Retinohypothalamic tract stimulation activates thermogenesis in brown adipose tissue in the rat

We have shown that injecting glutamate into the hypothalamic suprachiasmatic nucleus (SCN) stimulates interscapular brown adipose tissue (IBAT) thermogenesis in rats. In the present study, electrical stimulation of the retinohypothalamic tract (RHT), a projection of retinal ganglion cell axons that course via the optic tract and terminate in the SCN, mimicked the effect of intra-SCN glutamate injection on IBAT thermogenesis. This effect could be blocked by prior injection of the local anesthetic procaine or the excitatory amino acid blockers kynurenic acid and delta-D-glutamylaminomethyl-phosphonic acid into the contralateral SCN, implicating the involvement of SCN glutamate receptors. The primary function of the RHT is to communicate photic information to the SCN, and it has been shown that glutamate might be involved in mediating this effect. On the basis of these observations and our finding of thermogenic responses to RHT stimulation or intra-SCN glutamate, we conclude that photic signals transmitted to the SCN via release of glutamate from the RHT might be capable of acutely modulating thermogenesis in IBAT.