Disruption of maternal behaviour by acute conspecific interaction induces selective activation of the lateral periaqueductal grey.

Maternal behaviour is sensitive to stress and opioidergic activation. The periaqueductal grey (PAG) is involved in coping strategies to stress, whereas morphine inhibition of maternal behaviour depends on the activation of the PAG. The aim of this study was to investigate whether the PAG is activated by disrupting maternal behaviour. Lactating Wistar rats were assigned to four groups: C (control); E1 (acute exposure to a male rat); E2 (daily 2-h exposure to another lactating female and a male rat from Day 3 to 6 of lactation); and E1 + 2 (treated first as E2 and, on Day 9, as E1). Maternal behaviour was recorded on Day 9 of lactation and analysed for 1 h. The E1 group spent more time retrieving their pups, took longer to initiate nursing, had shorter nursing bouts and spent more time in non-maternal activities compared with control. Rats submitted to E2 or E1 + 2 did not differ from the control. In another experiment, lactating rats were treated as above, except that 90 min after the end of the observation period the rats were killed and their brains were processed for immunohistochemical detection of Fos protein in the PAG. Fos increased in the lateral PAG only in the E1 group. We also observed that neurons activated by acute conspecific interaction in the PAG could be responsible for an opioid-dependent decrease in maternal behaviour as this effect was reversed by a microinjection of naltrexone, nor-binaltorphimine or naloxonazine into the lateral PAG. Chronic conspecific interaction alters the way this circuitry responds to acute conspecific interaction.