Hypothalamic Monoaminergic Activity in 11-Week-Old Cold-Exposed Female Lean(Fa/Fa)and Obese(fa/fa)Zucker Rats

We previously reported that serotonergic activity was reduced in the ventromedial hypothalamic nucleus (VMN) of obese vs. lean male Zucker rats. To verify that this reduction was associated with genotype rather than gender, we measured monoamines and their major metabolites in hypothalamic nuclei of 11-week-old female lean (Fa/Fa) and obese (fa/fa) Zucker rats. In addition, since the thermic response to cold is reported to differ between lean and obese rats, some rats were also exposed to 9 degrees or 22 degrees C for 2h to determine if cold exposure altered hypothalamic monoaminergic activity. As in males, levels of 5-hydroxyindoleacetic acid [5-HIAA; major metabolite of serotonin (5-HT)] and the ratio of 5-HIAA/5-HT were lower in the VMN of obese vs. lean females (P = 0.008, 0.001, respectively). 5-HIAA/5-HT was also reduced in the paraventricular (PVN) and suprachiasmatic nuclei (SCN) of the obese compared to the lean females. Cold exposure significantly stimulated brown fat mitochondrial GDP binding in lean but not obese rats. Similarly, levels of norepinephrine, dopamine (DA), 5-HIAA, and 5-HT in the PVN, and 5-HIAA in the SCN increased in cold-exposed lean but not obese rats. In contrast, VMN and preoptic 3,4-dihydroxyphenylacetic acid (DOPAC; major metabolite of DA) increased in the cold-exposed obese but not lean animals. We conclude that: (1) the blunted peripheral response to cold in obese vs. lean Zucker rats is accompanied by altered hypothalamic monoaminergic activity, the physiological role of which needs further evaluation; and 2) depressed VMN serotonergic activity is associated with the obese genotype (fa/fa) rather than gender and as such may contribute to the reduced sympathetic and enhanced parasympathetic outflow from the VMN.

The response of glucose-excited neurones in the ventromedial hypothalamus to decreased glucose is enhanced in a murine model of type 2 diabetes mellitus

Obesity and type 2 diabetes mellitus (T2DM) are associated with dysfunctional insulin signalling and impaired central glucose sensing. Glucose sensing neurones reside in key areas of the brain involved in glucose and energy homeostasis (e.g. ventromedial hypothalamus; VMH). We have recently shown that insulin attenuates the ability of glucose-excited (GE) neurones to sense decreased glucose. We hypothesise that this effect of insulin on VMH GE neurones is impaired during T2DM when insulin signalling is dysfunctional. To test our hypotheses, we used whole cell patch clamp recording techniques to evaluate the effects of insulin on VMH GE neurones in brain slices from wild-type and diabetic (db/db) mice. The effects of decreasing glucose from 2.5 to 0.1 mM on VMH GE neurones were similar in wild-type and db/db mice. However, decreasing glucose from 2.5 to 0.5 mM decreased the action potential frequency, membrane potential and input resistance of VMH GE neurones to a significantly greater extent in db/db versus wild-type mice. Furthermore, insulin (5 nM) blunted the effects of decreased glucose in wild-type, but not db/db mice. These differences in both glucose and insulin sensitivity between wild-type and db/db mice were completely ameliorated by the insulin sensitiser, Compound 2 (300 nM). These data are consistent with our hypothesis that impaired insulin signalling in T2DM sensitises VMH GE neurones to decreased glucose.

Effects of Oleic Acid on Distinct Populations of Neurons in the Hypothalamic Arcuate Nucleus Are Dependent on Extracellular Glucose Levels

Pharmacological manipulation of fatty acid metabolism in the hypothalamic arcuate nucleus (ARC) alters energy balance and glucose homeostasis. Thus, we tested the hypotheses that distinctive populations of ARC neurons are oleic acid (OA) sensors that exhibit a glucose dependency, independent of whether some of these OA sensors are also glucose-sensing neurons. We used patch-clamp recordings to investigate the effects of OA on ARC neurons in brain slices from 14- to 21-day-old Sprague–Dawley (SD) rats. Additionally, we recorded spontaneous discharge rate in ARC neurons in 8-wk-old fed and fasted SD rats in vivo. Patch-clamp studies showed that in 2.5 mM glucose 12 of 94 (13%) ARC neurons were excited by 2 μM OA (OA-excited or OAE neurons), whereas six of 94 (6%) were inhibited (OA-inhibited2.5or OAI2.5neurons). In contrast, in 0.1 mM glucose, OA inhibited six of 20 (30%) ARC neurons (OAI0.1neurons); none was excited. None of the OAI0.1neurons responded to OA in 2.5 mM glucose. Thus OAI2.5and OAI0.1neurons are distinct. Similarly, in seven of 20 fed rats (35%) the overall response was OAE-like, whereas in three of 20 (15%) it was OAI-like. In contrast, in fasted rats only OAI-like response were observed (three of 15; 20%). There was minimal overlap between OA-sensing neurons and glucose-sensing neurons. In conclusion, OA regulated three distinct subpopulations of ARC neurons in a glucose-dependent fashion. These data suggest that an interaction between glucose and fatty acids regulates OA sensing in ARC neurons.

Differential effects of glucose and lactate on glucosensing neurons in the ventromedial hypothalamic nucleus

Glucose directly alters the action potential frequency of glucosensing neurons in the ventromedial hypothalamic nucleus (VMN). Glucose-excited neurons increase, and glucose-inhibited neurons decrease, their action potential frequency as glucose increases from 0.1 to 2.5 mmol/l. Glucose-excited neurons utilize the ATP-sensitive K(+) channel (K(ATP) channel) to sense glucose, whereas glucose opens a chloride channel in glucose-inhibited neurons. We tested the hypothesis that lactate, an alternate energy substrate, also regulates the action potential frequency of VMN glucose-excited and -inhibited but not nonglucosensing neurons. As expected, lactate reversed the inhibitory effects of decreased glucose on VMN glucose-excited neurons via closure of the K(ATP) channel. Although increasing glucose from 2.5 to 5 mmol/l did not affect the activity of glucose-excited neurons, the addition of 0.5 mmol/l lactate or the K(ATP) channel blocker tolbutamide increased their action potential frequency. In contrast to the glucose-excited neurons, lactate did not reverse the effects of decreased glucose on VMN glucose-inhibited neurons. In fact, it increased their action potential frequency in both low and 2.5 mmol/l glucose. This effect was mediated by both K(ATP) and chloride channels. Nonglucosensing neurons were not affected by lactate. Thus, glucose and lactate have similar effects on VMN glucose-excited neurons, but they have opposing effects on VMN glucose-inhibited neurons.

The regulation of glucose-excited neurons in the hypothalamic arcuate nucleus by glucose and feeding-relevant peptides

Glucosensing neurons in the hypothalamic arcuate nucleus (ARC) were studied using electrophysiological and immunocytochemical techniques in neonatal male Sprague-Dawley rats. We identified glucose-excited and -inhibited neurons, which increase and decrease, respectively, their action potential frequency (APF) as extracellular glucose levels increase throughout the physiological range. Glucose-inhibited neurons were found predominantly in the medial ARC, whereas glucose-excited neurons were found in the lateral ARC. ARC glucose-excited neurons in brain slices dose-dependently increased their APF and decreased their ATP-sensitive K+ channel (KATP channel) currents as extracellular glucose levels increased from 0.1 to 10 mmol/l. However, glucose sensitivity was greatest as extracellular glucose decreased to <2.5 mmol/l. The glucokinase inhibitor alloxan increases KATP single-channel currents in glucose-excited neurons in a manner similar to low glucose. Leptin did not alter the activity of ARC glucose-excited neurons. Although insulin did not affect ARC glucose-excited neurons in the presence of 2.5 mmol/l (steady-state) glucose, they were stimulated by insulin in the presence of 0.1 mmol/l glucose. Neuropeptide Y (NPY) inhibited and alpha-melanocyte-stimulating hormone stimulated ARC glucose-excited neurons. ARC glucose-excited neurons did not show pro-opiomelanocortin immunoreactivity. These data suggest that ARC glucose-excited neurons may serve an integrative role in the regulation of energy balance.

Convergence of pre- and postsynaptic influences on glucosensing neurons in the ventromedial hypothalamic nucleus

Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) were studied using visually guided slice-patch recording techniques in brain slices from 14- to 21-day-old male Sprague-Dawley rats. Whole-cell current-clamp recordings were made as extracellular glucose levels were increased (from 2.5 to 5 or 10 mmol/l) or decreased (from 2.5 to 0.1 mmol/l). Using these physiological conditions to define glucosensing neurons, two subtypes of VMN glucosensing neurons were directly responsive to alterations in extracellular glucose levels. Another three subtypes were not directly glucose-sensing themselves, but rather were presynaptically modulated by changes in extracellular glucose. Of the VMN neurons, 14% were directly inhibited by decreases in extracellular glucose (glucose-excited [GE]), and 3% were directly excited by decreases in extracellular glucose (glucose-inhibited [GI]). An additional 14% were presynaptically excited by decreased glucose (PED neurons). The other two subtypes of glucosensing neurons were either presynaptically inhibited (PIR; 11%) or excited (PER; 8%) when extracellular glucose was raised to > 2.5 mmol/l. GE neurons sensed decreased glucose via an ATP-sensitive K(+) (K(ATP)) channel. The inhibitory effect of increased glucose on PIR neurons appears to be mediated by a presynaptic gamma-aminobutyric acid-ergic glucosensing neuron that probably originates outside the VMN. Finally, all types of glucosensing neurons were both fewer in number and showed abnormal responses to glucose in a rodent model of diet-induced obesity and type 2 diabetes.

Insulin activates ATP-sensitive K+ channels in hypothalamic neurons of lean, but not obese rats

Insulin and leptin receptors are present in hypothalamic regions that control energy homeostasis, and these hormones reduce food intake and body weight in lean, but not obese, Zucker rats. Here we demonstrate that insulin, like leptin, hyperpolarizes lean rat hypothalamic glucose-responsive (GR) neurons by opening KATP channels. These findings suggest hypothalamic K ATP channel function is crucial to physiological regulation of food intake and body weight.

Brain glucose sensing and body energy homeostasis: role in obesity and diabetes

The brain has evolved mechanisms for sensing and regulating glucose metabolism. It receives neural inputs from glucosensors in the periphery but also contains neurons that directly sense changes in glucose levels by using glucose as a signal to alter their firing rate. Glucose-responsive (GR) neurons increase and glucose-sensitive (GS) decrease their firing rate when brain glucose levels rise. GR neurons use an ATP-sensitive K+ channel to regulate their firing. The mechanism regulating GS firing is less certain. Both GR and GS neurons respond to, and participate in, the changes in food intake, sympathoadrenal activity, and energy expenditure produced by extremes of hyper- and hypoglycemia. It is less certain that they respond to the small swings in plasma glucose required for the more physiological regulation of energy homeostasis. Both obesity and diabetes are associated with several alterations in brain glucose sensing. In rats with diet-induced obesity and hyperinsulinemia, GR neurons are hyporesponsive to glucose. Insulin-dependent diabetic rats also have abnormalities of GR neurons and neurotransmitter systems potentially involved in glucose sensing. Thus the challenge for the future is to define the role of brain glucose sensing in the physiological regulation of energy balance and in the pathophysiology of obesity and diabetes.

Adiposity and serum leptin increase in fatty (fa/fa) BNZ neonates without decreased VMH serotonergic activity

Decreased ventromedial hypothalamic (VMH) serotonergic activity occurs in genetic and diet-induced animal models of obesity. We previously found that this activity was lower in adult and in 12-day-old Zucker fa/fa vs. Fa/Fa pups, the fa/fa animals being identified by their greater adiposity. In the present study, we evaluated fa/fa rats (Brown Norway-Zucker hybrids) at ages 2, 4, 7, and 12 days to test the hypothesis that lower VMH serotonergic activity occurs before increased adiposity and/or attenuated energy expenditure. Our results negate this hypothesis. VMH serotonergic activity showed no consistent genotype differences even at 12 days of age. In contrast, by day 7, fa/fa vs. Fa/Fa pups had higher serum leptin concentrations, greater percent body fat, lower resting and cold-induced energy expenditure, and lower activity of brown fat thyroxine 5'-deiodinase, an enzyme that converts thyroxine to triiodothyronine. We conclude that the onset of increased adiposity induced by the fa gene does not require decreased VMH serotonergic activity and that the lower serotonergic activity seen in older fa/fa pups may be secondary to metabolic consequences of the disruption of the leptin regulatory pathway.

Phosphorylation modulates the activity of the ATP-sensitive K+ channel in the ventromedial hypothalamic nucleus

Regulation of the ATP-sensitive K+ (K-ATP) channel was examined in cell-attached and inside-out membrane patches of freshly isolated neurons from the ventromedial hypothalamic nucleus (VMN) of 7-14 day old male Sprague-Dawley rats. When inside-out patches were exposed to symmetrical K+, the reversal potential was -2.85 +/- 1.65 mV, the single channel conductance 46 pS, and the total conductance varied as a multiple of this value. Glucose (10 mM) reversibly inhibited channel activity in cell-attached preparations by 81%. In the presence of 0.1 mM ADP, 10, 5, and 1 mM ATP reversibly inhibited VMN K-ATP channels in inside-out patches by 88, 83, and 60%, respectively. This inhibition was not dependent on phosphorylation since 5 mM AMPPNP, the non-hydrolyzable analog of ATP, reversibly inhibited channel activity by 67%. Relatively high concentrations of glibenclamide (100 microM) also reversibly inhibited VMN K-ATP channel activity in cell attached and inside-out patches by 67 and 79%, respectively. Finally, the non-specific kinase inhibitor H7 (200 microM) decreased channel activity by 53% while the non-specific phosphatase inhibitor microcystin (250 nM) increased channel activity by 218%. These data suggest that while the inhibitory effect of ATP is not phosphorylation dependent, phosphorylation state is an important regulator of the VMN K-ATP channel.

A novel technique for producing antibody-coated microprobes using a thiol-terminal silane and a heterobifunctional crosslinker

Antibody-coated microprobes are used to measure neuropeptide release in the central nervous system. Although they are not quantitative, they provide the most precise spatial resolution of the location of in vivo release of any currently available method. Previous methods of coating antibody microprobes are difficult and time-consuming. Moreover, using these methods we were unable to produce evenly coated antibody microprobes. This paper describes a novel method for the production of antibody microprobes using thiol-terminal silanes and the heterobifunctional crosslinker, 4-(4-N-maleimidophenyl)butyric acid hydrazide HCl 1/2 dioxane (MPBH). Following silation, glass micropipettes are incubated with antibody to substance P (SP) that has been conjugated to MPBH. This method results in a dense, even coating of antibody without decreasing the biological activity of the antibody. Additionally, this method takes considerably less time than previously described methods without sacrificing the use of antibody microprobes as micropipettes. The sensitivity of the microprobes for SP is in the picomolar range, and there is a linear correlation between the log of SP concentration (M) and B/B0 (r2 = 0.98). The microprobes are stable for up to 3 weeks when stored in 0.1 M sodium phosphate buffer with 50 mM NaCl (pH 7.4) at 5 degrees C. Finally, insertion into the exposed spinal cord of an anesthetized rat for 15 min produces no damage to the antibody coating.

Low-affinity sulfonylurea binding sites reside on neuronal cell bodies in the brain

The antidiabetic sulfonylurea drugs bind to sites associated with an ATP-sensitive potassium (Katp) channel on cell bodies and terminals of neurons which increase their firing rates or transmitter release when glucose concentrations rise or sulfonylureas are present. High-affinity sulfonylurea binding sites are concentrated in areas such as the substantia nigra (SN) where glucose and sulfonylureas increase transmitter release from GABA neurons. But there is a paucity of high-affinity sites in areas such as the hypothalamic ventromedial nucleus (VMN) where many neurons increase their activity when glucose rises. Here we assessed both high- and low--affinity sulfonylurea binding autoradiographically with 20 nM [3H]glyburide in the presence of absence of Gpp(NH)p. Neurotoxin lesions with 6-hydroxydopamine (6-OHDA), 5,7-dihydroxytryptamine (5,7-DHT) and ibotenic acid were used to elucidate the cellular location of the two sites in the VMN, SN and locus coeruleus (LC). In the VMN, 25% of the sites were of low affinity. Neither 6-OHDA nor 5,7-DHT affected [3H]glyburide binding, while ibotenic acid reduced the number of VMN neurons and abolished low-affinity without changing high-affinity binding. In cell-attached patches of isolated VMN neurons, both 10 mM glucose and 100 microM glyburide decreased the open probability of the Katp channel suggesting that the low-affinity binding site resides on these neurons. In the SN pars reticulata, ibotenic acid reduced the number of neurons and high-affinity [3H]glyburide binding was decreased by 20%, while 6-OHDA had no effect. In the SN pars compacta, both 6-OHDA and ibotenic acid destroyed endogenous dopamine neurons and selectivity ablated low-affinity binding. In the LC, 6-OHDA destroyed norepinephrine neurons and abolished low-affinity binding. These data suggest that low-affinity sulfonylurea binding sites reside on cell bodies on VMN, SN dopamine and LC norepinephrine neuron cell bodies and that high-affinity sites may be on axon terminals of GABA neurons in the SN.

Role of the brain in energy balance and obesity

Energy balance and body weight are regulated in short, intermediate, and long cycles that are superimposed on each other. We propose that the brain is the primary center of this regulation. The brain has evolved mechanisms for sensing the energy status of the body using neural and metabolic signals such as glucose, insulin, and leptin. It has central processing and storage capacity for handling this afferent information and can change both structurally and functionally in response to its internal and external milieu. The brain regulates energy balance through its control of energy intake on the one hand and expenditure and storage on the other using neurohumoral mechanisms that include the autonomic nervous system. Work in animal models suggests that the brain of obese individuals largely ignores signals of excess adiposity from the periphery, keeping the body weight set point at pathologically high levels. Disordered regulation of neuropeptide Y and monoamine metabolism within the ventromedial hypothalamus is a consistent finding in the brains of obesity-prone and obese rodents. Such dysregulation causes inappropriate neurohumoral control of metabolism and autonomic output to organs such as the pancreas, resulting in increased metabolic efficiency and persistent adiposity. The high recidivism rate in the treatment of obesity suggests that central dysfunction may be due to long-term reorganization of the nervous system in such a way as to perpetuate the abnormally high set point of body weight.

Adrenalectomy increases serotonin turnover in brains of obese Zucker rats

Because adrenalectomy tends to normalize many metabolic abnormalities of obese Zucker rats, we hypothesized that it would also normalize the depressed serotonergic turnover in their ventromedial nucleus (VMN). Lean (Fa/Fa) and obese (fa/fa) male Zucker rats were adrenalectomized or sham operated when 5 wks old and sacrificed at 11 wks. Their brains were frozen, and 13 areas were dissected for HPLC-EC analysis of monoamines and metabolites. Consistent with previous studies, VMN serotonin turnover (indexed by 5-HIAA/5-HT) was lower in obese than lean sham-operated rats. Monoamine and metabolite concentrations were altered in several other brain areas as well. Adrenalectomy reduced percent body fat and elevated VMN serotonergic turnover more in obese than in lean rats. It also stimulated serotonergic turnover in almost every brain area examined. We conclude that in obese Zucker rats: monoaminergic activity is altered in several brain areas involved in regulating energy balance; adrenalectomy normalizes the reduced VMN serotonergic turnover seen in the obese rats; and adrenalectomy results in a generalized increase in central serotonergic turnover. These data are consistent with serotonin's role in inhibiting food intake and enhancing sympathetic stimulation of energy metabolism.

Tachykinin receptors in the spinal cord

In summary, all three tachykinin receptors appear to be important modulators of physiological systems in the spinal cord. However, although there is a good deal of data concerning binding characteristics in peripheral tissues, work done in the spinal cord is scanty, leading to a number of unanswered questions. Firstly, Lui et al. (1993) have suggested a discrepancy between the location of SP binding sites and SP containing terminals. This might explain the conflicting evidence on the role of NK1 receptors in the dorsal horn. Furthermore, evidence that NK2 receptors are involved in nociception is increasing, however binding sites for these receptors in the spinal cord have not been demonstrated. This appears to be due to the difficulty in locating an ideal receptor specific ligand. The role of NK2 receptors in autonomic function is also unclear, perhaps for the same reason. Finally, there is evidence indicating that NK3 binding sites are increased following transection of the LIV-VI dorsal roots, however, studies on the effects of inflammation have not been done, as they have with the NK1 and NK2 receptors. All of these and many more unanswered questions require further investigation.

Serotonergic activity is depressed in the ventromedial hypothalamic nucleus of 12-day-old obese Zucker rats

We previously reported lower ventromedial hypothalamic nucleus (VMN) serotonergic activity in 11-wk genetically obese vs. lean Zucker rats. To determine whether the activity was secondary to metabolic alterations associated with this established obesity (e.g., significant hyperphagia and hyperinsulinemia), we examined monoaminergic activity in various brain nuclei of 12-day lean (Fa/Fa and Fa/fa) and obese (fa/fa) rats early in the development of obesity. Obese pups had greater percent carcass fat than heterozygotes, both of which were fatter than homozygous lean rats. Obese, but not heterozygous lean, pups were hyperinsulinemic vs. Fa/Fa pups. VMN 5-hydroxy-3-indoleacetic acid levels, an index of serotonin release, were lower in obese and heterozygous than in homozygous lean pups and were not correlated with plasma insulin levels. Although monoamine differences also occurred in several other nuclei, for the most part they appeared to be unrelated to the obese genotype. We conclude that blunted VMN serotonergic activity is not secondary to the obese rat's hyperinsulinemia and may play a significant role in the development of obesity.

Altered alpha 1-adrenoceptor binding in intact and adrenalectomized obese Zucker rats (fa/fa)

While many autonomic and metabolic defects associated with genetic obesity in the Zucker rat are corrected by adrenalectomy (Adx), brain adrenoceptor function has not been examined in this context. Here, 3 weeks after Adx or sham surgery, brains of 11 weeks old lean (Fa/Fa) and obese (fa/fa) male Zucker rats were assayed for alpha 1-([3H]prazosin; [3H]PRZ) and alpha 2-adrenoceptor ([3H]paraminoclonidine; [3H]PAC) binding by autoradiography. By genotype, obese rats had 19-256% higher [3H]PRZ binding than lean rats in the amygdala (central [ACN], basolateral [ABL], basomedial [ABM] and medial [MAN] nuclei [n.]), hypothalamus (dorsomedial n. [DMN] and lateral [LH]) and somatosensory cortex. In the ABL and ACN, increased maximal binding (Bmax) in obese rats was associated with decreased affinity (increased Kd). Three weeks after surgery, sham-operated obese rats gained 27% more weight than lean rats but lean and obese Adx rats gained the same amount of weight. Adx reduced [3H]PRZ binding in both lean and obese rats by 37-70% in the amygdala (ABM, ACN, MAN) compared to sham-operated rats. But, Adx selectively reduced [3H]PRZ binding only in lean rats in the ABL, DMN, ventromedial hypothalamic n. (VMN) and ventroposteromedial thalamic n. In most areas, decreases in maximal binding (Bmax) associated with Adx were accompanied by decreases in Kd. Unlike [3H]PRZ binding, there was no consistent genotype difference in [3H]PAC binding although Adx was followed by increased binding in obese and decreased binding in lean rats in the ABL. In only the VMN, obese rats had a 21% higher alpha 2- to alpha 1-adrenoceptor ratio than lean rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Litter size, adrenalectomy and high fat diet alter hypothalamic monoamines in genetically lean (Fa/Fa) Zucker rats

To determine if diet-induced obesity is associated with depressed serotonergic activity (as is genetic obesity), we examined hypothalamic biogenic amines in 11-wk-old genetically lean (Fa/Fa) male Zucker rats raised in small (3 pups/dam) or control (8-9 pups/dam) litters. Five-week-old rats were adrenalectomized or sham-operated and, 1 wk later, fed either 11% of energy as fat (low fat) or 68% of energy as fat (high fat) diets for 5 wk. Tissue punches from the ventromedial nucleus (VMN), the paraventricular nucleus and the preoptic area were assayed via HPLC. Rats fed high vs. low fat had a greater percentage of body fat and brown fat mitochondrial GDP binding, whereas serotonergic turnover was lower. Small litter vs. control litter animals had lower VMN and preoptic concentrations of 3,4-dihydroxyphenylacetic acid, a major metabolite of dopamine. Although adrenalectomy resulted in smaller, leaner rats, it did not differentially affect the rats that became fatter. Because VMN and preoptic dopaminergic activities were depressed in small litter vs. control litter rats but the percentage of body fat was unchanged, this decreased dopamine metabolism is probably not causal to the obesity development. However, the same cannot be said for the attenuated serotonergic activity, although such activity may not be directly related to the degree of obesity.

Neuronal activity in hypothalamic nuclei of obese and lean Zucker rats

Central neural activity was assessed by measuring relative cytochrome oxidase (CO) activity in the ventromedial nucleus (VMN; thermogenesis regulation), the parvocellular paraventricular nucleus (PVN; feeding regulation), and the magnocellular PVN (secretion of vasopressin and oxytocin) in 10 age-matched pairs of 39- to 42-day-old Zucker rats. When obese (fa/fa) were compared to lean (Fa/Fa) rats, relative CO activity was significantly lower (approximately 10 percent) in the VMN and parvocellular PVN, but not in the magnocellular PVN. Cell diameters did not differ. To determine if there were corresponding differences in levels or release of hypothalamic monoamines, we compared 7 pairs of 90- to 94-day-old lean (Fa/?) and obese (fa/fa) rats at rest and after 2 h of 9 degrees C. Tissue punches from frozen PVN, VMN, and preoptic area (the latter being a site of thermosensitive units modulating VMN output) were assayed. In obese vs. lean noncold-exposed rats, we observed lower concentrations of: 5-hydroxyindoleacetic acid (5HIAA; metabolite of serotonin, 5HT) in the VMN; 3-methoxy-4-hydroxyphenylglycol (MHPG; metabolite of norepinephrine, NE) and NE + MHPG (index of total NE) in the preoptic area; and 3,4-dihydroxyphenylacetic acid (DOPAC; metabolite of dopamine, DA) in the PVN. Additionally, in the VMN, cold exposure resulted in: elevated concentrations of MHPG and MHPG + NE in both lean and obese rats; elevated concentrations of 5HT, 5HIAA, and 5HT + 5HIAA in obese rats, with no significant changes in these variables in lean animals; decreased ratio of 5HIAA/5HT in obese rats and increased ratio in leans. In the preoptic region, cold exposure led to increased concentrations of MHPG, NE + MHPG, 5HT, and 5HT + 5HIAA in obese but not lean rats. In the PVN, 5HT concentrations were increased in cold-exposed obese but not lean rats. Our data support the hypothesis that neuronal activity in obese rats differs from that of lean rats at rest and during cold exposure and suggest that several monoamine systems play a role in such differences.