Actions of Acidic Amino Acids on the Excitability of Medial Hypothalamic Neurons in the Rat

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Abstract
Systemic administration of acidic amino acids produces selective neurotoxic effects on neurons in the arcuate nucleus. Little is known on the comparative or topographical actions of various acidic amino acids within the hypothalamus. Microiontophoretic methods were utilized to compare the sensitivity of arcuate nucleus neurons with other medial hypothalamic neurons during applications of glutamate, aspartate, homocysteate and 2 conformationally restricted glutamate analogs, kainate and ibotenate. An increase in excitability to all test substances was observed from the majority of 62 neurons identified both as to location (arcuate, ventromedial and dorsomedial nuclei and periventricular area) and to response to electrical stimulation in the median eminence, amygdala and posterior pituitary. There was little difference in the sensitivity of neurons to glutamate, aspartate, or homocysteate. Kainate produced a more prominent and long-lasting excitation at lower application currents, frequently leading to spike inactivation and excessive depolarization followed by a prolonged recovery period. Ibotenate was also a potent excitatory agent, but its actions were biphasic, with an initial excitation followed by a decrease in excitability that influenced both spontaneous and glutamate-evoked firing; the latter is considered due to conversion of ibotenate to muscimol, a GABA analog. A homogeneity among arcuate and other medial hypothalamic neurons is suggested in terms of their membrane receptors and sensitivity to all acidic amino acids tested. Since acidic amino acids are also viewed as possible excitatory neurotransmitters within the hypothalamus, the diethyl esters of glutamic acid (GDEE) and kainic acid (KDEE), 2 proposed glutamate antagonists, were applied by microiontophoresis and assessed for their ability to alter glutamate- and aspartate-evoked activations and synaptically evoked excitations. Each agent depressed both glutamate- and aspartate-evolked activity nonselectively; thus, while they do affect acidic amino acid actions, they appear to be of limited use in differentiating between glutamate- and aspartate-activated receptors on medial hypothalamic neurons. Neither GDEE nor KDEE altered synaptically evoked excitations from the median eminence, amygdala, or posterior pituitary. Owing to the extensive dendritic arborization of hypothalamic neurons, it is possible that these agents attained a low potency at active synaptic sites. Further evaluations of the role of acidic amino acids as medial hypothalamic neurotransmitters include in vitro testing where it is possible to achieve a uniform application of their antagonists in known concentrations.