A role for the periaqueductal grey in opioidergic inhibition of maternal behaviour

Opiates are known to be involved in the regulation of various events surrounding parturition and lactation, such as maternal behaviour in rats. The onset of this behaviour has been closely linked to opiate action in the medial pre-optic area, where administration of morphine disrupts maternal behaviour during lactation. By combining the use of Fos protein immunohistochemical detection and pharmacological manipulations, in the present paper we show that the periaqueductal grey (PAG) is another region critically involved in the opioidergic blockade of maternal behaviour. According to our observations, a critical level of morphine-induced activation of the rostral lateral PAG appears to be required to inhibit maternal behaviour in lactating rats. This hypothesis was further confirmed in experiments showing that morphine's inhibitory effect on maternal responsiveness was blocked by unilateral naloxone injection into the rostral PAG, but not into nearby regions of the mesencephalic reticular nucleus. Therefore, only a partial inhibition of the opiate's effect on the rostral PAG was needed to block the inhibitory effect of morphine on maternal behaviour. Further studies are needed to ascertain whether the rostral lateral PAG plays a role in the natural onset of maternal behaviour, playing a complementary role to the medial pre-optic area, or merely inhibits maternal behaviour in response to this specific pharmacological challenge. Conversely, the present findings may well reflect a more general role of the PAG, seemingly providing an important piece of information for proposing a hitherto unexplored concept of the PAG as an important centre for the selection of adaptive behavioural responses.