Tuesday, 24 February 2015

Sensory Detection of Food Rapidly Modulates Arcuate Feeding Circuit

http://www.sciencedirect.com/science/article/pii/S0092867415000768

Yiming Chen, Yen-Chu Liu, Tzu-Wei Kuo, Zachary A Knight

Summary: AgRP and POMC neurons have been observed via optogenetic methods with sub-second time resolution in actively behaving mice. Neural activity fluctuates in response to food presentation and feeding, and has characteristic dynamics within both meals and bouts. This represents a fast-responding, learning/knowledge-based modulation to the slower-fluctuating endocrine-mediated homeostatic regulation.

In this paper, the authors use optogenetic techniques to image hypothalamic activity during feeding behaviour, specifically activity of AgRP and POMC neurons. These share downstream POMC-activated/AgRP-inhibited melanocortin receptors. The paper addresses a particular knowledge gap: short-term dynamics of AgRP and POMC before, during and immediately following feeding.

AgRP neurons (which stimulate feeding) and POMC (which inhibit it) are both believed to have their activity modulated by endocrine signals encoding information about nutritional state, which is supported by this paper. A challenge with ghrelin (an endocrine indicator of hunger) led to AgRP activity almost doubling and POMC activity dropping to nearly half, and this change was reversed after food was consumed.

Surprisingly, AgRP & POMC can be strongly regulated simply by observing food, rather than longer-term endocrine-mediated homeostatic responses.  In fasted mice both POMC and AgRP neurons show a strong and rapid response to presentation of food, with most of the response having already taken place before the first bite occurs. In mice that were previously fed ad libitum this is not observed. Food removal was found to reverse the effects of food presentation, but on a slower timescale.

Detectable but inaccessible food (either in a cage or hidden from view) led to a much smaller and transient response, with the hidden food evoking a smaller change. The appeal of the food also influences the strength of the neural response, with highly appetising foods (peanut butter and chocolate) causing larger responses.

POMC and AgRP neurons also fluctuate within feeding bouts in a meal, with both AgRP and POMC having consistent fluctuations lasting approximately 30 seconds. These resemble the responses on first presentation of food, but at a much smaller level of variation.

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