Activation of PKCα or PKCε as an approach to increase morphine tolerance in respiratory depression and lethal overdose

Abstract Long-term use of opioids is hindered by respiratory depression and the possibility for fatal overdose in drug abusers. This is due to higher levels of tolerance that develops against antinociception than to respiratory depression. Identifying important mechanisms that would increase morphine respiratory depression and overdose tolerance could lead to the safer use of opioids. Since PKC activity mediates the development and maintenance of morphine antinociceptive tolerance, we hypothesized that activating PKCα or ε at the pre-Botzinger complex (preBotC) can increase morphine tolerance in respiration and overdose. Laser microdissection and qRT-PCR were used to compare the relative mRNA abundances of PKCα, γ, and ε between vlPAG vs. preBotC. To test whether PKCα or ε could enhance morphine tolerance in respiratory depression and overdose, lentivirus carrying the wild type (WT), constitutively activated mutants (CA), and siRNA against PKCα or ε were stereotaxically injected into the preBotC. Expressing CAPKCα or ε, but not WTPKCα or ε, at the preBotC allowed rats to develop tolerance to morphine respiratory depression. In terms of lethality, expressing either WTPKCε, CAPKCα, or CAPKCε at preBotC increased morphine tolerance to lethal overdose. CAPKCε expressing rats developed the highest level of respiratory depression tolerance. Furthermore, when CAPKCε lentivirus was injected into the vlPAG, rats were able to develop significant antinociceptive tolerance at low doses of morphine that normally do not cause tolerance. The approach of increasing morphine respiratory depression and lethality tolerance by increasing PKCα or ε activity at preBotC could be used to make opioids safer for chronic use.