Quantification of the analgesic activity of narcotic antagonists by a modified hot-plate procedure.

TLDR

The study used two hot plates (49.5 °C and 54.5 °C) to evaluate the analgesic activity of morphine and several narcotic antagonists in rats. All tested narcotic antagonists produced dose‑related analgesia on the 49.5 °C plate, with only high‑dose pentazocine active at 54.5 °C; naloxone blocked morphine and antagonist effects but was inactive alone, aspirin had no effect, and physostigmine increased latencies only at doses causing motor deficits, confirming the low‑temperature plate yields reproducible, large‑magnitude, specific results.

Abstract

The analgesic activity of morphine and the narcotic antagonists, pentazocine, cyclazocine, levallorphan and nalorphine, was assessed in the rat using two hot plates: one maintained at 49.5 degrees C and the other at the "standard" 54.5 degrees C. The analgesic effects of morphine using the 49.5 degrees C hot plate were of a significantly greater magnitude than its effects using the 54.5 degrees C hot plate for both nontolerant and morphine-tolerant subjects. Dose-related effects were observed with all of the narcotic antagonists tested using the 49.5 degrees C hot plate; only the highest dose of pentazocine exhibited activity using the 54.5 degrees C hot plate. Naloxone (3.0 mg/kg) antagonized the analgesic effects of morphine and all of the narcotic antagonists, but was without activity itself when tested using the 49.5 degrees C hot plate at doses as high as 30 mg/kg. Aspirin was also inactive using the 49.5 degrees C plate, whereas physostigmine increased latencies only at a dose that produced severe motor deficit. A low temperature hot plate is recommended for evaluating the analgesic activity of narcotic antagonists in the rat. This simple procedure provides results which are dose-related, quantifiable, reproducible, of a large magnitude and specific.