Thermal, But Not Mechanical, Nociceptive Behavior is Altered in the Zucker Diabetic Fatty Rat and Is Independent of Glycemic Status

Abstract

This study investigated the possible link between developing hyperglycemia and mechanical and/or thermal hyperalgesia in the Zucker Diabetic Fatty (ZDF) rat. When normoglycemic (nonfasting blood glucose levels of 6 mM), 6-week-old ZDF rats were glucose intolerant compared to the nondiabetic Zucker lean control (ZL) rats, but there was no difference in their response to a noxious mechanical (paw pressure test) or thermal (hot plate) stimulus (mechanical nociceptive thresholds: ZDF 176.7 ± 14.4 g, ZL 161.7 ± 13.3 g; latencies to response to the thermal stimulus: ZDF 13.1 ± 1.6 sec, ZL 16.7 ± 1.5 sec). Blood glucose levels in untreated ZDF rats increased to 28.4 ± 2.9 mM by 20 weeks of age, while ZDF rats treated with the insulin sensitizer, rosiglitazone, and ZL rats remained normoglycemic (⩽ 8 mM) throughout the study. Hyperglycaemia in ZDF rats was not associated with mechanical hyperalgesia, as the nociceptive threshold remained constant in both the rosiglitazone-treated and untreated ZDF rats and in the ZL rats throughout the study. In contrast, the latency to response to the thermal stimulus increased with time in ZL rats, but remained constant in hyperglycaemic ZDF rats such that the difference reached significance by 9 weeks of age (ZDF 11.6 ± 1.7 sec, ZL 21.8 ± 2.7 sec, p < 0.01) and is consistent with hyperalgesia in the ZDF phenotype. However, this difference was not moderated by maintaining normoglycaemia in rosiglitazone-treated ZDF rats (12.8 ± 1.3 sec). Together, the data suggest that hyperglycemia does not play a central role in the development of hyperalgesia in the ZDF rat.