Abstract
Tolerance to the antinociceptive effects of opioids is a major concern. Studies
have shown that chronic use of non-steroidal anti-inflammatory (NSAIDs) causes
significant tolerance and cross-tolerance to morphine. Chronic NSAIDs use can
increase the risk of certain diseases, such as peptic ulcers, and exacerbate
others, like heart failure. Therefore, developing novel pharmacological
approaches could provide considerable benefits for chronic therapeutic
procedures. Isovaline with a chemical structure similar to glycine and GABA
induce a significant analgesic effect through GABA-B receptors. In this study,
we investigated the impact of both short-term and long-term use of isovaline on
the immediate response to pain, as well as the development of analgesic
tolerance through daily injection (i.p.) of isovaline (100 mg/kg) for 5 days in
male Balb/c mice. Additionally, on day 6, we examined the potential for
cross-tolerance between isovaline and sodium salicylate (300 mg/kg) or morphine
(5 mg/kg). The findings showed that isovaline injection resulted in a delayed
onset of analgesic effect, a lowered peak effect, and less cumulative pain
relief compared with sodium salicylate and morphine. This analgesic effect
gradually decreased over the five days of isovaline injection. When sodium
salicylate was injected into isovaline-tolerant mice, the antinociceptive effect
decreased, suggesting cross-tolerance to sodium salicylate. However, no such
tolerance was observed following morphine injection. Accordingly, it seems that
chronic isovaline may interact with the sodium salicylate analgesic pathway but
not with morphine.
Keywords
anti-nociception - morphine - salicylate - isovaline - cross-tolerance - hot plate