The current study provides supporting evidence in an animal model that experimental type 1-like diabetes produces a number of depressive phenotypes such as increased immobility in the TST, decreased hedonic state as measured by ICSS, and reduced hippocampal cell proliferation. Furthermore, these phenotypes were normalized by treatment with insulin. Reductions of reward-related behaviors and hippocampal cell proliferation may be markers for identifying pathophysiological mechanisms of depression associated with diabetes and markers for successful intervention. Since STZ-induced diabetes models type 1 diabetes, additional experiments are necessary to determine whether the results from this study might generalize to animal models for other types of diabetes.
The TST has mostly been used to measure the acute effects of antidepressant drugs [23
], as shown previously under these testing procedures. However, increased baseline TST immobility after exposure to infection, hormones, or stress may reflect induction of depressive-like behavior because these conditions also cause depression in humans [7
]. Similar to previous studies [7
], mice in the current study developed increased TST immobility after the onset of diabetes suggesting that diabetes increased depressive-like behavior. The present study further showed that insulin treatment reversed the augmentation of TST immobility. Diabetes also decreased locomotor activity, which could indicate that increased TST immobility resulted from nonspecific effects of diabetes on motor behavior. However, insulin treatment also decreased locomotor activity yet still restored TST behavior indicating that the changes in these measures produced by diabetes were independent. The general literature also shows that the effects of antidepressant drug testing on TST performance is independent of locomotor activity [23
The ICSS testing paradigm measures affective behavior related to the motivation for obtaining reward. ICSS can be used as a measure of anhedonia, or diminished capacity to experience pleasure or reward, which is a cardinal symptom of major depressive disorder [26
]. This is the first study to examine the effects of diabetes on ICSS performance in mice. By 3 weeks post STZ injection, diabetes resulted in shifting the ICSS curve to the right. This indicated the development of a decreased hedonic state, as the mouse no longer responded to stimulation frequencies that were previously reinforcing. One week of insulin treatment reduced hyperglycemia and shifted the curve back to baseline ICSS thresholds. The changes in reward sensitivity (i.e. during Week 3 of untreated diabetes and back to baseline during Week 4 with insulin treatment) without altering mean maximal response rate values suggest that rightward shifts in ICSS threshold are likely due to the effects of diabetes on hedonic state rather than on task performance.
The effects of diabetes were also measured on hippocampal cell proliferation and BDNF levels, two measures of hippocampal cellular plasticity. A number of rodent models have shown that experimental diabetes leads to decreased hippocampal cell proliferation and neurogenesis [13
]. Hippocampal neurogenesis has been associated with the anhedonic effects of stress in recent studies [27
]. Because hippocampal neurogenesis is also reduced in rodents by exposure to stress and models of depression and is reversed by antidepressant treatments [28
], this process and related deficits of neuroplasticity could mediate the interface between diabetes and affective behavior. In the present study, exposure to insulin reduced hyperglycemia and increased hippocampal cell proliferation back to control values in STZ-treated mice. This important finding suggests that reversing the deficits in hippocampal cell proliferation could prevent some of the progressive complications in diabetes. However, additional studies showing that increases in cell proliferation/neurogenesis can restore ICSS performance or another behavioral measure of depression independent of restoring plasma glucose and corticosterone levels are needed to test this hypothesis.
Changes in hippocampal cell proliferation in diabetes, and its reversal by insulin, could have involved changes in BDNF levels because BDNF levels have been associated with diabetes, insulin and stress. Previous studies in rats have reported that STZ lowers BDNF levels [29
] and BDNF gene expression was decreased in the hippocampus of the nonobese diabetic mouse [32
]. Exogenous BDNF augmented the hypoglycemic effects of insulin in STZ-treated mice, although the treatment was ineffective when given alone [33
]. Stress decreases hippocampal and cortical BDNF levels and gene expression and lowers neurogenesis, while antidepressant medications appear to increase neurogenesis by increasing BDNF levels [28
]. In the present study, BDNF levels were reduced by STZ-induced diabetes in the frontal cortex, but not the hippocampus. Although insulin treatment restored hippocampal cell proliferation, this effect was not due to increases in BDNF levels in the cortex and hippocampus. Thus, endogenous BDNF may contribute to the pathology of diabetes, based on the correlation between diabetes and lower BDNF levels, but the restoration of hippocampal cell proliferation in diabetes by insulin treatment may involve other mechanisms.
Plasma CORT levels, on the other hand, increase with diabetes and likely contribute to the diabetes-associated decrease in hippocampal cell proliferation and neurogenesis [16
]. Results from the current study provide corroborating evidence for the involvement of CORT in regulating hippocampal cellular plasticity in diabetes. Treatment of diabetic mice with insulin reduced plasma CORT levels and produced a corresponding restoration of hippocampal cell proliferation. The involvement of CORT acting directly at glucocorticoid receptors provides the best explanation for how insulin restored hippocampal cell proliferation in diabetes [16
], although other mechanisms associated with stress and may yet be shown to be involved in regulating hippocampal neurogenesis in diabetes.
Although anxiety is a common problem in depression, STZ-induced diabetes evoked depressive more than anxious phenotypes in C57BL/6 mice. Anxiety tests that rely on hunger and satiety signals, like the novelty induced hypophagia test or punished operant responding, would not be suitable for measuring anxiety in diabetic animals since diabetes increases appetite. Hyperphagia in diabetes can make it difficult to interpret the data from these tests as representing changes only in emotional behavior (data not shown). Thus, we used the EZM, which does not depend on hunger and satiety signals, to examine the effects of diabetes on anxiety. Results from the EZM indicated that experimental diabetes in C57BL/6 mice did not affect anxiety. These results do not corroborate with results from previous studies where STZ-induced diabetes led to anxiety-like behavior [34
]. Differences in results may be due to differences in species used, mouse versus rat, and choice of test used, EZM versus elevated plus maze or open field test, to measure anxiety. Future studies need to determine if certain tests of anxiety are more sensitive or appropriate for measuring anxiety in diabetic mice.