Effect of Rohypnol on Pain Threshold and Acoustic Startle Reflex in Wistar Rats

Some physical and psychological factors resulting in heterogeneous pain have been implicated to either increase or decrease pain thresholds. Rohypnol which produces both sedative and muscle relaxant effects, is a choice drug for abusers and addicts. This study investigated the effect of Rohypnol on pain threshold and acoustic startle reflex in Wistar rats.

Twenty-five (25) female Wistar rats (160–180g) were randomly grouped into five groups (A-E) of five rats each, sequel to their acclimatization under standard ethical conditions. Rats in group A served as the control and received 0.5mL distilled water once daily. Rats in group B received 2mg/kg Diclophenac Sodium (DIC) once daily; while rats in groups C, D and E received 3mg/kg Rohypnol, once, twice and thrice daily, respectively. Treatment to rats in all groups lasted eight days. All rats received standard feed and water ad libitum throughout the study. Pain threshold was assessed using analgesy-meter and tail-clip tests; while acoustic startle reflex (ASR) response time was assessed using an acoustic bell.

The results revealed a significant increase in pain threshold in groups B and E on days 2, 4, 6 and 8 (6.40±0.48 and 8.32±0.20; 6.93±0.32 and 7.00±0.20; 6.95±0.31 and 8.93±1.30; 7.05±0.29 and 9.66±0.71 seconds, respectively) compared to the control (4.7±0.23; 5.26±0.44; 5.01±0.38; 5.06±0.30 seconds). Pain threshold in group D significantly increased on days 6 and 8 (7.00±0.12 and 7.03±0.34 seconds, respectively). An increase in ASR response time was observed in the treated rat groups compared to the control, except on day 4 where the response time in group D was significantly increased compared to the control (2.47±0.08 vs. 1.80±0.13 seconds).

This finding highlights the analgesic effect of Rohypnol in reducing pain hypersensitivity, which may be attributed to the muscle tone depression, lack of muscle alertness, and the increase in ASR response associated with the drug.