Experience-dependent c-Fos expression in the primary chemosensory cortices of the rat

Abstract

Eating a new food is a unique event that guides future food choices. A key element for these choices is the perception of flavor (odor-taste associations), a multisensory process dependent upon taste and smell. The two primary cortical areas for taste and smell, gustatory cortex and piriform cortex, are thought to be crucial regions for processing and responding to odor-taste mixtures. To determine how previous experience impacts the primary chemosensory cortices, we compared the expression of the immediate early gene, c-Fos, between rats presented with a taste, an odor, or an odor-taste mixture for the first-time with rats that had many days of prior experience. Compared to rats with prior experience, we found that first-time sampling of all three chemosensory stimuli led to significantly greater c-Fos expression in gustatory cortex. In piriform cortex, only the novel chemosensory stimuli containing odors showed greater c-Fos expression. These results indicate that prior experience with taste, odor, or odor-taste stimuli habituates responses in the primary chemosensory cortices and adds further evidence supporting gustatory cortex as a fundamental node for the integration of gustatory and olfactory signals.

Keywords: Gustatory cortex; Habituation; Odor-taste; Piriform cortex; Rat; c-Fos.

Figure 1:

Schematic outline of the single bottle experience protocol. Rats were divided into two groups: novel and experience. Rats in the novel group were given 10 ml of water on days 1–6. On the final day (experimental day 7), rats in the novel group received, for the first time, 0.1 M sucrose (novel taste), 0.01% isoamyl acetate (novel odor) or a mixture of 0.01% isoamyl acetate-0.1 M sucrose (novel odor-taste). Rats assigned to the experience group received 10 ml water on days 1–3. On days 4–6, and the final experimental day, rats in the experience group received 0.1 M sucrose (experience taste), 0.01% isoamyl acetate (experience odor) or a mixture of 0.01% isoamyl acetate-0.1 M sucrose (experience odor-taste). On the final experimental day, all groups received only 5 ml of liquid.

Figure 2:

Novel chemosensory stimuli evoked greater c-Fos expression in gustatory cortex. Compared to rats with prior chemosensory experience (experience; dark grey bars; triangles represent individual count), rats that first sampled (novel; light grey bars; circles represent individual count) a taste (0.1 M sucrose), an odor (0.01% isoamyl acetate), or an odor-taste mixture (0.01% isoamyl acetate-0.1 M sucrose) exhibited significantly greater c-Fos expression in gustatory cortex. * Significant at p < 0.05.

Figure 3:

Representative images of gustatory cortex and ROIs. Novel sampling (left column) of a taste (top), an odor (middle), or an odor-taste mixture (bottom) induced significantly greater expression of c-Fos compared to rats with prior chemosensory experience (right column). Middle column shows enlarged images of the boxed areas for each rows novel (top) and experience (bottom) groups. There were no significant differences in ROI areas across sections or groups. Scale bar: 1mm low magnification; 250 µm high magnification. Abbreviations: Cl, Claustrum; CPu, caudate putamen; ec, external capsule; GC, gustatory cortex.

Figure 4:

Novel odorized stimuli, but not taste alone, evoked greater c-Fos expression in piriform cortex. Rats that first sampled an odor and an odor-taste mixture, but not a taste, (novel; light grey bars; circles represent individual count) had significantly higher c-Fos expression in piriform cortex compared to animals with previous chemosensory experience (experience; dark grey bars; triangles represent individual count). * Significant at p < 0.05; ** Significant at p < 0.01.

Figure 5:

Representative images of piriform cortex and ROIs. Novel sampling (left column) of an odor (middle) or an odor-taste mixture (bottom), but not a taste (top), induced significantly greater expression of c-Fos compared to rats with prior chemosensory experience (right column). Middle column shows enlarged images of the boxed areas for each rows novel (top) and experience (bottom) groups. There were no significant differences in ROI areas across sections or groups. Scale bar: 1mm low magnification; 250 µm high magnification. Abbreviations: DEn, dorsal endopiriform; IEn, intermediate endopiriform; lo, lateral olfactory tract; piriform cortex, PIR.

Figure 6:

Rats drank significantly less novel odor compared to novel taste and novel odor-taste mixture. On the fourth day of training, rats in the experience group were given chemosensory stimuli for the first-time. Rats consumed significantly less odor (0.01% isoamyl acetate), compared to taste (0.1 M sucrose) and odor-taste mixture (0.01% isoamyl acetate-0.1 M sucrose). * Significant at p < 0.05; ** Significant at p < 0.01.