Metaplasticity in the Ventral Pallidum as a Potential Marker for the Propensity to Gain Weight in Chronic High-Calorie Diet

Abstract

A major driver of obesity is the increasing palatability of processed foods. Although reward circuits promote the consumption of palatable food, their involvement in obesity remains unclear. The ventral pallidum (VP) is a key hub in the reward system that encodes the hedonic aspects of palatable food consumption and participates in various proposed feeding circuits. However, there is still no evidence for its involvement in developing diet-induced obesity. Here we examine, using male C57BL6/J mice and patch-clamp electrophysiology, how chronic high-fat high-sugar (HFHS) diet changes the physiology of the VP and whether mice that gain the most weight differ in their VP physiology from others. We found that 10-12 weeks of HFHS diet hyperpolarized and decreased the firing rate of VP neurons without a major change in synaptic inhibitory input. Within the HFHS group, the top 33% weight gainers (WGs) had a more hyperpolarized VP with longer latency to fire action potentials on depolarization compared with bottom 33% of weight gainers (i.e., non-weight gainers). WGs also showed synaptic potentiation of inhibitory inputs both at the millisecond and minute ranges. Moreover, we found that the tendency to potentiate the inhibitory inputs to the VP might exist in overeating mice even before exposure to HFHS, thus making it a potential property of being an overeater. These data point to the VP as a critical player in obesity and suggest that hyperpolarized membrane potential of, and potentiated inhibitory inputs to, VP neurons may play a significant role in promoting the overeating of palatable food.SIGNIFICANCE STATEMENT In modern world, where highly palatable food is readily available, overeating is often driven by motivational, rather than metabolic, needs. It is thus conceivable that reward circuits differ between obese and normal-weight individuals. But is such difference, if it exists, innate or does it develop with overeating? Here we reveal synaptic properties in the ventral pallidum, a central hub of reward circuits, that differ between mice that gain the most and the least weight when given unlimited access to highly palatable food. We show that these synaptic differences also exist without exposure to palatable food, potentially making them innate properties that render some more susceptible than others to overeat. Thus, the propensity to overeat may have a strong innate component embedded in reward circuits.

Keywords: motivation; obesity; overeating; patch clamp; synaptic plasticity; ventral pallidum.

Figure 1.

Chronic high-fat high-sugar diet hyperpolarizes ventral pallidum neurons and inhibits their firing rate. A, Experimental timeline. Mice in the HFHS group received an HFHS diet ad libitum in their home cages for 10–12 weeks and then returned to chow diet for 2 additional weeks before recordings. The control Chow group received regular chow during the same period of time. B, Recording setup. VP neurons were recorded from in sagittal slices while activating incoming axons using a bipolar stimulating electrode placed ∼300 μm rostral to recorded cells. AC, Anterior commissure; NA, nucleus accumbens. C, D, VP neurons of HFHS mice had lower membrane potential (C) and lower action potential (AP) frequency (D) compared with chow mice (two-tailed unpaired t tests). E, Representative current-clamp traces. F, G, In a series of incrementing depolarizing steps the firing rate was significantly lower in VP neurons of HFHS mice compared with chow mice (F; two-way ANOVA), but there was no difference in the minimal latency to the first action potential (G). H, Representative traces at three depolarizing steps. Data taken from 32–46 cells in 14–21 mice/group. Data are presented as the mean ± SEM.

Figure 2.

Chronic HFHS diet alters spontaneous but not evoked GABA neurotransmission in the ventral pallidum. A, The cumulative probability plot of sIPSC amplitude (amp.) is shifted to the left in HFHS mice, pointing to reduced sIPSC amplitude (Kolmogorov–Smirnov test). Inset, Median sIPSC amplitude was lower in HFHS mice (unpaired two-tailed t test). B, HFHS diet did not affect the interevent interval (IEI) of sIPSCs in the VP (Kolmogorov–Smirnov test). Inset, Comparison of medians (unpaired two-tailed t test). C, Representative sIPSC traces. D, E, HFHS diet did not affect short-term plasticity induced by five consecutive stimulations at 10 Hz (D) or 50 Hz (E; two-way ANCOVA). Insets, Representative traces synchronized with x-axis. Stimulation artifacts were truncated. F, G, The amplitude of evoked IPSCs after an HFS did not differ between chow and HFHS mice (mixed-effects ANOVA), or from baseline (one-sample two-tailed t test on minutes 13–19). Inset, Representative traces. H, Post-tetanic potentiation, measured at the first time point after the HFS, did not differ between groups (unpaired two-tailed t test). Data taken from 19–41 cells in 13–24 mice/group. Data are presented as the mean ± SEM. n.s., not significant.

Figure 3.

Ventral pallidum neurons of weight-gainer mice are more hyperpolarized and slower to fire compared with non-weight gainer mice. A, Increase in body weight gain (expressed as a percentage of the first day) during 10–12 weeks of chronic HFHS diet for individual mice. Mice were split into non-weight gainer (bottom 33% of weight gainers on the last day of HFHS diet) and weight-gainer (top 33% of weight gainers on last day of HFHS diet) groups according to final weight gain. B, VP neurons of WG mice were more hyperpolarized than those of NWG mice (unpaired two-tailed t test). C, There was no significant difference in the baseline firing frequency of VP neurons between WG and NWG mice (unpaired two-tailed t test). D, VP membrane potential was negatively correlated with weight gain (Middle, middle 33% weight gainers). Correlation was assessed using nonparametric Spearman correlation. E, F, The firing rates during incrementing depolarization steps did not differ between WG and NWG mice (two-way ANOVA), but the minimal latency to the first action potential was longer in HFHS mice (two-way ANOVA), possibly reflecting decreased excitability. G, Representative traces. Data are taken from 9–16 cells in 7 mice/group. Data are presented as the mean ± SEM. AP, Action potential.

Figure 4.

Weight-gainer and non-weight gainer mice show different synaptic plasticities in their GABA input to the VP. A, B, sIPSC amplitude (amp.; A) and IEI (B) were similar between WG and NWG mice (Kolmogorov–Smirnov tests). Insets, Comparison of medians (unpaired two-tailed t tests). C, Representative sIPSC traces. D, E, Five consecutive pulses at 10 Hz (D) or 50 Hz (E) generated short-term potentiation of IPSCs in WG but not NWG mice (two-way ANCOVA tests). Insets, Representative traces synchronized with x-axis. Stimulation artifacts were truncated. F, G, HFS protocol given to inhibitory inputs to the VP generated a transient potentiation in WG mice but not in NWG mice (mixed-effects ANOVA; one-sample t test for comparing minutes 13–19, 100% of baseline). F, Inset, Representative traces. H, HFS-induced plasticity in VP neurons was positively correlated with weight gain (Middle, middle 33% weight gainers), going from depression in NWG mice to potentiation in WG mice. Correlation was assessed using nonparametric Spearman correlation. I, Post-tetanic potentiation, measured at the first time point after the HFS, was stronger in WG mice (unpaired two-tailed t test). Data are taken from 8–17 cells in 7 mice/group. Data are presented as the mean ± SEM.

Figure 5.

Differences in synaptic plasticity seen between WG and NWG mice exist also between high and low food seekers without exposure to HFHS diet. A, Experimental protocol. Mice were first trained to press a lever for 20 mg of chow precision pellets. After 3 weeks of training, mice were left in their home cages and fed on chow for 10–12 weeks. B, Active and inactive lever pressing, pellets rewarded, and head entries of mice during training. C–E, After the HFHS diet mice were tested in the progressive ratio paradigm and lever pressing (C), break point (D), and head entries (E) were recorded. Mice were split according to the head entries to the food receptacle (E) into high seekers (top third) and low-seekers (bottom third). F, G, There was no correlation between head entries and lever presses (F) or break point (G). Correlations were assessed using nonparametric Spearman correlation. H, There was no difference in the minimal delay to the first action potential in a series of depolarizing steps (from +20 to + 80 pA) between high-seekers and low-seekers (mixed-effects ANOVA). I, There was no difference in short-term plasticity between high-seekers and low-seekers (five stimulations at 50 Hz; two-way ANCOVA). J, K, HFS protocol potentiated eIPSCs in the high-seekers and depressed eIPSCs in the low-seekers (mixed-effects ANOVA comparing minutes 13–19; one-sample t test for comparing minutes 13–19, 100% of baseline). The level of HFS-induced potentiation was similar between high-seekers and WG mice (mixed-effects ANOVA). This was also the case between low-seekers and NWG mice, although note that only the change in low-seekers was significantly different from baseline (p > 0.05 using two-tailed unpaired t test when comparing minutes 13–19 in high-seekers to WG mice or low-seekers to NWG mice). L, Post-tetanic potentiation, measured at the first time point after the HFS, was stronger in high-seekers compared with low-seekers (unpaired one-tailed t test). Data are taken from 4–9 cells in 5 mice/group. Data are presented as the mean ± SEM. Amplitude, amp. n.s., not significant.