Preclinical studies support a critical role of 5-HT4 receptors (5-HT4Rs) in depression and anxiety, but their influence in depression- and anxiety-like behaviours and the effects of antidepressants remain partly unknown. We evaluated 5-HT4R knockout (KO) mice in different anxiety and depression paradigms and mRNA expression of some neuroplasticity markers (BDNF, trkB and Arc) and the functionality of 5-HT1AR. Moreover, the implication of 5-HT4Rs in the behavioural and molecular effects of chronically administered fluoxetine was assessed in naïve and olfactory bulbectomized mice (OBX) of both genotypes. 5-HT4R KO mice displayed few specific behavioural impairments including reduced central activity in the open-field (anxiety), and decreased sucrose consumption and nesting behaviour (anhedonia). In these mice, we measured increased levels of BDNF and Arc mRNA and reduced levels of trkB mRNA in the hippocampus, and a desensitization of 5-HT1A autoreceptors. Chronic administration of fluoxetine elicited similar behavioural effects in WT and 5-HT4R KO mice on anxiety-and depression-related tests. Following OBX, locomotor hyperactivity and anxiety were similar in both genotypes. Interestingly, chronic fluoxetine failed to reverse this OBX-induced syndrome in 5-HT4R KO mice, a response associated with differential effects in hippocampal neuroplasticity biomarkers. Fluoxetine reduced hippocampal Arc and BDNF mRNA expressions in WT but not 5-HT4R KO mice subjected to OBX. These results demonstrate that the absence of 5-HT4Rs triggers adaptive changes that could maintain emotional states, and that the behavioural and molecular effects of fluoxetine under pathological depression appear to be critically dependent on 5-HT4Rs.