Effect of XingPiJieYu decoction on spatial learning and memory and cAMP-PKA-CREB-BDNF pathway in rat model of depression through chronic unpredictable stress

The present study utilized broadly accepted traditional animal models – CUS model to examine depressive behaviors, the spatial learning and memory capability and the expressions of mRNAs/proteins of the cAMP-PKA-CREB-BDNF signaling. The results indicated that the body weight gain, sucrose preference, total ambulation, percentage of central ambulation and rearing after CUS for 21 were significantly decreased. It indicates that the depression model was successfully established. Then the spatial cognitive performance in the Morris water maze task was also decreased, which demonstrated that depression by CUS had a dramatic influence on spatial cognitive performance in the MWN test, while the treatment of the rats with XPJY significantly reversed these changes. Furthermore, the expressions of mRNAs/proteins in the cascade of cAMP-PKA-CREB-BDNF signaling were decreased by CUS, too. The administration of XPJY to stressed rats prevented such metabolite reductions. These results suggest that XPJY could improve depression and related learning/memory impairment through the cAMP-PKA-CREB-BDNF signal cascade. Depression is a highly debilitating and widely distributed mental illness in the general population with a lifetime incidence of 15-25%, ranking it as one of the most burdensome diseases of society [5‐7], besides enormous personal suffering and increased mortality risk [28]. Depression may display many different syndromes like a pervasive low mood and loss of pleasure or interest in usual activities [1]. The patients also demonstrate sleeplessness, decreased body-weight [3, 4], and obvious cognitive function impairments, such as learning and memory impairment, etc. [5‐7]. Cognitive function related to depression has received far more attention. A growing body of research suggests that depressive symptoms and cognitive impairment are common often coexist in an individual patient, especially in older adults. And late-life depression even is a risk factor for cognitive decline [10]. Another clinical research also found that depressed participants, in the normal aging older adults, had a lower performance compared to non-depressed participants in cognitive domains, and the depressive symptoms may have a distinct impact on cognitive performance [29]. The studies show that the pattern of cognitive impairment associated with depressive symptoms involves executive dysfunction, reduced processing speed, and deficits in episodic memory [30‐32], while global intellectual ability, language skills, visuospatial abilities, and semantic processing are usually spared [33].

Among them, learning and memory impairment is one of the important cognitive impairments and residual symptoms, which has a strong impact on function of patients both at home and workplace, and the life quality of patients. Many studies also have reported learning and memory deficits in depressed subjects [3]. Further, it is becoming increasingly clear that the disturbance of cognitive processes, especially the impairment of learning and memory, plays an important role in the development and complete recurrence of depression [8, 9]. In addition, cognitive impairment is also one of the typical features of recurrent depressive disorder, predominantly connected with episodic memory processes and working memory [11‐13]. And cognitive impairment, linked with the earlier onset of depressive symptoms and episode prolongation, may in return lead to an ineffective antidepressant therapy and impede full recovery [14].

In this study, the chronic unpredictable stress for 21 days was used to establish depression model of rat and simulate the long-term negative modes and life events of humans. The chronic unpredictable stress procedure is one of the well-validated animal models of depression [34], that has good face validity in rodents as it can elicit depression like symptoms such as lack of sucrose preference [35, 36] interpreted as anhedonia [37] and reduced locomotor activity [38]. Anhedonia-like behavior is the core symptom of human depression [39], which means inability to experience pleasure. Anhedonia has been defined as decreased responsiveness to rewards [40, 41], and it is measured originally by declining intake of a palatable sweet solution. In this experimental conditions, there has been a significantly reduction of sucrose preference in CUS group compared with the control, which was reduced to approximately 27% at day 21 after the beginning of stress exposure. Reduced locomotor activity of rats in open field test may mimic some aspects of human psychomotor retardation [42], which is an accompanying symptom of depression in humans [40]. In the experimental conditions, chronic unpredictable stress rats also exhibited depressive behavior which is displayed by decreased total ambulation, central ambulation, rearing in comparison to control rats. Depression may also display many other different syndromes like decreased body weight [43] and impaired learning/memory [44].

The Morris water maze has been widely used all over the world to detecting spatial learning memory capability [45], to objectively reflect their cognitive levels. The rats are trained to learn to use the relationship between environmental labels and latent platforms in order to judge the positions of the platforms in the water. They may thus form stable spatial cognition. The rats were allowed to swim from the original position to the latent platform under water by utilizing the indications at the distal end. Their spatial learning was evaluated by repeated training. When the platforms under water were withdrawn, the reference memory of the animals was determined by using the frequency of penetration of the platform position [7]. The present study found that the escape latency in the XPJY 1.4 g/kg group on the first day was significantly shortened, but the difference for the sertraline group was not statistically significant. This results shows XPJY could increase the ability of spatial learning of depression rats by CUS. In contrast, the frequency for penetrating the central areas increased both in the XPJY 1.4 g/kg group and in the sertraline group in the spatial exploration test on the fifth day. It indicated that they can both achieve improvement with regard to rats’ memory capability, while the efficacy of the XPJY 1.4 g/kg group was more significant. This results shows XPJY increased the ability of spatial learning memory better than sertraline.

Previous studies have shown that chronic stressful life events are major reactions for inducing depression and a decrease in learning memory capability [44], as the major target for stress. The experimental results are consistent with actual situation. The primary findings of the present study show that CUS causes cognitive decline and depression like symptoms whereas XPJY showed ameliorating potential against detrimental effect of CUS. These results are in agreement with several other studies which also showed that UCMS causes cognitive decline and depression like behavior in animal models [38, 46].

The neurobiological mechanisms connecting the depressive symptoms with cognitive and functional performance are heterogeneous and have not been completely elucidated. A number of studies have shown that abnormalities in the hippocampus are closely associated with the occurrence and development of depression [47‐50]. Depression may affect learning memory capability by injuring hippocampus neurons [51], which has strong connections with depression and learning memory capability [52]. The mechanisms for cognitive disorder in depression mainly have two dimensions: neurobiological and vascular factors, which may mediate the cognitive and functional changes associated with depression [53], including changes in monoamine systems dysfunction, hormonal and immunologic changes, inflammatory processes, and alterations on genes expression [54]. Such as, the hypothalamic-pituitary-adrenal axis dysfunction in depression, which relates to hippocampal atrophy, may be a neurobiological causal factor to the episodic memory impairment in depressed subjects [54, 55]; and white matter lesions as vascular burden have be found in depressed subjects [56]. Our earlier studies have shown that learning and memory ability in depression rats might be related to reduce the inflammatory factors level, such as IL-1β, IL-6 and TNF-α, in serum and hippocampus [57].

Pathophysiological studies on depression have recently been gradually transferred to the intracellular secondary messenger system. As an intracellular secondary messenger, cAMP can promote neuronal differentiation and survival [58, 59] as well as outgrowth, regeneration [60‐62] and guidance of neuronal processes [63, 64], whose signaling has been shown to be implicated in mechanism of reduced synaptic plasticity, that may contribute to the pathophysiology of depression [65, 66]. cAMP can activate cAMP-dependent protein kinase (PKA), and subsequently PKA is able to activate CREB directly by phosphorylation of the transcription factor CREB [67] or indirectly [58, 68], thus further mediating multiple signaling molecules, like CREB and BDNF which play important roles in the signaling pathway of learning memory and depression [10].

CREB is a kind of regulatory factor in nuclei, as an important component of multiple intracellular signaling pathways in the nervous system, and is capable of regulating transcription by autophosphorylation. Many intracellular signal transduction cascades can influence, directly or indirectly, the activation of CREB. Some of the enzymes that participate in these cascades are PKA, protein kinase C (PKC), Ca2+/calmodulindependent protein kinase (CAMKII), extracellular-regulated protein kinase (ERK), phosphoinositide 3-kinase (PI3K), and glycogen synthase kinase 3 (GSK-3) [69, 70]. The downstream actions of CREB include the influences on neuron synaptic plasticity and the formation of long term memory [71, 72]. CREB is critical for the formation of hippocampally-dependent long-term memory [73]. In addition, CREB also decreases in the brains of patients with depression, which are elevated in those patients who have been using antidepressants [74].

BDNF is one of the downstream target genes of CREB, which is the most prevalent neurotrophic factor in the brain. It can be induced by the phosphorylation of CREB and responsible for neuronal survival, maintenance and growth. Some studies show that a decreased expression of BDNF, a key target implicated both in the etiology of depression [75], appears to be associated with depression symptoms both in animals and humans. Depressive patients have a decrease in serum, plasma and hippocampal BDNF levels in depressive patients [76, 77]. And a decreased serum BDNF level may be an indicator of vulnerability to develop depression [78]. On the other hand, several animal models of depression have also shown a reduced expression of BDNF in brain regions [79‐82] and produce an antidepressant-like behavior [83]. Furthermore, BDNF plays important roles in facilitating both early and late phase of LTP [84, 85].

Then, CREB can induce the expression of BDNF in general [86, 87], while BDNF can also activate the production of CREB [11]. They form a positive feedback ring. BDNF is the best studied neurotrophic factor implicated in depression, which is also concerned to neuroplasticity and memory. Previous studies have found that the expression level of BDNF in the hippocampus of CUS induced mice markedly reduced [88]. BDNF also could adjust the plasticity of neurons, such as plasticity of 5-HT neurons in the central nervous system [89, 90].

It has also been indicated that the intracellular cAMP-PKA-CREB-BDNF signaling pathway can be activated after anti-depression therapy for a period. [11]. Previous studies have revealed that the cAMP-PKA-CREB -BDNF signaling is involved in depressive behaviors in animals [18, 19, 91]. Consistently, the present study showed XPJY prevented the reduction of the cAMP-PKA-CREB -BDNF signaling cascade induced by CUS while improving the depressive behaviors. The most important, the present results show the cAMP-PKA-CREB -BDNF signaling may be also involved in the ability of spatial learning memory of depressive rats. The ability of spatial cognitive recovered with the increasement of cAMP-PKA-CREB -BDNF signaling, suggesting the effects of XPJY may be through regulation of the signaling pathway. Some recent studies also supported that the ability of learning and memory improved through cAMP-PKA-CREB-BDNF signaling pathway [12, 92]. Interestingly, sertraline in the present study showed the effect of antidepression and increasement of signaling pathway as same as XPJY, but didn’t in spatial learning memory test. The XPJY 1.4 g/kg group was more advantageous than sertraline in improving the learning memory ethology of depression rats. There are two probabilities: sertraline may need more time to work, or XPJY also work though other signaling pathway about learning and memory. These studies indicated that XPJY plays antidepression and anti learning memory impairment effect through the cAMP-PKA-CREB -BDNF signaling pathway, which remains to be further studied using specific blockers of the signal pathway. Other mechanisms may also be involved, and this still requires further study.

Recently, TCM received more and more attention in treating depression and related syndromes [15‐17]. XPJY is a common prescription in clinical practices and shows relatively satisfactory therapeutic effect in improving depression and cognitive dysfunction. In our previous studies, we established the depression model in the same way, and the expression of serum 5-HT and corticosterone were examined with Elisa. The results demonstrated that rats subjected to CUS exhibited decreases in serum 5-HT and increases in serum corticosterone. The results have shown that XPJY reversed the depression-like behaviors, increased serum 5-HT and decreased serum corticosterone induced by CUS in rats as the same as sertraline could [93]. In summary, the present study indicated that XPJY could improve the depression and related syndromes in rat CUS model through cAMP-PKA-CREB-BDNF pathway. These findings are important for understanding therapeutic effects of Chinese medicine XPJY on the impairment of learning and memory in depression.