Long-term Relationships between Cholinergic Tone, Synchronous Bursting and Synaptic Remodeling

Abstract

Cholinergic neuromodulation plays key roles in the regulation of neuronal excitability, network activity, arousal, and behavior. On longer time scales, cholinergic systems play essential roles in cortical development, maturation, and plasticity. Presumably, these processes are associated with substantial synaptic remodeling, yet to date, long-term relationships between cholinergic tone and synaptic remodeling remain largely unknown. Here we used automated microscopy combined with multielectrode array recordings to study long-term relationships between cholinergic tone, excitatory synapse remodeling, and network activity characteristics in networks of cortical neurons grown on multielectrode array substrates. Experimental elevations of cholinergic tone led to the abrupt suppression of episodic synchronous bursting activity (but not of general activity), followed by a gradual growth of excitatory synapses over hours. Subsequent blockage of cholinergic receptors led to an immediate restoration of synchronous bursting and the gradual reversal of synaptic growth. Neither synaptic growth nor downsizing was governed by multiplicative scaling rules. Instead, these occurred in a subset of synapses, irrespective of initial synaptic size. Synaptic growth seemed to depend on intrinsic network activity, but not on the degree to which bursting was suppressed. Intriguingly, sustained elevations of cholinergic tone were associated with a gradual recovery of synchronous bursting but not with a reversal of synaptic growth. These findings show that cholinergic tone can strongly affect synaptic remodeling and synchronous bursting activity, but do not support a strict coupling between the two. Finally, the reemergence of synchronous bursting in the presence of elevated cholinergic tone indicates that the capacity of cholinergic neuromodulation to indefinitely suppress synchronous bursting might be inherently limited.

Adaptation to prolonged neuromodulation in cortical cultures: an invariable return to network synchrony

“…Pulsed ACh applications effectively suppress network synchrony, but synchrony eventually reemerges Under baseline conditions, and in agreement with many reports [27,[30][31][32][33][34][35][36]70], spontaneous activity in the cortical networks used here occurs as periods of synchronous, network-wide bursting activity which lasts for several hundreds of milliseconds, separated by longer periods (1 to 10 seconds) of near-complete quiescence or sparse, asynchronous action potentials ( Figure 2a). As mentioned in the introduction, these network activity patterns share many similarities with the forms of synchrony observed in vivo under regimes of low neuromodulatory levels.…”

“…Indeed, in a prior study [35] we found that the chronic, prolonged (many hour) exposure of networks of cultured cortical neurons to a cholinergic agonist is associated with the gradual growth of excitatory synapses and, intriguingly, to the gradual reemergence of synchrony (see also [36]). If neuronal networks adapt to neuromodulatory input, it might be asked how the necessity to suppress network synchrony is ultimately addressed, in particular given that this activity form seems to be incompatible with attentive states and arousal [66,67].…”

“…Both in vivo [10,15,17,21,44] and in vitro [27,33,34,37,[45][46][47] experiments, as well as modeling studies [21,39,40,48,49], indicate that these forms of synchrony are not imposed by some external circuitry, global inhibition or pacemaker cells, but probably arise from the interplay of spontaneous synaptic activity, nonlinear neuronal recruitment cascades, refractoriness and network wide synaptic depression (summarized in [17]), effectively giving rise to a default activity mode, as it has been referred to [35,36] (see also [39]). Furthermore, and in full concordance with their activities in vivo [44,[50][51][52][53][54][55][56][57][58][59][60][61], cholinergic and adrenergic agonists suppress network synchrony in cell culture and slice preparations, shifting spontaneous activity away from this 'default' mode towards desynchronized, tonic firing modes [35,36,[62][63][64][65]. Thus, while synchrony in networks of cultured cortical neurons does not fully replicate the forms of synchrony related to low neuromodulatory tone in the intact brain, the similarities in underlying mechanisms and the comparable effects of neuromodulation suggest that this preparation is a useful mode...…”

“…In contrast, reduced activity of these systems, which occurs mainly during periods of NREM (non rapid eye movement) sleep, is associated with the prominent appearance of network synchrony as defined above. Importantly, however, neither these forms of network synchrony nor their modulation by acetylcholine (ACh) and noradrenaline (NA) are limited to the intact cortex, as similar activity patterns occur in brain slabs [21], acute and organotypic cortical preparations (for example, [22][23][24][25][26]) and even in networks of dissociated cortical neurons (for example, [27][28][29][30][31][32][33][34][35][36][37]; reviewed in [38]). …”

“…Where networks of dissociated cortical neurons in culture are concerned, synchrony takes the form of networkwide bursting activity which lasts for several hundreds of milliseconds, separated by longer periods (1 to 10 seconds) of near-complete quiescence or sparse, asynchronous action potentials [27][28][29][30][31][32][33][34][35][36][37]. These network-wide bursts are less frequent and more stereotyped as compared to those observed in the intact brain, which might be expected given the smaller size and lower connection density of these networks as well as the lack of reentrant pathways [21,[39][40][41]).…”