Impaired tunnel-maze behavior in rats with sensory lesions: vestibular and auditory systems

Maze behavior of rodents provides insight into processes of learning and memory and also serves to assess cognitive functions in neurotoxicity tests. Neurotoxic agents may impair maze behavior by acting upon different parts of the nervous system. To assess the dependence of maze learning upon vestibular and/or auditory input, the two systems were lesioned. Daily treatment of rat pups with streptomycin (400 mg/kg sc) on postnatal day 11 to 22 caused irreversible impairment of vestibular and auditory functions, whereas, 20 injections of neomycin in adult rats (100 mg/kg sc, postnatal weeks 8 to 11) led to hearing loss only. Hearing loss was assessed by absence of Preyer's reflex and impaired vestibular function by loss or shortened duration of postrotatory nystagmus. Learning in the unbaited 6-arm radial maze was tested at the age of 2 to 3 mon using a maze configuration that allowed to assess order of arm entries ("working memory") and left-right discrimination within each arm ("reference memory"). Treatment with streptomycin but not with neomycin led to impaired order of arm entries. Since treatment with streptomycin failed to induce any signs of brain lesions, impaired maze learning is considered to result from destruction of vestibular hair cell receptors with subsequent vestibular impairment and not from hearing loss or cognitive impairment.