Impact of serotonin transporter deficiency on parvalbumin- and neuropeptide Y-producing neurons of the lateral and basolateral amygdalar nuclei.

Hyperexcitability of lateral (La) and basolateral (BL) amygdalar nuclei (BLA) is a hallmark of anxiety-related disorders in humans. Excitation of BLA projection neurons (PN) is fine-tuned by inhibitory interneurons (INs), and monoaminergic afferents to the BLA modulate PN and IN activity. In the present study, BLA-neurons immunoreactive(ir) for parvalbumin (PV) or neuropeptide Y (NPY) and their interrelations with serotonergic and catecholaminergic afferents were analyzed in wildtype (WT) and in serotonin (5-HT) transporter knockout (5-HTT KO) mice, a mouse model for anxiety- and stress-related disorders. In both genotypes, PV- and NPY-ir neurons possess perisomatic appositions by serotonergic and tyrosine hydroxylase-ir afferents. Dual immunolabeling shows no colocalization of PV and NPY. Qualitative analysis of NPY/somatostatin(SOM) dual labeling in WT and 5-HTT KO mice documents colocalization of the peptides in neurons with predominantly fusiform somata, and single labeling for NPY in neurons with predominantly round somata. Quantification of PV- and NPY-ir neurons documents a reduction in numbers and densities of NPY-ir neurons of the BLA in 5-HTT KO mice while PV-ir neuron numbers and densities remain unchanged. Quantitative PCR shows increased expression of Npy receptor 2, Som receptor 4, and corticotropin releasing factor receptor 1 in the BLA of 5-HTT KO mice. mRNA for the peptides is unchanged, indicating that it may be NPY propeptide translation which is reduced in 5-HTT KO mice. Taken together, the results document an effect of life-long serotonin imbalance on the BLA NPY-system, which may contribute to increased anxiety-like behavior in 5-HTT KO mice.