Figure 5. Effect of local administration of naloxone on antinociceptive response to nitrous oxide. Six cohorts of rats (n = 6 per group) received cannulae positioned either intrathecally (A) or intracerebroventricularly (B). Rats received injections of either saline or naloxone (10, 100 micro gram, in 10 micro liter intrathecally; 5, 10 micro gram in 10 micro liter intracerebroventricularly) 10 min before the latency to eliciting a tail flick in response to a heat stimulus was measured. Subsequently, the animals were exposed to nitrous oxide (70% v/v) for 30 min and the tail-flick latency measurement was repeated. Data are expressed as mean +/- SEM. *= significantly different (P < 0.05) from baseline measurement.