Characterization of a Prawn OA/TA Receptor in Xenopus Oocytes Suggests Functional Selectivity between Octopamine and Tyramine
Figure 4
Tyramine (TA) selectively evokes an intracellular process that limits the response amplitude.
(A) A concentration series applied to a single oocyte. Color-coding corresponds to the same responses examined in more detail in B and C. (B) The low amplitude of the TA response was not due to cumulative buildup of desensitization because it could be observed upon the first high-concentration application. An initial 100 µM TA response (black) is aligned by the stimulus application (black bar) with a second 100 µM response (grey). The plateau amplitude changes very little between the first and second response indicating that the small size is not a function of repetitive application. Desensitization of the second response is apparent in the tail as a reduction in amplitude, and as a loss of minor oscillation. (C) The plateau limits he amplitude of the TA response. Responses from A (color coded from 0.1 µM to 1000 µM) are aligned by the response onset and overlaid. The rank-order of rise rate at the initial rise (asterisk) and amplitudes at points indicated are listed from lowest to highest. The rise rate appears to be a direct function of concentration. The tail amplitude after the second rise is also a direct function of concentration, whereas the plateau amplitude becomes an inverse function at higher concentrations. This indicates that rise rate and plateau amplitude are distinct functions of concentration. (D) A quantitative analysis of the observations in C shows that the process underlying the plateau is specific to TA. The scatter-plot shows the amplitude of each individual response plotted against its maximum rise-rate. Data include all responses from experiments plotted in Figure 3C. The Spearman rank-order correlation (rS) between rise rate and peak amplitude for TA evoked responses is relatively weak (rS = 0.52, p = 0.046), whereas for octopamine (OA) (rS = 0.93, p = 2e–7) and dopamine (DA) (rS = 0.97, p = 2e–7) it is strong. These correlations differ significantly between TA and the other two amines (TA vs OA, p = 0.002; TA vs DA, p = 0.0006) but not between OA and DA (OA vs DA, p = 0.382) (Fisher's z transformation for correlation coefficients, two-tailed Student's t-test). Each measurable response to an application of TA (4 oocytes, n = 15 responses), OA (6 oocytes, n = 24 responses), or DA (5 oocytes, n = 14 responses) was treated as an independent sample.