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This information converges in the nucleus tractus solitarius (NTS), an area of the caudal brainstem that integrates sensory information from the gastrointestinal tract and abdominal viscera, as well as taste information from the oral cavity92. Satiety-inducing signals that reach the NTS are initiated by mechanical or chemical stimulation of the stomach and small intestine during food ingestion, neural input related to energy metabolism in the liver93 and humoral signals such as cholecystokinin (CCK) that are released upon nutrient stimulation of neuroendocrine secretory cells lining the intestinal lumen94. Meal termination induced by such satiety signals can be demonstrated even when all neuronal connections between forebrain and hindbrain are severed95. The basic process of terminating a meal, therefore, involves brain areas that can function in the absence of hypothalamic influences. How then is the forebrain response to adiposity signals coupled to changes in the size of single meals? The hypothesis that such responses ultimately involve an interaction with hindbrain areas that control satiety is supported by the ability of both leptin96 and insulin87 to enhance the satiating effect of CCK. This interaction may be explained by the ability of central effector pathways to influence the response of NTS neurons to input from vagal afferents that convey satiety-related stimuli (Fig. 6). Recent evidence that leptin potentiates the effect of CCK to activate NTS 668

neurons demonstrates clearly that signals involved in energy homeostasis modulate the response of NTS neurons to input related to satiety97. Several caveats follow as a result from the hypothesis that NTS neurons are themselves responsible for integrating afferent information related to satiety with descending inputs from forebrain neurons involved in energy homeostasis. First, NTS neurons have reciprocal interconnections with forebrain areas such as the PVN98, so the integration of satiety and energy homeostasis information probably involves multiple brain areas. In addition, the neuronal substrates for responding to central effector peptides involved in energy homeostasis are present locally within the NTS, as well as in the hypothalamus. For example, MC4 receptors are expressed in the NTS54, and local administration of MC4-receptor agonists or antagonists into the fourth ventricle (which is adjacent to the NTS) elicits feeding responses that are indistinguishable from those induced by injecting these compounds into the more rostral lateral ventricle99.