Simultaneous wireless and high-resolution detection of nucleus accumbens shell ethanol concentrations and free motion of rats upon voluntary ethanol intake.

Highly sensitive detection of ethanol concentrations in discrete brain regions of rats voluntarily accessing ethanol, with high temporal resolution, would represent a source of greatly desirable data in studies devoted to understanding the kinetics of the neurobiological basis of ethanol's ability to impact behavior. In the present study, we present a series of experiments aiming to validate and apply an original high-tech implantable device, consisting of the coupling, for the first time, of an amperometric biosensor for brain ethanol detection, with a sensor for detecting the microvibrations of the animal. This device allows the real-time comparison between the ethanol intake, its cerebral concentrations, and their effect on the motion when the animal is in the condition of voluntary drinking. To this end, we assessed in vitro the efficiency of three different biosensor designs loading diverse alcohol oxidase enzymes (AOx) obtained from three different AOx-donor strains: Hansenula polymorpha, Candida boidinii, and Pichia pastoris. In vitro data disclosed that the devices loading H. polymorpha and C. boidinii were similarly efficient (respectively, linear region slope [LRS]: 1.98 ± 0.07 and 1.38 ± 0.04 nA/mM) but significantly less than the P. pastoris-loaded one (LRS: 7.57 ± 0.12 nA/mM). The in vivo results indicate that this last biosensor design detected the rise of ethanol in the nucleus accumbens shell (AcbSh) after 15 minutes of voluntary 10% ethanol solution intake. At the same time, the microvibration sensor detected a significant increase in the rat's motion signal. Notably, both the biosensor and microvibration sensor described similar and parallel time-dependent U-shaped curves, thus providing a highly sensitive and time-locked high-resolution detection of the neurochemical and behavioral kinetics upon voluntary ethanol intake. The results overall indicate that such a dual telemetry unit represents a powerful device which, implanted in different brain areas, may boost further investigations on the neurobiological mechanisms that underlie ethanol-induced motor activity and reward.

Modulation of Delta(9)-THC-induced increase of cortical and hippocampal acetylcholine release by micro opioid and D(1) dopamine receptors.

The administration of Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and synthetic cannabinoids stimulates acetylcholine (ACh) release in the rat prefrontal cortex (PFCx) and hippocampus as estimated by brain microdialysis. The present study was aimed at assessing whether the ability of Delta(9)-THC to stimulate ACh release is dependent upon opioid and dopamine (DA) receptors. Administration of the micro opioid receptor antagonists naloxone and naltrexone prevented the Delta(9)-THC-induced release of ACh in the PFCx and hippocampus. Similarly, bilateral infusion in the ventral tegmental area (VTA), 24h before Delta(9)-THC, of the pseudo-irreversible micro(1) antagonist naloxonazine completely prevented the increase of ACh release by Delta(9)-THC. Pre-treatment with the D(1) receptor antagonist SCH 39,166 reduced Delta(9)-THC-induced ACh release both in the PFCx and in the hippocampus. Since Delta(9)-THC has been shown to increase DA release in the nucleus accumbens (NAc) shell via a micro(1)-opioid receptor mediated mechanism located in the VTA (Tanda, G., Pontieri, F.E., Di Chiara, G., 1997. Cannabinoid and heroin activation of mesolimbic dopamine transmission by a common micro(1) opioid receptor mechanism. Science 276, 2048-2050.), we hypothesize that Delta(9)-THC-induced stimulation of ACh release in the PFCx and hippocampus is related to stimulation of endogenous opioids release in the VTA with secondary activation of DA neurons projecting to the NAc shell.

Behavioural sensitization after repeated exposure to Delta 9-tetrahydrocannabinol and cross-sensitization with morphine.

RATIONALE: Repeated exposure to several drugs of abuse has been reported to induce behavioural sensitization. So far no evidence has been provided that such a phenomenon also applies to cannabinoids. OBJECTIVES: In this study we investigated if repeated exposure to Delta(9)-tetrahydrocannabinol (Delta(9)-THC) induces behavioural sensitization. In addition we tested the possibility of cross-sensitization between Delta(9)-THC and morphine. METHODS: Male Sprague-Dawley rats were administered for 3 days, twice daily, with increasing doses of Delta(9)-tetrahydrocannabinol (2, 4 and 8 mg/kg i.p.) or increasing doses of morphine (10, 20 and 40 mg/kg s.c.) or vehicle. After a washout of 14 days the animals were challenged with Delta(9)-THC (75 and 150 microg/kg i.v.), with a synthetic cannabinoid agonist WIN55212-2 (75 and 150 microg/kg i.v.) or with morphine (0.5 mg/kg i.v.), through a catheter inserted into the left femoral vein 24 h before, and the behaviour recorded. RESULTS: Rats previously administered with Delta(9)-THC showed a greater behavioural activation compared to controls in response to challenge with Delta(9)-THC (150 microg/kg i.v.) and to challenge with morphine (0.5 mg/kg i.v.). Similar to that observed after repeated opiates, this behavioural sensitization was characterized by stereotyped activity. Animals administered with a schedule of morphine that induces behavioural sensitization to morphine also showed a behavioural sensitization to challenge with cannabinoids (Delta(9)-HC and WIN55212-2, 75 and 150 microg/kg i.v.). The effect of the challenge with Delta(9)-THC was prevented by the administration of the CB1 antagonist SR141716A (1 mg/kg i.p.), 40 min beforehand. CONCLUSIONS: The results of the present study demonstrate that repeated exposure to Delta(9)-THC induces behavioural sensitization not only to cannabinoids but also to opiates. This cross-sensitization was symmetrical since rats behaviourally sensitized to morphine were also sensitized to cannabinoids. These observations further support the evidence of an interaction between the opioid and the cannabinoid system and might provide a neurobiological basis for a relationship between cannabis use and opiate abuse.

Role of dopamine D1 receptors in the control of striatal acetylcholine release by endogenous dopamine.

In order to determine the role of dopamine (DA) D1 receptors in the control of striatal acetylcholine (ACh) transmission, we studied the effects of SCH 39166 (D1 receptor antagonist), alone or in combination with quinpirole (D2/D3 agonist) or PD 128,907 (D3 agonist) on ACh and DA release. Quinpirole reduced DA and ACh release; PD 128,907 decreased DA but not ACh release. SCH 39166 stimulated DA and decreased ACh release. Pretreatment with quinpirole reduced or prevented (depending on the dose) the stimulation of DA release while potentiating the decrease of ACh release elicited by SCH 39166. Similarly, SCH 39166 administered following PD 128,907 did not stimulate DA release, further decreasing ACh release. These results indicate that quinpirole or PD 128,907 affect the actions of SCH 39166 on DA and ACh release in opposite manner, counteracting the increase of DA release and potentiating the reduction of ACh release. These data support the tenet that endogenous DA exerts a stimulatory input on striatal ACh neurotransmission mediated by D1 receptors.

Role of striatal acetylcholine on dopamine D1 receptor agonist-induced turning behavior in 6-hydroxydopamine lesioned rats: a microdialysis-behavioral study.

The effects of MK-801, a non-competitive N-methyl D-aspartate (NMDA) receptor antagonist, of quinpirole, a dopamine (DA) D2 receptor agonist, and of SCH 58261, an A2A adenosine antagonist, were studied on acetylcholine (ACh) release in the striatum of 6-hydroxydopamine (60HDA) lesioned rats and on turning behavior induced by the administration of the DA D1 agonist CY 208-243. Administration of CY 208-243 to 6OHDA lesioned rats induced turning behavior and dose-dependently stimulated ACh release. At the dose of 50 microg/kg, MK-801 failed to affect basal ACh, while at 100 microg/kg MK-801 reduced it; however, MK-801 (50 and 100 microg/kg) potentiated the turning behavior elicited by CY 208-243, but failed to affect the CY 208-243-induced increase of striatal ACh release. The administration of quinpirole induced low-intensity turning behavior and decreased basal ACh release; on the other hand, quinpirole potentiated the turning behavior induced by CY 208-243, but failed to affect the CY 208-243-elicited increase of ACh release. Finally, intravenous administration of SCH 58261 stimulated basal ACh release but not turning behavior; SCH 58261, however, potentiated turning behavior induced by CY 208-243, while failing to affect the D1-elicited increase of ACh release. These results indicate that potentiation of D1-dependent turning behavior by MK-801, quinpirole and SCH 58261 is not mediated by a reduced ability of D1-agonists to stimulate ACh release from the denervated striatum.

Delta9-tetrahydrocannabinol enhances cortical and hippocampal acetylcholine release in vivo: a microdialysis study.

The intravenous administration of synthetic cannabinoid agonists was recently shown to dose dependently increase acetylcholine release from the rat prefrontal cortex and hippocampus (Eur. J. Pharmacol. 401 (2000) 179]. We report here that the active ingredient of cannabis preparations, delta9-tetrahydrocannabinol, administered at 10, 37.5, 75 and 150 microg/kg, dose dependently stimulated acetylcholine release from rat prefrontal cortex and hippocampus estimated by means of in vivo brain microdialysis with vertical concentric probes. At these doses, delta9-tetrahydrocannabinol induced behavioural stimulation. The administration of the CB1 receptor antagonist, ([N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3carboxamide]HCl) SR 141716A (200 microg/kg i.p.) significantly reduced the effect of delta9-tetrahydrocannabinol (75 microg/kg i.v.) on acetylcholine release from rat prefrontal cortex and hippocampus.

Intravenous administration of ecstasy (3,4-methylendioxymethamphetamine) enhances cortical and striatal acetylcholine release in vivo.

The effect of intravenous administration of 3,4-methylendioxymethamphetamine (MDMA), in a range of doses (0.32-3.2 mg/kg) that have been shown to maintain self-administration behaviour in rats, on in vivo acetylcholine release from rat prefrontal cortex and dorsal striatum was studied by means of microdialysis with vertical concentric probes. Intravenous administration of MDMA dose-dependently increased basal acetylcholine release from the prefrontal cortex to 57+/-21%, 98+/-20%, 102+/-7% and 141+/-14% above baseline, at doses of 0.32, 0.64, 1.0 and 3.2 mg/kg, respectively. MDMA also stimulated striatal acetylcholine release at the dose of 3.2 mg/kg i.v. (the maximal increase being 32+/-3% above baseline) while at the dose of 1 mg/kg i.v., MDMA failed to affect basal acetylcholine output. Administration of MDMA also dose-dependently stimulated behaviour. The results of the present study show that MDMA affects measures of central cholinergic neurotransmission in vivo and suggest that at least some of the psychomotor stimulant actions of MDMA might be positively coupled with an increase in prefrontal cortical and striatal acetylcholine release.

Cannabinoid CB(1) receptor agonists increase rat cortical and hippocampal acetylcholine release in vivo.

Intravenous administration of the cannabinoid CB(1) receptor agonists (R-(+)-[2, 3-Dihydro-5-methyl-3[morpholinyl)methyl]-pyrrolo[1,2,3-de]-1, 4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate), WIN 55,212-2 (10, 37.5, 75 and 150 microg/kg), and ((6aR)-trans-3-(1, 1-Dimethylheptyl)-6a,7,10,10a-tetrahydro-1-hydroxy-6, 6-dimethyl-6H-dibenzo[b,d]pyran-9-methanol), HU 210 (1 and 4 microg/kg) dose-dependently increased acetylcholine release in dialysates from the prefrontal cortex and the hippocampus of freely moving rats. Administration of the cannabinoid receptor antagonist [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3 carboxamide]HCl, SR 141716A, at a dose that per se did not affect basal acetylcholine release (2. 5 microg/kg), prevented the increase of acetylcholine release by WIN 55,212-2 (150 microg/kg i.v.) or by HU 210 (4 microg/kg i.v.) in both areas. These data demonstrate that, at low i.v. doses, the synthetic cannabinoid CB(1) receptor agonists, WIN 55,212-2 and HU 210 stimulate cortical and hippocampal acetylcholine release.

Local application of SCH 39166 reversibly and dose-dependently decreases acetylcholine release in the rat striatum.

The effect of local application by reverse dialysis of the dopamine D(1) receptor antagonist (-)-trans-6,7,7a,8,9, 13b-exahydro-3-chloro-2-hydroxy-N-methyl-5H-benzo-[d]-nap hto-[2, 1b]-azepine hydrochloride (SCH 39166) on acetylcholine release was studied in awake, freely moving rats implanted with concentric microdialysis probes in the dorsal striatum. In these experiments, the reversible acetylcholine esterase inhibitor, neostigmine, was added to the perfusion solution at two different concentrations, 0.01 and 0.1 microM. SCH 39166 (1, 5 and 10 microM), in the presence of 0.01 microM neostigmine, reversibly decreased striatal acetylcholine release (1 microM SCH 39166 by 8+/-4%; 5 microM SCH 39166 by 24+/-5%; 10 microM SCH 39166 by 27+/-7%, from basal). Similarly, SCH 39166, applied in the presence of a higher neostigmine concentration (0.1 microM), decreased striatal acetylcholine release by 14+/-4% at 1 microM, by 28+/-8% at 5 microM and by 30+/-5% at 10 microM, in a dose-dependent and time-dependent manner. These results are consistent with the existence of a facilitatory tone of dopamine on striatal acetylcholine transmission mediated by dopamine D(1) receptors located on striatal cholinergic interneurons.

Dopamine D(1) receptor-mediated control of striatal acetylcholine release by endogenous dopamine.

The role of dopamine D(1) and D(2) receptors in the control of acetylcholine release in the dorsal striatum by endogenous dopamine was investigated by monitoring with microdialysis the effect of the separate or combined administration of the dopamine D(1) receptor antagonist, SCH 39166 ¿(-)-trans-6,7,7a,8,9, 13b-exahydro-3-chloro-2-hydroxy-N-methyl-5H-benzo-[d]-nap hto-[2, 1b]-azepine hydrochloride¿ (50 microg/kg subcutaneous (s.c.)), of the dopamine D(2)/D(3) receptor agonist, quinpirole (trans-(-)-4aR, 4a,5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolo-(3,4-g)-quinoline hydrochloride) (5 and 10 microg/kg s.c.), and of the D(3) receptor selective agonist, PD 128,907 [S(+)-(4aR,10bR)-3,4,4a, 10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin -9-ol hydrochloride] (50 microg/kg s.c.), on in vivo dopamine and acetylcholine release. Microdialysis was performed with a Ringer containing low concentrations (0.01 microM) of the acetylcholinesterase inhibitor, neostigmine. Quinpirole (10 microg/kg s.c.) decreased striatal dopamine and acetylcholine release. Administration of PD 128,907 (50 microg/kg) decreased dopamine but failed to affect acetylcholine release. SCH 39166 (50 microg/kg s.c.) stimulated dopamine release and reduced acetylcholine release. Pretreatment with quinpirole reduced (5 microg/kg s.c.) or completely prevented (10 microg/kg s.c.) the stimulation of dopamine release elicited by SCH 39166 (50 microg/kg s.c.); on the other hand, pretreatment with quinpirole (5 and 10 microg/kg) potentiated the reduction of striatal acetylcholine release induced by SCH 39166 (50 microg/kg s.c.). Similarly, pretreatment with PD 128,907 (50 microg/kg) which prevented the increase of dopamine release induced by SCH 39166 (50 microg/kg), potentiated the reduction of striatal acetylcholine transmission elicited by SCH 39166. Thus, pretreatment with low doses of quinpirole or PD 128,907 influences in opposite manner the effect of SCH 39166 on striatal dopamine and acetylcholine release, counteracting the increase of dopamine release and potentiating the decrease in acetylcholine release. These results provide further evidence for the existence of a tonic stimulatory input of endogenous dopamine on striatal acetylcholine transmission mediated by dopamine D(1) receptors.

A within-subjects microdialysis/behavioural study of the role of striatal acetylcholine in D1-dependent turning.

In rats lesioned with 6-hydroxydopamine (6-OHDA) the effect of the noncompetitive N-methyl D-aspartate (NMDA) receptor antagonist, MK-801, the dopamine (DA) D2 receptor agonist quinpirole and the A2A adenosine antagonist SCH 58261 was studied on acetylcholine (ACh) release in the lesioned striatum and contralateral turning behaviour stimulated by the administration of the DA D1 receptor agonist CY 208-243. Administration of CY 208-243 (75, 100 and 200 microg/kg) to 6-OHDA-lesioned rats dose-dependently stimulated ACh release and induced contralateral turning. MK-801 (50 and 100 microg/kg) reduced basal ACh release (max 22%) and did not elicit any turning. MK-801 (50 and 100 microg/kg) potentiated the contralateral turning, but failed to modify the stimulation of ACh release elicited by 100 and 200 microg/kg of CY 208-243. MK-801 (100 microg/kg) prevented the increase in striatal ACh release evoked by the lower dose of CY 208-243 (75 microg/kg) but contralateral turning was not observed. The D2 receptor agonist quinpirole (30 and 60 microg/kg) elicited low-intensity contralateral turning and decreased basal ACh release. Quinpirole potentiated the D1-mediated contralateral turning behaviour elicited by CY 208-243 (100 microg/kg), but failed to affect the increase in ACh release elicited by the D1 agonist. The adenosine A2A receptor antagonist SCH 58261 (1 microg/kg i.v.) failed per se to elicit contralateral turning behaviour. SCH 58261 potentiated the contraversive turning induced by CY 208-243 but failed to affect the increase of ACh release. The results of the present study indicate that blockade of NMDA receptors by MK-801. stimulation of DA D2 receptors by quinpirole and blockade of adenosine A2A receptors by SCH 58261 potentiate the D1-mediated contralateral turning behaviour in DA denervated rats without affecting the action of the D1 agonist on ACh release. These observations do not support the hypothesis that the potentiation of D1-dependent contralateral turning by MK-801, quinpirole or SCH 58261 is mediated by changes in D1-stimulated release of ACh in the striatum.

Drug addiction as a disorder of associative learning. Role of nucleus accumbens shell/extended amygdala dopamine.

Conventional reinforcers phasically stimulate dopamine transmission in the nucleus accumbens shell. This property undergoes one-trial habituation consistent with a role of nucleus accumbens shell dopamine in associative learning. Experimental studies with place- and taste-conditioning paradigms confirm this role. Addictive drugs share with conventional reinforcers the property of stimulating dopamine transmission in the nucleus accumbens shell. This response, however, undergoes one-trial habituation in the case of conventional reinforcers but not of drugs. Resistance to habituation allows drugs to repetitively activate dopamine transmission in the shell upon repeated self-administration. This process abnormally facilitates associative learning, leading to the attribution of excessive motivational value to discrete stimuli or contexts predictive of drug availability. Addiction is therefore the expression of the excessive control over behavior acquired by drug-related stimuli as a result of abnormal strenghtening of stimulus-drug contingencies by nondecremental drug-induced stimulation of dopamine transmission in the nucleus accumbens shell.

Pharmacology of sensory stimulation-evoked increases in frontal cortical acetylcholine release.

Recent research has demonstrated that a variety of sensory stimuli can increase acetylcholine release in the frontal cortex of rats. The aim of the present experiments was to investigate the pharmacological regulation of sensory stimulation-induced increases in the activity of basal forebrain cholinergic neurons. To this end, the effects of agonists and antagonists at a variety of neurotransmitter receptors on basal and tactile stimulation-evoked increases in frontal cortical acetylcholine release were studied using in vivo brain microdialysis. Tactile stimulation, produced by gently stroking the rat's neck with a nylon brush for 20 min, significantly increased frontal cortical acetylcholine release by more than 100% above baseline. The noradrenergic alpha2 agonist clonidine (0.1 or 0.2 mg/kg) and alpha1 antagonist prazosin (1 mg/kg) failed to affect basal cortical acetylcholine release; however, both compounds significantly reduced the increases evoked by sensory stimulation. In contrast, the alpha2 antagonist yohimbine (3 mg/kg) increased basal cortical acetylcholine release, thereby preventing meaningful investigation of its effects on tactile stimulation-evoked increases. The benzodiazepine agonist diazepam (5 mg/kg) reduced, and the GABA(A) receptor antagonist picrotoxin (2 mg/kg) increased basal cortical acetylcholine release; in addition, diazepam attenuated the increases in cortical acetylcholine release evoked by tactile stimulation. While dopaminergic D1 (SCH 23390, 0.15 mg/kg) and D2 (raclopride, 1 mg/kg) receptor antagonists did not by themselves significantly influence the increases evoked by tactile stimulation, their co-administration produced a significant reduction. The opioid receptor antagonist naltrexone (1.5 mg/kg) failed to affect either basal or tactile stimulation-evoked increases in acetylcholine overflow. Finally, the non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (MK-801; 0.025 and 0.05 mg/kg) increased basal cortical acetylcholine release. These results confirm that cortically projecting cholinergic neurons are activated by sensory stimuli, and indicate that the increases in cortical acetylcholine release produced by tactile stimulation are inhibited by stimulation of alpha2 or blockade of alpha1 noradrenergic receptors, and by enhanced GABAergic transmission. In addition, simultaneous blockade of dopamine D1 and D2 receptors appears necessary to achieve a significant reduction of sensory stimulation-evoked acetylcholine release in the frontal cortex. The results are consistent with the hypothesis that cortical acetylcholine release is a component of the neurochemistry of arousal and/or attention and indicate that this is modulated by GABAergic, noradrenergic and dopaminergic systems. In contrast, endogenous opioid actions do not appear to be involved.

Dopaminergic regulation of striatal acetylcholine release: the critical role of acetylcholinesterase inhibition.

This study examined the effects of different levels of acetylcholinesterase (AChE) inhibition on dopaminergic regulation of striatal acetylcholine (ACh) release as estimated by in vivo brain microdialysis. Systemic administration of d-amphetamine (2 or 10 mg/kg) increased the striatal output of ACh when the AChE inhibitor neostigmine (0.1 microM) was present in the perfusion fluid. In contrast, when the same experiments were conducted at 0.01 microM neostigmine, d-amphetamine failed to affect (2 mg/kg) or significantly decreased (10 mg/kg) striatal ACh output. The inhibitory action of the D2 receptor agonist quinpirole (0.2 mg/kg) was significantly greater at 0.01 microM than at 0.1 microM neostigmine. Similarly, there was a nonsignificant trend for the D2 antagonist raclopride (1 mg/kg) to stimulate ACh release to a greater extent at the low neostigmine concentration. In contrast, the stimulant effects of systemic administration of the D1 agonist A-77636 (1.46 mg/kg) on striatal ACh release were the same at the two neostigmine concentrations. These results demonstrate that the concentration of an AChE inhibitor in the perfusion solution can quantitatively and even qualitatively influence the manner in which dopaminergic agents regulate ACh overflow in the striatum. On comparing the present results with earlier reports concerning the effects of d-amphetamine on tissue concentrations of ACh, it is tentatively concluded that a low neostigmine concentration is the more physiologically relevant condition. Under such conditions, at moderate doses d-amphetamine does not appear to alter striatal ACh release, with this likely being due to the opposing actions of D1 and D2 receptors. Nevertheless, until the endogenous interstitial concentrations of striatal ACh can be measured by other methods, the physiological relevance of ACh microdialysis studies in the striatum will remain uncertain.

Nonstriatal dopamine D1 receptors regulate striatal acetylcholine release in vivo.

The role of dopamine (DA) D1 receptors in the regulation of acetylcholine (ACh) release in the striatum was studied using in vivo microdialysis in freely moving rats. Systemic administration of the full D1 DA receptor agonist A-77636 (4 micromol/kg) increased striatal ACh release by 53% above the base line and decreased DA release by 33%. Local application of A-77636 (10 and 100 microM) by reverse dialysis was without effect on either striatal ACh or DA release. Systemic administration of the D1 DA receptor antagonist SCH 23390 (0.74 micromol/kg) or SCH 39166 (1.42 micromol/kg) blocked the stimulation of striatal ACh release produced by systemic A-77636 (4 micromol/kg). Local perfusion of either SCH 23390 or SCH 39166 did not decrease basal ACh release. Furthermore, when applied locally via the dialysis probe, SCH 23390 (12 microM) or SCH 39166 (50 microM) failed to attenuate the stimulation of striatal ACh release produced by systemic A-77636. Similarly, d-amphetamine (5.42 micromol/kg)-induced increases in striatal ACh release were not modified by simultaneous local perfusion with SCH 39166 (50 microM). These findings are consistent with the hypothesis that D1 receptor activation stimulates ACh release in the striatum. However, because local application of D1 receptor agonists and antagonists fail to influence ACh release, the relevant D1 receptors are not located in the striatum. The use of unphysiological dialysis conditions (high concentrations of acetylcholinesterase (AChE) inhibitors, high Ca++ concentrations and an absence of Mg++ in the perfusion fluid) may account for some earlier suggestions that local D1 receptors regulate ACh release in the striatum.

Chronic lithium attenuates dopamine D1-receptor mediated increases in acetylcholine release in rat frontal cortex.

The effects of chronic lithium treatment on methylphenidate-, D1 dopamine receptor agonist (A-77636)-, and tactile stimulation-induced increases in frontal cortical acetylcholine release were studied in the rat using in vivo brain microdialysis. Cortical acetylcholine release in control rats was maximally stimulated by methylphenidate (1.25 and 2.5 mg/kg) to 173% and 212% above baseline, respectively. The effect of methylphenidate (2.5 mg/kg) was blocked by pretreatment with the dopamine D1 receptor antagonist SCH 23390 (0.3 mg/kg). Chronic treatment with lithium chloride (3-4 weeks) produced plasma lithium concentrations of 0.45 +/- 0.02 meq/l. Chronic lithium significantly reduced increases in cortical acetylcholine release produced by methylphenidate. Stimulation of dopamine D1 receptors with the full D1 receptor agonist A-77636 (0.73 mg/kg) increased cortical acetylcholine release. Chronic lithium significantly reduced this effect of A-77636. In contrast, lithium failed to influence the increases of cortical acetylcholine release produced by tactile stimulation. These results suggest that while lithium does not influence normal, arousal-related increase in cortical acetylcholine release, this ion selectively attenuates dopamine mediated increases and/or abnormally large increases, which in the present circumstances were pharmacologically induced. The relevance of these findings to the antimanic actions of lithium is discussed.

Conditioned and unconditioned stimuli increase frontal cortical and hippocampal acetylcholine release: effects of novelty, habituation, and fear.

Recent evidence showing that basal forebrain cholinergic neurons with projections to the frontal cortex and hippocampus are activated by behaviorally salient stimuli suggests that these neurons are involved in arousal and/or attentional processes. We sought in the present experiments to test this hypothesis by examining whether unconditioned stimuli (a tone and flashing light) that normally increase cortical nad hippocampal acetylcholine (ACh) release would fail to do so after habituation (i.e., repeated presentation with no programmed consequences). In addition, the extent to which presentation of these stimuli would continue to increase ACh release when they had previously been paired with an aversive stimulus was investigated. Three experimental groups were used: habituation, novel stimuli, and conditioned fear. Subjects in each of these groups were placed in a training apparatus for twelve 200 min sessions. While the habituation group received extensive exposure to the tone and light during the training sessions, subjects in the novel stimuli group were placed in the apparatus but were never exposed to the tone or light during these sessions. The conditioned fear group was treated identically to the habituation group, with the addition that the tone and light were paired with footshock. On completion of these training schedules, all animals were implanted with microdialysis probes in the frontal cortex and hippocampus. Two days later, they were placed in the apparatus and the tone and light were presented to all subjects during microdialysis. In the novel stimuli group, the tone and light (unconditioned stimuli) produced significant increases in frontal cortical and hippocampal ACh release. Similarly, in the conditioned fear group, presentation of the tone and light (conditioned stimuli) also significantly increased ACh release in frontal cortex and hippocampus. In contrast, in the habituation group the tone and light failed to significantly enhance ACh release in either structure. During the test session, the tone and light elicited a variety of arousal- and fear-related behaviors in the novel stimuli and conditioned fear groups. In contrast, subjects in the habituation group generally failed to respond to these stimuli. These data indicate that cortically and hippocampally projecting basal forebrain cholinergic neurons are activated by conditioned and unconditioned stimuli that produce arousal in rats (novelty or conditioned fear). In contrast, presentation of these stimuli to habituated animals fails to enhance ACh release. These findings are consistent with a growing body of information indicating that ACh release in the cortex and hippocampus is reliably activated by behaviorally relevant stimuli. They also provide strong support for the hypothesis that cholinergic neurons in the basal forebrain are involved in arousal and/or attentional processes.

Ethanol as a neurochemical surrogate of conventional reinforcers: the dopamine-opioid link.

Various lines of evidence support the view that ethanol is a neurochemical surrogate of conventional reinforcers, such as food and sex. In fact, ethanol activates central neuronal systems that utilize dopamine, opioids, and gamma-aminobutyric acid (GABA) as neurotransmitters and also are activated by conventional reinforcers. These neurotransmitter systems are likely to mediate specific aspects of ethanol's reinforcing properties. Activation of the mesolimbic dopamine and endogenous opioid systems might be the substrate of the incentive and rewarding (ergotropic) properties of ethanol (arousal, euphoria, motor stimulation) and of the process of acquiring ethanol-related secondary reinforcers (incentive learning) and ethanol self-administration habits. Stimulation of the endogenous GABAergic system might mediate the sedative and drive-reducing (trophotropic) properties of ethanol. The dopamine and opioid systems are largely interconnected. Thus, pharmacological blockade of the endogenous opioid system by mu- or delta-opioid receptor antagonists prevents ethanol's activation of the dopamine system and reduces ethanol consumption. This interaction might contribute to naltrexone's effectiveness in reducing alcohol craving in humans.

D1 receptor blockade stereospecifically impairs the acquisition of drug-conditioned place preference and place aversion.

The motivational effects of dopamine (DA) D1 receptor blockade and its influence on the motivational effects of amphetamine (1.0mg/kg s.c.), morphine (1.0mg/kg s.c.) and lithium (40mg/kg s.c.) were studied in a place-conditioning paradigm. Drugs tested were two potent D1 receptor antagonists, SCH 23390 and SCH 39166, that differ in the poor affinity of the latter for 5-HT(2) receptors, and SCH 23388, the inactive enantiomer of SCH 23390. SCH 23390 and SCH 39166, at low doses (12.5 and 25µg/kg s.c.), paired for 30min with one compartment, elicited place aversion. Higher doses of the D1 antagonists or pairing for 60min with one compartment failed to elicit place aversion. SCH 39166 (50µg/kg s.c.) paired with both compartments completely prevented the place-aversion elicited by SCH 23390 (12.5µg/kg s.c.). SCH 23390 and SCH 39166 at low doses (12.5 and 25µg/kg s.c. respectively), paired with both compartments, abolished amphetamine-induced place preference. The D1 antagonists also impaired the acquisition of morphine-induced place preference and lithium-induced place aversion but only at higher doses (50 and 100µg/kg s.c.). These effects were stereospecific as the inactive enantiomer SCH 23388, up to a dose of 500µg/kg s.c. failed to impair the acquisition of amphetamine and morphine-induced place preference. It is concluded that DA plays a dual role in motivation: one role is that of assigning motivational valence to stimuli in relation to changes in DA transmission; another role of DA relates to the learning process involved in the acquisition of positive as well as negative incentive properties by otherwise neutral stimuli (incentive learning).