Maternal absence of the parathyroid hormone 2 receptor affects postnatal pup development

Abstract

During the lactation period, mothers have a variety of adaptive changes in brain physiology and behaviour that allow them to properly raise their pups. The exact circuitries and mechanisms responsible for these changes are not fully understood. Recent evidence suggests that the neuropeptide tuberoinfundibular peptide of 39 residues (TIP39) and its receptor, the parathyroid hormone 2 receptor (PTH2-R), contribute to these mechanisms. To further investigate this idea, we evaluated the growth rate of pups from dams with a genetically inactivated PTH2-R (PTH2-R-KO), as well as maternal behavioural and neuroendocrine parameters. We observed that PTH2-R-KO-reared pups had a slowed growth rate. This was associated with a reduced volume of milk yielded by PTH2-R-KO dams after 30-min suckling compared to wild-type (WT) dams when pups were returned after 5 h of separation. Our data suggest a reduced sensitivity of PTH2-R-KO dams to pup stimulation. We also observed a significant reduction in suckling-induced c-Fos expression in the paraventricular hypothalamic nucleus and signs of lower prolactin levels in the PTH2-R-KO dams. Our data suggest that the reduced growth rate of PTH2-R-KO-reared pups was likely the result of alterations in the milk-production pathway rather than modifications in behaviour. Although PTH2-R-KO dams showed increased anxiety in the elevated zero-maze test, no differences from WT dams in maternal behaviour were observed. Taken together, our findings suggest the involvement of the TIP39/PTH2-R system in the pathways involved in the successful development of the pups.

Figure 1

(a) Average pup body weight during the lactation period in litters reared by WT and PTH2-R-KO dams. * main genotype effect; p= 0.043. (b) Average body weight of WT and PTH2-R-KO dams before pregnancy and throughout the lactation period. PND: postnatal day.

Figure 2

Representation of the maternal behavior observed in WT and PTH2-R-KO dams as a percentage of scans spent in active nursing (a and b) or spent in non-nursing behavior (c and d). (a) and (c) the percentage of scans spent in each behavior over the 6 time-windows was averaged to obtain one value per PND (post-natal day); (b) and (d) the percentage of scans spent in each behavior over the 3 PND was averaged to obtain one value per hour. The black bar represents the dark phase of the light/dark cycle.

Figure 3

Milk yield (in grams) by WT and PTH2-R-KO dams after a 30-min or a 90-min suckling period (a) and basal serum prolactin level (ng/ml) of WT and PTH2-R-KO dams at postnatal day (PND) 5, 10 and 19 (b). * p=0.031.

Figure 4

Number of c-Fos positive cells counted in the paraventricular nucleus of the supraoptic nucleus (SON), the hypothalamus (PVN), the ventral bed nucleus of the stria terminalis (vBNST), the arcuate nucleus (ArcN), and the medial pre-optic area (MPA) in WT and PTH2-R-KO dams after a 90-min suckling stimulus. * p<0.05.

Figure 5

Representative micrographs showing the expression of c-Fos protein within the paraventricular nucleus of the hypothalamus (PVN) of WT and PTH2-R-KO dams (a and b, respectively) after a 90-min suckling stimulus. 3V: 3^rd ventricule. The bar scale represents 100μm.