Synaptic plasticity of 5‐hydroxytryptamine–immunoreactive terminals in the phrenic nucleus following spinal cord injury: A quantitative electron microscopic analysis

Abstract

The present study was conducted to examine the plasticity of 5-hydroxytryptamine (5-HT)-immunoreactive terminals in the rat phrenic nucleus following an ipsilateral C2 spinal cord hemisection and 30-day survival period. A retrograde horseradish peroxidase (HRP) labeling technique was used to identify the phrenic motoneurons at the electron microscopic (EM) level. After employing a pre-embedding immunocytochemical technique, the ultrastructural characteristics of 5-HT-immunoreactive terminals were qualitatively and then quantitatively analyzed with a computerized morphometric system before and after injury in separate groups of rats. The results indicated that the majority of the 5-HT-labeled terminals formed axodendritic contacts, but some 5-HT-labeled terminals made axosomatic contacts. 5-HT terminals were associated with either asymmetrical or symmetrical synapses, and some displayed postsynaptic dense bodies. Approximately 2% of the 5-HT terminals had dense-core vesicles. Although the total number of labeled and unlabeled terminals in the phrenic nucleus was reduced after hemisection, the number of 5-HT terminals in the hemisected group was greater than that of the control group. Moreover, the total number and length of asymmetrical and symmetrical synaptic active zones per 5-HT terminal were significantly greater after injury. Finally, the total number of 5-HT terminals with multiple synapses was significantly greater in the hemisected group as compared to controls. It is possible that 5-HT synaptic plasticity may be part of the morphological substrate for the unmasking of the latent crossed phrenic pathway which mediates recovery of the ipsilateral hemidiaphragm paralyzed by C2 spinal cord hemisection.

Do synaptic rearrangements underlie compensatory reflex enhancement in spinal motoneurons after partial cell loss?

“…Double synapses, defined as axon terminals forming synaptic contacts with two different postsynaptic profiles (Tai et al, 1997) were observed on both the control (n ϭ 2), and experimental (n ϭ 3) sides. Ipsilaterally, three boutons were of the M-type and 25 of the S-type, while the corresponding numbers for the contralateral side were 2 and 22, respectively.…”

Localization of serotoninergic neurons that participate in regulating diaphragm activity in the cat

“…These influences on respiration are mediated both directly through serotoninergic projections to respiratory motoneurons (Holtman et al, 1984; Pilowsky et al, 1990; Tai et al, 1997) and indirectly via serotoninergic inputs to the brainstem respiratory groups (Connelly et al, 1989; Cream et al, 2002; Lalley et al, 1994; 1995; Mulkey et al, 2007; Voss et al, 1990). There is evidence that medullary serotonin-containing neurons serve as central chemoreceptors (Bernard et al, 1996; Li et al, 2006a; Messier et al, 2002; 2004; Nattie, 1999; 2001; Nattie and Li, 2001; Nucci et al, 2008; Penatti et al, 2006; Taylor et al, 2004; 2005; 2006), mediate long-term facilitation of respiratory motor output following episodic hypoxia or electrical stimulation of carotid chemoafferent neurons (Fuller et al, 2000; 2001; Ling et al, 2001; Millhorn et al, 1980; Millhorn, 1986; Mitchell et al, 2001), and are necessary for recovery of respiratory activity following upper cervical spinal injury (Fuller et al, 2005; Tai et al, 1997; Zhou and Goshgarian, 1999; 2000; Zimmer and Goshgarian, 2006). Furthermore, neurons near the medullary midline in felines have been shown to participate in eliciting cough (Baekey et al, 2003; Jakus et al, 1998) and emesis (Miller et al, 1996), although it is unknown whether these cells contain serotonin.…”

Cough following low thoracic hemisection in the cat

“…Following upper cervical lateral hemisection, and interruption of inspiratory drive on one side of the spinal cord, the ipsilateral hemiparetic diaphragm spontaneously regains function following contralateral phrenic nerve transection. This injury-induced respiratory plasticity occurs via activation of pre-existing crossed pathways within the spinal cord (Goshgarian and Guth, 1977; Guth, 1976) and is largely serotonin dependent (Hadley et al, 1999; Tai et al, 1997). To our knowledge, only one paper has assessed respiratory function following incomplete experimental thoracic injuries.…”