The role of the aural head shake reflex in serotonin-mediated head shaking behavior

Abstract

The head shake reflex is a rapid rhythmic shaking of the head in a radial motion and is a prominent part of the behavior of most mammalian species. The administration of agonists at 5-hydroxytryptamine (5-HT) receptors to rats increases apparently-spontaneous head shaking behavior. The present study examined the relationship between the head shake reflex, elicited by stimulation of the aural ampullae with Tween 80, with a similar-appearing behavior, the head shake response caused by the administration of 5-HT agonists to rats. Head shaking was attenuated by the subcutaneous infiltration of the local anesthetic procaine into the posterior border of the external auditory meatus. However, the local anesthetic did not alter head shake behavior produced by administering either the 5-HT agonist quipazine or the 5-HT precursor 5-hydroxy-L-tryptophan (l-5-HTP). The magnitude of the head shake reflex was also diminished after habituation of the reflex by repeatedly applying Tween 80 to the ampullae, yet this treatment had no effect on the head shaking behavior caused by quipazine. In a complementary manner, pretreatment with the 5-HT₂ receptor antagonist ketanserin potently blocked shaking behavior caused by quipazine without significantly altering the head shake reflex. Chronic administration of the atypical antidepressant drug iprindole to rats for 7 days reduced quipazine-induced shaking behavior without affecting the head shake reflex. In contrast, chronic administration of the monoamine oxidase inhibitor phenelzine to rats for 7 days reduced head shaking behavior caused by either stimulus, indicating that an attenuation of motor reflex activity could play a role in the reduced response to quipazine. When shaking behavior caused by the auricular application of Tween 80 was combined simultaneously with quipazine, these two treatments produced additive effects on shaking behavior. No support was found in these experiments using rats that head shaking behavior caused by 5-HT agonists involved the enhanced perception of normally-subliminal cutaneous stimuli from the pinna region that usually elicit the head shake reflex. Instead, 5-HT agonists and aural pinna stimulation appeared to produce head shaking behavior in rats by independent physiological mechanisms. However, these mechanisms may intersect insofar as the motor pathways of the head shake reflex may be common with the motor pathway for eliciting shaking behavior by the stimulation of central 5-HT receptors.