Sensitivity of rubrospinal neurons to excitatory amino acids in the rat red nucleus in vivo

Distribution of the Extracellular Matrix in the Pararubral Area of the Rat

Previously we described similarities and differences in the organization and molecular composition of an aggrecan based extracellular matrix (ECM) in three precerebellar nuclei, the inferior olive, the prepositus hypoglossi nucleus and the red nucleus of the rat associated with their specific cytoarchitecture, connection and function in the vestibular system. The aim of present study is to map the ECM pattern in a mesencephalic precerebellar nucleus, the pararubral area, which has a unique function among the precerebellar nuclei with its retinal connection and involvement in the circadian rhythm regulation. Using histochemistry and immunohistochemistry we have described for the first time the presence of major ECM components, the hyaluronan, aggrecan, versican, neurocan, brevican, tenascin-R (TN-R), and the HAPLN1 link protein in the pararubral area. The most common form of the aggrecan based ECM was the diffuse network in the neuropil, but each type of the condensed forms was also recognizable. Characteristic perineuronal nets (PNNs) were only recognizable with Wisteria floribunda agglutinin (WFA) and aggrecan staining around some of the medium-sized neurons, whereas the small cells were rarely surrounded by a weakly stained PNNs. The moderate expression of key molecules of PNN, the hyaluronan (HA) and HAPLN1 suggests that the lesser stability of ECM assembly around the pararubral neurons may allow quicker response to the modified neuronal activity and contributes to the high level of plasticity in the vestibular system.

Immunohistochemical localization of ionotropic glutamate receptors in the rat red nucleus

In this study, we aimed to determine the presence as well as the diverse distribution of N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptor subunits in the rat red nucleus. Using adult Sprague-Dawley rats as the experimental animals, immunohistochemistry was performed on 30 µm thick coronal brain sections with antibodies against α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (GluA1-4), kainate (GluK1, GluK2/3, and GluK5), and NMDA (GluN1 and GluN2A) receptor subunits. The results showed that all ionotropic glutamate receptor subunits are expressed in the red nucleus. Specific staining was localized in the neuron bodies and processes. However, the pattern of immunoreactivity and the number of labeled neurons changed depending on the type of ionotropic glutamate receptor subunits and the localization of neurons in the red nucleus. The neurons localized in the magnocellular part of the red nucleus were particularly immunopositive for GluA2, GluA4, GluK2/3, GluK5, GluN1, and GluN2A receptor proteins. In the parvocellular part of the red nucleus, ionotropic glutamate receptor subunit immunoreactivity of variable intensity (lightly to moderately stained) was detected in the neurons. These results suggest that red nucleus neurons in rat heterogeneously express ionotropic glutamate receptor subunits to form functional receptor channels. In addition, the likelihood of the coexpression of different subunits in the same subgroup of neurons suggests the formation of receptor channels with diverse structure by way of different subunit combination, and the possibility of various neuronal functions through these channels in the red nucleus.

Noradrenergic modulation of glutamate-induced excitatory responses in single neurons of the red nucleus: an electrophysiological study

The effect induced by noradrenaline (NA) on the spiking activity evoked by glutamate (Glu) on single neurons of the mesencephalic red nucleus (RN) of the rat was studied extracellularly. Long-lasting microiontophoretic applications of the amine induced a significant and reversible depression of the responsiveness of RN neurons to Glu. This effect was mediated by noradrenergic alpha2 receptors since it was mimicked by application of clonidine, an alpha2 adrenoceptor agonist, and blocked or at least reduced by application of yohimbine, an antagonist of NA for the same receptors. The effect appears homogeneously throughout the nucleus and is independent of the effect of NA on baseline firing rate. Application of isoproterenol, a beta adrenoceptor agonist, either enhanced or depressed neuronal responses to Glu in a high percentage (86%) of the tested neurons. Moreover, application of timolol, a beta adrenoceptor antagonist, was able to strengthen the depressive effects induced by NA application on neuronal responsiveness to Glu. Although these data suggest some involvement of beta adrenergic receptors in the modulation of neuronal responsiveness to Glu, the overall results indicate a short-term depressive action of NA, mediated by alpha2 receptors, on the responsiveness of RN neurons and suggest that stress initially leads to an attenuation of the relay function of the RN.

Red nucleus neurons actively contribute to the acquisition of classically conditioned eyelid responses in rabbits

The red nucleus (RN) is a midbrain premotor center that has been suggested as being involved in the acquisition and/or performance of classically conditioned nictitating membrane/eyelid responses. We recorded in rabbits the activity of RN and pararubral neurons during classical eyeblink conditioning using a delay paradigm. Neurons were identified by their antidromic activation from contralateral facial and accessory abducens nuclei and by their synaptic activation from the ipsilateral motor cortex (MC) and the contralateral cerebellar interpositus (IP) nucleus. For conditioning, we used a tone as a conditioned stimulus (CS) followed 250 ms later by a 100 ms air puff as an unconditioned stimulus (US) coterminating with it. Conditioned responses (CRs) were determined from the evoked changes in the electromyographic activity of the orbicularis oculi (OO) muscle. Recorded neurons were classified by their antidromic activation and by their changes in firing rate during the CS-US interval. Identified neurons increased their firing rates in relation to the successive conditioning sessions, but their discharge rates were related more to the EMG activity of the OO muscle than to the learning curves. Reversible inactivation of the IP nucleus with lidocaine during conditioning evoked a complete disappearance of both conditioned and unconditioned eyelid responses, and a progressive decrease in CR-related activity of RN neurons. In contrast, MC inactivation evoked a decrease in the acquisition process and an initial disfacilitation of neuronal firing (which was later recovered), together with the late appearance of CRs. Thus, RN neurons presented learning-dependent changes in activity following MC inactivation.

Serotonin modifies the neuronal inhibitory responses to gamma-aminobutyric acid in the red nucleus: a microiontophoretic study in the rat

The effects of 5-hydroxytryptamine (5-HT) on the inhibitory responses evoked by gamma-aminobutyric acid (GABA) in neurons of the red nucleus (RN) were studied using a microiontophoretic technique. Extracellular unitary recordings performed in anesthetized rats demonstrated that 5-HT ejection influenced GABA-evoked inhibition in 94% of RN neurons, enhancing them in 52% and depressing them in 46% of cases. Both effects were specific and dose-dependent,although enhancements or depressions of the GABA responses were respectively inversely and directly related to the doses of 5-HT applied. The type of modulation exerted by 5-HT on the GABA responses was independent of the action of the amine on background firing. In fact, 5-HT induced an enhancement of the GABA responses in neurons mostly located in the rostral RN and a depression in those in the caudal RN. The application of 8-hydroxy-2(di-n-propylamino)tetralin, a specific 5-HT(1A) receptor agonist, enhanced GABA responses, whereas alpha-methyl-5-hydroxytryptamine, a 5-HT(2A) receptor agonist, depressed them. Both the 5-HT(2) antagonist methysergide and the 5-HT(2A) selective antagonist ketanserin were able to block partially or totally the depressive action of 5-HT on GABA responses. In contrast, the same 5-HT antagonists mimicked the enhancing action of 5-HT on the GABA responses or were ineffective. Application of bicuculline, a GABA(A) receptor antagonist, enhanced the excitatory action of 5-HT on the background firing and slightly reduced the inhibitory action. It is concluded that 5-HT is able to modulate GABA-evoked responses in RN neurons by acting on both 5-HT(1A) and 5-HT(2A) receptors. The functional significance of a serotonergic control on GABAergic inhibitory effects in RN is discussed.