Treatment schedule as a determinant of the development of tolerance to haloperidol.

Three groups of rats received haloperidol 0.5 mg/kg IP twice daily for 20 days, twice daily for 10 days, or every other day for 40 days. The rats in control groups received saline injections according to the same schedules as the experimental groups. During the chronic treatments, spontaneous motor activity was measured as an indicator of behavioral tolerance, and at the completion of treatments, limbic and striatal homovanillic acid (HVA) levels were determined in order to provide a biochemical indication of tolerance. Both of the haloperidol groups on twice-daily injection schedules exhibited a trend towards recovery of spontaneous motor activity during treatment, indicative of behavioral tolerance, as well as reduced HVA levels indicative of near complete biochemical tolerance. The group receiving haloperidol every other day exhibited a trend toward behavioral intolerance to haloperidol, along with elevated HVA levels that indicated a complete absence of tolerance. The suggested importance of treatment schedule rather than cumulative drug dosage in the development of tolerance to haloperidol may have significance to long-term side effects of chronic neuroleptic treatment such as tardive dyskinesia and clinical issues such as drug holidays.