Postnatal development of chorda tympani axons in the rat nucleus of the solitary tract

Abstract

The chorda tympani nerve (CT), one of three nerves that convey gustatory information to nucleus of the solitary tract (NTS), displays terminal field reorganization after postnatal day 15 in the rat. Aiming to gain insight into mechanisms of this phenomenon, CT axon projection field and terminal morphology in NTS subdivisions were examined using tract tracing, light microscopy, and immuno-electron microscopy at four postnatal ages: P15, P25, P35, and adult. The CT axons that innervated NTS rostrolateral subdivision both in the adult and in P15 rats were morphologically distinct from those that innervated the rostrocentral, gustatory subdivision. In both subdivisions, CT terminals reached morphological maturity before P15. Rostrolateral, but not rostrocentral axons, went through substantial axonal branch elimination after P15. Rostrocentral CT synapses, however, redistribute onto postsynaptic targets in the following weeks. CT terminal preference for GABAergic postsynaptic targets was drastically reduced after P15. Furthermore, CT synapses became a smaller component of the total synaptic input to the rostrocentral NTS after P35. The results underlined that CT axons in rostrocentral and rostrolateral subdivisions represent two distinct populations of CT input, displaying different morphological properties and structural reorganization mechanisms during postnatal development.

The μ-opioid receptor agonist DAMGO presynaptically suppresses solitary tract-evoked input to neurons in the rostral solitary nucleus

“…These results are similar to those from the cNST, where 50 -70% of GABAergic neurons have been shown to be monosynaptically innervated by the ST (Bailey et al 2008;Glatzer et al 2007). However, our report of a high proportion of rNST GAD67ϩ neurons with monosynaptic input appears at odds with a recent anatomical report (Wang et al 2012) that showed that only a small fraction of identified CT afferent fibers make synaptic contact with GABAergic neurons in the adult rat. It is important to note that we report the percentage of GABAergic neurons receiving monosynaptic ST input, while Wang et al report the percentage of CT terminals that synapse onto GABAergic neurons.…”

“…In contrast to the commercially available "GIN" mouse, which only expresses GFP in a particular subset of rNST GABAergic neurons (Travers et al 2007;Wang and Bradley 2010a), this GAD67-GFP knockin strain has been shown to produce EGFP in virtually all cells expressing GAD67 (Brown et al 2008;Kaneko et al 2008;Ono et al 2005;Tamamaki et al 2003), allowing for a more complete identification of GABAergic neurons in the NTS. Because of reports that monosynaptic inputs to GABAergic neurons in the rNST may diminish during development in rat (Wang et al 2012) but evidence for presynaptic inhibition of monosynaptic vagal inputs to cNST GABAergic neurons in mature as well as young mice (Glatzer et al 2007), we included both mouse pups (P14 -18) and adult mice (6 -11 wk) in our study. Initially we tested mouse pup brain slices in artificial cerebrospinal fluid (ACSF) and sometimes observed both inhibitory and excitatory inputs after afferent stimulation that precluded measurement of jitter essential to an interpretation of monosynaptic input.…”

Transcriptomes and neurotransmitter profiles of classes of gustatory and somatosensory neurons in the geniculate ganglion

“…Synapses of chorda tympani neurons in the NST differ between the rostral-central vs. the rostral-lateral subdivisions. Based on their larger size and other ultrastructural features, the rostral-lateral terminals were proposed to represent mechanosensation, while taste inputs were mostly in the rostral-central subdivision 53 . Our evidence, albeit incomplete, is that the putative mechanosensory T2 neurons of the geniculate ganglion also terminate in the same rostral-lateral subdivision and may represent at least part of the non-chemosensory input into the gustatory portion of the NST.…”

Selective Removal of Sodium Salt Taste Disrupts the Maintenance of Dendritic Architecture of Gustatory Relay Neurons in the Mouse Nucleus of the Solitary Tract

“…While a causal relationship between the development of presynaptic and postsynaptic elements of this circuit is not readily apparent, understanding changes at the synaptic level may provide useful. Wang et al (2012) found an age-dependent decrease in the number of CT terminal synapses in the rat NST, as would be predicted by the developmental decreases in total CT terminal field volume in rat ( May and Hill, 2006 ; Mangold and Hill, 2008 ) and mouse ( Fig. 5 ; Zheng et al, 2014 ).…”

“…5 ; Zheng et al, 2014 ). Interestingly, they report a substantial developmental decrease in number of CT synapses onto GABAergic targets ( Wang et al, 2012 ). There is also evidence that the decrease in terminal field size and numbers of synapses are paradoxically accompanied by an age-related increase in functional taste responses from the NST, especially to NaCl ( Hill et al, 1983 ).…”