Relationships between hypothalamic neuropeptide Y and food intake in the lactating rat.

Abstract

hypoinsulinaemic [2]. Certain NPYergic neurones in the hypothalamic ARC that prolect to release NPY in the PVN are postulated to regulate food intake and energy balance [3-71. The ARC-PVN NPYergic projection is overactive in lactation, as shown recently by increased NPY concentrations in the ARC and PVN [8]and by increased NPY gene expression in the ARC [9,lO]. Increased NPY 1-elease in this projection is thought to drive hyperphagic? ~n lactation. The objectives of our studies were to determine if the increased hypothalamic NPY levels found during lactation are the cause or the effect of the hyperphagia, and to assess further the possible role of insulin in mediating these adaptations. Lactating rats were either freely-fed or restricted to control levels of food intake for 3 days during peak hyperphagia (13-17 days postpaltum) . To measure regional hypothalamic NPY concentrations, individual nuclei were punched out of hypothalamic sections cut on cl vibratome and assayed for NPY by RIA [Ill. Total RNA WdS extracted from whole hypothalamic blocks using the guanidium t h i o c y a n a t e p h e n o l c h l o r o f o r m method [121. RNAs were separated by gel electrophoresis and hybridized with a 'ZP-labelled rat cDNA probe. The signal intensities for NPY mRNA were normalized using the signal similarly obtained for tubulin mRNA and quantified using scanning densitometry. Plasma insulin concentrations were also determined by RIA from blood obtained at sacrifice by cardiac puncture. Compared with non-lactating controls (n=lO), freely-fed lactating rats (n=9) showed significantly increased ( p 0 . 0 5 ) NPY levels ~n the ARC and MPO (Fig. l ) , but there was no significant increase in whole hypothalamic NPY mRNA levels. Food-restricted lactating rats (n=8) showed generally higher hypothalamic NPY levels than both other groups, with significantly higher concentrations than controls (p<0.051 in the ARC, MPO, PVN and LHA (Fig. I); this group also showed NPY mRNA levels that were significantly increased (p<O. 05) beyond those of cont 1-01s. Across all three experimental groups, there was a significant inverse correlation between serum insulin ConcentidtioIl and hypochaldmic NP'Y mRNA levels (r=-0.39, p < 0 . 0 1 ) . These findings exclude the possibility that hyperphagia is the cause of the rise in hypothalamic NPY expression and argue that insulin may be an important inhibitory regulator of NPY expression in the ARC in lactation. this, lactating animals ale usually