We have investigated the distribution and morphology of hippocampal interneurons that express the neurotrophins nerve growth factor (NGF) and neurotrophin-3 (NT-3) in the rat. For this study, we combined in situ hybridization for the detection of NGF and NT-3 mRNAs and immunocytochemistry against the calcium-binding proteins parvalbumin (PARV), calretinin (CALR), and calbindin (CALB). Whereas the majority of PARV ϩ interneurons expressed NGF mRNA, only subsets of CALR-and CALB-immunoreactive interneurons (23% and 24%, respectively) displayed NGF hybridization. Most CALB/NGF ϩ cells were located in the stratum oriens/alveus of the CA3-CA1 regions, suggesting that they may include the population of CALB ϩ , hippocamposeptal, nonpyramidal neurons. Most of the nonspiny CALR/NGF ϩ neurons were located within or in the vicinity of the pyramidal layer and had faint CALR immunostaining and stellate, thin dendrites. Regarding the spiny CALRimmunoreactive cells, we found that most of these neurons in the hilus were NGF ϩ , whereas only 59% of displayed NGF hybridization in the stratum lucidum of the CA3 region.
A small subset of PARV-and CALR-immunoreactive cells expressed NT-3 mRNA (16% and 13%, respectively). NT-3 message was not found in the large basket cells of the dentate gyrus, whereas the distribution and morphology of CALR/NT-3 ϩ cells were similar to those of nonspiny CALR/NGF ϩ cells. In fact, double in situ hybridization analysis confirmed that most NT-3 ϩ neurons also expressed NGF mRNA, indicating coexpression of both neurotrophins in subpopulations of PARV ϩ and CALR ϩ neurons. Moreover, the level of NGF mRNA expression was higher in PARV ϩ neurons than in CALR-and CALB-immunoreactive interneurons, whereas NT-3 message was expressed similarly in PARV ϩ and CALR ϩ neurons. The present findings show a differential expression of NGF and NT-3 mRNAs in subsets of hippocampal interneurons and suggest that the expression of these transcripts depends on factors intrinsic to particular cell types.