We have examined the onset and subsequent development of chemosensitivity in Rohon-Beard neurones from the Xenopus spinal cord. These cells become sensitive to bath-applied γ-aminobutyric acid (GABA) around stage 25 (early tailbud, about 1 d old), and remain so at least until stage 49 (9 d old). In contrast, a number of other neurotransmitter candidates tested caused no potential or conductance change during the same period. We examined ionophoretic dose-response relations of the cells at stage 26, a couple of hours after the first acquisition of GABA sensitivity. Sensitivities as high as 450 mV/nC were recorded. Comparable sensitivities were recorded between stages 46-49 (5-9 d old). Measurements of ionophoretic sensitivities and input resistances during several periods from stage 26 to maturity show that the underlying conductance change for a given GABA dose is likely to increase steadily during this time. A 'sensitivity index' (ionophoretic sensitivity/input resistance) was calculated, which is low at stage 26, higher at intermediate stages (stages 31-42), and highest for mature cells (stage 46-49; 5-9 d of development). The reversal potential of the ionophoretic GABA response is the same at stage 26 (-30 mV) as it is in mature cells. Ion substitution expriments show that Na+ and K+, but not Cl- or Ca2+, are involved in the response. GABA responses at stage 26 are pharmacologically similar to those of mature cells. The responses are blocked by 10 μM-picrotoxin or curare, and muscimol is an agonist in concentrations as low as 1 μM. GABA responses at stage 26 desensitize in a manner similar to that seen for mature cells, either with prolonged bath application of GABA or with repetitive ionophoretic application. Nearly half of the cells tested at stage 26 respond to glycine, in concentrations as low as 5 μM. This sensitivity is absent by 3 1/2 d of development. The responses of Rohon-Beard neurones to GABA are similar to those of other cells in that they involve a conductance increase, are mimicked by muscimol, and are blocked by picrotoxin. These responses are different in that they do not involve Cl- and are blocked by low concentrations of curare. Many of the characteristics of GABA receptors, i.e. the reversal potential, desensitization, and pharmacology, are constant during development. However, the sensitivity of the cells to GABA and the spectrum of transmitters to which they are sensitive appear to change.
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