Insufficient developmental excitatory neuronal activity fails to foster establishment of normal levels of inhibitory neuronal activity

Abstract

The nervous system is composed of excitatory and inhibitory neurons. One major class of inhibitory neurons release the neurotransmitter γ-Aminobutyric acid (GABA). GABAergic inhibitory activity maintains the balance that is disrupted in conditions such as epilepsy. At least some GABAergic neurons are initially excitatory and undergo a developmental conversion to convert to inhibitory neurons. The mechanism(s) behind this conversion are thought to include a critical developmental increase in excitatory activity. To test this hypothesis, we subjected ex vivo developing neuronal networks on multi-electrode arrays to various stimulation and pharmacological regimens. Synaptic activity of networks initially consists of epileptiform-like high-amplitude individual "spikes", which convert to organized bursts of activity over the course of approximately 1 month. Stimulation of networks with a digitized synaptic signal for 5days hastened the decrease of epileptiform activity. By contrast, stimulation for a single day delayed the appearance of bursts and instead increased epileptiform signaling. GABA treatment reduced total signals in unstimulated networks and networks stimulated for 5days, but instead increased signaling in networks stimulated for 1day. This increase was prevented by co-treatment with (2R)-amino-5-phosphonopentanoate and 6-cyano-7-nitroquinoxaline-2,3-dione, confirming that GABA invoked excitatory activity in networks stimulated for 1day. Glutamate increased signals in networks subjected to all stimulation regimens; the GABA receptor antagonist bicuculline prevented this increase only in networks stimulated for 1day. These latter findings are consistent with the induction of so-called "mixed" synapses (which release a combination of excitatory and inhibitory neurotransmitters) in networks stimulated for 1day, and support the hypothesis that a critical level of excitatory activity fosters the developmental transition of GABAergic neurons from excitatory to inhibitory.

An Overview of Studies Demonstrating that ex vivo Neuronal Networks Display Multiple Complex Behaviors: Emergent Properties of Nearest-Neighbor Interactions of Excitatory and Inhibitory Neurons.

“…This is underscored by studies with ex vivo neuronal networks. Our studies and those of other laboratories demonstrate that ex vivo neuronal networks can model multiple aspects of the nervous system, including development [42], maturation [36], longterm potentiation [43], environmental interaction [43,57], aging [31] and ultimate neurodegeneration [72 -75]. While the "default" interpretation of neuronal behavior is perhaps to consider stimulation as evoking response, our studies underscore the additional requirement of inhibitory neuronal activity for the achievement of both the development as well as maintenance of complex "responsive" actions of the nervous system.…”

“…This transient excitatory activity of GABAergic neurons also contributed to the level of total excitatory signaling required to trigger the developmental conversion of initially excitatory GABAergic neurons to inhibitory neurons [42]. Moreover, insufficient GABAergic excitatory activity actually inhibited this developmental transition.…”

“…Moreover, insufficient GABAergic excitatory activity actually inhibited this developmental transition. This leaves open the possibility that failure to sufficient GABAergic neurons to convert from excitatory to inhibitory could leave an individual at a threshold for seizures [42]. One interpretation of these latter findings is that epilepsy could arise not from excessive excitatory activity but rather from a lack of sufficient inhibitory activity.…”

Robot-Embodied Neuronal Networks as an Interactive Model of Learning

“…These analyses may contribute to optimization of neuronal-computer interfaces for perceptive and locomotive prosthetic applications. Manipulation of inhibitory neuronal activity and initial connectivity may provide insight into perturbations of balance in excitatory/inhibitory activity that promote seizures and/or perturb motor activity in epilepsy and Parkinson’s disease [ 4 - 6 , 29 ]. In addition, the inherent potentially-interfering basal activity of an amplitude matching sensory input in our system may provide a useful system for analyses of conditions such as attention deficit hyperactivity disorder, where competing sensory and internal distractions can distract an individual from task recognition and completion.…”

Amyotrophic Lateral Sclerosis Mechanism: Insights from the Caenorhabditis elegans Models

“…C. elegans possesses an ortholog of FUS, known as FUST-1, sharing approximately 50% identity at the protein level. Previous studies reported that deletions in fust-1 led to neuronal degeneration and paralysis, while overexpression did not exhibit any discernible effects [ 57 ]. Studies in C. elegans have unveiled structural and functional similarities between FUS and TDP-43, showing comparable outcomes when mutated ALS-associated variants are expressed [ 48 ].…”