Effect of gastric inflammation on psychological behaviors in male and female rats
In view of the growing evidence that pain-related and inflammatory GI diseases are linked to anxiety and depression symptoms, the overall aim of our study was to test a possible relationship between gastric inflammation and psychological behaviors in rats. While it has previously been shown that psychological stressors such as experimentally induced depression- and anxiety-like phenotypes enhance the vulnerability to intestinal inflammation [
33‐
35], we here explore whether GI inflammation has an impact on anxiety- and depression-like behaviors in rats.
The IAA added to drinking water at a concentration of 0.1% for 7 days has been previously shown to elicit murine gastric inflammation. In the present study we observed that the gastric inflammation was successfully induced by IAA treatment in rats of either gender, as assessed by increased MPO activities in IAA-treated rats. A potential drawback of IAA-induced gastritis model is the significant reduction of water intake [
36,
37]. Nonetheless, previous reported analysis of the circadian activity patterns revealed that IAA reduced drinking, feeding and locomotor activity only during the dark phase to a significant extent, suggesting that the reduction of water intake is not primarily taste-related but, together with the decrease in feeding and locomotor activity, reflects a behavioral consequence of gastritis [
22]. The normal appetite for sucrose and water should be reserved in gastritis rats. Moreover, the sucrose preference test has been used extensively to measure anhedonia, a core symptom of depression, in mild stress models of depression [
38‐
40]. Strekalova
et al.[
24] found that anhedonia was associated with analogues of depressive symptoms, such as increased floating during forced swimming and decreased exploration of novelty. Thus, the sucrose preference of IAA-treated rats reflects their depression scores.
The significant interaction between the factors treatment and gender on anxiety- and depression-like behaviors may indicate that the IAA-induced gastritis has a gender-related effect on psychological behaviors. The female rats with IAA-induced gastritis exhibited anxiety- and depression-like behaviors as assessed in SPT, OPT and EPM tests. Nevertheless, IAA treatment failed to affect the psychological behaviors in male rats. Although it could be argued that IAA treatment failed to reduce the time and distance that male rats spent and traveled in the central zone of the open field, because male control rats spent significantly less time and traveled less distance in the central zone than female control rats, this argument is not applicable to the time spent and the distance traveled in the open arms of EPM in which male and female control rats did not significantly differ.
The gender-related behavioral response to experimental gastritis is not an unexpected finding. Firstly, there is clinical evidence for a gender-related co-morbidity of functional dyspepsia with anxiety and depression as we have discussed before [
6,
8,
19]. Secondly, the IAA-induced gastritis in rats has previously been shown to elicit hypersensitivity to both mechanical and chemical noxious stimulation of the stomach, and has been proposed to represent an experimental model of functional dyspepsia [
13,
37,
41].
The gastric inflammation could influence the psychological behavior via the three potential pathways of GI-brain axis including immune, vagal nerve and neuroendocrine pathways. The gender-related increase in anxiety and depression due to gastritis went in parallel with the changes in the three GI-to-brain pathways. The IAA treatment had no effect on pro-inflammatory cytokines levels in plasma. In male rats, a normal behavior is in line with the lack of effect of IAA treatment on pro-inflammatory cytokines. However, the lack of effect of IAA treatment on plasma levels of pro-inflammatory cytokines (IL-6, TNF-α and INF-γ) in female rats may suggest that the gender-related impact of gastric inflammation on psychological behaviors is not mediated by the circulating immune pathway. The vagus nerve is another route for the GI-to-brain signaling. When the vagus nerve is activated, information carried by vagal afferent from gastrointestinal tract is transmitted to the nucleus tractus solitaries (NTS), the site of primary afferent termination of the vagus nerve — before areas involved in the stress response such as the paraventricular nucleus of the hypothalamus [
42‐
44]. It has been shown that stimuli induces expression of c-fos expression in neurons in the NTS, and also in the PVN of hypothalamus [
45,
46]. Moreover, data provided by studies in vagotomy animals showed that the stress-induced c-fos expression in PVN was reduced by vagotomy [
47‐
49], suggesting the neuronal activity in PVN can reflect the vagus nerves activity.
Expression of c-fos, either
C-
fos mRNA or c-Fos protein levels (c-Fos protein-positive neurons), is an indirect marker of neuronal activity because c-fos is often expressed when neurons fire action potentials. Emerging body of literature presents ample evidences that
C-fos mRNA expression can be used as a tool to assess neuronal activation [
50‐
55]. Moreover, data from previous studies have shown that the
C-fos mRNA expression was significantly correlated with c-Fos protein expression in neurons of brain regions [
56‐
59]. Therefore, we evaluated the vagus nerve activity by measuring the mRNA expression of
C-fos in the hypothalamus. There is a nonsignificant interaction between the treatment and gender on hypothalamic
C-fos expressions. No significant difference in hypothalamic
C-fos mRNA expressions between female rats with IAA-induced gastritis and female controls may indicate that the vagal nerves do not mediate the gender-related impact of gastric inflammation on psychological behavior in the present study. Nevertheless, it is important to emphasize that the entire hypothalamus may be a relatively extensive size of brain region compared to PVN, and may not be effective in the assessment of vagus afferent activity. Therefore, these results must be interpreted with caution. The observed lack of effect of IAA treatment on hypothalamic c-fos expression seems to be in contrast to the many reports that describe an induction of c-fos expression in a variety of brain areas in response to a wide range of stressors, including restraint, swimming, audiogenic noise and immune challenge [
60‐
63]. However, these stressors that induce c-fos expression are acute stressors. Data from ample studies have suggested that habituation to chronic stress could induce the lack of stress-induced c-fos expression in the PVN of hypothalamus [
64‐
67]. Tan
et al.[
64] have shown that expression of c-fos in the hypothalamic PVN region of the brain was induced and reached a peak at 0.5 hours for
C-fos mRNA and 4 hours for c-Fos protein, but disappeared at 2 hours for
C-fos mRNA and 16 hours for c-Fos protein during continuous restraint stress. Moreover, despite the lack of effect of chronic stress on c-fos expression in the PVN, the CRF expression in PVN and plasma CORT levels were found to be markedly changed during chronic stress [
64,
68,
69]. This dissociation between c-fos and CRF expression in PVN during chronic stress is in line with the results of the current study that the 7-day-IAA induced gastritis causes gender-related alterations in hypothalamic mRNA expressions of
Crf and
Gr and CORT levels in plasma, but had no effect on hypothalamic C-
fos mRNA expression.
Either neural excitatory input to the PVN (represented primarily by c-fos expression) or the ability of cells in the PVN to respond to that input (represented primarily by CRF expression) could activate the HPA axis negative feedback [
70]. Double-labelling studies have shown that the majority of cells in the PVN that express c-Fos in response to stressful stimuli can also express CRF [
71]. The dissociation between hypothalamic C-
fos and
Crf mRNA expressions observed in our study may indicate that dysregulation of HPA axis induced by gastritis stress is not a result of increased excitatory neural input to the PVN, but instead depends on some direct effect of gastritis on cells intrinsic to the HPA axis. Similar results were reported in the Sprague–Dawley rat following restraint stress with glucocorticoid pretreatment [
70].
The hypothalamic-pituitary-adrenal (HPA) axis plays a prominent role in the neuroendocrine route for the GI-to-brain signaling. The HPA axis activation is a homeostatic mechanism that is triggered by a physical or psychological stress to release corticotropin-releasing factor (CRF) and terminated by the negative feedback action that elevations in circulating CORT inhibit the HPA axis by acting via the glucocorticoid receptor (GR) [
72]. The significant interaction between the factors treatment and gender on plasma levels of corticosterone and hypothalamic mRNA expressions of
Gr and
Crf may suggest that the IAA-induced gastritis has a gender-related effect on the HPA axis activity. The HPA axis activity was altered in female but not male rats with IAA-induced gastritis, as reflected by decreases in hypothalamic
Gr mRNA expression and blood CORT levels, and an increase in hypothalamic
Crf mRNA expression. These alterations are likely to indicate a hyperactivity of HPA axis due to reduced sensitivity to the negative feedback action of CORT. HPA axis hyperactivity is a consistent finding among patients with major depression disorder [
73], and anxiety disorders including panic disorder [
74], social anxiety disorder [
75] and generalized anxiety disorder [
76]. Although it could be argued that IAA treatment failed to reduce the hypothalamic
Gr mRNA expression and plasma CORT levels in male rats, because the hypothalamic
Gr mRNA expression and plasma CORT levels were significantly less in male than in female control rats, this argument is not applicable to the hypothalamic
Crf mRNA expression in which male and female control rats did not significantly differ.
Hyperactivity of HPA axis is primarily characterized by an elevation in central CRF expression which also has been demonstrated to play a prominent role in the etiology of anxiety and depression. Intracerebroventricular administration of CRF reduces the open arm exploration in the EPM and also has anxiogenic effect in other anxiety tests [
77]. In clinical studies, individuals with depression, anxiety or suicide exhibit more CRF neurons in hypothalamus than normal individuals [
78]. Altogether, the gastric inflammation, the hyperactivity of HPA axis, in combination with the anxiety- and depression-like behaviors in female IAA-treated rats suggest that gastric inflammation has a gender-related impact on psychological behaviors via neuroendocrine (HPA axis) pathway.
Rats developed gastric inflammation following 7-day IAA treatment, since the IAA treatment increased the gastric MPO levels in rats of either gender. Furthermore, a significant interaction between the factors treatment and gender in MPO activity may suggest there is a gender difference in sensitivity of IAA-induced gastritis. Although female control rats have a higher MPO levels than male control rats, the IAA treatment still significantly increases the MPO activity in both genders. It may suggest that female rats are more vulnerable to IAA-induced gastritis than male rats. This gender difference in sensitivity of IAA-induced gastritis is consistent with the clinical findings that GI disorder has a considerably higher prevalence in women than in man [
15‐
17]. Therefore, we consider that the gender-related effect of gastritis on anxiety and depression as well as HPA axis activity may be related to the sensitivity to gastritis which was greater in female than in male rats.
Association between sex hormones and the gender-related anxiety- and depression-like behaviors
Data from animal and clinical studies may also provide some evidences for clarifying the mechanism by which gastric inflammation affects the HPA axis activity and psychological behaviors in a gender-related manner. There is an ample body of literatures that propose the sex hormone as an obvious candidate to explain the behavioral and physiological gender differences [
20,
21,
79].
The sex hormones in female and male rats respectively are 17ß-estradiol and testosterone which are demonstrated to modulate the HPA axis response to stressors. In the rodent, the HPA axis responses to endotoxin and to IL-1 are enhanced by gonadectomy and attenuated by estradiol and testosterone replacement [
80]. Data from clinical study have also shown that following a precipitous decline in estradiol levels during pregnancy, postpartum women experience greater HPA axis response to stressors [
81]. Therefore, in our study, a decline in estradiol level in female IAA-treated rats may lead to an enhanced HPA axis response to gastric inflammation, while an increase in testosterone level in male IAA-treated rats may reserve a normal HPA axis response to gastric inflammation.
Furthermore, since the HPA axis activity has been shown to regulate psychological behaviors, the alterations in sex hormones may be relevant to the gender-related anxiety- and depression-like behaviors in rats with IAA-induced gastritis. Data from previous studies have provided evidences for an association between decreased sex hormones and increased susceptibility to anxiety and depression. Postpartum depression, as well as premenstrual syndrome, premenstrual dysphoric disorder and menopause depression are all associated with a drop in circulating estrogen [
81]. In addition, men with hypogonadism, a condition where the body produces no or low testosterone, suffer increased levels of depression and anxiety, while testosterone replacement therapy has been shown to effectively improve mood [
82,
83].
It is well known that cyclical female sex hormone variation has a profound effect on behavioral and neurochemical parameters. Although this can be important, the short duration of the estrous cycle in rats coupled with the duration of the study and the fact that female rats housed in the same room can phase into the same estrous cycle stage [
84] would most likely result in not having a differential predominance of animals in any one particular phase of the cycle across groups [
85], and exclude the possibility of confounding behavioral effects of different phases of the cycle during the same testing sessions [
86‐
89].