Proposed animal neurosensitization model for multiple chemical sensitivity in studies with formalin

A potentially promising line of animal research relevant to multiple chemical sensitivity (MCS) is that of sensitization in the central nervous system (CNS), particularly limbic pathways in the brain. Sensitization is the progressive and enduring enhancement in behavioral and neurochemical responses that occurs after repeated exposure to psychostimulants or environmental stressors. Since the onset and progression of sensitization has many parallels with that of MCS, it has been proposed that MCS may be initiated through a mechanism similar to the sensitization of CNS components occurring in the rodent. To test this hypothesis, female Sprague-Dawley rats were exposed to formalin vapors (FORM, 11 ppm) or water vapor (control) 1 h/day for 7 days. The next day, a saline injection was given followed by a cocaine injection (15 mg/kg, i.p.) 24 h later, and locomotor activity was monitored. Animals pretreated with repeated FORM inhalation demonstrated a significantly enhanced locomotor response to cocaine compared to controls, an indicator that specific limbic pathways may have been sensitized. At 4 weeks of withdrawal from FORM exposure, a subset of animals remained sensitized to a cocaine challenge. No differences were found between groups after a saline injection. In a second experiment, animals were screened prior to FORM or water exposure for their response to a novel situation, a measure believed to reflect an animal's general responsiveness to stimuli. Rats were divided into high responders (HR) or low responders (LR), based on their locomotion in a novel cage. Results from three behavioral tests demonstrated that HR and LR were differentially affected by exposure to FORM. In a passive avoidance test, HR and LR appeared to be different in their distribution of responses, while HR and LR responses in the FORM group were nearly identical. On the elevated plus maze test of anxiety, HR spent more time on the open arms than LR in both treatment groups, with significant differences between HR and LR in the FORM, but not water, treated group. On a hot plate test to measure nociceptive levels, no differences occurred between HR and LR in the control group, whereas nociception of LR tended toward an increase compared to HR in the FORM-exposed group. Results from the second experiment suggest that the effects of FORM exposure may be obscured by examining behavior in a heterogeneous population (HR and LR). This approach using animal models may help define neural substrates that mediate the amplification of responses of a subpopulation of individuals to chemicals in the environment.