Expression of TrkB and TrkC but not BDNF mRNA in neurochemically identified interneurons in rat visual cortex in vivo and in organotypic cultures

The mammalian visual cortex contains morphologically diverse populations of interneurons whose neurochemical properties are believed to be regulated b y neurotrophic factors. This requires the expression of neurotrophin recept ors. We have analysed whether brain-derived neurotrophic factor (BDNF), its receptor trkB and the NT-3 receptor trkC are expressed in interneurons of rat visual cortex in vivo, and in organotypic visual cortex cultures, payin g particular attention to the subsets of neuropeptidergic neurons. In situ hybridization in combination with immunofluorescence for calcium-binding pr oteins and neuropeptides revealed that BDNF is not expressed in interneuron s in vivo or in vitro. For the neurotrophin receptors we found in vivo at p ostnatal day 70 (P70) that approximate to 80% of the parvalbumin-immunoreac tive (-ir), but only 50% of the intensely calbindin-ir, and only 20% of the calretinin-ir neurons express trkB. Double labelling with neuropeptides re vealed that approximate to 50% of the neuropeptide Y-ir and approximate to 50% of the somatostatin-ir neurons express trkB in a laminar-specific way. Only 25% of the vasoactive intestinal polypeptide (VIP)-ir neurons coexpres s trkB. The coexpression of neuropeptide Y with trkB, but not with BDNF or trkC, was confirmed with a double in situ hybridization. In contrast, the p ercentages differed in the immature cortex; at P14 70% of the NPY-ir neuron s and 46% of the calretinin-ir neurons revealed trkB expression, while the ratio for calbindin-ir cells was fairly constant (59%). From the interneuro n populations studied, only 12% of the parvalbumin-ir neurons expressed trk C. A triple labelling revealed that some neurons coexpressed both trk mRNAs , while others had only trkC. The analysis of interneurons in organotypic c ultures yielded very similar results. The results indicate that trkB ligand s synthesized by pyramidal neurons influence neuropeptide or calcium-bindin g protein expression in a paracrine or transsynaptic manner. However, in co ntrast to current belief, in the adult only about half of all interneurons appear responsive to trkB ligands. Although the proportion is higher in the immature cortex, not all of the interneurons appear neurotrophin-receptive . With regard to the presence or absence of neurotrophin receptors, the mol ecular heterogeneity of GABAergic interneurons in the visual cortex is high er than currently assumed, and the responsiveness to neurotrophins changes with development in a cell type-specific way.