Olfactory discrimination varies in mice with different levels of α7-nicotinic acetylcholine receptor expression

Abstract

Previous studies have shown that schizophrenics have decreased expression of α7-nicotinic acetylcholine (α7) receptors in the hippocampus and other brain regions, paranoid delusions, disorganized speech, deficits in auditory gating (i.e., inability to inhibit neuronal responses to repetitive auditory stimuli), and difficulties in odor discrimination and detection. Here we use mice with decreased α7 expression that also show a deficit in auditory gating to determine if these mice have similar deficits in olfaction. In the adult mouse olfactory bulb (OB), α7 expression localizes in the glomerular layer; however, the functional role of α7 is unknown. We show that inbred mouse strains (i.e., C3H and C57) with varying α7 expressions (e.g., α7 wild-type [α7+/+], α7 heterozygous knock-out [α7+/-] and α7 homozygous knock-out mice [α7-/-]) significantly differ in odor discrimination and detection of chemically-related odorant pairs. Using [(125)I] α-bungarotoxin (α-BGT) autoradiography, α7 expression was measured in the OB. As previously demonstrated, α-BGT binding was localized to the glomerular layer. Significantly more expression of α7 was observed in C57 α7+/+ mice compared to C3H α7+/+ mice. Furthermore, C57 α7+/+ mice were able to detect a significantly lower concentration of an odor in a mixture compared to C3H α7+/+ mice. Both C57 and C3H α7+/+ mice discriminated between chemically-related odorants sooner than α7+/- or α7-/- mice. These data suggest that α7-nicotinic-receptors contribute strongly to olfactory discrimination and detection in mice and may be one of the mechanisms producing olfactory dysfunction in schizophrenics.

Figure 1

[¹²⁵I] α-BGT binding is localized to the glomerular layer of the OB (arrows). A) Nonspecific binding. Representative binding in (B–D) C57 and (E–G) C3H mice. Scale bar = 1 mm.

Figure 2

Mean specific tissue equivalent α-BGT binding in mice. Unlike the hippocampus (see Supplementary Figures/Results), C57 α7+/+ mice have significantly more binding in the OB compared to C3H α7+/+ mice (n = 5–8 mice/strain). Significantly less α-BGT binding was observed in α7+/− and α7−/− compared to α7+/+ mice. No significant differences were observed between α7+/− or α7−/− mice (α7+/−: n = 4–5 mice/strain; α7−/−: n = 3 mice/strain, ** = p<0.01, ANOVA with multiple comparisons, Newman-Keuls).

Figure 3

Learning a “go-no-go” odor task. A) Sample data from a C57 α7+/+ mouse, where 1% isoamyl acetate was the rewarded response and mineral oil was the unrewarded response. Mice had a 50% chance of getting the correct response, but performance of ≥ 85% correct for three consecutive blocks was defined as reaching criterion or learning the task. Blocks consisted of 20 pseudo-randomly presented rewarded (10) and unrewarded (10) odor trials. B) Control data showing that mice cued on the odor for achieving criterion. As soon as the odor was removed, mice achieved the rewarded response by chance. When the odor was returned, mice reached criterion again. C) Despite the background of the mouse (C57 or C3H), all genotypes learned the go-no-go odor task at the same rate (ANOVA with multiple comparisons, Newman-Keuls, p = 0.9, n = 8–13 for each group).

Figure 4

Before mice were tested with the MLPEST program, the animals were required to complete S+/S− to ensure they were able to detect the differences between the alcohol pair. A) C3H α7+/+ mice reached criterion by the 6^th block the first time they were exposed to the odor pair (n = 8). B) In the last presentation, by the 3^rd block, all C3H mice achieved criterion (α7+/−: n = 7; α7−/−: n = 6). C) Similar to C3H α7+/+ mice, C57 α7+/+ mice reached criterion by the 7^th block in the first presentation. D) All C57 genotypes completed S+/S− paradigm by the 3^rd block in the last presentation of the alcohol pair (n = 7 for all groups).

Figure 5

Mean discrimination thresholds for α7+/+, α7+/−, and α7−/− littermates. Odors were mixed with 0.1% octaldehyde (A) or 0.1% propyl acetate (B). Odorant concentrations decreased by 0.5 log units for each new concentration, ranging from 0.1% to 0.0001%. A) For the discrimination task using an aldehyde odor pair, WT mice discriminated an entire log unit lower compared to HET and KO in both strains of mice (n = 5–11 mice; * p<0.05, ANOVA with multiple comparisons, Newman-Keuls). B) However for an acetate odor pair, only the C57 WT mice had a threshold half a log unit lower compared to all other groups of mice (n = 5–8 mice for aldehydes; * p < 0.05, ANOVA with multiple comparisons, Newman-Keuls).