Effects of excitatory amino acid receptor antagonists and agonists on suprachiasmatic nucleus responses to retinohypothalamic tract volleys

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.

Glutamate injection into the suprachiasmatic nucleus stimulates brown fat thermogenesis in the rat

Injection of glutamate (100 mM to 1 M, in 0.25 micrograms saline) into the hypothalamic suprachiasmatic nucleus (SCN) dose-dependently increased interscapular brown adipose tissue (IBAT) and core temperatures in the urethane-anaesthetized rat. This effect was more pronounced in rats tested during the light-off period than in animals tested during the light-on period. Prior injection of the local anaesthetic, procaine (5% in 0.5 microliter saline), into the ipsilateral ventromedial hypothalamic nucleus (VMH) attenuated the increases in IBAT and core temperatures induced by intra-SCN glutamate. The VMH has previously been implicated in the central regulation of BAT thermogenesis; the present results suggest the pathway arising in the SCN exerts an excitatory influence on VMH neurons involved in the control of BAT function.