Somatosensory contributions to c-fos activation within the caudal periaqueductal gray of lactating rats: effects of perioral, rooting, and suckling stimuli from pups.

In lactating rats, the immediate-early gene c-fos was previously shown to be highly activated in several brain sites by physical interaction with either suckling or nonsuckling pups but not by distal stimuli from pups, a non-pup stimulus, or no stimulation. Further, even greater levels of Fos-immunoreactivity (ir) occurred following suckling versus nonsuckling contact with pups in only 1 of over 25 sites--the caudal periaqueductal gray (cPAG) at an intercollicular level, lesions of which severely reduced the typical suckling-induced kyphotic nursing posture. Herein we further evaluated the effects and site-specificity of various somatosensory cues received from pups during 60 min on Fos-ir in the PAG of day 7 postpartum rats after a 48-h dam-litter separation. Dams interacting with suckling versus nonsuckling pups showed relatively high numbers of Fos-ir cells in the intercollicular cPAG site identified earlier, but not in three other rostrocaudal planes of the PAG. Elimination of rooting on the dam's ventrum by use of fully anesthetized pups did not further diminish Fos-ir in maternally behaving, nonsuckled dams. Perioral anesthesia of dams prior to reunion with the litter prevented retrieval and licking of pups but not pup-initiated nursing behavior, the duration of which was positively correlated with Fos-ir levels within the intercollicular cPAG. Thus, various somatosensory stimuli from pups activate c-fos in a discrete region of the cPAG but only interactions that include suckling and its behavioral consequences elicit maximal expression, consistent with a role for this midbrain site in the sensorimotor control of kyphotic nursing in rats.