Glutamatergic stimulation of the left dentate gyrus abolishes depressive-like behaviors in a rat learned helplessness paradigm

Loss of sodium leak channel (NALCN) in the ventral dentate gyrus impairs neuronal activity of the glutamatergic neurons for inflammation-induced depression in male mice

BACKGROUND

The dentate gyrus (DG) has been implicated in the pathophysiology of depression. Many studies have revealed the cellular types, neural circuits, and morphological changes of the DG involved in the development of depression. However, the molecular regulating its intrinsic activity in depression is unknown.

METHODS

Utilizing the mode of depression induced by lipopolysaccharide (LPS), we investigate the involvement of the sodium leak channel (NALCN) in inflammation-induced depressive-like behaviors of male mice. The expression of NALCN was detected by immunohistochemistry and real-time polymerase chain reaction. DG microinjection of the adeno-associated virus or lentivirus was carried out using a stereotaxic instrument and followed by behavioral tests. Neuronal excitability and NALCN conductance were recorded by whole-cell patch-clamp techniques.

RESULTS

The expression and function of NALCN were reduced in both the dorsal and ventral DG in LPS-treated mice; whereas, only knocking down NALCN in the ventral pole produced depressive-like behaviors and this effect of NALCN was specific to ventral glutamatergic neurons. The excitability of ventral glutamatergic neurons was impaired by both the knockdown of NALCN and/or the treatment of LPS. Then, the overexpression of NALCN in the ventral glutamatergic neurons decreased the susceptibility of mice to inflammation-induced depression, and the intracranial injection of substance P (non-selective NALCN activator) into the ventral DG rapidly ameliorated inflammation-induced depression-like behaviors in an NALCN-dependent manner.

CONCLUSIONS

NALCN, which drives the neuronal activity of the ventral DG glutamatergic neurons, uniquely regulates depressive-like behaviors and susceptibility to depression. Therefore, the NALCN of glutamatergic neurons in the ventral DG may present a molecular target for rapid antidepressant drugs.

PET imaging of animal models with depressive-like phenotypes

Major depressive disorder is a growing and poorly understood pathology. Due to technical and ethical limitations, a significant proportion of the research on depressive disorders cannot be performed on patients, but needs to be investigated in animal paradigms. Over the years, animal studies have provided new insight in the mechanisms underlying depression. Several of these studies have used PET imaging for the non-invasive and longitudinal investigation of the brain physiology. This review summarises the findings of preclinical PET imaging in different experimental paradigms of depression and compares these findings with observations from human studies. Preclinical PET studies in animal models of depression can be divided into three main different approaches: (a) investigation of glucose metabolism as a biomarker for regional and network involvement, (b) evaluation of the availability of different neuroreceptor populations associated with depressive phenotypes, and (c) monitoring of the inflammatory response in phenotypes of depression. This review also assesses the relevance of the use of PET imaging techniques in animal paradigms for the understanding of specific aspects of the depressive-like phenotypes, in particular whether it might contribute to achieve a more detailed characterisation of the clinical depressive phenotypes for the development of new therapies for depression.

Opioid modulation of depression: A focus on imaging studies

Depression is the leading cause of disability worldwide, with over 300 million people affected. Almost all currently available antidepressant treatments target monoamine neurotransmitter systems and have a delayed onset of action up to several weeks that can be associated with low rates of treatment response. The endogenous opioid system has been identified as a potential target for the development of novel antidepressants due to its high opioid receptor concentrations in central limbic areas that are also implicated in physiological processes including regulation of mood and emotion. Genetic depletion, pharmacological manipulation, and preclinical models have been widely used to characterize the role of opioid transmission in depressive states. Neuroimaging studies have been carried out in clinical populations to investigate opioid transmission in mood and emotion in an attempt to identify those regional anatomical and functional brain changes that are associated with depression. Great insight has been provided into the cerebral structural and functional changes associated with depression but there remains a need to tie the functional theories of depression to anatomical localization and further neuroimaging studies are best placed to do this.