Preferential expression of the neuropeptide Y Y1 over the Y2 receptor subtype in cultured hippocampal neurones and cloning of the rat Y2 receptor

1. Neuropeptide Y (NPY) and NPY receptors are most abundant in the hippocampal formation where they modulate cognitive functions. Expression of NPY receptors in rat cultured primary hippocampal cells was investigated in the present study by use of combined molecular, pharmacological and immunohistochemical approaches, including the cloning of the rat Y₂ receptor described here for the first time.
2. More than 70% of the hippocampal neurones were endowed with [¹²⁵I]-[Leu³¹,Pro³⁴]PYY Y₁-like receptor silver grain accumulations and Y₁ receptor immunostaining. These radio- and immuno-labelling signals were distributed over cell bodies and processes of bipolar, stellate and pyramidal-like neuronal cells, as confirmed by neurone-specific enolase and MAP-2 staining.
3. Competition binding profiles revealed that specific [¹²⁵I]-[Leu³¹,Pro³⁴]PYY binding was competitively displaced according to a ligand selectivity pattern prototypical of the Y₁ receptor sub-type with [Leu³¹,Pro³⁴]substituted NPY/PYY analogues>>C-terminal fragments=pancreatic polypeptides, with the non-peptide antagonist BIBP3226 being most potent. This profile excludes the possible labelling by [¹²⁵I]-[Leu³¹,Pro³⁴]PYY of the newly cloned Y₄, Y₅ and Y₆ receptors.
4. The expression of the genuine Y₁ receptor was confirmed by RT–PCR in hippocampal cultures. In contrast, negligible levels of Y₂-like/[¹²⁵I]-PYY_3–36 binding were detected in these cultures in spite of the presence of its mRNA, as characterized by RT–PCR. The expression of both the Y₁ and the Y₂ receptor mRNAs was also noted in normal embryonic hippocampal tissues showing that signals expressed in cultured neurones were also present in utero.
5. Taken together, these results suggest that the Y₁ receptor subtype may be of critical importance in the normal functioning of the rat hippocampus, especially during brain development and maturation.