Akira Niijima

Characterization of the effects of pancreatic polypeptide in the regulation of energy balance

BACKGROUND & AIMS Pancreatic polypeptide (PP) belongs to a family of peptides including neuropeptide Y and peptide YY. We examined the role of PP in the regulation of body weight as well as the therapeutic potential of PP. METHODS We measured food intake, gastric emptying, oxygen consumption, and gene expression of hypothalamic neuropeptides, gastric ghrelin, and adipocytokines in mice after administering PP intraperitoneally. Peptide gene expression was also examined in PP-overexpressing mice. Vagal and sympathetic nerve activities were recorded after intravenous administration in rats. Effects of repeated administrations of PP on energy balance and on glucose and lipid metabolism were examined in both ob/ob obese mice and fatty liver Shionogi (FLS)-ob/ob obese mice. RESULTS Peripherally administered PP induced negative energy balance by decreasing food intake and gastric emptying while increasing energy expenditure. The mechanism involved modification of expression of feeding-regulatory peptides (decrease in orexigenic neuropeptide Y, orexin, and ghrelin along with an increase in anorexigenic urocortin) and activity of the vagovagal or vagosympathetic reflex arc. PP reduced leptin in white adipose tissue and corticotropin-releasing factor gene expression. The expression of gastric ghrelin and hypothalamic orexin was decreased in PP-overexpressing mice. Repeated administrations of PP decreased body weight gain and ameliorated insulin resistance and hyperlipidemia in both ob/ob obese mice and FLS-ob/ob obese mice. Liver enzyme abnormalities in FLS-ob/ob obese mice were also ameliorated by PP. CONCLUSIONS These observations indicate that PP may influence food intake, energy metabolism, and the expression of hypothalamic peptides and gastric ghrelin.

The suprachiasmatic nucleus balances sympathetic and parasympathetic output to peripheral organs through separate preautonomic neurons

Opposing parasympathetic and sympathetic signals determine the autonomic output of the brain to the body and the change in balance over the sleep‐wake cycle. The suprachiasmatic nucleus (SCN) organizes the activity/inactivity cycle and the behaviors that go along with it, but it is unclear how the hypothalamus, in particular the SCN, with its high daytime electrical activity, influences this differentiated autonomic balance. In a first series of experiments, we visualized hypothalamic pre‐sympathetic neurons by injecting the retrograde tracer Fluoro‐Gold into the thoracic sympathetic nuclei of the spinal cord. Pre‐parasympathetic neurons were revealed by injection of the retrograde trans‐synaptic tracer pseudorabies virus (PRV) into the liver and by sympathetic liver denervation, forcing the virus to infect via the vagus nerve only. This approach revealed separate pre‐sympathetic and pre‐parasympathetic neurons in the brainstem and hypothalamus. Next, selective retrograde tracing with two unique reporter PRV strains, one injected into the adrenal and the other into the sympathetic denervated liver, demonstrated that there are two separate populations of pre‐sympathetic and pre‐parasympathetic neurons within the paraventricular nucleus of the hypothalamus. Interestingly, this segregation persists into the SCN, where, as a result, the day‐night balance in autonomic function of the organs is affected by specialized pre‐sympathetic or pre‐parasympathetic SCN neurons. These separate preautonomic SCN neurons provide the anatomical basis for the circadian‐driven regulation of the parasympathetic and sympathetic autonomic output. J. Comp. Neurol. 464:36–48, 2003.

Parasympathetic and sympathetic control of the pancreas: A role for the suprachiasmatic nucleus and other hypothalamic centers that are involved in the regulation of food intake

To reveal brain regions and transmitter systems involved in control of pancreatic hormone secretion, specific vagal and sympathetic denervation were combined with injection of a retrograde transsynaptic tracer, pseudorabies virus (PRV), into the pancreas. After sympathetic or vagal transsection first‐order neurons were revealed in the dorsal motor nucleus of the vagus (DMV) or in preganglionic spinal cord neurons (SPN), respectively. Careful timing of the survival of the animals allowed the detection of cell groups in immediate control of these DMV or SPN neurons. A far larger number of cell groups is involved in the control of DMV than of SPN neurons. Examples are given of a high level of interaction between the sympathetic and parasympathetic nervous system. Several cell groups project to both branches of the autonomic nervous system, sometimes even the same neurotransmitter is used, e.g., oxytocin neurons in the paraventricular nucleus and melanin‐concentrating hormone and orexin neurons in the lateral hypothalamus project to both the DMV and SPN neurons. Moreover, the appearance of third‐order neurons located in the sympathetic SPN after complete sympathectomy and in the DMV after complete vagotomy illustrates the possibility that motor neurons of the sympathetic and parasympathetic system may exchange information by means of interneurons. The presence of second‐order neurons in prefrontal, gustatory, and piriform cortex may provide an anatomic basis for the involvement of these cortices in the cephalic insulin response. The observation that second‐order neurons in both vagal and sympathetic control of the pancreas contain neuropeptides that are known to play a role in food intake indicates a direct association between behavioral and autonomic functions. Finally, the observation of third‐order neurons in the suprachiasmatic nucleus and ventromedial hypothalamus shows the modulatory action of the time of the day and metabolic state, respectively. J. Comp. Neurol. 431:405–423, 2001.

Regulation of Pancreatic β Cell Mass by Neuronal Signals from the Liver

Metabolic regulation in mammals requires communication between multiple organs and tissues. The rise in the incidence of obesity and associated metabolic disorders, including type 2 diabetes, has renewed interest in interorgan communication. We used mouse models to explore the mechanism whereby obesity enhances pancreatic β cell mass, pathophysiological compensation for insulin resistance. We found that hepatic activation of extracellular regulated kinase (ERK) signaling induced pancreatic β cell proliferation through a neuronal-mediated relay of metabolic signals. This metabolic relay from the liver to the pancreas is involved in obesity-induced islet expansion. In mouse models of insulin-deficient diabetes, liver-selective activation of ERK signaling increased β cell mass and normalized serum glucose levels. Thus, interorgan metabolic relay systems may serve as valuable targets in regenerative treatments for diabetes.

Glucose-sensitive afferent nerve fibers in the liver and their role in food intake and blood glucose regulation

The discharge rates of glucose-sensitive hepatic vagal afferents and glucose concentrations in the portal vein showed an inverse relationship in experiments performed using isolated and perfused liver preparations and in in vivo experiments on the guinea pig. The rate of discharge of these afferents in the rat was facilitated following administration of insulin and inhibited after application of glucagon and CCK. Hepato-pancreatic, -adrenal and -hepatic reflexes initiated through vagal hepatic afferents were demonstrated in the rabbit, guinea pig or rat. The results obtained suggest that glucose-sensitive vagal afferents from the liver play an important role in the control of food intake as well as in the control of blood glucose levels.

Effects of hypothalamic lesion on pancreatic autonomic nerve activity in the rat.

The effects of hypothalamic lesions and intravenous glucose infusion on the efferent activity of vagal and splanchnic nerves to the pancreas were studied in anesthetized rats. Lesions of the ventromedial hypothalamic (VMH), the dorsomedial hypothalamic (DMH) and the paraventricular (PVN) nuclei increased vagal and reduced splanchnic nerve activity. Lesion of the lateral hypothalamic area (LHA) decreased pancreatic vagal nerve activity, and produced either increased or decreased activity of pancreatic splanchnic nerve. Intravenous glucose infusion increased activity of the vagal nerve and reduced that of the splanchnic nerve. These glucose responses were influenced by hypothalamic lesions only slightly or not at all. The findings suggest that hypothalamic modulation of pancreatic hormone secretion involves both the parasympathetic and sympathetic nervous systems, and provide evidence that not only the VMH and the LHA but also the DMH and the PVN are involved in this mechanism.

The effects of interleukin 1β on the activity of adrenal, splenic and renal sympathetic nerves in the rat

The effects of intravenous (i.v.) administration of recombinant human interleukin-1 beta (rhIL-1 beta) on the activity of adrenal, splenic and renal sympathetic nerves were observed in urethane-anesthetized rats. An i.v. injection of IL-1 beta in doses of 10 pg-20 ng per animal (300-400 g, b.w.) resulted in a dose-dependent increase in the activity of the adrenal and splenic nerves, which lasted for more than 2-6 h. On the other hand, the activity of renal nerves showed a transient increase which was followed by a long-lasting suppression after injection of rhIL-1 beta (100 pg, i.v.). An i.v. injection of cyclooxygenase inhibitors (6 mg ibuprofen or 20 mg sodium salicylate) suppressed almost completely the rhIL-1 beta (100 pg)-induced activity in adrenal and splenic nerves. Although rhIL-1 beta (100 pg, i.v.) produced a fall in arterial blood pressure, baroreceptor denervation did not affect the excitatory responses of the adrenal and splenic nerves to rhIL-1 beta. The results suggest the regional differentiation of activity in the visceral sympathetic nerves in response to rhIL-1 beta. The rhIL-1 beta-induced activation of splenic sympathetic nerves implicates their involvement in the modulation of immunity by brain.

The Autonomic Nervous System as a Communication Channel between the Brain and the Immune System

Much evidence from various fields has revealed multiple channels of communication between the brain and the immune system. Among the routes of signal transmission, this review focuses on the roles and mechanisms of neural communication between the two systems. As for the centrifugal neural pathway by which the brain modulates immunity, there are various requirements for the noradrenergic sympathetic innervation of the primary and secondary lymphoid organs. In addition to the presence of beta- and alpha-adrenergic receptors on different types of immunocompetent cells, histological studies have demonstrated direct contact between tyrosine-hydroxylase-positive nerve terminals and lymphocytes in the spleen and thymus. The exposure of lymphocytes and macrophages to adrenergic agonists in vitro modulates their functions. A surgical or chemical sympathectomy is known to alter the immune responses in rodents. Recent data from the rat show that stress-induced immunosuppression is only slightly affected, if at all, by hypophysectomy or adrenalectomy, whereas it is largely dependent on sympathetic innervation. The splenic sympathetic nerve alters the firing rate by an ablation or stimulation of the hypothalamus, the administration of cytokines or neuropeptides, and an exposure to stress. Furthermore, such procedures provoke the increase in the release of noradrenaline in the rat spleen as assessed by in vivo microdialysis. The altered activities of the splenic sympathetic nerves mentioned above have been found to be causally related to the alteration in immunological responses including natural killer cytotoxicity. The splenic sympathetic nerve may thus constitute a communication channel that mediates central modulation of peripheral cellular immunity. Although the roles and mechanisms of parasympathetic control of lymphoid organs still remain obscure, recent data suggest that the thymic vagal efferent nerve may be involved in central modulation of immunity. Finally, electrophysiological studies have shown that hepatic vagal afferents may be one of the pathways through which blood-borne cytokines signal the brain.

The hepatic vagal reception of intraportal GLP-1 is via receptor different from the pancreatic GLP-1 receptor.

Glucagon-like peptide-1 (7-36)amide (tGLP-1), a representative humoral incretin, released into the portal circulation in response to a meal ingestion, exerts insulinotropic action through binding to the tGLP-1 receptor known to be a single molecular form thus far. We previously reported that the hepatic vagal nerve is receptive to intraportal tGLP-1, but not to non-insulinotropic full-length GLP-1-(1-37), through a mechanism mediated by specific receptor to the hormone. In the present study, we aimed to examine how modification of the receptor function alters this neural reception of tGLP-1, by using the specific agonist, exendin-4, and the specific antagonist, exendin (9-39)amide, of the receptor at doses known to exert their effects on the insulinotropic action of tGLP-1. Intraportal injection of 0.2 or 4.0 pmol tGLP-1, a periphysiological and pharmacological dose, respectively, facilitated significantly the afferent impulse discharge rate of the hepatic vagus in anesthetized rats, as reported previously. However, unexpectedly, intraportal injection of exendin-4 at a dose of 0.2 or 4.0 pmol, or of even 40.0 pmol, did not facilitate the afferents at all. Moreover, intraportal injection of exendin (9-39)amide at 100 times or more molar dose to that of tGLP-1, either 5 min before or 10 min after injection of 0.2 or 4.0 pmol tGLP-1, failed to modify the tGLP-1-induced facilitation of the afferents. The present results suggest that the neural reception of tGLP-1 involves a receptor mechanism distinct from that in the well-known humoral insulinotropic action.

Olfactory stimulation with scent of grapefruit oil affects autonomic nerves, lipolysis and appetite in rats

In a previous study, we found that olfactory stimulation with scent of grapefruit oil (SGFO) excites the sympathetic nerve innervating the white adipose tissue in rats. Here we further examined the effects of SGFO in rats and observed that olfactory stimulation with SGFO excited the sympathetic nerves innervating the brown adipose tissue and adrenal gland and inhibited the parasympathetic gastric nerve. Local anesthesia of the nasal mucosa with xylocaine or anosmic treatment using ZnSO4 eliminated the autonomic changes caused by SGFO. Moreover, stimulation with SGFO elevated the plasma glycerol level, and treatment with either ZnSO4 or an intraperitoneal injection of diphenhydramine, a histamine H1 receptor-antagonist, abolished the glycerol elevation by SGFO. Furthermore, a 15-min exposure to SGFO three times a week reduced food intake and body weight. Finally, limonene, a component of grapefruit oil, induced responses similar to those caused by SGFO, and diphenhydramine eliminated the glycerol response to limonene. Thus, the scent of grapefruit oil, and particularly its primary component limonene, affects autonomic nerves, enhances lipolysis through a histaminergic response, and reduces appetite and body weight.