Interneurons: Definition and Central Nervous System Role
Interneurons are neurons that are not dedicated to sensory or motor information relay, but rather serve to integrate and modulate signals between afferent and efferent neurons, playing a critical modulatory role in limiting information flow to higher brain centers. 1
Core Definition and Function
Local circuit neurons (interneurons) integrate signals from afferent and efferent neurons rather than directly transmitting sensory or motor information. 1 These neurons are found throughout the nervous system, including intrathoracic ganglia, ganglionated plexi, and extensively within the central nervous system, where they communicate constantly with each other. 1
Neurotransmitter Systems and Inhibitory Function
Inhibitory interneurons utilize different neurochemical classes including cholinergic, opioidergic, and GABAergic neurotransmitters to limit nociceptive information flow to supraspinal areas. 1
The mechanisms of action include:
- Presynaptic inhibition: Interneurons inhibit the release of nociceptive substances from primary afferent neurons 1
- Postsynaptic inhibition: They reduce excitatory firing in target neurons 1
- Modulation of projection neurons: Interneurons can inhibit both projection neurons and excitatory interneurons in descending pain pathways 1
Cortical Interneuron Development and Diversity
GABAergic interneurons, which comprise 20-30% of the cerebral cortical neuronal population, are the major inhibitory cells of the CNS and play an essential role in modulating electrical activity of excitatory pyramidal cells. 2
Developmental Origins
Excitatory glutamatergic projection neurons are derived from the dorsal forebrain cortex, while inhibitory GABAergic cortical interneurons originate from the subcortical ventral forebrain. 1 During embryogenesis, interneurons are generated in specific compartments of the ventral telencephalon known as ganglionic eminences, then migrate long distances to reach their final destinations. 2
Functional Diversity
Cortical GABAergic interneurons constitute one of the most diverse groups of cells in the CNS, heterogeneous in morphology, molecular profiling, and electrical properties. 3, 4 This diversity includes:
- Fast-spiking, parvalbumin-positive (PV+) GABAergic interneurons that have become extensively characterized through patch-clamp techniques, optogenetics, and computational approaches 5
- Multiple phenotypes varying on morphological, physiological, and neurochemical axes 4
- Context-dependent maturation of defining features 4
Clinical Significance
Disruption of interneuron development or function, either from environmental insult or genetic mutations, can have devastating consequences on normal brain function. 2 Many neurologic and psychiatric disorders are marked by imbalances between neural excitation and inhibition. 6
Pathological States
In chronic pain states, there is a decline in inhibitory system activity at the spinal cord level, mainly due to loss of GABAergic interneurons, leading to amplification and diffusion of nociceptive pain. 1 This loss of inhibitory interneurons contributes to the transition from acute to chronic pain, where nociceptive input continuously reaches brain areas even when initial noxious stimulation ceases. 1
Therapeutic Implications
Interneuron transplantation provides a novel strategy for modifying neural circuits in rodent models of epilepsy, Parkinson's disease, mood disorders, and chronic pain. 6 When transplanted from embryos or in vitro culture preparations, immature interneurons can disperse and integrate into host brain circuits in both cerebral cortex and other CNS regions. 6
Integration with Other Neural Systems
Both inhibitory and excitatory postsynaptic currents contribute to local field potential signals, with perisomatic inhibition setting up dipoles in pyramidal cells. 1 The contributions of GABAergic interneurons to neural signaling may be amplified by their high synchrony and divergent projections onto pyramidal cells, causing correlated inhibitory postsynaptic currents in many pyramidal cells simultaneously. 1