Hyperphagia induced by GABAA receptor-mediated inhibition of the nucleus accumbens shell: dependence on intact neural output from the central amygdaloid region

To investigate the role of corticolimbic input in modulating feeding-related nucleus accumbens (Acb) circuitry, researchers temporarily deactivated sites within the basolateral amygdaloid complex (BLA) or central amygdaloid region (CeA) via GABA(A) agonist (muscimol) infusions and measured feeding responses following muscimol infusions into the Acb shell. Hyperphagia elicited by intra-Acb shell muscimol was not altered by coinfusions of intra-BLA muscimol. In contrast, muscimol infusions into the CeA dose-dependently reduced feeding elicited either by intra-Acb shell GABA(A) receptor stimulation or by food deprivation and produced a syndrome of forepaw treading. Intra-CeA tetrodotoxin infusions also blocked intra-Acb shell muscimol-induced hyperphagia. Hence, feeding elicited by intra-Acb shell GABA(A) receptor stimulation requires intact neural output from the CeA but not the BLA.     

Forebrain circuitry involved in effort-related choice: Injections of the GABAA agonist muscimol into ventral pallidum alter response allocation in food-seeking behavior

Organisms often make effort-related choices based upon assessments of motivational value and work requirements. Nucleus accumbens dopamine is a critical component of the brain circuitry regulating work output in reinforcement-seeking behavior. Rats with accumbens dopamine depletions reallocate their instrumental behavior away from food-reinforced tasks that have high response requirements, and instead they select a less-effortful type of food-seeking behavior. The ventral pallidum is a brain area that receives substantial GABAergic input from nucleus accumbens. It was hypothesized that stimulation of GABA(A) receptors in the ventral pallidum would result in behavioral effects that resemble those produced by interference with accumbens dopamine transmission. The present studies employed a concurrent choice lever pressing/chow intake procedure; with this task, interference with accumbens dopamine transmission shifts choice behavior such that lever pressing for food is decreased but chow intake is increased. In the present experiments, infusions of the GABA(A) agonist muscimol (5.0-10.0 ng) into the ventral pallidum decreased lever pressing for preferred food, but increased consumption of the less preferred chow. In contrast, ventral pallidal infusions of muscimol (10.0 ng) had no significant effect on preference for the palatable food in free-feeding choice tests. Furthermore, injections of muscimol into a control site dorsal to the ventral pallidum produced no significant effects on lever pressing and chow intake. These data indicate that stimulation of GABA receptors in ventral pallidum produces behavioral effects similar to those produced by accumbens dopamine depletions. Ventral pallidum appears to be a component of the brain circuitry regulating response allocation and effort-related choice behavior, and may act to convey information from nucleus accumbens to other parts of this circuitry. This research may have implications for understanding the brain mechanisms involved in energy-related psychiatric dysfunctions such as psychomotor retardation in depression, anergia, and apathy.     

Contrasting effects of systemic and central sibutramine administration on the intake of a palatable diet in the rat

Sibutramine hydrochloride monohydrate is the only centrally active weight-modifying agent currently approved by the FDA for long-term use in the treatment of obesity. Systemic sibutramine treatment has been shown to reduce food intake in humans and rodent models in a manner that is consistent with the enhancement of satiety mechanisms. Although it is generally assumed that the hypophagic effects of the drug are mediated by actions within the brain, the locus or loci of these effects remains unclear. These experiments compared the effects of systemic and intracranial injections of sibutramine on the intake of a palatable diet in non-deprived animals. Consistent with prior reports, systemic injections of sibutramine hydrochloride (at 0, 0.5, 1.0, or 3.0 mg/kg sibutramine i.p.) dose-dependently reduced feeding on a high fat/high sucrose diet across a 2-h feeding session, but did not alter water intake or locomotor activity. In contrast, bilateral injections of sibutramine (at 0.0, 2.0, 4.0 and 10.0 μg/0.5 μl/side) into either the paraventricular nucleus of the hypothalamus (PVN) or the medial nucleus accumbens shell (ACb) significantly and dose-dependently increased food intake of the sweetened fat diet. ACb treatment also modestly inhibited locomotor behavior; intracranial injections had no effect on water consumption. These experiments are the first to suggest that sibutramine treatment may have distinct actions upon separate neural circuits that modulate food intake behavior in the rat.     

Dynorphinergic GABA neurons are a target of both typical and atypical antipsychotic drugs in the nucleus accumbens shell, central amygdaloid nucleus and thalamic central medial nucleus

Administration of typical and atypical antipsychotic drugs leads to activation of cells in the nucleus accumbens shell, central amygdaloid nucleus, and midline thalamic central medial nucleus, implicating important shared effects of these drugs. However, the exact cell types responding to antipsychotic drugs in the nucleus accumbens shell, central amygdaloid nucleus, and midline thalamic central medial nucleus are unclear. We report here that, in a rat model, the results of studies using double immunofluorescence labeling with antibodies directed against markers specific to candidate cell types suggest that the cells responding to haloperidol and clozapine in all three sites are: 1) neurons, rather than astrocytes; 2) inhibitory GABA neurons, but not acetylcholinergic neurons; and 3) dynorphin-containing GABA neurons, but not M-enkephalin-containing GABA neurons. The present study provides pharmacological evidence, at the cellular level in vivo, that the shared effects of antipsychotic drugs, whether typical and atypical, is activation of dynorphinergic GABA neurons in the nucleus accumbens shell, central amygdaloid nucleus, and midline thalamic central medial nucleus. Alternative ways to modulate dynorphinergic GABA neuronal activity or its target receptors might present an important new avenue for the treatment of schizophrenia and other psychotic disorders.     

Gaba(A) receptors in the pedunculopontine tegmental nucleus play a crucial role in rat shell-specific dopamine-mediated, but not shell-specific acetylcholine-mediated, turning behaviour

The role of GABA(A) receptors in the pedunculopontine tegmental nucleus in turning behaviour of rats was studied. Unilateral injection of the GABA(A) receptor agonist, muscimol (25-100 ng), into the pedunculopontine tegmental nucleus dose-dependently produced contraversive pivoting, namely tight head-to-tail turning marked by abnormal hindlimb backward stepping. This effect was GABA(A) receptor specific, since it was prevented by the GABA(A) receptor antagonist, bicuculline (50 ng), which alone did not elicit turning behaviour. Unilateral injection of a mixture of dopamine D(1) ((+/-)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol [SKF 38393], 5 microg) and D(2) (quinpirole, 10 microg) receptor agonists into the nucleus accumbens shell has been found to elicit contraversive pivoting, whilst unilateral injection of the acetylcholine receptor agonist (carbachol, 5 microg) into the same site is known to elicit contraversive circling, namely turning marked by normal stepping. The pivoting induced by a mixture of SKF 38393 (5 microg) and quinpirole (10 microg) injected into the nucleus accumbens shell was significantly inhibited by bicuculline (50 ng) injected into the pedunculopontine tegmental nucleus, whereas muscimol (25 ng) had no effect. Neither muscimol (25 ng) nor bicuculline (50 ng) modulated the contraversive circling induced by carbachol (5 microg) injected into the nucleus accumbens shell. It is therefore concluded that unilateral stimulation of GABA(A) receptors in the pedunculopontine tegmental nucleus can elicit contraversive pivoting and that the pedunculopontine tegmental nucleus is one of the output stations of the accumbens region that mediates shell-specific, dopaminergic pivoting, but not of the accumbens region that mediates shell-specific, cholinergic circling.     

GABA(A) agents injected into the ventral pallidum differentially affect dopaminergic pivoting and cholinergic circling elicited from the shell of the nucleus accumbens

The ability of GABA(A) receptors in the ventral pallidum to modulate shell-specific behavior was studied. Injections of the non-selective acetylcholine receptor agonist, carbachol (5 microg), into the shell of the nucleus accumbens elicited contraversive circling, namely turning marked by normal stepping; in contrast, injections of a mixture of dopamine D(1) (SKF 38393, 5 microg) and D(2) (quinpirole, 10 microg) receptor agonists into this brain structure elicited contraversive pivoting, namely turning marked by abnormal hindlimb stepping. Unilateral injections of the GABA(A) receptor agonist muscimol (10, 25 and 50 ng) into the ventral pallidum dose-dependently mimicked shell-specific circling, especially when given at a level +8.6mm anterior to the interaural line; this effect was GABA(A) receptor specific, because it was prevented by the GABA(A) receptor antagonist bicuculline (150 ng). Unilateral pallidal injections of a dose of muscimol that was ineffective per se (10 ng) abolished contraversive pivoting elicited by shell injections of dopamine receptor agonists; instead, it elicited moderate ipsiversive pivoting. Pallidal injections of bicuculline (150 ng) replaced the contraversive pivoting elicited by dopamine receptor agonist with ipsiversive circling. In contrast, unilateral pallidal injections of 10 ng muscimol (anterior +8.6mm level) suppressed the contraversive circling elicited by shell injections of carbachol; instead, it elicited moderate ipsiversive pivoting. Pallidal injections of bicuculline (150 ng) produced short-lasting ipsiversive circling that was followed by contraversive pivoting.We conclude that the ventromedial portion of the ventral pallidum contains GABA(A) receptors that are crucial for the transmission of information from the shell of the nucleus accumbens via the ventral pallidum towards other brain structures; this holds especially for information about shell-specific circling elicited by carbachol. The same portion of the ventral pallidum also contains GABA(A) receptors that control the transfer of information from the nucleus accumbens towards structures outside the ventral pallidum; this holds especially for information about shell-specific pivoting elicited by dopaminergic agonists.     

The glutamatergic projection from the prefrontal cortex to the nucleus accumbens core is required for cocaine-induced decreases in ventral pallidal GABA

Cocaine-primed reinstatement of drug seeking is associated with a decrease in extracellular GABA in the ventral pallidum (VP). The present study investigated the neural mechanism of this cocaine-induced decrease in VP GABA by determining if activity of the glutamatergic projection from the medial prefrontal cortex (PFC) to the nucleus accumbens is required for the effect. Microdialysis was performed to measure extracellular GABA in the VP while simultaneously, either a combination of the GABA agonists baclofen and muscimol was microinjected into the PFC, or the AMPA/kainate glutamate receptor antagonist CNQX was microinjected into the accumbens core. Inhibition of the PFC with GABA agonists and blockade of AMPA glutamate receptors in the accumbens core were both sufficient to prevent the cocaine-induced decrease in VP GABA, further implicating increased activity of the cortico-striato-pallidal circuit in relapse to drug seeking.     

Once is too much: conditioned changes in accumbens dopamine following a single saccharin-morphine pairing

The present study tested whether presentation of a taste cue would support conditioned suppression of dopamine in the nucleus accumbens (NAcc) following a single taste-drug pairing. Nondeprived male Sprague-Dawley rats were given 20-min access to a 0.15% saccharin conditioned stimulus (CS). Immediately thereafter, experimental rats were injected with morphine (15 mg/kg ip); standard controls were injected with saline; and explicitly unpaired controls were injected with morphine, but approximately 24 hr later. All rats were then given one 20-min CS-only test. Microdialysis samples from the NAcc were measured over 20-min intervals before, during, and after CS access on the conditioning and test trial. The results showed that a single saccharin-morphine pairing led to a marked reduction in CS intake, and the reduction in intake was accompanied by a conditioned blunting of the accumbens dopamine response to the saccharin reward cue. In turn, a single exposure to the saccharin cue also blunted the unconditioned dopamine response to morphine. Reward comparison effects, then, are cross-modal, bidirectional, and immediate, resulting in both unconditioned and conditioned changes in brain and behavior.     

Nucleus accumbens opioids regulate flavor-based preferences in food consumption

Opioid signaling in the nucleus accumbens (NAcc) regulates feeding behavior, having particularly strong effects on consumption of highly palatable foods. Since macronutrient content may contribute to palatability, it is uncertain whether opioid regulation of food consumption is based primarily on its macronutrient content or its flavor per se. In order to isolate the effect of flavor, we manipulated opioid signaling in the NAcc in rats and quantified consumption of differently flavored but nutritionally identical pellets. When pellets of either flavor were presented alone, microinjection of d-Ala(2),N,Me-Phe(4),Gly-ol(5)-enkephalin (DAMGO (a mu opioid receptor (MOP) agonist)) into the NAcc increased consumption of pellets of both flavors equally. When both flavors of pellets were presented simultaneously, however, DAMGO in the NAcc selectively increased, while naltrexone (a non-selective opioid antagonist) in the NAcc selectively decreased, consumption of the more preferred flavor. Systemic naltrexone injection had no flavor specific effects, decreasing consumption of both flavors equally. Non-selective inactivation of NAcc neurons by local microinjection of muscimol (a GABA(A) agonist) increased consumption of both the more- and less-preferred flavors equally. These results indicate that opioid signaling directly regulates a subset of NAcc neurons that can selectively enhance consumption of preferred palatable foods based exclusively on flavor cues.     

Origins of lateral hypothalamic afferents associated with N-methyl-d-aspartic acid-elicited eating studied using reverse microdialysis of NMDA and Fluorogold

Afferent projections to the tuberal lateral hypothalamus (tLH), where excitatory amino acid application is most effective in eliciting feeding, and to the anterior, posterior and medial regions of the hypothalamus were studied using reverse microdialysis of N-methyl-D-aspartic acid (NMDA) and Fluorogold (FG). NMDA at 660 microM delivered for 10 min was effective in stimulating food intake only when administered into the tLH, causing a mean intake of 9.3 g compared to less than 1 g in any other site. Subsequent administration of FG through the dialysis probe retrogradely in labeled neurons in brain structures associated with the feeding response including the frontal cortex, amygdala, nucleus accumbens (NA), preoptic areas, substantia nigra, ventral tegmental area (VTA), parabrachial nucleus, and the nucleus of the solitary tract (NST). Labeling after anterior and posterior LH infusion of FG was similar to that seen after tLH delivery with some apparent differences, whereas FG administration into the medial hypothalamus produced a distinctly different pattern of labeling compared to the other groups. Some of the observed labeling appeared to be almost exclusively associated with the tLH where NMDA elicits feeding. In particular, amygdala, preoptic area and shell of the accumbens labeling was noticeably denser in tLH eaters than in all other groups. These findings are consistent with the role of LH glutamate and NMDA receptors in the regulation of food intake and identify afferents to the region which possibly mediate endogenous LH glutamate's effects on feeding.     

Nucleus accumbens opioid,GABaergic, and dopaminergic modulation of palatable food motivation: contrasting effects revealed by a progressive ratio study in the rat

The current studies were designed to evaluate whether incentive motivation for palatable food is altered after manipulations of opioid, GABAergic, and dopaminergic transmission within the nucleus accumbens. A progressive ratio schedule was used to measure lever-pressing for sugar pellets after microinfusion of drugs into the nucleus accumbens in non-food-deprived rats. The mu opioid agonist D-Ala2, NMe-Phe4, Glyo15-enkephalin and the indirect dopamine agonist amphetamine induced a marked increase in break point and correct lever-presses; the GABA(A) agonist muscimol did not affect breakpoint or lever-presses. The data suggest that opioid, dopaminergic, and GABAergic systems within the accumbens differentially modulate food-seeking behavior through mechanisms related to hedonic evaluation of food, incentive salience, and control of motor feeding circuits, respectively.     

Opposing roles for the nucleus accumbens core and shell in cue-induced reinstatement of food-seeking behavior

Reinstatement of previously extinguished instrumental responding for drug-related cues has been used as an animal model for relapse of drug abuse, and is differentially affected by inactivation of the core and shell subregions of the nucleus accumbens (NAc). To compare the roles of these subregions in reinstatement induced by cues associated with natural and drug rewards, the present study assessed the effects of inactivation of the NAc core and shell on cue-induced reinstatement of food-seeking behavior. Rats acquired a lever pressing response for food reward paired with a light/tone conditioned stimulus (CS). They were then subjected to extinction, where both food and the CS were withheld. Reinstatement of responding was measured during response-contingent presentations of the CS. Following saline infusions into the NAc core or shell, rats displayed a significant increase in lever pressing during reinstatement sessions. Inactivation of the core, induced by infusion of GABA agonists muscimol and baclofen, attenuated responding for the CS, but did not affect pavlovian approach toward the food receptacle. In contrast, inactivation of the shell had the opposite effect, potentiating responding relative to vehicle treatments. These data suggest that the NAc core and shell play opposing, yet complementary roles in mediating the influence that food-associated conditioned stimuli exert over behavior. The core enables reward-related stimuli to bias the direction and vigor of instrumental responding. In contrast, the shell facilitates alterations in behavior in response to changes in the incentive value of conditioned stimuli. The fact that the NAc core appears to play a similar role in cue-induced reinstatement induced by both natural and drug rewards suggests that this region of the ventral striatum may be a final common pathway through which both drug- and food-associated stimuli may influence the direction and magnitude of ongoing behavior.     

Direct and indirect effects of muscimol on medial vestibular nucleus neurones in guinea-pig brainstem slices

Inhibitory amino acids are considered as major transmitters in the vestibular system. Using intracellular recordings in slices, we applied gamma-aminobutyric acid (GABA) and muscimol (a specific agonist of the GABA_A receptor) to the two main types of medial vestibular nucleus neurones (A and B MVNn). In either a high Mg²⁺/low Ca²⁺ solution, or a solution containing tetrodotoxin, all MVNn were hyperpolarized by GABA and muscimol. This indicates that both types of MVNn are endowed with postsynaptic, hyperpolarising GABA_A receptors. In a normal medium, about half of A and B MVNn were, in contrast, depolarised by GABA and muscimol, whereas the remaining cells were hyperpolarised. These results could be due to a modulation by GABA and muscimol of a tonic GABA release in the slice. Such a release was, indeed, suggested by results showing the depolarising effect of either tetrodotoxin (TTX) or bicuculline, when applied alone. The cells that were depolarised by GABA or muscimol in control conditions were always hyperpolarised in the presence of TTX. Our data therefore suggest that GABA acting at GABA_A receptors in the medial vestibular nucleus can play a role either through a postsynaptic hyperpolarising action or indirectly by inhibiting a tonic GABA release, probably resulting from the spontaneous activity of local inhibitory interneurones. A GABAergic regulation of these interneurones could be important in processes of vestibular habituation and/or adaptation.     

GABA(A) receptor activation in the lateral parabrachial nucleus induces water and hypertonic NaCl intake

Inhibitory serotonergic and cholecystokinergic mechanisms in the lateral parabrachial nucleus and central GABAergic mechanisms are involved in the regulation of water and NaCl intake. In the present study we investigated if the GABA(A) receptors in the lateral parabrachial nucleus are involved in the control of water, NaCl and food intake in rats. Male Holtzman rats with stainless steel cannulas implanted bilaterally into the lateral parabrachial nucleus were used. Bilateral injections of muscimol (0.2 nmol/0.2 microl) into the lateral parabrachial nucleus strongly increased 0.3 M NaCl (20.3+/-7.2 vs. saline: 2.6+/-0.9 ml/180 min) without changing water intake induced by the treatment with the diuretic furosemide combined with low dose of the angiotensin converting enzyme inhibitor captopril s.c. In euhydrated and satiated rats, bilateral lateral parabrachial nucleus injections of muscimol (0.2 and 0.5 nmol/0.2 microl) induced 0.3 M NaCl intake (12.1+/-6.5 and 32.5+/-7.3 ml/180 min, respectively, vs. saline: 0.4+/-0.2 ml/180 min) and water intake (5.2+/-2.0 and 7.6+/-2.8 ml/180 min, respectively, vs. saline: 0.8+/-0.4 ml/180 min), but no food intake (2+/-0.4 g/240 min vs. saline: 1+/-0.3 g/240 min). Bilateral lateral parabrachial nucleus injections of the GABA(A) antagonist bicuculline (1.6 nmol/0.2 microl) abolished the effects of muscimol (0.5 nmol/0.2 microl) on 0.3 M NaCl and water intake. Muscimol (0.5 nmol/0.2 microl) into the lateral parabrachial nucleus also induced a slight ingestion of water (4.2+/-1.6 ml/240 min vs. saline: 1.1+/-0.3 ml/240 min) when only water was available, a long lasting (for at least 2 h) increase on mean arterial pressure (14+/-4 mm Hg, vs. saline: -1+/-1 mm Hg) and only a tendency to increase urinary volume and Na+ and K+ renal excretion. Therefore the activation of GABA(A) receptors in the lateral parabrachial nucleus induces strong NaCl intake, a small ingestion of water and pressor responses, without changes on food intake.     

The effects on feeding of galanin and M40 when injected into the nucleus of the solitary tract, the lateral parabrachial nucleus, and the third ventricle.

Several reports indicate that central injection of galanin stimulates feeding, and that there is macronutrient specificity in this response. In addition, the galanin receptor antagonist, M40, reduces food intake when injected centrally. The nucleus of the solitary tract (NTS) and the lateral parabrachial nucleus (PBN) contain galanin receptors, and are involved in the control of food intake. Hence, we sought to compare the feeding response to galanin injection into these areas with that of third ventricle (3V) galanin injection. The feeding response to injection of galanin was greatest for the 3V. Hindbrain injection of galanin stimulated food intake only at the beginning of the dark period. NTS injection of M40 inhibited intake of a macronutrient diet in food-deprived rats, but was ineffective at reducing dark-onset feeding or deprivation-induced chow intake. 3V injection of M40 did not reduce deprivation-induced intake. PBN injection of galanin at dark onset had no effect in a group of fat-preferring rats. These results suggest that hindbrain galanin may contribute to feeding by inhibiting satiety, and that hypothalamic galanin receptors are involved with stimulation of intake. Furthermore, the absence of a consistent pattern of the stimulation of macronutrient intake suggests that galanin may not be a significant effector of macronutrient selection during individual meals.     

Electrical activity and feeding correlates of intracranial hypothalamic injection of GABA, muscimol and picrotoxin in the rats

Assemblies of electrodes and a cannula were stereotaxically implanted in the ventromedial (VMH), lateral (LHA) and paraventricular (PVH) hypothalamic areas in male albino rats. Electrical activity of these regions was recorded electrographically before and following intracranial injection (ICI) of GABA, muscimol and picrotoxin. In another set of animals, food intake and water intake were also measured. The activity of the ventromedial hypothalamus changed from slow to fast after ICI of GABA and picrotoxin and fast to slow after muscimol. The activity of the lateral hypothalamus changed from slow to fast with ICI of muscimol and picrotoxin and from fast to slow with GABA, while that of the paraventricular hypothalamic nucleus changed from slow to fast with ICI GABA and fast to slow with muscimol and picrotoxin. ICI of GABA into VMH and LHA and muscimol in VMH, LHA and PVH caused a decrease in food intake. Water intake was also decreased after ICI of GABA in PVH and muscimol in LHA and PVH. On the opposite picrotoxin increased food intake in VMA and LHA and water intake in PVH. The possible interaction of GABAergic drugs with the areas of the brain controlling feeding and drinking is being discussed.     

Sucrose sham feeding decreases accumbens norepinephrine in the rat

Noradrenergic projections from the dorsomedial medulla reach the shell of the nucleus accumbens (NAcc), a structure implicated in both reward and feeding behavior. Despite this relationship, the effect of food reward on accumbens norepinephrine (NE) remains uninvestigated. In the course of assessing dopamine (DA) in the NAcc during sucrose ingestion [0.03, 0.1, and 0.3 M; Am. J. Physiol., Regul. Integr. Comp. Physiol., 286 (2004) R31], we also analyzed NE in the microdialysis samples from 14 ad-libitum-fed male rats. In contrast to DA, which increased with sucrose concentration (+20-47%) during sham feeding, in the same animals, NE levels were reduced (approximately -20%), regardless of sucrose concentration. These results demonstrate a novel relationship between accumbens DA and NE during orosensory stimulation with a preferred nutrient.     

The effects of dentate granule cell destruction on behavioral activity and Fos protein expression induced by systemic MDMA in rats

In this study, we examined the effect of the s.c. administration of (+/-) 3,4-methylenedioxymethamphetamine (MDMA) or saline on locomotor activity and Fos expression following the bilateral destruction of hippocampal dentate granule cells by colchicine in rats. The lesioned animals, when administered s.c. saline, showed a significantly greater increase in locomotor activity compared to the intact animals, and revealed a marginally significant level of increased locomotor activity compared to the sham-lesioned animals. In addition, when the lesioned animals were given s.c. saline or MDMA, there was a significant increase in Fos expression in the nucleus accumbens core, but not in the medial prefrontal cortex, dorsolateral prefrontal cortex, anterior cingulate cortex, piriform cortex, dorsal striatum, or nucleus accumbens shell, compared to the intact and sham-lesioned animals. Overall, these results suggest that the nucleus accumbens core may be involved in the enhancement of locomotor activity induced by the injection of saline alone (stress loading) or MDMA following bilateral destruction of hippocampal dentate granule cells by colchicine.