The roles of somatostatin-expressing (GIN) and fast-spiking inhibitory interneurons in UP-DOWN states of mouse neocortex

Abstract

The neocortex contains multiple types of inhibitory neurons whose properties suggest they may play different roles within the cortical circuit. By recording from three cell types during two distinct network states (UP and DOWN states) in vitro, we were able to quantify differences in firing characteristics between these cells during different network regimes. We recorded from regular-spiking (RS) excitatory cells and two types of inhibitory neurons, the fast-spiking (FS) neurons and GFP- (and somatostatin-) expressing inhibitory neurons (GIN), in layer 2/3 of slices from mouse somatosensory neocortex. Comparisons of firing characteristics between these cells during UP- and DOWN-states showed several patterns. First, of these cell types, only GIN cells fired persistently during DOWN-states, whereas all three cell types fired readily during UP-states. Second, the onset of firing and distribution of action potentials throughout UP-states differed by cell type, showing that FS cell UP-state firing occurred preferentially near the beginning of the UP-state, whereas the firing of RS cells was slower to develop at the start of the UP-state, and GIN cell firing was sustained throughout the duration of the UP-state. Finally, membrane potential and spike correlations between heterogeneous cell types were more pronounced during UP-states and, in the case of RS synapses onto GIN cells, varied throughout the UP-state. These results suggest that there is a division of labor between FS and GIN cells as the UP-state progresses and suggest that GIN cells could be important in the termination of UP-states.

Fig. 1.

up- and down-states in regular spiking (RS), fast spiking (FS), and GFP- (and somatostatin-) expressing inhibitory neurons (GIN) cells. A: simultaneously recorded RS and GIN cells during application of low-divalent artificial cerebrospinal fluid (ACSF). RS cells fired during up-states only, whereas GIN cells fired during both up- and down-states. B: period during horizontal line in A enlarged to show a single up-state in both cell types. C: up-state recorded in an FS cell and a different RS cell from that shown in A and B. In all 3 panels, action potentials are truncated for display purposes. Vertical dotted lines in B and C indicate start and end times of the up-states, as defined using the detection algorithm described in methods. In both cases, the RS cell was used to determine the up-state beginning and end.

Fig. 2.

up-state frequencies and duration by age. A: mean up-state duration decreased linearly with age from P12 to P17. Error bars represent ±SE. B: up-state frequencies in up-states per minute. Significant differences indicated with asterisks (P < 0.05). Number of cells analyzed: P12, 3; P13, 11; P14, 8; P15, 11; P16, 3; P17, 6.

Fig. 3.

Relationships between sequential interspike intervals indicate the degree of rhythmicity of firing patterns. A: duration of interspike interval i plotted against duration of interspike interval i + 1 for each cell type and state during which firing was observed. Plots incorporate all observed interspike intervals across all recorded cells. Solid lines indicate unity. B: median normalized distance from unity line for interspike interval pairs. All median values were significantly different from one another (P < 0.001). C: median CV2 ratios across cells for each type and state for which firing occurred. Median values were all significantly different from one another (P < 0.05). Whiskers on box-and-whisker plots indicate upper and lower quartile ranges (highest and lowest 25%); boxes indicate the interquartile range (the middle 50%), and the notches on the boxes indicate 95% CIs for the median. Number of cells used in each analysis: RS, 20; FS, 12; GIN, 21.

Fig. 4.

Time to 1st spike during a given up-state differs according to cell type. A: cumulative distribution plots of times in milliseconds to the 1st spike in an up-state averaged for each cell. B: median times to 1st spike for each cell type. FS median is significantly different from RS and GIN (P < 0.001). Conventions for box-and-whisker plots as in Fig. 3. C: times to 1st spike in up-states for the neuronal pair types indicated (RS-GIN, n = 14 pairs, 166 up-states; RS-FS, n = 3 pairs, 62 up-states; FS-GIN, n = 5 pairs, 111 up-states). Note that times to 1st spike that were >2 s were not included in graphs in C. Solid lines indicate unity.

Fig. 5.

Distribution of firing before, during, and after up-states. Firing rates are shown for the 5 s before and after up-states (left and right, respectively) and for up-states themselves (middle). up-states were normalized in time by up-state durations, i.e., spike times were calculated as a percent of the total up-state duration. Solid lines indicate means and dotted lines/shading indicate 95% CIs. Number of cells analyzed: RS, 20; FS, 12; GIN, 21.

Fig. 6.

Sub- and suprathreshold cross-correlations for each type of cell pair. A1–A3: subthreshold cross-correlations for RS-GIN (number of pairs = 14; number of up-states = 239), RS-FS (number of pairs = 3; number of up-states = 77), and FS-GIN (number of pairs = 5; number of up-states = 121) pairs, respectively, during down-states. Black lines indicate means of data and shuffled data. Dark and light shading indicate 95% CIs for nonshuffled data and shuffled data, respectively. B1–B3: action potential cross-correlations for the same 3 cell pairings as in A [number of spikes for RS-GIN pairs = 1,764 (RS) and 1,421 (GIN); number of spikes for RS-FS pairs: 1,079 (RS) and 10,471 (FS); number of spikes for FS-GIN pairs = 7,787 (FS) and 2,704 (GIN)]. C1–C3: spike cross-correlations for each heterogeneous cell pairing. CIs were not calculated for RS-FS pairs because n = 3 for this pairing. Number of pairs for RS-GIN = 14; number of pairs for FS-GIN = 5.

Fig. 7.

Spike-triggered averages for heterogeneous cell pairings during up-states. A–F indicate spike-triggered average membrane potentials for each cell pairing during up-states. First column is spike triggered averages for the entire up-state; 2nd column is for presynaptic action potentials that occurred during the 1st 3rd of the up-state; 3rd column is for presynaptic action potentials that occurred during the last 3rd of the up-states. Lines and shading as in Fig. 6. Numbers of pairs analyzed (number of spikes in whole up-states, 1st 3rd and last 3rd): GIN → RS: 4 (889, 336, 215); RS → GIN: 3 (322, 107, 34); FS → RS: 1 (2,169, 738, 324); RS → FS: 3 (503, 204, 67); GIN → FS: 5 (5,393, 1,629, 2,286); FS → GIN: 5 (6,056, 2,581, 1,089).