Circulating free amino acids in Aplysia californica

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.

Inhibition of the glutamate-induced K(+) current in identified Onchidium neurons by nitric oxide donors

Nitric oxide (NO) acts as a neurotransmitter and neuromodulator in the nervous system of many vertebrates and invertebrates. The effects of extracellularly applied sodium nitroprusside (SNP) and diethylamine NO (C(2)H(5))(2)N[N(O)NO]-Na(+) (DEA/NO), NO donors, on a glutamate (Glu)-induced K(+) current in identified Onchidium neurons were investigated using voltage clamp and pressure ejection techniques. Bath-applied SNP (10 microM) and DEA/NO (5-10 microM) reduced the Glu-induced K(+) current without affecting the resting membrane conductance and holding current. The Glu-induced K(+) current also was inhibited by the focal application of SNP to the neuron somata. The suppressing effects of NO donors were concentration-dependent and completely reversible. Pretreatment with hemoglobin (50 microM), a nitric oxide scavenger, and 1H-[1,2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 1 microM), a specific inhibitor of NO-stimulated guanylate cyclase, decreased the SNP-induced inhibition of the Glu-induced current. Bath-applied 50 microM 3-isobutyl-1-methylxanthine (IBMX), a nonspecific phosphodiesterase inhibitor, or intracellular injection of 1 mM guanosine 3',5'-cyclic monophosphate (cGMP) inhibited the Glu-induced current, mimicking the effect of NO donors. These results demonstrate that SNP and DEA/NO inhibit the Glu-induced K(+) current and that the mechanism of NO inhibition of the Glu-induced current involves cGMP-dependent protein kinase.

Inhibitory glutamate receptor channels

Inhibitory glutamate receptors (IGluRs) are a family of ion channel proteins closely related to ionotropic glycine and gamma-aminobutyric acid (GABA) receptors; They are gated directly by glutamate; the open channel is permeable to chloride and sometimes potassium. Physiologically and pharmacologically, IGluRs most closely resemble GABA receptors; they are picrotoxin-sensitive and sometimes crossdesensitized by GABA. However, the amino acid sequences of cloned IGluRs are most similar to those of glycine receptors. Ibotenic acid, a conformationally restricted glutamate analog closely related to muscimol, activates all IGluRs. Quisqualate is not an IGluR agonist except among pulmonate molluscs and for a unique multiagonist receptor in the crayfish Austropotamobius torrentium. Other excitatory amino acid agonists are generally ineffective. Avermectins have several effects on IGluRs, depending on concentration: potentiation, direct gating, and blockade, both reversible and irreversible. Since IGluRs have only been clearly described in protostomes and pseudocoelomates, these effects may mediate the powerful antihelminthic and insecticidal action of avermectins, while explaining their low toxicity to mammals. IGluRs mediate synaptic inhibition in neurons and are expressed extrajunctionally in striated muscles. The presence of IGluRs in a neuron or muscle is independent of the presence or absence of excitatory glutamate receptors or GABA receptors in the cell. Generally, extrajunctional IGluRs in muscle have a higher sensitivity to glutamate than do neuronal synaptic receptors. Some extrajunctional receptors are sensitive in the range of circulating plasma glutamate levels, suggesting a role for IGluRs in regulating muscle excitability The divergence of the IGlu/GABA/Gly/ACh receptor superfamily in protostomes could become a powerful model system for adaptive molecular evolution. Physiologically and pharmacologically, protostome receptors are considerably more diverse than their vertebrate counterparts. Antagonist profiles are only loosely correlated with agonist profiles (e.g., curare-sensitive GABA receptors, bicuculline-sensitive AChRs), and pharmacologically identical receptors may be either excitatory or inhibitory, and permeable to different ions. The assumption that agonist sensitivity reliably connotes discrete, homologous receptor families is contraindicated. Protostome ionotropic receptors are highly diverse and straightforward to assay; they provide an excellent system in which to study and integrate fundamental questions in molecular evolution and adaptation.