Dopamine and serotonin modulate human GABAρ1 receptors expressed in Xenopus laevis oocytes

Abstract

GABAρ1 receptors are highly expressed in bipolar neurons of the retina and to a lesser extent in several areas of the central nervous system (CNS), and dopamine and serotonin are also involved in the modulation of retinal neural transmission. Whether these biogenic amines have a direct effect on ionotropic GABA receptors was not known. Here, we report that GABAρ1 receptors, expressed in X. laevis oocytes, were negatively modulated by dopamine and serotonin and less so by octopamine and tyramine. Interestingly, these molecules did not have effects on GABA(A) receptors. 5-Carboxamido-tryptamine and apomorphine did not exert evident effects on any of the receptors. Schild plot analyses of the inhibitory actions of dopamine and serotonin on currents elicited by GABA showed slopes of 2.7 ± 0.3 and 6.1 ± 1.8, respectively, indicating a noncompetitive mechanism of inhibition. The inhibition of GABAρ1 currents was independent of the membrane potential and was insensitive to picrotoxin, a GABA receptor channel blocker and to the GABAρ-specific antagonist (1,2,5,6-tetrahydropyridine-4-yl)methyl phosphinic acid (TPMPA). Dopamine and serotonin changed the sensitivity of GABAρ1 receptors to the inhibitory actions of Zn(2+). In contrast, La(3+) potentiated the amplitude of the GABA currents generated during negative modulation by dopamine (EC(50) 146 μM) and serotonin (EC(50) 196 μM). The functional role of the direct modulation of GABAρ receptors by dopamine and serotonin remains to be elucidated; however, it may represent an important modulatory pathway in the retina, where GABAρ receptors are highly expressed and where these biogenic amines are abundant.

Keywords: 5-HT; GABAA; GABAρ1; Xenopus oocyte; dopamine; receptor modulation.

Figure 1

Effects of the biogenic amines on GABA-currents in oocytes expressing GABAρ1 receptors. (A) Effect of biogenic amines at several concentrations upon currents activated by 3.5 μM GABA. (B) Monoamine concentration–response relationships showing the IC₅₀ for each one. Data were normalized to the maximal GABA response (3.5 μM) of each oocyte (C) DA and 5-HT did not modulate GABA-A receptors. DA (■), 5-HT (●), Tyra (▲), and Octo (▼). Data points are the means ± SEM obtained from at least 9 oocytes (n = 9) from 5 different frogs (N = 5).

Figure 2

Competition assays. (A) Sample currents and (B) concentration–response relationships of GABA-current modulation by DA (105, 210, 400, and 750 μM) in oocytes expressing GABAρ1. DA concentration–response curves of currents elicited by 1.7, 3.5, 7, and 21 μM GABA. Notice that DA did not shift the GABA dose–responses curve (P > 0.05). Data points are the means ± SEM obtained from at least 9–11 oocytes from 5 frogs.

Figure 3

Competition assays. (A) Sample currents and (B) concentration–response relation of GABA-current modulation by 5-HT (140, 340, 680, and 900 μM) in oocytes expressing GABAρl (C) 5-HT concentration–response curves of currents elicited by 1.7, 3.5, 7, and 21 μM GABA Notice that 5-HT did not affect the GABA dose–responses curve (P > 0.05). Data points are the means ± SEM obtained from at least 5 oocytes (n = 4–7) from 5 frogs (N = 5).

Figure 4

Effect of the GABAρ receptor antagonist TPMPA on the GABA-elicited currents modulated by DA. (A) Sample records showing that TPMPA effectively and reversibly blocked the currents generated by 3.5 μM GABA and those modulated by 105, 210, and 750 μM DA. Note that the coapplication of DA concentration, higher than 105 μM, and TPMPA enhanced the inhibitory effect. (B) TPMPA antagonism on currents elicited by coapplication of GABA and either DA at 105, 210, and 750 μM. Data points are the means ± SEM from 6 oocytes (n = 6) from 5 frogs (N = 3).

Figure 5

Effect of the GABAρ receptor antagonist TPMPA on the GABA-elicited currents modulated by 5-HT. (A) Sample records showing that TPMPA effectively and reversibly blocked the currents generated by 3.5 μM GABA and modulated by 175, 340, and 900 μM 5-HT. (B) TPMPA antagonism on currents elicited by coapplication of GABA and either 5-HT at 175, 340, and 900 μM. The currents were adjusted to the amplitude the maximum current for each curve. Data points are the means ± SEM from 6 oocytes (n = 6) from 5 frogs (N = 5).

Figure 6

Current–voltage relationships for GABA-elicited currents modulated by DA and 5-HT at indicated concentrations. Note that in all cases the current–voltage relationship is linear and not affected by DA or 5-HT (A). The extent of inhibition is the same at −120 and −60 mV (B). Data points are the means ± SEM obtained from at least 9 oocytes (n = 9) from 5 frogs (N = 5).

Figure 7

Effect of Picrotoxin on the GABA-elicited currents modulated by DA and 5-HT. (A) Sample records showing that Ptx reversibly blocked the currents generated by 3.5 μM GABA and modulated by 210 μM DA and 340 μM 5-HT. Note that the coapplication of the biogenic amines and Ptx did not enhance the inhibitory effect. (B) Picrotoxin antagonism of the currents elicited by coapplication of GABA and either DA or 5-HT. The currents were normalized to the maximum amplitude elicited by GABA+DA or GABA+5HT in absence of Ptx. Data points are the means ± SEM from at least 8 oocytes (n = 8) from 5 frogs (N = 5).

Figure 8

Lanthanum modulation. (A) Sample records illustrating the activation of GABAρ1 receptors and the potentiation of GABA-currents by La³⁺ and their modulation by DA and 5-HT. (B) La³⁺ concentration–response relations of the currents generated by 3.5 μM GABA and modulated by 210 μM DA and 340 μM 5-HT. Observe the blocking effect of DA and 5-HT on the GABA-elicited current in presence of La³⁺. The currents were normalized to the amplitude of that elicited by 3.5 μM GABA alone. Data points are the means ± SEM from at least 8 oocytes (n = 8) from 5 frogs (N = 5).

Figure 9

Zinc inhibition. (A) Inhibition of the currents elicited by GABA alone and modulated by DA and 5-HT. The coapplication of either DA or 5-HT with Zn²⁺ enhances the inhibitory effect. (B) Concentration–response relationships of the currents elicited by 3.5 μM GABA and modulated by 210 μM DA and 340 μM 5-HT. Note the displacement of the curves in the presence of DA or 5-HT. The currents were normalized to the amplitude of that elicited by GABA+DA or GABA+5HT in absence of Zn²⁺. Data points are the means ± SEM from at least 7 oocytes (n = 7) from 5 frogs (N = 5).