BDNF and major depressive disorder: Moreover, KEGG pathway enrichment analysis showed that common DEGs associated with depression-like behaviours were enriched in KEGG pathways associated with nicotine addiction, insulin resistance, synaptic vesicle cycle, ether lipid metabolism, inflammatory mediator regulation of TRP channel, neurotrophin signalling pathway, glutamatergic synapse, Th1 and Th2 cell differentiation, and renin secretion (Fig. 3d).