Polysynaptic neural pathways between the hypothalamus, including the suprachiasmatic nucleus, and the liver

The suprachiasmatic nucleus of the hypothalamus is responsible for a 24-h r hythm in basal glucose levels in the rat. The neural pathways used by the s uprachiasmatic nucleus to mediate this rhythm in plasma glucose have not ye t been identified. In the present study we examined whether there are any c onnections between hypothalamic centers, including the suprachiasmatic nucl eus, and the liver, which is the main site for glucose production and stora ge. Transneuronal virus tracing from the liver showed that after injection of pseudorabies virus, specific neuronal cell populations in the central ne rvous system were labeled retrogradely, suggesting that specific sites in t he central nervous system may control liver metabolism. First-order neurons belonged to the sympathetic and parasympathetic system, while second-order and third-order neurons were present in both the brainstem and hypothalamu s. The presence of third-order neurons in the suprachiasmatic nucleus sugge sts an involvement of the biological clock in the neural control of the liv er. (C) 2000 Elsevier Science BN. All rights reserved.