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"The axon terminal labeling of NPY[neuropeptide Y]-expressing neurons is generally limited with current immunohistochemical methods, but some terminals are evident in the outer molecular layer, suggesting that many of the NPY-containing neurons are HIPP [hilar perforant path-associated] cells and resemble somatostatin neurons."
"Although the neurochemical marker content of these intracellularly labelled neurons is largely unknown, in one case a HIPP [hilar perforant pathway-associated] cell was shown to contain neuropeptide Y (NPY). [Fruend and Buszaki 1996, Sik 1995] Since NPY and somatostatin co-localize in many hilar interneurons, [Kohler 1987] it indicates that HIPP cells express somatostatin as well."
"The hilar region neurons with outer molecular layer projections were
strikingly similar to the HIPP [hilar interneurons with perforant-path associated projection] cells described by Han et al. (1993)
in vitro and Buckmaster and Schwartzkroin (1995a) in vivo. Axon
terminals in the outer molecular layer are believed to derive from
somatostatin/neuropeptide Y (NPY)-positive neurons in the hilus
(Bakst et al., 1985; Kohler et al., 1986; Sloviter and Nilaver, 1987;
Deller and Leranth, 1990; Milner and Veznedaroglu, 1992). Like our
filled neuron, NPY-positive boutons have been observed to establish
symmetrical synapses on the dendritic shafts and spines of presumed
granule cells (Deller and Leranth, 1990; Milner and Veznedaroglu,
1992)."
"The correspondence between NPY[neuropeptide Y]-containing neurons and
morphologically identified cell types is quite complex. SOM [somatostatin]-positive
hilar cells with hilar dendritic arbors project to the outer
molecular layer and thus most likely correspond to HIPP [hilar perforant path-associated] cells. Direct evidence for the prediction that hilar NPY cells also correspond
to HIPP cells has been recently provided by the immunocytochemical
demonstration of NPY in an intracellularly
filled HIPP cell (Fig. 21; Sik et al., submitted)."
"However, since HIPP cells probably correspond to somatostatin/neuropeptide Y-immunopositive neurons (Han et al., 1993), it is noteworthy that GABA/somatostatin-immunoreactive neurons gave a weaker GABA immunoreaction than somatostatin-negative, GABA-immunopositive cells (Somogyi et al., 1984). Therefore, it is possible that the normally lower GABA level in the HIPP cell contributed to the lack of GABA immunoreactivity in the slice preparation. In perfusion-fixed material of normal rat dentate gyrus almost all boutons forming type 2 synapses in the outer molecular layer, where the HIPP cell terminates, were GABA-immunopositive (K. Halasy and P. Somogyi, unpublished result). Considering these indirect lines of evidence together with the technical difficulties of immunoreaction in slice tissue, it seems probable that the HIPP cell is a GABA- and somatostatin-containing neuron."