Promethazine affects optokinetic but not vestibular responses in monkeys

Area of Science:

• Neuroscience
• Ophthalmology
• Aerospace Medicine

Background:

• Promethazine is a common treatment for motion sickness, including Space Adaptation Syndrome.
• Limited data exists on promethazine's specific effects on vestibular and optokinetic responses.

Purpose of the Study:

• To investigate the impact of promethazine on vestibular and optokinetic responses in a primate model.
• To elucidate the mechanisms by which promethazine affects visual-oculomotor pathways.

Main Methods:

• Monkeys received promethazine at human-equivalent spaceflight dosages.
• Vestibular and optokinetic nystagmus were recorded using eye coils.
• Data was analyzed to assess effects on eye velocity, reflex gains, and time constants.

Main Results:

• Promethazine significantly reduced the initial eye velocity gain during optokinetic nystagmus (OKN).
• Angular vestibulo-ocular reflex (aVOR) gains and time constants, saccades, and ocular counter-rolling remained unaffected.
• A two-component model indicated reduced gain in the rapid OKN component.

Conclusions:

• Promethazine impairs the direct visual-oculomotor pathway by reducing OKN gain.
• The drug has minimal impact on saccades, aVOR, and low-frequency vestibulo-ocular reflexes.
• Findings align with human studies showing reduced ocular pursuit and VOR suppression after promethazine.