Metabolic and orexigenic effects of intracerebroventricular neuropeptide Y are attenuated by food deprivation.

Administration of neuropeptide Y (NPY) into the hypothalamus or cerebral ventricles has been shown to increase food intake, the secretion of hormones such as insulin, glucagon and corticosterone and to alter the metabolism of carbohydrate and lipids. It has been suggested that metabolic effects of hypothalamic NPY may contribute to fat accretion in some types of obesity and to the metabolic and behavioural adaptation to food deprivation. However, it is currently unknown if different nutritional states alter the responses to hypothalamic NPY. Consequently, we have compared the effects of NPY injected into the third ventricle (ICV) in the fed and overnight-fasted state on ingestive behaviour, on insulin, glucagon and corticosterone secretion before, and following, an IV glucose bolus (IVGTT) and on blood glucose following an intra-arterial insulin bolus (ITT). Studies were performed on conscious, unrestrained adult female rats. In the fed state, 2 and 6 micrograms ICV NPY produced a potent orexigenic and dypsogenic effect. In the fasted state, the 2 micrograms dose had a dypsogenic effect, while only the 6 micrograms dose had a significant orexigenic effect. In the fed but not fasted state, 3 micrograms ICV NPY increased plasma glucagon and corticosterone levels and attenuated the decline in blood glucose during the ITT. By contrast, in both fed and fasted groups, 3 micrograms ICV NPY potentiated the insulin secretory responses during the IVGTT. We conclude that, apart from stimulating insulin secretion, the acute metabolic and orexigenic responses to ICV NPY in this study were substantially reduced or abolished by overnight fasting. Therefore, behavioural and metabolic responses to endogenous hypothalamic NPY may also be more significant in the fed than the fasted state.