding effects of cocaine are mediated, in part, by the mesoaccumbens dopamine (DA) system projecting from A10 DA cells in the ventral tegmental area (VTA) to the nucleus accumbens (NAc). Previous electrophysiological studies from our laboratory have indicated that cocaine (i.v.) exerts inhibitory effects on A10 DA neurons, due to enhanced stimulation by DA at DA autoreceptors are well as by activation of NAc-VTA feedback pathways. In the present experiments, extracellular single-unit recording and microiontophoretic techniques were used to determine the possible alterations in the mesoaccumbens DA system after repeated cocaine administration. Twice daily injections of cocaine (10 mg/kg i.p., 14 days) caused significant subsensitivity to the inhibitory effects of low i.v. doses of the DA agonist apomorphine in comparison to rats receiving similar treatments with saline or procaine. Iontophoretic application of DA to A10 DA neurons in rats treated repeatedly with cocaine (2X10 mg/kg, 14 days) also produced significantly less inhibition as compared to control rats. Cell population analysis of the VTA revealed that autoreceptor subsensitivity in cocaine-treated rats resulted in a significantly greater number of spontaneously active A10 DA neurons, and a significantly higher firing rate as compared to A10 DA neurons in control rats. In striking contrast to A10 DA cells, recordings from NAc neurons in cocaine-treated rats (2X10 mg/kg, 14 days) indicated that these cells were supersensitive to the inhibitory effects of iontophoretic DA. Although the mechanism underlying such supersensitivity remains unclear, the increased sensitivity of postsynaptic NAc DA receptors combined with the subsensitivity of A10 DA autoreceptors could lead to greatly enhanced DA transmission and may help to explain some aspects of cocaine-induced behavioral sensitization.     

Evidence for autoreceptor modulation of endogenous dopamine release from rabbit caudate nucleus in vitro

This study was designed to determine if the release of endogenous dopamine (DA), like [3H]DA, is modulated by inhibitory autoreceptors. A high-performance liquid chromatographic assay was developed which was capable of detecting the basal efflux and electrically evoked overflow of endogenous DA and dihydroxyphenylacetic acid (DOPAC), the primary DA metabolite. In the absence of neuronal uptake inhibitors the stimulation-evoked overflow of endogenous DA was entirely in the form of DOPAC, whereas overflow consisted primarily of DA in the presence of uptake inhibition. The evoked overflow of DA and DOPAC was abolished by reduction of the Ca++ concentration of the superfusion medium from 1.3 to 0.13 mM. The DA receptor antagonist sulpiride (1 microM) increased DOPAC overflow by 41%. Nomifensine (10 microM) increased slightly and cocaine (10 microM) decreased slightly the total overflow of endogenous compounds (DA plus DOPAC). Combination of nomifensine and sulpiride or cocaine and sulpiride increased total overflow of endogenous compounds by 217 and 120%, respectively, as compared to the neuronal uptake inhibitors alone. The DA receptor agonists apomorphine (0.3 microM) and bromocriptine (1 microM) inhibited DOPAC overflow by 92 and 83%, respectively. However, apomorphine and bromocriptine failed to inhibit endogenous DA release in the presence of nomifensine. Sulpiride antagonized the inhibitory effects of both apomorphine and bromocriptine. In experiments in which [3H]DA and endogenous DA overflow were measured simultaneously, radiolabeled DA behaved exactly like the endogenous transmitter.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential effects of cocaine on firing rate and pattern of dopamine neurons: role of alpha1 receptors and comparison with L-dopa and apomorphine

Psychostimulants, including cocaine, have two opposing effects on dopamine (DA) neurons: a DA-mediated inhibition and a non-DA-mediated excitation. The latter, expressed as an increase in both firing rate and a slow oscillation (SO) in firing pattern, has been shown to require forebrain inputs to DA neurons and activation of adrenergic alpha(1) receptors. However, since the effect was observed when the DA-mediated inhibition was blocked by a D2 antagonist, it is uncertain whether the underlying mechanism also plays a role in cocaine's effects in normal animals where D2-like receptors are not blocked. This study showed that under such conditions, cocaine decreased firing rate and bursting without significantly inhibiting the SO in DA neurons recorded in the ventral tegmental area. Different from cocaine, l-dopa and apomorphine, two nonpsychostimulant DA agonists known to lack the alpha(1)-mediated excitatory effect, consistently inhibited all three measures of DA cell activity. Blockade of alpha(1) receptors by prazosin did not enhance cocaine's ability to inhibit firing rate and bursting, but it did enable cocaine to inhibit the SO. These results suggest that in control rats where D2-like receptors are not blocked, alpha(1) receptors play an important role in cocaine's effect on the SO but not in its effect on firing rate and bursting of DA neurons. The maintained SO after cocaine injection may reflect continued modulation of DA neurons by forebrain inputs, regulate the pattern of DA release, and provide a temporal structure for selection of synaptic inputs to DA neurons.     

Neurochemical and behavioral responses to cocaine in adult male rats with neonatal isolation experience

Our research demonstrates that neonatal isolation (ISO; 1 h/day isolation; postnatal days 2-9) enhances extracellular, ventral striatal dopamine (DA) responses to psychostimulants in infant and juvenile rats. In adult rats, we find ISO facilitates acquisition and maintenance of cocaine self-administration. We now test whether ISO enhances cocaine-induced accumbens DA levels in adults using in vivo microdialysis. Behavioral responses to cocaine and DA antagonists were also examined. Adult male rats were derived from litters subjected to ISO or nonhandled (NH) control conditions. In experiment 1, microdialysis probes were aimed at accumbens core and separate groups administered vehicle or cocaine (5 and 10 mg/kg i.p.). Samples were analyzed for DA levels via high-performance liquid chromatography. In experiment 2, ISO and NH rats were administered one of these cocaine doses, and locomotor activity was assessed. Effects of cocaine (0.3-30 mg/kg), the D(1) antagonist SCH23390 [R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (0.003-0.03 mg/kg)], and the D(2) antagonist eticlopride (0.01-0.1 mg/kg) on disruption of responding for food were examined in experiment 3. Cocaine plasma levels were assessed in experiment 4. ISO enhanced cocaine-induced increases in accumbens DA levels. Furthermore, the D(2), but not D(1), antagonist disrupted behavior to a greater extent in ISO versus NH rats. Yet, ISO did not significantly alter behavioral responses to cocaine or cocaine plasma levels. These data show that the ability of ISO to enhance accumbens DA responses to cocaine endures into adulthood. Moreover, that ISO rats are more sensitive to a D(2) antagonist may reflect decreased levels of this receptor type as we showed previously in infant rats.     

Ethanol withdrawal reduces the number of spontaneously active ventral tegmental area dopamine neurons in conscious animals

Withdrawal from chronic ethanol treatment leads to a reduction in the electrical activity in dopamine (DA) neurons in the ventral tegmental area (VTA). However, there is disagreement on how the electrical activity is reduced (i.e., in the number of spontaneously active DA neurons or their firing rates and burst firing activity) and on the underlying mechanisms. The use of general anesthesia has been suggested to cause this discrepancy. In the present study, we demonstrate that ethanol withdrawal, in conscious animals, causes a reduction in the number of spontaneously active VTA DA neurons, but not in their firing rate or burst firing activity. Similar results were obtained in a previous study using anesthetized preparation, showing that general anesthesia does not cause this difference. Ethanol withdrawal-induced reduction in a number of spontaneously active VTA DA neurons could be mediated by depolarization inactivation because this effect was reversed by systemic administration of amphetamine, which inhibits VTA DA neurons by hyperpolarization. In addition, the withdrawal effect was normalized by acute ethanol administration, suggesting that the decrease in the number of spontaneously active VTA DA neurons represents an adaptational change to chronic ethanol treatment. Because the electrical activity of DA neurons controls the release of DA, it is possible that the decreased DA release during ethanol withdrawal observed in previous studies is caused by the reduction in the electrical activity of VTA DA neurons.     

Methylphenidate restores ventral tegmental area dopamine neuron activity in prenatal ethanol-exposed rats by augmenting dopamine neurotransmission

Altered neurotransmission in the mesolimbic dopamine (DA) system has been suggested to be the underlying cause of attention problems commonly observed in children with fetal alcohol spectrum disorder (FASD). Methylphenidate is effective in treating attention problems in children with FASD. However, the underlying mechanism is currently unknown. We have shown previously that reduced ventral tegmental area (VTA) DA neuron activity in prenatal ethanol-exposed animals can be normalized by DA agonist treatment. In the present study, we investigated the possibility that similar mechanism mediates the effect of methylphenidate using the in vivo extracellular single-unit recording technique in anesthetized animals. We observed that reduced VTA DA neuron activity in prenatal ethanol-exposed animals was normalized by methylphenidate. The effect of methylphenidate was mediated by increased extracellular levels of DA instead of norepinephrine because this effect was not altered by the coadministration of prazosin, an alpha(1) receptor antagonist, and was mimicked by the application of DA transporter blockers, nomifensine and 1-2(-[bis(4-flurophenyl)methoxy]ethyl)-4-(3-phenyl)piperazine dihydrochloride (GBR 12909). These observations support our hypothesis that depolarization inactivation is the cause of prenatal ethanol exposure-induced reduction in VTA DA neuron activity. We speculate that methylphenidate normalized the activity of VTA DA neurons by increasing extracellular DA levels in the VTA and the activation of somatodendritic DA autoreceptors. As a result, the depolarization inactivation was removed by hyperpolarization. The normalized VTA DA neuron activity in prenatal ethanol-exposed animals may contribute to a restoration of DA neurotransmission and the therapeutic effect of methylphenidate in attention problems in children with FASD.     

Antagonism of 5-hydroxytryptamine(4) receptors attenuates hyperactivity induced by cocaine: putative role for 5-hydroxytryptamine(4) receptors in the nucleus accumbens shell.

The localization of 5-hydroxytryptamine(4) (5-HT(4)) receptors suggests their role in the regulation of dopamine (DA) neurotransmission, a speculation that has been supported by neurochemical studies. Mesolimbic DA systems play a prominent role in mediating the behavioral effects of the abused psychostimulant cocaine, and the intent of the present study was to assess the role of 5-HT(4) receptors in the control of spontaneous and cocaine-induced activity. Systemic administration of the 5-HT(4) receptor partial agonist 1-(4-amino-5-chloro-2-methoxyphenyl)-3-[1-butyl-4-piperidinyl]1-propa none hydrochloride (RS 67333; 0.0001-1 mg/kg) or the 5-HT(4) receptor antagonist 4-amino-5-chloro-2-methoxy-benzoic acid-(diethylamino)ethyl ester hydrochloride (SDZ 205,557; 0.0001-1 mg/kg) did not significantly alter spontaneous activity, whereas SDZ 205,557 significantly attenuated cocaine-induced horizontal activity and rearing. To test the hypothesis that cocaine-elicited behaviors were modulated by 5-HT(4) receptors in the nucleus accumbens (NAc) shell, two separate groups of male rats were implanted with bilateral cannulas aimed at the NAc shell. Intra-NAc shell microinjections of either RS 67333 (1 or 3 microgram/0.2 microliter/side) or SDZ 205,557 (1-5 microgram/0.2 microliter/side) did not alter spontaneous activity observed after a systemic saline injection but did significantly attenuate the hyperactivity induced by systemic cocaine injection (10 mg/kg). These results support an involvement of 5-HT(4) receptors, particularly those in the NAc shell, in the locomotor stimulatory effects of cocaine. Furthermore, these data suggest that 5-HT(4) receptors may regulate behavioral processes dependent on mesolimbic DA pathways and may provide a novel target for the development of medications useful in the treatment of both drug dependence and psychiatric disorders.     

Electrophysiological evaluation of a partial agonist of dopamine receptors

The ergot derivative trans-dihydrolisuride (TDHL) was tested for its effects on firing rates of dopaminergic (DA) neurons located in the substantia nigra pars compacta of chloral hydrate-anesthetized rats. Using extracellular single-barreled microelectrodes, DA neurons were identified by their long duration, positive-negative action potentials and their slow bursting pattern of spontaneous firing, as well as by the location of recording sites through histological recoveries of dye deposits. Like haloperidol and clozapine, which are full DA receptor antagonists, TDHL antagonized the depression in DA neuron firing induced by systemic amphetamine. However, where full antagonists completely reversed the amphetamine effect, TDHL could do so only partially, the maximal effect being around half. Like DA agonists, but unlike DA antagonists, TDHL also depressed the spontaneous firing rates of DA neurons. But whereas the full agonist apomorphine completely inhibited firing of DA neurons, TDHL only depressed firing rates by about half, even at high doses. These data support the contention that TDHL is a partial DA agonist.     

Acute cocaine differentially alters accumbens and striatal dopamine clearance in low and high cocaine locomotor responders: behavioral and electrochemical recordings in freely moving rats

Behavioral responses of rodents to cocaine are characterized by marked individual variability. Here, outbred male Sprague-Dawley rats were profiled based on concomitant recording of behavioral and electrochemical responses. Rats were categorized as either low or high cocaine responders (LCRs or HCRs, respectively) based on their differential locomotor responsiveness to an acute, low-dose injection of cocaine (10 mg/kg i.p.). LCRs and HCRs also differed in other cocaine-induced behaviors. The role of the dopamine transporter (DAT) in mediating the behavioral differences in cocaine responsiveness in LCRs and HCRs was investigated by high-speed chronoamperometric recording of exogenous dopamine (DA) clearance signals in nucleus accumbens (NAc) and dorsal striatum (dSTR). Higher volumes of DA were required in NAc of HCRs, than of LCRs, to produce equivalent peak DA signal amplitude (A(max)) responses. In HCRs, systemic cocaine administration evoked an immediate and prolonged 2-fold augmentation in A(max) in both brain regions, coincident with locomotor activation. The cocaine-induced decrease in the efficiency of DA clearance (k) in NAc of HCRs was more immediate and prolonged than in dSTR, where the transient decrease coincided with maximal stereotypic behavior. In contrast, in LCRs, A(max) was not altered by cocaine, and decay rate constant (k) was transiently attenuated only in dSTR. Correlation analyses of individual responses revealed that cocaine-induced changes in DA clearance signal parameters accounted for 20 to 40% of the variation in behavioral responsiveness to cocaine. Overall, our findings emphasize the importance of characterizing individual responses to understand more fully the range of functional consequences resulting from DAT inhibition.     

Changes in sensitivity of release modulating dopamine autoreceptors after chronic treatment with haloperidol

The release of recently taken up [3H]dopamine ([3H]DA) elicited by electrical stimulation (3 Hz, 2 min, 16 mA) from slices of the rabbit caudate nucleus is inhibited by apomorphine (0.01-0.1 microM) in a concentration-dependent manner. This action is mediated through the activation of presynaptic inhibitory DA autoreceptors. The inhibition of [3H]DA release by apomorphine (0.1 microM) was antagonized 2 hr, but not 24 hr after the single administration of haloperidol (1 mg/kg s.c.). After 2 days of withdrawal after 28 days of chronic treatment with haloperidol (1 mg/kg s.c.) once daily, apomorphine (0.01-0.1 microM) was more effective in inhibiting [3H]DA release elicited by electrical stimulation when compared with rabbits injected chronically with either the vehicle for haloperidol or with saline. In superfused slices of the rabbit caudate nucleus, exposure to S-sulpiride (0.1 and 1 microM) increased in a concentration-dependent manner the release of [3H] DA elicited by electrical stimulation. After 28 days of chronic treatment with haloperidol, the facilitation of [3H]DA release by S-sulpiride was significantly reduced when compared with the controls. The inhibition of central noradrenergic transmission by DA receptor agonists was studied in hypothalamic slices prelabeled with [3H]norepinephrine ([3H-NE]). Apomorphine (0.01-1 microM) inhibited the electrically evoked (5 Hz, 2 min, 26 mA) release of [3H]NE from hypothalamic slices of untreated rabbits. The sensitivity to the inhibitory effect of apomorphine on [3H]NE overflow remained unaffected after 2 days of withdrawal following 28 days of chronic treatment with haloperidol. In summary, our results indicate that chronic haloperidol administration induces changes in sensitivity of the DA autoreceptors regulating dopaminergic neurotransmission but does not affect the sensitivity of DA receptors modulating NE release in the central nervous system. These results suggest that the DA autoreceptors that regulate dopaminergic neurotransmission may play a physiological role in the modulation of transmitter release and consequently are susceptible to the development of changes in sensitivity after chronic receptor blockade. The possible implication of changes in sensitivity of the DA autoreceptor during the treatment of schizophrenia with neuroleptics is discussed.     

Persistence of neurochemical changes in dopamine systems after repeated cocaine administration

The purpose of this study was to test whether persistent changes consistent with behavioral sensitization occur in dopamine (DA) uptake, release or receptors following repeated cocaine administration. Our neurochemical experiments focused primarily on the striatum; however, quantitative autoradiography was used to measure D-1 and D-2 DA receptors in both cell body and terminal regions of the nigrostriatal and mesolimbic dopaminergic pathways. After receiving eight once-daily injections of cocaine (10 mg/kg, i.p.), rats remained behaviorally sensitized for 1 week. This repeated treatment with cocaine induced two changes consistent with increased dopaminergic transmission. Postsynaptic D-2 DA receptors were selectively increased in nucleus accumbens one day after termination of the repeated cocaine administration; however, these receptors returned to control levels one week after cocaine administration had been terminated. In contrast, amphetamine-stimulated [3H] DA release from striatal slices was increased in rats receiving repeated cocaine injections, but this increase was not apparent until 1 week after the drug administration had been terminated. While neither of these two changes is sufficient to explain cocaine-induced behavioral sensitization, both are consistent with increased dopaminergic responsiveness and may contribute to sensitization.     

Antagonism of 5-hydroxytryptamine(2a) receptors attenuates the behavioral effects of cocaine in rats

Serotonin (5-hydroxytryptamine; 5-HT) 5-HT(2A) receptors have been shown to modulate dopamine (DA) function and a more thorough appreciation of this modulatory interaction between 5-HT2A receptors and DA systems may yield insight into novel approaches to treatment of cocaine dependence. The present study examined the effects of two ligands with varying selectivity for 5-HT2A receptors on the locomotor stimulant and discriminative stimulus effects of cocaine in male rats. Locomotor activity was measured following intraperitoneal injection of vehicle (1 ml/kg), the selective 5-HT2A receptor antagonist M100907 [R-(+)-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol] (0.02-2.0 mg/kg), or the 5-HT(2) receptor antagonist ketanserin (0.04-4 mg/kg) 45 min before administration of saline (1 ml/kg) or cocaine (10 mg/kg); monitoring of activity in photobeam chambers began at once and proceeded for 1 h. Neither M100907 nor ketanserin significantly altered basal locomotor activity, but both drugs attenuated cocaine-induced hyperactivity (p < 0.05). In drug discrimination studies, rats were trained to discriminate cocaine (10 mg/kg) from saline (1 ml/kg) in a two-lever, water-reinforced operant task. M100907 (0.05-1.6 mg/kg) and ketanserin (0.05-4 mg/kg) evoked a dose-related attenuation of the stimulus effects of cocaine (5 mg/kg, p < 0.05). These results suggest that 5-HT2A receptors play an important role in the behavioral effects of cocaine and that 5-HT2A receptors should be considered a viable target for analysis in the search for pharmacotherapies useful in the treatment of cocaine dependence.     

Stimulation of serotonin2C receptors blocks the hyperactivation of midbrain dopamine neurons induced by nicotine administration

In vivo electrophysiological techniques were used to study the effect of nicotine on the basal activity of dopamine (DA)-containing neurons in the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) of chloral hydrate-anesthetized rats. Acute i.v. injections of nicotine (25-400 microg/kg) caused a dose-dependent increase of the firing rate and the bursting activity of DA neurons both in the SNc and the VTA. Repeated daily injection of nicotine (1 mg/kg i.p.) for 10 consecutive days did not cause any significant change in the basal activity of DA neurons in the SNc and the VTA. Acute challenge with nicotine (25-400 microg/kg i.v.) in animals treated repeatedly with this drug caused a dose-related excitation of DA neurons in both areas. To test the hypothesis that stimulation of 5-hydroxytryptamine (5-HT, serotonin)(2C) receptors could affect nicotine-induced stimulation of DA neuronal activity, the selective 5-HT(2C) receptor agonist RO 60-0175 was used. Pretreatment with 100 microg/kg i.v. (S)-2-(chloro-5-fluoro-indo-l-yl)-l-methylethylamine 1:1 C(4)H(4)O(4) (RO 60-0175) prevented the enhancement in DA neuronal firing rate elicited by acute nicotine (25-400 microg/kg i.v.) in the SNc of both drug naive and chronically treated rats but was devoid of any significant effect in the VTA. Moreover, the dose of 300 microg/kg i.v. RO 60-0175 significantly reduced the stimulatory effect of VTA DA neurons induced by acute challenge with nicotine (25-400 microg/kg i.v.) both in drug naive and chronically treated rats. It is concluded that selective activation of 5-HT(2C) receptors can block the stimulatory action of nicotine on midbrain DA neuronal activity.     

Relative selectivity of 6,7-dihydroxy-2-dimethylaminotetralin, N-n-propyl-3-(3-hydroxyphenyl)piperidine, N-n-propylnorapomorphine and pergolide as agonists at striatal dopamine autoreceptors and postsynaptic dopamine receptors

6,7-Dihydroxy-2-dimethylaminotetralin (TL-99), N-n-propyl-3-(3-hydroxyphenylpiperidine [(+/-)-3-PPP], N-n-propylnorapomorphine and pergolide were evaluated for activity on a number of biochemical parameters that are presumed to indicate an agonist effect at dopamine (DA) autoreceptors (antagonism of the gamma-hydroxybutyrate-induced increase in dopa formation), at postsynaptic DA receptors (elevation of acetylcholine levels) or at both types of DA receptors (diminution of DA synthesis and homovanillic acid levels) in rat striatum. All four agents decreased striatal dopa accumulation (in the presence and in the absence of gamma-hydroxybutyrate). N-propylnorapomorphine, pergolide and TL-99 also reduced homovanillic acid levels and increased acetylcholine concentrations in striatum whereas (+/-)-3-PPP was inactive. The compounds were all more potent in diminishing dopa accumulation caused by gamma-hydroxybutyrate treatment than in increasing acetylcholine levels [(+/-)-3-PPP showing the highest dissociation] indicating a preferential agonist activity at DA autoreceptors. The relative selectivity of the compounds for DA autoreceptors and postsynaptic DA receptors was evaluated further by studying the antagonism by these drugs of the activation of striatal dopa formation (index of both DA autoreceptor and postsynaptic DA receptor stimulation) and tyrosine hydroxylase (index of postsynaptic DA receptor stimulation only) induced by haloperidol or reserpine. The DA agonists were all more potent in antagonizing the neuroleptic-induced increase in DA synthesis than in counteracting the drug-induced activation of tyrosine hydroxylase, with (+/-)-3PPP exhibiting the highest dissociation. The present results indicate that the DA agonists studied possess some selectivity for striatal DA autoreceptors, (+/-)-3-PPP being the most selective in this respect.     

5-HT2 receptor blockade by ICI 169,369 and other 5-HT2 antagonists modulates the effects of D-2 dopamine receptor blockade

The effects of D-2 dopamine (DA) receptor blockade were modulated by ICI 169,369, a selective 5-hydroxytryptamine (5-HT)2 receptor antagonist, and by other 5-HT2 antagonists. Specifically, it appears that blockade of 5-HT2 receptors can attenuate the effects of D-2 receptor blockade on rat striatal dopaminergic transmission. Thus, the blockade of D-2 receptors by haloperidol results in a compensatory increase in rat striatal DA metabolism, which is enhanced by ICI 169,369. By itself, ICI 169,369 did not significantly alter DA metabolism. Conversely, several compounds which possess appreciable activity at 5-HT2 sites in ex vivo binding assays, but possess little activity at D-2 sites (i.e., pirenperone, setoperone, fluperlapine and clozapine), all produced large increases in striatal DA metabolism. Therefore, these data suggest that the 5-HT2 component of these compounds, by enhancing DA metabolism, may act to attenuate the blockade of striatal D-2 receptors by these compounds. Consistent with this hypothesis, the chronic blockade of D-2 receptors by haloperidol increases the number of striatal D-2 DA receptors, and these increases are attenuated by the coadministration of ICI 169,369. Likewise, pirenperone and clozapine, at doses which acutely produced elevations in DA metabolism which were similar to those produced by haloperidol, failed to increase the number of D-2 receptors in striatum. Interestingly, 5-HT2 receptor blockade did not appear to potently modulate the effects of D-2 receptor blockade in the olfactory tubercle, a brain region which is innervated by mesolimbic DA-containing neurons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of prenatal ethanol exposure on the excitability of ventral tegmental area dopamine neurons in vitro

Prenatal ethanol exposure leads to a persistent reduction in the number of spontaneously active dopaminergic (DA) neurons (DA neuron population activity) in the ventral tegmental area (VTA) in developing and adult animals. This effect might contribute to the dysfunction of the mesolimbic/cortical DA system and attention problems in children with fetal alcohol spectrum disorders. To characterize the underlying cellular mechanism for prenatal ethanol exposure-induced reduction in VTA DA neuron population activity, we used the whole-cell patch-clamp technique to study the membrane properties of putative VTA DA neurons in brain slices in 2- to 3-week-old control and prenatal ethanol-exposed animals. The results show that prenatal ethanol exposure did not impair the spontaneous pacemaker activity in putative VTA DA neurons but reduced the frequency of evoked action potentials. In addition, prenatal ethanol exposure led to a reduction in hyperpolarization-induced cation current (I(h)) and an up-regulation of somatodendritic DA autoreceptors. The above prenatal ethanol exposure-induced changes could decrease the excitability of VTA DA neurons. However, they do not seem to play a role in reduced VTA DA neuron population activity in vivo, an effect thought to be mediated by excessive excitation leading to depolarization inactivation. Taken together, the above results indicate that prenatal ethanol exposure-induced reduction in VTA DA neuron population activity in vivo is not caused by changes in the intrinsic pacemaker activity or other membrane properties and could instead be caused by altered inputs to VTA DA neurons.     

Individual differences in cocaine-induced locomotor sensitization in low and high cocaine locomotor-responding rats are associated with differential inhibition of dopamine clearance in nucleus accumbens

Behavioral sensitization to cocaine reflects neuroadaptive changes that intensify drug effects. However, repeated cocaine administration does not induce behavioral sensitization in all male Sprague-Dawley rats. Because cocaine inhibits the dopamine (DA) transporter (DAT), we investigated whether altered DAT function contributes to these individual differences. Freely moving rats had electrochemical microelectrode/microcannulae assemblies chronically implanted in the nucleus accumbens so that exogenous DA clearance signals were recorded simultaneous with behavior. The peak DA signal amplitude (A(max)) and efficiency of clearance (k) were used as indices of in vivo DAT function. Low and high cocaine responders (LCRs and HCRs, respectively) were identified based on their locomotor responsiveness to an initial injection of cocaine (10 mg/kg i.p.). Consistent with DAT inhibition, cocaine elevated A(max) and reduced k in HCRs, but not in LCRs. The same dose of cocaine was administered for six additional days and after a 7-day withdrawal. Baseline behavioral and dopamine clearance indices were unaltered by repeated cocaine or after withdrawal. Only LCRs expressed cocaine-induced sensitized locomotor activation, and this was accompanied by cocaine-induced elevations in A(max) and reductions in k. These sensitized responses to cocaine persisted in LCRs after withdrawal. In contrast, neither locomotor nor electrochemical responses were altered by repeated saline administration or a saline challenge after repeated cocaine administration, suggesting that conditioning did not significantly contribute. Our results suggest that increased DAT inhibition by cocaine is associated with locomotor sensitization and that DAT serves as a common substrate for mediating both the initial and sensitized locomotor responsiveness to cocaine.     

Cocaine potentiates ethanol-induced excitation of dopaminergic reward neurons in the ventral tegmental area

The coabuse of cocaine and ethanol is one of the most frequently used substance abuse combinations in the United States. The dopamine (DA) neurons in the ventral tegmental area (VTA) are important in the rewarding mechanism of these two substances. Cocaine is known to block the reuptake of DA and serotonin (5-HT). At concentrations below 1 microM, cocaine preferentially blocks the reuptake of 5-HT compared with DA. We have previously shown that ethanol increases the firing rate of DA neurons in the VTA, and that this excitation is enhanced by 5-HT. Extracellular single-unit recordings were made from VTA dopaminergic neurons in coronal brain slices from young adult Fischer 344 rats. Cocaine (1-10 microM) reduced the spontaneous firing rate in VTA dopaminergic neurons in a concentration-related manner. A lower concentration of cocaine (500 nM), which is a concentration that is pharmacologically relevant in addicts, produced only a very small decrease in the firing rate of VTA neurons but potentiated ethanol excitation of these neurons. Higher concentrations of cocaine (1 microM) did not enhance ethanol excitation. Ethanol-induced excitation was potentiated by the higher concentrations of cocaine (1 and 2 microM) in the presence of the D(2) receptor antagonist sulpiride (1 microM). Furthermore, cocaine potentiation of ethanol-induced excitation was reversed by ketanserin (2 microM), a 5-HT(2) antagonist. The enhanced ethanol excitation of VTA dopaminergic neurons caused by cocaine may partially explain the high incidence of the coabuse of these two substances.