NORADRENERGIC MODULATION OF NEURONAL RESPONSES TO N-METHYL-D-ASPARTATE IN THE VESTIBULAR NUCLEI: AN ELECTROPHYSIOLOGICAL AND IMMUNOHISTOCHEMICAL STUDY M. BARRESI, * C. GRASSO, F. LICATA AND G. LI VOLSI Department of Biomedical Sciences, Section of Physiology, University of Catania, Italy Abstract—Excitatory responses evoked by N-methyl-D- aspartate (NMDA) in the vestibular nuclei (VN) of the rat were studied in vivo during microiontophoretic application of noradrenaline (NA) and/or its agonists and antagonists. Ejection of NA-modified excitatory responses mediated by NMDA receptors (NMDAR) in all neurons tested; the effect was enhancement in 59% of cases and depression in the remaining 41%. Enhancements prevailed in all VN with the exception of the lateral vestibular nucleus, where both effects were recorded in an equal number of cases. The enhancing action of NA on NMDAR-mediated responses was mimicked by the noradrenergic beta-receptor agonist isoproterenol, the beta 1 specific agonist denopamine and the alpha 2 agonist clonidine. These effects were blocked respectively by the generic beta-receptor antagonist timo- lol, the beta 1 antagonist atenolol and the alpha 2 antagonist yohimbine. In contrast, application of the alpha 1 receptor agonist cirazoline and the specific alpha 1 antagonist prazo- sin respectively mimicked and partially antagonized the depression of NMDAR-mediated excitations induced by NA. Double-labeling immunohistochemical techniques demonstrated broad colocalization of NMDAR (specifically NR1 and NR2 subunits) with noradrenergic receptors (alpha 1 , alpha 2 and beta 1 ) in many VN neurons; only minor differences were found between nuclei. These results indicate that NA can produce generalized modulation of NMDAR-mediated excitatory neurotransmission in VN, which may in turn modify synaptic plasticity within the nuclei. Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: NMDA receptors, noradrenaline, vestibular complex, firing rate, microiontophoresis. INTRODUCTION The vestibular nuclei (VN) constitute a sensorimotor complex involved in the control of posture (Pompeiano, 1972; Sarkisian, 2000), eye movements (Pompeiano, 1972; Ito, 1991; Sarkisian, 2000) and motor learning (Broussard and Kassardjian, 2004). Primary vestibular afferents to the VN are mostly glutamatergic (Zhang et al., 2011). The important role played by glutamate in these nuclei is confirmed by the high concentration of the neurotransmitter (Li et al., 1996) and its receptors (De Waele et al., 1994; Vidal et al., 1996), found throughout the whole complex. A significant and selective noradrenergic projection from the locus coeruleus is also delivered to VN (Schuerger and Balaban, 1993, 1999). Modulation of glutamatergic neurotransmission by noradrenaline (NA) has been described in various central structures such as the hippocampus (Stanton et al., 1989; Segal et al., 1991), prefrontal cortex (Ji et al., 2008a,b), amygdala (Ferry et al., 1997) and cerebellar cortex (Pompeiano, 2006). We found in a previous study of VN that GLU-evoked excitatory effects on secondary vestibular neurons are modulated by NA (Barresi et al., 2009). Ionotropic AMPA and N-methyl-D-aspartate receptors (NMDAR) are widely colocalized in VN (Chen et al., 2000). The NMDAR participate in various learning mechanisms throughout the CNS (Malenka and Nicoll, 1993; Dineley et al., 2001; Antic et al., 2010). Specifically, in the VN, a structure characterized by intrinsic plasticity (Gittis and du Lac, 2006), NMDAR are related to long-term potentiation and depression (Scarduzio et al., 2012). We set out to determine whether NA application could modulate NMDAR-mediated responses in VN, and which types of noradrenergic receptor may be involved. We also explored the extent to which NMDAR are co-localized with noradrenergic receptors in single neurons of the VN. Noradrenergic effects, mediated by various receptors, and implicated in learning have been described (Cahill and McGaugh, 1996; Gibbs et al., 2010; McIntyre et al., 2012). A possible involvement of NA in mechanisms of synaptic plasticity in VN by a modulation of NMDAR- mediated responses is discussed. http://dx.doi.org/10.1016/j.neuroscience.2014.01.054 0306-4522/Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. * Corresponding author. Address: Department of Biomedical Sci- ences, Section of Physiology, University of Catania, Viale Andrea Doria 6, I-95125 Catania, Italy. Tel: +39-95-7384221; fax: +39-95- 7384217. E-mail addresses: mbarresi@unict.it (M. Barresi), clgrasso@unict.it (C. Grasso), flicata@unict.it (F. Licata), glivolsi@unict.it (G. Li Volsi). Abbreviations: ANOVA, analysis of variance; ATE, atenolol; CIRA, cirazoline hydrochloride; CLO, clonidine hydrochloride; DENO, denopamine; ISO, L-isoproterenol hydrochloride; LVN, lateral vestibular nucleus; MBA, mean background activity; MVN, medial vestibular nucleus; NA, noradrenaline; NMDA, N-methyl-D-aspartate; NMDAR, NMDA receptors; PBS, phosphate-buffered saline; PRA, prazosin hydrochloride; SD, standard deviation; SIOC, semi- quantitative indicator of co-localization; SpVN, spinal vestibular nucleus; SVN, superior vestibular nucleus; VN, vestibular nuclei; YO, yohimbine hydrochloride. Neuroscience 265 (2014) 172–183 172