Postprandial effects of cholecystokinin are mediated by brainstem PrRP neurons

Cholecystokinin (CCK) is a gut-derived hormone that regulates satiety through neural mechanisms. It binds to CCK1 receptors located on vagal afferents that project to the brainstem and activate circuits in the dorsal-vagal complex. This vagal pathway is critical for CCK-induced satiation, but little is known about the circuitry that is activated downstream. Our lab has shown previously that CCK-induced satiety requires signalling through receptors for prolactin-releasing peptide (PrRP) in the brain(1-3), and here we pinpoint the role of a specific population of PrRP-expressing neurons in the nucleus of the solitary tract (NTS) in the brainstem. PrRP neurons are found in three discrete populations in the brain: one in the NTS, another in the ventrolateral medulla of the brainstem and a third in the dorsomedial nucleus of the hypothalamus. To dissect these populations, we first generated a knockout mouse by inserting a lox-STOP-lox (LSL) codon between the PrRP promoter and exon 1 of the gene. Unlike those in the hypothalamus, PrRP neurons in the brainstem are noradrenergic and express the enzyme tyrosine hydroxylase (TH). Thus, expression of PrRP in the brainstem, but not the hypothalamus, was rescued by crossing LSL-PrRP mice with TH-Cre mice. Subsequent experiments were performed on littermates. All data are presented as mean ± SEM. Rescue was confirmed by qPCR and immunohistochemistry. LSL-PrRP mice displayed late-onset obesity and, in contrast to wild-type littermates, did not respond to intraperitoneal CCK with reduced food intake (WT: 0.57±0.05g vs. 0.24±0.02g [P < 0.05]; LSL-PrRP: 0.44±0.04g vs. 0.39±0.02g [P > 0.05]; vehicle vs. CCK; n=6/7; ANOVA). Restoration of PrRP in the brainstem was sufficient to restore a normal body-weight phenotype in TH-Cre::LSL-PrRP mice. Furthermore, these animals responded to intraperitoneal CCK (food intake: 0.54±0.06g vs. 0.25±0.02g; vehicle vs. CCK; n=6; P < 0.05; ANOVA in same experiment as above). Thus, the brainstem population is sufficient to mediate the satiating actions of CCK. To show that PrRP neurons in the NTS alone can mediate reductions in food intake, we injected an adeno-associated virus expressing a ‘designer receptor exclusively activated by designer drug’(4) into the NTS of PrRP-Cre mice under 2% isoflurane anaesthesia. This resulted in the transfection of NTS PrRP neurons, permitting us to activate these neurons selectively by intraperitoneal injection of the designer drug, clozapine-N-oxide (CNO)(4). Activation of PrRP neurons in the NTS resulted in a significant reduction in fast-induced food intake (0.68±0.07g vs. 0.31±0.06g; vehicle vs. CNO; n=10; P < 0.05; student’s t-test). Together our results highlight the importance of central PrRP signalling in regulating body-weight and show, for the first time, that a specific population of PrRP neurons in NTS is necessary for the satiating effects of CCK. 1. Lawrence CB et al. (2000) Nat neurosci 3, 645-646. 2. Lawrence CB et al. (2002) Endocrinology 143, 360-367. 3. Bechtold DA & Luckman SM (2006) Endocrinology 147, 4723-4729. 4. Wess J et al. (2013) Trends in pharmacological sciences 34, 385-392.