Synaptic inhibition is an important feature of the circuitry in the cerebral cortex. How would you determine whether GABA or Gly, or both, or neither, is the inhibitory neurotransmitter of the cortex?

Sourav PanSeptember 24, 2024

Answered

To determine whether GABA (gamma-aminobutyric acid) or glycine (Gly), or both, or neither, is the inhibitory neurotransmitter in the cerebral cortex, several experimental approaches can be employed:

1. Pharmacological Manipulation:
   • Receptor Antagonists: Use specific antagonists for GABA and glycine receptors (e.g., bicuculline for GABA receptors and strychnine for glycine receptors). By applying these antagonists to cortical slices or in vivo, researchers can observe changes in inhibitory postsynaptic potentials (IPSPs) or overall neuronal activity. If blocking GABA receptors abolishes inhibition, GABA is likely the primary inhibitory neurotransmitter. If glycine receptor blockade affects inhibition, then glycine may also play a role.
2. Electrophysiological Recordings:
   • Patch-Clamp Technique: Use this technique to record postsynaptic currents in cortical neurons. By applying GABA or glycine directly to the neurons and measuring the resulting currents, researchers can determine which neurotransmitter is responsible for the observed inhibitory effects. The reversal potential of the currents can also indicate the ion selectivity of the receptors involved.
   • Stimulus-evoked Responses: Record the synaptic responses in cortical neurons during stimulation of presynaptic inhibitory neurons. Analyze the characteristics of the IPSPs to see if they are sensitive to GABA or glycine receptor antagonists.
3. Immunohistochemistry and In Situ Hybridization:
   • Localization of Receptors: Use immunohistochemistry to visualize the presence of GABA and glycine receptors in cortical tissue. In situ hybridization can be used to detect mRNA for GABA and glycine receptors, indicating their synthesis in specific neurons. This can help identify which neurotransmitter is present and potentially active in the cortex.
4. Genetic Manipulation:
   • Knockout Models: Create or use existing genetic knockout models for GABA or glycine receptors. Observing the effects on inhibitory transmission in the cortex of these models can provide insights into the roles of each neurotransmitter. If the knockout of GABA receptors leads to a significant loss of inhibition, it suggests that GABA is the primary inhibitory neurotransmitter.
5. Calcium Imaging:
   • Monitoring Neuronal Activity: Use calcium imaging techniques to monitor the activity of cortical neurons in response to GABA or glycine application. Changes in intracellular calcium levels can indicate the activation of inhibitory pathways and help distinguish the contributions of GABA and glycine.

By combining these methods, researchers can comprehensively assess the roles of GABA and glycine in synaptic inhibition within the cerebral cortex, determining whether one, both, or neither serves as the primary inhibitory neurotransmitter.