Which neurotransmitter is primarily responsible for presynaptic inhibition of nociceptive input at the dorsal horn of the spinal cord?

Explanation

## Presynaptic Inhibition in the Dorsal Horn **Key Point:** GABA is the primary neurotransmitter responsible for presynaptic inhibition of nociceptive input at the dorsal horn of the spinal cord. ### Mechanism of Action GABA mediates presynaptic inhibition primarily via **GABA-B receptors** located on the presynaptic terminals of primary afferent (C-fiber and Aδ-fiber) neurons. Activation of these Gi-coupled receptors reduces calcium influx and increases potassium conductance, thereby decreasing the release of excitatory neurotransmitters (glutamate, substance P) from nociceptive afferents. This is the classical mechanism of presynaptic inhibition as described in Kandel's *Principles of Neural Science* and Guyton & Hall's *Textbook of Medical Physiology*. ### Distinction from Other Neurotransmitters | Neurotransmitter | Primary Site of Action | Type of Inhibition | Effect | |---|---|---|---| | **GABA** | **Presynaptic terminal (GABA-B)** | **Presynaptic** | **Reduces nociceptor transmitter release — PRIMARY** | | Glycine | Postsynaptic membrane | Postsynaptic | Hyperpolarizes second-order neuron | | Enkephalin | Presynaptic terminal (μ-opioid) | Presynaptic (modulatory) | Reduces nociceptor transmitter release | | Substance P | Postsynaptic receptor | Excitatory | Facilitates pain signal transmission | ### Why Not Enkephalin? While enkephalins (endogenous opioids) do act presynaptically at μ-opioid receptors on primary afferent terminals to reduce neurotransmitter release, GABA is considered the **primary** mediator of presynaptic inhibition in the dorsal horn. GABAergic interneurons in the substantia gelatinosa (lamina II) are the principal source of tonic presynaptic inhibitory control. Enkephalins play an important but secondary/modulatory role in this process. The question stem asks for the neurotransmitter "primarily responsible" for presynaptic inhibition — this unambiguously points to GABA, not enkephalin. ### Why Not Glycine? Glycine acts predominantly at **postsynaptic** glycine receptors (GlyR, a Cl⁻ channel) to hyperpolarize second-order neurons. Although some evidence exists for presynaptic glycine receptors, glycine is not considered the primary mediator of presynaptic inhibition in the dorsal horn. ### Why Not Substance P? Substance P is an excitatory neuropeptide released from primary afferent C-fibers. It acts on postsynaptic NK-1 receptors to facilitate pain transmission — the opposite of inhibition. **High-Yield:** GABA-B receptor agonists (e.g., baclofen) are used clinically to reduce spasticity and pain precisely because they exploit this presynaptic inhibitory mechanism at the spinal cord level. **Clinical Pearl:** The Gate Control Theory (Melzack & Wall) incorporates presynaptic inhibition: activation of large-diameter Aβ fibers (touch, pressure) stimulates inhibitory interneurons (releasing GABA and enkephalins) in the substantia gelatinosa, "closing the gate" to pain signals from smaller nociceptive fibers. GABAergic interneurons are the primary cellular substrate of this gate. *Reference: Kandel ER et al., Principles of Neural Science, 6th ed.; Guyton & Hall, Textbook of Medical Physiology, 14th ed.*