Excitability of neurons in the ventromedial nucleus in rat hypothalamic slices: modulation by amino acids at cerebrospinal fluid levels.

Free amino acid composition of cerebrospinal fluid (CSF) in the rat was measured to examine if electrical activity of neurons in the ventromedial nucleus of the hypothalamus (VMH) is modulated by amino acids at CSF levels. In CSF collected through a cannula in the cerebromedullary cistern of freely moving rats, the total concentration of amino acids was 979 microM, while that in plasma was 3.50 mM. Glutamine was the major (60%) component and each of other amino acids was less than 8.0% (lysine) of total. Among neutral amino acids, concentrations of serine, threonine and alanine were much higher than those of others. Although concentrations of basic amino acids such as lysine and arginine were also relatively high, those of acidic amino acids such as glutamate and aspartate were very low. Spontaneous firing rates of 73% of the neurons in the VMH in brain slices in vitro were facilitated by changing the perfusion medium from artificial CSF (ACSF) without amino acids to one containing each amino acid close to CSF levels (AA-ACSF). Glutamine (600 microM) alone similarly facilitated the activity in 64% of VMH neurons, though the facilitatory effect of amino acids was blocked by simultaneous application of neither AP-5 nor CNQX, antagonists for NMDA or non-NMDA receptors, respectively, to AA-ACSF at 100 microM.(ABSTRACT TRUNCATED AT 250 WORDS)