Distribtuion of gamma-aminobutyric acid (GABA) in the rat hypothalamus: functional correlates of GABA wtih activities of appetite controlling mechanisms

Insulin effects on core neurotransmitter pathways involved in schizophrenia neurobiology: a meta-analysis of preclinical studies. Implications for the treatment

Impairment of insulin action and metabolic dysregulation have traditionally been associated with schizophrenia, although the molecular basis of such association remains still elusive. The present meta-analysis aims to assess the impact of insulin action manipulations (i.e., hyperinsulinemia, hypoinsulinemia, systemic or brain insulin resistance) on glutamatergic, dopaminergic, γ-aminobutyric acid (GABA)ergic, and serotonergic pathways in the central nervous system. More than one hundred outcomes, including transcript or protein levels, kinetic parameters, and other components of the neurotransmitter pathways, were collected from cultured cells, animals, or humans, and meta-analyzed by applying a random-effects model and adopting Hedges'g to compare means. Two hundred fifteen studies met the inclusion criteria, of which 180 entered the quantitative synthesis. Significant impairments in key regulators of synaptic plasticity processes were detected as the result of insulin handlings. Specifically, protein levels of N-methyl-D-aspartate receptor (NMDAR) subunits including type 2A (NR2A) (Hedges' g = -0.95, 95%C.I. = -1.50, -0.39; p = 0.001; I2 = 47.46%) and 2B (NR2B) (Hedges'g = -0.69, 95%C.I. = -1.35, -0.02; p = 0.043; I2 = 62.09%), and Postsynaptic density protein 95 (PSD-95) (Hedges'g = -0.91, 95%C.I. = -1.51, -0.32; p = 0.003; I2 = 77.81%) were found reduced in insulin-resistant animal models. Moreover, insulin-resistant animals showed significantly impaired dopamine transporter activity, whereas the dopamine D2 receptor mRNA expression (Hedges'g = 3.259; 95%C.I. = 0.497, 6.020; p = 0.021; I2 = 90.61%) increased under insulin deficiency conditions. Insulin action modulated glutamate and GABA release, as well as several enzymes involved in GABA and serotonin synthesis. These results suggest that brain neurotransmitter systems are susceptible to insulin signaling abnormalities, resembling the discrete psychotic disorders' neurobiology and possibly contributing to the development of neurobiological hallmarks of treatment-resistant schizophrenia.

Effect of oral administration of GABA on thermoregulation in athletes during exercise in cold environments: A preliminary study

Background

γ-aminobutyric acid (GABA), a common ingredient in sports supplements and other health products, regulates body temperature in the preoptic area and anterior hypothalamus (PO/AH). To date, no study has examined the effect of GABA on thermoregulation during exercise in humans in a cold temperature environment (11 ± 0.3°C, 45% ± 2% relative humidity).

Methods

We performed a randomized, double-blind study. Ten trained male athletes consumed either a drink (3 ml/kg weight) containing GABA (1,000 mg, trial G) or an equivalent amount of placebo drink (trial C) before exercise. They rested for 20 min and then cycled at 60% of maximum output power for 40 min, pedaling at 60 rpm, and recovered for 20 min. Core temperature (T_c), skin temperature (upper arm, chest, thigh, calf), and heart rate (HR) were monitored at rest (T₀), exercise begins (T₂₀), 20 min of exercise (T₄₀), the exercise ends (T₆₀), and at recovery (T₈₀).

Results

Compared to T₀, T_c decreased significantly at T₂₀ and increased significantly at T₄₀, T₆₀ and T₈₀ (p < 0.01). From 35–80 min, the T_c was higher in trial G (peaked at 37.96 ± 0.25°C) than in trial C (37.89 ± 0.37°C), but it failed to reach significant difference (p > 0.05); T_sk continued to increase during exercise and was significantly higher than T₀ at T₄₀ (p < 0.05), T₆₀ and T₈₀ (p < 0.01). There was no significant difference in T_sk between the two trials (p > 0.05).

Conclusion

Our findings provide initial evidence that oral administration of GABA does not affect thermoregulation and has no adverse effects on the body as an ergogenic exercise supplement during exercise in cold environments.

Effects of GABA and Leptin Receptor-Expressing Neurons in the Lateral Hypothalamus on Feeding, Locomotion, and Thermogenesis

OBJECTIVE

The lateral hypothalamus (LH) is known for its role in feeding, and it also regulates other aspects of energy homeostasis. How genetically defined LH neuronal subpopulations mediate LH effects on energy homeostasis remains poorly understood. The behavioral effects of chemogenetically activating LH gamma-aminobutyric acid (GABA) and the more selective population of LH GABA neurons that coexpress the leptin receptor (LepR) were compared.

METHODS

LepR-cre and VGAT-cre mice were injected with AAV5-hSyn-DIO-hM3DGq-mCherry in the LH. The behavioral effects of LH GABA or LH LepR neuronal activation on feeding, locomotion, thermogenesis, and body weight were assessed.

RESULTS

The activation of LH GABA neurons increased body temperature (P ≤ 0.008) and decreased body weight (P ≤ 0.01) despite decreased locomotor activity (P = 0.03) and transiently increased chow intake (P ≤ 0.009). Also, similar to other studies, this study found that activation of LH GABA neurons induced gnawing on both food and nonfood (P = 0.001) items. Activation of LH LepR neurons decreased body weight (P ≤ 0.01) and chow intake when presented on the cage floor (P ≤ 0.04) but not when presented in the cage top and increased locomotor activity (P = 0.002) and body temperature (P = 0.03).

CONCLUSIONS

LH LepR neurons are a subset of LH GABA neurons, and LH LepR activation more specifically regulates energy homeostasis to promote a negative energy balance.

The role of corticostriatal-hypothalamic neural circuits in feeding behaviour: implications for obesity

Emerging evidence from human imaging studies suggests that obese individuals have altered connectivity between the hypothalamus, the key brain region controlling energy homeostasis, and cortical regions involved in decision-making and reward processing. Historically, animal studies have demonstrated that the lateral hypothalamus is the key hypothalamic region involved in feeding and reward. The lateral hypothalamus is a heterogeneous structure comprised of several distinct types of neurons which are scattered throughout. In addition, the lateral hypothalamus receives inputs from a number of cortical brain regions suggesting that it is uniquely positioned to be a key integrator of cortical information and metabolic feedback. In this review, we summarize how human brain imaging can inform detailed animal studies to investigate neural pathways connecting cortical regions and the hypothalamus. Here, we discuss key cortical brain regions that are reciprocally connected to the lateral hypothalamus and are implicated in decision-making processes surrounding food.

Visualizing hypothalamic network dynamics for appetitive and consummatory behaviors

Optimally orchestrating complex behavioral states, such as the pursuit and consumption of food, is critical for an organism's survival. The lateral hypothalamus (LH) is a neuroanatomical region essential for appetitive and consummatory behaviors, but whether individual neurons within the LH differentially contribute to these interconnected processes is unknown. Here, we show that selective optogenetic stimulation of a molecularly defined subset of LH GABAergic (Vgat-expressing) neurons enhances both appetitive and consummatory behaviors, whereas genetic ablation of these neurons reduced these phenotypes. Furthermore, this targeted LH subpopulation is distinct from cells containing the feeding-related neuropeptides, melanin-concentrating hormone (MCH), and orexin (Orx). Employing in vivo calcium imaging in freely behaving mice to record activity dynamics from hundreds of cells, we identified individual LH GABAergic neurons that preferentially encode aspects of either appetitive or consummatory behaviors, but rarely both. These tightly regulated, yet highly intertwined, behavioral processes are thus dissociable at the cellular level.

Inhibition of deprivation-induced food intake by GABA(A) antagonists: roles of the hypothalamic, endocrine and alimentary mechanisms

The role of gamma amino butyric acid A receptors/neurons of the hypothalamic, endocrine and alimentary systems in the food intake seen in hunger was studied in 20 h food-deprived rats. Food deprivation decreased blood glucose, serum insulin and produced hyperphagia. The hyperphagia was inhibited by subcutaneous or ventromedial hypothalamic administration of gamma amino butyric acid A antagonists picrotoxin or bicuculline. Although results of blood glucose was variable, insulin level was increased by picrotoxin or bicuculline. In contrast, lateral hypothalamic administration of these agents failed to reproduce the above changes. Subcutaneous administration of picrotoxin or bicuculline increased gastric content, decreased gastric motility and small bowel transit. In contrast, ventromedial or lateral hypothalamic administration of picrotoxin or bicuculline failed to alter the gastric content but decreased the small bowel transit. The results of alimentary studies suggest that gamma amino butyric acid neurons of both ventromedial and lateral hypothalamus selectively regulate small bowel transit but not the gastric content. It may be concluded that ventromedial hypothalamus plays a dominant role in the regulation of food intake and that picrotoxin or bicuculline inhibited food intake by inhibiting gamma amino butyric acid receptors of the ventromedial hypothalamus, increasing insulin level and decreasing the gut motility.

Appetite at high altitude: an fMRI study on the impact of prolonged high-altitude residence on gustatory neural processing

Regulation of food intake is very important for health. It has been reported that people have decreased appetite at high altitude (HA). The current study recruited long-term HA residents to participate in an fMRI experiment which involved food craving. Result shows that the HA group showed decreased activation in the neural circuit for food craving, accompanied by decreased activation in regions for cognitive control and increased activation in regions for emotional processing. Such results also reflect the decreased gray matter volume and the hypometabolism mechanism under prolonged hypoxia stress at HA.

Effects of intraperitoneal administration of the GABA B receptor agonist baclofen on food intake in rats measured under different feeding conditions

The effects of intraperitoneal (i.p.) administration of the GABA(B) receptor agonist baclofen were assessed in rats under different feeding conditions. In Experiment 1, it was observed that baclofen (1-4 mg/kg) significantly (at least, P<0.05) increased cumulative food intake in non-deprived rats during the 120 min measurement period during the early light phase of the light-dark cycle. By contrast, during the early dark phase of the light-dark cycle in non-deprived rats, the 1mg/kg doses of baclofen significantly increased cumulative feeding at 30, 60 and 120 min (at least P<0.05), the 2mg/kg dose significantly increased feeding at 30 and 60 min (at least P<0.05) and the 4 mg/kg dose had no effects on feeding. In Experiment 2, baclofen (1-4 mg/kg) was found to produce no significant effects on food intake in rats that were food-deprived for 22 h. In Experiment 3, the effects of baclofen were investigated on food intake in 16 h food-deprived rats that had received an oral preload for 2h prior to drug administration. Baclofen (1-4 mg/kg) significantly increased cumulative food consumption (at least, P<0.05) only during the first 30 min after administration in these animals. The results of this study indicate that the effects of baclofen on food intake may be related to the state of hunger or satiety of the animals and the time during the light-dark cycle when the drug is administered.

The effects of acute multiple intraperitoneal injections of the GABAB receptor agonist baclofen on food intake in rats

This study was undertaken to examine the effects of acute repeated administration of the GABA(B) receptor agonist baclofen on food intake in rats. In Experiment 1, the effects of repeated intraperitoneal (i.p.) injections of the GABA(B) receptor agonist baclofen (1 and 2 mg/kg) at 2 h intervals were investigated on food intake in non-deprived male Wistar rats. Both doses of baclofen significantly increased food intake after the 1st injection (P<0.05), but had no effects on intake following the 2nd and 3rd injections. By contrast, in Experiment 2, diazepam (1 and 2 mg/kg, i.p.) significantly increased food intake (at least, P<0.05) after each of 3 injection separated by 2 h in non-deprived rats. These data show that tolerance occurs to the hyperphagic effects of baclofen with acute multiple injections, and may have important implications for future studies investigating the effects of GABA(B) receptor agonists on food intake and energy homeostasis.

Neural connectivity in the mediobasal hypothalamus of the sheep brain

The ventromedial nucleus of the hypothalamus (VMN) and the arcuate nucleus (ARC) are two centres regulating energy balance and food intake, but inter-connectivity of these nuclei is not well defined in non-rodent species. In this study, we performed retrograde tracing and immunohistochemistry in the ovine brain with ewes receiving FluoroGold (FG) injections into either ARC or VMN for the mapping of retrogradely labelled cells. Strong reciprocal connections were found between the two regions. The distribution of the FG labelled neurons in other regions of the hypothalamus and brain stem was also mapped. Some of the cells projecting from ARC to VMN were immunopositive for neuropeptide Y, galanin, adrenocorticotropin (marker of pro-opiomelanocortin cells) or tyrosine hydroxylase (marker of dopaminergic cells). Melanin-concentrating hormone and orexin neurons in the lateral hypothalamic area were also found to provide input to the VMN and ARC. This observed interconnectivity between regions important for metabolic regulation and other neuroendocrine functions presumably allows coordinated functions. Input to both the ARC and VMN from other brain regions, such as brain stem cell groups, provides a further level of regulation. These data provide a substrate upon which further understanding of appetite regulation and neuroendocrine function can be derived in this species.

The effects of intraperitoneal and intracerebroventricular administration of the GABAB receptor antagonist CGP 35348 on food intake in rats

In order to test the hypothesis that endogenous gamma-aminobutyric acid (GABA), acting at central GABAB receptors, plays a physiological role in the control of feeding behaviour, it was reasoned that blocking these receptors with a centrally active GABAB receptor antagonist should reduce food intake in hungry rats. In the present study, experiments were carried out to test this possibility using the GABAB receptor antagonist 3-aminopropyl-diethoxy-methyl-phosphinic acid (CGP 35348), which is water-soluble and can penetrate the blood-brain barrier from the systemic circulation. CGP 35348 (50 and 100 mg/kg, i.p.) had no effect on food intake in 22-h fasted rats, but a higher dose (i.e. 500 mg/kg., i.p.) significantly reduced cumulative food consumption. These findings are consistent with previous observations that high systemic doses of CGP 35348 are needed to block central GABAB receptors. However, to eliminate the possibility that the 500 mg/kg dose of CGP 35348 decreased food intake by a peripheral, rather than a central mode of action, further experiments were undertaken where the drug was given directly into the brain by the intracerebroventricular (i.c.v.) route. I.c.v. administration of CGP 35348 (5 and 10 microg) significantly decreased cumulative food intake food intake in rats that had been fasted for 22 h. By contrast, i.c.v. administration of CGP 35348 (10 microg) had no effect on water intake in 16-h water-deprived rats. The results indicate that CGP 35348 reduces food consumption in hungry rats by blocking central GABAB receptors in a behaviourally specific manner. These findings suggest that endogenous GABA acting at central GABAB receptors plays a physiological role in the regulation of feeding behaviour.

Adeno-associated viral glutamate decarboxylase expression in the lateral nucleus of the rat hypothalamus reduces feeding behavior

In vivo gene transfer of glutamate decarboxylase (GAD) has been explored as a means of inducing or increasing the production of the inhibitory amino-acid neurotransmitter, GABA. This strategy has been applied to neuroprotection, seizure prevention, and neuromodulation. In the present experiment, AAV2 was used to transfer the genes for green fluorescence protein (GFP) and GAD65 into the lateral nucleus of the rat hypothalamus. Microinjection of 500 nl of AAV2 resulted in transduction of a 0.25+/-0.04 mm(3) with targeting errors of X=0.48 mm, Y=0.18 mm, Z=0.37 mm using standard stereotactic technique. Pre- and postinjection food and water consumption, urine and feces production, and weight were recorded. In comparison with rAAVCAGGFP- and PBS-injected animals, rats treated with rAAVCAGGAD65 demonstrated reduced weight gain (P<0.014) and transiently reduced daily food consumption (P<0.007) during the postoperative period. No changes in water consumption or waste production were recorded. Effective GAD65 gene transfer was confirmed with in situ hybridization using a probe to the woodchuck post-transcriptional regulatory element sequence included in the vector. These findings suggest that increased GABA production in lateral nucleus of the hypothalamus induced by GAD65 gene transfer may reduce weight gain through reduced feeding.

Central pipecolic acid increases food intake under ad libitum feeding conditions in the neonatal chick

It has been demonstrated that L-pipecolic acid (L-PA) is a major metabolic intermediate of L-lysine in the mammalian and chicken brain. A previous study showed that intracerebroventricular (i.c.v.) injection of L-PA suppressed feeding in neonatal chicks, and the actions were associated with gamma-aminobutyric acid (GABA)-B receptor activation. It has been reported that endogenous L-PA in the brain fluctuated under different feeding conditions. In the present study, we investigated the effect of i.c.v. injection of L-PA on food intake in the neonatal chick under ad libitum feeding conditions. The food intake was increased by 0.5 or 1.0 mg L-PA under ad libitum feeding conditions contrary to previous studies using fasted birds. A hyperphagic effect of L-PA (0.5 mg) was attenuated by both GABA-A receptor antagonist (picrotoxin, 0.5 microg) and GABA-B receptor antagonist (CGP54626, 21.0 ng). These results indicate that a hyperphagic effect of L-PA is mediated by both GABA-A and GABA-B receptors and L-PA differentially affects food intake under different feeding conditions in the neonatal chick.

Molecular biology and gene therapy in the treatment of chronic pain

Technologic advancements have made cell type-specific targeting, expression control, and safe and stable gene transfer possible. Animal research has provided increasing experience with gene transfer to the nervous system and sensory neurons in particular. Gene-based neuromodultion can be achieved through neuronal delivery of transgenes capable of altering synaptic function. Alternatively, ex vivo gene transfer can be used to create cell lines capable of secreting analgesic neurepeptides. Translatation of these grafts and direct gene-based neuromoduation can be applied to the control of pain and the root causes of pain. These approaches combine anatomic and pharmacologic specificity. As the technology continues to improve, clinical application of cellular and molecular pain control is likely.

Extracellular glutamate increases in the lateral hypothalamus during meal initiation, and GABA peaks during satiation: microdialysis measurements every 30 s

Glutamate injected into the lateral hypothalamus can initiate eating, and gamma-aminobutyric acid (GABA) can stop it. This leads to the hypothesis that glutamate inputs are active at the beginning of a meal, and GABA is released at the end. To test this theory, the authors used microdialysis to sample glutamate and GABA simultaneously before, during, and after a meal. Food-deprived rats ate a meal of chow. Glutamate increased during the first third of the meal, then decreased to below baseline while the rats were still eating. GABA also increased at the start of the meal but continued rising and peaked during the last third of the meal. Glutamate may drive a hypothalamic system for eating, and GABA may oppose it.

Effects of subcutaneous administration of the gamma-aminobutyric acid(A) receptor agonist muscimol on water intake in water-deprived rats

The effects of the gamma-aminobutyric acid(A) (GABA(A)) receptor agonist muscimol were investigated on water intake in rats that had been deprived of water for 16 h. Muscimol (0.5-2.0 mg/kg sc) produced a dose-related inhibition of water consumption in both male (n=8) and female (n=8) rats, with maximal suppression of drinking occurring during the first 30 min after administration. Doses of 1 and 2 mg/kg produced significant decreases in water intake (P<.01), while a lower dose of 0.5 mg/kg was without effect. The hypodipsic effect of muscimol (1.0 mg/kg sc) was abolished by pretreatment of the animals with the GABA(A) receptor antagonist bicuculline (1 mg/kg sc). Furthermore, muscimol (2 mg/kg sc) did not produce aversion in a two-bottle conditioned taste aversion test, indicating that the suppressant effects of muscimol on water intake are not due to drug-induced malaise. The results suggest that systemic administration of muscimol produces a behaviourally specific suppression of primary drinking in rats by a GABA(A) receptor-mediated mechanism. Moreover, this action of muscimol appears to be independent of the gender of the animals.

GABA(A) and GABA(B) receptors in the anterior piriform cortex modulate feeding in rats

The effects of GABA(A) and GABA(B) receptors in the anterior piriform cortex (APC) on intake of an amino acid imbalanced diet and a basal diet were evaluated in rats. Administration of muscimol (GABA(A) receptor agonist) to the APC immediately suppressed ingestion of both amino acid imbalanced and basal diets. Central administration of bicuculline (a GABA(A) receptor antagonist) stimulated feeding of the amino acid imbalanced diet but had no effect on intake of the basal diet. The GABA(B) receptor antagonist phaclofen decreased consumption of the basal diet but did not affect consumption of the amino acid imbalanced diet. These findings demonstrate that manipulation of GABA-sensitive cells in the APC can have a pronounced effect on feeding behavior that is not selective to aminoprivic feeding. However, these data suggest that GABA(A) and GABA(B) receptors may function as regulators that are activated by monoaminergic systems and neuropeptides in response to amino acid imbalanced diet intake. Inhibitory effects of GABA(A) and GABA(B) receptors may modulate the pyramidal cells, contributing to the reduced feeding response to the amino acid imbalanced diet. Also, transcription of mRNA for both GABA receptors and the GABA reuptake transporter was affected by a threonine deficient but not a corrected diet, compared to the basal diet. Taken together, these results support the involvement of GABA receptors in the APC in feeding in general and the responses to amino acid deprivation in vivo.

The influence of isolation and aminooxyacetic acid (AOAA) on GABA in muricidal rats

The influence of isolation was studied on the development of muricidal behavior and on brain GABA. GABA was significantly lower in several parts of the limbic system of brain taken from muricidal rats, when compared to their non-muricidal counterparts. Isolation potentiated the development of muricidal behavior and lowered GABA levels. Administration of aminooxyacetic acid (AOAA) resulted in a preferential elevation of GABA in the olfactory lobes. Muricidal behavior was inhibited during the AOAA-induced increase in GABA.

Noradrenergic and GABAergic systems in the medial hypothalamus are activated during hypoglycemia

Noradrenergic and GABAergic systems in the medial hypothalamus influence plasma glucose and may be activated during glucoprivation. Microdialysis probes were placed into the ventromedial nucleus (VMH), lateral hypothalamus (LHA), and paraventricular nucleus (PVH) of male Sprague-Dawley rats to monitor extracellular concentrations of norepinephrine (NE) and GABA. During systemic hypoglycemia, induced by insulin (1.0 U/kg), NE concentrations increased in the VMH (P < 0.05) and PVH (P = 0.06) in a bimodal fashion during the first 10 min and 20-30 min after insulin administration. In the VMH, GABA concentrations increased (P < 0.05) in a similar manner as NE. Extracellular NE concentrations in the LHA were slightly lower (P = 0.13), and GABA levels remained at baseline. The increases in NE and GABA in the VMH were absent during euglycemic clamp; however, NE in the PVH still increased, reflecting a direct response to hyperinsulinemia. On the basis of these data, we propose that the activity of noradrenergic afferents to the medial hypothalamus is increased during hypoglycemia and influences the activity of local GABAergic systems to activate appropriate physiological compensatory mechanisms.

Effects of three neurochemical stimuli on delayed feeding and energy metabolism

Infusions of norepinephrine (NE), the gamma-aminobutyric acid agonist, muscimol (MUS), or neuropeptide Y (NPY) into the paraventricular nucleus (PVN) of the hypothalamus all increase food intake. Such feeding may be due to direct activation of behavioral processes driving ingestion and/or to alterations in nutrient metabolism that feeding serves to normalize. To examine these possibilities, male Sprague-Dawley rats received PVN infusions of vehicle, 20 nmol NE, 1 nmol MUS or 100 pmol NPY at dark onset, then food intake was measured under three feeding conditions: (1) 1 and 2 h immediately after injections, (2) 1 h after a 1 h delay between injections and access to food, and (3) 1 h after a 1 h feeding delay, but with injections occurring just before presenting food. Measures of energy expenditure (EE) and respiratory quotients (RQs) in the absence of food were made over 2 h in parallel experiments. Results confirmed that NE, MUS and NPY all increased dark-onset feeding, but only NPY increased intake above control levels after a 1 h feeding delay. No neurochemically-induced changes in EE were observed, nor were there changes in RQs after NE or MUS. However, NPY reliably enhanced RQs from 30 to 120 min of testing. Our findings imply that NE and MUS initiate relatively immediate, short-term feeding that is not associated with changes in nutrient metabolism and does not summate with cues stimulated by delayed access to food. NPY initiates more protracted feeding temporally linked to enhanced carbohydrate metabolism. This may indicate that part of NPY's feeding stimulatory effects are secondary to physiological processes driving ingestion.

Influence of benzodiazepines on body weight and food intake in obese and lean Zucker rats

1. The gamma-aminobutyric acid (GABA)-ergic system, which is functionally altered in obese (fa/fa) Zucker rats, plays an important role in controlling energy balance within the central nervous system. 2. GABA receptors seem to be involved in the dysfunction of the hypothalamic energy homeostasis-controlling mechanisms in these animals due to a genetically-induced defect of the leptin-neuropeptide Y system. 3. To shed further light on the possible role played by the GABA system in the pathogenesis of this rat model, two benzodiazepine (BDZ) receptor agonists (diazepam and clonazepam) and one BDZ antagonist (flumazenil) were administered intraperitoneally in obese and lean Zucker rats. 4. Body weight gain was reduced by the BDZ agonists in both phenotypes, and one receptor-agonist (diazepam) lowered insulin concentration in obese rats. In GABA-antagonist-treated obese rats, the daily amount of body weight gain and food intake acquired an oscillatory rhythm similar to that of normal rodents. 5. By demonstrating the role of BDZ receptors, these findings may help clarify the pathophysiology of obesity and insulin resistance in fatty Zucker rats.

In the ventromedial nucleus of the rat hypothalamus, GABA-immunolabeled neurons are abundant and are innervated by both enkephalin- and GABA-immunolabeled axon terminals

Immunohistochemical-labeling for the neurochemicals gamma-aminobutyric acid (GABA) and enkephalin are abundant in the ventromedial nucleus of the hypothalamus (VMN). In VMN, both GABA and enkephalin may function to regulate feeding behavior, as well as other hormone-controlled behaviors. Importantly, in several brain areas, enkephalin is often thought to modulate GABAergic neurotransmission. Therefore, we used dual-labeling immunohistochemistry with electron microscopic analysis to study the circuitry of neurons containing GABA- and/or enkephalin-labeling within the VMN. Somato-dendritic profiles containing GABA-labeling were three fold more abundant than GABA-labeled axon terminals (117 soma or dendrites vs. 34 axons). In addition, axon terminals containing GABA-labeling sometimes synapsed onto GABA-labeled somata or dendrites (25% or 9/34). In contrast, under these conditions labeling for enkephalin was primarily restricted to axon terminals, which were very abundant throughout VMN. Enkephalin-containing terminals accounted for a large fraction (25% 23/92) of the axons in contact with GABA-labeled dendrites, although they also contacted unlabeled dendrites. These observations suggest that a population of VMN neurons are GABAergic. These may be either local circuit 'interneurons' or projection neurons. In addition, GABA-labeled VMN neurons may be regulated by either enkephalin or GABA. These morphologic observations provide the basis for disinhibitory mechanisms to function within the VMN.

Effect of glucose and 2-deoxyglucose on hypothalamic GABA release in lactating rats

Gamma-amino butyric acid (GABA), an inhibitory neurotransmitter, has been implicated in the control of feeding behavior. This study was conducted to investigate the in vitro release of GABA in the basal medial hypothalamus (BMH) of hyperphagic lactating (L) and control nonlactating (NL) rats. Pregnant Sprague-Dawley rats (n = 10) were ad lib fed a semipurified powdered diet during the last 6 days of pregnancy until day 19 of lactation. Nonpregnant (n = 10) animals served as controls. Body weights and food intake were recorded every other day. Lactating rats demonstrated an increase in body weight as well as food intake as compared to nonlactating animals. At sacrifice, the BMH was removed and perfused (0.1 ml/min) with Kreb's Ringer buffer (KRB) ("basal" medium) using a Brandel perifusion system. KRB containing glucose (100 mM) or 2-deoxyglucose (2DG) (100 mM) was also applied to the tissue. Potassium stimulation was carried out to test for the viability of the tissues. Samples were collected every 10 min, derivatized with O-Phthalaldehyde and analyzed via HPLC. Glucose depressed, and 2DG enhanced GABA release compared to basal levels. There were no significant differences in GABA release between lactating and nonlactating groups. These data suggest that GABA release is responsive to metabolic changes in the brain.

The lateral hypothalamic area revisited: ingestive behavior

This article discusses the role of the lateral hypothalamic area (LHA) in feeding and drinking and draws on data obtained from lesion and stimulation studies and neurochemical and electrophysiological manipulations of the area. The LHA is involved in catecholaminergic and serotonergic feeding systems and plays a role in circadian feeding, sex differences in feeding and spontaneous activity. This article discusses the LHA regarding dietary self-selection, responses to high-protein diets, amino acid imbalances, liquid and cafeteria diets, placentophagia, "stress eating," finickiness, diet texture, consistency and taste, aversion learning, olfaction and the effects of post-operative period manipulations by hormonal and other means. Glucose-sensitive neurons have been identified in the LHA and their manipulation by insulin and 2-deoxy-D-glucose is discussed. The effects on feeding of numerous transmitters, hormones and appetite depressants are described, as is the role of the LHA in salivation, lacrimation, gastric motility and secretion, and sensorimotor deficits. The LHA is also illuminated as regards temperature and feeding, circumventricular organs and thirst and electrolyte dynamics. A discussion of its role in the ischymetric hypothesis as an integrative Gestalt concept concludes the review.

Effects of GABA and L-glutamate on the gastric acid secretion and gastric defensive mechanisms in rat lateral hypothalamus

The effects of administration of an inhibitory GABAergic or excitatory glutaminergic neurotransmitter into the lateral hypothalamic area (LHA) on gastric acids, an aggressive mechanism, and transepithelial potential difference (PD) and mucosal blood flow (MBF), defensive mechanisms, were examined in anesthetized rats, since lesions of LHA in these animals cause gastric mucosal damage and electrical stimulation stimulates gastric acids and antral contractions. Microinfusion of the inhibitory neurotransmitter, muscimol (GABA agonist) resulted in an increase in gastric acid secretion and in PD and MBF. The GABA antagonists picrotoxin and bicuculline methiodide, in contrast, decreased these three factors. The excitatory neurotransmitter L-glutamate induced only an increase of MBF. Thus, the GABAergic system in LHA stimulates the gastric functions, both defensive and aggressive mechanisms, while the glutaminergic system increases only a portion of the defensive system. The results suggest that there is a significant interaction between LHA and stomach functions.

Insulin resistance syndrome: defective GABA neuromodulation as a possible hereditary pathogenetic factor (the 'GABA hypothesis')

The origin of the insulin resistance syndrome which, mostly through its cardiovascular implications, is characterized by a high incidence of death, is still practically unknown. Energy and glucose homeostasis are under the control of CNS centers and the neurotransmitter GABA modulates the activity of these centers. Alteration of the biochemical structure of GABA receptors is suggested as an interpretation of the origin of the syndrome.

Intrahypothalamic, but not hippocampal, administration of muscimol suppresses hyperglycemia induced by hippocampal neostigmine in anesthetized rats

We investigated the effects of intrahypothalamic or hippocampal injection of GABA receptor agonists on hyperglycemia induced by hippocampal neostigmine. Prior to the injection of neostigmine (50 nmol) into the hippocampus (HPC), muscimol (0.01-1 nmol) or baclofen (1 nmol) was injected into the bilateral ventromedial hypothalamus (VMH). Muscimol suppressed the hyperglycemia in a dose-dependent manner, but baclofen affected it only minimally. In contrast, neither hippocampal muscimol (1 or 2.5 nmol) nor baclofen (1 nmol) suppressed the hippocampal neostigmine-dependent hyperglycemia. Intrahypothalamic muscimol (1 nmol) also decreased the changes in hepatic venous plasma glucagon and epinephrine significantly. These results indicate that intrahypothalamic muscimol suppresses hyperglycemia caused by cholinergic neurons originating from the HPC, indicating existence of the location specificity.

Lateral preoptic neurons inhibit thirst in the rat

Kainic acid (KA) and muscimol were injected into the lateral preoptic area (LPO) of the rat to study their effects on drinking behavior. A low dose (5 ng) of KA, which stimulates neurons, decreased the amount of water intake induced by hypertonic saline (IP) and angiotensin II (SC). Injection of 2 ng muscimol, a potent GABAA receptor agonist that suppresses neurons, facilitated drinking responses induced by hypertonic saline, but did not affect angiotensin II-induced drinking. Rats injected with a high dose (150 ng) of KA, which destroys neurons, showed marked polydipsia accompanied by increased urination. One week after the KA lesion, drinking and urine output recovered to normal. During the polydipsia, a small volume of concentrated urine could be excreted if water intake was restricted. After recovery, excessive drinking responses followed water deprivation and hypertonic saline load. The rats normally drank water in response to angiotensin II and to polyethylene glycol solution. The results show that activation of LPO neurons inhibits water intake, and that suppression of LPO neurons facilitates osmotically induced water intake. Therefore, LPO neurons are probably involved in the inhibition of thirst.

GABAA receptors in the amygdala: role in feeding in fasted and satiated rats

The purpose of this study was to clarify further the site of action in the amygdala as well as functional characteristics of feeding in response to two GABA receptor agonists. Guide cannulae for microinjection were implanted stereotaxically in the rat just above the central nucleus of the amygdala (CNA). Microinjections of 0.05, 0.25, 0.5 or 1.0 nmol muscimol, a GABAA-selective receptor agonist, produced a dose- and time-dependent decrease of food intake in both the satiated and fasted rat. The bilateral injection of muscimol into the amygdala was more effective than a unilateral injection during the first 2 h, although the overall effects were similar. Microinjection of 0.1 nmol bicuculline methiodide, a GABAA receptor antagonist, into the CNA significantly blocked this inhibitory effect of 0.05 and 0.5 nmol muscimol again in both the satiated and fasted rat. Doses of 0.05, 0.5, 5.0 and 10.0 nmol of the selective GABAB agonist, baclofen, injected into homologous sites in the CNA did not alter food intake. These findings support the viewpoint that the amygdala and its central nucleus comprise a pivotal region involved in the mechanisms underlying the control of feeding behavior. Further, it is envisaged that hypophagic or anorexic responses are induced through the activation of GABAA receptors by the presynaptic release of GABA from neurons which form a component of the anatomical system for hunger and satiety.

Systemic administration of baclofen inhibits water intake in rats

1. The present experiments were carried out to investigate the effects of systemic administration of baclofen on water intake in rats. 2. Baclofen (1, 2 and 4 mg/kg, s.c.) inhibited water intake in 16 hr water-deprived rats in a dose-related manner, with maximal effects occurring during the first 30 min after administration. 3. Baclofen (0.25 and 2 mg/kg, s.c.) had no effects on water intake in non-deprived rats. 4. Baclofen (2 mg/kg) inhibited water intake elicited by i.p. injection of hypertonic NaCl in rats. 5. Baclofen (1 mg/kg) did not produce taste aversion in a taste aversion experiment. This indicates that the effects of baclofen on water intake is not due to an aversive effect of the drug.

The effect of systemic administration of baclofen on food intake in rats

The effects of systemic administration of the GABAB agonist, baclofen was investigated on food intake in non-fasted rats. Baclofen (1.0, 2.0 and 4.0 mg/kg, s.c.) produced a dose-related increase in food intake in a free-feeding paradigm during the first 90 min after administration, with maximum increases occurring at a dose of 2 mg/kg (Experiment 1). Baclofen (0.5 and 1.0 mg/kg, s.c.) also increased food intake in the 40 min post-drug recording period in non-fasted rats, trained to make operant responses for food on a fixed-ratio schedule (Experiment 2). These results demonstrate that systemic administration of baclofen can stimulate ingestive behaviour in satiated rats and suggest a possible role for a GABAB receptor-mediated mechanism in the control of food intake.

Induction of food intake by a GABAergic mechanism in the turkey

The effect of GABAergic influences on food and water intake in Large White turkeys was investigated. In Experiment 1, food and water intake were monitored in hens injected intracerebroventricularly (ICV) with varying doses of muscimol, a GABA agonist. In Experiment 2, hens were pretreated with ICV injections of picrotoxin, a GABA antagonist, prior to the injection of muscimol. In Experiment 3, the effect of ICV injections of muscimol on water intake was determined in hens not allowed access to food. The ICV injection of muscimol caused a dose-dependent increase in food intake with 75 nmole being the most efficacious dose. Water intake was stimulated following the injection of muscimol when food and water were available ad lib. However, water intake was not affected by the ICV injection of muscimol if food was not simultaneously available, indicating that the increased water intake was a result of increased food intake. The effect of muscimol on food intake was significantly attenuated by pretreatment with picrotoxin. It appears, therefore, that GABA acts within the brain of the turkey to increase food, but not water, intake.

Effect of glucoprivation on glutamate decarboxylase activity in the ventromedial nucleus

Activity of the GABA-synthesizing enzyme, glutamate decarboxylase (GAD), was measured in brain areas involved in glucoregulation 60 min after a glucoprivie challenge. The rate of GAD activity in the ventromedial nucleus of the hypothalamus (VMN) increased in a dose-dependent manner in response to intraperitoneal injection of 2-deoxy-D-glucose (2-DG). The increase in VMN GAD activity was significantly correlated with an increase in food intake (r2 = .77, p less than 0.01). The increase in VMN GAD activity was not due to the higher food intake since rats receiving 2-DG and denied access to food also had elevated rates of VMN GAD activity. VMN GAD activity was increased 28% and 32% after intracerebroventricular injection of 2-DG or 5-thioglucose, respectively. The rates of GAD activity in the lateral hypothalamus and area postrema were not affected by either peripherally or centrally administered 2-DG. The increase in VMN GAD activity after glucoprivation may be involved in the regulation of blood glucose by influencing food intake.

GABA-related feeding control in genetically obese rats

Feeding in response to glucoprivation induced by 2-deoxy-D-glucose (2-DG) is impaired in genetically obese (Zucker) rats. Muscimol, a GABAA-agonist (0.5 nmol/0.5 microliter in each area) increased food intake in lean rats over 3 h but in fatty rats only at 30 min after infusion into the VMH. Injection of muscimol into the DMH and PVN increased feeding of both phenotypes. Picrotoxin, a non-competitive GABAA-antagonist (0.1 nmol/0.5 microliter) increased food intake after infusion into the LH of both phenotypes and decreased food intake over a 3 h period when infused into the VMH. DMH and PVN of fatty rats. In the lean littermates, picrotoxin was only effective in reducing food intake at 30 min after infusion into the VMH and PVN but not the DMH. The present results suggest that the fatty Zucker rat has a disturbance in the GABA-related regulatory mechanism of feeding behavior in the ventromedial hypothalamus, which may be responsible for the impaired response to glucoprivation found in these rats.

Effects of intracerebroventricular injection of muscimol or GABA on operant feeding in pigs

Young pigs were prepared with lateral intracerebroventricular (ICV) cannulae. They were housed individually in cages fitted with operant panels and could obtain food and water ad lib. The GABA-A receptor agonist muscimol (25-200 nmol) ICV produced an increase in food intake in which the dose-response relation was most obvious 30-60 min after dosing. The 25-nmol dose had no effect on feeding. However, muscimol (50 nmol) caused a significant increase in feeding (p less than 0.01) during the first 30 min after injection, while the 100- and 200-nmol doses increased food intake (p less than 0.01) during the first 60 min. The effect of muscimol (100 nmol) on food intake was completely abolished by the simultaneous administration of the GABA-A receptor antagonist bicuculline (100 nmol). GABA (40-1600 nmol) ICV also produced a dose-related increase in food intake (p less than 0.01) in the 15 min after injection. Only doses of 800 nmol and above were effective. The effects of GABA (1600 nmol) were completely abolished by the simultaneous administration of bicuculline (50 nmol). Neither muscimol nor GABA influenced food intake for the 24-hr time period or water intake during any time period. The results indicate that stimulation of central GABA-A receptors induces operant feeding in the satiated pig.

Direct evidence indicating that a GABA-mimetic stimulates acid secretion through central mechanisms

Acid secretagogue effects of central and peripheral baclofen were compared in the rat. Intravenously (0.1-1.0 mg/kg) and intracerebroventricularly (0.1-1.0 micrograms) administered baclofen augmented acid output to the same degree in a dose-dependent manner. GABA microinjected into the lateral hypothalamus (200 and 400 micrograms) significantly increased acid output. These support the proposal that baclofen stimulates acid secretion through central mechanisms and also the possible role of GABA in central regulation mechanisms of acid secretion.

Gastric acid inhibitory action of a GABA-related compound, 3-amino-3-phenylpropionic acid, in the rat

The acid inhibitory properties of 3-amino-3-phenylpropionic acid, a structural GABA analogue, were studied in the perfused rat stomach preparation. 3-Amino-3-phenylpropionic acid, 10 and 30 mg/kg i.v., dose dependently suppressed the gastric acid secretion induced by baclofen (2 mg/kg s.c.). This secretagogue action had been shown to be unaffected by either GABAA or GABAB receptor antagonists. The i.v. administration of 3-amino-3-phenylpropionic acid (3 and 10 mg/kg) was also effective to abolish the acid stimulatory effects of muscimol (1 mg/kg i.v.) and 2-deoxy-D-glucose (200 mg/kg i.v.). 3-Amino-3-phenylpropionic acid, even at the high dose (30 mg/kg i.v.) had no influence on the acid output in response to histamine and bethanechol. Furthermore, 3-amino-3-phenylpropionic acid had no significant effect on the acid secretion induced by electrical vagal stimulation. These results indicate that the antisecretory effect of 3-amino-3-phenylpropionic acid is different from those of antimuscarinics, H2-receptor antagonists and vagal blockade. Together, the results suggest that 3-amino-3-phenylpropionic acid might act in the brain to inhibit central regulation mechanisms of gastric acid secretion, probably through GABA mechanisms.

Neurons containing messenger RNA encoding glutamate decarboxylase in rat hypothalamus demonstrated by in situ hybridization, with special emphasis on cell groups in medial preoptic area, anterior hypothalamic area and dorsomedial hypothalamic nucleus

Previous deafferentation studies have suggested that most hypothalamic GABAergic innervation originates from neurons within the hypothalamus. We have investigated the distribution of GABAergic cell groups in the rat hypothalamus by means of the in situ hybridization technique, using a cDNA probe for messenger RNA encoding glutamate decarboxylase. Several major GABAergic cell groups were demonstrated, including cells of the tuberomammillary nucleus, arcuate nucleus, suprachiasmatic nucleus, medial preoptic area, anterior hypothalamic area, the dorsomedial hypothalamic nucleus, perifornical area, and lateral hypothalamic area. The most prominent glutamate decarboxylase mRNA-containing cell groups were located in the medial preoptic area, anterior hypothalamic area and dorsomedial hypothalamic nucleus, and were composed of small- to medium-sized neurons. Compared to previously well-characterized GABAergic cell groups in the tuberomammillary nucleus, reticular thalamic nucleus, and non-pyramidal cells of cerebral cortex, the cells of these GABAergic groups demonstrated only weak cDNA labelling, indicating that they contain lower levels of glutamate decarboxylase mRNA. Several types of control experiments supported the specificity of this cDNA labelling, and the GABAergic nature of these cell populations was further supported by detection of glutamate decarboxylase and GABA immunoreactivity. Abundance of GABAergic cells in many hypothalamic nuclei indicates that GABA represents quantitatively the most important transmitter of hypothalamic neurons, and may be involved in neuroendocrine and autonomic regulatory functions.

Brain glutamate and gamma-aminobutyrate (GABA) metabolism in thiamin-deficient rats

The brain metabolism of glutamate and gamma-aminobutyrate (GABA) was investigated in thiamin-deficient and pair-fed control rats, in order to determine whether the GABA shunt may provide an important alternative to 2-oxo-glutarate dehydrogenase (EC 1.2.4.2) in energy-yielding metabolism in thiamin deficiency. Brains from thiamin-deficient animals contained less glutamate, 2-oxo-glutarate and GABA than those from control animals. The brain content of ATP was unaffected by thiamin deficiency. After intracerebroventricular injection of [14C]glutamate, the specific radioactivity of GABA in the brains from deficient animals was 45-50% higher than that in controls, suggesting a considerable increase in the metabolic flux through the GABA shunt in thiamin deficiency. Brain GABA showed a marked circumannual variation, with a peak in mid-summer and a minimum value in mid-winter.

GABA receptors: are cellular differences reflected in function?

The putative involvement of GABAA and GABAB receptors in various behavioral and physiological effects is summarized in Table III. A division of function among the two types of GABA receptors appears to exist. GABAA receptors mediate feeding, cardiovascular regulation, anxiolytic effects, and anticonvulsive activity. GABAB receptors, on the other hand, are involved in analgesia, cardiovascular regulation, and depression. Although there is some overlap and shared functions among the receptor types, it is evident that GABAA and GABAB receptors have different behavioral and physiological profiles. Feeding, anticonvulsive activity and anxiety, for example, primarily involve GABAA receptors. Analgesia and depression, on the other hand, are GABAB effects. In those cases where GABAA and GABAB receptors mediate similar functions (e.g. cardiovascular regulation), they do so by affecting different transmitter systems and cellular mechanisms. It is proposed, therefore, that GABAA and GABAB receptors differ not only at the cellular level, but that they also have different functions in the mammalian central nervous system. The association of different subtypes of a receptor with different functions and mechanisms of action is not unique to the GABA system. D1 and D2 receptors in the dopamine system, for example, also exhibit some separation of function as do the mu, delta and kappa types of opiate receptors. Different subtypes of neurotransmitter receptors, therefore, appear to be a general organizing principle used by the brain to transduce chemical signals into different functional responses. A better understanding of the exact processes through which cellular signals are transformed into functional responses is a goal of future research.

Brain 3-hydroxybutyrate, glutamate, and GABA in a rat model of dietary obesity

Whole brain concentrations of 3-hydroxybutyrate, glutamate and gamma-aminobutyric acid (GABA) have been measured in two strains of rats with differing susceptibility to obesity. S 5B/Pl rats are resistant to developing obesity when eating a high-fat diet, whereas Osborne-Mendel rats readily develop obesity when eating the same diet. We tested the hypotheses that brain 3-hydroxybutyrate, glutamate and GABA differ between S 5B/Pl rats and Osborne-Mendel rats, and that these substrates/neuroregulators are altered when eating a high-fat diet primarily in S 5B/Pl (resistant) rats. Blood and brain 3-hydroxybutyrate concentrations were higher in S 5B/Pl rats than in Osborne-Mendel rats (p less than 0.05) but diet effects were not significant. Brain glutamate concentration, like 3-hydroxybutyrate, was higher in S 5B/Pl rats than in Osborne-Mendel rats (p less than 0.01) and was not affected by adding fat to the diet. Brain GABA differed only slightly between strains but increased after adding fat to the diet (p less than 0.05) in both strains with a greater increase occurring in S 5B/Pl rats. The brains of S 5B/Pl rats are chronically exposed to higher levels of 3-hydroxybutyrate and glutamate than are those of Osborne-Mendel rats. Thus, 3-hydroxybutyrate is a potential signal in the regulation of body weight. Brain GABA increases with fat feeding, especially in S 5B/Pl rats, suggesting that the ability to adjust to an energy dense diet may be through suppression of food intake by elevated brain GABA.

Food intake suppressant effect of baclofen in rats

1. Baclofen given intraperitoneally (i.p.) to rats caused a dose-dependent decrease in food intake. 2. Bicuculline or picrotoxin (GABAA-antagonist) and methergoline (5-HT antagonist) decreased the anorectic effect of baclofen. 3. Pimozide (dopamine receptor blocker), phenoxybenzamine and propranolol (alpha and beta adrenergic blockers) did not diminish the baclofen effect, but even increased the anorexia induced by the drug. 4. It can be postulated that, at least partially, GABAA receptor mechanism, GABA-5HT receptor complex and/or 5-HT mechanism may be involved in baclofen induced anorexia.

Inhibition constants and GABA-shifts at 2 degrees C and 37 degrees C for a spectrum of ligands acting on the benzodiazepine receptors of guinea pig hippocampus

1. Dissociation constants and Hill coefficients were determined for 13 ligands inhibiting the binding of [3H]flunitrazepam to the benzodiazepine receptors of guinea pig hippocampus at 2 degrees C and 37 degrees C. 2. The ratio of I50 in the absence of gamma-aminobutyric acid (GABA) to that in the presence of 10(-5) M GABA (the GABA-shift) was found to vary from 0.4 to 2.5. 3. There was no correlation between the pharmacological activity of the ligand and the GABA-shift, or between the pharmacological activity and the magnitude of the increase in affinity of the ligand for the receptor as the temperature decreased from 37 degrees C to 2 degrees C. 4. A correlation was observed between the temperature-induced affinity change and the GABA-shift at 37 degrees C.

Experimental obesity: a homeostatic failure due to defective nutrient stimulation of the sympathetic nervous system

The basic hypothesis of this review is that studies on models of experimental obesity can provide insight into the control systems regulating body nutrient stores in humans. In this homeostatic or feedback approach to analysis of the nutrient control system, we have examined the afferent feedback signals, the central controller, and the efferent control elements regulating the controlled system of nutrient intake, storage, and oxidation. The mechanisms involved in the beginning and ending of single meals must clearly be related to the long-term changes in fat stores, although this relationship is far from clear. Changes in total nutrient storage in adipose tissue can arise as a consequence of changes in the quantity of nutrients ingested in one form or another or a decrease in the utilization of the ingested nutrients. A change in energy intake can be effected by increased size of individual meals, increased number of meals in a 24-hour period, or a combination of these events. Similarly, a decrease in utilization of these nutrients can develop through changes in resting metabolic energy expenditure which are associated with one of more of the biological cycles such as protein metabolism, triglyceride for glycogen synthesis and breakdown, or maintenance of ionic gradients for Na+ + K+ across cell walls. In addition, differences in energy expenditure related to the thermogenesis of eating or to the level of physical activity may account for differences in nutrient utilization.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.