Catecholaminergic neurons in synaptic connections with pre-Bötzinger complex neurons in the rostral ventrolateral medulla in normoxic and daily acute intermittent hypoxic rats

Abstract

The rostral ventrolateral medulla (RVLM) contains cardiovascular-related catecholaminergic neurons and respiratory-related pre-Bötzinger complex (pre-BötC) neurons, which are intermingled and functionally connected for coordinating cardiorespiratory activities. Daily acute intermittent hypoxia (dAIH) is known to elicit respiratory plasticity. However, it is unclear if the catecholaminergic neurons directly synapse onto pre-BötC neurons, and if the local circuitry exhibits plasticity when exposed to dAIH. The present study was aimed to determine the synaptic phenotypes between dopamine-β-hydroxylase (DβH)-immunoreactive (ir) catecholaminergic neurons and neurokinin 1 receptor (NK1R)-ir pre-BötC neurons, and the effect of dAIH on the neuronal network. Immunofluorescence histochemistry was used to reveal immunoreactivities of DβH and NK1R in the RVLM of normoxic and dAIH rats. Synaptic phenotypes were examined with double-labeling immunoelectron microscopy. We found that DβH immunoreactivity was expressed in somata and processes, some of which were in close apposition to NK1R-ir pre-BötC neurons. DβH-ir gold particles were localized to somata, dendrites, and axonal terminals. DβH-ir terminals formed asymmetric synapses, and occasionally, symmetric synapses in the pre-BötC, featuring the local circuitry. Of the synapses, 28% in normoxic and 29.6% in dAIH groups were apposed to NK1R-ir dendrites. Significant increases in DβH expression and NK1R-ir processes were found in the dAIH group. Moreover, the area and number of processes in close appositions were significantly elevated, strongly suggesting that dAIH induced plasticity with increased connections and interactions between the cardiovascular- and respiratory-related neurons in the local circuitry. In conclusion, asymmetric synapses are predominant in the crosstalk between catecholaminergic and pre-BötC neurons in the RVLM, elaborating excitatory transmission driving the coupling of cardiorespiratory activities. The neural network manifests plasticity in response to dAIH challenge.