Embryonic hypothalamic expression of functional glutamate receptors.

Glutamate can play a number of roles in the developing brain, including modulation of gene expression, cell motility, neurite growth and neuronal survival, all critical for the final organization and function of the mature brain. These functions are dependent on the early expression of glutamate receptors and on glutamate release in developing neurons. This subject has received little attention in the hypothalamus, despite glutamate's critical role as an excitatory transmitter in hypothalamic control of circadian rhythms, endocrine secretion, temperature regulation, and autonomic control. A total of 10,922 rat hypothalamic neurons were studied with digital Ca2+ imaging with the ratiometric dye fura-2 to examine their responses to glutamate receptor agonists and antagonists during embryonic development and maturation in vitro. Functional glutamate receptors were found very early in development (embryonic day 15-E15) with both Ca2+ imaging and with patch clamp recording. This is a time when the hypothalamus is beginning to undergo neurogenesis. Ca2+ responses from N-methyl-D-aspartate receptors developed later than those from non-N-methyl-D-aspartate ionotropic receptors that responded to kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate. The responses of immature E15 cells after one day in vitro were compared with more mature cells after six days in vitro to examine the response to repeated 3 min applications of 100 microM kainate (n = 108). Immature cells showed similar Ca2+ rises (+232nM Ca2+) with each kainate stimulation. In contrast, more mature cells showed an initial Ca2+ rise of 307 nM, with the second rise only to 147 nM above the initial baseline. Immature cells more quickly returned to their pre-kainate baseline than did older cells. The expression of metabotropic glutamate receptors was studied with the selective agonist trans-1-amino-cyclopentyl-1,3-dicarboxylic acid and with glutamate stimulation in the absence of extracellular Ca2+ and presence of 1 mM EGTA. After five days in vitro. E16 astrocytes showed a greater response than did neurons to conditions that would activate the metabotropic glutamate receptor. A dramatic increase in the percentage of cells that responded to N-methyl-D-aspartate was found after only a few days in culture. Only a small number of E15 cells studied on the day of culture (4% of 694 cells) showed a response to 100 microM N-methyl-D-aspartate. Thirty-eight percent of 120 E18 cells cultured for one day in vitro showed an N-methyl-D-aspartate response.(ABSTRACT TRUNCATED AT 400 WORDS)