Cerebral blood flow modulation by transcutaneous cranial electrical stimulation with Limoge's current.

OBJECTIVES: Transcutaneous cranial electrical stimulation (TCES) delivers a high-frequency (166 kHz) pulsed biphasic balanced current with a pulse repetition frequency of 100 Hz with 40% duty cycle through a negative electrode and two positive electrodes over the skull. TCES has a proven ability to potentiate anesthesia and analgesia, although the physiological mechanisms of this effect remain unclear. We hypothesized that the mechanism is a modulation of CBF in the central endogenous opioid system. This study aimed at determining the effects of TCES on CBF to elucidate its physiological mechanism. METHODS: Thirty-six healthy volunteers were randomly assigned to active or placebo TCES, and all assessments were double blind. TCES was performed using the Anesthelec™ device. In the stimulated group, an active cable was used, and in the control group (sham), the cable was inactive. CBF was measured by XeCT™ before and after two hours of TCES. RESULTS: Globally, CBF was unchanged by TCES. However, locally, TCES induced a significant CBF decrease in the brainstem and thalamus, which are structures involved in pain and anxiety (TCES and control CBF decrease were 18.5 and 11.9 mL/100g brain tissue/min, respectively). CONCLUSION: TCES can modulate local CBF but it has no effect on overall CBF. [Clinical Trials. gov number: NCT00273663].

Dopaminergic terminals in the rat cortex.

The destruction of ascending noradreniergic pathways by bilateral microinjections of 6-hydroxydopamnine made laterally to the pedunculus cerebellaris superior completely abolished the in vitro synthesis of [(3)H]norepinephrine from L-[(3)H]tyrosine in slices and in synaptosomes of the rat cortex. However, normal [(3)H]dopamine synthesis could still be observed in both cortical preparations from animals with lesions. These results provide the first biochemical support for the existence of dopaminergic terminals independent of noradrenergic terminals in the rat cortex.

Reactivity and plasticity in the amygdala nuclei during opiate withdrawal conditioning: differential expression of c-fos and arc immediate early genes.

Opiate withdrawal leads to the emergence of an aversive state that can be conditioned to a specific environment. Reactivation of these withdrawal memories has been suggested to be involved in relapse to drug-seeking of abstinent opiate addicts. Among the limbic areas that are likely to mediate these features of opiate dependence, amygdala nuclei represent critical neural substrates. Using a conditioned place aversion paradigm (CPA), we have previously shown specific opposite patterns of reactivity in the basolateral (BLA) and the central (CeA) amygdala, when comparing the experience of acute opiate withdrawal with the re-exposure to a withdrawal-paired environment. These data gave clues about the potential mechanisms by which amygdala nuclei may be involved in withdrawal memories. To extend these results, the present study aimed to assess the cellular reactivity and plasticity of amygdala nuclei during the opiate withdrawal conditioning process. For this, we have quantified c-fos and arc expression using in situ hybridization in rats, following each of the three conditioning sessions during CPA, and after re-exposure to the withdrawal-paired environment. BLA output neurons showed an increase in the expression of the plasticity-related arc gene during conditioning that was also increased by re-exposure to the withdrawal-paired environment. Interestingly, the CeA showed an opposite pattern of responding, and the intercalated cell masses (ITC), a possible inhibitory interface between the BLA and CeA, showed a persistent activation of c-fos and arc mRNA. We report here specific c-fos and arc patterns of reactivity in amygdala nuclei during withdrawal conditioning. These findings improve our understanding of the involvement of the amygdala network in the formation and retrieval of withdrawal memories. Plasticity processes within BLA output neurons during conditioning, may participate in increasing the BLA reactivity to conditioned stimuli, which could in turn (by the control of downstream nuclei) reinforce and drive the motivational properties of withdrawal over drug consumption.

Neural substrates of opiate withdrawal.

Drug withdrawal is an integral part of most types of dependence and, to a large extent, opiate withdrawal has been considered the prototypic, classic measure of opiate dependence. The opiate withdrawal syndrome is characterized by multiple behavioral and physiological signs such as behavioral activation, ptosis, diarrhea, 'wet dog' shakes and motivational dysfunction, which may be represented in the CNS at multiple sites. It seems that the activating effects associated with the opiate withdrawal syndrome may be mediated by the nucleus locus coeruleus. Other signs such as wet dog shakes may involve sites in the hypothalamus important for temperature regulation. Certain other signs such as diarrhea and lacrimation may be dependent on peripheral opiate receptors. The motivational aspects of opiate withdrawal as demonstrated by the aversive stimulus effects or negative reinforcing effects (e.g. disrupted lever-pressing for food and place aversions) may involve those elements of the nucleus accumbens that are known to be important for the acute reinforcing effects of opiates in nondependent rats. Evidence exists at the cellular and molecular level for both 'within-system' and 'between-system' adaptations to dependence. Elucidation of the neural networks, cellular mechanisms and molecular elements involved in opiate withdrawal may provide not only a model for our understanding of the adaptive processes associated with drug dependence but also of those associated with other chronic insults to CNS function.

Opponent process theory of motivation: neurobiological evidence from studies of opiate dependence.

One hypothetical model for a mechanism of drug dependence involves the development of an adaptive process that is initiated to counter the acute effects of the drug. This adaptive process persists after the drug has been cleared from the brain, leaving an opposing reaction unopposed (abstinence signs). From a motivational perspective a particularly attractive hypothesis has been that of opponent process theory (32). Here many reinforcers elicit positive affective and hedonic processes that are opposed by negative affective and hedonic processes. Thus the intense pleasure of the opiate drug "rush" or "high" would be opposed by aversive withdrawal symptoms. The present paper presents neurobiological evidence to support the opponent process concept and suggests neural circuitry that may be involved. The region of the nucleus accumbens in the forebrain of the rat has been shown to be a particularly sensitive substrate not only for the acute reinforcing properties of opiate drugs, but also for the response disruptive effects of opiate antagonists in opiate dependent rats. This region also appears to be particularly sensitive to the aversive stimulus effects of opiate antagonists using a place aversion measure in dependent rats. These results suggest that the region of the nucleus accumbens and its neural circuitry may be an important neural substrate for both the positive and negative motivational aspects of drug dependence.

Transcutaneous electrical stimulation with Limoge current potentiates morphine analgesia and attenuates opiate abstinence syndrome.

Transcutaneous electrostimulation is a somewhat controversial technique used in the management of the opiate withdrawal syndrome. We report an animal study of a particular transcutaneous electrostimulation called transcutaneous cranial electrostimulation, based on a technique used for many years on heroin addicts for the rapid severance of their addiction, which has been validated in a clinical setting by a double-blind trial. This technique involves the application of an intermittent high-frequency current (Limoge's current). Our experimental data show that this transcutaneous cranial electrostimulation increases morphine analgesia by threefold on the tail flick latency measure and produces a 48% attenuation of the abstinence syndrome observed after abrupt cessation of morphine administration. These results were obtained using a double-blind paradigm.

Chronic treatment with five different neuroleptics elicits behavioral supersensitivity to opiate infusion into the nucleus accumbens.

It has previously been demonstrated that direct opiate infusion into nucleus accumbens elicits psychomotor activation in rats. In the present study, the effects of chronic treatment with five different neuroleptics on this behavioral response were investigated. All neuroleptics tested (haloperidol, sulpiride, flupentixol decanoate, perphenazine enanthate, fluphenazine decanoate, palmitic ester of pipotiazine) induced a marked behavioral supersensitivity to intraaccumbens opiate infusion. A similarly enhanced sensitivity was observed in chronic reserpine-treated rats. The maximum sensitivity to opiates appeared 2-3 weeks after the beginning of neuroleptic treatment and was present up to 1 month after the end of treatment. Naloxone blocked the neuroleptic-induced enhanced response to opiates. It is concluded that chronic blockade of dopaminergic transmission results in considerable functional alterations of the endogenous opiate systems. The results are discussed in terms of possible underlying neuronal mechanisms, and important clinical implications are noted.

Small doses of apomorphine and chronic administration of d-amphetamine reduce locomotor hyperactivity produced by radiofrequency lesions of dopaminergic A10 neurons area.

The lesion of the ventral mesencephalic tegmentum region (VMT) induces a behavioral syndrome characterized mainly by locomotor hyperactivity and reduction of attention processes. Previous data indicated that this syndrome was due at least in part to the destruction of dopaminergic (DA) A10 neurons. In order to test this hypothesis, we examined the effects of two dopaminomimetics drugs: apomorphine (APO) and d-amphetamine (d-AMPH) on the VMT behavioral syndrome. The acute administration of very low doses of APO (30 microgram/KG; Sc) reduces the behavioral deficits; similarly a chronic administration of d-aMPH (two injections daily for 43 days) reduces locomotor hyperactivity. In these two cases, the lesioned rats activity reaches the control level. These results confirm the primary role of DA-A10 neurons in the VMT behavioral syndrome. Acute APO and chronic d-AMPH effects are discussed in terms of (i) reactivation of DA postsynaptic receptors disafferented after DA-A10 group destruction and (ii) strengthening of the hyperfunctioning of the remaining DA-A10 neurons. VMT-A10 syndrome could be a good animal model for pathophysiological studies.

Chronic flupentixol treatment potentiates the reinforcing properties of systemic heroin administration.

The behavioral effects of systemic heroin administration were examined in rats subjected to flupentixol impregnation prior to and during behavioral testing. In the first experiment, the dose of heroin required to produce a place preference was determined in two groups of rats, one of which had received chronic flupentixol decanoate (12 mg/kg, sc, every 10 days for 6 weeks) and the other which had received the palm oil vehicle during the same time period. It was found that whereas 60 micrograms/kg of heroin was required to produce a place preference in control rats, only 7.5 micrograms/kg was sufficient to do so in chronic neuroleptic-treated rats. In a second experiment, the locomotor activating effect of heroin was evaluated in two groups of rats that had been subjected to the same chronic regimen as in Experiment 1. Locomotor activity was enhanced in both groups following 120 micrograms/kg heroin, whereas 30 micrograms/kg was ineffective in either group. Finally, it was found that neuroleptic-treated, but not control, rats rapidly learned to self-administer intravenous infusions of an unusually low dose of heroin (4 micrograms) and to discriminate these infusions from vehicle infusions. Together, these data show that chronic dopamine (DA) receptor blockade produces a marked increase in the sensitivity to the reinforcing and discriminative stimulus properties of systemic heroin administration and that this increase is not attributable to heroin-induced locomotor activation. The results are discussed in terms of the role of DA systems in opiate reinforcement processes.

Idazoxan and 8-OH-DPAT modify the behavioral effects induced by either NA, or 5-HT, or dual NA/5-HT reuptake inhibition in the rat forced swimming test.

The rat forced swimming test (FST) predicts the efficacy of antidepressants, which decrease immobility duration in the test, and can distinguish selective serotonin (5-HT) and noradrenaline (NA) reuptake inhibitors, which, respectively, increase swimming and climbing behaviors. However, dual 5-HT and NA reuptake-inhibition produces climbing behavior solely, thereby suggesting with other data that the NA-system mediates inhibiting interactions on 5-HT-induced swimming in the FST. Since alpha(2)-adrenoreceptors and 5-HT(1A)-receptors have important regulatory functions and are involved in 5-HT/NA interactions, we examined whether the alpha(2)-receptor-antagonist idazoxan and the 5-HT(1A)-receptor-agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) would modify the behavioral pattern induced in the FST by either selective or non-selective antidepressant treatments. The rats were treated subacutely (3 injections IP over 48 h) with: (a) idazoxan (0.5-10 mg/kg) alone, and in combination with desipramine (10 mg/kg), or desipramine + fluoxetine (10/10 mg/kg), or the dual serotonin/noradrenaline reuptake-inhibitor milnacipran (20 mg/kg). (b) 8-OH-DPAT (0.25-1 mg/kg) alone, and in combination with either desipramine (10 mg/kg) or fluoxetine (10 mg/kg). The results indicated: (a) Idazoxan (0.5, 5, 10 mg/kg) produced no anti-immobility effects per se in the FST, antagonized the effects of the NA-reuptake-inhibitor desipramine, and allowed desipramine + fluoxetine, as well as milnacipran, to increase swimming behavior. (b) 8-OH-DPAT produced non-significant effects per se, potentiated desipramine-induced antidepressant-like effects on immobility and climbing, and both antagonized swimming and produced climbing behavior in combination with fluoxetine. Our data support clinical trials suggesting that alpha(2)-receptor-antagonists and 5-HT(1A)-receptor-agonists may be of interest in augmentation strategies for antidepressant treatments. The scoring of active behaviors in the FST appears to be an interesting tool for studying 5-HT/NA interactions induced by antidepressants, as well as for the testing of augmentation strategies.

Clonidine blocks acquisition but not expression of conditioned opiate withdrawal in rats.

Previous studies in rodents have reported that clonidine, an alpha 2-adrenergic receptor agonist, attenuated conditioned aversions to naloxone-precipitated opiate withdrawal when administered prior to each withdrawal conditioning episode. The current study was designed to determine whether clonidine could modify the expression of previously established conditioned place aversions and conditioned suppression of operant responding. Dose- and time-dependent effects of clonidine on activity and suppression of operant responding for food identified appropriate treatment parameters for subsequent studies in which rats rendered dependent on opiates through implantation of morphine pellets were tested for: (1) conditioned place aversion; and (2) conditioned suppression of operant responding for food (fixed ratio-15 schedule), in a paradigm wherein rats received four pairings of naloxone with a distinct tone and odor stimulus. Clonidine dose-dependently blocked the acquisition of both conditioned behaviors when administered prior to naloxone on each conditioning trial, but was ineffective in blocking the expression of these conditioned withdrawal signs when administered prior to the test session.

Behavioral expression of opiate withdrawal is altered after prefrontocortical dopamine depletion in rats: monoaminergic correlates.

The objective of this study was to establish the effects of prefrontocortical dopamine depletion on opiate withdrawal and prefrontocortical neurochemical changes elicited by morphine dependence and withdrawal. The dopaminergic content was also measured in the nucleus accumbens during withdrawal, in order to detect reactive changes induced by prefrontocortical lesion. Withdrawal was induced by naloxone in morphine-dependent rats. Monoamine levels were analyzed post-mortem by high performance liquid cromatography. The results showed that chronic morphine dependence did not modify basal levels of monoamines in sham rats, revealing neuroadaptation of prefrontocortical dopamine, noradrenaline and serotonin systems to chronic morphine. The neuroadaptive phenomenon remained after prefrontocortical lesion (> 79% dopamine depletion). On the other hand, a strong increase of dopamine, noradrenaline, and serotonin contents in the medial prefrontal cortex of sham rats was detected during opiate withdrawal. However, in lesioned rats, the increase of prefrontocortical dopamine and serotonin content, but not that of noradrenaline, was much lower. In the nucleus accumbens, prefrontocortical lesion reactively enhanced the dopaminergic tone and, although opiate withdrawal reduced dopaminergic activity in both sham and lesioned rats, this reduction was less intense in the latter group. At a behavioral level, some symptoms of physical opiate withdrawal were exacerbated in lesioned rats (writhing, mastication, teeth-chattering, global score) and exploration was reduced. The findings hence indicate that: (i) prefrontocortical monoaminergic changes play a role in the behavioral expression of opiate withdrawal; (ii) the severity of some withdrawal signs are related to the dopaminergic and serotonergic tone of the medial prefrontal cortex rather than to the noradrenergic one, and (iii) an inverse relationship between mesocortical and mesolimbic dopaminergic systems exists.

Effect of injections of 6-OHDA into either nucleus accumbens septi or frontal cortex on spontaneous and drug-induced activity.

The effect of injections of 6-hydroxydopamine (6-OHDA) into either frontal cortex (FCx) or nucleus accumbens (NAS) on spontaneous, amphetamine and apomorphine photocell cage activity was studied. Both lesions groups had significant noradrenaline depletion in frontal cortex but only the FCx group had significant dopamine depletion in frontal cortex. Whereas NAS 6-OHDA rats exhibited enhanced apomorphine- and decreased amphetamine-activity there were no differences in activity of the FCx group. 6-OHDA NAS rats also exhibited spontaneous hypoactivity on the third but not the seventh post-operative day; there were no differences in spontaneous activity on either days in the FCx group. In 1975 Kelly, Seviour and Iversen demonstrated that destruction of forebrain dopaminergic terminals induced with injection sof 6-hydroxydopamine (6-OHDA) into the nucleus accumbens septi (NAS) attenuated the locomotor response to 1.5 mg/kg d-amphetamine without affecting stereotypy seen at 5.0 mg/kg. In addition, these rats exhibited an enhanced locomotor response to the dopamine receptor agonist apomorphine, an effect thought to reflect receptor supersensitivity induced by dopamine denervation (Ungerstedt, 1971). Biochemical assay data revealed that dopamine levels were significantly reduced both in nucleus accumbens septi (NAS) and olfactory tubercle (OT) but not neostriatum; thus it was concluded that amphetamine- and apomorphine-induced locomotor activity is mediated by the mesolimbic dopamine system. Since then it has become clear that the A10 group of dopamine (DA) cells bodies in the ventral tegmental area (VTA), give rise to dopamine fibres which innervate not only NAS and OT, but also frontal cortex (Bjorklund and Lindvall, 1978).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence of a complete independence of the neurobiological substrates for the induction and expression of behavioral sensitization to amphetamine.

The repeated administration of amphetamine in rats produces behavioral sensitization which is characterized either by a progressive enhancement of the locomotor activity induced by the drug or by an enduring behavioral hypersensitivity to the drug after the cessation of the treatment. Some authors have suggested that the action of amphetamine at the level of the nucleus accumbens is responsible for the expression of behavioral sensitization, whereas the action of amphetamine at the level of the dopamine cell bodies in the ventral tegmental area induces some changes responsible for the initiation of the phenomenon. The present study fully tested this hypothesis. In two separate experiments, the effects of different doses of amphetamine repeatedly administered in the ventral tegmental area or in the nucleus accumbens were tested on the later behavioral reactivity to the administration of amphetamine in the nucleus accumbens. Independent groups of rats received five repeated administrations (one injection every other day) of different doses of amphetamine either in the ventral tegmental area (0, 1, 2.5, 5 micrograms/0.5 microliters per side) or in the nucleus accumbens (0, 1, 3, 10 micrograms/l microliters per side). Two days following the last intracerebral amphetamine injection, each group received a phosphate buffer solution challenge directly into the nucleus accumbens followed two days later by an amphetamine challenge (1 microgram/l microliters per side) in the nucleus accumbens and two days later by a peripheral challenge with amphetamine (0.5 mg/kg, s.c.). Locomotor responses were recorded following each injection. Results showed that injections of amphetamine into the nucleus accumbens induced a dose-dependent increase in locomotor activity which remained identical with the repetition of the injections. No difference between the different intra-accumbens pretreated groups was observed following the diverse phosphate-buffered saline solution and amphetamine challenges. In contrast, intra-ventral tegmental area administration of amphetamine did not produce any modification of locomotor activity. However, whereas no difference between the differently pretreated groups was observed following phosphate-buffered saline administration into the nucleus accumbens, a potentiation of the locomotor response to a challenge dose of amphetamine into the nucleus accumbens was observed which was dependent on the dose of amphetamine pretreatment into the ventral tegmental area. Similar potentiation was observed following peripheral challenge with amphetamine. Finally, cross-sensitization was observed when a challenge dose of cocaine (10 micrograms/1 microliter per side) was injected into the nucleus accumbens, as well as when a peripheral challenge dose of morphine (2.5 mg/kg, s.c.) was administered to the ventral tegmental area-pretreated groups.(ABSTRACT TRUNCATED AT 400 WORDS)

Nucleus accumbens and amygdala are possible substrates for the aversive stimulus effects of opiate withdrawal.

Specific brain sites for the opiate abstinence syndrome syndrome have been elusive to delineate, and the classic overt signs of withdrawal such as wet dog shakes, ptosis and teeth chattering appear to be widely represented in the brain. Using a more general motivational test involving a disruption of operant behavior in dependent rats, the brain site most sensitive to the response disruptive effects of intracerebral administration of the opiate antagonist, methylnaloxonium, was the region of the nucleus accumbens, a site also implicated in the acute reinforcing properties of opiates. This disruption of operant responding was hypothesized to reflect the aversive properties of opiate withdrawal. The present study directly tested that hypothesis by exploring whether intercerebral administration of methylnaloxonium produced aversive stimulus effects as measured by the formation of place aversions. Rats implanted intracerebroventricularly or with bilateral cannulae aimed at the medial dorsal thalamus, periaqueductal gray, ventral tegmental area, amygdala or nucleus accumbens were made dependent on morphine by subcutaneous implantation of two 75-mg morphine pellets. The animals were then subjected to place aversion training by pairing of a distinct environment (one of three arms of a three-armed box with distinct texture, markings and smell) with a single injection of methylnaloxonium intracerebroventricularly or intracerebrally. Results showed that at high doses of methylnaloxonium (1000-2000 ng) all sites produced a place aversion. However, lower doses (250-500 ng) produced a significant brain site selectivity with the region of the nucleus accumbens the most sensitive. Observational measurements taken during the postinjection period with the high dose of methylnaloxonium showed that agitation was particularly observed following methylnaloxonium administration into the nucleus accumbens and periaqueductal gray.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between neuropeptide FF and opioids in the ventral tegmental area in the behavioral response to novelty.

Considerable evidence has focused on the interaction between endogenous opioid peptides and the dopaminergic mesocorticolimbic system in behavioral responses to stress. Recently, it has been proposed that the CNS synthesizes and secretes neuropeptides that act as part of a homeostatic system to attenuate the effects of morphine or endogenous opioid peptides. Among these antiopioids, neuropeptide FF (NPFF) is particularly interesting since both NPFF immunoreactive-like terminals and NPFF binding sites are located in the vicinity of the dopaminergic cell bodies within the ventral tegmental area (VTA) suggesting an interaction at this level. The purpose of the present study was to evaluate the respective implication of opioid and antiopioid peptides at the level of the VTA in the locomotor response to novelty in rats. The results indicate that s.c. naloxone pretreatment, an opiate receptor antagonist, reduced locomotor activity in rats placed in a novel environment without having any effect in a familiar environment. This effect takes place in the VTA since intra-VTA administration of naloxone methobromide diminished similarly and dose-dependently the motor response to novelty. This effect is mainly dependent on opioid peptides released at VTA level since local injections of thiorphan, an inhibitor of enkephalin degradation, strongly increased locomotor response to novelty and this effect is completely prevented by the co-administration of naloxone methobromide. When injected in the VTA, NPFF is acting as an antiopioid compound, i.e. it reduces the locomotor activity triggered by exposure to novelty to the level recorded in a familiar environment. Moreover, NPFF decreased dose-dependently the potentiation of novelty-induced locomotor response produced by VTA injection of thiorphan. Taken together, these results suggest that NPFF neurons may participate at the level of the VTA to a homeostatic regulating process counteracting opioid effects induced by a mild stress such as novelty.

D-amphetamine-induced behavioral sensitization: implication of a glutamatergic medial prefrontal cortex-ventral tegmental area innervation.

Behavioral sensitization to amphetamine is expressed as a progressive enhancement of the behavioral activating effects of the drug when repeated injections are performed as well as a long-lasting hypersensitivity to later environmental or pharmacological challenges. The mesoaccumbens dopamine system has been proposed to be the major candidate so far responsible for the induction and expression of this process, which are dependent on the action of amphetamine in the ventral tegmental area and nucleus accumbens, respectively. The development of this process has been proposed to be the result of an interaction between somatodendritically released dopamine and dopaminergic D1 receptors localized on different inputs to the ventral tegmental area, including glutamate afferents arising in part from mesocorticolimbic areas such as the medial prefrontal cortex and the amygdala. Three groups of experiments were designed to test the role of each of these components in the behavioral sensitization to amphetamine. First, the intervention of the glutamatergic transmission of the ventral tegmental area in the induction of sensitization to amphetamine was tested. The effects of an N-methyl-D-aspartate antagonist, 3-(R-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid. on the behavioral sensitization induced by amphetamine administered repeatedly in the ventral tegmental area was tested. It was found that the blockade of N-methyl-D-aspartate receptors with 3-(R-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid coadministered with amphetamine in the ventral tegmental area dose-dependently prevented the induction of sensitization. In a second step, the role of the structures which send glutamatergic inputs to the ventral tegmental area in the process of behavioral sensitization was tested. We evaluated the effects of ibotenic acid lesion of the medial prefrontal cortex and the amygdala on behavioral sensitization induced by peripheral or intra-ventral tegmental area administration of amphetamine. We found that ibotenic acid lesion of the medial prefrontal cortex blocked the behavioral sensitization induced by both intra-ventral tegmental area and peripheral treatment with amphetamine. In contrast, ibotenic acid lesion of the amygdala produced no effect on behavioral sensitization induced peripherally or centrally. These experiments confirmed (i) that the ventral tegmental area, where dopaminergic cell bodies are located, is a critical site for the induction of behavioral sensitization, (ii) that this process implicates the glutamatergic transmission in the ventral tegmental area, and (iii) that the medial prefrontal cortex is crucially implicated merely because of its direct glutamatergic inputs on to ventral tegmental area neurons. Together, these results reinforce the view that the behavioral sensitization to amphetamine implicates not only the mesoaccumbens dopaminergic neurons, but also other structures of the mesocorticolimbic system, such as the medial prefrontal cortex and more specifically its glutamatergic component.

The cytology of dopaminergic and nondopaminergic neurons in the substantia nigra and ventral tegmental area of the rat: a light- and electron-microscopic study.

The results of this study support the conclusion that dopaminergic cells can be distinguished from non-dopaminergic cells, at both the light- and electron-microscopic level, by cytological features, and particularly by the pattern of Nissl substance. In both the substantia nigra and the ventral tegmental area, two main categories of cell type can be identified in Nissl preparations: (1) dark-staining, basophilic cells with large masses of Nissl substance and (2) light-staining cells with more translucent cytoplasm. The following findings provide evidence that the basophilic cells of both substantia nigra and ventral tegmental area are the dopaminergic cells. (1) There is a good correlation between the topographic distribution of basophilic cells and that of dopaminergic cells mapped by both histofluorescence and immunohistochemical methods. (2) After unilateral destruction of the dopaminergic neurons by intracerebral injection of 6-hydroxydopamine in the dopaminergic pathway, the basophilic cells in the substantia nigra and ventral tegmental area disappeared on the lesion side, while the lighter-staining cells appeared unaffected. (3) In normal rats, and in rats with unilateral 6-hydroxydopamine lesions, intraventricular injection of [3H]norepinephrine was used for specific labeling of dopaminergic neurons. In autoradiograms of semithin sections, such labeling was observed only in dark-staining and not in light-staining cells, and in cases of unilateral 6-hydroxydopamine lesion was totally absent on the lesion side. Electron-microscopy showed much of the cytoplasm of the basophilic dopaminergic cells to be densely filled with free ribosomes associated with large, well organized complexes of rough endoplasmic reticulum. The cytoplasm of the light, non-dopaminergic cells contains only sparse free ribosomes and small, widely spaced aggregates of rough endoplasmic reticulum. Both cell types occur in a similar variety of size and shape.

Neuropeptide FLFQRFamide receptors within the ventral mesenchephalon and dopaminergic terminal areas: localization and functional antiopioid involvement.

The neuropeptide FLFQPQRF amide is a FMRFamide-like peptide with some anti-opiate properties. Neuropeptide FLFQPQRFamide receptors are present in the mammalian central nervous system and have been clearly identified as different from opiate receptors. Autoradiography has revealed neuropeptide FLFQPQRFamide receptor localization within the ventral mesencephalon, where opiate receptors are also located. In order to delineate anatomical localization of neuropeptide FLFQPQRFamide receptors, we used selective chemical lesions of dopaminergic cells and intrinsic perikarya of the ventral mesencephalon, coupled with in vitro autoradiographic techniques. We show that: (i) unilateral lesions of dopaminergic perikarya produced by 6-hydroxydopamine did not affect either ipsi or contralateral neuropeptide FLFQPQRFamide receptor density within the mesencephalon; (ii) unilateral lesions of intrinsic perikarya by ibotenic acid injected into the ventral tegmental area produced a significant reduction of neuropeptide FLFQPQRFamide receptors (40%) in this region; (iii) in the substantia nigra pars compacta, ibotenic acid unilateral lesions did not affect the density of neuropeptide FLFQPQRF-amide receptors; (iv) unilateral 6-hydroxydopamine or ibotenic acid lesions failed to affect neuropeptide FLFQPQRFamide binding in the dopaminergic projection areas. These results suggest that, like opiate receptors, the neuropeptide FLFQPQRFamide binding sites are not localized on dopaminergic neurons but are distributed on both soma of non dopaminergic cells in the ventral tegmental area and on fibers afferent to the ventral tegmental area and substantia nigra pars compacta. To evaluate the possibility that the stimulation of neuropeptide FLFQPQRFamide receptors may affect the opioid modulation of mesocorticolimbic dopaminergic neuron activity, we tested the effects of neuropeptide FLFQPQRFamide ventral tegmental area infusion (0.25-10 micrograms) on the behavioral activation induced by intra-ventral tegmental area morphine infusion. We observed that in the ventral tegmental area, the stimulation of neuropeptide FLFQPQRFamide receptors inhibits morphine-induced locomotor hyperactivity. These results suggest that in the ventral tegmental area, neuropeptide FLFQPQRFamide may participate, via an indirect mechanism, to the control of the mesocorticolimbic dopaminergic system activity by counteracting the effect of opioids.

Ventral tegmental area infusion of substance P, neurotensin and enkephalin: differential effects on feeding behavior.

The neuropeptides substance P, neurotensin and [Met]enkephalin are found in the ventral tegmental area, site of the A10 dopamine cell bodies. Evidence suggests a functional interaction between these peptides and the dopaminergic neurons. All three peptides have been shown to exert an activating effect on these neurons. The present study analyzed the effects of ventral tegmental area infusion of neurotensin, substance P and D-ala-[Met]enkephalin on feeding behavior. These effects were studied in both food-deprived and satiated rats. During a 30 min test, the following parameters were registered: latency to eat, total food intake, food spillage, number of eating bouts and duration of eating. Similar measures were taken for drinking. In deprived rats substance P (0.5, 3.0 micrograms) increased latency to eat but did not affect other parameters, and substance P did not affect eating in satiated rats. Neurotensin (0.5, 2.5 micrograms) increased latency to eat and markedly reduced food consumption in deprived rats and had no effect in satiated rats. D-Ala-[Met]enkephalin (0.1, 1.0 micrograms) stimulated feeding behavior in both deprived and satiated rats. These results show that although the different peptides are presumed to activate the dopaminergic A10 neurons, their effects on feeding behavior can be differentiated. The findings are discussed in terms of motor and motivational mechanisms, and the relative contributions of specific and non-specific influences on feeding are considered.