Involvement of the protein kinase Cγ isoform in development of tolerance to nitrous oxide–induced antinociception in mice

Abstract Prolonged exposure to nitrous oxide (N 2 O) results in development of acute tolerance to its antinociceptive effect. Cross-tolerance to N 2 O-induced antinociception is also observed in morphine-tolerant animals. Despite increasing evidence of tolerance development to N 2 O-induced antinociception, the details of the mechanisms that underlie this tolerance remain unknown. The present study was conducted to investigate the involvement of brain protein kinase C (PKC) isoform in these two types of tolerance to N 2 O-induced antinociception in mice. Prolonged exposure (41 min in total, including 30 min pre-exposure and 11 min of antinociceptive testing) to 70% N 2 O produced a reduction in N 2 O-induced antinociception, indicating development of acute tolerance. The prolonged exposure to 70% N 2 O caused an activation of PKCγ isoform in the brain, but not the PKCe isoform. Pretreatment with a PKCγ-antisense oligonucleotide but not the corresponding mismatch oligonucleotide (i.c.v.) prevented the development of acute tolerance to N 2 O-induced antinociception. Chronic morphine treatment (10 mg/kg, s.c., b.i.d. for 5 days) resulted in development of tolerance to morphine-induced antinociception and cross-tolerance to N 2 O-induced antinociception. The development of tolerance to morphine and cross-tolerance to N 2 O were both inhibited by pretreatment with PKC inhibitor, chelerythrine (1 nmol, i.c.v.). Morphine-tolerant mice showed an activation of PKC within the brain, which was suppressed by pretreatment with chelerythrine (1 nmol, i.c.v.). Thus, activation of brain PKC, in particular, the PKCγ isoform, appears to play an important role in the development of both acute tolerance and cross-tolerance to N 2 O-induced antinociception in mice.