Odorant category profile selectivity of olfactory cortex neurons

Abstract

The olfactory cortex receives converging axonal inputs from many mitral and tufted cells in the olfactory bulb. Recent studies indicate that single cortical neurons integrate signals from diverse odorants. However, there remains a basic question, namely, the signals from which kinds of odorants are integrated by the individual cortical neurons? The present study examined the possibility that some cortical neurons integrate signals from distinct component odorants of natural foods because individual foods produce a fixed combination of odorants. Previous psychophysical studies of core odorants emitted by fruits and vegetables suggest that the olfactory images of individual natural foods are basically characterized by the profile of structural and perceptual categories of food-born odorants. The single-unit spike responses of neurons in the dorsoposterior part of rat anterior piriform cortex to a panel of eight food-related categories of odorants were herein examined. The results showed that many cortical neurons in this region are tuned selectively to either a single category or a specific combination of distinct categories. The cortical neurons showed mixture facilitation and mixture inhibition when stimulated with mixtures of distinct categories, thus suggesting that olfactory circuits may play a role in enhancing the category-profile selectivity of individual neurons. These results indicate that signals from distinct categories of food-born odorants are integrated in these cortical neurons. This suggests that these cortical neurons detect the odorant-category profile of foods to distinguish distinct food odors.

Figure 1.

Odorant-category profiles of natural foods. Categories of the core odorants of 33 natural foods (15 vegetables and 18 fruits). Each row shows an individual natural food. Core odorants were classified into 14 categories according to their molecular structure and perceived odor. Each column shows an individual category. On the top of these columns, the characteristic molecular structure of each category is shown. The categories are Sul, alcohols, MPZ, C6C9, aldehydes, ketones, ITC, tHC, esters, tAl, NH₂, acids, lactones, and phenols. The eight categories examined in the present study are indicated by the columns with color. Note that individual foods show distinct odorant-category profiles. The names of the odorants in the boxes a–z, aa–az, ba–bz, and ca–cr are listed in the supplemental Figures 1 and 2 (available at www.jneurosci.org as supplemental material).

Figure 2.

A panel of odorant categories used for the stimulation (A) and the position of recording area in the aPC (B–D). A, The panel consists of eight distinct categories (Sul, MPZ, C6C9, ITC, tHC, ester, tAl, and NH₂) of core odorants of natural foods. Each column shows (from top to bottom) the name of the category, abbreviation, molecular structure, component odorants in the mixture (3–7 odorants) used for the stimulation with each category, and the perceptual quality. B, Ventrolateral view of the rat brain. The analysis focused on neurons located within the green rectangular area (an arrow with *). Boundaries of the olfactory cortex are shown by broken lines. LOT is indicated with a dotted line. C, A schematic diagram illustrating the dorsal part of the aPC that includes the green rectangular area in B. With a reference to the intersection point (black dot) between MCA and the dorsalmost portion of the LOT, a 500 × 500 μm hypothetical grid was made on the surface of the olfactory cortex. Recording area is indicated by a green rectangle (an arrow with *). Recording microelectrodes were inserted into the green rectangular area under the visual guidance. D, Coronal section of aPC stained with cresyl violet. The arrow indicates the recording point marked by the Chicago Sky Blue dye. Scale bar, 500 μm.

Figure 3.

Olfactory cortex neurons in the area of aPC respond to food odors. A, Spike traces and peristimulus time histogram of a neuron in the area of the aPC, showing strong responses to the mixture of odorants that mimics the grape odor. This neuron was found in layer 3. B, Responses of another neuron (layer 2) to the artificial odor of watermelon. C, Responses of another neuron (layer 3) to the odor of raw carrot. aFR, Averaged firing rate. The bar under the histogram indicates the period of odor stimulation (3 s).

Figure 4.

Response selectivity of a neuron in the area among the eight odorant categories examined. A, Spike responses of an aPC neuron to MPZ (middle trace). The top trace indicates the period of MPZ stimulation (upward deflection, 3 s). The bottom trace shows the respiration pattern. Upward deflection indicates inhalation. Scale bar, 1 s. B, Spike traces and peristimulus time histogram of responses of the same neuron to the stimulation with individual odorant categories. This neuron showed strong responses to MPZ, ITC, and NH₂ but showed almost no response to the other categories. C, Schematic representation of the odorant-category selectivity of this neuron (neuron 42). Excitatory responses were shown by the upward columns. This neuron was found in layer 2.

Figure 5.

Odorant-category selectivity of individual neurons in the aPC. Based on the category selectivity, 70 odor-responsive neurons were classified into three groups: single category neurons (left box), multiple category neurons (middle and right top boxes), and neurons with only inhibitory responses (right bottom box). Individual neurons are shown by a thick horizontal line with neuron number. Layers of the recorded neurons are shown in the parentheses under the neuron number. L1, Layer 1; L2, layer 2; L3, layer 3. Statistically significant excitatory responses are shown by upward columns. Column size reflects the aMOR (see Materials and Methods). Truncated columns were shown by a ceiling with a broken line, and their aMORs are shown by numbers within the column. Statistically significant inhibitory responses are shown by downward hatched columns with the same size. Column size does not reflect the magnitude of the inhibitory response. For each neuron, the most adequate category that induced the largest excitatory response was indicated by an asterisk. Note a wide variety of category-profile selectivity of individual neurons in the aPC.

Figure 6.

A matrix representation of the number of recorded neurons that received excitatory inputs from a specific binary combination of the eight categories. The number of dots in each rectangle indicates the number of aPC neurons that responded to both the category shown at the horizontal axis (bottom row) and the category shown at the vertical axis (leftmost column). This matrix was based on the data of 32 multiple category neurons (excluding neuron 57) in Figure 5. Note that all of the binary combinations of the eight categories were observed in the recorded neurons.

Figure 7.

Mixture facilitation of odor-evoked responses of aPC neurons. A, Spike traces and peristimulus time histogram of a neuron in the area of the aPC. This neuron (layer 2) showed small inhibitory responses to ester (left histogram) but showed no significant response to ITC (middle). When stimulated with the mixture of these categories, however, this neuron showed strong excitatory responses (right). B, Another aPC neuron (layer 3) showing only weak excitatory responses to Sul (left) and C6C9 (middle). This neuron showed a robust response when stimulated with the mixture of Sul and C6C9. aFR, Averaged firing rate. The bar under the histogram indicates the period of odor stimulation (3 s).

Figure 8.

Mixture inhibition of odor-evoked responses of aPC neurons. A, Spike traces and peristimulus time histogram of a neuron in the area of the aPC (layer 2), showing simple type of mixture inhibition. This neuron showed no response to Sul (left) and a strong response to ITC (middle). When stimulated with a mixture of Sul and ITC (Sul+ITC), this neuron showed no response. Thus, the response to ITC was suppressed by mixing ITC with Sul. Same cell as in Figure 4 B, Another neuron in the area of the aPC (layer 3), showing a complex type of mixture inhibition. This neuron showed strong responses to Sul (left) and ester (middle). However, when stimulated with a mixture of Sul and ester (Sul+Ester), this neuron showed no significant response. aFR, Averaged firing rate. The bar under the histogram indicates the period of odor stimulation (3 s).

Figure 9.

Category-combination selectivity of mixture inhibition of aPC neurons. A, Spike traces and peristimulus time histogram of a neuron in the area of the aPC (layer 1). This neuron showed excitatory responses to MPZ (left column) but also showed no significant response to other seven categories (middle column). The excitatory response to MPZ was suppressed when mixed with Sul, C6C9, tHC, ester or NH₂. However, the mixture with ITC or tAl did not effectively suppress the MPZ-evoked response. B, A schematic diagram of the category-combination selectivity of mixture inhibition of the neuron shown in A. The horizontal bar with color indicates the category (MPZ) that evoked excitatory response in this neuron. The excitatory response to MPZ was suppressed when mixed with the categories that are linked by the dotted line (shown by ellipses with color). Mixture with categories shown by ellipses without the dotted line did not suppress the MPZ-induced response. C, Another neuron (layer 3) showing excitatory responses to Sul. The remaining seven categories showed no significant response. The excitatory response to Sul was suppressed when mixed with MPZ or ester. However, mixture with other categories did not suppress the Sul-evoked response. D, A schematic diagram of the category-combination selectivity of mixture inhibition of the neuron in C. E, Another neuron (layer 2) showing the complex type of mixture inhibition. This neuron showed excitatory response to NH₂, MPZ, and ITC. Mixture inhibition was observed when two of them were mixed [NH₂+MPZ, NH₂+ITC (right column) and ITC+MPZ (data not shown)]. F, Schematic diagram of the complex type of mixture inhibition of the neuron in E. The MPZ-induced response (horizontal bar with MPZ) was suppressed when MPZ was mixed with Sul, C6C9, ITC, tHC, ester, tAl, or NH₂. ITC-induced response (horizontal bar with ITC) was suppressed when ITC was mixed with Sul, MPZ, C6C9, tHC, ester, tAl, or NH₂. The NH₂-induced response (horizontal bar with NH₂) was suppressed when NH₂ was mixed with Sul, MPZ, C6C9, ITC, ester, or tAl. aFR, Averaged firing rate. The bar under the histogram indicates the period of odor stimulation (3 s).