User:Stempera/Synaptic gating draft

Synaptic gating is the ability of neural circuits to "gate" inputs by either suppressing or facilitating specific synaptic activity. Selective inhibition of certain synapses has been studied thoroughly (see Gate theory of pain), and recent studies have supported the existence of permissively gated synaptic transmission. In general, synaptic gating involves a mechanism of central control over neuronal output. It includes a sort of "gatekeeper" neuron, which has the ability to influence transmission of information to selected targets independently of the parts of the synapse upon which it exerts its action (see also neuromodulation).

Bistable neurons have the ability to oscillate between a hyperpolarized (down state) and a polarized (up state) resting membrane potential without firing an action potential. These neurons can thus be referred to as "up/down" neurons. According to one model, this ability is linked to the presence of NMDA and AMPA glutamate receptors. External stimulation of the NMDA receptors is responsible for moving the neuron from the down state to the up state, while the stimulation of AMPA receptors allows the neuron to reach and surpass the threshold potential. Neurons that have this bistable ability have the potential to be gated because outside "gatekeeper" neurons can modulate the membrane potential of the gated neuron by selectively shifting them from the "up" state to the "down" state. Such mechanisms have been observed in the nucleus accumbens, with gatekeepers originating in the cortex, thalamus and basal ganglia.

Inhibition

Modulation of interneurons

Gating can occur by shunting inhibition. Gating signal from the “gatekeeper” triggers inhibitory interneurons in order to prevent one set of neurons from firing even when stimulated by another set. In this state, the gate is “closed”.

Stimulation of an inhibitory interneuron can also be variable, such that the gatekeeper's influence "opens" a gate that is already closed. This phenomenon, known as inhibitory modulation, reduces the strength of the stimulation of the inhibitory neuron. As a result, the postsynaptic set of neurons exists under less inhibition, thereby opening the gate.

Role in spatial attention

Due to the brain's limited capacity to process information, it becomes necessary that the brain have the ability filter out unnecessary information, and select important information. Input, especially to the visual field, competes for selective attention. Models for gating mechanisms in the process of attention have been explored by many groups of researchers, however, a consensus on the role of synaptic gating in attention has not been reached.

Permissive gating

The firing of an action potential, and consequently the release of neurotransmitters, occurs by this gating mechanism. In synaptic gating, in order for an action potential to occur, there must be more than one input to produce a single output in the neuron being gated. The interaction between these sets of neurons creates a biological AND gate. The neuron being gated is bistable and must be brought to the "up" state before it can fire an action potential. When this bistable neuron is in the "up" state, the gate is open. A "gatekeeper" neuron is responsible for stimulating the bistable neuron by shifting it from a "down" state to an "up" state and thus, opening the gate. Once the gate is open, an excitatory neuron can cause the bistable neuron to further depolarize and reach threshold causing and action potential to occur. If the "gatekeeper" does not shift the bistable neuron from "down" to "up", the excitatory neuron will not be able to fire an action potential in the bistable neuron. Both the "gatekeeper" neuron and excitatory neuron are necessary to fire an action potential in the bistable neuron, but neither is sufficient to do so alone.

Synaptic gating involves a variety of mechanisms by which the efficacy of neuronal activity is modulated. Recent studies demonstrate the permissive properties of synaptic gating. In certain instances, membrane depolarization will cause an opening of the 'gates' that previously had an inhibitory effect on the neuron they were gating. This permissive gating is more than a matter of simple summation. Summation is the convergence of many EPSPs at the axon hillock (either from a single neuron firing at a high frequency or from many neurons firing at once) that depolarizes the membrane potential to the point of threshold. The membrane depolarization caused by the opening of synaptic gates causes an additional increase in intracellular calcium that facilitates the release of neurotransmitters; thus, it is able to selectively distribute information from the presynaptic cell.

Synaptic gating and disease

Comorbidity of ADHD and anxiety

Studies of children diagnosed with ADHD have shown considerably higher scores on the Anxious/Depressed scale of the Achenbach Child Behavior Checklist, which implies a comorbidity of ADHD and anxiety. It has been suggested that impaired synaptic gating processes in the nucleus accumbens are the underlying cause of this comorbidity . This defect causes a reduction in synaptic gating of dopamine input from the prefrontal cortex and hippocampus on the nucleus accumbens. One theory supposes that this defect reduces the individual's ability to selectively inhibit fear responses from the amygdala, leading to anxiety. There are several theories, however, on how this impairment ultimately affects those with ADHD.

Schizophrenia

Schizophrenics often suffer from an inability to illustrate context-dependent memory, an inability to show affective valence – proper emotions, and an inability for attentional and temporal processes. Synaptic gating seems to illustrate why all of these inabilities develop. In particular, hippocampal input into the nucleus accumbens, a region of the basal ganglia, acts as a “gate” creating a more depolarized “up” state within the accumbens neurons allowing them to be more receptive to innervation from the PFC. In addition, amygdala input, in much the same way, acts as a gate creating a more depolarized state within the accumbens neurons although this depolarized state is much more transient. All in all, nucleus accumbens neurons are bistable. Schizophrenics have damage to the hippocampus and amygdala illustrating improper gating and resulting in nucleus accumbens neurons being in the “down” position. This illustrates schizophrenics inability to demonstrate context-dependent memory and their inability to show proper affective valence. In addition, because accumbens neurons are in the “down” position they are not as receptive to PFC stimulation and thus schizophrenics show problems in attentional deficits. In conclusion, the gating theory of schizophrenia posits a bistable nucleus accumbens neuron that when gated improperly, as in schizophrenics, leads to a wealth of behavioral and memory deficits.