326 Brain Research, 527 (1990) 326-329 Elsevier BRES 24275 Short Communications Insulin co-inject n suppmuel the ther ic to pJtmlmte microinjecUon into the in rats Shimon Amir, Alessandra Schiavetto and Robin Pollock Centerfor Studies in Behavioral Neurobiology, Departmentof Psychology, Concordia University, Montreal, Que. (Canada) (Accepted 29 May 1990) Key words: Insulin; Glutamate; Ventromediai hypothalamic nucleus; Brown adipose tissue; Thermogenesis; Rat Selective stimulation of ventromedial hypothalamic nucleus (VMH) neurons by microinjection of the excitatory amino acid glutamate sharply increased interscapular brown adipose tissue (IBAT) and core temperatures in urethane-anaesthetized rats. This effect was blocked by co-injection of insulin (0.1-1/~g) though not an inactive insulin analog, TNB 3 insulin. Injection of insulin (1/~g) into the contralateral VMH or systemic administration of insulin (1/~g) had no effect on the thermogenic response to intra-VMH glutamate. These results complement those showing that intra-VMH insulin suppresses the basal firing rate of sympathetic nerves to IBAT and diminishes cold-induced nonshivering thermogenesis in BAT and add support to the view that insulin functions as an inhibitory signal on VMH neurons controlling thermogenesis in BAT. Heat generation in brown adipose tissue (BAT) is influenced by insulin, but the mechanisms mediating the effect of insulin on BAT are incompletely understood 27. Insulin can influence heat production directly by influ- encing BAT metabolism 34, as well as indirectly, via effects in the central nervous system. An important central nervous system region concerned with neural control of BAT thermogenesis is the ventromedial hypo- thalamic nucleus (VMH) 25. In previous experiments, injection of insulin into the VMH in rats was found to suppress the basal firing rate of sympathetic nerves supplying BAT, suggesting that insulin has a direct inhibitory action on the neural outflow from the VMH to BAT 3°,32. However, the effect of intra-VMH insulin on heat production in BAT induced either by physiological or neural stimuli was not reported. Recently, we have shown that injection of insulin into the VMH suppresses cold-induced stimulation of BAT thermogenesis in rats 2. In the present study we examined the effect of intra- VMH insulin on the thermogenic response resulting from direct stimulation of VMH neurons with the excitatory amino acid glutamate. In a previous study, we demon- strated that injection of glutamate into the VMH acti- vates BAT thermogenesis via the sympathetic outflow, leading to a sharp increase in interscapular BAT (IBAT) and core temperatures 4. We report here that insulin co-injection blocks the thermogenic effect of intra-VMH injection of glutamate in a direct and specific manner. Normally fed, male Wistar rats (275-325 g) were anaesthetized intraperitoneally with urethane (1.4 g/kg) and mounted in a stereotaxic instrument equipped with a thermostatically-controlled heating blanket calibrated to keep body temperature above 36 °C. A small incision was made 5 cm posterior to the scapulae and a thermistor probe (YSI 409B) was inserted and placed under the right lobe of IBAT. A second thermistor probe (YSI 401) was inserted 5 cm into the rectum. A 28 gauge needle connected to a Hamilton microsyringe containing gluta- mate (1 M), insulin (bovine insulin; Sigma) or glutamate plus insulin was then introduced into the right VMH using the following coordinates from the Paxinos and Watson 22 atlas: AP -2.5, L 0.6, V 9'.5. The position of the needles was later verified histologically. Interscapular BAT and core temperatures were monitored continu- ously using YSI telethermometers (model 41) connected to a flat bed recorder. Intra-VMH injection of saline (0.25 /~1, n = 4) or insulin (0.1, 0.5 or 1/~g, in 0.25/~1 saline; n = 4 for each dose of insulin) had no effect on basal IBAT or core temperatures of urethane-anaesthetized rats (data not shown). In contrast, intra-VMH injection of insulin suppressed the thermogenic response to intra-VMH co-injection of glutamate. Fig. 1 shows examples of the effect of injecting glutamate alone (0.25 ~tl of 1 M Correspondence: S. Amir, Center for Studies in Behavioral Neurobiology, Department of Psychology, Concordia University, 1455 de Maisonneuve Blvd. West, Montreal, Que., Canada H3G IMS. 0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)