Tonic GABA_AReceptor-Mediated Currents of Human Cortical GABAergic Interneurons Vary Amongst Cell Types

Abstract

Persistent anion conductances through GABA A receptors (GABA A R) are important modulators of neuronal excitability. However, it is currently unknown how the amplitudes of these currents vary amongst different cell types in the human neocortex, particularly amongst diverse GABAergic interneurons. We have recorded 101 interneurons in and near layer 1 from cortical tissue surgically resected from both male and female patients, visualised 84 of them and measured tonic GABA A R currents in 48 cells with an intracellular [Cl -] of 65 mM and in the presence of 5 µM GABA. We compare these tonic currents amongst five groups of interneurons divided by firing properties and four types of interneuron defined by axonal distributions; rosehip, neurogliaform, stalked-bouton, layers 2-3 innervating and a pool of other cells. Interestingly, the rosehip cell, a type of interneuron only described thus far in human tissue, and layers 2-3 innervating cells exhibit larger tonic currents than other layer 1 interneurons, such as neurogliaform and stalked-bouton cells; the latter two groups showing no difference. The positive allosteric modulators of GABA A Rs allopregnanolone and DS2 also induced larger current shifts in the rosehip and layer 2-3 innervating cells, consistent with higher expression of the δ-subunit of the GABA A R in these neurons. We have also examined how patient parameters, such as age, seizures, type of cancer and anticonvulsant treatment may alter tonic inhibitory currents in human neurons.The cell type specific differences in tonic inhibitory currents could potentially be used to selectively modulate cortical circuitry. Significance statementTonic currents through GABA A receptors are a potential therapeutic target for a number of neurological and psychiatric conditions. Here we show that these currents in human cerebral cortical GABAergic neurons display cell-type specific differences in their amplitudes 4 which implies differential modulation of their excitability. Additionally, we examine whether the amplitudes of the tonic currents measured in our study show any differences between patient populations, finding some evidence that age, seizures, type of cancer, and anticonvulsant treatment may alter tonic inhibition in human tissue. These results advance our understanding of how pathology affects neuronal excitability and could potentially be used to selectively modulate cortical circuitry.

Differential effects of group III metabotropic glutamate receptors on spontaneous inhibitory synaptic currents in spine-innervating double bouquet and parvalbumin-expressing dendrite-targeting GABAergic interneurons in human neocortex

“…Further selective effects could emerge from the differential allocation of synaptic and extrasynaptic GABA A receptor species (Farrant & Nusser, 2005; Field et al, 2021), as well as from the location of pre- and postsynaptic GABA B receptors. Some GABAergic interneuron types, such as neurogliaform cells, have been shown to evoke both slow phasic and tonic GABA A receptor signalling through extrasynaptic GABA A and GABA B receptors (Tamás et al, 2003; Oláh et al, 2007; Oláh et al, 2009).…”

“…For samples from cases O-W (see table 2) the fixative also contained 0.05% (w/v) glutaraldehyde (GA). Next, slices were thoroughly washed and re-sectioned into 3-5, ∼60 µm thick sections on a vibratome as described previously (Field et al, 2021). From each slice two sections, including the one in which the soma was predicted to be found, were selected for fluorescent visualization.…”

“…Samples were collected between 10 am and 5 pm (UK time) as described previously (Bocchio et al, 2019; Field et al, 2021). A small (0.5–2 cm 3 ) block of neocortex was dropped into cold cutting artificial cerebrospinal fluid (ACSF) saturated with carbogen (95 % O 2 , 5 % CO 2 ), and divided into smaller blocks as necessary.…”

“…Samples were transported over 15-20 min to the laboratory in a sealed transport bottle. Slices of ∼350 µm thickness were prepared as described in Field et al (2021), except for samples A-C the cutting ACSF was gradually replaced by recording ACSF, in which they were stored at room temperature until recording. The recording ACSF contained the following (in mM): 130 NaCl, 3.5 KCl, 1.3 NaH 2 PO 4 , 24 NaHCO 3 , 3 CaCl 2 , 1.5 MgSO 4 , 12.5 glucose, (pH ∼7.3, ∼300 mOsm/L).…”

“…Electrophysiological recordings were performed over 10-16 hours after slicing between 12 PM and 4 AM the next day as described previously (Bocchio et al, 2019; Field et al, 2021). Slices in the recording chamber were perfused continuously with recording ACSF saturated with carbogen at a flow rate of ∼10 ml/min using a peristaltic pump (Gilson).…”