Localization of Insulin and Type 1 IGF Receptors in Rat Brain by in Vitro Autoradiography and in Situ Hybridization
Insulin and the insulin-like growth factors (IGFs) are structurally related, circulating hormones that regulate growth and intermediary metabolism in virtually all tissues. In many organs their function has been well defined but in the adult CNS no general understanding of their action and importance is available. For many years it has been known that circulating insulin can affect the CNS directly to regulate peripheral metabolism (1). The hypothalamus is considered the most likely site of these actions (2). Our laboratory has been particularly interested in the ability of insulin to reduce food intake and act as a regulator of body weight. This has been demonstrated by peripheral infusions of insulin (3) or by injecting insulin into the third ventricle (4) or into the hypothalamus (5). Electrophysiological studies in hypothalamus (6) and hippocampus (7) support this hypothesis by demonstrating that insulin can affect the firing rate of neurons and therefore can act as a neuromodulator or possibly neurotransmitter. Insulin’s inhibition of norepinephrine uptake in cultured neonatal neurones supports a neuromodulatory role for insulin (8). Recently, Figlewicz has confirmed this finding in hippocampal slices from adult rats (9). A role for IGFs as neuromodulators in the adult brain has also been suggested. IGF-1 can affect the release of acetylcholine from hippocampal slices (10) while IGF-2 but not IGF-1 reduces food intake when injected into the third ventricle and may be a physiological satiety factor (11). In the hypothalamus, IGF-1 may modulate release of somatostatin and thereby regulate growth hormone release from the pituitary (12).
KeywordsOlfactory Bulb Receptor mRNA Arcuate Nucleus Granule Cell Layer Insulin Binding
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