Optogenetic Activation of the Basolateral Amygdala Promotes Both Appetitive Conditioning and the Instrumental Pursuit of Reward Cues.

Reward-associated stimuli can both evoke conditioned responses and acquire reinforcing properties in their own right, becoming avidly pursued. Such conditioned stimuli (CS) can guide reward-seeking behavior in adaptive (e.g., locating food) and maladaptive (e.g., binge eating) ways. The basolateral amygdala (BLA) regulates conditioned responses evoked by appetitive CS, but less is known about how the BLA contributes to the instrumental pursuit of CS. Here we studied the influence of BLA neuron activity on both behavioral effects. Water-restricted male rats learned to associate a light-tone cue (CS) with water delivery into a port. During these Pavlovian conditioning sessions, we paired CS presentations with photo-stimulation of channelrhodopsin-2 (ChR2)-expressing BLA neurons. BLA photo-stimulation potentiated CS-evoked port entries during conditioning, indicating enhanced conditioned approach and appetitive conditioning. Next, new rats received Pavlovian conditioning without photo-stimulation. These rats then received instrumental conditioning sessions where they could press an inactive lever or an active lever that produced CS presentation, without water delivery. Rats pressed more on the active versus inactive lever, and pairing CS presentation with BLA-ChR2 photo-stimulation intensified responding for the CS. This suggests that BLA-ChR2 photo-stimulation enhanced CS incentive value. In a separate experiment, rats did not reliably self-administer BLA-ChR2 stimulations, suggesting that BLA neurons do not carry a primary reward signal. Last, intra-BLA infusions of d-amphetamine also intensified lever-pressing for the CS. The findings suggest that BLA-mediated activity facilitates CS control over behavior by enhancing both appetitive Pavlovian conditioning and instrumental pursuit of CS.SIGNIFICANCE STATEMENT Cues paired with rewards can guide animals to valuable resources such as food. Cues can also promote dysfunctional reward-seeking behavior, as in overeating. Reward-paired cues influence reward seeking through two major mechanisms. First, reward-paired cues evoke conditioned anticipatory behaviors to prepare for impending rewards. Second, reward-paired cues are powerful motivators and they can evoke pursuit in their own right. Here we show that increasing neural activity in the basolateral amygdala enhances both conditioned anticipatory behaviors and pursuit of reward-paired cues. The basolateral amygdala therefore facilitates cue-induced control over behavior by both increasing anticipation of impending rewards and making reward cues more attractive.

Sensitization to amphetamine psychostimulant effect: A key role for ventral tegmental area neurotensin type 2 receptors and MAP kinase pathway.

Neurotensin is an endogenous neuropeptide that acts as a potent modulator of ventral tegmental area (VTA) neurotransmission. The present study was aimed at determining VTA cell population and neurotensin receptor subtype responsible for the initiation of amphetamine-induced psychomotor activity and extracellular signal-regulated kinases (ERK1/2) sensitization. During an induction phase, rats were injected intra-VTA on two occasions, every second day, with [D-Tyr11 ]-neurotensin (D-Tyr-NT), SR142948 (a mix Ntsr1/Ntsr2 receptor subtype antagonist), SR48692 (a Ntsr1 antagonist), D-Tyr-NT + SR142498, D-Tyr-NT + SR48692, or the vehicle. Effects of intra-VTA drugs were evaluated at locomotor activity and ERK1/2 phosphorylation. Five days after the last VTA microinjection, the effect of a systemic injection of amphetamine was tested (sensitization test). Results show that D-Tyr-NT stimulated locomotor activity during the induction phase, an effect that was blocked by SR142948, but not SR48692. Amphetamine also induced significantly higher ambulatory activity in rats preinjected with D-Tyr-NT than in rats preinjected with the vehicle. This sensitization effect was again attenuated by SR142948, but not SR48692, hence suggesting that this effect is mediated by Ntsr2 receptors. To confirm this, we tested a highly selective Ntsr2 peptide-peptoid hybrid ligand, NT150. At the concentration tested, NT150 stimulated locomotor activity and lead to sensitized locomotor activity and a selective neurochemical (pERK1/2) response in tyrosine hydroxylase-positive neurons of the VTA. Both effects were prevented by SR142948. Taken together, these results show that neurotensin, acting on Ntsr2 receptor subtypes, stimulates locomotor activity and initiates neural changes (ERK1/2 phosphorylation) that lead to amphetamine-induced sensitization.

Optogenetic activation of basolateral amygdala-to-nucleus accumbens core neurons promotes Pavlovian approach responses but not instrumental pursuit of reward cues.

Reward-associated conditioned stimuli (CSs) can acquire predictive value, evoking conditioned approach behaviours that prepare animals to engage with forthcoming rewards. Such CSs can also acquire conditioned reinforcing value, becoming attractive and pursued. Through their conditioned effects, CSs can promote adaptive (e.g., locating food) but also maladaptive behaviours (e.g., drug use). Basolateral amygdala neurons projecting to the nucleus accumbens core (BLA→NAc core neurons) mediate the response to appetitive CSs, but the extent to which this involves effects on the predictive and/or conditioned reinforcing properties of CSs is unclear. Thus, we examined the effects of optogenetic stimulation of BLA→NAc core neurons on i) CS-triggered approach to the site of reward delivery, a Pavlovian conditioned approach response and ii) the instrumental pursuit of a CS, a measure of conditioned reinforcement. Water-restricted, adult male rats learned that a light-tone compound cue (the CS) predicts water delivery into a receptacle. Pairing optogenetic stimulation of BLA→NAc core neurons with CS presentation potentiated CS-triggered water receptacle visits. This suggests that activity in BLA→NAc core neurons promotes Pavlovian goal-approach behaviour. Next, rats could lever press for CS presentations, without water delivery. Optogenetic stimulation of BLA→NAc core neurons either during instrumental test sessions or during prior CS-water conditioning did not influence lever responding for the CS. This suggests that activity in BLA→NAc core neurons does not influence the instrumental pursuit of a water-paired CS. We conclude that activation of BLA→NAc core neurons promotes cue-induced control over behaviour by increasing conditioned goal-approach responses, without affecting the operant pursuit of reward cues.

Dorsal raphe stimulation relays a reward signal to the ventral tegmental area via GluN2C NMDA receptors.

BACKGROUND: Glutamate relays a reward signal from the dorsal raphe (DR) to the ventral tegmental area (VTA). However, the role of the different subtypes of N-methyl-D-aspartate (NMDA) receptors is complex and not clearly understood. Therefore, we measured NMDA receptors subunits expression in limbic brain areas. In addition, we studied the effects of VTA down-regulation of GluN2C NMDA receptor on the reward signal that arises from DR electrical stimulation. METHODS: Using qPCR, we identified the relative composition of the different Grin2a-d subunits of the NMDA receptors in several brain areas. Then, we used fluorescent in situ hybridization (FISH) to evaluate the colocalization of Grin2c and tyrosine hydroxylase (TH) mRNA in VTA neurons. To assess the role of GluN2C in brain stimulation reward, we downregulated this receptor using small interfering RNA (siRNA) in rats self-stimulating for electrical pulses delivered to the DR. To delineate further the specific role of GluN2C in relaying the reward signal, we pharmacologically altered the function of VTA NMDA receptors by bilaterally microinjecting the NMDA receptor antagonist PPPA. RESULTS: We identified GluN2C as the most abundant subunit of the NMDA receptor expressed in the VTA. FISH revealed that about 50% of TH-positive neurons colocalize with Grin2c transcript. siRNA manipulation produced a selective down-regulation of the GluN2C protein subunit and a significant reduction in brain stimulation reward. Interestingly, PPPA enhanced brain stimulation reward, but only in rats that received the nonactive RNA sequence. CONCLUSION: The present results suggest that VTA glutamate neurotransmission relays a reward signal initiated by DR stimulation by acting on GluN2C NMDA receptors.

Dopaminergic mechanisms underlying the expression of antipsychotic-induced dopamine supersensitivity in rats.

Antipsychotic treatment can produce a dopamine-supersensitive state, potentiating the response to dopamine receptor stimulation. In both schizophrenia patients and rats, this is linked to tolerance to ongoing antipsychotic treatment. In rodents, dopamine supersensitivity is often confirmed by an exaggerated psychomotor response to d-amphetamine after discontinuation of antipsychotic exposure. Here we examined in rats the dopaminergic mechanisms mediating this enhanced behavioural response, as this could uncover pathophysiological processes underlying the expression of antipsychotic-evoked dopamine supersensitivity. Rats received 0.5 mg/kg/day haloperidol via osmotic minipump for 2 weeks, before treatment was discontinued. After cessation of antipsychotic treatment, rats showed a supersensitive psychomotor response to the D2 agonist quinpirole, but not to the D1 partial agonist SKF38393 or the dopamine reuptake blocker GBR12783. Furthermore, acute D1 receptor blockade (using SCH39166) decreased the exaggerated psychomotor response to d-amphetamine in haloperidol-pretreated rats, whereas acute D2 receptor blockade (using sulpiride) enhanced it. Thus, after discontinuation of antipsychotic treatment, D1- and D2-mediated transmission differentially modulate the expression of a supersensitive response to d-amphetamine. This supersensitive behavioural response was accompanied by enhanced GSK3ß activity and suppressed ERK1/2 activity in the nucleus accumbens (but not caudate-putamen), suggesting increased mesolimbic D2 transmission. Finally, after discontinuing haloperidol treatment, neither increasing ventral midbrain dopamine impulse flow nor infusing d-amphetamine into the cerebral ventricles triggered the expression of already established dopamine supersensitivity, suggesting that peripheral effects are required. Thus, while dopamine receptor-mediated signalling regulates the expression of antipsychotic-evoked dopamine supersensitivity, a simple increase in central dopamine neurotransmission is insufficient to trigger this supersensitivity.

Antipsychotic agents for the treatment of substance use disorders in patients with and without comorbid psychosis.

Substance dependence has serious negative consequences upon society such as increased health care costs, loss of productivity, and rising crime rates. Although there is some preliminary evidence that atypical antipsychotic agents may be effective in treating substance dependence, results have been mixed, with some studies demonstrating positive and others negative or no effect. The present study was aimed at determining whether this disparity originates from that reviewers separately discussed trials in patients with (DD) and without (SD) comorbid psychosis. Using electronic databases, we screened the relevant literature, leaving only studies that used a randomized, double-blind, placebo-controlled or case-control design that had a duration of 4 weeks or longer. A total of 43 studies were identified; of these, 23 fell into the category of DD and 20 into the category of SD. Studies in the DD category suggest that atypical antipsychotic agents, especially clozapine, may decrease substance use in individuals with alcohol and drug (mostly cannabis) use disorders. Studies in the SD category suggest that atypical antipsychotic agents may be beneficial for the treatment of alcohol dependence, at least in some subpopulations of alcoholics. They also suggest that these agents are not effective at treating stimulant dependence and may aggravate the condition in some cases.

Modulation of brain stimulation reward and locomotor activity by ionotropic glutamate receptors of the tail of the ventral tegmental area.

The rostromedial tegmental nucleus also referred to as the tail of the ventral tegmental area (tVTA) contains a cluster of gamma-aminobutyric acid (GABA)ergic neurons that receive dense glutamatergic afferents from the lateral habenula (LHb), and project to dopamine (DA) neurons of the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). In light of previous evidence implicating glutamate transmission in the regulation of midbrain DA neuronal activity, we first assessed the impact of intra-tVTA microinjection of NBQX (0.8 nmol/side) and PPPA (0.825 nmol/side), respectively AMPA and NMDA receptor antagonists, on reward induced by intracranial self-stimulation (ICSS) and on locomotor activity. Since the tVTA contains a large concentration of mu opioid receptors, additional measures were obtained following microinjection of endomorphin-1 (EM-1, 1 nmol/side) to confirm tVTA placements. Then, using small interfering RNAs (siRNAs), we tested the effect of tVTA downregulation of the GluN1 subunit of the NMDA receptor on reward and locomotor activity. Results show that NBQX, PPPA and EM-1 all enhance reward and locomotor activity, effects that were of different magnitude in rostral and intermediate parts of the tVTA. On the other hand, a reduction in GluN1 subunits used a marked decrease in operant responding for ICSS, but failed to alter ICSS reward and the reward-enhancing effect of PPPA. Our results support a role for the tVTA as a main inhibitory component of DA-dependent behavioral measures, and suggest that tVTA NMDA receptors that modulate reward are most likely expressed on tVTA afferent terminals.

Neurotensin modulation of spontaneous EPSCs in the nucleus accumbens of Lewis and Fischer 344 rats.

Fischer 344 (F344) and Lewis (LEW) rats are inbred strains that are differentially sensitive to drugs of abuse and that respond differently to the endogenous neuropeptide neurotensin (NT). To understand the mechanisms involved we used whole cell patch clamp recording technique to study the effects of an equimolar concentration of NT and its active analog, d-Tyr[11]neurotensin (d-NT), on the amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in nucleus accumbens medium spiny (MS) neurons in brain slices. NT and d-NT produced an increase in the amplitude but not in the frequency of sEPSCs in all neurons tested in both F344 and LEW rats. In LEW rats, NT and d-NT produced an increase in sEPSCs of the same magnitude. In contrast, in F344 rats, d-NT produced an increase in sEPSCs that was 2.4 times larger than that of NT. Moreover, the effect of d-NT in F344 rats was also significantly larger than that measured in LEW rats whereas NT produced an effect of the same magnitude in both strains. These results demonstrate that MS neurons in F344 rats are more responsive to the activation of NT receptors sensitive to d-NT than LEW animals. This finding parallels previous behavioral data and provides additional evidence that the NT circuitry differs in the two strains, in a brain region known to play a key role in the rewarding effects of drugs of abuse.

Evidence for a role of endogenous neurotensin in the development of sensitization to the locomotor stimulant effect of morphine.

This experiment was aimed at exploring the role of endogenous neurotensin in the development of sensitization to the locomotor stimulant effect of morphine. During the induction phase (Days 1, 3, 5 and 7), male Long-Evans adult rats were treated with the neurotensin antagonist SR-48692 (160, 320 or 640 microg/kg, i.p.) or its vehicle, followed by morphine (5.0 mg/kg, i.p.) or its vehicle, and their locomotor activity (ambulatory, non-ambulatory and vertical activity) was measured for 2 h. One week after the last injection, each group received a single injection of morphine (2.5 mg/kg, i.p.) and their locomotor activity was again measured for 2 h (sensitization test, day 14). Results show that SR-48692 alone did not change locomotion. Morphine stimulated locomotor activity, an effect that was stronger on day 7 than on day 1. The two higher doses of SR-48692 attenuated the acute stimulant effect of morphine and prevented the observed increase from day 1 to day 7. The sensitization test on day 14 showed that rats pre-treated with morphine alone displayed significantly stronger ambulatory and vertical activity than vehicle pre-treated rats, a sensitization effect that was attenuated by SR-48692. The present results suggest that endogenous neurotensin contributes to the acute locomotor stimulant effect of morphine and to the induction of its sensitization.

Blockade of 5-HT2a receptors reduces haloperidol-induced attenuation of reward.

Previous studies have shown that effective antipsychotic medications attenuate reward, an effect that is generally attributed to their effectiveness at blocking the dopamine D2-like receptors. As blockade of the serotonin type 2a (5-HT2a) receptors is a common property of the newer antipsychotics, the present study compared the effect of haloperidol, clozapine, and M100907 (a selective 5-HT2a antagonist) and the combined effect of haloperidol and M100907 treatment on brain stimulation reward (BSR). Experiments were performed on male Sprague-Dawley rats trained to produce an operant response to obtain electrical stimulation in the lateral hypothalamus. Measures of reward threshold were determined in different groups of rats using the curve-shift method using fixed current intensity and variable frequency before and at different times after injection of haloperidol (0.01, 0.05, 0.1, and 0.25 mg/kg), clozapine (1, 7.5, 15, and 30 mg/kg), M100907 (0.033, 0.1, and 0.3 mg/kg), or their vehicle. The effect of M100907 (0.3 mg/kg) on the attenuation of BSR by a sub- and suprathreshold dose of haloperidol was studied in another group of rats. Clozapine produced a dose-orderly increase in reward threshold with a mean maximal increase of 50%; at high doses, clozapine induced cessation of responding in several animals at different time periods. Haloperidol induced a dose-dependent increase in reward threshold, with the mean maximal increase (75%) being observed at the highest dose; it also produced a dose-dependent reduction of maximum rates of responding. M100907 failed to alter reward at any of the doses tested and had no effect on the subthreshold dose (0.01 mg/kg) of haloperidol. But when combined with a suprathreshold dose of haloperidol, M100907 reduced the reward-attenuating effect of haloperidol. These results show that 5-HT2a receptors are unlikely to constitute a component of the reward-relevant pathway activated by lateral hypothalamic stimulation. However, blockade of 5-HT2a receptors may account for the relatively lower level of reward attenuation produced by clozapine, and predict that antipsychotic medications that have a high affinity for the 5-HT2a receptor may be less likely to induce dysphoria.

Increased striatal gray matter densities in patients with schizophrenia and substance use disorder: a voxel-based morphometry study.

We sought to investigate the link between substance abuse and increased striatal gray matter densities (GMD) in schizophrenia, using voxel-based morphometry (VBM). Increased striatal GMD were found in patients with schizophrenia and substance use disorder (n=12), but not schizophrenia only patients (n=11), compared to healthy volunteers (n=15).

Repeated ventral midbrain neurotensin injections sensitize to amphetamine-induced locomotion and ERK activation: A role for NMDA receptors.

Previous studies have shown that activation of ventral midbrain NMDA receptors is required to initiate sensitization by amphetamine. In view of the recent evidence that neurotensin modulates ventral midbrain glutamate neurotransmission, we tested the hypothesis that neurotensin is acting upstream to glutamate to initiate sensitization to the behavioral and neurochemical effects of amphetamine. During a first testing phase, adult male rats implanted with bilateral ventral midbrain cannulae were injected every second day for three days with D-[Tyr11]neurotensin (1.5 nmol/side), the preferred NMDA GluN2A/B antagonist, CPP (40 or 120 pmol/side), the selective GluN2B antagonist, Ro04-5595 (200 or 1200 pmol/side), CPP (40 or 120 pmol/side) + D-[Tyr11]neurotensin (1.5 nmol/side) or Ro04-5595 (200 or 1200 pmol/side) + D-[Tyr11]neurotensin (1.5 nmol/side) and locomotor activity was measured immediately after the injection. Five days after the last central injection, the locomotor response or the expression of phosphorylated extracellular signal-regulated kinases 1/2 (pERK1/2) in neurons of different limbic nuclei was measured following a systemic injection of amphetamine sulfate (0.75 mg/kg, i.p.). Results show that amphetamine induced significantly stronger locomotor activity and pERK1/2 expression in the nucleus accumbens shell and infralimbic cortex in neurotensin pre-exposed animals than in controls (vehicle pre-exposed). These sensitization effects initiated by neurotensin were prevented by CPP, but not Ro04-5595. These results support the hypothesis that neurotensin is stimulating glutamate neurotransmission to initiate neural changes that sub-serve amphetamine sensitization and that glutamate is acting on NMDA receptors that are mostly likely composed of GluN2A, but not GluN2B, subunits. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Neurotensin in the nucleus accumbens reverses dopamine supersensitivity evoked by antipsychotic treatment.

Long-term exposure to antipsychotics like haloperidol can increase sensitivity to dopamine agonist stimulation. This could contribute to treatment failure and increase relapse to psychosis. Chronic antipsychotic treatment elevates neurotensin levels in the nucleus accumbens (NAc), where the neuropeptide modulates dopamine function by signalling through NTS1 receptors. We hypothesized that increasing neurotensin activity in the NAc attenuates the expression of antipsychotic-induced dopamine supersensitivity, which is indicated by a potentiated psychomotor response to amphetamine. Rats received either continuous (CONT-HAL; achieved via subcutaneous osmotic minipump) or intermittent (INT-HAL; achieved via daily subcutaneous injection) haloperidol treatment for 16-17 days. Three to 5 days later, we injected neurotensin into the NAc and measured amphetamine-induced locomotion. Only CONT-HAL rats showed potentiated amphetamine-induced locomotion, indicating dopamine supersensitivity. Compared to intra-NAc saline, intra-NAc neurotensin suppressed amphetamine-induced locomotion in CONT-HAL rats, but not in INT-HAL or control rats. In a new cohort of CONT-HAL and INT-HAL rats, we measured striatal levels of proneurotensin mRNA and NTS1 receptors. The two treatments led to overlapping but also distinct neurochemical profiles. Neither treatment altered NTS1 receptor levels in the NAc, but both increased proneurotensin mRNA levels in the NAc core. In the caudate-putamen, only INT-HAL increased NTS1 receptor levels, while only CONT-HAL increased proneurotensin mRNA expression. Thus, antipsychotic-induced dopamine supersensitivity enhances the ability of neurotensin in the NAc to regulate dopamine-mediated behaviours, and this likely does not involve changes in local levels of NTS1 receptors or proneurotensin mRNA. We conclude that increasing neurotensin activity could be considered to attenuate antipsychotic-induced dopamine supersensitivity.

Chronic fluoxetine rescues changes in plasma membrane density of 5-HT1A autoreceptors and serotonin transporters in the olfactory bulbectomy rodent model of depression.

Reduced serotonin (5-HT) neurotransmission is postulated to underlie the pathogenesis of depression. The serotonin transporter (SERT) and 5-HT1A auto-receptors act in concert to ensure homeostasis of serotonin (5-HT) neurotransmission and regulation of their cell surface expression represent efficient mechanisms to maintain this homeostasis. Thus, we investigated the changes in the subcellular distribution of SERT and 5-HT1A receptors (5-HT1AR) in the rat olfactory bulbectomy model of depression using immuno-gold labeling and electron microscopy, and examined the effect of chronic treatment with the antidepressant, fluoxetine, a serotonin reuptake inhibitor, on the subcellular distribution of SERT and 5-HT1AR. The density of plasma membrane labeling of 5-HT1A auto-receptors on dendrites of dorsal raphe neurons was increased after bulbectomy, but the 5-HT1A hetero-receptor membrane labeling on dendrites of CA3 hippocampal neurons was not. The density of membrane labeling of SERTs was increased both in dendrites of dorsal raphe neuron and axon terminals in the hippocampus after bulbectomy. However, the proportion of 5-HT1AR and SERT membrane labeling relative to total labeling was unchanged, suggesting an increase in protein levels. The increases in 5-HT1AR and SERTs membrane labeling induced by bulbectomy were reversed by chronic fluoxetine treatment, and these changes were associated with a reduction in the relative proportion of membrane versus total labeling, consistent with a protein shift between subcellular compartments. Our findings support the hypothesis that changes in efficacy of serotonergic neurotransmission in this model of depression depends on both activity and density of cell surface-expressed SERT and 5-HT1A auto-receptors.

Cross-fostering does not alter the differential sensitivity of Fischer and Lewis rats to central neurotensin-induced locomotion and hypothermia.

A cross-fostering paradigm was used to determine whether the differential locomotor and hypothermic responses to neurotensin (NT) in Fischer (F344) and Lewis (LEW) rats are mediated by the post-natal environment. From post-natal day (PD) 1 to PD 21, male pups from each strain were assigned to a same-strain dam (in-fostered) or were cross-fostered, and at adulthood were implanted with a guide cannula over the lateral ventricle. They were then tested for locomotion and hypothermia following injection of vehicle, 0.18, 1.8 or 18nmol of NT or D-Tyr([11])NT. In-fostered LEW, but not F344, displayed a strong dose-orderly hypothermic response to NT and to D-Tyr([11])NT while in-fostered F344, but not LEW, rats displayed strong locomotor responses to D-Tyr([11])NT. Cross-fostering had no effect on D-Tyr([11])NT-induced locomotor responses in either strain; it had no effect also on NT- and D-Tyr([11])NT-induced hypothermia in F344 rats while it slightly increased the sensitivity to NT in LEW rats. The results show that these NT-mediated actions are not influenced by cross-fostering or the pre-weaning environment.

Neurotensin receptor activation sensitizes to the locomotor stimulant effect of cocaine: a role for NMDA receptors.

This study was aimed at determining whether repeated activation of neurotensin receptors sensitizes to cocaine-induced locomotor activity and whether this effect can be prevented by blockade of N-methyl-d-aspartate receptors. Independent groups of male rats were injected on four occasions, every other day (training phase), with vehicle or one of two doses (4 and 8 mg/kg) of the NMDA antagonist CPP [(+/-)-3-(2-carboxypiperazine-4-yl)-propanephosphonic)] followed by an intracerebroventricular injection of 18 nmol/10 microl of d-Tyr[(11)]neurotensin, or its vehicle. Ambulatory, non-ambulatory and vertical movements were measured for 2 h on every test day. One week after the last day of the training phase, locomotor responses to a single injection of cocaine (7.5 mg/kg, ip) were measured in all rats; a second cocaine challenge test was performed 3 weeks post-training. Results show that during the training phase d-Tyr[(11)]neurotensin produced an initial suppression of all locomotor responses followed by an augmentation of ambulatory and non-ambulatory activity compared to controls, effects that were only slightly altered by CPP. Cocaine produced higher ambulatory and non-ambulatory activity in animals pre-exposed to neurotensin than in the vehicle pre-exposed animals, a sensitization effect that was not prevented by CPP at 1 week post-training but that was blocked at 3 weeks at the high dose. When given alone, the low dose of CPP produced an effect very similar to that of neurotensin on cocaine sensitization. These results further confirm that neurotensin plays a role in sensitization to psychostimulant drugs and suggests that NMDA receptors are involved in the long-term effect of exposure to neurotensin.

Effects of excitotoxic lesions of the medial prefrontal cortex on density of high affinity [125I-Tyr3]neurotensin binding sites within the ventral midbrain and striatum.

The present study was aimed at determining the extent to which excitotoxic lesions of the medial prefrontal cortex reduce neurotensin receptors within the striatum, the nucleus accumbens, the ventral tegmental area and the substantia nigra. The medial prefrontal cortex was unilaterally lesioned with ibotenic acid and 10 days later brain sections were processed for neurotensin receptor autoradiographic analysis using 0.1 nM [(125)I-Tyr3]neurotensin with, or without, levocabastine. Analysis revealed at least two sites, one levocabastine-insensitive neurotensin NT(1) and one levocabastine-sensitive neurotensin NT(2)-like. The proportion of the latter site was high within the caudal striatum, the nucleus accumbens and the medial prefrontal cortex. Lesions produced a 60% to 80% reduction in neurotensin NT(1) within the ipsilateral medial prefrontal cortex, but no change in the sub-cortical nuclei. An increase in neurotensin NT(2)-like receptors was found in ipsilateral dorso-caudal caudate. These results show that a significant amount of neurotensin NT(1) receptors are located on neurons within the medial prefrontal cortex but not on their efferent terminals.

Role of the dorsal diencephalic conduction system in the brain reward circuitry.

Previous work with psychophysically based studies suggests that electrolytic lesion of the habenula, which lies in the dorsal diencephalic conduction system (DDC), degrades the intracranial self-stimulation (ICSS). This experiment was aimed at studying the importance of the DDC in brain stimulation reward, and its connections with other areas that support operant responding for brain stimulation. For this purpose, rats were implanted with stimulating electrodes at the dorsal raphe (DR) and lateral hypothalamus (LH), and lesioning electrodes in the medial forebrain bundle (MFB) and the DDC. Rats were trained to self-administer the stimulation at three different current intensities and were tested daily for changes in reward thresholds, defined as the pulse frequency required for half-maximal responding. The lesions were done at the DDC and the MFB, and were separated by two weeks interval during which the rats were tested for self-stimulation. At the end of the experiment, rats were transcardially perfused and their brains collected to determine the extent of the lesions and the locations of the stimulation sites. Results show that lesions at both the DDC and MFB produce larger and longer-lasting increases in the reward thresholds (upto 0.40 log10 units) than lesions at either pathway alone (upto 0.25 log10 units), and were more effective in attenuating the reward induced by the LH stimulation. These results suggest that there exist two parallel pathways, the MFB and the DDC, which could constitute a viable route for the reward signal triggered by ICSS.

Effect of electrolytic lesions of the dorsal diencephalic conduction system on the distribution of Fos-like immunoreactivity induced by rewarding electrical stimulation.

The dorsal diencephalic conduction system (DDC) is an important pathway of the brain reward circuitry, linking together forebrain and midbrain structures. The present work was aimed at describing the effect of a DDC lesion on the distribution of Fos-like immunoreactivity (FLIR) following intracranial self-stimulation (ICSS) of the lateral hypothalamus (LH). Rats were implanted with monopolar electrodes and divided into three groups; the first two groups were trained to self-stimulate at the LH, whereas the third group received no stimulation and served as a control. Among the two groups that were trained for ICSS, one of them received a lesion at the DDC and was tested for ICSS on the subsequent 5days. On the last day of testing, control rats were placed in operant chambers without receiving any stimulation, and the remaining rats were allowed to receive the stimulation for 1h. All rats were then processed for FLIR. As previously shown, a lesion at the DDC resulted in significant attenuations of the rewarding effectiveness of LH stimulation. Results also show a higher FLIR in several reward-related areas following LH stimulation, especially in the hemisphere ipsilateral to the stimulation electrode. Compared to non-lesioned rats, lesioned animals had lower FLIR in certain brain regions, suggesting that those regions that were activated by the rewarding stimulation may be functionally interconnected with the DDC.

Ventral Midbrain NMDA Receptor Blockade: From Enhanced Reward and Dopamine Inactivation.

Glutamate stimulates ventral midbrain (VM) N-Methyl-D-Aspartate receptors (NMDAR) to initiate dopamine (DA) burst firing activity, a mode of discharge associated with enhanced DA release and reward. Blockade of VM NMDAR, however, enhances brain stimulation reward (BSR), the results can be explained by a reduction in the inhibitory drive on DA neurons that is also under the control of glutamate. In this study, we used fast-scan cyclic voltammetry (FSCV) in anesthetized animals to determine whether this enhancement is associated with a change in phasic DA release in the nucleus accumbens. Rats were implanted with a stimulation electrode in the dorsal-raphe (DR) and bilateral cannulae above the VM and trained to self-administer trains of electrical stimulation. The curve-shift method was used to evaluate the effect of a single dose (0.825 nmol/0.5 µl/side) of the NMDAR antagonist, (2R,4S)-4-(3-Phosphopropyl)-2-piperidinecarboxylic acid (PPPA), on reward. These animals were then anesthetized and DA release was measured during delivery of electrical stimulation before and after VM microinjection of the vehicle followed by PPPA. As expected, phasic DA release and operant responding depended similarly on the frequency of rewarding electrical stimulation. As anticipated, PPPA produced a significant reward enhancement. Unexpectedly, PPPA produced a decrease in the magnitude of DA transients at all tested frequencies. To test whether this decrease resulted from excessive activation of DA neurons, we injected apomorphine 20 min after PPPA microinjection. At a dose (100 µg s.c.) sufficient to reduce DA firing under control conditions, apomorphine restored electrical stimulation-induced DA transients. These findings show that combined electrical stimulation and VM NMDARs blockade induce DA inactivation, an effect that indirectly demonstrates that VM NMDARs blockade enhances reward by potentiating stimulation-induced excitation in the mesoaccumbens DA pathway.