The vestibular complex (VC) is a sensorimotor structure involved in the control of posture and eye movements. We had demonstrated that noradrenaline (NA) is able to modulate glutamate-elicited excitations of VC neurons by acting on a and b adrenoceptors. In this study we analysed the effect of b₁-adrenoceptors on NMDA-evoked responses. Our aim was to evaluate the participation of b adrenergic receptors to the modulation of a type of glutamate receptor mostly involved in long-term effects.

Extracellular recordings of unitary neuronal activities were performed in the VC of adult male anesthetized Wistar rats. NMDA-evoked excitations were studied during microiontophoretic applications of either NA or denopamine (DEN), a NA agonist for b₁-adrenergic receptors. The effects of atenolol (ATE), a NA antagonist for b₁-adrenergic receptors were tested also.

NA ejection enhanced NMDA-evoked responses in 5 out of 10 neurons tested, DEN in 8 out of 9 units. Both NA- and DEN-induced enhancements of NMDA responses (5 and 8 respectively) were antagonized by application of ATE. In contrast, depressions of NMDA responses induced by NA applications in 5 units were never antagonized and enhanced in 2 cases by ATE ejection. The immunofluorescence staining technique verified a diffuse co-localization of NMDA and b₁ adrenoreceptors throughout the VC.

These results demonstrate that in VC the activation of b₁ adrenoceptors facilitates and enhances the excitatory responses mediated by NMDA receptors.