In vivo electroretinographic studies of the role of GABAC receptors in retinal signal processing

Abstract

All three classes of receptors for the inhibitory neurotransmitter GABA (GABAR) are expressed in the retina. This study investigated roles of GABAR, especially GABACR (GABA(A)-ρ), in retinal signaling in vivo by studying effects on the mouse electroretinogram (ERG) of genetic deletion of GABACR versus pharmacological blockade using receptor antagonists. Brief full-field flash ERGs were recorded from anesthetized GABACR−/− mice, and WT C57BL/6 (B6) mice, before and after intravitreal injection of GABACR antagonists, TPMPA, 3-APMPA, or the more recently developed 2-AEMP; GABAAR antagonist, SR95531; GABABR antagonist, CGP, and agonist, baclofen. Intravitreal injections of TPMPA and SR95531 were also made in Brown Norway rats. The effect of 2-AEMP on GABA- induced current was tested directly in isolated rat rod bipolar cells, and 2-AEMP was found to preferentially block GABACR in those cells. Maximum amplitudes of dark (DA) and light-adapted (LA) ERG b-waves were reduced in GABACR−/− mice, compared to B6 mice, by 30–60%; a-waves were unaltered and oscillatory potential amplitudes were increased. In B6 mice, after injection of TPMPA (also in rats), 3-APMPA or 2-AEMP, ERGs became similar to ERGs of GABACR−/− mice. Blockade of GABAARs and GABABRs, or agonism of GABABRs did not alter B6 DA b-wave amplitude. The negative scotopic threshold response (nSTR) was slightly less sensitive in GABACR−/− than in B6 mice, and unaltered by 2-AEMP. However, amplitudes of nSTR and photopic negative response (PhNR), both of which originate from inner retina, were enhanced by TPMPA and 3-APMPA, each of which has GABAB agonist properties, and further increased by baclofen. The finding that genetic deletion of GABACR, the GABACR antagonist 2-AEMP, and other antagonists all reduced ERG b-wave amplitude, supports a role for GABACR in determining the maximum response amplitude of bipolar cells contributing to the b-wave. GABACR antagonists differed in their effects on nSTR and PhNR; antagonists with GABAB agonist properties enhanced light-driven responses whereas 2-AEMP did not.

Contribution of GABAa, GABAc and glycine receptors to rat dark-adapted oscillatory potentials in the time and frequency domain

“…We also found that blocking GABAc receptors increased the amplitude of OPs and delayed the time-to-peak of OPs, which is in agreement with previous reports in both rats and mice [ 34 – 36 ]. However, other groups have suggested that GABAc receptors are minimally involved with OPs [ 32 ].…”

“…Here, we showed that inhibiting GABAa by intravitreal injection of SR significantly decreased the amplitude of the later OPs, although it also slightly increased the amplitude of the b-wave. In mice, GABAa-blockade by SR causes complete attenuation of OPs, except for an OP1-like wavelet [ 34 ], and it has been reported that SR delays the time-to-peak of the b-wave and increases the amplitude of the b-wave [ 55 ]. We suggest that the differences between our b-wave findings and previous reports may result from use of different animal models (rats vs. mice).…”

“…The fundus was examined using direct ophthalmoscopy. As described previously [ 34 ], we made solutions of phosphate buffered saline containing 20 mM GABA, 4 mM 2-amino-4-phosphonobutyric acid (APB), 100 μM (1,2,5,6-tetrahydropyridin-4-yl) methylphosphinic acid (TPMPA), 5 μM tetrodotoxin (TTX), 100 μM strychnine (STR), 1mM isoguvacine (ISO) and 100 μM SR95531 (SR). 2 μL of prepared solution was delivered to the vitreous using a Hamilton microsyringe, under dim red light (λ > 620 nm), and the same volume of phosphate-buffered saline (PBS) was injected into the other eye as a vehicle control.…”

“…Three subtypes of GABA receptor – GABAa, GABAb and GABAc – have been found in the retina [ 33 ]. Either antagonism or agonism of GABAa can remove OPs [ 32 , 34 , 35 ]. Some studies have shown that GABAb and GABAc receptor blockade has no significant effect on OPs in rats [ 32 ], but others report that OPs are increased after blocking GABAc receptors in rats [ 34 , 35 ] and in GABAc receptor-knockout mice [ 36 ].…”

Altered visual function in a larval zebrafish knockout of neurodevelopmental risk genepdzk1

“…If enhanced behavioural contrast sensitivity in pdzk1-KO zebrafish arises from deficits of inhibitory mechanisms, altered bipolar cell responses underlying the b-wave may be modulated by a more generalised change in retinal inhibition. In rodents, antagonist blockage or genetic knockout of the GABAc receptor, a key inhibitory receptor on bipolar cells, leads to a reduced ERG b-wave but increased spontaneous and light-evoked activity in retinal ganglion cells[39][40][41]. How PDZK1 might be involved in the crosstalk between bipolar, amacrine and ganglion cells remains to be investigated.While the observed b-wave deficit points to bipolar or third-order retinal dysfunction, there could be other potential causes.…”