Why do you eat? We’ve come to realise that it’s far more complex than simply saying you eat because you are hungry. Anyone who’s been reading my blog will understand the complexity of the neural networks involved in the control feeding behaviour.
A paper came out a couple of weeks ago that splits out a number of the drivers for food intake by differentiating a subpopulation within the lateral hypothalamus (LH)1. This stems from an earlier paper that found that GABAergic (VGAT-expressing) neurones of the LH show heterogeneous feeding behaviours2:
- Consummatory behaviour – the classic homeostatic drive to consume calories
- Appetitive behaviour – the more complex drive for reward and rewarding foods
Anyway, back to the recent Siemian paper, where they investigate leptin receptor (LEPR)-expressing neurones of the LH as a subpopulation (~20 %) of the larger VGAT population. They start with my favourite method for inhibiting neurones, which is to ablate them using cre-dependent caspase (AAV-FlEX-tsCasp3-TEVp; Figure 1A/B). Ablating LH VGAT neurones with caspase causes a lean phenotype with reduced food intake (“consummatory behaviour”; Figure 1C/D), with delayed learning to cue-stimulated sucrose intake (“appetitive behaviour”; Figure 1E). However, LH Lepr-ablated mice had no change to gross food intake or body weight (Figure 1F/G), but did learn a cue-stimulated appetitive response (Figure 1H).
In order to directly control feeding behaviour, they next use my favourite method for driving neuronal activity, which is optogenetic stimulation of ChR2. Injecting cre-dependent ChR2 or the inhibitory NpHR into the LH of VGAT-cre or Lepr-cre mice (Figure 2A/B), they run two behaviour tests to mimic the data seen with caspase. In this case, because optogenetic stimulation is instantaneous (rather than the long-term chronic caspase), they use acute tests.
First is free-access feeding to assess consummatory behaviour; second is real-time place preference to assess appetitive behaviour. I use both these tests frequently, and find them to be both robust and informative. The data show, as expected, that optogenetic stimulation of VGAT neurones drives consummatory behaviour (Figure 2D/E) and a strong preference (Figure 2H/I), whereas stimulation of Lepr neurones induced place preference (Figure 2J/K) without any impact on consummatory behaviour (Figure 2F/G).
Next comes the data that I think is the most interesting part of this paper, and also the most complex, where the authors use a fluorescent miniscope to investigate GCaMP activity in the two populations of interest (Figure 3A/B). What’s really interesting here is how they split out the neuronal populations based on the timing of their response to whether they were responsive during the cue or after it (they show “pre-responsive” neurones, but in a world without midi-chlorians, I don’t think we should put too much stock in neurones that predict when a cue is going to happen).
So anyway, what we’re interested in is the subpopulations of neurones that respond differently between the CS+ and CS– stimuli, which for LHVGAT is the post-lick reward-responsive neurones (Figure 3G), but for LHLEPR is both the cue-responsive and reward-responsive neurones (Figure 3J/K). This is quantified in Figure 3L-O, where they show that LHLEPR neurones are strongly and significantly predictive of reward from cues. This means that in addition to the general LH drive for reward, the LHLEPR neurones are the ones that can distinguish cues and therefore may be important for the behavioural discrimination of food cues.
There are a couple more figures that go into further detail to show the importance of the LHLEPR projections to the VTA for mediating appetitive learning, and then show that the LHLEPR neurones are not relevant for cocaine preference. But, I’ve shown here the results that I found most interesting, and how they inform us on the control of feeding behaviour. In particular, I want to highlight the use of miniscopes to pick out subpopulations of neurones based on behavioural responses.
1. Siemian et al. Cell Reports 36, 109615 (2021) Lateral hypothalamic LEPR neurons drive appetitive but no consummatory behaviors
2. Jennings et al. Cell 160, 516-527 (2015) Visualizing hypothalamic network dynamics for appetitive and consummatory behaviors.