Alterations in blood glucose levels under hyperinsulinemia affect accumbens dopamine

Abstract

Dopaminergic systems have been implicated in diabetes and obesity. Notwithstanding, the most basic relationship between dopamine and plasma insulin as well as glucose levels yet remains unknown. The present experiments were designed to investigate the effects of acute hyperinsulinemia on basal dopamine levels in the nucleus accumbens of the rat under chloral hydrate anesthesia using acute microdialysis in combination with the hyperinsulinemic-glycemic clamping procedure. In Experiment 1, each rat was infused with one of the three concentrations of insulin (2.4, 4.8, or 9.6 mU/kg per min) while plasma glucose levels were maintained at euglycemia (approximately 5.5 mmol/L). Dopamine, dihydroxyphenylacetic acid and homovanillic acid were not significantly different from baseline during either the clamp or post-clamp periods for all insulin concentrations. In Experiment 2, rats were infused with the highest concentration of insulin (9.6 mU/kg per min) and plasma glucose levels were maintained at either hypoglycemia (approximately 3 mmol/L) or hyperglycemia (approximately 14 mmol/L). Dopamine was elevated at 100 min (+113% above basal levels) and 120 min (+117%) in the hypoglycemic condition and at 120 min (+121%) in the hyperglycemic condition. In the hyperglycemic post-clamp period, homovanillic acid was decreased below basal levels (approximately -32%). These results together suggest that short-term blood glucose deviations coupled with acute hyperinsulinemia affect the mesoaccumbens dopamine system.

Figure 1

Hyperinsulinemia-euglycemic clamp procedure. Plasma insulin (A) and blood glucose (B) during baseline, clamp, and post-clamp periods. Each rat was exposed to one of three insulin concentrations. The continuous intravenous insulin infusion rate for the “low” group was 2.4 mU/kg/ min, “moderate” dose was 4.8 mU/kg/min and “high” dose was 9.6 mU/kg per min. The high insulin concentration significantly elevated plasma insulin levels compared with the other groups (* p < 0.01).

Figure 2

In vivo microdialysis of the nucleus accumbens during hyperinsulinemic-euglycemic clamping. Dopamine (A), DOPAC (B), HVA (C) during the clamp (left) and post clamp (right) periods. Microdialysis sampling began 1 h prior to the intravenous infusions to establish basal levels, was maintained during the 2 h clamp, and the 1 h post clamp periods. The microdialysis sampling period 1 h before the infusions represented the basal levels for DA and the metabolites.

Figure 3

Hyperinsulinemia- glycemic clamp procedure. The graphs show plasma insulin (A) and blood glucose (B) during baseline, clamp, and post-clamp periods. The hyperinsulinemia condition was maintained by a continuous intravenous infusion of 9.6 mU/kg/min. During the clamp period hyperglycemia was maintained at approximately 14 mmol/L and hypoglycemia was maintained at approximately 3 mmol/L.

Figure 4

In vivo microdialysis of the nucleus accumbens during hyperinsulinemic-glycemic clamping. Dopamine (A), DOPAC (B), HVA (C) during the clamp (left) and post clamp (right) periods. Differences in extracellular levels of DA were observed in both group and in HVA in the hyperinsulinemic-hyperglycemic group. Significant changes from basal levels are marked accordingly (* p< 0.05; # p<0.03).

Figure 5

Schematics of coronal sections of the rat brain depicting approximate microdialysis probe sites in the nucleus accumbens. Probe placements for Experiment 1(A) and Experiment 2 (B) is represented by gray bars depicting the extent of the active membrane (0.2 mm X 2 mm). Black areas represent the degree overlap within each experiment. Probes were implanted bilaterally, but data was analyzed from only one probe from each rat. Plates were modified from [24] and approximate anterior position in mm from Bregma are in bold.