Connexin36 expression in major centers of the auditory system in the CNS of mouse and rat: Evidence for neurons forming purely electrical synapses and morphologically mixed synapses

Abstract

Electrical synapses formed by gap junctions composed of connexin36 (Cx36) are widely distributed in the mammalian central nervous system (CNS). Here, we used immunofluorescence methods to document the expression of Cx36 in the cochlear nucleus and in various structures of the auditory pathway of rat and mouse. Labelling of Cx36 visualized exclusively as Cx36-puncta was densely distributed primarily on the somata and initial dendrites of neuronal populations in the ventral cochlear nucleus, and was abundant in superficial layers of the dorsal cochlear nucleus. Other auditory centers displaying Cx36-puncta included the medial nucleus of the trapezoid body (MNTB), regions surrounding the lateral superior olivary nucleus, the dorsal nucleus of the medial lemniscus, the nucleus sagulum, all subnuclei of the inferior colliculus, and the auditory cerebral cortex. In EGFP-Cx36 transgenic mice, EGFP reporter was detected in neurons located in each of auditory centers that harboured Cx36-puncta. In the ventral cochlear nuclei and the MNTB, many neuronal somata were heavily innervated by nerve terminals containing vesicular glutamate transporter-1 (vglut1) and Cx36 was frequently localized at these terminals. Cochlear ablation caused a near total depletion of vglut1-positive terminals in the ventral cochlear nuclei, with a commensurate loss of labelling for Cx36 around most neuronal somata, but preserved Cx36-puncta at somatic neuronal appositions. The results suggest that electrical synapses formed by Cx36-containing gap junctions occur in most of the widely distributed centers of the auditory system. Further, it appears that morphologically mixed chemical/electrical synapses formed by nerve terminals are abundant in the ventral cochlear nucleus, including those at endbulbs of Held formed by cochlear primary afferent fibers, and those at calyx of Held synapses on MNTB neurons.

Immunocytochemical Evidence for Electrical Synapses in the Dorsal Descending and Dorsal Anterior Octaval Nuclei in the Goldfish, <b><i>Carassius auratus</i></b>

“…As mentioned previously, the overlap of eighth nerve terminals with Cx35 puncta in dDO and dAO requires further study, but it is consistent with such observations on the goldfish lateral dendrite of the Mauthner neuron [Pereda et al, 2003] and in rodents, where unilateral labyrinthectomy results in almost complete homolateral depletion of Cx36 puncta in the ventral cochlear nucleus as well as in primary vestibular nuclei [Nagy et al, 2013;Rubio and Nagy, 2015]. This observation is also consistent with other morphological and/or physiological reports of mixed synapses between eighth nerve fibers and vestibular nuclei in agnathan and gnathostome fishes [Stefanelli and Caravita, 1970;Hinojosa, 1973;Rovainen, 1974;Korn et al, 1977], frogs [Precht et al, 1974;Babalian and Shapovalov, 1984], lizards [Richter Brain Behav Evol 2019;93:34-49 DOI: 10.115993:34-49 DOI: 10.…”

“…Among primary vestibular nuclei, morphological studies similarly demonstrated that the eighth nerve forms mixed synapses with octaval nuclei in the lamprey [Stefanelli and Caravita, 1970;Rovainen, 1974], with the tangential nucleus (T) in the goldfish [Hinojosa, 1973], and with the lateral vestibular nucleus (or nucleus magnocellularis, MG) in the toadfish, where these synapses were physiologically confirmed [Korn et al, 1977]. The eighth nerve also forms mixed synapses with specific vestibular nuclei in species of amphibians [Precht et al, 1974;Babalian and Shapovalov, 1984], reptiles [Richter et al, 1975], birds [Hinojosa and Robertson, 1967;Wilson and Wylie, 1970], and mammals [Korn et al, 1973;Rubio and Nagy, 2015]. In the mammalian auditory system, two studies morphologically identified mixed synapses in the ventral cochlear nucleus [Sotello and Triller, 1982;Rubio and Nagy, 2015].…”

“…The eighth nerve also forms mixed synapses with specific vestibular nuclei in species of amphibians [Precht et al, 1974;Babalian and Shapovalov, 1984], reptiles [Richter et al, 1975], birds [Hinojosa and Robertson, 1967;Wilson and Wylie, 1970], and mammals [Korn et al, 1973;Rubio and Nagy, 2015]. In the mammalian auditory system, two studies morphologically identified mixed synapses in the ventral cochlear nucleus [Sotello and Triller, 1982;Rubio and Nagy, 2015]. Hence, electrical and chemical transmission by mixed synapses between a subset of eighth nerve afferents and their specific targets may be a primitive feature that is retained across vertebrates.…”

“…Cx35 is a synapse-specific piscine orthologue of mammalian connexin 36 [O'Brien et al, 1996[O'Brien et al, , 1998] that is localized to the presynaptic hemiplaque of the gap junction synapse . Cx35 immunocytochemical staining demonstrates gap junctions in mixed synapses as well as in electrical, also referred to as "purely electrical" [Rubio and Nagy, 2015;Nagy et al, 2017], gap junction synapses; purely electrical synapses are located at contacts that do not include chemical synapses. Whereas mixed synapses are formed between an axon terminal and a soma or dendrite, purely electrical synapses are somato-somatic, somato-dendritic, dendro-dendritic, or axo-axonic [Nagy et al, 2017].…”

“…Whereas mixed synapses are formed between an axon terminal and a soma or dendrite, purely electrical synapses are somato-somatic, somato-dendritic, dendro-dendritic, or axo-axonic [Nagy et al, 2017]. Purely electrical synapses are phylogenetically widespread among invertebrates and vertebrates and are found in many areas of the vertebrate neuraxis [SerranoVélez et al, 2014;reviewed in: Connors and Long, 2004;Pereda, 2014;Curti and O'Brien, 2016], including specific regions of the mammalian dorsal and ventral cochlear nuclei Rubio, 2009, 2011;Apostolides and Trussell, 2013;Rubio and Nagy, 2015].…”