Influence of sex and estrous cycle on blood glucose levels, body weight gain, and depressive-like behavior in streptozotocin-induced diabetic rats

Highlights

• Male rats are more susceptible to STZ-treatment than naturally cycling (NC) females.

• Both, male and NC female STZ-treated rats exhibited higher immobility in the forced swim test.

• This depressive-like profile was more pronounced in females in proestrus/estrus than in metestrus/diestrus.

• NC females, regardless of their estrous cycle, were more active than males.

Abstract

Depression is the most common psychiatric disorder in diabetic patients, showing higher rates in women than in men. This comorbidity has been studied in rodents using the streptozotocin (STZ)-induced diabetes (DM) model, consistently reporting a depressive-like profile in males. Few articles have examined these disturbances in females (ovariectomized or combined with male rats) yielding controversial results. This work was aimed to study whether there are sex differences in the depressive-like profile of STZ-treated male and naturally cycling female Wistar rats. We also analyzed the possible influence of the estrous cycle in females. DM was induced by injecting STZ (50 mg/kg, i.p.) in 2 consecutive days. Ten days later, the depressive-like profile was assessed in the Forced-Swim Test (FST). Locomotion and motor coordination were also evaluated. Body weight and blood glucose levels were registered at the beginning and at the end of the experiment; the estrous cycle, food and water intake were daily monitored. All diabetic subjects showed increased blood glucose levels, polyphagia, polydipsia and decreased body weight as compared to controls, but males were more susceptible to STZ-treatment than females pooled in all phases of the estrous cycle. After treatment with STZ, males and females in proestrus/estrus (P/E) exhibited a depressive-like profile in the FST (increased immobility and reduced swimming); females in metestrus/diestrus were unaffected. The only sex difference observed was a more pronounced reduced swimming in STZ-treated P/E females compared with hyperglycemic males. No changes in locomotion or motor coordination were found. This work emphasizes estrous cycle differences in STZ-treated rats, and in the resultant depressive-like profile. It also supports clinical evidences made in women with DM and stresses the importance of studying STZ-treated naturally cycling females and their estrous cycle phases.

Introduction

It is well known that Diabetes Mellitus (DM) provokes a large number of clinical alterations. The consequences of DM in the Central Nervous System were ignored until 1950, when it was recognized that this system is also affected by hyperglycemia, causing cognitive dysfunction and behavioral disturbances. These effects have been defined as “diabetic encephalopathy or central neuropathy” [14, 43, 60, 64, 76, 82].

There is strong evidence supporting that DM increases the risk for affective disorders. Depression is considered the most common psychiatric disorder in diabetic patients. Its prevalence has been estimated to be twice than that in the general population [4, 38, 57]. It has also been demonstrated that the course of depression is more severe in diabetic patients [65], and that this mood disorder is related with a poor glycemic control and inadequate treatment adherence [57]. Moreover, depressed diabetic patients show less antidepressant effects, increased clinical morbidity and mortality and development of DM-related complications [2, 20, 28, 57, 65]. All these factors cause a reduced life quality and enhanced costs of health care [20, 57].

Interestingly, as seen in the general population, there is a higher rate of depression in diabetic women than in men [4, 65]. The mechanisms underlying the relationship between DM and depression are not well known. In the last decades, ample evidence has been published relating the depressive-like profile of experimental DM in rodents using both, animal models of DM type 1 (most of them using streptozotocin (STZ), a cytotoxic compound that destroys pancreatic β cells) and type 2 (ob/ob and db/db mice [74, 85], fa/fa rats [55], high-fat/high-carbohydrate diets [1, 75], among others). These studies have mainly utilized two different paradigms: the forced swim (FST) and the tail suspension tests. Regarding the studies using the STZ-induced DM model, they have consistently reported a depressive-like profile in male rodents when compared to their respective controls [18, 24, 29, 30, 41, 44, 62, 82]. However, only few articles have examined these disturbances in STZ-induced diabetic female rats: Khanam and Pillai [49] and Aswar and colleagues [8], used a mixed group including males and naturally cycling females, while others used independent groups of ovariectomized (OVX) female rats [9, 35]. The results of these studies have yielded controversial results in the FST: thus, unaltered profiles in depressive-like behaviors where found when comparing hyperglycemic OVX females with non-hyperglycemic OVX rats [35], or STZ-treated animals (males + females) with a group of rats administered with vehicle [49]. In contrast, an enhanced depressive-like profile was reported by Bansal and Chopra comparing OVX STZ-treated females with a non-hyperglycemic sham-operated group [9], as well as in the work of Aswar using STZ-treated animals regardless of their sex with a group of vehicle-treated rats [8].

The FST is the most popular behavioral paradigm to study antidepressant-like effects of drugs and non-pharmacological treatments for depression [17, 56, 67]. Using this model, others and we have found a sex differential behavioral profile. For example, higher immobility scores (considered as a depressive-like index) have been observed in male than in female rats [3, 21, 37, 42]. Moreover, the FST is sensitive to the female's endocrine variations during the estrous cycle [34], showing lower immobility scores when the levels of estrogens and progesterone are high [6, 10]. Notably, the studies that considered sex differences and estrous cycle effects in the FST, have reported conflicting results that have been carefully reviewed by the group of Christina Dalla [50] with heterogenic profiles: enhanced, reduced or equal immobility scores have been found in males when compared with females. These heterogenic profiles within females were also found when the estrous cycle phase was considered.

As mentioned earlier, available data concerning the depressive-like profile of naturally cycling diabetic female rats -which model women in the reproductive age- is controversial; in addition, the effect of the estrous cycle on this behavioral profile in these animals has never been analyzed. Therefore, the main goal of the present work was to analyze the behavior of male and naturally cycling female rats in the FST in different endocrine conditions (proestrus/estrus (P/E) vs. metestrus/diestrus (M/D)), using the STZ-induced DM model.