Research paperEarly mood behavioral changes following exposure to monotonous environment during isolation stress is associated with altered hippocampal synaptic plasticity in male rats

- Monotony during social isolation stress leads to early alteration of mood state.
- Hippocampal pyknosis and apical dendritic loss was augmented during monotony.
- Monotony stress altered the hippocampal asymmetric and symmetric synapse density.
- Fluoxetine improved the mood status by augmenting hippocampal serotonin level.
- Monotony stress altered the expression of pCREB and synaptophysin in hippocampus.

Social isolation stress and its effect on mood have been well reported, but the effect of monotony (a state of repetition of events for a considerable period of time without variation) on mood and hippocampal synaptic plasticity needs to be addressed. Present study was conducted on male Sprague-Dawley rats. Singly housed (SH) rats were subjected to monotony stress by physical, visual and pheromonal separation in specially designed animal segregation chamber. Fluoxetine (a selective serotonin reuptake inhibitor) was administered orally. Behavioral assessment showed anxiety and depression like traits in SH group. Monotony stress exposure to SH group resulted in increased pyknosis, decreased apical dendritic arborization and increased asymmetric (excitatory) synapses with the corresponding decrease in the symmetric (inhibitory) synapses in the hippocampal CA3 region. Monotonous environment during isolation stress also decreased the serotonin level and reduced the expression of synaptophysin and pCREB in the hippocampus. Fluoxetine administration to singly housed rats resulted in amelioration of altered mood along with improvement in serotonin and decrease in excitatory synaptic density but no change in altered inhibitory synaptic density in the hippocampus. These findings suggest that monotony during isolation contributes to early impairment in mood state by altering hippocampal synaptic density and neuronal morphology.