Tizanidine Mechanism of Action
Tizanidine is an α2-adrenergic receptor agonist that reduces spasticity by increasing presynaptic inhibition of motor neurons in the spinal cord. 1
Primary Mechanism
Tizanidine acts as an agonist at α2-adrenergic receptor sites, which produces its antispasticity effects through several interconnected pathways 1:
- Increases presynaptic inhibition of excitatory neurotransmitter release from spinal interneurons
- Reduces facilitation of spinal motor neurons by acting primarily on polysynaptic pathways
- Does not directly affect skeletal muscle fibers or the neuromuscular junction
- Has minimal effect on monosynaptic spinal reflexes
The overall effect is thought to reduce the facilitation of spinal motor neurons, thereby decreasing muscle tone and spasticity 1, 2.
Receptor Specificity
The drug's mechanism involves both α2-adrenergic and imidazoline receptors 3:
- α2A adrenergic receptors appear to be the primary mediators of the antispasticity effect
- Imidazoline receptors also contribute to the therapeutic action
- α2B adrenergic receptors do not appear to mediate the antispasticity effect
Research demonstrates that the antispasticity effect is reversed by α2A antagonists and imidazoline receptor antagonists, but not by α2B antagonists 3.
Structural Relationship to Clonidine
Tizanidine's imidazoline chemical structure is related to clonidine, another α2-adrenergic agonist 1. However, critical differences exist:
- Tizanidine has one-tenth to one-fiftieth (1/10 to 1/50) the potency of clonidine in lowering blood pressure
- This separation between muscle relaxant effects and cardiovascular effects makes tizanidine more favorable for treating spasticity 2
Specific Neurophysiologic Effects
Electrophysiologic studies reveal that tizanidine acts on multiple spinal cord circuits 4:
- Reinforces presynaptic inhibition (reducing brisk tendon reflexes)
- Enhances Ia reciprocal inhibition (postsynaptic mechanism)
- Increases Ib nonreciprocal inhibition (reducing muscle hypertonia)
- Markedly reduces flexor reflexes
This broad spectrum of activity suggests tizanidine corrects multiple pathophysiologic abnormalities in spasticity, not just a single mechanism 4.
Site of Action
The drug acts at both spinal and potentially supraspinal sites 3:
- Targets α2A receptors on spinal neurons
- Acts on primary afferent terminals
- May affect activated astrocytes (which show upregulation of α2A receptors in spasticity)
- Inhibits facilitatory coeruleospinal pathways from brainstem to spinal cord 2
Clinical Correlation
System identification studies in spinal cord injury patients demonstrate that tizanidine reduces reflex mechanical responses substantially (15-60% reduction in peak torque) without inducing comparable changes in intrinsic muscle properties 5. This confirms the drug's primary action is on reflex pathways rather than direct muscle effects.
Human studies show tizanidine decreases alpha motoneuron excitability and increases presynaptic inhibition, making it particularly effective when spasticity involves decreased presynaptic inhibition and increased motoneuron excitability 6.
Additional Pharmacologic Properties
Beyond antispasticity effects, α2-receptor-mediated mechanisms also underlie: