Olfactory bulb mitral-tufted cell plasticity: odorant-specific tuning reflects previous odorant exposure

Abstract

Olfactory system second-order neurons, mitral-tufted cells, have odorant receptive fields (ORFs) (molecular receptive ranges in odorant space for carbon chain length in organic odorant molecules). This study quantified several dimensions of these excitatory odorant receptive fields to novel odorants in rats and then examined the effects of passive odorant exposure on the shape of the ORF-tuning curve. ORFs for carbon chain length of novel ethyl esters (pure odorants that the animals had not been exposed to previously) were determined before and after a 50 sec prolonged exposure to one of the odorants. In response to novel odorants, quantitative analysis of mitral-tufted cell excitatory ORFs revealed that the median ORF width spanned 3-4 carbons, generally with a single-most excitatory odorant. Exposure to either the most excitatory odorant (ON-PEAK) or an odorant that was two carbons longer (OFF-PEAK) for 50 sec produced whole ORF suppression immediately after the end of the prolonged exposure, with the ON-PEAK exposure producing the greatest suppression. These results are consistent with a feature-detecting function for mitral-tufted cells. Redetermination of the ORF 15 and 60 min after the exposure revealed that OFF-PEAK exposure produced a reduction in responsiveness to the best odorant and an increase in responsiveness to the exposed odorant. In contrast, exposure to the ON-PEAK odorant or no odorant did not affect ORFs. Given that mitral-tufted cells receive exclusive excitatory input from olfactory receptor neurons expressing identical receptor proteins, it is hypothesized that experience-induced mitral-tufted cell ORF changes reflect modulation of lateral and centrifugal olfactory bulb circuits.

Figure 1.

Representative example of single-unit mitral-tufted cell responses to a novel series of ethyl esters. A, Sample traces of spike activity before, during, and after 2 sec odorant presentations of a homologous series of ethyl esters differing in carbon chain length B, Antidromic LOT-evoked spike recorded from the neuron shown in A (arrow, evoked spike) and histological verification of electrode location near the mitral-tufted cell layer (asterisk, recording site). The time scale for antidromic response is 5 msec. C, ORF for cell in A is based on mean odorant-evoked changes in cell firing for each odorant. D, ORF of same cell remapped as a percentage of the BEST-ODORANT response (normalized). Replotting the ORF as a percentage of BEST-ODORANT response does not change overall ORF shape.

Figure 2.

A--D, Individual examples of mitral-tufted cell ORFs to novel ethyl esters. Normalized ORFs were mapped as a percentage of the BEST-ODORANT response for each cell. Individual cells show differences in ORF shape and some have suppressive responses to odorants with longer and shorter chains surrounding excitatory stimuli. Responses to AA were also mapped. Some cells that responded to ethyl esters were also responsive to AA stimulation.

Figure 3.

Response specificities of mitral-tufted cells to novel ethyl esters. A cell was considered to respond to an odorant if the odorant-evoked activity was >0 (see Materials and Methods). A, Mean novel responses for all cells (n = 72) based on odorant-evoked changes in cell firing to each ethyl ester and AA. B, Percentage of excitatory responses evoked by each odorant for all cells tested. C, Mean number of odorants eliciting responses in each cell. On the basis of excitatory responses alone, most cells displayed relatively broad RFs. D, Molecular structure and name of all odorants used.

Figure 4.

Response specificities of mitral-tufted cells based on statistically defined responses (see Materials and Methods). Responses were considered significant only if the odorant-evoked firing was statistically different from baseline activity. A, Mean novel responses for all cells based on significant odorant-evoked changes in cell firing to each ethyl ester and AA. B, Mean number of odorants eliciting responses in each cell. On the basis of significant response, most cells displayed more narrow ORFs with fewer odorants eliciting responses.

Figure 5.

Conditional probability of an individual cell responding to specific odorants on the basis of all ORFs determined in this experiment (n = 72). Assuming a response to a given odorant (listed as the ordinate in the pseudocolor graph on the right), the probability of response to other odorants is displayed with higher probabilities (red). A histogram representation of the same data is shown on the left (odorant that the cell responds to is labeled above each histogram). If a given cell responded to an odorant of specific carbon chain length, it had a high probability of responding to shorter chained-related odorants. Response probabilities were based on odorant-induced responses that were statistically significant from baseline activity.

Figure 6.

Mean single-unit ORF changes (odor-evoked spikes) immediately after 50 sec odorant exposure. A, Mean single-unit changes immediately after exposure to the ON-PEAK odorant (n = 20). B, Mean single-unit changes immediately after exposure to the OFF-PEAK odorant (n = 28). Arrows represent the carbon chain length of odorant presented during exposure. Asterisks represent a significant difference between postexposure odorant responses and preexposure responses (p < 0.05).

Figure 7.

Mean normalized single-unit ORF changes immediately after 50 sec odorant exposure. ORFs from Figure 6 were normalized as a percentage of the preexposure BEST-ODORANT response. A, Mean single-unit changes immediately after exposure to the ON-PEAK odorant (n = 20). B, Mean single-unit changes immediately after exposure to the OFF-PEAK odorant (n = 28). Arrows represent the carbon chain length of odorant presented during exposure. Asterisks represent a significant difference between postexposure odorant responses and preexposure responses (p < 0.05).

Figure 8.

Examples of individual mitral-tufted cell odorant ORFs before and 60 min after a single 50 sec odorant exposure. ORFs are normalized on the basis of the preexposure BEST-ODORANT response. A, ORF changes in a mitral-tufted cell after ON-PEAK odorant exposure. In this case, the overall ORF shows little change, although with enhanced suppression of odorants similar to the BEST-ODORANT. B, ORF changes in a mitral-tufted cell with OFF-PEAK odorant exposure. In this cell, ORF changes were seen with an overall shift of the ORF toward the experienced odorant as well as suppression of the BEST-ODORANT response. Arrows represent the carbon chain length of the odorant presented during the 50 sec exposure.

Figure 9.

Mean single-unit odorant ORF changes after a single 50 sec odorant exposure. ORFs were normalized as a percentage of the preexposure BEST-ODORANT response. A, Mean single-unit changes 15 min and 1 hr after exposure to the ON-PEAK odorant (n = 11). B, Mean single-unit changes 15 min and 1 hr after experience to the OFF-PEAK odorant (n = 10). Arrows represent the carbon chain length of odorant presented during exposure. Asterisks represent a significant difference between postexposure odorant responses and preexposure responses (p < 0.05).

Figure 10.

Pseudocolor representation of ORF changes over the course of 60 min after a single 50 sec odorant exposure. A, Mean single-unit changes after exposure to the ON-PEAK odorant (n = 11). Immediately after exposure, the ORF is suppressed but appears to recover within 15 min. After this, the ORF remains relatively stable over time, with no apparent shift, and the BEST-ODORANT remains the same. B, Mean single-unit changes after exposure to the OFF-PEAK odorant (n = 10). In contrast to the ON-PEAK exposed cells, after the initial suppression was brought about through OFF-PEAK exposure, the ORF displays major changes throughout the 60 min. The ORF shape changes with the responses to the BEST-ODORANT being suppressed, and responses to the experienced odorant being enhanced. The horizontal bar represents the carbon chain length of odorant presented during exposure. The color bar represents the amount of odorant-induced activity, with red being excitatory odorant responses and blue being suppression relative to baseline.

Figure 11.

Mitral-tufted cells show narrowing of responsiveness to all esters after 50 sec of exposure with one of the esters in the series. As a population, both ON-PEAK- and OFF-PEAK exposed groups showed ORF narrowing with a significant decrease in the percentage of cells showing excitatory responses to the esters 60 min after the 50 sec odorant exposure. A statistical comparison revealed a significant difference between preexposure and postexposure for all cells (p < 0.05).

Figure 12.

Responses to isoamyl acetate before and after experience with ethyl esters. Response magnitudes are expressed as a percentage of the initial isoamyl acetate response. Similar to the other odorants, adaptation to one of the ethyl esters caused responses to be significantly suppressed. The responses seemed to recover over the course of the experiment and being similar to baseline after 60 min (n = 13).