Excitatory effect of amino acids on identified neuron R14 of Aplysia. I. Glycine-induced depolarization and its ionic mechanism

Physiological and pharmacological characteristics of quisqualic acid-induced K(+)-current response in the ganglion cells of Aplysia

The extracellular application of either quisqualic acid (QA) or Phe-Met-Arg-Phe-NH2 (FMRFamide) induces an outward current in identified neurons of Aplysia ganglion under voltage clamp. The time course of the QA-induced response is significantly slower than that induced by FMRFamide. The reversal potential for both responses was -92 mV and was shifted 17 mV in a positive direction for a twofold increase in the extracellular K(+) concentration. The QA-induced response was markedly depressed in the presence of Ba(2+), a blocker of inward rectifier K(+)-channel, whereas TEA, a Ca(2+)-activated K(+)-channel (BK(Ca)) blocker, or 4-AP, a transient K(+) (A)-channel blocker, had no effect on the response. The QA-induced K(+)-current was significantly suppressed by CNQX and GYKI52466, antagonists of non-NMDA receptors. However, the application of either kainate or AMPA, agonists for non-NMDA receptors, produced no type of response in the same neurons. The QA-induced K(+)-current response was not depressed at all by an intracellular injection of either guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS) or guanosine 5'-O-(3-thiotriphosphate) (GTP-gammaS), but the FMRFamide-induced response was markedly blocked by both GDP-betaS and GTP-gammaS in the same cell. Furthermore, the QA- and FMRFamide-induced K(+)-current responses were both decreased markedly when the temperature was lowered to 15 degrees C, from 23 degrees C. These results suggested that the QA-induced K(+)-current response is produced by an activation of a novel type of QA-receptor and that this response is not produced by an activation of the G protein.

Circulating free amino acids in Aplysia californica

1. Questions regarding the availability of free amino acids and their importance as precursors and direct participants in neural functions are essential to our understanding but cannot be answered without basic data. 2. A profile of 22 circulating amino acids was developed for the often studied Aplysia californica.

Glycine, like glutamate, microinjected into the nucleus tractus solitarii of rat decreases arterial pressure and heart rate

Glycinergic mechanisms have been implicated in central cardiovascular regulation. However, the inhibitory amino acid's role in the nucleus tractus solitarii (NTS), the site of termination of cardiovascular afferents, has not been clarified. Thus, we sought to determine if the microinjection of glycine into the NTS alters arterial pressure and heart rate. Microinjections of glycine, like glutamate, confined to the NTS decreased arterial pressure and heart rate in a neurally mediated, dose-dependent manner. The glycine antagonist strychnine completely blocked these effects of glycine but did not itself alter arterial pressure or heart rate, or interfere with the baroreceptor reflex. The acute hypotensive, bradycardic response to glycine was followed by a period during which glycine essentially eliminated the cardiovascular responses to the microinjection into the NTS of glutamate, an amino acid reputed to be a transmitter in the baroreceptor reflex arc. These data suggest that glycine is involved in cardiovascular regulation by the NTS but do not support its being an integral transmitter in the baroreceptor reflex.

Excitatory effect of amino acids on identified neuron R14 of Aplysia. II. Neutral amino acids and structure-activity relationships

The effects of 20 amino acids on identified neuron R14 of Aplysia kurodai were studied by conventional intracellular recording and voltage-clamp techniques. Neutral alpha-amino acids caused marked dose-dependent depolarizations of the neuron. The effects of several amino acids applied simultaneously were additive. L-isomers were much more effective than the corresponding D-isomers. R14 was at most slightly depolarized by perfusion with 2-aminoisobutyric acid and the non-alpha-amino acids beta-alanine and beta- and gamma-aminobutyric acid. Putative neurotransmitters ACh, 5-HT, dopamine, GABA, and taurine were much less effective in depolarizing R14 than neutral amino acids. Values of the Hill coefficient (nH) and the apparent dissociation constant (KA) were approximately 0.64 and 80 mM, respectively, for glycine. These results suggest that R14 has a general sensitivity to neutral amino acids and that amino acids in the hemolymph may be able to influence the electrical activity of R14.