Main and accessory olfactory bulbs and their projections in the brain anticipate feeding in food-entrained rats

Jun 11, 2014Chronobiology international

Smell-related brain areas and their connections respond before feeding in rats trained to eat on a schedule

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Abstract

The olfactory bulb shows high activation before food presentation in adult rats under a restricted feeding schedule.

The olfactory bulb has a circadian clock independent of the suprachiasmatic nucleus.
High activation of the olfactory bulb is associated with food anticipatory activity before food presentation.
In rats with ad libitum access to food, FOS levels in the olfactory bulb were low during the day and high at night.
Rats under restricted feeding conditions exhibited high FOS levels in the olfactory bulb just before food presentation and during consumption.
FOS levels in the brain targets, such as the piriform cortex and bed nuclei of the accessory olfactory tract, mirrored the pattern observed in the olfactory bulb.

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The olfactory bulb (OB) has a circadian clock independent of the suprachiasmatic nucleus, but very little is known about the functional significance of its oscillations. The OB plays a major role in food intake as it contributes to the evaluation of the hedonic properties of food, it is necessary for a normal pattern of locomotor behavior and their ablation disrupts feeding patterns. Previously we demonstrated that OB of rabbit pups can be entrained by periodic nursing but it was not clear whether food was the entraining signal. Here we hypothesized that OB can be entrained by a food pulse during the day in adult rats under a restricted feeding schedule. Then we expect that OB will have a high activation before food presentation when animals show food anticipatory activity (FAA). To this aim we determined by immunohistochemistry the expression of FOS protein, as an indicator of neural activation, in the mitral and granular cell layers of the main and accessory OB. Additionally we also explored two of the OB brain targets, the piriform cortex (PC) and bed nuclei of the accessory olfactory tract (BAOT), in three groups: ad libitum (ALF), restricted feeding (RF), and fasted rats after restricted feeding (RF-F). In ALF group FOS levels in both main and accessory OB were low during the day and high during the night at the normal onset of the increase of activity, in agreement with previous reports. On the contrary in RF and RF-F groups FOS was high at the time of FAA, just before food presentation, when animals are in a state of high arousal and during food consumption but was low during the night. In their brain targets, we observed a similar pattern as OB in all groups with the only difference being that FOS levels remained high during the night in RF-F group. We conclude that the OB is entrained by food restriction by showing high activation at the time of food presentation, which persists during fasting and impose a similar FOS pattern to the two brain targets explored only in fed animals.

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Main and accessory olfactory bulbs and their projections in the brain anticipate feeding in food-entrained rats

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