Intracranial Self-stimulation Research Articles

Dopaminergic lesions of the dorsolateral striatum in rats increase delay discounting in an impulsive choice task.

Dysregulated dopamine transmission in striatal circuitry is associated with impulsivity. The current study evaluated the influence of dopaminergic inputs to the dorsolateral striatum on impulsive choice, one aspect of impulsive behavior. We implemented an operant task that measures impulsive choice in rats via delay discounting wherein intracranial self-stimulation (ICSS) was used as the positive reinforcer. To do so, rats were anesthetized to allow implanting of a stimulating electrode within the lateral hypothalamus of one hemisphere and bilateral dorsal striatal injections of the dopaminergic toxin, 6-OHDA (lesioned) or its vehicle (sham). Following recovery, rats were trained in a delay discounting task wherein they selected between a small ICSS current presented immediately after lever pressing, and a large ICSS current presented following a 0 to 15s delay upon pressing the alternate lever. Task acquisition and reinforcer discrimination were similar for lesioned and sham rats. All rats exhibited an initial preference for the large reinforcer, and as the delay was increased, preference for the large reinforcer was decreased indicating that the subjective value of the large reinforcer was discounted as a function of delay time. However, this discounting effect was significantly enhanced in lesioned rats for the longer delays. These data reveal a contribution of dopaminergic inputs to the dorsolateral striatum on impulsive choice behavior, and provide new insights into neural substrates underlying discounting behaviors.

Brain Rewarding Stimulation Reduces Extracellular Glutamate Through Glial Modulation in Medial Prefrontal Cortex of Rats.

Growing evidence implicates a critical involvement of prefrontal glial modulation of extracellular glutamate (GLU) in aversive behaviors. However, nothing is known about whether prefrontal glial cells modulate GLU levels in rewarding behaviors. To address this question, we measured GLU efflux in the medial prefrontal cortex (PFC) of rats associated with rewarding behaviors. We used intracranial self-stimulation (ICSS) of the medial forebrain bundle (MFB) as the rewarding behavior. GLU was indirectly measured using microdialysis combined with on-line fluorometric detection of NADH resulting from the reaction of GLU and NAD(+) catalyzed by GLU dehydrogenase with a time resolution of 1 min. ICSS caused a minute-by-minute change of extracellular GLU in the medial PFC, with a slight decrease during the stimulation, followed by an increase afterward. This bidirectional change was tetrodotoxin insensitive and abolished by the gliotoxin fluorocitrate. To confirm and extend the previous studies of aversion-induced increase of extracellular GLU in the medial PFC, we also measured prefrontal GLU efflux associated with an aversive stimulation, immobilization stress. The temporal change in extracellular GLU caused by this stress was markedly different from that observed during ICSS. A rapid increase in GLU was detected during the aversive stimulation, followed by a large increase afterward. This bimodal change was tetrodotoxin insensitive, similar to that detected for ICSS. These findings indicate a bidirectional regulation of extracellular GLU by prefrontal glial cells associated with rat ICSS behavior, and reveal that glial modulation of GLU neurochemistry in the medial PFC contributes to rewarding as well as aversive behaviors in rats.

Central muscarinic cholinergic involvement in serial pattern learning: Atropine impairs acquisition and retention in a serial multiple choice (SMC) task in rats

Atropine sulfate is a muscarinic cholinergic antagonist which impairs acquisition and retention performance on a variety of cognitive tasks. The present study examined the effects of atropine on acquisition and retention of a highly-structured serial pattern in a serial multiple choice (SMC) task. Rats were given daily intraperitoneal injections of either saline or atropine sulfate (50mg/kg) and trained in an octagonal operant chamber equipped with a lever on each wall. They learned to press the levers in a particular order (the serial pattern) for brain-stimulation reward in a discrete-trial procedure with correction. The two groups learned a pattern composed of eight 3-element chunks ending with a violation element: 123–234–345–456–567–678–781–818 where the digits represent the clock-wise positions of levers in the chamber, dashes indicate 3-s pauses, and other intertrial intervals were 1s. Central muscarinic cholinergic blockade by atropine caused profound impairments during acquisition, specifically in the encoding of chunk-boundary elements (the first element of chunks) and the violation element of the pattern, but had a significant but negligible effect on the encoding of within-chunk elements relative to saline-injected rats. These effects persisted when atropine was removed, and similar impairments were also observed in retention performance. The results indicate that intact central muscarinic cholinergic systems are necessary for learning and producing appropriate responses at places in sequences where pattern structure changes. The results also provide further evidence that multiple cognitive systems are recruited to learn and perform within-chunk, chunk-boundary, and violation elements of a serial pattern.

Receptor Reserve Moderates Mesolimbic Responses to Opioids in a Humanized Mouse Model of the OPRM1 A118G Polymorphism.

The OPRM1 A118G polymorphism is the most widely studied μ-opioid receptor (MOR) variant. Although its involvement in acute alcohol effects is well characterized, less is known about the extent to which it alters responses to opioids. Prior work has shown that both electrophysiological and analgesic responses to morphine but not to fentanyl are moderated by OPRM1 A118G variation, but the mechanism behind this dissociation is not known. Here we found that humanized mice carrying the 118GG allele (h/mOPRM1-118GG) were less sensitive than h/mOPRM1-118AA littermates to the rewarding effects of morphine and hydrocodone but not those of other opioids measured with intracranial self-stimulation. Reduced morphine reward in 118GG mice was associated with decreased dopamine release in the nucleus accumbens and reduced effects on GABA release in the ventral tegmental area that were not due to changes in drug potency or efficacy in vitro or receptor-binding affinity. Fewer MOR-binding sites were observed in h/mOPRM1-118GG mice, and pharmacological reduction of MOR availability unmasked genotypic differences in fentanyl sensitivity. These findings suggest that the OPRM1 A118G polymorphism decreases sensitivity to low-potency agonists by decreasing receptor reserve without significantly altering receptor function.

Quantitative structure-activity relationship analysis of the pharmacology of para-substituted methcathinone analogues.

Methcathinone (MCAT) is a potent monoamine releaser and parent compound to emerging drugs of abuse including mephedrone (4-CH3 MCAT), the para-methyl analogue of MCAT. This study examined quantitative structure-activity relationships (QSAR) for MCAT and six para-substituted MCAT analogues on (a) in vitro potency to promote monoamine release via dopamine and serotonin transporters (DAT and SERT, respectively), and (b) in vivo modulation of intracranial self-stimulation (ICSS), a behavioural procedure used to evaluate abuse potential. Neurochemical and behavioural effects were correlated with steric (Es ), electronic (σp ) and lipophilic (πp ) parameters of the para substituents. For neurochemical studies, drug effects on monoamine release through DAT and SERT were evaluated in rat brain synaptosomes. For behavioural studies, drug effects were tested in male Sprague-Dawley rats implanted with electrodes targeting the medial forebrain bundle and trained to lever-press for electrical brain stimulation. MCAT and all six para-substituted analogues increased monoamine release via DAT and SERT and dose- and time-dependently modulated ICSS. In vitro selectivity for DAT versus SERT correlated with in vivo efficacy to produce abuse-related ICSS facilitation. In addition, the Es values of the para substituents correlated with both selectivity for DAT versus SERT and magnitude of ICSS facilitation. Selectivity for DAT versus SERT in vitro is a key determinant of abuse-related ICSS facilitation by these MCAT analogues, and steric aspects of the para substituent of the MCAT scaffold (indicated by Es ) are key determinants of this selectivity.

Heterodimerization of human orexin receptor 1 and kappa opioid receptor promotes protein kinase A/cAMP-response element binding protein signaling via a Gαs-mediated mechanism

Orexin and dynorphin are co-expressed in the same synaptic vesicles of hypothalamic neurons and play opposing roles in cocaine self-administration, brain stimulation reward, and impulsivity in ventral tegmental area (VTA), where dopamine neurons express both OX1R and KORs. However, detailed mechanisms of how the coreleased peptides and both receptors fine-tune their signalings and physiological/behavioral effects together remain unclear. Here we explore the possibility of heterodimerization between OX1R and KOR and reveal novel signal transduction mechanisms. First, we demonstrated co-expression of OX1R and KOR in rat hippocampal neurons by single-cell PCR. Furthermore, heterodimerization between OX1R and KOR was examined using bioluminescence and fluorescence resonance energy transfer (BRET and FRET). Our data revealed that human OX1R and KOR heterodimerize, and this heterodimer associates with Gαs, leading to increased protein kinase A (PKA) signaling pathway activity, including upregulation of intracellular cAMP levels and cAMP-response element (CRE) luciferase reporter activity, resulting in increased cAMP-response element binding protein (CREB) phosphorylation. These results support the view that OX1R and KOR heterodimerization might have an anti-depressive role.

Effect of GABAA Partial Inverse Agonists on Drug‐facilitated ICSS Reward

We examined the ability of gamma-aminobutyric acid A receptor (GABAA) partial inverse agonists Ro15-4513 and FG-7142 and the antagonist flumazenil to block the reward-facilitating effects of methamphetamine on intracranial self-stimulation (ICSS) using a rate-frequency procedure. A total of 13 adult male C57BL6/J mice implanted with chronic bipolar stimulating electrodes directed toward the medial forebrain bundle and served as subjects. Following surgery, mice were trained to respond on an operant lever for electrical stimulation in 10 one min trials descending each subsequent min. A dose of 3 mg/kg methamphetamine robustly facilitated operant responding for ICSS (Top Figure). When administered alone, doses of 1 and 3 mg/kg flumazenil; 0.3 and 1 mg/kg RO15-4513; 0.1, 0.3 and 1 mg/kg FG-7142 did not significantly affect ICSS compared to the vehicle control. Higher doses of RO15-4513 and FG-7142 significantly decreased responding for ICSS relative to the vehicle control. Preadministration of flumazenil had no effect on methamphetamine facilitated ICSS. However, doses of 0.3 and 1 mg/kg RO15-4513, which themselves did not significantly attenuate ICSS, decreased the ICSS facilitation produced by 3 mg/kg methamphetamine (Bottom Figure). Similarly, 0.3 and 1 mg/kg FG-7142 dose-dependently decreased methamphetamine-facilitated ICSS. These results suggest an important role of GABAA receptors in modulating methamphetamine's reward-facilitating effects as well as the possibility that negative GABAA receptor modulation may have therapeutic value for the treatment of substance abuse disorders.

Agonist and antagonist effects of cytisine in vivo

Varenicline, the most successful smoking cessation aid, is a selective partial agonists at α4β2* nicotinic receptors. Its efficacy is likely to be shared by other drugs with similar receptor action, including cytisine. The present study aimed to characterize behavioral effects of cytisine compared with nicotine using locomotor activity tests, intracranial self-stimulation of ventral tegmental area (discrete-trial threshold current intensity titration procedure), drug discrimination (0.6 mg/kg nicotine from vehicle), physical dependence (osmotic minipumps delivering 6 mg/kg/day of nicotine) and intravenous nicotine self-administration (0.01 mg/kg per infusion) in adult Wistar rats. Cytisine (1–3 mg/kg) partially substituted for nicotine and at the highest dose tended to antagonize nicotine's discriminative stimulus effects. Nicotine (0.05–0.4 mg/kg), but not cytisine (0.3–3 mg/kg), lowered ICSS thresholds and cytisine dose-dependently reversed effects of nicotine. Nicotine (0.15–0.6 mg/kg), but not cytisine (0.3–3 mg/kg), stimulated locomotor activity and cytisine (3 mg/kg) fully reversed these effects of nicotine. Acute pretreatment with nicotine (0.15–0.6 mg/kg), but not cytisine (0.3–3 mg/kg), reinstated extinguished nicotine self-administration. Continuous infusion of nicotine induced physical dependence, as indicated by reduced rates of food-reinforced responding induced by a challenge dose of mecamylamine. At the highest tested dose (3 mg/kg), cytisine tended to reduce response rates irrespective of whether the rats were continuously exposed to nicotine or saline. Cytisine behaves like a weak partial agonist, mimicking effects of nicotine to a limited degree. Although cytisine reversed several effects of nicotine, it seemed to have a reduced potential to produce withdrawal signs in nicotine-dependent subjects.

The Serotonin 2C Receptor Agonist Lorcaserin Attenuates Intracranial Self-Stimulation and Blocks the Reward-Enhancing Effects of Nicotine.

Lorcaserin, a serotonin (5-hydroxytryptamine, 5-HT) 2C receptor agonist, was recently approved for the treatment of obesity. We previously suggested that 5-HT2C receptor agonists affect reward processes and reduce the rewarding effects of drugs of abuse. Here, we determined whether lorcaserin (1) decreases responding for brain stimulation reward (BSR) and (2) prevents nicotine from enhancing the efficacy of BSR. Rats were trained on the intracranial self-stimulation (ICSS) paradigm to nosepoke for BSR of either the dorsal raphé nucleus or left medial forebrain bundle. In Experiment 1, lorcaserin (0.3-1.0 mg/kg) dose-dependently reduced the efficacy of BSR. This effect was blocked by prior administration of the 5-HT2C receptor antagonist SB242084. In Experiment 2, separate groups of rats received saline or nicotine (0.4 mg/kg) for eight sessions prior to testing. Although thresholds were unaltered in saline-treated rats, nicotine reduced reward thresholds. An injection of lorcaserin (0.3 mg/kg) prior to nicotine prevented the reward-enhancing effect of nicotine across multiple test sessions. These results demonstrated that lorcaserin reduces the rewarding value of BSR and also prevents nicotine from facilitating ICSS. Hence, lorcaserin may be effective in treating psychiatric disorders, including obesity and nicotine addiction, by reducing the value of food or drug rewards.

The anti-(+)-methamphetamine monoclonal antibody mAb7F9 attenuates acute (+)-methamphetamine effects on intracranial self-stimulation in rats.

Passive immunization with monoclonal antibodies (mAbs) against (+)-methamphetamine (METH) is being evaluated for the treatment of METH addiction. A human/mouse chimeric form of the murine anti-METH mAb7F9 has entered clinical trials. This study examined the effects of murine mAb7F9 on certain addiction-related behavioral effects of METH in rats as measured using intracranial self-stimulation (ICSS). Initial studies indicated that acute METH (0.1-0.56 mg/kg, s.c.) lowered the minimal (threshold) stimulation intensity that maintained ICSS. METH (0.3 mg/kg, s.c.) also blocked elevations in ICSS thresholds (anhedonia-like behavior) during spontaneous withdrawal from a chronic METH infusion (10 mg/kg/day x 7 days). In studies examining effects of i.v. pretreatment with mAb7F9 (at 30, 100, or 200 mg/kg), 200 mg/kg blocked the ability of an initial injection of METH (0.3 mg/kg, s.c.) to reduce baseline ICSS thresholds, but was less capable of attenuating the effect of subsequent daily injections of METH. MAb7F9 (200 mg/kg) also produced a small but significant reduction in the ability of METH (0.3 mg/kg, s.c.) to reverse METH withdrawal-induced elevations in ICSS thresholds. These studies demonstrate that mAb7F9 can partially attenuate some addiction-related effects of acute METH in an ICSS model, and provide some support for the therapeutic potential of mAb7F9 for the treatment of METH addiction.

Stereoselective Actions of Methylenedioxypyrovalerone (MDPV) To Inhibit Dopamine and Norepinephrine Transporters and Facilitate Intracranial Self-Stimulation in Rats.

The designer stimulant methylenedioxypyrovalerone (MDPV) is a potent reuptake inhibitor at transporters for dopamine (DAT) and norepinephrine (NET) that produces a constellation of abuse-related behavioral effects. MDPV possesses a chiral center, and the abused formulation of the drug is a racemic mixture, but no data are available on the pharmacology of its isomers. Here, the individual optical isomers of MDPV were prepared and examined with respect to their neurochemical actions on neurotransmitter reuptake and behavioral effects in an assay of intracranial self-stimulation (ICSS) in rats. In assays of DAT uptake inhibition, S(+)MDPV (EC50 = 2.13 nM) was more potent than either (±)MDPV (EC50 = 4.85 nM) or R(-)MDPV (EC50 = 382.80 nM); the three drugs were less potent at NET uptake inhibition, with the same rank order of potency. Neither racemic MDPV nor its optical isomers inhibited the reuptake of serotonin at concentrations up to 10 μM. S(+)MDPV produced an abuse-related and dose-dependent facilitation of ICSS, and the potency of S(+)MDPV (significant facilitation at doses ≥ 0.1 mg/kg) was greater than that of the racemate (significant facilitation at doses ≥ 0.32 mg/kg). R(-)MDPV failed to alter ICSS at doses up to 100 times greater than the lowest effective dose of S(+)MDPV. The results indicate that abuse-related neurochemical and behavioral effects of racemic MDPV reside primarily with its S(+) isomer.

Steric parameters, molecular modeling and hydropathic interaction analysis of the pharmacology of para-substituted methcathinone analogues.

There is growing concern over the abuse of certain psychostimulant methcathinone (MCAT) analogues. This study extends an initial quantitative structure-activity relationship (QSAR) investigation that demonstrated important steric considerations of seven 4- (or para-)substituted analogues of MCAT. Specifically, the steric character (Taft's steric ES ) of the 4-position substituent affected in vitro potency to induce monoamine release via dopamine and 5-HT transporters (DAT and SERT) and in vivo modulation of intracranial self-stimulation (ICSS). Here, we have assessed the effects of other steric properties of the 4-position substituents. Definitive steric parameters that more explicitly focus on the volume, width and length of the MCAT 4-position substituents were assessed. In addition, homology models of human DAT and human SERT based upon the crystallized Drosophila DAT were constructed and docking studies were performed, followed by hydropathic interaction (HINT) analysis of the docking results. The potency of seven MCAT analogues at DAT was negatively correlated with the volume and maximal width of their 4-position substituents, whereas potency at SERT increased as substituent volume and length increased. SERT/DAT selectivity, as well as abuse-related drug effects in the ICSS procedure, also correlated with the same parameters. Docking solutions offered a means of visualizing these findings. These results suggest that steric aspects of the 4-position substituents of MCAT analogues are key determinants of their action and selectivity, and that the hydrophobic nature of these substituents is involved in their potency at SERT.

Medial Forebrain Bundle Deep Brain Stimulation has Symptom-specific Anti-depressant Effects in Rats and as Opposed to Ventromedial Prefrontal Cortex Stimulation Interacts With the Reward System.

In recent years, deep brain stimulation (DBS) has emerged as a promising treatment option for patients suffering from treatment-resistant depression (TRD). Several stimulation targets have successfully been tested in clinical settings, including the subgenual cingulum (Cg25) and the medial forebrain bundle (MFB). MFB-DBS has led to remarkable results, surpassing the effect of previous targets in terms of response latency and number of responders. However, the question remains as to which mechanisms underlie this difference. The aim of the present study was to thoroughly study the anti-depressant effect of MFB-DBS in the Flinders sensitive line (FSL) rat model of depression as well as to investigate whether MFB-DBS and Cg25-DBS operate through the same neurobiological circuits. FSL and control rats received bilateral high-frequency stimulation to the MFB at the level of the lateral hypothalamus, while being subjected to a variety of depression- and anxiety-related behavioral paradigms. To further compare the effects of MFB-DBS and Cg25-DBS on reward-related behavior, animals were stimulated in either the MFB or ventromedial prefrontal cortex (vmPFC, rodent analog to Cg25), while being tested in the intra-cranial self-stimulation paradigm. A marked symptom-specific anti-depressant effect of MFB-DBS was demonstrated. The ICSS-paradigm revealed that MFB-DBS, as opposed to vmPFC-DBS interacts with the reward system. Our data suggest that MFB-DBS and Cg25-DBS do not operate via the same neurobiological circuits. This differentiation might be of interest when selecting patients for either Cg25- or MFB-DBS.

Mouse model of OPRM1 (A118G) polymorphism increases sociability and dominance and confers resilience to social defeat.

A single nucleotide polymorphism (SNP) in the human μ-opioid receptor gene (OPRM1 A118G) has been widely studied for its association in drug addiction, pain sensitivity, and, more recently, social behavior. The endogenous opioid system has been shown to regulate social distress and reward in a variety of animal models. However, mechanisms underlying the associations between the OPRM1 A118G SNP and these behaviors have not been clarified. We used a mouse model possessing the human equivalent nucleotide/amino acid substitution to study social affiliation and social defeat behaviors. In mice with the Oprm1 A112G SNP, we demonstrate that the G allele is associated with an increase in home-cage dominance and increased motivation for nonaggressive social interactions, similar to what is reported in human populations. When challenged by a resident aggressor, G-allele carriers expressed less submissive behavior and exhibited resilience to social defeat, demonstrated by a lack of subsequent social avoidance and reductions in anhedonia as measured by intracranial self-stimulation. Protection from social defeat in G-allele carriers was associated with a greater induction of c-fos in a resilience circuit comprising the nucleus accumbens and periaqueductal gray. These findings led us to test the role of endogenous opioids in the A112G mice. We demonstrate that the increase in social affiliation in G carriers is blocked by pretreatment with naloxone. Together, these data suggest a mechanism involving altered hedonic state and neural activation as well as altered endogenous opioid tone in the differential response to aversive and rewarding social stimuli in G-allele carriers.

Neuronal correlates of attention and its disengagement in the superior colliculus of rat.

Orienting attention to a new target requires prior disengagement of attention from the current focus. Previous studies indicate that the superior colliculus (SC) plays an important role in attention. However, recordings of responses of SC neurons during attentional disengagement have not yet been reported. Here, we analyzed rat SC neuronal activity during performance of an attention-shift task with and without disengagement. In this task, conditioned stimuli (CSs; right and/or left light-flash or sound) were sequentially presented. To obtain an intracranial self-stimulation reward, rats were required to lick a spout when an infrequent conditioned stimulus appeared (reward trials). In the disengagement reward trials, configural stimuli consisting of an infrequent stimulus and frequent stimulus in the former trials were presented; in the non-disengagement reward trials, only an infrequent stimulus was presented. Of the 186 SC neurons responding to the CSs, 41 showed stronger responses to the CSs in the disengagement reward trials than in the non-disengagement reward trials (disengagement-related neurons). Furthermore, lick latencies in the disengagement reward trials were negatively correlated with response magnitudes to the CSs in half of the disengagement-related neurons. These disengagement-related neurons were located mainly in the deep layers of the SC. Another 70 SC neurons responded to the CSs in both disengagement and non-disengagement reward trials, suggesting that these neurons were involved in attention engagement. Our results suggest complementary mechanisms of attentional shift based on two subpopulations of neurons in the SC.

Chronic administration of ketamine induces depression in Wistar rat, but does not affect sexual function

Schizophrenia is a mental disorder that is characterized by periodic shift in positive symptoms (conceptual disorganization, delusions, hallucinations, hyperactivity, stereotypy behaviour etc.) and negative symptoms (social withdrawal, loss of function, anhedonia, decreased emotional expression, impaired concentration, and diminished social engagement).1,2 Anhedonia in experimental rats is marked by decrease in intracranial self-stimulation, consumption of sucrose solution, and sexual behavior.3 Depression is associated with different degrees of sexual dysfunction and population suffering from sexual dysfunction are frequently depressed.3,4 Read More...,

Effects of acute and repeated dosing of the synthetic cannabinoid CP55,940 on intracranial self-stimulation in mice

BackgroundSynthetic cannabinoids have emerged as a significant public health concern. To increase the knowledge of how these molecules interact on brain reward processes, we investigated the effects of CP55,940, a high efficacy synthetic CB1 receptor agonist, in a frequency-rate intracranial self-stimulation (ICSS) procedure. MethodsThe impact of acute and repeated administration (seven days) of CP55,940 on operant responding for electrical brain stimulation of the medial forebrain bundle was investigated in C57BL/6J mice. ResultsCP55,940 attenuated ICSS in a dose-related fashion (ED50 (95% C.L.)=0.15 (0.12–0.18)mg/kg). This effect was blocked by the CB1 receptor antagonist rimonabant. Tolerance developed quickly, though not completely, to the rate-decreasing effects of CP55,940 (0.3mg/kg). Abrupt discontinuation of drug did not alter baseline responding for up to seven days. Moreover, rimonabant (10mg/kg) challenge did not alter ICSS responding in mice treated repeatedly with CP55,940. ConclusionsThe finding that CP55,940 reduced ICSS in mice with no evidence of facilitation at any dose is consistent with synthetic cannabinoid effects on ICSS in rats. CP55,940-induced ICSS depression was mediated through a CB1 receptor mechanism. Additionally, tolerance and dependence following repeated CP55,940 administration were dissociable. Thus, CP55,940 does not produce reward-like effects in ICSS under these conditions.

Blockade of the GLT-1 Transporter in the Central Nucleus of the Amygdala Induces both Anxiety and Depressive-Like Symptoms.

Depression has been associated with abnormalities in glutamatergic neurotransmission and decreased astrocyte number in limbic areas. We previously demonstrated that global and prefrontal cortical blockade of the astrocytic glutamate transporter (GLT-1) induces anhedonia and c-Fos expression in areas that regulate anxiety, including the central amygdala (CEA). Given the role of the amygdala in anxiety and the high degree of comorbidity between anxiety and depression, we hypothesized that GLT-1 blockade in the CEA would induce symptoms of anhedonia and anxiety in rats. We microinjected the GLT-1 inhibitor, dihydrokainic acid (DHK), into the CEA and examined effects on intracranial self-stimulation (ICSS) as an index of hedonic state, and on behavior in two anxiety paradigms, elevated plus maze (EPM) and fear conditioning. At lower doses, intra-CEA DHK produced modest increases in ICSS responding (T0). Higher doses resulted in complete cessation of responding for 15 min, suggesting an anhedonic or depressive-like effect. Intra-CEA DHK also increased anxiety-like behavior such that percent time in the open arms and total entries were decreased in the EPM and acquisition of freezing behavior to the tone was increased in a fear-conditioning paradigm. These effects did not appear to be explained by non-specific changes in activity, because effects on fear conditioning were assessed in a drug-free state, and a separate activity test showed no significant effects of intra-CEA DHK on locomotion. Taken together, these studies suggest that blockade of GLT-1 in the CEA is sufficient to induce both anhedonia and anxiety and therefore that a lack of glutamate uptake resulting from glial deficits may contribute to the comorbidity of depression and anxiety.

Intracranial self-stimulation reward thresholds during morphine withdrawal in rats bred for high (HiS) and low (LoS) saccharin intake

RationaleSweet preference is a marker of vulnerability to substance use disorders, and rats selectively bred for high (HiS) vs. low saccharin (LoS) intake display potentiated drug-seeking behaviors. Recent work indicated that LoS rats were more responsive to the negative effects of drugs in several assays. ObjectiveThe current study used the intracranial self-stimulation (ICSS) procedure to investigate the anhedonic component of morphine withdrawal in male HiS and LoS rats. MethodsRats were administered morphine (10mg/kg) or saline for 8 days. To evaluate withdrawal effects, reward thresholds were measured 24 and 28h following the 8th morphine injection (spontaneous withdrawal) and again for 4 days following daily acute morphine and naloxone (1mg/kg) administration (precipitated withdrawal). Results24h following the final morphine injection, reward thresholds in LoS rats were significantly elevated compared to reward thresholds in LoS controls, indicating spontaneous withdrawal. This effect was not observed in HiS rats. LoS rats also showed greater elevations of reward thresholds on several days during naloxone-precipitated withdrawal compared to their HiS counterparts. ConclusionsLoS rats were more sensitive to morphine withdrawal-mediated elevations in ICSS thresholds than HiS rats. While these differences were generally modest, our data suggest that severity of the negative affective component of opiate withdrawal may be influenced by genotypes related to addiction vulnerability.

Effects of the triple monoamine uptake inhibitor amitifadine on pain-related depression of behavior and mesolimbic dopamine release in rats.

Pain-related depression of behavior and mood is a key therapeutic target in the treatment of pain. Clinical evidence suggests a role for decreased dopamine (DA) signaling in pain-related depression of behavior and mood. Similarly, in rats, intraperitoneal injection of dilute lactic acid (IP acid) serves as a chemical noxious stimulus to produce analgesic-reversible decreases in both (1) extracellular DA levels in nucleus accumbens (NAc) and (2) intracranial self-stimulation (ICSS), an operant behavior reliant on NAc DA. Intraperitonial acid-induced depression of ICSS is blocked by DA transporter (DAT) inhibitors, but clinical viability of selective DAT inhibitors as analgesics is limited by abuse potential. Drugs that produce combined inhibition of both DA and serotonin transporters may retain efficacy to block pain-related behavioral depression with reduced abuse liability. Amitifadine is a "triple uptake inhibitor" that inhibits DAT with approximately 5- to 10-fold weaker potency than it inhibits serotonin and norepinephrine transporters. This study compared amitifadine effects on IP acid-induced depression of NAc DA and ICSS and IP acid-stimulated stretching in male Sprague-Dawley rats. Amitifadine blocked IP acid-induced depression of both NAc DA and ICSS and IP acid-stimulated stretching. In the absence of the noxious stimulus, amitifadine increased NAc levels of both DA and serotonin, and behaviorally, amitifadine produced significant but weak abuse-related ICSS facilitation. Moreover, amitifadine was more potent to block IP acid-induced depression of ICSS than to facilitate control ICSS. These results support consideration of amitifadine and related monoamine uptake inhibitors as candidate analgesics for treatment of pain-related behavioral depression.