Anti-opioid effects of neuropeptide FF receptors in the ventral tegmental area

The aim of this study was to establish the modulation pattern of the soleus H-reflex and excitability changes of interneurones mediating presynaptic inhibition from tibialis anterior to soleus Ia afferents when the right foot approached and withdrew from a step in standing humans. The task was conducted at 40 beats per minute, and this tempo corresponded to the rhythm of a half full movement cycle. Each subject was instructed not to load and not to move forward. Soleus H-reflexes were elicited once in every full movement cycle that lasted approximately 3s. The ipsilateral knee joint angle and activity of leg muscles were recorded through a twin-axis goniometer and surface EMG, respectively. In all subjects, the soleus H-reflex was modulated in a phase-dependent pattern. The H-reflex was significantly depressed during the approach phase of the motor task and when the foot was on the step, and facilitated during the withdrawal phase and when the foot was on the ground. The soleus H-reflex conditioned by common peroneal nerve stimulation at tibialis anterior motor threshold at a long conditioning test interval was increased during the withdrawal phase or while the foot was on the ground suggesting that presynaptic inhibition was decreased. In most subjects, knee extensor activity was small, while ankle flexors and extensors were active in a reciprocal pattern. This study provides evidence that the soleus H-reflex is modulated in a phasic pattern during one-legged foot reaching and withdrawal by changes occurring at a premotoneuronal level.     

Multiplexed neurochemical signaling by neurons of the ventral tegmental area

The ventral tegmental area (VTA) is an evolutionarily conserved structure that has roles in reward-seeking, safety-seeking, learning, motivation, and neuropsychiatric disorders such as addiction and depression. The involvement of the VTA in these various behaviors and disorders is paralleled by its diverse signaling mechanisms. Here we review recent advances in our understanding of neuronal diversity in the VTA with a focus on cell phenotypes that participate in ‘multiplexed’ neurotransmission involving distinct signaling mechanisms. First, we describe the cellular diversity within the VTA, including neurons capable of transmitting dopamine, glutamate or GABA as well as neurons capable of multiplexing combinations of these neurotransmitters. Next, we describe the complex synaptic architecture used by VTA neurons in order to accommodate the transmission of multiple transmitters. We specifically cover recent findings showing that VTA multiplexed neurotransmission may be mediated by either the segregation of dopamine and glutamate into distinct microdomains within a single axon or by the integration of glutamate and GABA into a single axon terminal. In addition, we discuss our current understanding of the functional role that these multiplexed signaling pathways have in the lateral habenula and the nucleus accumbens. Finally, we consider the putative roles of VTA multiplexed neurotransmission in synaptic plasticity and discuss how changes in VTA multiplexed neurons may relate to various psychopathologies including drug addiction and depression.     

Opposite alterations of NPFF1 and NPFF2 neuropeptide FF receptor density in the triple MOR/DOR/KOR-opioid receptor knockout mouse brains

Mice lacking the mu-delta-kappa-opioid receptor (MOR/DOR/KOR) genes and their corresponding wild-type littermates have been used to quantify NPFF(1) and NPFF(2) (neuropeptide FF) receptors by in vitro autoradiography in the central nervous tissues. Adjacent coronal sections were labelled with [125I]YVP ([125I]YVPNLPQRF-NH(2)) and [125I]EYF ([125I]EYWSLAAPQRF-NH(2)) as specific radioligands for NPFF(1) and NPFF(2) receptors, respectively. NPFF(2) receptors are predominantly expressed in both genotypes, but their density increases significantly in non cortical regions of mutant mice: 64% in the amygdaloid area, 89, 308, 1214 and 49% in the nucleus of the vertical limb of the diagonal band, substantia nigra, the vestibular nucleus and the spinal cord, respectively. In contrast, the density of the NPFF(1) subtype is lower than NPFF(2) in both genotypes and significantly decreased in some brain areas of mutant mice: -99, -90 and -90% in the nucleus of the vertical limb of the diagonal band, substantia nigra and the spinal cord, respectively. This study shows that mice lacking opioid receptors have brain region-dependent increases (NPFF(2)) and decreases (NPFF(1)) in NPFF receptors densities and suggests a different functional participation of each NPFF receptor subtype in the actions of opioids.     

P>海洛因依赖对大鼠中脑腹侧被盖区、伏隔核神经元P物质和神经肽Y表达的影响 /P>

目的 探讨海洛因依赖对大鼠中脑腹侧被盖区（VTA）、伏隔核（NAc）神经元P物质（SP）、神经肽Y（NPY）表达的影响,为进一步揭示海洛因依赖的中枢机制提供重要的原位形态学资料。 方法 成年雄性SD大鼠55只,随机分为海洛因依赖组、生理盐水对照组及正常对照组。建立海洛因依赖模型,并分别于第10、17、24、31、38天取脑组织,免疫组织化学SABC法显示VTA、NAc区神经元SP、NPY免疫反应细胞并用图像分析法测定平均灰度值。结果 海洛因依赖组VTA、 NAc区的SP、NPY免疫反应细胞与生理盐水及正常对照组比较,免疫反应强度明显减弱、染色变浅。平均灰度值测定,海洛因依赖组各时间点大鼠VTA、NAc区 的SP、NPY免疫反应细胞平均灰度值均高于正常对照组及生理盐水对照组,差异有统计学意义（P ＜0.05）。结论 实验结果提示,海洛因依赖使VTA、NAc区SP、NPY的分泌受到抑制,可能导致内源性阿片肽的分泌受影响,SP、NPY可能是药物依赖形成机制中的关键信号分子。     

Distribution of serotonin 5-HT2C receptors in the ventral tegmental area

Serotonin 2C receptors (5-HT2CR) appear to exert tonic inhibitory influence over dopamine (DA) neurotransmission in the ventral tegmental area (VTA), the origin of the mesolimbic DA system, thought to be important in psychiatric disorders including addiction and schizophrenia. Current literature suggests that the inhibitory influence of 5-HT2CR on DA neurotransmission occurs via indirect activation of GABA inhibitory neurons, rather than via a direct action of 5-HT2CR on DA neurons. The present experiments were performed to establish the distribution of 5-HT2CR protein on DA and GABA neurons in the VTA of male rats via double-label immunofluorescence techniques. The 5-HT2CR protein was found to be co-localized with the GABA synthetic enzyme glutamic acid decarboxylase (GAD), confirming the presence of the 5-HT2CR on GABA neurons within the VTA. The 5-HT2CR immunoreactivity was also present in cells that contained immunoreactivity for tyrosine hydroxylase (TH), the DA synthetic enzyme, validating the localization of 5-HT2CR to DA neurons in the VTA. While the degree of 5-HT2CR+GAD co-localization was similar across the rostro-caudal levels of VTA subnuclei, 5-HT2CR+TH co-localization was highest in the middle relative to rostral and caudal levels of the VTA, particularly in the paranigral, parabrachial, and interfascicular subnuclei. The present results suggest that the inhibitory influence of the 5-HT2CR over DA neurotransmission in the VTA is a multifaceted and complex interplay of 5-HT2CR control of the output of both GABA and DA neurons within this region.     

Ultrastructural localization of enkephalin and mu-opioid receptors in the rat ventral tegmental area

Enkephalins are endogenous ligands for opioid receptors whose activation potently modulates the output of mesocorticolimbic dopaminergic neurons within the ventral tegmental area. Many of the reinforcing effects of enkephalins in the mesocorticolimbic system are mediated by mu-opioid receptors. To determine the sites for Leu(5)-enkephalin activation of mu-opioid receptors in the ventral tegmental area, we examined the dual electron microscopic immunocytochemical localization of their respective antigens in this region of rat brain. Enkephalin immunoperoxidase reaction product and mu-opioid receptor immunogold-silver labeling showed similar cellular and subcellular distribution in both the paranigral and parabrachial subdivisions of the ventral tegmental area. Enkephalin immunoreactivity was mainly localized in small unmyelinated axons (50.4%) and in axon terminals (40.4%). The majority of these terminals formed symmetric, inhibitory-type synapses, many of which were on dendrites expressing plasmalemmal mu-opioid receptors. Appositional contacts were also often seen between axons or terminals that were differentially labeled for the two antigens. In addition, some of the enkephalin-labeled terminals and a few somatodendritic profiles showed a plasmalemmal or vesicular localization of mu-opioid receptors.Our results indicate that dendritic targets of inhibitory terminals, as well as nearby axon terminals, are potential sites for enkephalin activation of mu-opioid receptors throughout the ventral tegmental area. Moreover, co-localization of enkephalin and mu-opioid receptors in selective neuronal profiles may indicate an autoregulatory role for these receptors or their internalization along with the bound ligand in this brain region.     

Histamine excites GABAergic cells in the rat substantia nigra and ventral tegmental area in vitro

We have investigated the effect of histamine (HA) on spontaneous firing of dopaminergic (DA) and GABAergic neurons in the substantia nigra (SN) and the ventral tegmental area (VTA) of the rat in vitro. Single-unit extracellular recordings were obtained and drugs were bath applied. In both regions application of HA (10 and 100 μM) did not affect the firing frequency of DAergic cells, but increased the firing of GABAergic neurons. The histamine-induced excitation was blocked by the H₁ receptor antagonist mepyramine (1 μM), but was unaffected by application of the H₂ antagonist cimetidine (50 μM) or the H₃ antagonist thioperamide (10 μM). Our results suggest that histamine does not directly inhibit dopaminergic neurons in SN and VTA, but rather that this inhibition is mediated through histamine-induced excitation of GABAergic neurons.     

Organization of ventral tegmental area projections to the ventral tegmental area-nigral complex in the rat

The ventral tegmental area (VTA) is a nodal link in reward circuitry. Based on its striatal output, it has been subdivided in a caudomedial part which targets the ventromedial striatum, and a lateral part which targets the ventrolateral striatum [Ikemoto S (2007) Dopamine reward circuitry: two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex. Brain Res Rev 56:27-78]. Whether these two VTA parts are interconnected and to what extent the VTA innervates the substantia nigra compacta (SNc) and retrorubral nucleus (RR) are critical issues for understanding information processing in the basal ganglia. Here, VTA projections to the VTA-nigral complex were examined in rats, using Phaseolus vulgaris leucoagglutinin (PHA-L) as anterograde tracer. The results show that the dorsolateral VTA projects to itself, as well as to the dorsal tier of the SNc and RR, largely avoiding the caudomedial VTA. The ventrolateral VTA innervates mainly the interfascicular nucleus. The components of the caudomedial VTA (the interfascicular, paranigral and caudal linear nuclei) are connected with each other. In addition, the caudomedial VTA (especially the paranigral and caudal linear nuclei) innervates the lateral VTA, and, to a lesser degree, the SNc and RR. The caudal pole of the VTA sends robust, bilateral projections to virtually all the VTA-nigral complex, which terminate in the dorsal and ventral tiers. Modest inputs from the medial supramammillary nucleus to ventromedial parts of the VTA-nigral complex were also identified. In double-immunostained sections, PHA-L-labeled varicosities were sometimes found apposed to tyrosine hydroxylase-positive neurons in the ventral mesencephalon. Overall, the results underscore that VTA projections to the VTA-nigral complex are substantial and topically organized. In general, these projections, like the spiralated striato-nigro-striatal loops, display a medial-to-lateral organization. This anatomical arrangement conceivably permits the ventromedial striatum to influence the activity of the lateral striatum. The caudal pole of the VTA appears to be a critical site for a global recruitment of the mesotelencephalic system.     

Mapping of the colocalization of calretinin and tyrosine hydroxylase in the rat substantia nigra and ventral tegmental area

The distribution of calretinin (CR), a calcium binding protein, was compared with that of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of dopamine, throughout the rostrocaudal extent of the rat subsantia nigra (SN) and ventral tegmental area (VTA). After mapping the cells using double-labelling immunofluorescence, it was possible to distinguish three distinct cell types: cells immunoreactive for CR only, cells immunoreactive for TH only, and cells in which the two proteins were colocalized (CR+TH). Colocalized cells in rat brain sections comprised approximately 40–55% of the fluorescent labelled cells in the SN compacta, 30–40% in the VTA, and 55–80% in the SN lateralis. Colocalized cells in the SN reticulata were infrequent except in the more caudal sections where a majority of the TH-immunoreactive cells also contained CR. The percentage of CR cells that contained TH was approximately 80% in the SN compacta and averaged 65% in the VTA. Overall, the percentage of TH-immunoreactive cells which also contained CR was approximately 50% in the SN compacta and 45% in the VTA. These data reveal a significant degree of colocalization of CR in dopamine-producing cells of the SN and VTA and suggest the need for studies concerning the fate of these individual cell types following experimental manipulations.     

Properties of distinct ventral tegmental area synapses activated via pedunculopontine or ventral tegmental area stimulation in vitro

Anatomical studies indicate that synaptic inputs from many cortical and subcortical structures converge on neurons of the ventral tegmental area (VTA). Although in vitro electrophysiological studies have examined synaptic inputs to dopamine (DA) and non-DA neurons in the VTA, they have largely relied upon local electrical stimulation to activate these synapses. This provides little information regarding the distinct properties of synapses originating from different brain areas. Using whole-cell recordings in parasagittal rat brain slices that preserved subcortical axons from the pedunculopontine nucleus (PPN) to the VTA, we compared these synapses with those activated by intra-VTA stimulation. PPN-evoked currents demonstrated longer latencies than intra-VTA-evoked currents, and both VTA and PPN responses were mediated by GABA_A and AMPA receptors. However, unlike VTA-evoked currents, PPN currents were exclusively mediated by glutamate in 25–40% of the VTA neurons. Consistent with a cholinergic projection from the PPN to the VTA, nicotinic acetylcholine receptors (nAChR) were activated by endogenous acetylcholine released during PPN, but not VTA, stimulation. This was seen as a reduction of PPN-evoked, and not VTA-evoked, synaptic currents by the α7-nAChR antagonist methyllycaconitine (MLA) and the agonist nicotine. The β2-nAChR subunit antagonist dihydro-β-erythroidine had no effect on VTA- or PPN-evoked synaptic currents. The effects of MLA on PPN-evoked currents were unchanged by the GABA_A receptor blocker picrotoxin, indicating that α7-nAChRs presynaptically modulated glutamate and not GABA release. These differences in physiological and pharmacological properties demonstrate that ascending PPN and presumed descending inputs to VTA utilize distinct mechanisms to differentially modulate neuronal activity and encode cortical and subcortical information.     

Projections from the ventral tegmental area and mesencephalic raphe to the dorsal raphe nucleus in the rat

Summary The origins of the dopaminergic innervation of the rat dorsal raphe nucleus (NRD) have been investigated using a combination of fluorescent retrograde tracing and fluorescence histochemistry. Stereotaxic microinjections of True Blue were placed in the central, caudal and lateral portions of the NRD, and after 6–12 days survival the brains were processed for fluorescence histochemical detection of catecholamines. Retrogradely labeled neurons were searched for in the diencephalic A11 and A13 dopaminergic cell groups, substantia nigra, ventral tegmental area (VTA) and the linear, central superior and dorsal raphe nuclei. The various NRD injections consistently resulted in retrograde labeling of a small number of catecholamine-containing, presumed dopaminergic cell bodies, confined mainly to three regions: the VTA, the linear and central superior raphe nuclei and the NRD itself. The present findings indicate that not only dopaminergic neurons in the VTA but also the system of catecholamine-containing cells, extending dorsally and caudally from the VTA within the midline raphe area, project to the NRD. Although often similar in size, shape and distribution to the catecholaminergic neurons the majority of retrogradely labeled cells in these regions were, however, found to be non-catecholaminergic.Abbreviations 3 Principal oculomotor nucleus - 4 Trochlear nucleus - Aq Cerebral aqueduct - cp cerebral peduncle - cst cortico-spinal tract - dscp decussation of the superior cerebellar peduncle - DTg Dorsal tegmental nucleus - fr fasciculus retroflexus - IF Interfascicular nucleus - IP Interpeduncular nucleus - LL nucleus of the lateral lemniscus - ml medial lemniscus - mlf medial longitudinal fasciculus - mNV mesencephalic trigeminal nucleus - NLC Nucleus linearis caudalis - NLR Nucleus linearis rostralis - NRD Dorsal raphe nucleus - PAG Periaqueductal grey - PN Pontine nucleus - PRN Pontine raphe nucleus - R Red nucleus - RCS Nucleus raphe centralis superior - SN Substantia nigra - VTA Ventral tegmental area - VTg Ventral tegmental nucleus     

Opiate state controls bi-directional reward signaling via GABAA receptors in the ventral tegmental area

The neural mechanisms that mediate the transition from a drug-naive state to a state of drug dependence and addiction are not yet known. Here we show that a discrete population of GABA(A) receptors in the mammalian ventral tegmental area (VTA) serves as a potential addiction switching mechanism by gating reward transmission through one of two neural motivational systems: either a dopamine-independent (opiate-naive) or a dopaminergic (opiate-dependent or opiate-withdrawn) system. Bi-directional transmission of reward signals through this GABA(A) receptor substrate is dynamically controlled by the opiate state of the organism and involves a molecular alteration of the GABA(A) receptor. After opiate exposure and subsequent withdrawal, the functional conductance properties of the rat VTA GABA(A) receptor switch from an inhibitory to an excitatory signaling mode.     

6-Hydroxydopamine lesions of the ventral tegmental area suppress ghrelin's ability to elicit food-reinforced behavior

While past research suggests that ghrelin stimulates appetite through an action on hypothalamic signaling, recent evidence indicates that the peptide acts via mesotelencephalic dopamine neurons to alter appetitive motivation. In the present study, rats were trained to operantly respond for food on a progressive ratio PR5 schedule until stable breakpoints were established. Ghrelin (30-300 pmol) was then injected directly into the ventral tegmental area (VTA) and the 300 pmol dose was observed to increase breakpoint. The dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA, 6 μg) was subsequently administered into the VTA resulting in a significant depletion of striatal dopamine. Stable breakpoints were then re-established. When ghrelin's effects were reassessed, the peptide's ability to alter operant responding for food was reliably reduced. Our findings demonstrate that ghrelin induces food-reinforced behavior in the mesotelencephalic reward pathway and that this effect is dependent on intact dopaminergic signaling. We conclude that the metabolic peptide ghrelin interacts with dopamine, within reward circuitry, to modulate appetitive behavior.     

Modulation of morphine state-dependent learning by muscarinic cholinergic receptors of the ventral tegmental area

In the present study, the effects of bilateral intra-ventral tegmental area (intra-VTA) injections of an anticholinesterase, physostigmine and/or muscarinic acetylcholine receptor antagonist, atropine on memory retention and morphine state-dependent learning were examined in adult male Wistar rats. As a model of learning, a step-through passive avoidance task was used. Post-training subcutaneous administration of morphine (0.5, 2.5 and 5 mg/kg) dose-dependently impaired memory retrieval on the test day. Pre-test administration of morphine (2.5 and 5 mg/kg) induced state-dependent retrieval of the memory acquired under post-training morphine influence. Pre-test intra-VTA microinjection of physostigmine (0.5, 1 and 2 microg/rat) or atropine (1, 2 and 3 microg/rat) alone cannot affect memory retention. Interestingly, pre-test intra-VTA administration of physostigmine (1 and 2 microg/rat) reversed post-training morphine (5 mg/kg, s.c.)-induced retrieval impairment. Furthermore, pre-test intra-VTA microinjection of physostigmine (1 and 2 mug/rat) with an ineffective dose of morphine (0.5 mg/kg), synergistically improved memory performance impaired by post-training morphine. On the other hand, pre-test intra-VTA microinjection of atropine (2 and 3 microg/rat) 5 min before the administration of morphine (5 mg/kg, s.c.) dose-dependently inhibited morphine state-dependent memory. Pre-test atropine microinjection also reversed the influence of physostigmine on morphine response. It may be concluded that the muscarinic acetylcholine receptors of the VTA play an important role in morphine-induced recovery of memory, on the test day.     

Direct monitoring of dopamine and 5-HT release in substantia nigra and ventral tegmental area in vitro

Fast-scan cyclic voltammetry with carbon fibre microelectrodes was used to detect endogenous dopamine (DA) and 5-hydroxytryptamine (5-HT) release from three distinct regions of guinea-pig mid-brain in vitro : rostral and caudal substantia nigra (SN) and the ventral tegmental area (VTA). Previous electrophysiological studies have demonstrated that cells of the caudal SN and the VTA have similar characteristics, whereas cells in the rostral SN have distinctly different properties. In the present study, we confirmed that each region has tyrosine hydroxylase-positive neurons and determined, using high-performance liquid chromatography, that DA levels were similar in rostral and caudal SN, but lower in SN than in VTA. In each region, application of veratrine, which was shown by intracellular recordings to have a reversible depolarising action, evoked a signal attributable to DA and distinguishable from that of 5-HT. Release signals were monitored every 250 ms with a spatial resolution of less than 50 μm. DA release was calcium-dependent and was not detectable in a catecholamine-poor area such as the cerebellum, or in mid-brain tissue pre-treated with reserpine. Within the normal mid-brain, the amount of DA released was correlated with tissue content in that it was higher in the VTA than in either region of SN. It is concluded that DA released from somato-dendritic parts of mid-brain neurons exhibits site-specific variation. This is the first report of direct monitoring of DA and 5-HT relase from these regions with in situ electrodes and demonstrates the utility of fast-scan cyclic voltammetry to investigate the mechanisms and possible non-classical functions of somato-dendritic DA release.     

Combined grafts of the ventral tegmental area and nucleus accumbens in oculo. Histochemical and electrophysiological characterization

Summary Dissection techniques and optimal donor stages have been established for constructing an isolated intraocular model of the ventral tegmental area (VTA)-accumbens system using intraocular sequential grafting. Single grafts including accumbens and VTA respectively survived and developed many organotypic features when taken from 15–17 day fetuses. Falck-Hillarp fluorescence histochemistry showed dopamine neurons and terminals in single VTA grafts, no or almost no catecholamine fibers in single accumbens grafts, and a well-developed VTA-accumbens dopamine pathway in combined grafts where cell bodies in the VTA part provided the accumbens part with a rich terminal network. A similar distribution was found using immunohistochemistry with antibodies directed against tyrosine hydroxylase. CCK-like immunoreactivity had a distribution that mimicked that of the catecholamine-containing system. Enkephaline-like immunoreactivity was found both in single VTA and in single accumbens pieces as well as in both parts of the double grafts. Cells with long-duration action potentials typical of dopamine neurons discharged at approximately 8 Hz in single VTA grafts and below 1 Hz in the VTA part of VTA-accumbens double grafts. Cells in the accumbens portion of double grafts had shorter action potential durations and fired at 10–20 Hz. Haloperidol increased discharge frequency in VTA neurons with long action potential durations while apomorphine reduced discharge markedly. Antidromic activation of putative dopamine neurons in the VTA part was obtained by electrical stimulation of the accumbens part. The indirect dopamine agonist + 3-methyl-phencyclidine slowed firing rates of neurons in the accumbens part of double grafts. Taken together, the histochemical and the electrophysiological data show that the intraocular VTA-accumbens system retains several of its normal structural and functional characteristics. It is proposed that the isolated VTA-accumbens projection can be used as a model to study the cellular mechanism of action of stimulant and opiate drugs of abuse.     

Augmentation of ventral tegmental area stimulation-induced feeding by both stimulation and lesion of the contralateral ventral tegmental area in the rat

Unilateral lesions of the ventral tegmental area (VTA) facilitate behavioral responses (feeding and exploration) induced by electrical stimulation of the VTA in the contralateral hemisphere. It was hypothesized that this facilitation may result from a lesion-induced compensatory increase in dopamine transmission in the intact hemisphere. In the present study we tested on the functional level the hypothesis that the activity of bilateral mesocorticolimbic systems is inversely related. For this purpose we compared the effect of unilateral subthreshold activation with the effect of subsequent unilateral lesion of VTA on feeding response evoked by electrical stimulation of the contralateral VTA. In male Wistar rats implanted with bilateral VTA electrodes stimulation-induced feeding was tested in a latency to feed-stimulation frequency curve-shift paradigm. One electrode was used for induction of feeding reaction and the other electrode was used for concurrent stimulation (with the subthreshold current) and subsequent electrolytic lesioning of the contralateral VTA. It was found that both contralateral stimulation and subsequent lesion performed through the same electrode facilitated a feeding response that manifested as a decrease in the reaction's threshold and a leftward shift of the latency-frequency curve. The paradoxical similarity of the effects of the stimulation and lesion is discussed in terms of functional organization of the mesocorticolimbic system and adaptive changes in dopaminergic transmission.     

Direct monitoring of dopamine and 5-HT release in substantia nigra and ventral tegmental area in vitro

Fast-scan cyclic voltammetry with carbon fibre microelectrodes was used to detect endogenous dopamine (DA) and 5-hydroxytryptamine (5-HT) release from three distinct regions of guinea-pig mid-brain in vitro: rostral and caudal substantia nigra (SN) and the ventral tegmental area (VTA). Previous electrophysiological studies have demonstrated that cells of the caudal SN and the VTA have similar characteristics, whereas cells in the rostral SN have distinctly different properties. In the present study, we confirmed that each region has tyrosine hydroxylase-positive neurons and determined, using high-performance liquid chromatography, that DA levels were similar in rostral and caudal SN, but lower in SN than in VTA. In each region, application of veratrine, which was shown by intracellular recordings to have a reversible depolarising action, evoked a signal attributable to DA and distinguishable from that of 5-HT. Release signals were monitored every 250 ms with a spatial resolution of less than 50 m. DA release was calcium-dependent and was not detectable in a catecholamine-poor area such as the cerebellum, or in mid-brain tissue pre-treated with reserpine. Within the normal mid-brain, the amount of DA released was correlated with tissue content in that it was higher in the VTA than in either region of SN. It is concluded that DA released from somato-dendritic parts of mid-brain neurons exhibits site-specific variation. This is the first report of direct monitoring of DA and 5-HT relase from these regions with in situ electrodes and demonstrates the utility of fast-scan cyclic voltammetry to investigate the mechanisms and possible non-classical functions of somato-dendritic DA release.     

Decreased motivational properties of morphine in mouse models of cancerous- or inflammatory-chronic pain: implication of supraspinal neuropeptide FF(2) receptors

Our main purpose was to evaluate the influence of cancer pain on the rewarding properties of morphine. Opioids are very addictive when used by healthy persons, conversely the occurrence of an opioid addiction seems very low when patients suffering from cancer are treated with morphine. We investigated the reinforcing properties of morphine in the place preference paradigm on a new model of mice suffering from a cancer pain induced by syngenic melanoma cells injected in the hind paw. These data were compared with mice suffering either from a short-term- or a chronic-inflammatory pain induced respectively by injection of carrageenan or complete Freund's adjuvant. Remarkably, mice suffering from cancer pain or chronic inflammatory pain did not develop any preference for the environment associated with the injection of morphine. In mice injected with melanoma cells, the specific binding of [(125)I]EYWSLAAPQRF-NH(2), an agonist of neuropeptide FF(2) receptors, was increased in several brain areas involved in the rewarding properties of opiates, including the shell of the nucleus accumbens, the major islands of Calleja, the ventral endopiriform nucleus and the amygdaloid area. Our study is the first to reveal a modification of morphine rewarding properties under cancer pain in rodents. We postulate that anti-opioid neuropeptides might contribute to the suppression of morphine rewarding effects in this murine model of cancer pain.