Blocking opioid receptors alters short-term feed intake and oro-sensorial preferences in weaned calves

Abstract

Opioid peptides may participate in the control of feed intake through mechanisms involving pleasure reward linked to consumption of palatable feed. The objective of this study was to determine whether blocking opioid receptors might void oro-sensorial preferences of calves, and affect circulating glucose, insulin, and anorexigenic hormones in fasted and fed calves. Two experiments involved 32 Holstein calves [body weight (BW)=86.5±1.73kg, age=72±0.6 d]. In experiment 1, all calves received an ad libitum choice of the same feed either unflavored or flavored with a sweetener (Luctarom SFS-R, Lucta, Montornès del Vallès, Spain). Feed consumption was recorded every 2h from 0800 to 1400h for 3 consecutive days to verify the establishment of an oro-sensorial preference for sweet feed (SF). The SF was preferred over the control feed (CF) at all recorded times. In experiment 2, calves were subjected to a 2 × 2 factorial design to study the interaction between opioid activity and metabolic state. Half of the calves were fasted for 14h (FAS), whereas the other half remained well fed (FED). Within each of these groups, at feeding time (0800h), half of the calves received an i.v. injection of naloxone (NAL, an opioid receptor antagonist; 1mg/kg of BW) and the other half was injected with saline solution (SAL; 0.9% NaCl). Therefore, treatments were FED-NAL, FED-SAL, FAS-NAL, and FAS-SAL. Blood samples were taken at −10, 20, 180, and 240min relative to NAL or SAL injections. As expected, cumulative consumption of starter feed was greater in FAS than in FED calves. Total feed consumption 2h after feeding was lower in NAL than in SAL calves. Calves in the FAS group did not discern between CF and SF during the first 4h after feed offer. Preference for SF was greater in SAL than in NAL calves. Calves in the FED-SAL treatment preferred SF at 2 and 6h after feed offer and tended to prefer SF at 4h after feeding. However, FED-NAL calves did not discern between SF and CF during the first 4h after feeding and tended to prefer SF only after 6h from feeding. Plasma glucose, insulin, and cholecystokinin concentrations were greater in FED than in FAS calves. Injection of naloxone decreased plasma glucagon-like peptide-1 (GLP-1) in NAL calves. Blocking opioid receptors reduced intake the first 2h after naloxone injection in FED calves, altered oro-sensorial preferences, and reduced plasma GLP-1 concentration. In conclusion, the opioid peptide system may control short-term feed intake by modulating the oro-sensorial response triggered by feed consumption, especially when calves are fed ad libitum.