Aim: studies have demonstrated that pregnant and lactating rats showed alterations in pain threshold that may be opioid-dependent. The periaqueductal gray (PAG) has been reported as a potential site for opioid regulation of behavioral selection and nociceptive site. In this study the objective was to evaluate opioid receptor Oprm1, Oprk1 and Oprd1 gene activity (for mu, kappa and delta opioid receptor respectively), and MOR, KOR and DOR protein expression in the PAG of female rats in different lactation stages.

Methods and Results: Female Wistar rats weighing 180-200g at the beginning of the study were used divided into five groups. For mating, two female rats were placed overnight with one sexually experienced male rat. The day on which sperm was observed in the vaginal smear, was designated day 1 of pregnancy. In the 2, 7, 14 or 21 lactation day the rats were sacrificed and the brains were collected. Each group of animals had seven subjects for PCR and six individuals for Western Blot analysis. The RT-PCR results show that there was an increase in Oprk1 gene expression and the KOR expression only on the 21 lactation day. The animals used in this study were maintained in accordance with the guidelines from the COBEA and CEEA from Universidade de São Paulo, registration number 112, page 2, vol. 2.

Conclusion: These data suggest that the final of lactation may influence gene activity of opioid receptor in the PAG. Thus, opioid transmission in the PAG might be modulated by different reproductive states plasticity according to the opioid receptor type and that may influence the nociception system.