Expression of NPY Y₁ and Y₅ receptors in the hypothalamic paraventricular nucleus of aged Fischer 344 rats

Many mammals, nearing the end of life, spontaneously decrease their food intake and body weight, a stage we refer to as senescence. The spontaneous decrease in food intake and body weight is associated with attenuated responses to intracerebroventricular injections of neuropeptide Y (NPY) compared with old presenescent or with young adult rats. In the present study, we tested the hypothesis that this blunted responsiveness involves the number and expression of hypothalamic paraventricular nucleus (PVN) Y₁ and/or Y₅ NPY receptors, both of which are thought to mediate NPY-induced food intake. We found no significant difference in mRNA levels, via quantitative PCR, for Y₁ and Y₅ receptors in the PVN of senescent vs. presenescent rats. In contrast, immunohistochemistry indicated that the number of PVN neurons staining for Y₁ receptor protein was greater in presenescent compared with senescent rats. We conclude that a decreased expression and number of Y₁ or Y₅ receptors in the PVN cannot explain the attenuated responsiveness of the senescent rats to exogenous NPY.