Electrophysiological evidence for a reciprocal interaction between amphetamine and cocaine-related drugs on rat midbrain dopaminergic neurons

To determine the functional interactions occurring between amphetamine and cocaine-like drugs on a single neuron, we used intracellular single-electrode voltage-clamp recordings from dopaminergic cells of the rat midbrain maintained in vitro. In the presence of cocaine (3-30 microM), the outward current caused by amphetamine (100 microM) on cells held at about -60 mV was attenuated. The degree of attenuation of the amphetamine-induced response was almost the same for 3 and 30 microM cocaine (44 and 51% of control, respectively). This effect of cocaine was reversible. We also tested other DA-uptake inhibitors (nomifensine and 4-phenyltetrahydroisoquinoline) against the amphetamine-induced outward current. Both drugs enhanced the effects of dopamine (DA) while reducing the outward response caused by amphetamine. Pretreatment of the animals with reserpine (12 mg/kg/i.p.), which irreversibly depletes the vesicular DA stores, neither affected the amplitude of the current caused by amphetamine nor changed the cocaine-induced attenuation of the membrane responses to amphetamine. Interestingly, when amphetamine (3 microM) was superfused on the dopaminergic neurons prior and during the application of cocaine, the DA-uptake blocker was no longer able to potentiate the outward response caused by the superfusion of DA. Taken together, these data suggest that: (i) amphetamine and cocaine interact with the DA transporter to produce distinct actions which under certain circumstances can compete with each other; (ii) the amphetamine-induced release of DA from the somata and dendrites of the dopaminergic cells is, at least in part, related to the reverse operation of the DA transporter and is not dependent on the integrity of the vesicular content of the catecholamine.     

诺米芬新对慢性吗啡处理后小鸡自发活动量的影响

目的 :探求吗啡对刚出生的小鸡自发活动量的影响以及行为敏感化的多巴胺神经机制。方法 :连续 7d每天给予小鸡ip不同剂量的吗啡 (5、10、2 0mg·kg- 1 ) ,观察其行为敏感化效应 ,撤药 3d后观察多巴胺转运体阻断剂诺米芬新 (0 5、1mg·kg- 1 )的干预效果。结果 :反复给药后 ,5mg·kg- 1 吗啡显著增加小鸡的自发活动量 (P <0 0 5 ,P <0 0 1) ,而且其自发活动量随给药次数的增多而不断增高 ;2 0mg·kg- 1 吗啡则明显降低小鸡的自发活动量 (P <0 0 5 )。诺米芬新呈剂量依赖性增加 5mg·kg- 1 组小鸡的自发性活动量 (P <0 0 5 )。结论 :反复小剂量吗啡处理能够导致鸟类行为敏感化效应 ,多巴胺可能在其中起重要作用。     

Influence of serotoninergic drugs on in vivo dopamine extracellular output in rat striatum

In vivo microdialysis was used to investigate the mechanism behind the increase in extracellular dopamine (DA) induced by increase in extracellular serotonin (5-HT) level and 5-HT₁ and 5-HT₂ receptor activation. The following serotoninergic drugs were perfused in the absence or presence of nomifensine (5 μM)or tetrodotoxin (TTX; 2 μM): clomipramine (10, 500 and 1,000 μM), a selective 5-HT reuptake inhibitor; 8-OH-DPAT (50 and 500 μM), a 5-HT_1A receptor agonist; and α-methyl-5-HT (1, 5 and 50 μM), a 5-HT₂ receptor agonist. All the serotoninergic drugs studied increased DA extracellular output in a dose-dependent manner. The presence of nomifensine attenuated the effect of perfusion of clomipramine (500 μM) and completely abolished the effect of perfusion of 8-OH-DPAT (500 μM) and α-methyl-5-HT (5 μM) on DA extracellular output. Clomipramine (100–1,000 μM) perfusion produced a dose dependent increase in DOPAC extracellular output, which was stronger when clomipramine (500 μM) was co-perfused with nomifensine. 8-OH-DPAT and α-methyl-5-HT perfusion decreased DOPAC overflow. Addition of TTX to the perfusion fluid one hour before serotoninergic drugs perfusion, did not completely abolish the effect on dopamine extracellular output produced by the serotoninergic drugs. These data seem to indicate that increase in extracellular 5-HT level and 5-HT₁ and 5-HT₂ receptor activation increase in vivo DA extracellular output in the striatum mainly by a non-exocytotic mechanism involving DA uptake sites and, secondarily, by activation of 5-HT receptors. J. Neurosci. Res. 52:591–598, 1998. © 1998 Wiley-Liss, Inc.     

Studies of the biogenic amine transporters. VII. Characterization of a novel cocaine binding site identified with [125I]RTI-55 in membranes prepared from human,monkey and guinea pig caudate

[¹²⁵I]RTI-55 is a cocaine analog with high affinity for dopamine (DA) and serotonin (5-HT) transporters. Quantitative ligand binding studies revealed a novel high affinity [¹²⁵I]RTI-55 binding site assayed under 5-HT transporter (SERT) conditions which has low affinity for almost all classic biogenic amine transporter ligands, including high affinity 5-HT transporter inhibitors such as paroxetine, but which retains high affinity for cocaine analogs. This site, termed SERT_site2 for its detection under 5-HT transporter conditions (not for an association with the SERT) occurs in monkey caudate, human caudate, and guinea pig caudate membranes, but not in rat caudate membranes. SERT_site2 is distinguished from the DA transporter (DAT) and SERT by several criteria, including a distinct ligand-selectivity profile, the inability to detect SERT_site2 in cells stably expressing the cloned human DAT, and insensitivity to irreversible ligands which inhibit [¹²⁵I]RTI-55 binding to the DAT and SERT. Perhaps the most striking finding about SERT_site2 is that a wide range of representative antidepressant agents have very low affinity for SERT_site2. The affinity of cocaine for this site is not very different from the concentration cocaine achieves in the brain at pharmacological doses. Viewed collectively with the observation that ligands with high affinity for SERT_site2 are mostly cocaine analogs, these data lead us to speculate that actions of cocaine which differ from those of classic biogenic amine uptake inhibitors may be mediated in part via SERT_site2. Synapse 28:322–338, 1998. Published 1998 Wiley-Liss, Inc.     

Place preference conditioning with methylphenidate and nomifensine

The role of central catecholaminergic systems in place preference conditioning produced by methylphenidate and nomifensine was investigated. Several doses of either methylphenidate or nomifensine produced alterations in place preferences, while desipramine (10 mg/kg), a relatively selective noradrenergic uptake inhibitor, was ineffective, Haloperidol (0.15-0.5 mg/kg) did not attenuate place preferences induced with methylphenidate (2.5 and 5.0 mg/kg) or nomifensine (5.0 mg/kg), although conditioning with methylphenidate (5.0 mg/kg) was blocked after injections of a high dose of haloperidol (1.0 mg/kg). Intraventricular injections of 6-hydroxydopamine had no effect on methylphenidate (2.5 mg/kg) induced place preferences. In contrast, pretreatment with haloperidol (0.2 mg/kg) or intraventricular injections of 6-hydroxydopamine attenuated locomotor activity induced by methylphenidate (5.0 mg/kg). These results suggest that methylphenidate and nomifensine produce place preferences via mechanisms that are either qualitatively or quantitatively distinct from their catecholamine-dependent stimulant actions.     

Characterization of sodium-dependent, high-affinity serotonin uptake in rat spinal cord synaptosomes

Synaptosomal accumulation of [3H]serotonin was used to determine if the rat spinal cord possesses a high-affinity neuronal uptake system for serotonin. Two temperature-dependent accumulation processes were found, one sodium-dependent, the second sodium-independent. Sodium-dependent [3H]serotonin accumulation was linear with sodium concentrations up to 143 mM, was associated with the purified synaptosomal fraction (P2B), and decreased 76% by osmotic lysis, 88% by sonication, and 96% by 0.1% Triton X-100. Drug inhibition studies demonstrated fluoxetine to be the most potent inhibitor of this system (IC50 0.075 microM) while desipramine (IC50 0.43 microM) and nomifensine (IC50 0.95 microM) were less potent. Kinetic analysis revealed that sodium-dependent accumulation in purified synaptosomes was saturable at low [3H]serotonin concentrations (Ku = 50 nM, Vmax = 4 pmol/mg protein/min). Sodium-independent [3H]5-HT accumulation was substantially less sensitive to fluoxetine, desipramine and nomifensine. While sodium-independent accumulation was not significantly affected by osmotic lysis, it was markedly increased by prior sonication of tissue. Also, in contrast to sodium-dependent accumulation, sodium-independent accumulation was evenly distributed in all tissue fractions, and was not saturable at low [3H]serotonin concentrations. It is concluded that sodium-dependent [3H]serotonin accumulation reflects uptake into spinal serotonergic nerve terminals while sodium-independent accumulation probably reflects a temperature-sensitive binding to membrane fragments. Comparison to brain uptake of serotonin and the necessity for using 37 degrees C sodium-free blanks rather than 0 degree C blanks in spinal cord homogenates is discussed.     

Evidence for central selective dopamine receptor stimulation in the mediation of nomifensine-induced hyperalgesia and the effects of opiate antagonists

The hyperalgesic effects of nomifensine were studied using a modified tail immersion test in mice. Intracerebroventricular injection of the peripherally acting dopamine antagonist domperidone totally antagonized nomifensine-induced hyperalgesia but this blockade was not evident after peripheral administration of domperidone suggesting mediation of hyperalgesia at a central location. The dopamine antagonist cis-flupenthixol and also bromocriptine shifted the nomifensine-induced hyperalgesia dose-response curve to the right, whereas sulpiride had no antagonistic effect. These results are discussed in relation to the D-1 and D-2 classification of dopamine receptors, and their putative agonists and antagonists. Naloxone produced a blockade of nomifensine-induced hyperatgesia, but another opiate antagonist MR 1452, though inherently hyperalgesic, did not modify the hyperalgesia produced by nomifensine. These findings do not rule out the participation of an opiate receptor in the mechanism of opiate antagonist blockade of nomifensine-induced hyperalgesia. However, they may suggest that MR 1452-induced hyperalgesia depends on a different mechanism from that of induced by nomifensine since they were not synergistic.     

Clinical investigations into antidepressive mechanisms. II. Dexamethasone suppression test predicts response to nomifensine or amitriptyline

This prospective study investigates the possibility of a central noradrenergic-cholinergic imbalance in subgroups of depressed inpatients using the dexamethasone suppression test (DST) as one peripheral indicator. The DST was performed in 43 depressed inpatients. Subsequently, a group (n = 20) of DST suppressors (DST-) and a group (n = 23) of DST nonsuppressors (DST+) were treated under double blind conditions with either nomifensine (NOM) a noradrenaline potentiating drug, or amitriptyline (AMI) a noradrenaline potentiating and strong anticholinergic compound. DST+ depressives responded favorably to AMI, but not to NOM. Conversely, DST- depressives responded favorably to NOM but less well to AMI. Together with other biochemical findings this data suggests: 1) a hypofunction of the noradrenergic system in DST- patients who may, from a clinical point of view, usually show minor or 'neurotic' depressions; 2) a hypofunction of the noradrenergic and a hyperfunction of the cholinergic system in DST+ patients who may present a more severe or 'endogenous' depression. These data suggest a biochemical heterogeneity of depression and offer an aid for a more specific antidepressive drug therapy.     

In vivo evaluation of striatal dopamine reuptake sites using 11C-nomifensine and positron emission tomography

In vitro nomifensine demonstrates high affinity and specificity for dopamine reuptake sites in the brain. In the present study 11C-nomifensine was administered i.v. in trace amounts (10-50 micrograms) to ketamine anaesthetized Rhesus monkeys (6-10 kg b.w.) and the time-course of radioactivity within different brain regions was measured by positron emission tomography (PET). Six base-line experiments lasting for 60-80 min were performed. The procedure was repeated after pretreatment with nomifensine (2-6 mg/kg i.v.), another reuptake inhibitor, mazindol (0.3 mg/kg i.v.), desipramine (0.5 mg/kg i.v) or spiperone (0.3 mg/kg i.v.) before the administration of a second 11C-nomifensine dose. The highest radioactivity uptake was found in the dopamine innervated striatum and the lowest in a region containing the cerebellum, known to be almost devoid of dopaminergic neurons. The difference between striatal and cerebellar uptake of 11C-nomifensine derived radioactivity was markedly reduced after nomifensine and mazindol but not after desipramine and spiperone. These results indicate that in vivo the striatal uptake of 11C-nomifensine, as measured with PET, involves specific binding with the dopamine reuptake sites. In the first human applications of 11C-nomifensine and PET in a healthy volunteer, the regional uptake of radioactivity was similar to that in base-line experiments with Rhesus monkeys. In the healthy subject the striatal/cerebellar ratio was 1.6, 50 min after the injection of 11C-nomifensine. In a hemi-parkinsonian patient this ratio was 1.1 contralaterally and 1.3 ipsilaterally to the affected side. 11C-nomifensine and PET seems to be an auspicious method to measure the striatal dopaminergic nerve terminals of man in vivo.     

Application of fixed-interval schedules to intracranial self-stimulation for assessing psychomotor stimulants

Rats were implanted with stimulating electrodes aimed at the medial forebrain bundlelateral hypothalamus (MFB-LH) and were trained to lever-press for brain stimulation on a fixed-interval 15-sec schedule of reinforcement. When behavior had stabilized, the animals were tested with graded doses of d-amphetamine (0.1–1.0 mg/kg), cocaine (3.0–30 mg/kg), and nomifensine (1.0–10 mg/kg). Separate groups of animals were tested in a locomotor activity procedure over the same dose ranges. All three drugs produced graded increases in rates of responding which ranged from 180–295% of saline control levels following the highest dose of each drug. In contrast, there were no significant changes in numbers of reinforcements at any of the doses used. When tested in a locomotor activity device, all three drugs produced similar increases in activity with increasing dosage. Although currently little used, the fixed-interval schedule of brain self-stimulation can be helpful in making comparisons of the stimulant properties of abused drugs, such as amphetamine and cocaine, and less well-known drugs with abuse potential such as nomifensine. © 1992 Wiley-Liss, Inc.