Conductive hearing loss does not affect spatial learning and memory in middle-aged guinea pigs

Abstract

Hearing loss (HL) in mid-life has been suggested as a risk factor for cognitive decline. It is unclear whether this relationship is due to deprivation of auditory input alone, degenerative processes, or a combination. Animal models are useful to investigate underlying neural mechanisms as human studies can be confounded by various factors. However, most animal studies use young animals and often exclude females. We used middle-aged guinea pigs of both sexes to investigate whether 8 weeks of auditory deprivation due to conductive HL caused spatial learning and memory impairments. Forty guinea pigs (20 M, 20 F, ~ 12 months) were tested in the Morris Water Maze (MWM) to assess baseline spatial learning and memory. In 20 of these animals (10 M, 10 F) the ear canal was plugged and 8 weeks later, animals were again assessed in MWM. No deficits in spatial learning or memory were observed in either sex. HL caused a small decline in body weight suggesting some stress associated with conductive HL, although adrenal weight, corrected for body weight, did not change. Our data suggest that auditory input deprivation alone does not affect spatial cognition in middle-age, in line with recent human data suggesting that additional risk factors need to be present.

Keywords: Ageing; Cognition; Hearing; Morris water maze.

Fig. 1

Thresholds of Auditory Brainstem Response (ABR) (dB SPL) in control middle-aged animals (n = 9; black circles) and in middle-aged animals with ears plugged (n = 9; open circles) showing consistent and significant threshold shift across the frequencies. Data mean ± SEM. ** p < 0.001, *** p < 0.0001 between control and HL groups at each frequency.

Fig. 2

(A): Graph showing baseline data from learning trials MWM of middle-aged (n = 40) and young animals (n = 9). * p < 0.05 between young and middle-aged animals. For differences between days within groups see text. (B): Graph showing baseline data from learning trials MWM of middle-aged good learners (n = 24) and bad learners (n = 16) as well as young animals (n = 9). * p < 0.01 between middle-aged good and bad learners, # p < 0.01 between young and middle-aged bad learners, + p < 0.05 between young and middle-aged good learners. For further details see text. (C, D): Bar graphs showing percentage time spent in different quadrants in 60 s in the memory trial, comparing young and middle-aged animals (C) and young, and middle-aged good and bad learners (D). Only middle-aged animals as a group (C) and middle-aged good learners (D) showed a significant (* p < 0.01) preference for the target quadrant. (E): Graph showing baseline data from learning trials MWM of middle-aged male (n = 20) and female animals (n = 20). (F): scatterplot showing correlation between swimming speed time to reach platform on day 5 in all middle-aged animals. Also shown are young animals for comparison (not included in trendline).

Fig. 3

(A) Graph showing post-intervention data from learning trials MWM of middle-aged animals with (n = 19) and without conductive HL (n = 20). No statistically significant differences were observed at any day between control and HL animals. For details of differences within each group versus time see text. (B) Bar graphs showing percentage time spent in different quadrants in 60 s in the memory trial, comparing middle-aged animals with (n-19) and without a conductive HL (n = 20). Both groups showed a statistically significant increased time in the target quadrant (* p < 0.01) compared to the other quadrants.