Postnatal maturation of somatostatin-expressing inhibitory cells in the somatosensory cortex of GIN mice

Abstract

Postnatal inhibitory neuron development affects mammalian brain function, and failure of this maturation process may underlie pathological conditions such as epilepsy, schizophrenia, and depression. Furthermore, understanding how physiological properties of inhibitory neurons change throughout development is critical to understanding the role(s) these cells play in cortical processing. One subset of inhibitory neurons that may be affected during postnatal development is somatostatin-expressing (SOM) cells. A subset of these cells is labeled with green-fluorescent protein (GFP) in a line of mice known as the GFP-positive inhibitory neurons (GIN) line. Here, we studied how intrinsic electrophysiological properties of these cells changed in the somatosensory cortex of GIN mice between postnatal ages P11 and P32+. GIN cells were targeted for whole-cell current-clamp recordings and ranges of positive and negative current steps were presented to each cell. The results showed that as the neocortical circuitry matured during this critical time period multiple intrinsic and firing properties of GIN inhibitory neurons, as well as those of excitatory (regular-spiking [RS]) cells, were altered. Furthermore, these changes were such that the output of GIN cells, but not RS cells, increased over this developmental period. We quantified changes in excitability by examining the input-output relationship of both GIN and RS cells. We found that the firing frequency of GIN cells increased with age, while the rheobase current remained constant across development. This created a multiplicative increase in the input-output relationship of the GIN cells, leading to increases in gain with age. The input-output relationship of the RS cells, on the other hand, showed primarily a subtractive shift with age, but no substantial change in gain. These results suggest that as the neocortex matures, inhibition coming from GIN cells may become more influential in the circuit and play a greater role in the modulation of neocortical activity.

Keywords: inhibition; postnatal development; somatosensory cortex; somatostatin.

Figure 1

Responses of GIN and RS cells to current steps. (A) Voltage responses to current steps injected into a P11 GIN cell. (B) Voltage responses to current steps injected into a P32+ GIN cell. (C) Top: Voltage responses to current steps injected into a P11 RS cell. Bottom: Current steps representing −100, −60, −20, and +100 pA current steps, each of 600 ms duration. The same amplitude current steps were used for A–D. (D) Voltage responses to current steps injected into a P32+ RS cell.

Figure 2

Intrinsic properties of GIN cells and RS cells change over development. (A) Average input resistance of GIN cells (black) and RS cells (gray) decreases as a function of age. (B) The average membrane time constant also decreases with age in both the GIN cells (black) and the RS cells (gray). (C) The rheobase current increases from P11 to P12 in the GIN cells, then remains constant into maturity (black). The rheobase current in the RS cells increases significantly with age (gray). Data for each cell type were fit with an exponential function (A and B) or a sigmoidal function (C).

Figure 3

Sag currents increase in GIN cells with age but decrease in RS cells. (A) Top: Average sag current traces for GIN cell traces (left) and RS cell (right) by age. The colors correspond to the following ages: P11 (red), P13 (orange), P15 (green), P17 (blue), and P32+ (black). Bottom: Response of a GIN cell to a hyperpolarizing current step. The dotted box denotes the region illustrated above in (A). The arrows describe the sag magnitude [shown in (B)] and the rebound depolarization [shown in (C)]. For descriptions of how these quantities were measured, see Materials and Methods. (B) The magnitude of the sag current in GIN cells increases with age (black), whereas the sag current decreases with age in the RS cells (gray). (C) The rebound depolarization also increases in GIN cells (black) and decreases in RS cells, following a similar time course as the sag current (gray). Data for each cell type were fit with a sigmoidal function (B and C).

Figure 4

Action potential properties change as a function of age in GIN and RS cells. (A) Across development, action potential half-width decreases in GIN (black) and RS cells (gray). (B) Left: Overlaid action potential waveforms of a P11 GIN cell (red) and a P32+ GIN cell (black). Dashed black box indicates region shown above, which illustrates the change in the AHP with age in GIN cells. Right: Overlaid action potential waveforms of a P11 RS cell (red) and a P32+ RS cell (black). Dashed black box indicates region shown above, which illustrates the change in the afterhyperpolarization (AHP) with age in RS cells. The colors correspond to the following ages: P11 (red), P13 (orange), P15 (green), P17 (blue), and P32+ (black). (C) Quantification of AHP magnitude (measured as the most negative point of the AHP, minus the action potential threshold) for GIN cells (black) and RS cells (gray). (D) Quantification of the AHP slope for GIN cells (black) and RS cells (gray). Data for each cell type were fit with a sigmoidal function (C and D; black) or a second degree polynomial (C and D; gray).

Figure 5

GIN cells show increased firing rates with age. (A) Instantaneous firing rate across a 200 pA current step of 600 ms duration for GIN cells. (B) Instantaneous firing rate across a 200 pA current step of 600 ms duration for RS cells. (C) The adaptation ratio (first ISI/average of last three ISIs) decreased slightly with age in GIN cells (black) but did not change in RS cells (gray). (D) The frequency/current (f/I) curve of GIN cells calculated from 20 to 300 pA across age showed a multiplicative increase. (E) The f/I curve of RS cells calculated from 20 to 300 pA across age shows primarily a rightward shift. (F) The gain (calculated as the slope of the linear portion of the f/I curve) for GIN cells (black) increased with age, but not in RS cells (gray). The colors in A, B, D, and E correspond to the following ages: P11 (red), P13 (orange), P15 (green), P17 (blue), and P32+ (black). Data for each cell type were fit by a linear fit (C, black; F, gray), a second degree polynomial (C, gray), or a sigmoidal function (F, black).