Sniff adjustment in an odor discrimination task in the rat: analytical or synthetic strategy?

Abstract

A growing body of evidence suggests that sniffing is not only the mode of delivery for odorant molecules but also contributes to olfactory perception. However, the precise role of sniffing variations remains unknown. The zonation hypothesis suggests that animals use sniffing variations to optimize the deposition of odorant molecules on the most receptive areas of the olfactory epithelium (OE). Sniffing would thus depend on the physicochemical properties of odorants, particularly their sorption. Rojas-Líbano and Kay (2012) tested this hypothesis and showed that rats used different sniff strategies when they had to target a high-sorption (HS) molecule or a low-sorption (LS) molecule in a binary mixture. Which sniffing strategy is used by rats when they are confronted to discrimination between two similarly sorbent odorants remains unanswered. Particularly, is sniffing adjusted independently for each odorant according to its sorption properties (analytical processing), or is sniffing adjusted based on the pairing context (synthetic processing)? We tested these hypotheses on rats performing a two-alternative choice discrimination of odorants with similar sorption properties. We recorded sniffing in a non-invasive manner using whole-body plethysmography during the behavioral task. We found that sniffing variations were not only a matter of odorant sorption properties and that the same odorant was sniffed differently depending on the odor pair in which it was presented. These results suggest that rather than being adjusted analytically, sniffing is instead adjusted synthetically and depends on the pair of odorants presented during the discrimination task. Our results show that sniffing is a specific sensorimotor act that depends on complex synthetic processes.

Keywords: discrimination; olfaction; olfactomotor act; rat; sniffing; sorption properties.

Figure 1

Non-invasive sniff recording. (A) Schematic representation of the whole-body plethysmography. This setup allows us to access the respiratory signal of the animal in a non-invasive manner. The plethysmograph is equipped with three ports: an odor port (pink, central) connected to the olfactometer and two reward ports (black and white circles surrounded with blue lines) equidistant from the central odor port. (B) Example of one trial. The rat starts the trial by poking its nose into the central port; this motion triggers the delivery of an odorant for 3 s (green). Each odorant was associated with a reward port: odorant A/left port and odorant B/right port. If the rat makes the correct choice, water is available at the reward port for 6 s from the beginning of the trial. The first lick (light green) triggers the delivery of 60 μL of water (blue).

Figure 2

Sniffing signal processing. (A) Top: Raw sniffing signal recorded by the plethysmograph. An algorithm was applied to detect the zero-crossing points. Bottom: The blue squares represent the detection of the beginning of the inspiratory phase, and the violet squares represent the beginning of the expiratory phase. (B1) Sampling duration and number of sniffs. Sampling duration (Sd) is defined as the time spent in the odor port. The number of sniffs (Ns) is defined as the number of sniffs occurring during the sampling period (pink square). (B2) A representative sniff cycle is shown to illustrate the parameters measured: inspiration duration (ID), expiration duration (ED), and inspiration peak flow rate (IPF).

Figure 3

Enantiomers are sniffed similarly. (A) Global sampling parameters (mean ± s.e.m.), Sd (left) and Ns (right), for each odorant in each enantiomer odor pair: L-car (magenta)/D-car (red) n = 195; L-lim (cyan)/D-lim (dark blue) n = 211. (B) Modulation of sniff parameters in the first, second, and third cycles for (left to right) L-car/D-car and L-lim/D-lim. From top to bottom: mean (± s.e.m.) normalized ID, IPF, and ED. Same colors as in (A). The number of trials for each odorant and cycle in L-car/D-car pair is: n_{cycle 1} = 195, n_{cycle 2} = 188, and n_{cycle 3} = 175 and in L-lim/D-lim pair is: n_{cycle 1} = 211, n_{cycle 2} = 201, and n_{cycle 3} = 162. Data were analyzed using a paired t-test; ^*p < 0.05; and ^**p < 0.01.

Figure 4

Odorants with similar sorption properties but which are not enantiomers can induce different sniffing strategies. (A) Global sampling parameters (mean ± s.e.m.), Sd (left), and Ns (right), for non-enantiomer odor pairs: hept (green)/mbz (orange) n = 168; cum (brown)/cyc (gray) n = 166. (B) Modulation of respiratory parameters in the first, second, and third cycles for (left to right) hept/mbz and cum/cyc. From top to bottom: mean (± s.e.m.) normalized ID, IPF, and ED. Same colors as in (A). The number of trials for each odorant and cycle in hept/mbz pair is: n_{cycle 1} = 168, n_{cycle 2} = 163, and n_{cycle 3} = 144 and in cum/cyc pair is: n_{cycle 1} = 166, n_{cycle 2} = 160, and n_{cycle 3} = 110. Data were analyzed using a paired t-test; ^**p < 0.01; and ^***p < 0.001.

Figure 5

Sorption is not the only parameter that determines global sniffing variations. (A) Global sampling parameters (mean ± s.e.m.), Sd (left), and Ns (right), for non-enantiomer odor pair: hept (green)/iso (black) n = 150. (B) Modulation of respiratory parameters in the first, second, and third cycles for hept/iso. From top to bottom: mean (± s.e.m.) normalized ID, IPF, and ED. Same colors as in (A). The number of trials for each odorant and cycle in hept/iso pair is: n_{cycle 1} = 150, n_{cycle 2} = 137, n_{cycle 3} = 65. Data were analyzed using a paired t-test; ^*p < 0.05; and ^***p < 0.001.

Figure 6

Sniffing variations depend on the pair in which an odorant is presented. (A) Global sampling parameters (mean ± s.e.m.), Sd (left), and Ns (right), for each odorant depending on the pair in which it was presented. D-car (red, n = 206) in the D-car/L-car pair (filled triangle) and in the D-car/D-lim pair (empty triangle); hept (green, n = 155) in the hept/iso pair (filled square) and the hept/mbz (empty square); D-lim (dark blue, n = 206) in the D-lim/L-lim pair (filled circle) and the D-car/D-lim pair (empty circle). (B) Modulation of respiratory parameters in the first, second, and third cycles for the same odorant presented in two different pairs. Left: D-car in the enantiomer pair or the D-car/D-lim pair; middle: hept in the hept/iso pair or the hept/mbz pair; right: D-lim in the enantiomer pair or the D-car/D-lim pair. From top to bottom: mean (±s.e.m.) of the normalized ID, IPF, and ED. The number of trials for each odorant and cycle is: D-car: n_{cycle 1} = 206, n_{cycle 2} = 197, and n_{cycle 3} = 181; hept: n_{cycle 1} = 155, n_{cycle 2} = 148, and n_{cycle 3} = 121; D-lim: n_{cycle 1} = 206, n_{cycle 2} = 200, and n_{cycle 3} = 157. Data were analyzed using a paired t-test; ^*p < 0.05; ^**p < 0.01; ^***p < 0.001.