BACKGROUND: Protein absorption occurs mostly in the form of di- and tri-peptides transported exclusively by the intestinal H⁺/peptide cotransporter 1 (PEPT1). We demonstrated a diurnal variation in expression and function of duodenal and jejunal but not ileal PEPT1 in rat small intestine (unpublished data). Various factors (diet, hormones) appear to regulate this rhythm; the role of CNS neural regulation is unexplored. AIM: To define the role of vagal innervation in control of gene expression, function, and diurnal rhythm of rat intestinal PEPT1. HYPOTHESIS: Complete abdominal vagotomy abolishes diurnal variations in gene expression and transport function of PEPT1 in rat small intestine. METHODS: 24 Lewis rats underwent total abdominal vagotomy; 24 served as normal controls. Rats were kept in a 12-h light/dark room [lights 6AM to 6PM] with free access to chow and water; their feeding pattern was monitored. Rats were sacrificed 4 wks later, and duodenal, jejunal, and ileal segments were harvested at 9AM, 3PM, 9PM, and 3AM (n=6 rats at each time point). mRNA and protein levels (expressed relative to the housekeeping gene GAPDH) were determined by real time RT-PCR and Western blots, respectively, and transporter-mediated uptake of di-peptide (Gly-Sar) was measured by everted sleeve technique. Histomorphology was quantitated. RESULTS: Diurnal variation in expression of mRNA for PEPT1, as in controls, was retained in duodenum and jejunum after vagotomy (peak at 3PM, p<0.05) but not in ileum; however, diurnal variation in expressions of protein and Gly-Sar uptake was abolished throughout the small intestine of vagotomized rats (p>0.3). As in controls, uptake was greater in jejunum compared to duodenum and ileum at all time points after vagotomy (V_max in nmol/cm/min: jejunum vs duodenum and ileum; 163 vs 88 and 71 at 3AM; p<0.04); K_m remained unchanged. There was no difference in protein expression between anatomic segments. Nocturnal feeding of rats resumed at 2nd wk post-op (>70% of food intake occurred during dark cycle). Villous height remained unchanged after vagotomy. SUMMARY: After vagotomy, PEPT1 retained a diurnal pattern of mRNA expression in duodenum and jejunum but not in ileum, similar to controls. Diurnal variation in protein levels and transport function of PEPT1 was abolished in all three intestinal segments after vagotomy. Maximal uptake was in jejunum. CONCLUSIONS: Vagal innervation does not mediate diurnal rhythm of mRNA expression of PEPT1, but does appear to mediate in part diurnal variation in protein expression and transport function of PEPT1, possibly via posttranscriptional/posttranslational processing. (Support: DK39937 MGS).