Altered neuronal responses to feeding-relevant peptides as sign of developmental plasticity in the hypothalamic regulatory system of body weight.

Neuronal plasticity during the critical postnatal period of development seems to promote a change in the function of the hypothalamic regulatory system of body weight. Rats raised in small litters (SL) of only three pups per mother compared to ten or twelve in control litters (CL) gain significantly more weight than normal rats till weaning and are overweight also in later life. These rats are known to express hyperleptinemia, hyperglycemia and hyperinsulinemia. The review summarizes the results of action of leptin and insulin as well as of several feeding-relevant neuropeptides on neuronal activity of hypothalamic regulatory centres in overweight SL rats compared to controls. The study was performed on brain slices perfused with solution containing 10 mM glucose. Whereas a normally inhibitory action of leptin and insulin on medial arcuate neurons (ArcM) is reduced in SL rats and partly replaced by activation, the normally activating effect of these hormones on ventromedial (VMH) neurons is altered to predominant inhibition. Inhibition of ArcM neurons may decrease the release of the orexigenic neuropeptide Y (NPY) and agouti gene-related protein (AGRP). Thus, the negative feedback by leptin and insulin on food intake is replaced by diminished response and partly positive feedback processes in SL rats. The action of NPY and AGRP as well as of the orexigenic melanin-concentrating hormone on paraventricular (PVH) and VMH neurons is also shaped from activation or bimodal effects to predominant inhibition. Such inhibition of PVH and VMH might lead to reduced energy expenditure in small litter rats. Also the anorexigenic melanocortin alpha-MSH seems to contribute into increased energy storage. These altered responses of hypothalamic neurons in overweight small litter rats might reflect a general mechanism of neurochemical plasticity and "malprogramming" of hypothalamic neuropeptidergic systems leading to a permanently altered regulatory function.