Tactile-kinesthetic system of rats as an animal model for minimal brain dysfunction

A previous study showed that rats exposed to methylmercury during development exhibit effects similar to those described for children with minimal brain dysfunction (MBD), namely, hyperactivity, altered locomotion structure, and unaltered learning ability, but reduced and more variable attention spans induced by increasing difficulties within an operant learning paradigm. Psychopathological studies suggest that behavioral disturbances of the MBD type may originate directly or indirectly from deficiencies in the tactile-kinesthetic system. This sensory modality is the main mechanism by which an individual organism assimilates reality. Deficiencies in the tactilekinesthetic system impair the action of the equilibration processes (in Piaget's sense) which ensure that the stages of psychological development occur in an orderly sequence. The lack of this control over development may result in the behavioral characteristics of MBD. Problems with the tactile-kinesthetic system may also be the reason for the deficiencies of fine motor control in MBD children. In an atempt to extrapolate this interpretation of human psychopathological mechanisms to experimental animals, an operant paradigm was developed for the assessment of the tactile-kinesthetic system of rats, the schedule of differential reinforcement of force range (DRF). Rats were trained to press in discrete trials a force sensitive lever during at least 1 s between two force thresholds of 60 and 80 g without any feedback other than the rats' own tactilekinesthetic perception. Offspring of rat dams exposed to 1.5 and 5 mg/l methylmercury-chloride in their drinking water from 2 weeks before pairing until weaning, exhibited a clearcut performance deficit (approximately 25% correct responses compared to approximately 50% of the controls). Moreover, they showed a dose-dependent increase in the percentage of responses in which the force excerted exceeded the upper threshold. Using Powers' model of behavior (behavior is the control of perception) the observed effects may be interpreted as a methylmercury-induced deficit in the tactile-kinesthetic sensory system. This result is in accordance both with the earlier interpretation of observed perinatal methylmercury effects on behavior and with the tactile-kinesthetic hypothesis for MBD. The DRF schedule is proposed as a sensitive tool in experimental behavioral teratology for the preclinical assessment of MBD hazard.Dedicated to Professor Dr. Gerhard Zbinden on the occasion of his retirement