Cholecystokinin Octapeptide Increases Spontaneous Glutamatergic Synaptic Transmission to Neurons of the Nucleus Tractus Solitarius Centralis

Abstract

Cholecystokinin (CCK) is released from enteroendocrine cells after ingestion of nutrients and induces multiple effects along the gastrointestinal tract, including gastric relaxation and short-term satiety. We used whole cell patch-clamp and immunohistochemical techniques in rat brain stem slices to characterize the effects of CCK. In 45% of the neurons of nucleus tractus solitarius subnucleus centralis (cNTS), perfusion with the sulfated form of CCK (CCK-8s) increased the frequency of spontaneous excitatory currents (sEPSCs) in a concentration-dependent manner (1-300 nM). The threshold for the CCK-8s excitatory effect was 1 nM, the EC(50) was 20 nM, and E(max) was 100 nM. The excitatory effects of CCK-8s were still present when the slices were preincubated with tetrodotoxin or bicuculline or when the recordings were conducted with Cs(+) electrodes. Pretreatment with the CCK-A receptor antagonist, lorglumide (1 microM), antagonized the effects of CCK-8s, whereas perfusion with the CCK-B preferring agonist CCK-8 nonsulfated (CCK-ns, 1 microM) did not affect the frequency of sEPSCs. Similarly, pretreatment with the CCK-B receptor antagonist, triglumide (1 microM), did not prevent the actions of CCK-8s. Although the majority (i.e., 76%) of CCK-8s unresponsive cNTS neurons had a bipolar somata shape and were TH-IR negative, no differences were found in either the morphological or the neurochemical phenotype of cNTS neurons responsive to CCK-8s. Our results suggest that the excitatory effects of CCK-8s on terminals impinging on a subpopulation of cNTS neurons are mediated by CCK-A receptors; these responsive neurons, however, do not have morphological or neurochemical characteristics that automatically distinguish them from nonresponsive neurons.

Presynaptic Melanocortin-4 Receptors on Vagal Afferent Fibers Modulate the Excitability of Rat Nucleus Tractus Solitarius Neurons

“…This c-Fos expression is likely caused by increased glutamate release, because administration of glutamatergic antagonists decreases satiety (Covasa et al, 2004a,b;Zheng et al, 1999Zheng et al, , 2002a. Furthermore, perfusion with anorexigenic hormones, such as CCK, increase vagal glutamatergic inputs to NTS (Simasko and Ritter, 2003;Baptista et al, 2005a). Our data are consistent with the idea that brainstem melanocortins suppress food intake by enhancing vagal satiation signals.…”

“…Pretreatment with either kynurenic acid (1 mM) or a solution containing CNQX and APV (10 and 30 M, respectively) abolished the events, indicating that the sEPSC were glutamatergic (Baptista et al, 2005a). In 11 of 31 NTS neurons (i.e., 35%), perfusion with ␣MSH (500 nM) increased the frequency of sEPSCs from 1.1 Ϯ 0.23 to 2.7 Ϯ 0.59 events/s (i.e., 233 Ϯ 21.6% of control).…”

“…The methods of preparing the brainstem slices and the identification of NTS neurons have been described previously (Baptista et al, 2005a(Baptista et al, ,b, 2007. Briefly, 25-to 35-d-old Sprague Dawley rats of either sex were anesthetized with isoflurane (abolition of the foot pinch withdrawal reflex) before being killed by administration of a bilateral pneumothorax.…”

“…Satiation signals such as gastric distention or nutrients in the intestine reach the nucleus tractus solitarius (NTS) via vagal afferent fibers or circulating hormones. NTS neurons respond, principally, with activation of glutamatergic vagal inputs (Fraser et al, 1995;Willing and Berthoud, 1997;Rinaman et al, 1998;Berthoud et al, 2001;Emond et al, 2001;Baptista et al, 2005a), resulting in the reduction of meal size (Zheng et al, 1999;Treece et al, 2000;Covasa et al, 2004a;Gillespie et al, 2005).…”

“…Furthermore, the classical satiety hormone cholecystokinin (CCK) and the longer-lasting ␣MSH synthetic analog melanotan II (MTII) use the cAMP-ERK (extracellular signal-regulated kinase)-cAMP response elementbinding protein cascade in NTS circuits as a common pathway to integrate satiety signals from the gut and adiposity signals from the hypothalamus in the control of meal size and food intake (Sutton et al, 2005). We have also shown that CCK modulates NTScentralis neurons via an increase of glutamatergic synaptic inputs (Baptista et al, 2005a(Baptista et al, , 2007, an effect that occurs also in POMC neurons of the caudal NTS (Appleyard et al, 2005).…”