Regulation of locomotor activity by metabotropic glutamate receptors in the nucleus accumbens and ventral tegmental area

Glutamatergic innervation of the ventral tegmental area (VTA) and the nucleus accumbens (NA) regulates locomotor activity. The present study was designed to evaluate the involvement of metabotropic glutamate receptors (mGluRs) in motor activity. Agonists selective for each of the three subgroups of mGluRs were microinjected into the VTA or NA, and motor activity was monitored. The group I agonist (S)-3,5-dihydroxyphenylglycine elicited a dose-dependent elevation in motor activity after microinjection into either the VTA or NA. The effect in the NA was blocked by the mGluR1-specific antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester. The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine also elicited a short-duration motor activation after microinjection into either structure. The dose response in the VTA was biphasic, and the coadministration of the group II/III-specific antagonist (RS)-alpha-methyl-4-phosphonophenylglycine partially blocked motor activation in both the NA and VTA. Although the group III agonist L-(+)-2-amino-4-phosphonobutyric acid produced a relatively modest behavioral stimulation after microinjection into the NA, it was without effect in the VTA. These data indicate a role for mGluR subgroups in the regulation of motor activity in the VTA and NA.     

Systemic nicotine stimulates dopamine release in nucleus accumbens: re-evaluation of the role of N-methyl-D-aspartate receptors in the ventral tegmental area

Systemic nicotine stimulates dopamine (DA) release in the nucleus accumbens (NAcc), and N-methyl-D-aspartate (NMDA) receptors in the ventral tegmental area (VTA) appear to be involved. However, it is not known whether the secretion of DA elicited by nicotine depends on the tonic and/or phasic activation of NMDA receptors by glutamate (Glu). To clarify this, in vivo microdialysis was conducted in freely moving, alert rats to measure DA and Glu overflows in the NAcc and Glu in the VTA. Nicotine (0.065, 0.09, or 0.135 mg/kg delivered i.v. at 0.09 mg/kg/60 s via a jugular cannula) dose dependently stimulated NAcc DA secretion (P <.05). However, 0.065 mg/kg nicotine failed to stimulate Glu release in the VTA, whereas higher doses of nicotine (> or =0.09 mg/kg) were effective (P <.05). Administering the competitive NMDA receptor antagonists, 2-amino-5-phosphonopentanoic acid (AP-5; 1 mM) or 0.2 mM cis-4-phosphonomethyl-2-piperidine carboxylic acid (CGS 19755) through the VTA probe, abolished NAcc DA release after 0.065 mg/kg nicotine (P <.01) and reduced the response to 0.09 mg/kg nicotine. Therefore, the NAcc DA response to a relatively low dose of nicotine depends on the tonic activation of NMDA receptors in the VTA. In contrast, infusing 1 mM 2-amino-5-phosphonopentanoic acid or 1 mM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor antagonist, into the NAcc through the microdialysis probe had no effect on NAcc DA secretion in response to 0.09 mg/kg nicotine. These findings, coupled with data showing that Glu secretion in the VTA was stimulated only by higher doses of nicotine, indicate that the phasic release of VTA Glu is involved in the NAcc DA response to higher doses of nicotine (> or =0.09 mg/kg).     

Differential conditioned place preference responses to endomorphin-1 and endomorphin-2 microinjected into the posterior nucleus accumbens shell and ventral tegmental area in the rat

An unbiased conditioned place preference (CPP) paradigm was used to evaluate the reward effects of endogenous mu-opioid receptor ligands endomorphin-1 (EM-1) and endomorphin-2 (EM-2) from the mesolimbic posterior nucleus accumbens (Acb) shell and the ventral tegmental area (VTA) in CD rats. EM-1 (1.6-8.1 nmol) microinjected into posterior Acb shell produced CPP, whereas EM-2 (8.7-17.5 nmol) given into the same Acb shell produced conditioned place aversion (CPA). EM-1 (1.6-16.3 nmol) microinjected into the VTA produced CPP, whereas EM-2 (8.7 and 17.5 nmol) given into the same VTA site did not produce any effect, but at a high dose (35 nmol) produced CPP. EM-1 (3.3 nmol) or EM-2 (17.5 nmol) microinjected into the nigrostriatal substantia nigra was not significantly different from vehicle-injected groups. D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) at 94.13 pmol or 3-methoxynaltrexone at 0.64 pmol microinjected into the posterior Acb shell blocked EM-1-induced CPP and EM-2-induced CPA. At a higher dose, CTOP (941.3 pmol) and 3-methoxynaltrexone (6.4 pmol) produced CPA and CPP, respectively. Coadministration with antiserum against dynorphin A(1-17) (Dyn) (10 microg) microinjected into the posterior Acb shell blocked EM-2-induced CPA. However, it did not affect EM-1-induced CPP. It is concluded that EM-1 and EM-2 produce site-dependent CPP and CPA, respectively, by stimulation of different subtypes of mu-opioid-receptors; stimulation of one subtype of mu-opioid-receptor at the posterior Acb shell and VTA by EM-1 induces CPP, whereas stimulation of another subtype of mu-opioid receptor at the posterior Acb shell, but not the VTA, by EM-2 induces the release of Dyn to produce CPA.     

Nicotinic acetylcholine receptors in the anterior, but not posterior, ventral tegmental area mediate ethanol-induced elevation of accumbal dopamine levels

Ethanol-induced elevations of accumbal dopamine levels have been linked to the reinforcing properties of the drug. However, it has not yet been demonstrated where the primary point of action of ethanol is in the mesolimbic dopamine system, and there appear to be conflicting findings depending on methodology (electrophysiology, microdialysis, or intracranial self-administration). We have suggested that ethanol acts in the nucleus accumbens (nAc), where it activates a neuronal loop involving ventral tegmental nicotinic acetylcholine receptors (nAChRs) to elevate dopamine levels in the nAc. Application of ethanol in the nAc results in elevated dopamine levels in the same brain region, whereas administration in the anterior ventral tegmental area (VTA) fails to influence dopamine output. In the present study, we were able to repeat these findings. In addition, application of ethanol in the posterior VTA also failed to influence nAc dopamine levels. Perfusion of the nAChR antagonist mecamylamine in the anterior VTA completely blocked the elevation of accumbal dopamine levels observed after ethanol perfusion in nAc, whereas mecamylamine in the posterior VTA had no effect. To detect a possible influence on phasic dopamine release, the dopamine transporter inhibitor nomifensine was included in the accumbal perfusate. In addition, under these conditions, ethanol in the anterior or posterior VTA failed to influence dopamine release in the nAc. These results support previous suggestions of distinct functions of the anterior and posterior VTA and give further evidence for our hypothesis of a nAc-anterior VTA-nAc neuronal circuitry involved in the dopamine-activating effects of ethanol.     

Increase of neurotensin content elicited by neuroleptics in nucleus accumbens

The content of neurotensin immunoreactive material (neurotensin-IR) of nucleus accumbens increases 16 hr after a single injection of 2 mg/kg i.p. of haloperidol; this increase persists for 8 hr or longer. Repeated injections of 2 mg/kg/day of haloperidol i.p. cause a gradual and progressive increase of neurotensin-IR. From 0.32 pmol/mg of protein, it increases to 0.57 pmol/mg (1 week) to 0.62 pmol/mg (2 weeks) and reaches 0.68 pmol/mg (3 weeks). A significant increase of neurotensin-IR content of nucleus accumbens is obtained with 0.5 mg/kg i.p. daily for 2 weeks. Maximal responses are obtained with 1 mg/kg/day in 3 weeks. The striatal neurotensin-IR content of rats injected for 2 weeks with 1 mg/kg/day of haloperidol is also increased. In these rats, the neurotensin-IR content of preoptic area hypothalamus, septum and amygdala failed to increase. The increase of neurotensin-IR material of nucleus accumbens was elicited also by chloropromazine (6 mg/kg), trifluoroperazine (2 mg/kg) and pimozide (1.5 mg/kg) while promazine (10 mg/kg) and promethazine (25 mg/kg) were ineffective. The increase of neurotensin-IR content caused by haloperidol chloropromazine, trifluoroperazine and pimozide in accumbens and striatum suggests a modulation of neurotensin metabolism, synthesis or utilization directly or indirectly through dopaminergic synapses.     

GABAergic actions mediate opposite ethanol effects on dopaminergic neurons in the anterior and posterior ventral tegmental area

It is known that the posterior ventral tegmental area (p-VTA) differs from the anterior VTA (a-VTA) in that rats learn to self-administer ethanol into the p-VTA, but not into the a-VTA. Because activation of VTA dopaminergic neurons by ethanol is a cellular mechanism underlying the reinforcement of ethanol consumption, we hypothesized that ethanol may exert different effects on dopaminergic neurons in the p-VTA and a-VTA. In patch-clamp recordings in midbrain slices from young rats (postnatal days 22-32), we detected no significant difference in electrophysiological properties between p-VTA and a-VTA dopaminergic neurons. However, acute exposure to ethanol (21-86 mM) stimulated p-VTA dopaminergic neurons but suppressed a-VTA dopaminergic neurons. Conversely, ethanol (>21 mM) dose-dependently reduced the frequency of the GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) generated by inhibitory neuronal firing but not miniature inhibitory postsynaptic currents (mIPSCs) in p-VTA dopaminergic neurons. By contrast, ethanol increased the frequency and amplitude of both sIPSCs and mIPSCs in a-VTA dopaminergic neurons. All of these effects of ethanol were abolished by a GABA(A) receptor antagonist. There was a strong negative correlation between ethanol-evoked modulation of sIPSCs and neuronal firing in VTA dopaminergic neurons. These results indicate that GABAergic inputs play an important role in ethanol's actions in the VTA. The differential effects of ethanol on sIPSCs and neuronal firing in the p-VTA and a-VTA could be the basis for ethanol reinforcement via the p-VTA.     

Acupuncture stimulates the release of serotonin, but not dopamine, in the rat nucleus accumbens

Acupuncture has been introduced as one of the available therapies widely used in alternative medicine, but it has not achieved widespread acceptance with scientific evidence. Furthermore there are still many unanswered questions about the basic mechanisms of acupuncture. To investigate the neuropharmacological mechanisms of oriental acupuncture, we studied the acupuncture-induced changes of in vivo monoamine release in the rat brain. A microdialysis guide cannula was implanted into the nucleus accumbens (ACC), which plays an important role in the brain reward system. Acupuncture treatment at the unilateral or bilateral Shenshu (bladder urinary channel 23) acupoints, located on the both sides of the spinous processes on the lower back, was carried out for 60 min in freely moving rats, and the dopamine (DA) and serotonin (5-HT) contents of the microdialysates in the ACC were measured simultaneously. In rats subjected to acupuncture at bilateral Shenshu acupoints, increases of 5-HT release in the ACC were observed at 20 min of acupuncture treatment and continued until 40 min after acupuncture was ended. Acupuncture at a unilateral Shenshu acupoint increased the release of 5-HT at 20 min compared with that in the sham-control group. Five-HT release returned to the baseline level at 120 min. The effects of acupuncture at bilateral Shenshu acupoints on the release of 5-HT in the ACC were greater than that of unilateral acupuncture treatment. In contrast, DA release in the ACC was not changed following acupuncture treatment. Effective acupuncture increased and prolonged the activity of serotonergic neurons in the reward system pathway of the brain. This suggests that oriental acupuncture therapy may be effective for the treatment of emotional disorders, drug abuse and alcoholism.     

Cocaine is self-administered into the shell but not the core of the nucleus accumbens of Wistar rats

The rewarding properties of cocaine have been postulated to be regulated, in part, by the mesolimbic dopamine system. However, the possibility that the rewarding properties of cocaine are mediated by direct activation of this system has yielded contradictory findings. The intracranial self-administration technique is used to identify specific brain regions involved in the initiation of response-contingent behaviors for the delivery of a reinforcer. The present study assessed whether adult Wistar rats would self-administer cocaine directly into the nucleus accumbens shell (AcbSh) and core (AcbC). For each subregion, subjects were placed in standard two-lever operant chambers and randomly assigned to one of five groups for each site that were given either artificial cerebrospinal fluid (aCSF), or 400, 800, 1200, or 1600 pmol of cocaine/100 nl to self-administer. The data indicate that rats with placements within the AcbSh readily self-administered 800 to 1600 pmol of cocaine/100 nl and responded significantly more on the active than inactive lever. These subjects also decreased responding on the active lever when aCSF was substituted for cocaine and reinstated responding on the active lever when cocaine was reintroduced. Coinfusion of the D2-like receptor antagonist sulpiride inhibited cocaine self-infusion in the AcbSh. In contrast to the AcbSh data, rats failed to self-administer any tested dose of cocaine into the AcbC or areas ventral to the AcbSh. These findings suggest that the AcbSh is a neuroanatomical substrate for the reinforcing effects of cocaine and that activation of D2-like receptors is involved.     

The posterior ventral tegmental area mediates alcohol-seeking behavior in alcohol-preferring rats

The mesolimbic dopamine (DA) system is involved in the rewarding process of drugs of abuse and is activated during the anticipation of drug availability. However, the neurocircuitry that regulates ethanol (EtOH)-seeking has not been adequately investigated. The objectives of the present study were to determine 1) whether the posterior ventral tegmental area (p-VTA) mediates EtOH-seeking, 2) whether microinjections of EtOH into the p-VTA could stimulate EtOH-seeking, and (3) the involvement of p-VTA DA neurons in EtOH-seeking. Alcohol-preferring rats were trained to self-administer 15% EtOH and water. After 10 weeks, rats underwent extinction training, followed by 2 weeks in their home cages. During the home-cage period, rats were then bilaterally implanted with guide cannulae aimed at the p-VTA or anterior ventral tegmental area (a-VTA). EtOH-seeking was assessed by the Pavlovian spontaneous recovery model. Separate experiments examined the effects of: 1) microinjection of quinpirole into the p-VTA, 2) EtOH microinjected into the p-VTA, 3) coadministration of EtOH and quinpirole into the p-VTA, 4) microinjection of quinpirole into the a-VTA, and 5) microinjection of EtOH into the a-VTA. Quinpirole microinjected into the p-VTA reduced EtOH-seeking. Microinjections of EtOH into the p-VTA increased EtOH-seeking. Pretreatment with both quinpirole and EtOH into the p-VTA reduced EtOH-seeking. Microinjections of quinpirole or EtOH into the a-VTA did not alter EtOH-seeking. Overall, the results suggest that the p-VTA is a neuroanatomical substrate mediating alcohol-seeking behavior and that activation of local DA neurons is involved.     

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.     

Dopamine-independent locomotion following blockade of N-methyl-D-aspartate receptors in the ventral tegmental area.

Compounds acting in the ventral tegmental area to increase motor activity are thought to do so by activating mesolimbic dopamine transmission. The present report demonstrates that the microinjection of N-methyl-D-aspartate (NMDA) antagonists into the ventral tegmental area produces a dose-dependent increase in motor activity. This effect was not mimicked by antagonizing either alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate or metabotropic glutamate receptors in the ventral tegmental area. Three experiments were conducted that indicated that the capacity of NMDA receptor antagonists to elevate motor activity did not involve increased dopamine transmission. 1) The systemic administration of a D1 dopamine receptor antagonist did not inhibit the motor stimulant response to NMDA antagonist injection into the ventral tegmental area except at doses that also inhibited motor activity after an injection of saline into the ventral tegmental area. 2) Stimulating orphanin receptors in the ventral tegmental area selectively inhibits dopamine cells, and this did not alter NMDA antagonist-induced motor activity. Whereas, stimulating gamma-aminobutyric acid (GABA)(B) receptors hyperpolarizes both dopamine and GABA cells in the ventral tegmental area, and this abolished NMDA antagonist-induced motor activity. 3) The microinjection of an NMDA antagonist into the ventral tegmental area did not increase dopamine metabolism in dopamine terminal fields, including the accumbens, striatum, or prefrontal cortex. Also consistent with a lack of dopamine involvement, repeated administration of NMDA antagonist into the ventral tegmental area did not produce behavioral sensitization. These data identify a mechanism to elicit a motor stimulant response from the ventral tegmental area that does not involve activating dopamine transmission.     

Adaptive responses of gamma-aminobutyric acid neurons in the ventral tegmental area to chronic ethanol

We have recently identified a homogeneous population of gamma-aminobutyric acid (GABA)-containing neurons in the ventral tegmental area (VTA), an area implicated in the reinforcing properties of alcohol. We evaluated the effects of local and systemic ethanol on VTA GABA neuron spontaneous activity in ethanol naive and chronically treated freely behaving rats and in anesthetized rats. In freely behaving animals, acute i.p. administration of 0.2 to 2.0 g/kg ethanol reduced the firing rate of VTA GABA neurons. Chronic administration of 2.0 g/kg i.p. ethanol enhanced baseline activity of VTA GABA neurons and induced tolerance to ethanol inhibition of their firing rate. In a separate group of freely behaving animals, tolerance to 0.4 to 2.0 g/kg i.p. ethanol-induced inhibition of VTA GABA neuron firing rate was observed following 2 weeks of chronic exposure to ethanol vapors producing intermittent blood alcohol levels of 158 mg/100 ml. In acute studies in halothane-anesthetized animals, ethanol applied locally into the VTA decreased the spontaneous firing rate of VTA GABA neurons, whereas systemic ethanol produced an early inhibition followed by a late excitation at 30 to 60 min after the ethanol injection, suggesting that ethanol modulation of an extrinsic input may excite VTA GABA neurons. Tolerance to local ethanol inhibition of VTA GABA neuron firing rate was produced by 2 weeks of chronic exposure to intermittent ethanol vapors. These results demonstrate the marked sensitivity of these neurons to ethanol and suggest that chronic ethanol administration produces selective adaptive circuit responses within the VTA or in extrategmental structures that regulate VTA GABA neuron activity.     

Neurochemical and behavioral effects of corticotropin-releasing factor in the ventral tegmental area of the rat

Corticotropin-releasing factor (CRF) is thought to have a neurotransmitter function in the mammalian central nervous system. The release of CRF into the hypothalamic-hypophyseal portal system in response to stress is known, and it has been suggested that CRF may function elsewhere in the central nervous system to promote stress-induced physiological and behavioral changes. Acute stress has been shown to activate dopamine (DA) neurons in the ventral tegmental area (VTA) that project to the prefrontal cortex. To determine whether CRF may act in the VTA to activate the mesocortical or mesolimbic DA systems, it was injected into the VTA of rats, and changes in spontaneous motor behavior and DA metabolism were measured. Intra-VTA injection of CRF produced a dose-dependent increase in horizontal and vertical photocell counts with a minimal effective dose of 0.01 and 0.1 nmol, respectively. In contrast, the minimal effective dose for CRF-stimulated motor behavior after injection into the lateral ventricles was 0.3 nmol for horizontal activity. Furthermore, in the open field, the behavioral profile of CRF (0.3 nmol) given intra-VTA differed from that observed after intraventricular injection. The motor stimulant effect of intra-VTA CRF was not blocked by pretreatment with the DA receptor antagonist haloperidol. After intra-VTA injection, CRF produced a dose-dependent decrease in DA metabolism in the prefrontal cortex. The decrease in DA metabolism in the prefrontal cortex was present at 30 and 60 min but not at 120 min after injection, and in the nucleus accumbens DA metabolism was increased only at 60 min after injection.(ABSTRACT TRUNCATED AT 250 WORDS)     

毁损伏隔核、腹侧苍白球对大鼠觅药行为的影响

目的探讨伏隔核和腹侧苍白球在药物强化中的作用。方法分别毁损成瘾大鼠伏隔核、腹侧苍白球,利用条件性地点偏好实验测定术前、术后成瘾大鼠对注射吗啡的偏好分,评价伏隔核和腹侧苍白球在药物强化效应的作用。结果毁损大鼠双侧伏隔核能够完全消除大鼠对注射吗啡侧的偏好,毁损腹侧苍白球明显减少成瘾大鼠的地点偏好行为。结论伏隔核和腹侧苍白球是调节强化作用的重要位置,伏隔核－腹侧苍白球通路是药物强化的共同环路。     

Ethanol potentiation of glycine-induced responses in dissociated neurons of rat ventral tegmental area

The potentiation of glycine-induced responses by ethanol (EtOH) was studied in neurons freshly dissociated from the ventral tegmental area (VTA) of 5- to 14-day-old postnatal rats using whole-cell and gramicidin-perforated patch-clamp techniques. Under current-clamp conditions, EtOH increased glycine-induced membrane depolarization and action potential firing. Under voltage-clamp conditions, EtOH (0. 1-40 mM) alone did not elicit a current. When coapplied with glycine, EtOH enhanced the glycine-induced current in 35% (180 of 474) of the neurons. The EtOH-induced enhancement of glycine current was independent of membrane potential (between -60 and +60 mV); the reversal potential was not changed. Concentration-response analysis showed that in the presence of EtOH (10 mM), the EC(50) for glycine decreased from 25 +/- 4 to 14 +/- 3 microM; the Hill coefficient increased from 1.5 +/- 0.2 to 1.9 +/- 0.3. Kinetic analysis of glycine currents indicated that EtOH decreased the time constant of activation and increased the time constant of deactivation of glycine-gated chloride channels. EtOH may accelerate glycine association with its receptor at the agonist binding site and increase the apparent agonist affinity. Our observations suggest that, at pharmacologically relevant concentrations, EtOH alters the function of glycine receptors and thus the excitability of neonatal VTA neurons. This action of EtOH may contribute to the neurobehavioral disturbances associated with fetal alcohol syndrome.     

M5 muscarinic receptor knockout mice show reduced morphine-induced locomotion but increased locomotion after cholinergic antagonism in the ventral tegmental area

M(5) muscarinic receptors are the only muscarinic receptor subtype expressed by mesencephalic dopamine neurons and provide an important excitatory input to mesolimbic and nigrostriatal dopamine systems. Here, we studied locomotion induced by systemic morphine (3, 10, and 30 mg/kg i.p.) in M(5) knockout mice of the C57BL/6 (B6) and CD1 x 129SvJ background strains. M(5) knockout mice of both strains showed reduced locomotion in response to 30 mg/kg morphine. B6 M(5) knockout mice were less sensitive to naltrexone in either the antagonism of morphine-induced locomotion or in the reduction of locomotion by naltrexone alone. This suggests that M(5) knockout mice are less sensitive to the effects of either exogenous or endogenous opiates on locomotion and that spontaneous locomotion in B6 mice is sustained by endogenous opiates. In B6 wild-type mice, ventral tegmental area (VTA) pretreatment with the muscarinic receptor antagonist atropine (3 microg bilateral), but not the nicotinic receptor antagonist mecamylamine (5 microg bilateral), reduced locomotion in response to 30 mg/kg morphine to a similar extent as systemic M(5) knockout, suggesting that reduced morphine-induced locomotion in M(5) knockout mice is due to the loss of M(5) receptors on VTA dopamine neurons. In contrast, in M(5) knockout mice, but not in wild-type mice, either intra-VTA atropine or mecamylamine alone increased locomotion by almost 3 times relative to saline and potentiated morphine-induced locomotion. Therefore, in M(5) knockout mice, blockade of either VTA muscarinic or nicotinic receptors increased locomotion, suggesting that in the absence of VTA M(5) receptors, VTA cholinergic inputs inhibit locomotion.     

Mu opioid receptor involvement in enkephalin activation of dopamine neurons in the ventral tegmental area

Many lines of evidence suggest that opioids act in the A10 dopamine (DA) region to activate DA neurons projecting to limbic terminal areas. Thus, injection of morphine and enkephalin analogs into the ventral tegmental area (a major subnucleus of the A10 DA region) produces an increase in spontaneous motor activity that is blocked by DA receptor antagonists and increases DA metabolism in the nucleus accumbens. The present study utilized enkephalin analogs specific for either the mu or delta opioid receptor to evaluate which receptor subtype(s) is activating the A10 DA neurons. It was found that the specific mu agonist, Try-D-Ala-Gly-NMe-Phe-Gly-ol, was significantly more potent than the specific delta agonist, [D-Pen2,5]-enkephalin, at increasing spontaneous motor activity or DA metabolism in the nucleus accumbens, septum, striatum and prefrontal cortex. Further, naloxonazine, a putative antagonist of the mu-1 isoreceptor, significantly attenuated the motor-stimulant effect and increase in DA metabolism produced by intra-ventral tegmental area injection of Tyr-D-Ala-Gly-NMe-Phe-Gly-ol. It was found that the disposition of microinjected Tyr-D-Ala-Gly-NMe-Phe-Gly-ol or [D-Pen2,5]-enkephalin was not responsible for the difference in their potency. It is concluded that the mu receptor and, perhaps, the mu-1 isoreceptor mediate a major portion of the activation of A10 DA neurons previously demonstrated with mixed mu and delta opioid agonists.     

Necessary role for ventral tegmental area adenylate cyclase and protein kinase A in induction of behavioral sensitization to intraventral tegmental area amphetamine

In the present study, we investigated the effects of selective activation or inhibition of ventral tegmental area (VTA) adenylate cyclase (AC) and protein kinase A (PKA) on long-term sensitization induced by repeated intra-VTA or peripheral amphetamine (AMPH). Selective inhibition of AC by SQ 22,536 (9-(tetrahydro-2-furanyl)-9H-purin-6-amine; 100 nmol/side bilateral into VTA) had no effect on acute basal locomotion but attenuated the locomotor stimulation induced by acute i.p. AMPH (1.5 mg/kg). Coinjection of SQ 22,536 (100 nmol/side) fully blocked the sensitization induced by repeated intra-VTA AMPH (15 nmol/side) but had no detectable effect on the sensitization induced by repeated i. p. AMPH. Persistent activation of AC by intra-VTA cholera toxin (500 ng/side) modestly increased acute locomotion and induced a robust sensitization to i.p. AMPH challenge 10 days after the last of three repeated VTA microinjections. Selective inhibition of PKA by Rp-adenosine-3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPS; 25 nmol/side) had no effect on acute basal or AMPH-stimulated locomotion. Coinjection of Rp-cAMPS (25 nmol/side) fully blocked the sensitization induced by repeated intra-VTA AMPH but had no effect on sensitization induced by repeated i.p. AMPH. Intra-VTA microinjection of the selective PKA activator Sp-adenosine-3',5'-cyclic monophosphothioate triethylamine (Sp-cAMPS; 25-100 nmol/side) dose-dependently stimulated acute locomotion and exerted synergistic effects on locomotor activity when coinfused into the VTA with AMPH but had no detectable effect on acute i.p. AMPH-induced locomotion. Repeated intra-VTA Sp-cAMPS did not induce sensitization to AMPH challenge but potentiated the sensitization induced by repeated i.p. AMPH. These results suggest that VTA cAMP signal transduction is necessary for the induction of persistent sensitization to intra-VTA amphetamine and that peripheral and intra-VTA AMPH may not induce behavioral sensitization by identical mechanisms.     

Ethanol excitation of dopaminergic ventral tegmental area neurons is blocked by quinidine

The dopaminergic (DA) neurons in the ventral tegmental area (VTA) are important for the reinforcing effects of ethanol. We have shown that ethanol directly excites DA VTA neurons and reduces the afterhyperpolarization (AHP) that follows spontaneous action potentials in these neurons. These data suggested that ethanol may be increasing the firing rate of DA VTA neurons by modulating currents that contribute to the AHP, either by reducing a K+ current or by increasing the inward current Ih. In the present study, different blockers of K+ channels and Ih were tested to determine whether any could prevent the ethanol excitation of DA VTA neurons. Extracellular single-unit recordings and whole-cell patch-clamp recordings were made from DA VTA neurons in brain slices from Fischer-344 rats and ethanol (40-120 mM) and channel blockers were applied in the bath. Ethanol excitation was not reduced by blockade of Ih with cesium (5 mM) or ZD7288 (30 microM), or by block of G-protein-coupled inwardly rectifying K+ channels with barium (500 microM). Tetraethylammonium (TEA) ion (2-10 mM), which blocks the large conductance calcium-dependent potassium K+ current and some types of delayed rectifier currents, had no effect on the ethanol-induced excitation. Interestingly, ethanol excitation of DA VTA neurons was blocked by quinidine (20-80 microM), a drug that blocks many types of delayed rectifier K+ channels, including some insensitive to TEA. This effect of quinidine was concentration-dependent and reversible. These results suggest that ethanol excites DA VTA neurons by reducing a quinidine-sensitive K+ current.     

Sigma receptor "antagonist" BMY 14802 increases neurotensin concentrations in the rat nucleus accumbens and caudate

Acute and chronic treatment with antipsychotic drugs, such as haloperidol, selectively increases the concentrations of neurotensin (NT) in the nucleus accumbens and caudate of the rat. These increases in NT concentration in the nucleus accumbens and caudate have been hypothesized to underlie the therapeutic and extrapyramidal effects of antipsychotic drugs, respectively. The present study evaluates the effects of the putative antipsychotic and selective sigma receptor "antagonist" BMY 14802 on regional brain NT concentrations. NT concentrations in discrete brain regions of adult, male, Sprague-Dawley rats were measured by a sensitive and specific radioimmunoassay. Like haloperidol (1 mg/kg i.p.), acute and chronic treatment with BMY 14802 (35 mg/kg/day i.p.) produced significant increases in the concentrations of NT in the nucleus accumbens and anterior and posterior caudate. This effect was dose-dependent. Maximal increases in NT concentration were observed 18 hr after a single dose of BMY 14802. Neither acute nor chronic treatment with the sigma "agonist" (+)-SKF 10,047 (20 mg/kg i.p.), the N-methyl-D-aspartate-phencyclidine binding site antagonist MK-801 (0.25 mg/kg i.p.) or the selective D2 antagonist sulpiride (100 mg/kg i.p.), produced the pattern of NT alterations observed after the administration of BMY 14802. These findings suggest that the blockade of sigma receptors modulates NT concentrations in these brain regions.