SLC6A4 and depressive disorder: Studies conducted in TLE patients with comorbid depression indicate abnormalities in serotonergic neurotransmission with comparably decreased 5-HT1A receptor binding and 5-HT-transporter activity in specific brain areas involved in both these disorders when compared to TLE patients without depression, independently of the side of the lesion and the degree of hippocampal sclerosis [128, 130-133].