GABAergic inhibition gates excitatory LTP in perirhinal cortex

Abstract

The perirhinal cortex (PRh) is a key region downstream of auditory cortex (ACx) that processes familiarity linked mnemonic signaling. In gerbils, ACx-driven EPSPs recorded in PRh neurons are largely shunted by GABAergic inhibition (Kotak et al., 2015, Frontiers in Neural Circuits, 9). To determine whether inhibitory shunting prevents the induction of excitatory long-term potentiation (e-LTP), we stimulated ACx-recipient PRh in a brain slice preparation using theta burst stimulation (TBS). Under control conditions, without GABA blockers, the majority of PRh neurons exhibited long-term depression. A very low concentration of bicuculline increased EPSP amplitude, but under this condition TBS did not significantly increase e-LTP induction. Since PRh synaptic inhibition included a GABA_B receptor-mediated component, we added a GABA_B receptor antagonist. When both GABA_A and GABA_B receptors were blocked, TBS reliably induced e-LTP in a majority of PRh neurons. We conclude that GABAergic transmission is a vital mechanism regulating e-LTP induction in the PRh, and may be associated with auditory learning.

Keywords: GABAA receptor; GABAB receptor; LTP; auditory cortex; perirhinal cortex.

Figure 1

ACx projection to PRh and brain slice preparation. A. The schematic (top) shows the ACx injection site for anatomical tracing with AAV-CaMKII-GFP, and the transverse section (bottom) shows the injection site. B. A low magnification image of the dense ACx terminal arborization in PRh (dashed white rectangle indicates region shown in panel C). rhinal fissure, rf. C. A high power image of ACx arbors within PRh L1. D. The schematic shows the ACx injection site for AAV-CaMKII-ChR2-mCh, and the projection to PRh. E. Brightfield (left) and fluorescent (right) images are shown at the PRh recording site. ChR2-expressing ACx fibers are labeled with mCherry. Asterisk, recording pipette. F. Blue light (470 nm)-evoked EPSPs were recorded in PRh in response to ChR2-expressing ACx fibers. G. The schematic shows the orientation of the ACx-PRh brain slice preparation (left) and the configuration of stimulation and recording sites (right). Medial geniculate nucleus, MG; hippocampus, Hipp. H. An ACx-evoked depolarizing PSP recorded in L1 of PRh. I. An L1-evoked PSP recorded in a PRh neuron. Note that optogenetically-evoked PSP amplitude was smaller than those recorded in response electrical stimulation.

Figure 2

GABAergic inhibition shunts depolarizing EPSPs recorded in PRh. A. A PRh L1-evoked PSP recorded in a PRh neuron without GABA receptor antagonists. Note a delayed hyperpolarizing response (arrow) B. When the PRh neuron was held at −50 mV, both short-latency GABA_A and delayed GABA_B (arrow) IPSP components were revealed. Note the small depolarizing PSP immediately after stimulus onset is largely shunted. C. In the presence of 2 μM BIC, the EPSP reached threshold to elicit an action potential. D. At a concentration of 1.5 μM BIC and 10 μM SCH, an evoked subthreshold EPSP could be recorded. E. An example of consecutive PRh neuron responses to L1-evoked TBS (arrowheads mark each stimulus pulse). Note that there was a gradual buildup of postsynaptic depolarization, leading to intense discharge in the PRh neuron. In this example, both BIC and SCH were in the bath.

Figure 3

GABAergic inhibition gates the induction of e-LTP. A. Example evoked responses are shown before (black) and after (red) TBS in a control neuron without GABA receptor antagonists. The time course for e-LTD in this neuron is shown in the middle panel. The summary distribution of normalized PSP change in all control neurons (n=13) is shown to the right. Dashed line is the mean percent change. B. Example evoked responses are shown before (black) and after (red) TBS in a neuron treated with a low concentration of BIC. The time course for e-LTP in this neuron is shown in the middle panel. The summary distribution shows the PSP change in all neurons treated with low BIC (n=18), and indicates that most neurons displayed e-LTD. The blue arrow indicates a neuron with a percent change of 140%. C. Example evoked responses are shown before (black) and after (red) TBS in a neuron treated with both low BIC and SCH. The time course for e-LTD in this neuron is shown in the middle. The summary distribution shows the PSP change in all neurons treated with low BIC and SCH (n=15), and indicates that most displayed e-LTP. The blue arrows indicate 4 neurons with a percent change of 137, 174, 201, and 206%. The distribution was significantly greater than those shown in panels A and B (red asterisk).