Effects of α-noradrenergic substances on the optokinetic and vestibulo-ocular responses in the rabbit: a study with systemic and intrafloccular injections

The effects of microinjection of α-noradrenergic agonists and antagonists in the flocculus on the basic gain and adaptability of vestibulo-ocular and optokinetic responses were investigated. A complementary, previous investigation ⁴⁶ had shown that the adaptation, but not the basic performance, of compensatory oculomotor responses were markedly influenced by ß-noradrenergic mechanisms in the flocculus. In contrast, the present experiments with bilateral, intrafloccular injections of phenylephrine, prazosin, clonidine and idazoxan failed to reveal any effect of α₁- or α₂-noradrenergic mechanisms on either basic performance or adaptation of compensatory eye movements. Intravenous administration of clonidine, however, reduced the gain of the optokinetic and vestibulo-ocular responses by about 70 and 50%, respectively, at dosages of 0.07 mg/kg. Recovery from this effect took about 1.5 h. A higher dosage of clonidine (0.7 mg/kg) had a similar, but longer lasting effect, and also markedly increased the frequency of spontaneous saccades. Intravenous administration of phenylephrine did not affect the oculomotor responses. It is concluded that the control of oculomotor responses is not susceptible to α-noradrenergic influences at the level of the flocculus, but that α₂-agonistic action inhibits these responses through an extra-floccular structure.