DEVELOPMENTAL REGULATION OF THE HYPOTHALAMIC METABOTROPIC GLUTAMATE-RECEPTOR MGLUR1

The expression of the metabotropic glutamate receptor mGluR1 was studi ed with Northern and Western blot analysis, with immunocytochemistry, and with Ca2+ digital imaging in the developing rat hypothalamus. mGlu R1 is coupled to a G protein and activation by glutamate and related a gonists leads to intracellular phosphotidylinositol hydrolysis and Ca2 + mobilization. mGluR1 RNA could be detected in embryonic hypothalamus , and by the day of birth and prior to the primary period of synaptoge nesis, both mGluR1 RNA and protein were strongly expressed. In paralle l experiments with digital imaging of cultured hypothalamic cells, som e embryonic day 18 hypothalamic neurons and many astrocytes after 3 d in vitro showed Ca2+ responses to quisqualate and t-ACPD, and to gluta mate in the absence of extracellular Ca2+. A greater number of embryon ic neurons responded to NMDA than to agonists of the metabotropic rece ptor. With increased development time in culture, the number of neuron s that responded to metabotropic glutamate receptor agonists increased . In the adult hypothalamus, mGluR1-immunoreactive neurons were widesp read, and particularly dense in the dorsomedial, lateral, and anterior hypothalamus/preoptic areas, and in the mammillary body. Strongly imm unoreactive cells were interspersed among neurons with no immunoreacti vity. In developing neurons a diffuse immunostaining appeared along de ndrites and somata. With time, beginning in the first week after birth , strongly stained puncta appeared, possibly associated with synaptic specializations. These puncta were numerous on dendrites of some adult neurons, and were the most strongly,stained regions of neurons. Neuro ns developing in vitro at low neuron densities showed a development of mGluR1 immunoreactivity similar to that of neurons in vivo, but with a delayed progression of immunostaining. We found no obvious staining of axons or of astrocytes. A strong expression of mGluR1 protein was f ound in the hypothalamus during the first 2 postnatal weeks; this expr ession was partially reduced in adults. In contrast, cerebellum showed no reduction in mGluR1 protein in adults. Together these data suggest a complex regulation of mGluR1 during development, with sufficient ex pression of functional receptors in the developing hypothalamus to mod ulate morphogenesis and synaptogenesis, and later to play a role in tr ansduction of glutamate signals in the adult. Different regions of the brain showed dramatic differences in the way each expresses mGluR1 du ring development.