Summary: Researchers report newborn granule cells in the dentate gyrus become less excitable after three weeks. The loss of excitability is crucial for the functioning of mature neurons.
The dentate gyrus of the hippocampus is part of the brain that helps form memories. It is also one of just two areas in the adult brain where new neurons are continuously formed.
The dentate gyrus is part of a circuit that receives electrical signals from an area of the brain cortex that processes sensory and spatial input from other areas of the brain. By combining this sensory and spatial information, the dentate gyrus can generate a unique memory of an experience.
Essential for the computational function of the dentate gyrus is a sparse neural activity — that is, the dentate gyrus must have nerve circuits that are electrically quiet. This is accomplished by strong inhibitory circuits and low excitability of the principal neurons of the dentate gyrus, the granule cells.
However, newborn granule cells show high excitability that disappears as the cells mature. Little has been known about the mechanisms that create low excitability in mature cells or how excitability of the newborn granule cells changes over time.
Now University of Alabama at Birmingham researchers led by Linda Overstreet-Wadiche, Ph.D., and Jacques Wadiche, Ph.D., associate professors in the UAB Department of Neurobiology, have described key roles for G protein-mediated signaling and the late maturation of an ion channel during the differentiation of granule cells. Their study is published in the Journal of Neuroscience. First author Jose Carlos Gonzalez, Ph.D., is a postdoctoral fellow in the Overstreet-Wadiche lab.
“Our goal was to characterize the function, maturation and sources of this signaling pathway, so that in the future it can be manipulated for therapeutic purposes,” Overstreet-Wadiche said.
A G protein is a molecular switch inside of cells that responds to stimuli outside the cell, and G proteins have been parts of numerous Nobel Prize-winning research. Ion channels in the cell membrane are gates that can open to allow ions to flow into or out of a cell. The flux of ions generates electrical currents across the cell membrane to control the excitability of individual neurons.
Overstreet-Wadiche and colleagues found that intact G protein signaling is required for low granule cell excitability. They also found that a potassium channel called GIRK, or G protein-activated inward rectifying potassium channel, is constantly active in mature dentate granule cells, and this made the neurons less excitable by lowering the cells’ resting membrane potential, as well as by other electrophysiological effects.
The UAB researchers also found that newborn granule cells, about 10 to 12 days old, do not have functional GIRK channels. At about three weeks, functional GIRK channels start to appear, and they also start being controlled by G protein signaling, via a cell receptor called the GABA B receptor. GABA, or gamma-aminobutyric acid, is the chief inhibitory neurotransmitter in mammals. Inhibitory neurons release GABA, and the GABA then binds to receptors on target neurons to inhibit neural circuits.
Thus, GIRK appears to lower excitability of mature dentate granule cells two ways. The first is through the intrinsic mechanism of constantly active GIRK channels that reduce resting membrane potentials and intrinsic excitability. The second is by phasic activation of GIRK signaling — this inhibition is accomplished by inhibitory neurons that release GABA at somatodendritic synapses with the granule cells. The GABA crosses the synapse to the dentate granule cell and activates GIRK channels through GABA-B-receptor/G-protein signaling.
Inhibitory interneurons that release GABA are known to belong to three groups, as identified by expression of various proteins. Overstreet-Wadiche and colleagues thus asked which interneuron subtypes formed inhibitory synapses with the dentate granule cells to initiate phasic GABA B-receptor/GIRK inhibition. They found that nNOS-expressing interneurons were the main source of the GABA-B-receptor-mediated inhibition. SST-expressing interneurons had a smaller effect, and PV-expressing interneurons had no inhibitory effect.
“The dentate gyrus is critical for controlling the flow of neural activity through the hippocampus, and this gatekeeping function is important, not only for normal cognitive functions, but also for suppressing seizure activity,” Overstreet-Wadiche said. “We have shown how a well-known signaling pathway makes a particularly strong contribution to suppressing excitability in this region, and this answered a longstanding question about why these neurons have such a low resting membrane potential compared to other neurons.”
Co-authors with Gonzalez, Overstreet-Wadiche and Wadiche for the study, “Constitutive and synaptic activation of GIRK channels differentiates mature and newborn dentate granule cells,” are Sean J. Markwardt, UAB Department of Neurobiology; and S. Alisha Epps, UAB Department of Physical Medicine and Rehabilitation. Markwardt now works for Avant Healthcare, Carmel, Indiana, and Epps is an assistant professor of psychology, Whitworth University, Spokane, Washington.
Funding: Support was provided by National Institutes of Health grants NS064025, NS065920 and NS075162.
Source: Jeff Hansen – UAB
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to HKUST.
Original Research: Abstract for “Constitutive and Synaptic Activation of GIRK Channels Differentiates Mature and Newborn Dentate Granule Cells” by Jose Carlos Gonzalez, S. Alisha Epps, Sean J. Markwardt, Jacques I. Wadiche and Linda Overstreet-Wadiche in Journal of Neuroscience. Published July 18 2018.
Constitutive and Synaptic Activation of GIRK Channels Differentiates Mature and Newborn Dentate Granule Cells
Sparse neural activity in the dentate gyrus is enforced by powerful networks of inhibitory GABAergic interneurons in combination with low intrinsic excitability of the principal neurons, the dentate granule cells (GCs). Although the cellular and circuit properties that dictate synaptic inhibition are well studied, less is known about mechanisms that confer low GC intrinsic excitability. Here we demonstrate that intact G protein-mediated signaling contributes to the characteristic low resting membrane potential that differentiates mature dentate GCs from CA1 pyramidal cells and developing adult-born GCs. In mature GCs from male and female mice, intact G protein signaling robustly reduces intrinsic excitability, whereas deletion of G protein-activated inwardly rectifying potassium channel 2 (GIRK2) increases excitability and blocks the effects of G protein signaling on intrinsic properties. Similarly, pharmacological manipulation of GABAB receptors (GABABRs) or GIRK channels alters intrinsic excitability and GC spiking behavior. However, adult-born new GCs lack functional GIRK activity, with phasic and constitutive GABABR-mediated GIRK signaling appearing after several weeks of maturation. Phasic activation is interneuron specific, arising primarily from nNOS-expressing interneurons rather than parvalbumin- or somatostatin-expressing interneurons. Together, these results demonstrate that G protein signaling contributes to the intrinsic excitability that differentiates mature and developing dentate GCs and further suggest that late maturation of GIRK channel activity is poised to convert early developmental functions of GABAB receptor signaling into GABABR-mediated inhibition.
The dentate gyrus exhibits sparse neural activity that is essential for the computational function of pattern separation. Sparse activity is ascribed to strong local inhibitory circuits in combination with low intrinsic excitability of the principal neurons, the granule cells. Here we show that constitutive activity of G protein-coupled inwardly rectifying potassium channels (GIRKs) underlies to the hallmark low resting membrane potential and input resistance of mature dentate neurons. Adult-born neurons initially lack functional GIRK channels, with constitutive and phasic GABAB receptor-mediated GIRK inhibition developing in tandem after several weeks of maturation. Our results reveal that GABAB/GIRK activity is an important determinant of low excitability of mature dentate granule cells that may contribute to sparse DG activity in vivo.