Strong expression of NETRIN-G2 in the monkey claustrum

The claustrum is a phylogenetically conserved structure, with extensive reciprocal connections with cortical regions, and has thus been considered important for sensory, motor, emotional, and mnemonic coordination or integration. Here, we show by in situ hybridization that the adult monkey claustrum is strongly positive for NETRIN-G2, a gene encoding a glycosyl phosphatidyl-inositol-linked membrane protein, which constitutes a subfamily with NETRIN-G1 within the netrin/UNC6 family. There is a conspicuous dorsal/ventral differentiation, where the label is stronger in the ventral claustrum. NETRIN-G2 positive neurons are not GABAergic, but rather correspond to claustrocortical projection neurons, as demonstrated by retrograde transport of Fast Blue from cortical injections and by double in situ hybridization for NETRIN-G2 and GAD67. Since NETRIN-G2 is known to be preferentially expressed in cortex, in contrast with the thalamically expressed NETRIN-G1, these results raise the possibility of some functional similarity in regulation of excitatory neural transmission in the claustrum and cortex.