Unfixed postmortem human fetal brains show early neuronal activity and characteristics of mature cortical behavior, as shown in a study conducted by a research team from the University of Connecticut Health Center.
Numerous qualities of adult brains, including glutamate and GABA receptor activity and spontaneous electrical activity of subplate neurons, were noted in fetal neurons as young as 20 gestational weeks.
The study focused on the subplate zone, which is crucial in providing guidance to axonal projections in pattern formation for developing brains. This area contains the earliest born neurons (pioneer neurons), and was thus predicted to have the earliest electrical activity.
A sample size of five fetal brains was used, and all tissue preparation and physiological recordings were performed the same day the samples arrived.
The neurons studied were chosen based on four criteria: location in the subplate zone, appearance (one primary neurite directing towards the subventricular zone and several secondary neurites), large sodium currents and repeated action potential firing with current injection.
Four patterns of spontaneous electrical activity were recorded. Three patterns include neurons that spend most of the time at resting potential, neurons with a sharp depolarization of short duration and neurons with long-lasting and action potentials with large amplitudes.
The most prevalent pattern showed long periods of long-lasting depolarization plateaus with bursts of full-size action potentials. This activity highly resembles adult brain activity during anesthesia and deep sleep.
Extracellular shocks were delivered to three locations to test for the presence of functional synapses: between the subplate and subventricular zone, between the suplate and cortical plate and lateral to the cell body. Results show that a significant percentage of subplate neurons are absent of synaptic activity, which may be caused by lack of ligand-gated receptors.
To determine the functionality of ligand-gated receptors, glutamate and GABA were separately injected. Glutamate injection was followed by an increase in amplitude of induced depolarization, while GABA injections caused robust hyperpolarization, suggesting that human brains contain functional neurotransmitter receptors as early as 20 gestational weeks.
Previous animal models showed changes in intracellular chloride concentration, causing GABAergic pathways to change from excitatory to inhibitory as neurons matured. Inhibitory activity of GABAergic neurons in fetal brains of this experiment were recorded under conditions in which relative chloride ion concentrations could not be preserved.
It is uncertain from this experiment as to whether GABA is excitatory or inhibitory in human brains at 20 gestational weeks.
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