(1/918) Age-related reductions in [3H]WIN 35,428 binding to the dopamine transporter in nigrostriatal and mesolimbic brain regions of the fischer 344 rat.
In the present study, we used the potent cocaine analog [3H]WIN 35, 428 to map and quantify binding to the dopamine transporter (DAT) within the dorsal striatum, nucleus accumbens, substantia nigra, and ventral tegmental area in young (6-month-old), middle-aged (12-month-old), and aged (18- and 24-month-old) Fischer 344 rats. Quantitative autoradiographic analysis of indirect [3H]WIN 35,428 saturation curves revealed two-site binding for all four brain regions in every age group. The percentage of binding to the high- or low-affinity sites did not differ with age or region and was approximately 50%. However, significant age-related decreases in the overall density (Bmax) of [3H]WIN 35,428-binding sites were observed in the striatum, nucleus accumbens, substantia nigra, and ventral tegmental area. The Bmax within all brain regions declined by more than 15% every 6 months, with the Bmax in the aged (24-month-old) group being approximately half that measured in the young adult (6-month-old) group. Competition experiments indicated that nomifensine also exhibited two-site binding to the DAT in Fischer 344 rats. No consistent age-related differences in binding affinities were noted with either [3H]WIN 35,428 or nomifensine. Taken together, these results support the hypothesis that functional DATs within the nigrostriatal and mesolimbic systems are down-regulated with age, without changing their affinity for ligands. (+info)
(2/918) Effects of tetrahydroprotoberberines on dopamine D2 receptors in ventral tegmental area of rat.
AIM: To compare the actions of tetrahydroprotoberberines (THPB) on dopamine (DA) D2 receptors in the ventral tegmental area (VTA) of rat. METHODS: Extracellular single unit recording technique was used in i.v. gallamine-paralyzed rats. RESULTS: Eleven THPB analogs tested completely attenuated the apomorphine (Apo, 20 micrograms.kg-1)-induced inhibition on VTA DA cell firing activity. The OH group on C2 at THPB was linked with the reversal of Apo-induced inhibition. Their reversal potencies (ED50, microgram.kg-1) for D2 receptors were: THPB-143 (5.6) > SPD (8.5) > Iso (17.0) > THP (33) > THB (48) > THPB-18 (66) > THPB-1 (179) > THPB-19 (408) > THPB-126 (510) > THPB-104 (1019) > THPB-10 (4815). CONCLUSION: Among these 11 THPB, the 2-hydroxyl-THPB (THPB-143) showed the strongest antagonistic action on D2 receptors. (+info)
(3/918) 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. (+info)
(4/918) Properties and plasticity of excitatory synapses on dopaminergic and GABAergic cells in the ventral tegmental area.
Excitatory inputs to the ventral tegmental area (VTA) influence the activity of both dopaminergic (DA) and GABAergic (GABA) cells, yet little is known about the basic properties of excitatory synapses on these two cell types. Using a midbrain slice preparation and whole-cell recording techniques, we found that excitatory synapses on DA and GABA cells display several differences. Synapses on DA cells exhibit a depression in response to repetitive activation, are minimally affected by the GABAB receptor agonist baclofen, and express NMDA receptor-dependent long-term potentiation (LTP). In contrast, synapses on GABA cells exhibit a facilitation in response to repetitive activation, are depressed significantly by baclofen, and do not express LTP. The relative contribution of NMDA and non-NMDA receptors to the synaptic currents recorded from the two cell types is the same as is the depression of synaptic transmission elicited by the application of adenosine, serotonin, or methionine enkephalin (met-enkephalin). The significant differences in the manner in which excitatory synaptic inputs to DA and GABA cells in the VTA can be modulated have potentially important implications for understanding the behavior of VTA neurons during normal behavior and during pathological states such as addiction. (+info)
(5/918) Altered activity of midbrain dopamine neurons following 7-day withdrawal from chronic cocaine abuse is normalized by D2 receptor stimulation during the early withdrawal phase.
Using in vivo single-unit recording in rats, we compared the effects of continuous cocaine infusion via minipump or single daily injections (both 40 mg/kg/d x 14 days, S.C.) on the activity of putative dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA). On days 1-5 after cocaine withdrawal, animals were further treated with single daily injections of DA agonists. On withdrawal day 7 continuous cocaine caused a reduction in spontaneously active neurons in the SNC and reduced bursting in the VTA. In contrast, intermittent cocaine resulted in an increase in the number of active neurons in the VTA. These changes were all reversed by apomorphine or quinpirole given during the first 5 withdrawal days. The D1 antagonist SCH 39166 did not antagonize the effects of apomorphine in either region. The role of D2 receptors in modulating baseline DA activity during intermediate cocaine withdrawal is discussed. (+info)
(6/918) Pharmacological reduction of small conductance calcium-activated potassium current (SK) potentiates the excitatory effect of ethanol on ventral tegmental area dopamine neurons.
Dopaminergic neurons in the ventral tegmental area (VTA) are important for the rewarding properties of drugs of abuse, including ethanol. We previously demonstrated that ethanol excites VTA neurons and that ethanol reduces the amplitude of the after hyperpolarization (AHP) that follows spontaneous action potentials. Because the small conductance calcium-activated potassium current (SK) is a component of the AHP of VTA neurons, we assessed the effect of the SK blockers apamin and d-tubocurarine (d-TC) on the action of ethanol on dopaminergic VTA neurons with intracellular and extracellular recording in rat brain slices. Apamin (1-200 nM) and d-TC (100 and 400 microM) caused concentration-dependent reductions in the AHP amplitude. Ethanol (80 mM) caused a small reduction in the AHP. In the presence of apamin (40 nM), ethanol (80 mM) caused a much larger reduction in AHP amplitude. Extracellular studies showed that apamin (20, 40, and 100 nM) and d-TC (400 microM) had no significant effect on the spontaneous firing rate of dopaminergic VTA neurons but enhanced the potency of ethanol to increase their firing rate. These results indicate that the ethanol-induced reduction of the AHP and excitation of VTA neurons is not due to a reduction in SK current. Furthermore, blockade of SK current by apamin or d-TC enhances the excitatory effect of ethanol on dopaminergic VTA neurons. These data suggest that the amount of SK current present affects the sensitivity of dopaminergic VTA neurons to ethanol excitation and that neurotransmitters that reduce SK current may increase the reward potency of ethanol. (+info)
(7/918) Baclofen inhibits heroin self-administration behavior and mesolimbic dopamine release.
An emerging hypothesis to explain the mechanism of heroin-induced positive reinforcement states that opiates inhibit gamma-aminobutyric acid (GABA)-ergic interneurons within the mesocorticolimbic dopamine (DA) system to disinhibit DA neurons. In support of this hypothesis, we report that the development of heroin self-administration (SA) behavior in drug-naive rats and the maintenance of SA behavior in heroin-trained rats were both suppressed when the GABA(B) receptor agonist baclofen was coadministered with heroin. Microinjections of baclofen into the ventral tegmental area (VTA), but not the nucleus accumbens, decreased heroin reinforcement as indicated by a compensatory increase in SA behavior. Additionally, baclofen administered alone or along with heroin dose-dependently reduced heroin-induced DA release. This effect was blocked partially by intra-VTA infusion of the GABA(B) antagonist 2-hydroxysaclofen, suggesting an additional, perhaps GABA(A) receptor-mediated, disinhibitory effect. Taken together, these experiments, for the first time, demonstrate that heroin-reinforced SA behavior and nucleus accumbens DA release are mediated predominantly by GABA(B) receptors in the VTA and suggest that baclofen may be an effective agent in the treatment of opiate abuse. (+info)
(8/918) Behavioral and biochemical manifestations of mecamylamine-precipitated nicotine withdrawal in the rat: role of nicotinic receptors in the ventral tegmental area.
Brain mesolimbic dopamine (DA) neurons are considered critical for the dependence-producing action of nicotine, and its stimulatory effect on behavior and DA neurotransmission appears largely mediated via nicotinic receptors (nAChRs) in the ventral tegmental area (VTA). The nAChR antagonist mecamylamine administered systemically in chronically nicotine-treated rats elicits a behavioral withdrawal syndrome concomitant with a reduced DA output in the nucleus accumbens (NAC). Here, we investigated the behavioral and biochemical consequences of intrategmental administration of mecamylamine in rats chronically infused with nicotine by means of minipumps for 14 days (9 mg/kg/day). Bilateral, intrategmental mecamylamine injections (1, 3 or 9 micrograms/0.5 microliter/side) dose-dependently increased abstinence signs such as gasps, teeth chatter, and reduced locomotor activity in nicotine-treated, but not in control animals. Moreover, a unilateral intrategmental injection of 9 micrograms mecamylamine reduced DA output in the ipsilateral NAC of chronically nicotine-treated rats, but not in control animals. Consequently, nAChRs in the VTA may be involved not only in the stimulatory effects of acute nicotine administration, but also in the withdrawal reaction following cessation of chronic nicotine treatment. (+info)