Latency-related development of functional connections in cultured cortical networks

Abstract

To study plasticity, we cultured cortical networks on multielectrode arrays, enabling simultaneous recording from multiple neurons. We used conditional firing probabilities to describe functional network connections by their strength and latency. These are abstract representations of neuronal pathways and may arise from direct pathways between two neurons or from a common input. Functional connections based on direct pathways should reflect synaptic properties. Therefore, we searched for long-term potentiation (this mechanism occurs in vivo when presynaptic action potentials precede postsynaptic ones with interspike intervals up to approximately 20 ms) in vitro. To investigate if the strength of functional connections showed a similar latency-related development, we selected periods of monotonously increasing or decreasing strength. We observed increased incidence of short latencies (5-30 ms) during strengthening, whereas these rarely occurred during weakening. Furthermore, we saw an increased incidence of 40-65 ms latencies during weakening. Conversely, functional connections tended to strengthen in periods with short latency, whereas strengthening was significantly less than average during long latency. Our data suggest that functional connections contain information about synaptic connections, that conditional firing probability analysis is sensitive enough to detect it and that a substantial fraction of all functional connections is based on direct pathways.

Figure 1

(A) MEA, used to record neuronal activity in cultured networks of cortical neurons. It is based on a glass substrate (5 × 5 cm) with 61 embedded electrodes in the center of the chamber, hexagonally spaced at a distance of 70 μm of each other. The glass ring (diameter 30 mm) glued on top was filled with glia-conditioned growth medium and firmly sealed. (B) Close up of one of the electrodes and several neurons. Electrode diameter, 12 μm. Most electrodes did not pick up signals from more than one neuron.

Figure 2

Example of estimated CFP (○, mean ± SD of five consecutive bins of 0.5 ms each). Solid line represents least mean square fit of Eq. 1. This fit is used to obtain values for strength (M_i,j) and latency (T_i,j) of the functional connection between a pair of electrodes (i,j).

Figure 3

Example of the development of strength (M_i,j) and latency (T_i,j) of a functional connection. A long-term recording was divided into data blocks. In each data block, strengths and latencies of functional connections between all pairs of active electrodes were determined. If a pair was not related in a data block, strength was set to zero. Only electrode pairs that were related in at least 100 data blocks were included for analysis. M-graphs were smoothed using a moving average filter to highlight longer-term trends (solid line). The filter averaged each point of the curves with its five neighbors on both sides. Thus, we observed the development of functional connections between pairs of electrodes. Shaded area at 31 DIV indicates a period >1 day of monotonous strengthening. Note also the period of monotonous weakening between 32 and 34 DIV.

Figure 4

Latency distribution of functional connections in periods of monotonous strengthening (▴) or weakening (○) with a minimum duration of one day (A), or 10 h (B). In graph A, latencies of 5 n ± 2.5 ms were pooled (n = 1…20). Graph B: similar, but now mean ± SD of two consecutive bins is shown. Data from all cultures were pooled for these curves. The figure shows that the effects are comparable, but less pronounced using the shorter threshold period length of monotonous strengthening or weakening.

Figure 5

Number of periods during which functional connections had a latency in the range 5–30 ms (N_5–30, ▴) or 40–65 ms (N_40–65, ж) as a function of the chosen threshold period length. For comparison the number of periods with the required length but without latency restrictions is also shown (N_all, ○). All periods had to contain at least 10 data blocks. At all minimum lengths we found N_40–65<N_5–30<N_all. At 24 h, N_40–65 was 35; at 48 h, it equaled 14.

Figure 6

Differences in development of the strength of functional connections between periods with latencies of 5–30 ms (▴), 40–65 ms (ж), or without latency restrictions (○). A shows the relative strengthening per hour, as a function of the chosen threshold period length. B shows the percentage of functional connections with increasing strength. Bold line: equilibrium between increasing and decreasing functional connections.

Figure 7

Strengthening during periods of low latency (5–30ms, ▴) or high latency (40–65 ms, ●), as a function of the initial strength of functional connections. A depicts strengthening as a percentage of the initial strength. The curves show that strengthening decreased with increasing initial strength and eventually even changed into weakening. However, B shows that the number of functional connections decreased rapidly with increasing initial strength. Weakening during short latency occurred infrequently as there were only 20 functional connections (1.3%) with initial strength >0.003.