Distribution of metabotropic glutamate receptor 1a in the rat hypothalamus: An immunocytochemical study using monoclonal and polyclonal antibody

L-glutamate appears to be a major excitatory neurotransmitter in the hypothalamus. Its action is mediated via ionotropic and metabotropic glutamate receptors (mGluR). Eight mGluRs have already been cloned. In the present study the hypothalamic distribution of mGluR1a has been investigated by immunocytochemistry using monoclonal antibodies recently produced by some of the present authors (T. J. G., R. K., T. K.). The observations have been compared with findings obtained with a polyclonal antibody. A widespread and heterogeneous distribution of mCluR1a was found with the monoclonal antibodies. Intense immunolabelling of perikarya and dendrites occurred in several hypothalamic cell groups including the suprachiasmatic, anterior periventricular, anterior hypothalamic (posterior part), paraventricular, supraoptic, arcuate, tuberal magnocellular, dorsomedial and mammillary nuclei (particularly in the medial). It was only the ventromedial nucleus in which several perikarya were stained by the polyclonal antibody but appeared to be negative by the monoclonal antibodies. The findings fit extremely well with the data on the hypothalamic distribution of mGluR1 mRNA with the exception of the ventromedial nucleus. It remains to be elucidated whether alternatively spliced variants of mGluR1 (mGluR1b and 1c) are expressed in this nucleus. Further, they confirm the results of former immunohistochemical studies. In addition, they indicate that a significant part of the neuroendocrine region of the hypothalamus (including the paraventricular, supraoptic and arcuate nuclei) also contains mGluR1 suggesting that this receptor may play a role also in neuroendocrine regulation.