Peripheral gastric leptin modulates brain stem neuronal activity in neonates

Gastric leptin: a novel role in cardiovascular regulation

Gastric-derived leptin affects satiety and gastrointestinal function via vagal mechanisms and has been shown to interact with the gut hormone cholecystokinin (CCK). CCK selectively inhibits splanchnic sympathetic nerve discharge (SND) and the activity of a subset of presympathetic vasomotor neurons in the rostroventrolateral medulla (RVLM). The present study sought to examine the effects of gastric leptin on arterial pressure (AP), heart rate (HR), SND, and RVLM neuronal activity to determine whether its effects on cardiovascular regulation are dependent on CCK(1) receptors and vagal afferent transmission. To mimic gastric leptin, leptin (15-30 microg/kg) was administered close to the coeliac artery in anesthetized, artificially ventilated Sprague-Dawley rats. Within 5 min, leptin selectively decreased the activity of RVLM neurons also inhibited by CCK (-27 +/- 4%; P < 0.001; n = 15); these inhibitory effects were abolished following administration of the CCK(1) receptor antagonist lorglumide. Leptin significantly decreased AP and HR (-10 +/- 2 mmHg, P < 0.001; and -8 +/- 2 beats/min, P < 0.01; n = 35) compared with saline (-1 +/- 2 mmHg, 3 +/- 2 beats/min; n = 30). In separate experiments, leptin inhibited splanchnic SND compared with saline (-9 +/- 2% vs. 2 +/- 3%, P < 0.01; n = 8). Bilateral cervical vagotomy abolished the sympathoinhibitory, hypotensive, and bradycardic effects of leptin (P < 0.05; n = 6). Our results suggest that gastric leptin may exert acute sympathoinhibitory and cardiovascular effects via vagal transmission and CCK(1) receptor activation and may play a separate role to adipose leptin in short-term cardiovascular regulation.

Leptin inhibits swallowing in rats

Swallowing is under the control of premotoneurons located in the medullary solitary tract nucleus. Although rats with transected midbrain do not seek out food, they are able to ingest food present near the mouth, and acute food deprivation induces an increase in food intake. Leptin is a satiety signal that regulates feeding behavior. Because leptin receptors are found within the caudal brainstem, and because food intake is regulated in midbrain transected rats, this study tested the hypothesis that leptin is able to modify the activity of premotoneurons involved in swallowing. Leptin was microinjected at the subpostremal level of the medullary solitary tract nucleus in anesthetized Wistar rats. Electromyographic electrodes in sublingual muscles allowed recording of swallowing induced by stimulation of sensitive fibers of the superior laryngeal nerve. Repeated stimulation induced rhythmic swallowing. Microinjection of leptin (0.1 pg and 0.1 ng) in the swallowing center induced an inhibition of rhythmic swallowing (latency of <30 s) as shown by the reduced number and strength of electromyographic activities, which could last several minutes. The threshold of the leptin-induced inhibition was close to 0.1 pg. Interestingly, the inhibitory effect of leptin was not observed in leptin receptor-deficient Zucker rats. Here we show that, in Wistar rats, leptin already known to modulate the discharge of medullary solitary tract nucleus-sensitive neurons involved in satiety reflexes can also modify the activity of swallowing premotoneurons, thereby inhibiting an essential motor component of feeding behavior.

Rapid inhibition of neural excitability in the nucleus tractus solitarii by leptin: implications for ingestive behaviour

The fat-derived peptide leptin regulates cellular activity in areas of the CNS related to feeding, and application of leptin to the fourth ventricle or the nucleus tractus solitarii (NTS) inhibits food intake and weight gain. The hypothesis that leptin modulates cellular activity in the NTS was tested using whole-cell patch-clamp recordings in brainstem slices. Leptin caused a rapid membrane hyperpolarization in 58% of rat NTS neurones, including neurones receiving tractus solitarius input (i.e. viscerosensory) and those involved in regulating output to the stomach, identified after gastric inoculation with a transneuronal retrograde viral label. The hyperpolarization was accompanied by a decrease in input resistance and cellular responsiveness, reversed near the K(+) equilibrium potential, and was prevented by intracellular Cs(+). Perfusion of tolbutamide (200 microm) or wortmannin (100-200 nm) prevented the hyperpolarization, indicating activation of an ATP-sensitive K(+) channel via a PI3 kinase-dependent mechanism. Constant latency tractus solitarius-evoked EPSCs were decreased in amplitude by leptin, and the paired-pulse ratio was increased, suggesting effects on evoked EPSCs involved activation of receptors on vagal afferent terminals. Leptin reduced the frequency of spontaneous and miniature EPSCs, whereas IPSCs were largely unaffected. Leptin's effects were observed in neurones from lean, but not obese, Zucker rats. Neurones that expressed enhanced green fluorescent protein (EGFP) in a subpopulation of putative GABAergic neurones in transgenic mice did not respond to leptin, whereas unlabelled murine neurones responded similarly to rat neurones. Leptin therefore directly and rapidly suppresses activity of excitatory NTS neurones likely to be involved in viscerosensory integration and/or premotor control of the stomach.

Leptin and CCK selectively activate vagal afferent neurons innervating the stomach and duodenum

The hormone leptin and the gut peptide CCK synergistically interact to enhance the process of satiation. Although this interaction may occur at several levels of the neuroaxis, our previous results indicate that leptin can specifically enhance the satiation effect of CCK by acting on subdiaphragmatic vagal afferent neurons. Because of this localized action, we hypothesized that a high proportion of vagal afferent neurons innervating the stomach or duodenum would be responsive to leptin and/or CCK. To test this hypothesis, we measured changes in cytosolic calcium levels induced by leptin and CCK in cultured nodose ganglion neurons labeled with a retrograde neuronal tracer injected into either the stomach or the duodenum. In the neurons labeled from the stomach, CCK activated 74% (39 of 53) compared with only 35% (34 of 97) of nonlabeled cells. Of the CCK-responsive neurons 60% (18 of 30) were capsaicin-sensitive. Leptin activated 42% (22 of 53) of the stomach innervating neurons compared with 26% of nonlabeled neurons. All of the leptin-sensitive neurons labeled from the stomach also responded to CCK. In the neurons labeled from the duodenum, CCK activated 71% (20 of 28). Of these CCK-responsive neurons 80% (12 of 15) were capsaicin sensitive. Leptin activated 46% (13 of 28) of these duodenal innervating neurons, of which 89% (8 of 9) were capsaicin-sensitive. Among neurons labeled from the duodenum 43% (12 of 28) were responsive to both leptin and CCK, compared with only 15% (15 of 97) of unlabeled neurons. Our results support the hypothesis that vagal afferent sensitivity to CCK and leptin is concentrated in neurons that innervate the stomach and duodenum. These specific visceral afferent populations are likely to comprise a substrate through which acute leptin/CCK interactions enhance satiation.

Leptin-induced satiation mediated by abdominal vagal afferents

Leptin is a hormone secreted into the systemic blood primarily by white adipose tissue. However, leptin also is synthesized and stored by cells in the gastric mucosa. Because gastric mucosal leptin is secreted in response to ingestion of a meal, we hypothesized that it might contribute to satiation (meal termination) by acting on gastrointestinal vagal afferent neurons. To test whether leptin is capable of acutely reducing short-term food intake, we measured consumption of a liquid meal (15% sucrose) following low-dose leptin administration via the celiac artery, which perfuses the upper gastrointestinal tract. Leptin (1, 3, 10 μg) was infused via a chronically implanted, nonocclusive celiac arterial catheter or via a jugular vein catheter with its tip in the right cardiac atrium. Fifteen percent sucrose intake was then measured for 30 min. We found that leptin dose dependently inhibited sucrose intake when infused through the celiac catheter but not when infused into the general circulation via a jugular catheter. Plasma leptin concentrations in the general circulation following celiac arterial or jugular leptin infusions were not significantly different. Celiac arterial leptin infusion did not reduce meal size in vagotomized or capsaicin-treated rats. Finally, we also found that reduction of meal size by celiac leptin infusion was markedly enhanced when coinfused with cholecystokinin, a gastrointestinal satiety peptide whose action depends on vagal afferent neurons. Our results support the hypothesis that leptin contributes to satiation by a mechanism dependent on gastrointestinal vagal afferent innervation of the upper gastrointestinal tract.

Effects of interactions between interleukin-1 beta and leptin on cat intestinal vagal mechanoreceptors

In a previous study, we established that leptin acts on chemosensitive intestinal vagal mechanoreceptors and that its excitatory effects are blocked by the endogenous interleukin-1beta receptor antagonist (Il-1ra). To determine how interleukin-1beta (Il-1beta) is involved in the action of leptin, we studied the effects of this drug on the single vagal afferent activities of intestinal mechanoreceptors in anaesthetized cats. For this purpose, the activity of 34 intestinal vagal mechanoreceptors was recorded via glass microelectrodes implanted in the nodose ganglion. Il-1beta (1 microg) administered into the artery irrigating the upper part of the intestine activated both the 16 leptin-activated units (type 1 units; P < 0.01) and the 12 leptin-inhibited units (type 2 units; P < 0.001), but had no effect on the six leptin-insensitive units. Cholecystokinin (CCK, 10 microg) induced an activatory response only in the two types of Il-1beta-sensitive units. When Il-1beta was administered after CCK, its excitatory effects on type 1 units were enhanced, whereas the excitatory effects on type 2 units were abolished. Pre-treatment with Il-1ra (250 microg) blocked all the effects of Il-1beta and the excitatory effects of leptin on type 1 units, whereas it enhanced the inhibitory effects of leptin on type 2 units. It can therefore be concluded that (i) leptin acts on intestinal vagal mechanoreceptors via Il-1beta in the case of the type 1 units and independently of Il-1beta in the case of the type 2 units, and (ii) type 1 and type 2 units belong to two different populations of vagal afferents that transmit different information about ingestion or inflammation to the CNS, depending on the chemical environment.

Effects of leptin on cat intestinal vagal mechanoreceptors

Vagal afferent nerve fibres are involved in the transmission to the central nervous system of information relating to food intake and immune reactions. Leptin is involved in the control of food intake and has specific receptors in afferent vagal neurones. To investigate the role of these receptors, we studied the effects of leptin on single vagal afferent activities from intestinal mechanoreceptors in anaesthetized cats. The activity of 35 intestinal vagal mechanoreceptors was recorded by means of glass microelectrodes implanted in the nodose ganglion. Leptin (10 microg), administered into the artery irrigating the upper part of the intestine, induced activation in 17 units (P < 0.001), inhibition in 8 units (P < 0.05), and had no effect on 10 units. The excitatory effects of leptin were blocked by the endogenous interleukine-1beta receptor antagonist, (Il-1ra, 250 microg, I.A.). Cholecystokinin (CCK, 10 microg, I.A.) induced an activatory response only in the two types of units which were responsive to leptin alone. When leptin was administered after CCK, its excitatory effects were enhanced and its inhibitory effects were blocked, whereas it had no effect on the units which were not affected by leptin alone. The interactions between leptin and CCK are specific ones, since other substances (phenylbiguanide, a serotoninergic agonist; substance P) known to activate the mechanoreceptors did not modify the effects of leptin. These results indicate that leptin appears to play a role in the control of immune responses and food intake via intestinal vagal afferent nerve fibres and that there is a functional link between leptin and Il-1beta.

Subdiaphragmatic vagotomy fails to inhibit intravenous leptin-induced IL-1beta expression in the hypothalamus

Leptin is known to be an important circulating signal for regulation of food intake and body weight. Recent evidence has suggested that leptin is involved in infection and inflammation. The afferent vagus nerve is known to be an important component for transmitting peripheral immune signals to the brain, such as interleukin (IL)-1beta expression in the brain, anorexia, and fever responses. In the present study, we investigated whether intravenous leptin-induced IL-1beta expression in the hypothalamus is mediated via afferent vagus nerve. IL-1beta transcripts in the hypothalamus were significantly increased on RT-PCR assessment 1 h after the administration of leptin (1 mg/kg iv) to mice. Subdiaphragmatic vagotomy did not significantly modify intravenous leptin-induced IL-1beta expression in the hypothalamus compared with that in sham-treated mice. These data suggest that circulating leptin directly acts in the brain independently of afferent vagus nerve input originating from the subdiaphragmatic organs.