Meal pattern analysis in neural-specific proopiomelanocortin-deficient mice

Abstract

The central melanocortin system, consisting of melanocortin peptides, agouti gene related peptide and their receptors plays a critical role in the homeostatic control of energy balance. Loss of function mutations in the genes encoding proopiomelanocortin or melanocortin MC(4) receptors cause profound obesity and hyperphagia. However, little is known about the functional relationship of melanocortin neurocircuits to the temporal organization of meal-taking behavior. We used an operant paradigm that combined lever pressing for food pellet deliveries with free water intake monitored by lickometers to quantify meal patterns in mutant mice that selectively lack proopiomelanocortin expression in hypothalamic neurons (nPOMCKO). Compared to wildtype siblings, nPOMCKO mice consumed 50% more food and water daily and exhibited a more stereotyped feeding pattern characterized by reduced inter-meal and inter-mouse variations. Average meals were larger in size but shorter in duration, with no change in meal number. Consequently, intermeal intervals were prolonged in nPOMCKO mice. Similar patterns were observed in pre-obese juvenile and frankly obese adult mice suggesting that neither age nor degree of obesity was responsible for the altered phenotypes. Spontaneous locomotion and wheel running were decreased in nPOMCKO mice, but circadian variations in locomotor and feeding activity were conserved. These data show that hyperphagia in male nPOMCKO mice is due to increased meal size but not meal number, and this pattern is established by age of 5weeks. The combination of larger, more rapidly consumed meals and prolonged intermeal intervals suggests that proopiomelanocortin peptides are necessary for normal meal termination, but not the maintenance of satiety.

Fig. 1

Nocturnal locomotor activity of neuron-specific POMC knockout (nPOMCKO) mice and wild-type littermates (nPOMCWT). (A) Ambulatory time in an open field recorded over 4h intervals between 19:00–07:00 from nPOMCKO (n = 7, open squares) and nPOMCWT (n = 6, filled squares) male mice. (B) Running wheel revolutions recorded over 4h intervals between 18:00–06:00 from nPOMCKO (n = 7, open squares) and nPOMCWT (n = 5, filled squares) male mice. Data are means ± SEM.

Fig. 2

Nocturnal feeding duration (A) and feeding frequency (B) of neuron-specific POMC knockout (nPOMCKO) mice and wild-type littermates (nPOMCWT) under free-feeding conditions in running wheel cages. Both measures of feeding activity were recorded over 4h intervals between 18:00–06:00 from nPOMCKO (n = 7, open squares) and nPOMCWT (n = 8, filled squares) male mice. Data are means ± SEM. *P < 0.05, between genotypes

Fig. 3

Daily body weights of male neuron-specific POMC knockout (nPOMCKO) mice and wild-type littermates (nPOMCWT) while living continuously in operant chambers. The dotted line indicates the end of the 5d training period, after which all mice responded for food pellets on an FR30 schedule. Adult nPOMCKO (n = 10), open squares; juvenile nPOMCKO (n = 6), open triangles; and adult nPOMCWT (n = 11), filled squares. Data are means ± SEM.

Fig. 4

Daily food (A) and water (B) intake of male neuron-specific POMC knockout (nPOMCKO) mice and wild-type littermates (nPOMCWT) while living continuously in operant chambers. The dotted line indicates the end of the 5d training period, after which all mice responded for food pellets on an FR30 schedule. Adult nPOMCKO (n = 10), open squares; juvenile nPOMCKO (n = 6), open triangles; and adult nPOMCWT (n = 11), filled squares. Data are means ± SEM.

Fig. 5

Average nocturnal meal values of male neuron-specific POMC knockout (nPOMCKO) mice and wild-type littermates (nPOMCWT). (A) Meal size. (B) Meal number. (C) Meal duration. (D) Intermeal duration. Adult nPOMCWT (n = 11), dark gray columns; adult nPOMCKO (n = 10), white columns; and juvenile nPOMCKO (n = 6), light gray columns. Data are means + SEM. * P < 0.05, ** P < 0.01, *** P < 0.001 compared to nPOMCWT; † compared to adult nPOMCKO.

Fig. 6

Frequency histograms for all individual meal sizes calculated from nine consecutive nocturnal (A, C) and diurnal periods (B, D). Adult male nPOMCWT mice (n = 11) (A, B). Adult male nPOMCKO mice (n = 10) (C, D). Bin size increments are based on 20 mg food pellets.

Fig. 7

Comparisons of sequential nocturnal meal pattern values of male neuron-specific POMC knockout (nPOMCKO) mice and wild-type littermates (nPOMCWT). (A) Sequential meal durations. (B) Sequential meal sizes. (C) Sequential intermeal intervals. Adult nPOMCKO (n = 10), open squares; juvenile nPOMCKO (n = 6), open triangles; and adult nPOMCWT (n = 11), filled squares. Data are means + SEM. * P < 0.05, ** P < 0.01, *** P < 0.001 compared to nPOMCWT.