Korean J Physiol Pharmacol.  2018 Nov;22(6):721-729. 10.4196/kjpp.2018.22.6.721.

Regional difference in spontaneous firing inhibition by GABA(A) and GABA(B) receptors in nigral dopamine neurons

• Kim Y
• Jang J
• Kim HJ
• Park MK

• KMID: 2430095
• DOI: http://doi.org/10.4196/kjpp.2018.22.6.721

Abstract

GABAergic control over dopamine (DA) neurons in the substantia nigra is crucial for determining firing rates and patterns. Although GABA activates both GABA(A) and GABA(B) receptors distributed throughout the somatodendritic tree, it is currently unclear how regional GABA receptors in the soma and dendritic compartments regulate spontaneous firing. Therefore, the objective of this study was to determine actions of regional GABA receptors on spontaneous firing in acutely dissociated DA neurons from the rat using patch-clamp and local GABA-uncaging techniques. Agonists and antagonists experiments showed that activation of either GABA(A) receptors or GABA(B) receptors in DA neurons is enough to completely abolish spontaneous firing. Local GABA-uncaging along the somatodendritic tree revealed that activation of regional GABA receptors limited within the soma, proximal, or distal dendritic region, can completely suppress spontaneous firing. However, activation of either GABA(A) or GABA(B) receptor equally suppressed spontaneous firing in the soma, whereas GABA(B) receptor inhibited spontaneous firing more strongly than GABA(A) receptor in the proximal and distal dendrites. These regional differences of GABA signals between the soma and dendritic compartments could contribute to our understanding of many diverse and complex actions of GABA in midbrain DA neurons.

Keyword

Dopamine neuron; GABA; GABA receptor distribution; Somatodendritic balance; Spontaneous firing inhibition

MeSH Terms

Animals
Carisoprodol
Dendrites
Dopamine*
Dopaminergic Neurons*
Fires*
gamma-Aminobutyric Acid
Mesencephalon
Neurons
Rats
Receptors, GABA
Receptors, GABA-A
Substantia Nigra
Trees
Carisoprodol
Dopamine
Receptors, GABA
Receptors, GABA-A
gamma-Aminobutyric Acid

Figure

• 

  Fig. 1 Dose dependence of GABA-induced inhibition of spontaneous firing in SNc DA neurons. (A and B) Identification of SNc DA neurons. Transmitted and immunofluorescence images of an acutely isolated DA neuron. Expression of tyrosine hydroxylase (TH) was confirmed by staining with THantibody. Spontaneous firing activity (b: cell-attached current-clamp mode, c: whole cell current-clamp mode) was recorded from acutely isolated DA neuron (B-a). DA neurons showed a large sag in voltage response to hyperpolarizing current steps (from - 800 pA with 200 pA increases for 500 ms). (C) Bath application of GABA (0.3 µM to 10 µM) inhibited spontaneous firing activity in a dose-dependent manner. (D) Dose-response curve of GABA-induced inhibition of firing rate (IC50=0.87 µM, n=18).

• 

  Fig. 2 GABA-induced currents by activation of GABAA and GABAB receptors in SNc DA neurons. Activation of GABAA and GABAB receptors in DA neurons evoked inward and outward currents at −50 mV, respectively. (A) Typical current responses to isoguvacine hydrochloride (a specific GABAA agonist, 20 µM) and (R)-Baclofen (a specific GABAB agonist, 100 µM). (B) Comparison of GABA, isoguvacine, and baclofen mediated currents (n=7). (C) GABA (10 µM) mediated current in the presence of GABAB receptor antagonist (CGP55845, 2 µM) or GABAA receptor antagonist (SR95531, 5 µM). Inset: 10 µM GABA induced inward current at −50 mV. (D) GABA-induced currents in the presence of GABA receptor antagonists are summarized (n=5). *p<0.05; ***p<0.001 by t-test.

• 

  Fig. 3 Inhibition of spontaneous firing by activation of GABAA or GABAB receptors. (A) Typical firing traces recorded from acutely dissociated DA neurons. Isoguvacine (20 µM) and (R)-Baclofen (100 µM) applications completely inhibited spontaneous firing activity of DA neurons. (B) Effects of GABAA agonist and GABAB agonist on spontaneous firing are summarized (n=9). (C) GABA induced firing inhibitions in the presence of CGP55845 (2 µM) or SR95531 (5 µM). (D) CGP55845 (2 µM) and SR95531 (5 µM) did not alter the effect of GABA (n=5). **p<0.01 by two sample t-test.

• 

  Fig. 4 Inhibition of spontaneous firing by caged-GABA uncaging on the soma and dendrites in DA neurons. (A) Transmitted image of SNc DA neuron with uncaging sites of (O)-CNB caged-GABA (20 µM). (B) Inhibition of spontaneous firing by uncaging of caged-GABA on the soma, proximal dendrite, and distal dendrite, respectively. Red areas indicated the duration of firing inhibition by caged-GABA uncaging (red triangles) (C) Duration of spontaneous firing inhibition by local caged-GABA uncaging is summarized (n=5). n.s., nonsignificant.

• 

  Fig. 5 Regionally different contribution of GABAA and GABAB receptors in the soma and dendrites to firing inhibition of SNc DA neurons. (A) Transmitted image of DA neuron with location of uncaging sites. (B) Spontaneous firing inhibited by caged-GABA uncaging in the soma, proximal dendrite, and distal dendrite (control). CGP55845 (2 µM), a GABAB receptor specific antagonist, significantly blocked GABA effects in the proximal and distal dendrite. In the soma, GABAA and GABAB receptor antagonists elicited partial block of GABA-induced firing inhibition. Spontaneous firing inhibitions by activation of GABAA and GABAB receptors in three different regions are summarized. *p<0.05; **p<0.01; ***p<0.001 by t-test, n.s., nonsignificant.

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