PKMζ alters oxycodone-taking in a dose- and sex-dependent manner

Abstract

Opioid use disorder involves disruptions to glutamate homeostasis and dendritic spine density in the reward system. PKMζ is an atypical isoform of protein kinase C that is expressed exclusively in neurons and plays a role in postsynaptic glutamate signaling and dendritic spine maturation. As opioid use leads to alterations in glutamate transmission and dendritic spine density, we hypothesized that PKMζ deletion would alter opioid-taking behaviors. The current study examined two doses of oxycodone self-administration in male and female mice with constitutive deletion of PKMζ compared to wildtype controls. At a dose of 0.25 mg/kg/infusion, PKMζ deletion significantly potentiated oxycodone self-administration in both male and female mice. However, increases in motivation for oxycodone, as indicated by increased breakpoint on a progressive ratio schedule, were only seen in male PKMζ knockout mice and not females. When we examined a lower dose of oxycodone, 0.125 mg/kg/infusion, PKMζ knockout led to increases in oxycodone self-administration only in female mice. Additionally, female PKMζ knockout mice exhibited higher breakpoints on a progressive ratio schedule at this dose compared to all other groups. In addition to the self-administration studies, we also examined locomotor sensitization in response to experimenter administered oxycodone. PKMζ KO decreased oxycodone induced locomotion in males and potentiated oxycodone sensitization in females. Together, these results suggest that PKMζ acts to dampen oxycodone taking in both sexes, but females may be more sensitive to its effects.

Keywords: Addiction; Motivation; Oxycodone; PKMζ; Self-administration; Sex differences.

Fig. 1.. PKMζ knockout potentiates 0.25 mg/kg/inf oxycodone self-administration in both sexes.

Constitutive PKMζ deletion leads to an increase in the number of oxycodone infusions earned in both females (n = 20) (A) and males (n = 20) (B) in comparison to female (n = 23) and male wildtype (n = 24) counterparts. Male PKMζ knockout mice also exhibit an increase in the number of active responses across 12-days of oxycodone self-administration (D), effect that is not seen in female PKMζ knockout mice (C). While there were no significant effects of PKMζ knockout on inactive responding in females (E), there was a decrease in inactive responding in male knockout mice (F). *p<.05, ***p<.001 main effect of genotype.

Fig. 2.. PKMζ knockout potentiates the final breakpoint in a progressive ratio paradigm at 0.25 mg/kg/inf oxycodone in males and females.

Constitutive PKMζ deletion leads to an increase in breakpoint for oxycodone at the 0.25 mg/kg/inf dose in males (n = 10) and females (n = 8) in comparison to male (n = 8) and female (n = 8) wildtype counterparts **p<.01 main effect of genotype.

Fig. 3.. PKMζ knockout potentiates 0.125 mg/kg/inf oxycodone self-administration exclusively in female animals.

Constitutive PKMζ deletion leads to an increase in the number of oxycodone infusions earned in females (n = 9) (A) but not males (n = 6) (B) in comparison to female (n = 9) and male (n = 11) wildtype counterparts. Female PKMζ knockout mice also exhibit an increase in the number of active responses across 12-days of oxycodone self-administration (C), effect that is not seen in male PKMζ knockout mice (D). There were no significant effects of PKMζ knockout on inactive responding in either females (E) or males (F). *p<.05, **p<.01 main effect of genotype.

Fig. 4.. PKMζ knockout potentiates the final breakpoint in a progressive ratio at 0.125 mg/kg/inf paradigm exclusively in female animals.

Constitutive PKMζ deletion leads to an increase in the breakpoint for oxycodone at the 0.125 mg/kg/inf dose in female, but not male, mice. **p<.01, pairwise comparison wildtype females vs. PKMζ knockout females due to significant interaction effect.

Fig. 5.. PKMζ knockout alters oxycodone-induced locomotion differentially in male and female mice.

Constitutive PKMζ deletion does not alter the locomotor response to the testing chamber during the habituation phase prior to oxycodone injection in either male (A) or female (B) mice (n = 6–8/group). For 60 min following each daily oxycodone injections (5 mg/kg, i.p.) we measured distance travelled in meters (m) and binned the data in 5-minute intervals (C-G). The influence of genotype on oxycodone-induced locomotor behavior varied by sex, an effect that is more clearly illustrated when we examined area under the curve across the five days. Male PKMζ knockout mice exhibit a decrease in oxycodone-induced locomotion across all five days (H, ***p<.001, effect of genotype). In contrast, female PKMζ knockout mice exhibit greater oxycodone-induced locomotion, particularly during the final two days of injections, where the wildtype mice appear to exhibit habituation (I, **p<.01, effect of genotype).