maLPA1-null mice as an endophenotype of anxious depression

Abstract

Anxious depression is a prevalent disease with devastating consequences and a poor prognosis. Nevertheless, the neurobiological mechanisms underlying this mood disorder remain poorly characterized. The LPA1 receptor is one of the six characterized G protein-coupled receptors (LPA1-6) through which lysophosphatidic acid acts as an intracellular signalling molecule. The loss of this receptor induces anxiety and several behavioural and neurobiological changes that have been strongly associated with depression. In this study, we sought to investigate the involvement of the LPA1 receptor in mood. We first examined hedonic and despair-like behaviours in wild-type and maLPA1 receptor null mice. Owing to the behavioural response exhibited by the maLPA1-null mice, the panic-like reaction was assessed. In addition, c-Fos expression was evaluated as a measure of the functional activity, followed by interregional correlation matrices to establish the brain map of functional activation. maLPA1-null mice exhibited anhedonia, agitation and increased stress reactivity, behaviours that are strongly associated with the psychopathological endophenotype of depression with anxiety features. Furthermore, the functional brain maps differed between the genotypes. The maLPA1-null mice showed increased limbic-system activation, similar to that observed in depressive patients. Antidepressant treatment induced behavioural improvements and functional brain normalisation. Finally, based on validity criteria, maLPA1-null mice are proposed as an animal model of anxious depression. Here, for we believe the first time, we have identified a possible relationship between the LPA1 receptor and anxious depression, shedding light on the unknown neurobiological basis of this subtype of depression and providing an opportunity to explore new therapeutic targets for the treatment of mood disorders, especially for the anxious subtype of depression.

Figure 1

Anhedonic behaviours in mice lacking the LPA1 receptor. (a) In the saccharin preference test, maLPA1-null mice displayed a preference below the anhedonic threshold set at 65% in all doses, showing significant differences from the preference of wt mice (n=7 per genotype). (b) Chronic desipramine treatment leads to improvement in motivation of maLPA1-null animals reaching levels above the anhedonic threshold (c) In female urine sniffing tests, maLPA1-null mice exhibited a significant reduction of the percentage of time sniffing oestrogenic female urine compared to wt animals (n= 6 per genotype). *P<0.05 time compared with wt. ^#P<0.05 time spent sniffing urine versus water. DMI, desipramine; FUST, female urine sniffing test; SPT, saccharin preference test.

Figure 2

Impaired nesting behaviour in maLPA1-null mice is reverted by desipramine treatment. The quality of the nest built by each genotype was assessed after 24 h (a) or after 1 h and 4 h (b) (n=10 per genotype). (c) Experimental procedure. (d) Nest scores 24 h after administration of DMI or vehicle. (e) Representative images from the test for both genotypes. *P<0.05 **P<0.01 difference from wt; ^#P<0.05 difference between time points. DMI, desipramine.

Figure 3

Mice lacking the LPA1 receptor showed altered stress-coping behaviour (FST and TST) together with fear and anxiety (ETM). (a) MaLPA1-null mice exhibited reduced immobility and increased climbing compared to wt mice. The behavioural patterns differed between the two genotypes for all tests (n=7 per genotype). (b) In TST, maLPA1-null mice exhibited lower immobility and higher energy and PM levels than the control group (n=8 per genotype). (c) Compared with the wt mice, maLPA1-null mice showed higher baseline avoidance in the ETM, indicating anxiety-like behaviour, without habituating during the test. In the third trial of the escape test, they took more time to leave the open arm, which is associated with the high proportion of freezing time (n=9 per genotype). *P<0.05 **P<0.01 ***P<0.001 difference from wt; ^#P<0.05 or ^&P<0.05 difference between time points in wt or maLPA1-null mice, respectively. ETM, elevated T-maze; FST, forced swim test; Imm, immobility; TST, tail suspension test.

Figure 4

Functional connectivity in wt and maLPA1-null mice. (a) Activity of the limbic and extralimbic regions involved in mood after behaviour was normalised to the relevant basal group. Structures were grouped into major brain subdivisions. (b) Significant correlations between c-Fos activation and behavioural parameters in the FUST and TST. (c, d) Data are presented for the three conditions: basal (upper), after FUST (middle) and after TST (lower). (c) Interregional correlation matrices reveal different limbic maps in the absence of LPA1 receptor, especially in basal and TST conditions. (d) Strongly correlated levels of c-Fos among structures (r⩾±0.8; P⩽0.05) are shown in the diagrams. Red lines indicate positive correlation, while blue lines illustrate negative correlations. BLA, basolateral amygdala; CA1, cornus ammonis 1; CA3, cornus ammonis 3; CE, central amygdala; d/vPAG, dorsal/ventral periaqueductal grey matter; DG, dentate gyrus; DRN, dorsal raphe nucleus; FUST, female urine sniffing test; IL, infralimbic cortex; lHb, lateral habenula; mHb, medial habenula; mPFC, medial prefrontal cortex; NAc, nucleus accumbens; NAcC, nucleus accumbens core; NAcSh, nucleus accumbens shell; PL, prelimbic cortex; PVN, paraventricular nucleus; TST, tail suspension test; VTA, ventral tegmental area.