Effect of i.c.v. infusion of the alpha-MSH agonist MTII on meal patterns in male rats following nicotine withdrawal

The present study explored the role of endogenous alpha-MSH in the alteration of meal patterns induced by nicotine (NIC) withdrawal. Male Sprague Dawley rats bearing third ventricle cannulas were placed in computerized food intake monitors. On days 1-21, the rats were given 4 mg/kg/day of NIC or saline (SAL) in four equal i.p. doses during the dark period. NIC suppressed (P < 0.05) food intake only during the first week. The normalization of food intake occurred when the reduced meal size of the NIC injected rats was countered by an increase in meal number. Despite the normalization of 24-h food intake, body weight in NIC rats was decreased (P < 0.05) for 21 days. On day 22, the rats were divided into 4 groups (n's = 7-8 each) and injected into the third ventricle with various doses of the alpha-MSH agonist MTII or artificial cerebrospinal fluid (aCSF): SAL + aCSF, SAL + MTII, NIC + aCSF, NIC + MTII. Infusion of MTII (30 ng/rat) suppressed (P < 0.01) dark phase food intake in both groups, but the NIC + MTII group ate (P < 0.05) more than the SAL + MTII group. Meal number during the dark phase was suppressed by MTII, but the NIC + MTII group took significantly more meals that the SAL + MTII group. Infusion of MTII suppressed meal size in SAL and NIC treated rats, but this effect was attenuated in NIC treated rats. All meal parameters normalized by the day after i.c.v. infusion. These data indicate that NIC treatment differentially affects the neural controls of meal number and meal size and attenuates the suppression by MTII of meal number and meal size.     

Paraventricular hypothalamic alpha-melanocyte-stimulating hormone and MTII reduce feeding without causing aversive effects

alpha-Melanocyte-stimulating hormone (alpha-MSH) appears to play a tonic inhibitory role in feeding and energy storage. MTII, a specific synthetic MC3-R/MC4-R agonist, has similar effects on feeding in rats. The current studies demonstrate that PVN administration of alpha-MSH or MTII decreases nocturnal and NPY-stimulated food intake without causing aversive effects. Co-administration with NPY of 600 pmol alpha-MSH or 1 pmol MTII into the PVN caused a significant decrease in NPY-induced feeding. PVN administration of MTII or alpha-MSH at doses effective to suppress feeding did not cause conditioned taste aversion (CTA). ICV administration of alpha-MSH, however, did cause weak CTA. These results indicate that the potent effects on feeding of MC3-R and MC4-R agonists when injected into the PVN are not due to aversive effects.     

Alpha-melanocyte stimulating hormone and ghrelin: central interaction in feeding control

Alpha-melanocyte stimulating hormone (alpha-MSH) and ghrelin play significant yet opposite roles in the regulation of feeding: alpha-MSH inhibits, whereas ghrelin stimulates consumption. The two peptidergic systems may interact in the process of food intake control. A single report published thus far has shown that a synthetic agonist of the melanocortin receptors, MTII, injected in the hypothalamic paraventricular nucleus (PVN) decreases feeding generated by ghrelin. We found that very low doses of alpha-MSH and MTII administered ICV significantly reduced ghrelin-dependent hyperphagia. However, an endogenous molecule, alpha-MSH, infused in the PVN did not exert an inhibitory effect on ghrelin-induced consumption, whereas the effective dose of PVN MTII exceeded that necessary to decrease short-term deprivation-induced feeding. We conclude that it is likely that in feeding regulation alpha-MSH and ghrelin "interact" at the central nervous system level, but the involvement of the PVN in this interaction appears questionable.     

Profound and rapid reduction in body temperature induced by the melanocortin receptor agonists

The melanocortin receptor 4 (MC4R) plays a major role in body weight regulation and its agonist MTII has been widely used to study the role of MC4Rs in energy expenditure promotion and feeding reduction. Unexpectedly, we observed that intraperitoneal (i.p.) administration of MTII induced a rapid reduction in both body temperature and energy expenditure, which was independent of its effect on feeding and followed by a prolonged increase in energy expenditure. The rapid reduction was at least partly mediated by brain neurons since intracerebroventricular (icv) administration of alpha melanocyte-stimulating hormone, an endogenous melanocortin receptor agonist, produced a similar response. In addition, the body temperature-lowering effect of MTII was independent of the presence of MC4Rs, but in a similar fashion to the previously shown effect on body temperature by 5′AMP. Moreover, β-adrenergic receptors (β-ARs) were required for the recovery from low body temperature induced by MTII and further pharmacological studies showed that the MTII’s effect on body temperature may be partially mediated by the vasopressin V1a receptors. Collectively, our results reveal a previously unappreciated role for the melanocortin pathway in rapidly lowering body temperature.     

Anatomical dissociation of melanocortin receptor agonist effects on taste- and gut-sensitive feeding processes

Injections of the melanocortin 3/4 receptor (MCR) agonist melanotan II (MTII) to a variety of brain structures produces anorexia, suggesting distributed brain MCR control of food intake. We performed a detailed analysis of feeding behavior (licking microstructure analysis) after a range of MTII doses (0.005 nM to 1 nM) was targeted to the forebrain (third ventricle, 3V) or hindbrain (fourth ventricle, 4V) regions. MTII (0.1 nM and 1 nM) delivered to the 3V or 4V significantly reduced 0.8 M sucrose intake. The anorexia was mediated by reductions in the number of licking bursts in the meal, intrameal ingestion rate, and meal duration; these measures have been associated with postingestive feedback inhibition of feeding. Anorexia after 3V but not 4V MTII injection was also associated with a reduced rate of licking in the first minute (initial lick rate) and reduced mean duration of licking bursts; these measures have been associated with taste evaluation. MTII effects on taste evaluation were further explored: In experiment 2, 3V MTII (1 nM) significantly reduced intake of noncaloric 4 mM saccharin and 0.1 M and 1 M sucrose solutions, but not water. The anorexia was again associated with reduced number of licking bursts, ingestion rate, meal duration, initial lick rate, and mean burst duration. In experiments 3 and 4, brief access (20 s) licking responses for sweet sucrose (0.015 M to 0.25 M) and bitter quinine hydrochloride (0.01 mM to 1 mM) solutions were evaluated. Licking responses for sucrose were suppressed, whereas those for quinine solutions were increased after 3V MTII, but not after 4V MTII injections (0.1 nM and 1 nM). The results suggest that multiple brain MCR sites influence sensitivity to visceral feedback, whereas forebrain MCR stimulation is necessary to influence taste responsiveness, possibly through attenuation of the perceived intensity of taste stimuli.     

Neurotensin: effects of hypothalamic and intravenous injections on eating and drinking in rats

To investigate a role for the brain-gut peptide neurotensin (NT) in ingestive behavior, changes in food and water intake of food-deprived rats were examined following injection of NT into the paraventricular hypothalamic nucleus (PVN) or the mesenteric vein. Unilateral PVN NT (2.5, 5.0, 10.0 micrograms/0.3 microliter) produced substantial dose-dependent reductions in total food intake 0.5, 1, and 4 hr postinjection. In contrast, PVN NT had no effect on water intake and produced no change in grooming, rearing, sleeping, resting or locomotor activity. Bilateral PVN NT at a high dose (10.0 micrograms/side) suppressed consumption of solid or liquid diet in food-deprived rats, but did not affect water intake in water-deprived rats. This specificity is consistent with a role for CNS NT in feeding behavior. Intravenous NT (1-1000 pmole/kg/min for 30 min) did not specifically suppress food intake; however, low doses did increase water intake in food-deprived rats. These findings do not support a role for plasma NT in feeding, but do suggest that it may play a role in drinking behavior.     

Effect of Chronic Caloric Restriction on the Synchronization of Various Physiological Measures in Old Female Fischer 344 Rats

-A variety of physiological and behavioral parameters which relate to metabolism were continuously monitored in 18 month old female Fischer 344 rats which were maintained on either ad libitum or reduced calorie diets. Caloric restriction (CR) stimulated average motor activity per day, the duration of each feeding episode, food consumed per episode, and water consumed per gram lean body mass (LBM). However, CR limited total food consumption, feeding time, number of feeding episodes per day, total eating and drinking time, and the daily ratio of food consumed to water consumed, CR also decreased average body temperature per day, O₂ consumption, CO₂, production, and respiratory quotient. A variety of parameters concerning water consumption were not affected. CR rats ate their food immediately when food was presented during the light span, while ad libitum fed animals ate numerous small meals throughout the entire dark span. An anticipatory response to restricted feeding was also noted. Total motor activity, metabolism, and body temperature increased just prior to scheduled feeding and reached maximum values shortly after feeding, suggesting that these parameters were highly synchronized to feeding. Females and males were found to respond to caloric restriction in a similar fashion. Dramatic changes in respiratory quotient and body temperature suggest rapid shifts between metabolic pathways (glycolysis to giuconeogenesis) to obtain optimal efficiency. Lower body temperature and metabolism may provide protection against DNA damage, thereby increasing the survival potential of restricted rats. These responses may provide insight into the mechanisms by which caloric restriction acts to extend life span.     

Maximum acute exercise tolerance in hyperthyroid and hypothyroid rats subjected to forced swimming

Thyroid dysfunction can compromise physical capacity. Here, we analyze the effects of hyperthyroidism and hypothyroidism on maximum swim time in rats subjected to acute forced swimming, as an indicator of anaerobic capacity. Animals were forced to swim against a load (5% of body weight) attached to the tail and were killed 48 hours after the last test. Hyperthyroid rats were treated with thyroxine (50 mug/100 g body weight, i. p. for 7 days). The hypothyroid group received 0.03% methimazole in the drinking water for 4 weeks. Thyroid state was confirmed by alterations in serum thyroid-stimulating hormone (TSH), triiodothyronine (T3), thyroxine (T4), and liver mitochondrial glycerol phosphate dehydrogenase (mGPD) activity. Hyperthyroid rats presented significantly lower visceral fat mass (VFM) and higher food intake (p<0.05) with unchanged body weight. Maximum swim time (MST), glycogen content (skeletal muscle and liver), and leptin levels were lower while corticosterone was higher (p<0.05). In hypothyroid rats body weight was lower (p<0.05), without changes in VFM. Tested at 7-day intervals, MST was lower for tests 2, 3, and 4 (p<0.05). Muscle glycogen was higher in extensor digitorum longus (EDL) and soleus (p<0.05), without changes in liver. Serum corticosterone was lower, while leptin was higher (p<0.05). These results suggest that in hyperthyroid and hypothyroid rats, thyroid hormones together with corticosterone and/or leptin may impair exercise capacity differently through its known effects on glycogen metabolism.     

Eating, drinking and temperature responses to intracerebroventricular cholecystokinin in the chick

The central effect of cholecystokinin-octapeptide (CCK), SQ 19,844 or sincalide, on the intake of food and water and on colonic temperature (Tc) was investigated using the broiler cockerel. Four-week old chicks were maintained in a thermoneutral environment of 23-24 degrees C. After food was removed for a 24 hr interval, CCK was infused in a volume of 10.0 microliters into the lateral cerebral ventricle (ICV) in doses ranging from 10-150 ng. Although lower doses of CCK had no effect on food intake, 100 or 150 ng of CCK significantly reduced consumption of food in a dose-dependent manner; water drinking was significantly decreased by 100 ng of CCK. In addition, CCK at doses of 100 and 150 ng prevented the slow rise in Tc observed following infusions of control CSF. This latter effect appeared to be a result of feeding activity associated with caloric intake and the heat increment in the control birds rather than a specific thermoregulatory effect. Overall, our results suggest that CCK may comprise a part of the central mechanism underlying the neural control of short term satiety in an avian species similar to that proposed for the mammal.     

Abnormal Motor Activity and Thermoregulation in a Schizophrenia Rat Model for Translational Science

Background

Schizophrenia is accompanied by altered motor activity and abnormal thermoregulation; therefore, the presence of these symptoms can enhance the face validity of a schizophrenia animal model. The goal was to characterize these parameters in freely moving condition of a new substrain of rats showing several schizophrenia-related alterations.

Methods

Male Wistar rats were used: the new substrain housed individually (for four weeks) and treated subchronically with ketamine, and naive animals without any manipulations. Adult animals were implanted with E-Mitter transponders intraabdominally to record body temperature and locomotor activity continuously. The circadian rhythm of these parameters and the acute effects of changes in light conditions were analyzed under undisturbed circumstances, and the effects of different interventions (handling, bed changing or intraperitoneal vehicle injection) were also determined.

Results

Decreased motor activity with fragmented pattern was observed in the new substrain. However, these animals had higher body temperature during the active phase, and they showed wider range of its alterations, too. The changes in light conditions and different interventions produced blunted hyperactivity and altered body temperature responses in the new substrain. Poincaré plot analysis of body temperature revealed enhanced short- and long-term variabilities during the active phase compared to the inactive phase in both groups. Furthermore, the new substrain showed increased short- and long-term variabilities with lower degree of asymmetry suggesting autonomic dysregulation.

Conclusions

In summary, the new substrain with schizophrenia-related phenomena showed disturbed motor activity and thermoregulation suggesting that these objectively determined parameters can be biomarkers in translational research.     

Estradiol treatment fails to affect the feeding responses to melanocortin-3/4 receptor agonism or antagonism in ovariectomized rats

The involvement of the hypothalamic melanocortin-3 and -4 (MC3/4) receptors system in the inhibitory actions of estradiol (E2) on feeding was investigated. Ovariectomized Long-Evans rats with lateral ventricular (LICV) injection cannulae were maintained on a near-physiological, cyclic schedule of E2 treatment. LICV injections of 0.5 nmol of the MC3/4 agonist MTII decreased feeding, and LICV injections of the MC3/4 antagonists SHU9119 (12.5-500 pmol) and AgRP (1.0 nmol) stimulated feeding. None of these effects was affected by E2 treatment. Thus, hypothalamic MC3/4 receptors play a physiological role in the control of feeding in female rats as in males but do not mediate E2's feeding effects during the ovarian cycle.     

Analogs of MTII, lactam derivatives of alpha-melanotropin, modified at the N-terminus, and their selectivity at human melanocortin receptors 3, 4, and 5.

In search for selective agonists at human melanocortin-4 receptor, proline-substituted analogs of MTII, a potent nonselective agonist at melanocortin receptors, were prepared by solid-phase syntheses and evaluated for their ability to bind and activate human MC-3, MC-4, and MC-5 receptors. Replacement of Nle(4) with Pro resulted in [Pro(4)]MTII with affinity to and agonist potency at hMC-4R similar to MTII, but with about 400-fold lower potency at hMC-5R and about 20-fold lower potency at hMC-3R. The substantial increase in selectivity of [Pro(4)]MTII with respect to hMC-5R prompted us to investigate additional analogs of MTII with modified N-termini. The Ac-Nle(4) segment, not encompassed in the lactam ring, was substituted with flexible, hydrophobic, or hydrophilic substituents, and also, with residues resembling proline. The similar agonist potency of these peptides to that of MTII at hMC-4R but significantly lower activity of these compounds at hMC-5R demonstrated that the N-terminal fragment of MTII has virtually no effect on the binding affinity and activation at hMC-4R, but it is essential for full potency at hMC-5R.     

Caffeine tolerance: behavioral, electrophysiological and neurochemical evidence

The development of tolerance to the stimulatory action of caffeine upon mesencephalic reticular neurons and upon spontaneous locomotor activity was evaluated in rats after two weeks of chronic exposure to low doses of caffeine (5-10 mg/kg/day via their drinking water). These doses are achievable through dietary intake of caffeine-containing beverages in man. Concomitant measurement of [3H]-CHA binding in the mesencephalic reticular formation was also carried out in order to explore the neurochemical basis of the development of tolerance. Caffeine, 2.5 mg/kg i.v., markedly increased the firing rate of reticular neurons in caffeine naive rats but failed to modify the neuronal activity in a group exposed chronically to low doses of caffeine. In addition, in spontaneous locomotor activity studies, our data show a distinct shift to the right of the caffeine dose-response curve in caffeine pretreated rats. These results clearly indicate that tolerance develops to the stimulatory action of caffeine upon the reticular formation at the single neuronal activity level as well as upon spontaneous locomotor activity. Furthermore, in chronically caffeine exposed rats, an increase in the number of binding sites for [3H]-CHA was observed in reticular formation membranes without any change in receptor affinity. We propose, therefore, that up-regulation of adenosine receptors may underlie the development of tolerance to the CNS effects of caffeine.     

Amylin infusion into rat nucleus accumbens potently depresses motor activity and ingestive behavior

Amylin, a calcitonin gene-related peptide-like peptide coreleased with insulin, exerts anorexic effects on central administration. Because previous studies revealed dense amylin binding in the nucleus accumbens (Acb), we investigated the behavioral effects of amylin infusions (10, 30, and 100 ng/side) into Acb subregions. Intra-Acb shell amylin infusions decreased ambulation, rearing, feeding, and drinking in either food-deprived rats or water-deprived rats; motor activity was affected more potently than ingestive behavior. Moreover, intra-Acb shell amylin reduced motor activity in nondeprived rats tested in the absence of food or water, indicating that the expression of amylin's effects is independent of drive or proximal incentives. Intra-Acb core amylin infusions in water-deprived rats also decreased ambulation and water intake, although anterior Acb placements were associated with smaller motor effects, regardless of Acb subregion. In contrast to amylin's effects, intra-Acb shell infusions of orexin-A (50, 100, and 500 ng/side) had no effects on motor activity, feeding, or drinking. Hence the Acb may be a target for behavioral regulation by satiety-related peptides like amylin.     

The hypothalamic paraventricular nucleus is not essential for orexigenic NPY or anorexigenic melanocortin action

Bilateral electrolytic lesions of the paraventricular nucleus of the hypothalamus (PVN) produce hyperphagia with excess weight gain. The orexigenic neuropeptide Y (NPY) system and the anorexigenic melanocortin system act in the PVN to regulate food intake, and participate in mediating the anorexic effects of leptin. We hypothesized that changes in the responsiveness of these systems may contribute to the hyperphagia observed in PVN-lesioned rats. Adult female Sprague-Dawley rats received either sham or electrolytic lesions in the PVN immediately followed by implantation of a guide cannula into the third cerebroventricle. Twenty-five days following surgery groups of sham and hyperphagic PVN-lesioned rats were injected intracerebroventricularly (i.c.v.) with either 118 pmole or 470 pmole of NPY and food intake was measured for 3 h. Food intake in response to NPY was nearly three-fold higher in PVN-lesioned rats as compared to sham rats. However, the response to 5 microg leptin i.c.v. was not different in lesioned versus sham rats. The effect of the melanocortin agonist MTII on food intake was tested in additional rats beginning either 7-14 days or 30-40 days following surgery. Doses of 0.1 nmole or 1.0 nmole of MTII were injected immediately before lights-off and food intake was measured at 2 h, 24 h and 48 h post-injection. Suppression of food intake in PVN-lesioned rats was not different from that in sham-lesioned rats. These data suggest that hyper-responsiveness to NPY may account in part for the hyperphagia observed in PVN-lesioned rats. Furthermore, based on the similarities of responses of PVN-lesioned and sham control rats to the anorexigenic agents MTII and leptin and the hypersensitivity of lesioned rats to NPY, we conclude that the PVN is not essential for NPY stimulation of food intake or for melanocortin suppression of food intake and that NPY and melanocortin receptors outside of the PVN are sufficient to produce these effects.     

Anthrax edema toxin has cAMP-mediated stimulatory effects and high-dose lethal toxin has depressant effects in an isolated perfused rat heart model

While anthrax edema toxin produces pronounced tachycardia and lethal toxin depresses left ventricular (LV) ejection fraction in in vivo models, whether these changes reflect direct cardiac effects as opposed to indirect ones related to preload or afterload alterations is unclear. In the present study, the effects of edema toxin and lethal toxin were investigated in a constant pressure isolated perfused rat heart model. Compared with control hearts, edema toxin at doses comparable to or less than a dose that produced an 80% lethality rate (LD(80)) in vivo in rats (200, 100, and 50 ng/ml) produced rapid increases in heart rate (HR), coronary flow (CF), LV developed pressure (LVDP), dP/dt(max), and rate-pressure product (RPP) that were most pronounced and persisted with the lowest dose (P ≤ 0.003). Edema toxin (50 ng/ml) increased effluent and myocardial cAMP levels (P ≤ 0.002). Compared with dobutamine, edema toxin produced similar myocardial changes, but these occurred more slowly and persisted longer. Increases in HR, CF, and cAMP with edema toxin were inhibited by a monoclonal antibody blocking toxin uptake and by adefovir, which inhibits the toxin's intracellular adenyl cyclase activity (P ≤ 0.05). Lethal toxin at an LD(80) dose (50 ng/ml) had no significant effect on heart function but a much higher dose (500 ng/ml) reduced all parameters (P ≤ 0.05). In conclusion, edema toxin produced cAMP-mediated myocardial chronotropic, inotropic, and vasodilatory effects. Vasodilation systemically with edema toxin could contribute to shock during anthrax while masking potential inotropic effects. Although lethal toxin produced myocardial depression, this only occurred at high doses, and its relevance to in vivo findings is unclear.     

Colocalization of Glucagon-Like Peptide-1 (GLP-1) Receptors, Glucose Transporter GLUT-2, and Glucokinase mRNAs in Rat Hypothalamic Cells: Evidence for a Role of GLP-1 Receptor Agonists as an Inhibitory Signal for Food and Water Intake

Abstract: This study was designed to determine the possible role of brain glucagon-like peptide-1 (GLP-1) receptors in feeding behavior. In situ hybridization showed colocalization of the mRNAs for GLP-1 receptors, glucokinase, and GLUT-2 in the third ventricle wall and adjacent arcuate nucleus, median eminence, and supraoptic nucleus. These brain areas are considered to contain glucose-sensitive neurons mediating feeding behavior. Because GLP-1 receptors, GLUT-2, and glucokinase are proteins involved in the multistep process of glucose sensing in pancreatic β cells, the colocalization of specific GLP-1 receptors and glucose sensing-related proteins in hypothalamic neurons supports a role of this peptide in the hypothalamic regulation of macronutrient and water intake. This hypothesis was confirmed by analyzing the effects of both systemic and central administration of GLP-1 receptor ligands. Acute or subchronic intraperitoneal administration of GLP-1 (7–36) amide did not modify food and water intake, although a dose-dependent loss of body weight gain was observed 24 h after acute administration of the higher dose of the peptide. By contrast, the intracerebroventricular (i.c.v.) administration of GLP-1 (7–36) amide produced a biphasic effect on food intake characterized by an increase in the amount of food intake after acute i.c.v. delivery of 100 ng of the peptide. There was a marked reduction of food ingestion with the 1,000 and 2,000 ng doses of the peptide, which also produced a significant decrease of water intake. These effects seemed to be specific because i.c.v. administration of GLP-1 (1–37), a peptide with lower biological activity than GLP-1 (7–36) amide, did not change feeding behavior in food-deprived animals. Exendin-4, when given by i.c.v. administration in a broad range of doses (0.2, 1, 5, 25, 100, and 500 ng), proved to be a potent agonist of GLP-1 (7–36) amide. It decreased, in a dose-dependent manner, both food and water intake, starting at the dose of 25 ng per injection. Pretreatment with an i.c.v. dose of a GLP-1 receptor antagonist [exendin (9–39); 2,500 ng] reversed the inhibitory effects of GLP-1 (7–36) amide (1,000 ng dose) and exendin-4 (25 ng dose) on food and water ingestion. These findings suggest that GLP-1 (7–36) amide may modulate both food and drink intake in the rat through a central mechanism.     

Stimulation and inhibition of food intake in sheep by centrally-administered hypothalamic releasing factors

Short-term effects of hypothalamic releasing factors on feeding behavior and digestive motility patterns were assessed in hay-fed sheep trained to eat more than half the total amount eaten over 8 h within the first 3 h after food presentation. Thyrotropin-releasing hormone (TRH) given intracerebroventricularly (ICV, 30 ng/kg) or intravenously at higher doses (IV, 3 micrograms/kg) reduced food consumption by 20 p. cent. The ICV or IV TRH-induced reduction was associated with behavioral excitation and stimulation of antroduodenal motor activity without changes in water intake. The ovine corticotropin releasing factor (oCRF 41) decreased food and water intake by 30-50% when administered ICV (60 ng/kg) but was not active when given systemically at doses up to 6 micrograms/kg. The synthetic human growth hormone releasing factor (hGRF 44) administered centrally (60 ng/kg) increased the amount of food intake and the antral motor activity without behavioral excitation. The results indicate a centrally-mediated facilitation of food intake by GRF and its inhibition by CRF which also affect water consumption. The presence of digestive motor effects suggests that extrapituitary pathways may be involved, as for TRH, in the action of both GRF and CRF.     

Activation of the melanocortin-4 receptor causes enhanced excitation in presympathetic paraventricular neurons in obese Zucker rats

Sympathetic nerve activity is increased in obesity-related hypertension. However, the central mechanisms involved in the increased sympathetic outflow remain unclear. The hypothalamic melanocortin system is important for regulating energy balance and sympathetic outflow. To understand the mechanisms by which the melanocortin systems regulates sympathetic outflow, we investigated the role of melanocortin 4 receptors (MC4R) in regulating presympathetic paraventricular nucleus (PVN) neurons. We performed whole-cell patch-clamp recordings on retrogradely labeled PVN neurons projecting to the rostral ventrolateral medulla in brain slices from obese zucker rats (OZRs) and lean zucker rats (LZRs). The MC4R agonists melanotan II (MTII) and α-melanocyte-stimulating hormone (α-MSH) increased the firing activity and depolarized the labeled PVN neurons from both LZRs and OZRs in a concentration-dependent manner. MTII produced significant greater increase in the firing activity in OZRs than in LZRs. Blocking MC4R with the specific antagonist SHU9119 had no effect on the basal firing rate but abolished the MTII-induced increase in the firing rate in both OZRs and LZRs. Furthermore, intracellular dialysis of guanosine 5'-O-(2-thodiphosphate), but not bath application of kynurenic acid and bicuculline, eliminated the MTII-induced increase in firing activity. In addition, MTII had no effect on the frequency and amplitude of glutamatergic excitatory postsynaptic currents and GABAergic inhibitory postsynaptic currents in labeled PVN neurons. Collectively, our findings suggest that MC4R contributes to the elevated excitability of PVN presympathetic neurons, which may be involved in obesity-related hypertension.