Hypothalamic Self-stimulation Research Articles

Lateral hypothalamic self-stimulation with threshold current intensity induces emotional overeating in self-deprivation paradigm in well-fed rats: Role of orexin and dopaminergic systems of the brain

BACKGROUND: Emotional overeating in forms of соmpulsive overeating, or bulimia nervosa, and binge eating disorder (ВED) is underlying the basis of eating disorders. In particular, binge eating disorder is included in ICD-10 as a form of nonchemical dependence.
 AIM: The participation of the orexin and dopamine systems of the brain was studied in conditions of food self-deprivation caused by self-stimulation of the lateral hypothalamus.
 METHODS: To reproduce the self-stimulation of the lateral hypothalamus, male Wistar rats were trained to press the pedal in Skinner box. After training, rats received a food deprivation, a feeder was placed in the Skinner box, and a conditioned reflex on food reinforcement was developed in rats for 5 days. The food self-deprivation reaction was observed in these rats with a stimulating current intensity of 10% and above the threshold for self-stimulation.
 RESULTS: Hungry animals pressed the pedal forhypothalamic self-stimulation and did not distract to the feeding trough. Food reactions were observed only when using the threshold current. Then rats received a free access to food, but the animals, pressing the pedal for self-stimulation, continued food intakes for many times and eat up to 60 seeds in 10 minutes of the experiment. Dopamine receptor D2/D3 antagonist sulpiride at doses of 5 and 20 mg/kg ip and orexin receptor antagonists OX1RSB-408124 and OX2R TCS 29 0.5 mg/ml, 20 l intranasally were used. Sulpiride administration in fed rats decreased both the eating behavior and the reinforcing properties of electrical stimulation in food self-deprivation testing. At the same time, SB-408124 administration induced a decrease in the number of seeds eaten, but the number of pedal presses increased. Administration of OX2R TCS 29 had no effect.
 CONCLUSIONS: The conclusion was made about the selectivity of the orexin A OX1RSB-408124 antagonist for emotional eating forms compared with the D2/D3 receptor antagonist dopamine sulpiride.

Study of reinforcing properties of new antagonists of glutamate receptors

Reinforcing properties of antagonists of NMDA receptors IEM-1921, IEM-1791, IEM-2181 and the antagonist of AMPA receptors IEM 1460 were investigated. Substances have been synthesized in S.V. Anichkov Dept. of Neuropharmacology Institute of Experimental Medicine. Electrodes were implanted into the lateral hypothalamus for brain stimulation reward. Rats were trained to press a pedal in Skinner box for receiving electric stimulation of the brain. IEM-1921 in doses of 1, 3 and 10 mg/kg enhanced number of pedal pressing and reduced self-stimulation thresholds more than phencyclidine (in doses 1, 3, 10 mg/kg) and MK-801 (in dose 1 and 3 mg/kg). IEM-1460 in doses of 1, 3 and 5 mg/kg reduced the frequency of hypothalamic self-stimulation and enhanced it thresholds. Also we investigated the conditional reinforcing properties of antagonists of glutamate receptors in conditional place preference test (CPP). The antagonist of NMDA receptors IEM 1921 did not cause CPP. The antagonist of AMPA receptors IEM 1460 caused CPP in dose of 3 mg/kg. IEM 1791 didn’t cause CPP. At the same time IEM 2181, the IEM 1791 water-soluble salt, caused CPP. Thus, the antagonist of NMDA of receptors of IEM-1921 increases the reinforcing properties of self-stimulation in rats more than phencyclidine and MK-801.This substance can be used as means analyzer for studying of the reinforcing properties of drugs. Antagonist of NMDA receptors IEM 2181cause CPP and has the conditional reinforcing properties. It is interesting that the antagonists of AMPA receptors IEM 1460 partially cause CPP, but reduced the reinforcing properties of self-stimulation.

Peculiarities of Behavior and Emotional Reactions of Old Rats under Conditions of Brain Self-Stimulation Combined with Injections of Melatonin

We studied the peculiarities of emotional reactions and behavior of old rats in the open field test under conditions of self-stimulation of the ventrolateral and ambivalent zones of the ventromedial hypothalamus either provided in isolation or combined with injections of melatonin. Isolated selfstimulation and self-stimulation combined with introduction of melatonin (six sessions) exerted mostly activating influences on the system of positive emotional reactions. Chronic self-stimulation exerted activating effects on the cerebral mechanisms responsible for negative emotional behavior and increased the levels of motor excitation and emotionality (autonomic behavior manifestations) in animals in the open field test, i.e., induced manifestations of emotional stress in experimental rats. Hypothalamic selfstimulation and injections of melatonin exerted modulating inhibitory effects on the system of negative reinforcement, i.e., decreased psychomotor excitation. The indices of orientation/searching (vertical) activity and the number of episodes of grooming increased, while the latency of coming out from the field center decreased. The data obtained indicate that melatonin possesses anxiolytic and antistress properties mediated by its influence on the reinforcing cerebral systems.

Mineralocorticoid receptor antagonism prevents hedonic deficits induced by a chronic sodium appetite.

Our laboratory has reported that manipulations that provoke a robust sodium appetite (e.g., sodium depletion, deoxycorticosterone acetate) decrease lateral hypothalamic self-stimulation (LHSS) reward if rats are denied access to hypertonic saline solutions. The following studies investigated the interaction between chronic sodium appetite and the renin-angiotensin-aldosterone system on LHSS reward. In Experiment 1, animals treated with the diuretic furosemide (20 mg/kg) when denied access to saline exhibited an increase in the current required to produce 50% of the maximum LHSS response rate (ECu50) 48 hr after extracellular volume depletion. Furosemide-depleted rats that were allowed to drink 0.3 M saline after depletion, or that were treated with the selective mineralocorticoid receptor (MR) antagonist spironolactone, which significantly reduced sodium appetite, did not show ECu50 changes. In Experiment 2 chronic intracerebroventricular administration of the selective MR antagonist RU 28318 (10 microg/microl/hr) prevented decreases in the ECu50 induced by deoxycorticosterone acetate-no salt treatment. We conclude that an unresolved sodium appetite will reduce responding for rewards and that experimental manipulations that reduce sodium appetite (e.g., access to saline or blockade of MR) decrease hedonic deficits.

Motivational Effect of Nomifensine in the Intracranial Self-Stimulation Behavior Using a Runway Method

Intracranial self-stimulation (ICSS) behavior is an experimental methodology to study reward and motivational effects. We have established a new paradigm to evaluate enhancing motivation by drugs in the runway method using the priming stimulation of ICSS. In the present study, we investigated the effects of nomifensine on the experimental extinction process of non-reinforcing reward and pre-trial electric priming stimulations in lateral hypothalamic self-stimulation. In this study, the experimental extinction process of the non-reinforcing reward means the experimental method of excluding reward effect in ICSS behavior. The extinction process in the runway method consisted of these 15 trials. Nomifensine, an antidepressant drug, delayed the running speed of the extinction process at doses of 5 and 10 mg/kg (i.p.) compared with the vehicle alone. This result suggests that the delay in the running speed of the extinction process promotes a motivational effect in rats. Previously, priming stimulation in the runway method was found to affect motivational function of ICSS. Therefore, our findings suggest the possible application of nomifensine for improving motivation.

Effect of Diazepam on the Runway Method Using Priming Stimulation of Intracranial Self Stimulation Behavior

Intracranial self-stimulation (ICSS) behavior is an experimental methodology to study reward and motivational effects. We have established a paradigm to evaluate enhancing motivation by drugs in the runway method using the priming stimulation of ICSS. In the present study, we investigated the effects of diazepam on the experimental extinction process of nonreinforcing reward and pre-trial electric priming stimulations in lateral hypothalamic self-stimulation. The extinction process in the runway method consisted of these 15 trials. Diazepam, an anti-anxiety drug, at doses of 0.5 and 1 mg/kg (i.p.) delayed the extinction of running behavior when priming stimulation was given. The GABAergic antagonist flumazenil at doses of 5 and 10 mg/kg (i.p.) totally prevented the effect of diazepam. These results demonstrate that diazepam delays the extinction of running behavior on ICSS in the runway method and flumazenil, a GABAergic antagonist, eliminates the delayed effect of diazepam, that is, indicating that the delayed extinction effect of diazepam may be related to facillitation of motivation, which was promoted via the GABAergic system in the ICSS behavior.

Intraventricular insulin and leptin decrease sucrose self-administration in rats

Data from our laboratory and others have demonstrated an effect of the candidate adiposity signals insulin and leptin to decrease brain reward function, as assessed by lateral hypothalamic self-stimulation and food-conditioned place preference. In this study, we evaluated the effect of centrally administrated insulin or leptin to acutely decrease motivated performance for 5% sucrose, i.e., progressive ratio (PR) sucrose self-administration. Consistent with findings using other behavioral assays, both insulin and leptin significantly decreased the number of bar presses (62 ± 7 and 76 ± 8% of paired controls respectively), and the number of sucrose rewards obtained (87 ± 4 and 91 ± 4% of paired controls respectively), relative to within-subjects' control day performance on PR sucrose self-administration, whereas acute intraventricular cerebrospinal fluid had no effect. Rats fed a higher fat diet for 5 weeks were resistant to the effects of the intraventricular insulin or leptin, suggesting a central resistance to their action. Thus the findings of this study extend and support previous observations which suggest that neuroendocrine signals which regulate energy homeostasis in the CNS may also play a role in modulating reward circuitry, and specifically, food reward.

Effects of central leptin infusion on the reward-potentiating effect of d-amphetamine

It was previously reported that chronic food restriction and maintenance of rats at 75–80% of initial body weight enhanced the reward-potentiating effect of d-amphetamine in the lateral hypothalamic self-stimulation (LHSS) paradigm. Moreover, the enhancement reversed in parallel with body weight recovery when ad libitum access to food was reinstated. The present study tested the hypothesis that hypoleptinemia during food restriction is necessary for expression of enhanced drug reward. In Experiment 1, intracerebroventricular (i.c.v.) infusion of leptin (0.5 μg/0.5 μl/hr for 8 days) in food-restricted rats did not alter the rewarding effect of d-amphetamine (0.5 mg/kg, i.p.). Considering that i.c.v. leptin may not diffuse into deep brain regions where direct effects on drug reward sensitivity may be exerted, effects of acute bilateral microinjection of leptin (0.5 μg) in ventral tegmental area and nucleus accumbens were tested in Experiment 2 and found to have no effect. In Experiment 3, chronic i.c.v. leptin infusion in ad libitum fed rats decreased food intake and body weight and enhanced the rewarding effect of d-amphetamine. Sensitivity to d-amphetamine returned to normal as body weight recovered following cessation of leptin infusion. This result suggests that weight loss, whether from hormone-induced appetite suppression or experimenter-imposed food restriction, is sufficient to enhance drug reward sensitivity. Experiment 4 tested whether food restriction in the absence of body weight loss alters drug reward sensitivity. Rats received chronic i.c.v. infusion of the orexigenic melanocortin receptor antagonist, SHU9119 (0.02 μg/0.5 μl/hr for 12 days), and a subset were pair-fed to vehicle-infused controls. Although these subjects ingested ∼50% of the amount of food ingested by free-feeding SHU9119-infused rats, they displayed no weight loss and no change in sensitivity to d-amphetamine. Together, results of this study support the importance of weight loss, but not leptin, in the enhancement of drug reward sensitivity.

The effects of deoxycorticosterone-induced sodium appetite on hedonic behaviors in the rat.

The authors tested the hypothesis that chronic treatment with a dose of deoxycorticosterone acetate (DOCA) known to elicit a robust sodium appetite can negatively affect the hedonic state of rats. Daily treatment with DOCA with no opportunity to ingest saline produced a rightward shift in the midpoint (effective current 50) of lateral hypothalamic self-stimulation (LHSS) current-response functions and reduced intakes of a palatable sucrose solution. Providing rats with 0.3 M saline during DOCA treatment prevented the rightward shift in LHSS response functions and the decrease in sucrose intake. The authors concluded that a chronic sodium appetite, with no opportunity to attenuate the appetite, can elicit a reduced responsiveness to reward.

Roles of pedunculopontine tegmental cholinergic receptors in brain stimulation reward in the rat

The brainstem pedunculopontine tegmental nucleus (PPTg) is proposed to mediate hypothalamic self-stimulation reward via cholinergic activation of the ventral tegmental area (VTA). However, to date there is little direct evidence to support this hypothesis. To further study the role of PPTg in hypothalamic self-stimulation reward. By using in vivo microdialysis, the levels of extracellular acetylcholine (ACh) in the PPTg and VTA were detected during lateral hypothalamic (LH) self-stimulation in rats. Rate-frequency curve shift procedure was used to evaluate the effects of nonselective muscarinic antagonist scopolamine (1 approximately 100 microg/microl) and nicotinic antagonist mecamylamine (5 approximately 100 microg/microl) microinjected into the PPTg on the rewarding efficacy of LH self-stimulation. Subsequently, the drugs were injected into the PPTg, and the extracellular ACh in the VTA was measured. LH self-stimulation produced a concurrent ACh release in the PPTg and VTA. Intra-PPTg injection of scopolamine (100 microg/microl) significantly reduced the frequency threshold for LH self-stimulation reward, but nicotinic antagonist mecamylamine did not shift the threshold. However, mecamylamine (10, 25 microg/microl) injected into the PPTg robustly diminished the nicotine-potentiated LH self-stimulation reward. The extracellular ACh in the VTA was dramatically increased by intra-PPTg scopolamine (10, 100 microg/microl), but not by mecamylamine. Results confirm that PPTg plays an important role in brain stimulation reward by modulating the cholinergic activity of the VTA. The PPTg muscarinic receptors contribute to an inhibitory modulation of reward effects by self-stimulation, whereas nicotinic receptors seem to be more involved in nicotine potentiation of brain stimulation reward.

Differential effects of intra-midbrain raph� and systemic 8-OH-DPAT on VTA self-stimulation thresholds in rats

Intra-median raphe nucleus (MRN) administration of the 5-HT(1A) receptor agonist 8-OH-DPAT decreases lateral hypothalamic self-stimulation thresholds and is reported to have biphasic effects following systemic administration. These experiments attempted to extend the previous findings to mesolimbic pathway self-stimulation at ventral tegmental area (VTA) electrodes. This study was conducted to provide comparative data for systemic and intra-dorsal raphe nucleus (DRN) and intra-MRN effects of 8-OH-DPAT on VTA self-stimulation. Male Sprague-Dawley rats with VTA electrodes were trained to respond for electrical stimulation. Systemic and intra-midbrain raphe 8-OH-DPAT effects on rate-frequency thresholds were measured. Systemic administration of WAY 100635 was used to confirm 5-HT(1A) receptor mediation of 8-OH-DPAT effects. 8-OH-DPAT (0.003-0.3 mg kg(-1) SC) increased rate-frequency thresholds and decreased maximal response rates. WAY 100635 alone (0.0125-0.1 mg kg(-1) SC) did not alter these measures. Intra-DRN and intra-MRN 8-OH-DPAT (5.0 microg) decreased rate-frequency thresholds without altering maximal response rates. Intra-DRN 8-OH-DPAT (0.1-5.0 microg) induced a slight decrease and intra-MRN 8-OH-DPAT a slight increase in locomotor activity. WAY 100635 (0.1 mg kg(-1)) blocked effects of 8-OH-DPAT on VTA self-stimulation. These results confirm threshold-decreasing effects of intra-MRN 8-OH-DPAT and extend this to the DRN and to VTA thresholds. Monophasic dose dependent increases in VTA thresholds following systemic 8-OH-DPAT are not equivalent to reports for hypothalamic self-stimulation. Differences between studies may be attributable to stimulation site and/or differences in threshold measurement procedures. Effects of WAY 100635 in this study indicate 5-HT(1A) receptor mediation of these 8-OH-DPAT effects.

Effects of anxiolytic drugs on rewarding and aversive behaviors induced by intracranial stimulation.

In considering the characteristics of the action of anxiolytic drugs and their mechanism in the brain, it may be necessary not only to study the behavioral pharmacology but also to perform brain site research. In the present study, the action of anxiolytic drugs was examined with respect to various behaviors that were induced by stimulating the brain areas related to emotions such as reward (pleasure) or aversion in rats. First, the low rate of response in lateral hypothalamic self-stimulation behavior was induced by schedules of low current brain stimulation, variable interval (VI) and differential reinforcement of low rate (DRL). Anxiolytic drugs such as benzodiazepines facilitated these low-rate responses. The drug susceptibility was highest in the low current stimulation, lower in the VI stimulation, and lowest in the DRL stimulation schedules. Furthermore, it was found by the auto-titration method in intracranial self-stimulation behavior that anxiolytic drugs decreased the threshold of stimulation reward. Second, it was recognized using the decremental lever pressing (DLP) paradigm that anxiolytic drugs increased the threshold of aversive stimulation of mesencephalic dorsal central gray (DCG), and this increasing effect of the drug was antagonized by GABA receptor blockers such as biccuculline. Finally, it was examined whether or not the conflict situation is established by combining brain stimulation reward and aversion, such as foot-shock or DCG stimulation. As a result, the conflict behavior was established by combining not only the brain stimulation reward and foot-shock aversion, but also the brain stimulation reward and DCG stimulation aversion. Further anxiolytic drugs exhibited anti-conflict action in both situations. The susceptibility of anxiolytic drugs was higher with respect to the conflict behavior induced by intracranial reward and aversion than to that induced by the conventional method based on milk reward and foot-shock aversion. These results suggest that behavioral methods using brain stimulation can examine the mechanisms of direct drug action at the brain stimulation site. Indeed, in the present brain stimulation behavioral study, anxiolytic drugs such as benzodiazepines increased the stimulation threshold in lateral hypothalamic self-stimulation and inhibited the DCG aversive stimulation, thus resulting in an anticonflict action of the drugs.

WIN 55,212-2 decreases the reinforcing actions of cocaine through CB 1 cannabinoid receptor stimulation

CB 1 cannabinoid receptor agonists show a different profile compared to other drugs of abuse on the basis of experimental data that reveal their reinforcing properties. Thus, there are controversial data in the literature concerning the ability of CB 1 receptor agonists to reinforce behavioral responses in experimental animals, i.e. to lower self-stimulation thresholds, and to support self-administration or conditioned place preference. The aim of the present study was to examine the effects of WIN 55,212-2, a potent CB 1 receptor agonist (graded doses 0.1, 0.3, 1 mg/kg, i.p.), on the rewarding efficacy of lateral hypothalamic self-stimulation and on the systemic cocaine-induced potentiation of brain-stimulation reward. WIN 55,212-2 did not affect lateral hypothalamic self-stimulation thresholds both in drug naı̈ve rats and in rats pretreated with the drug, whereas it produced a significant, dose-dependent decrease in the maximal rate of responding, i.e. in the performance of the animals. Cocaine (5.0 mg/kg, i.p.) produced a significant reduction in self-stimulation threshold, without altering maximal rates of responding. Importantly, WIN 55,212-2 attenuated the effect of cocaine at the two higher doses tested. The effects of the CB 1 receptor agonist were reversed by pretreatment with the selective CB 1 receptor antagonist SR 141716A (0.02 mg/kg, i.p.) that did not by itself affect cocaine’s action. These results indicate that acute stimulation of CB 1 receptors per se does not affect baseline self-stimulation, but reduces the reinforcing effects induced by cocaine. Taken together these findings suggest that cannabinoids may interfere with brain-reward systems responsible for the expression of acute reinforcing properties of drugs of abuse, such as cocaine, and provide evidence that the cannabinoid system could be an interesting drug discovery and development target for the treatment of drug addiction.

Exposure to chronic mild stress alters thresholds for lateral hypothalamic stimulation reward and subsequent responsiveness to amphetamine

Chronic mild stress in rodents has been proposed to model some of the environmental factors that contribute to the induction of depressive disorders in humans. This model is based on the hypothesis that chronic mild stress induces a change in brain reward function that resembles the symptomatology of major depression, namely, a decrease in responsiveness to rewarding stimuli. The purpose of the first experiment was to investigate whether chronic mild stress affects brain reward function as measured by alterations in lateral hypothalamic self-stimulation behavior in rats. Exposure to chronic mild stress induces a reduction in body weight which might affect brain reward function on its own. Therefore, the potential contribution of a reduction in body weight to the chronic mild stress-induced alterations in brain reward function was examined in a separate group of food-restricted rats. Thresholds for lateral hypothalamic self-stimulation were slightly but significantly lowered in animals exposed to chronic mild stress, indicating an enhancement of stimulation reward efficacy. Food restriction had no effect on brain reward function. The second experiment examined the interaction between prior exposure to chronic mild stress or food restriction and responsiveness to a pharmacological challenge, amphetamine, that enhances brain reward function. Acute administration of amphetamine produced a greater enhancement of lateral hypothalamic self-stimulation reward in animals exposed to chronic stress relative to non-stressed and food-restricted animals. Taken together, the present findings indicate that chronic mild stress sensitizes the neural substrates that mediate both lateral hypothalamic stimulation and psychostimulant drug reward. These findings support the hypothesis that prior exposure to stress affects the vulnerability for drug-taking behavior by increasing the positive reinforcing properties of drug of abuse.

Effects of the D(3) dopamine receptor antagonist, U99194A, on brain stimulation and d-amphetamine reward, motor activity, and c-fos expression in ad libitum fed and food-restricted rats.

Previous studies indicate that the D(3) dopamine (DA) receptor is preferentially expressed in limbic forebrain DA terminal areas and may mediate functional effects opposite those of the D(1) and D(2) receptor types. However, the locations of the D(3) receptors that regulate behavior, and the range of behavioral functions regulated, are not clear. The objective of this study was to evaluate behavioral and cellular effects of the preferential D(3) dopamine receptor antagonist, U99194A. In experiment 1, the rewarding effect of U99194A (5.0, 10.0 and 20.0 mg/kg, SC) was measured in terms of its ability to lower the threshold for lateral hypothalamic self-stimulation (LHSS) in ad libitum fed rats. To amplify a possibly weak reward signal, testing was also conducted in food-restricted rats. The ability of U99194A to alter the threshold-lowering effect of d-amphetamine was also assessed. In experiment 2, effects of U99194A on horizontal and vertical motor activity were compared in ad libitum fed and food-restricted rats. In experiment 3, effects of a behaviorally active dose of U99194A (5.0 mg/kg) on brain c-fos expression were measured and compared to those produced by d-amphetamine (0.5 mg/kg, IP). In experiment 4, the motor and cellular activating effects of U99194A were challenged with the D(1) dopamine receptor antagonist, SCH-23390 (0.1 mg/kg). U99194A displayed no rewarding efficacy in the LHSS paradigm. U99194A did, however, augment the rewarding effect of d-amphetamine. U99194A also produced a motor activating effect, reversible by SCH-23390, which was greater in food-restricted than ad libitum fed rats. The pattern and intensity of fos-like immunoreactivity (FLI) induced by U99194A was similar to that produced by d-amphetamine and was blocked, in caudate-putamen and nucleus accumbens, by SCH-23390. These results indicate that U99194A has psychostimulant-like effects on motor activity and striatal c-fos expression that are dependent upon the D(1) DA receptor. However, doses of U99194A that are adequate to stimulate motor activity and c-fos expression in striatal and limbic structures do not possess direct rewarding effects in the LHSS paradigm. Overall, these results seem consistent with the hypothesis that D(3) antagonism enhances D(1)/D(2) mediated signaling with behavioral effects dependent on both the density of D(3) receptors and the prevailing level of DA transmission in particular brain regions.

Augmentation of drug reward by chronic food restriction: Behavioral evidence and underlying mechanisms

Chronic food restriction and maintenance of low body weight have long been known to increase the self-administration and motor-activating effects of abused drugs. Using a lateral hypothalamic self-stimulation (LHSS) rate-frequency method, it is shown that chronic food restriction augments the rewarding (i.e., threshold lowering) effect of diverse drugs of abuse. Further, the effect is attributed to increased sensitivity of a neural substrate, rather than a change in drug bioavailability or pharmacokinetics, because it is preserved when drugs are injected directly into the lateral cerebral ventricle (intracerebroventricularly). The food restriction regimen that augments drug reward also increases the induction of c- fos, by intracerebroventricular amphetamine, in limbic forebrain dopamine (DA) terminal areas. The possibility of increased DA receptor function is suggested by findings that rewarding and motor-activating effects of direct DA receptor agonists are augmented by food restriction, and the augmented behavioral effects of amphetamine are reversed by an otherwise subthreshold dose of D-1 antagonist. Initial studies of DA receptor-mediated signal transduction, that are focused on the D-2 receptor, suggest increased functional coupling between receptor and G-protein (i.e., quinpirole-stimulated [ 35S]GTPγS binding) in dorsal striatum. Unlike behavioral sensitization induced by intermittent stress or psychostimulant treatment, which persist indefinitely following induction, the augmenting effect of food restriction abates within 1 week of restored ad libitum feeding and weight gain. The possible involvement of endocrine hormones and/or ‘feeding-related’ neuropeptides, whose levels change dynamically with depletion and repletion of adipose stores, is therefore under investigation. Initial tests have been limited to acute treatments aimed at attenuating the effects of hypoinsulinemia, hypoleptinemia and elevated corticosterone levels in food-restricted rats. None of these treatments has attenuated the behavioral effect of food restriction. While a melanocortin receptor agonist has been found to enhance drug reward, melanocortin receptors do not seem to mediate the augmenting effect of food restriction. Continuing investigations of endocrine adiposity signals, ‘feeding-related’ neuropeptides and dopaminergic signal transduction may further elucidate the way in which drugs of abuse exploit mechanisms that mediate survival-related behavior, and help explain the high comorbidity of drug abuse and eating disorders.

The melanocortin receptor agonist MTII augments the rewarding effect of amphetamine in ad-libitum-fed and food-restricted rats.

Numerous forms of evidence support a functional association between drug-seeking and ingestive behavior. One example is the augmentation of rewarding and cellular-activating effects of abused drugs by chronic food restriction. Within the past several years, a variety of orexigenic and anorexigenic neuropeptides that mediate adaptive responses to changes in energy balance and body weight have been identified. The involvement of these neuropeptidergic signaling systems in the modulation of drug reward has received little experimental attention. alpha-Melanocyte-stimulating hormone (alpha-MSH) and agouti-related protein, which act as agonist and antagonist, respectively, at melanocortin receptors (MCRs), are of particular interest because of their neuroanatomical distribution and opponent actions at a common receptor type. The objective of this study was to determine whether lateral ventricular (i.c.v.) injections of the MCR agonist MTII and MCR antagonist SHU9119 at doses that decrease and increase food intake, respectively, modify the rewarding effect of amphetamine. The rewarding effect of amphetamine was measured in terms of its ability to lower the threshold for lateral hypothalamic self-stimulation (LHSS) using a rate-frequency method. The modulatory effects of MTII and SHU9119 were evaluated by preceding i.c.v. amphetamine injection with i.c.v. injection of one of these MCR ligands or vehicle. Tests were conducted in both ad-libitum-fed and food-restricted subjects. SHU9119, the agouti-related protein-like antagonist, markedly increased overnight food intake of ad-libitum-fm-fed rats following i.c.v. injection of 1.0 microg and 0.5 microg. However, these injections had no effect on thresholds for LHSS tested 25 min or 60 min post-injection, nor did they alter the threshold-lowering effect of amphetamine (50 microg) tested 25 min, 100 min, or 24 h post-injection. MTII, the alpha-MSH-like agonist, suppressed overnight food intake of ad-libitum-fed rats following i.c.v. injection of 1.0 microg. MTII had no effect on thresholds for LHSS tested 60 min post-injection. However, this MCR agonist potentiated the threshold-lowering effect of amphetamine tested 100 min but not 24 h post-injection. This effect was evident in both ad-libitum-fed and food-restricted rats. These results indicate that agonist activity at MCRs potentiates amphetamine reward and that the anorexigenic neuropeptide alpha-MSH may exert this effect physiologically.

Dopamine and glutamate release in the nucleus accumbens and ventral tegmental area of rat following lateral hypothalamic self-stimulation

Rewarding hypothalamic brain stimulation is thought to depend on trans-synaptic activation of high-threshold (and thus rarely directly depolarized by rewarding stimulation) dopaminergic fibers of the medial forebrain bundle. We used in vivo microdialysis and high-performance liquid chromatography coupled with electrochemical or fluorometric detection to investigate the concurrent release of dopamine and glutamate in the nucleus accumbens septi and in the ventral tegmental area, as a function of lateral hypothalamic self-stimulation. Self-stimulation at a variety of stimulation frequencies and pulse widths increased levels of dopamine and its primary metabolites, dihydroxyphenylacetic acid and homovanillic acid in the nucleus accumbens. Lateral hypothalamic self-stimulation also induced significant increases in ventral tegmental area dopamine and metabolite levels, and the percentage increase of dopamine was higher in this region than in the nucleus accumbens. Local perfusion with the dopamine uptake inhibitor nomifensine (10 μM) increased dopamine levels in the nucleus accumbens about three-fold and potentiated the increase of dopamine levels induced by self-stimulation. Nomifensine perfusion also induced a delayed decrease in nucleus accumbens glutamate levels, and self-stimulation did not modify this effect of the drug. Local perfusion with the D2-type dopamine receptor antagonist raclopride significantly increased both basal and self-stimulation induced dopamine release in the nucleus accumbens. Neither nomifensine nor raclopride perfusion significantly affected the maximal rates of self-stimulation. Perfusion with tetrodotoxin (2 μM) into nucleus accumbens significantly decreased basal and prevented stimulation-induced increases in accumbens dopamine levels but only slightly decreased the rate of self-stimulation. In contrast, perfusion of tetrodotoxin (0.5 μM) into the ventral tegmental area decreased basal and blocked stimulation-induced increases in both nucleus accumbens and ventral tegmental area dopamine levels; this treatment also blocked or strongly inhibited self-stimulation. While it had no effect on glutamate levels in the nucleus accumbens, lateral hypothalamic self-stimulation induced a significant and tetrodotoxin-sensitive increase in glutamate levels in the ventral tegmental area. Taken together, the present results indicate that, across a broad range of stimulation parameters, rewarding lateral hypothalamus stimulation causes major and persistent activation of the mesolimbic dopamine system, and suggest descending glutamatergic fibers in the medial forebrain bundle as a candidate for the directly activated descending pathway in lateral hypothalamus brain stimulation reward.

Rewarding and locomotor-activating effects of direct dopamine receptor agonists are augmented by chronic food restriction in rats.

Previous studies indicate that chronic food restriction augments the rewarding and motor-activating effects of diverse drugs of abuse. The drugs that have so far proved susceptible to the augmenting effect of food restriction all increase synaptic concentrations of dopamine (DA). It is not known whether behavioral effects of selective, direct DA receptor agonists are also subject to the augmenting effect of food restriction. The first objective of this study was to investigate whether the rewarding and locomotor-activating effects of the D1 agonist, A77636, and the D2 agonist, quinpirole are augmented by chronic food restriction. The second purpose was to investigate whether the augmented rewarding and locomotor-activating effects of d-amphetamine in food-restricted rats are reversed by the D1 antagonist, SCH23390. Rewarding effects of drugs were measured in terms of their ability to lower the threshold for lateral hypothalamic self-stimulation (LHSS) using a rate-frequency method. Locomotor-activating effects were measured in terms of the number of midline crossings exhibited by rats in a shuttle apparatus. A77636 (1.0 and 2.5 mg/kg, i.p.) produced a greater threshold-lowering effect in food-restricted than ad libitum fed rats but produced variable effects on locomotor activity with no difference between groups. Quinpirole (0.2 and 0.5 mg/kg, i.p.) produced a marginally greater threshold-lowering effect in food-restricted rats and a dramatic locomotor response that was exclusive to food-restricted rats. The D1 antagonist, SCH23390, at a dose of 0.01 mg/kg (i.p.), had no effect on the lowering of LHSS threshold by amphetamine (0.5 mg/kg, i.p.) in ad libitum fed rats but blocked the augmentation otherwise observed in food-restricted rats. SCH23390, at a dose of 0.025 mg/kg, had no effect on locomotor activity induced by amphetamine (0.5 mg/kg) in ad libitum fed rats but blocked the augmentation otherwise observed in food-restricted rats. These results indicate that the augmentation of reward by food restriction extends to drugs that bypass the DA terminal and act postsynaptically. When taken together with prior immunohistochemical and behavioral findings, these results suggest that food restriction may increase the "enabling" effect of the D1 receptor on DA-mediated behaviors.

Dynamic changes in cytochrome oxidase activity in the amygdala following lesions of rewarding sites in the lateral hypothalamus

The aim of this study was to evaluate neural changes in oxidative metabolism in amygdaloid sub-nuclei following unilateral electrolytic lesions of lateral hypothalamic sites that supported brain stimulation reward. A histochemical analysis of cytochrome oxidase activity, comparing lesioned to non-lesioned sides in the amygdala, revealed a significant reduction of oxidative metabolism in the cortical nucleus and, to a lesser degree, in the adjacent piriform cortex; this effect was observed 2–4 weeks after the lesion, with complete recovery by the eighth week in the case of the cortical nucleus only. No particular pattern in cytochrome oxidase activity was detected in other amygdaloid sub-nuclei that were examined, including the basolateral and medial nucleus. Within both structures, the most pronounced decreases in metabolic activity were observed at roughly the same level, corresponding to the posterolateral and posteromedial levels of the cortical nucleus and just anterior to the amygdalopiriform transition. These results suggest that within the amygdaloid complex, the cortical sub-nuclei and possibly the neighbouring piriform cortex contribute more to modulating lateral hypothalamic self-stimulation than components of the central extended amygdala.