Opioids prolong and anoxia shortens delay between onset of preinspiratory (pFRG) and inspiratory (preBötC) network bursting in newborn rat brainstems

Abstract

Differential responses to opioids established the hypothesis that pre/postinspiratory (Pre-I) neurons of the parafacial respiratory group (pFRG) and inspiratory (Insp) neurons of the pre-Bötzinger complex (preBötC) constitute a dual brainstem respiratory center. For further analysis of pFRG/preBötC interactions, we studied in newborn rat brainstem-spinal cord preparations opioid and anoxia effects on histologically identified pFRG-driven "type-I" Insp preBötC neurons and Pre-I neurons from three distinct respiratory brainstem regions. The micro-opioid [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) slowed inspiratory-related cervical nerve bursts quantally, whereas anoxia induced nonquantal slowing and repetitive cervical bursts. DAMGO had no effect on membrane potential or input resistance of Pre-I neurons, while anoxia hyperpolarized them (approximately 5 mV) and decreased their resistance (approximately 30%). DAMGO prolonged the preinspiratory phase of Pre-I neuron bursting, whereas anoxia caused a shift to postinspiratory (48%) or inspiratory (22%) activity and silenced further 30% of cells. Pre-I neuron responses were not correlated with their rostrocaudal location or morphology. Neither DAMGO nor anoxia changed membrane potential of type-I neurons, but decreased their input resistance by 33% and 21%, respectively. The opposite DAMGO- and anoxia-evoked phase shifts of Pre-I neuron activity were reflected by corresponding shifts of pre/postinspiratory drive potentials in type-I neurons and, partly, by voltage-sensitive dye-imaged medullary neuronal population activities. The findings suggest that opioids presynaptically delay activation of type-I neurons as the target of drive from the pFRG to the preBötC. Contrary, anoxia seems to partly synchronize the pFRG and preBötC rhythm generators. This may enhance inspiratory and postinspiratory medullary activities for triggering multiple inspiratory motor bursts.

Phox2b-Expressing Neurons of the Parafacial Region Regulate Breathing Rate, Inspiration, and Expiration in Conscious Rats

“…This entrainment occurred over a limited range of breathing frequencies, possibly because only a small fraction ( ~14%) of the RTN neurons expressed ChR2. Phox2b-neuron stimulation produced quantal breathing only in anesthetized rats maintained slightly below apneic threshold consistent with the fact that this pattern is typically observed when the respiratory network is in a low excitability state (Mellen et al, 2003; Ballanyi et al, 2009). During low frequency stimulation, the greatest phase advance occurred when Phox2b neurons are stimulated during early- and mid-expiration, probably because these neurons can most productively activate pre-I/I “rhythmogenic” and early-I inhibitory neurons during this time window, thereby initiating a premature inspiration (Rubin et al, 2009).…”

Calcium imaging of neuronal activity in the most rostral parafacial respiratory group of the newborn rat

“…The group data from 14 experiments were: C4 burst rate 5.9 ± 1.5/min, pre-inspiratory phase duration 897 ± 590 ms, inspiratory phase 917 ± 101 ms, and the post-inspiratory phase duration 3593 ± 825 ms. This post-inspiratory phase duration was comparable to that of previous electrophysiological measurements [10]. In the present measurements, we could not clearly focus on individual stained cells because the dye injection method used here was effective in labeling cells located in regions slightly deeper than the cut surface (approximately 50 lm depth), but not for cells in the rostral cut surface plane due to insertion of dye-containing glass micropipettes into the tissue.…”

“…1). This cut was equivalent to a position approximately 0.5 mm rostral to the caudal end of the facial nucleus that corresponded to a level of the Xth cranial nerve root [7,10]. During the experiments, each preparation was placed in a 2-ml recording chamber and was superperfused continuously at the flow rate of 2.5-3 ml/min with the modified Krebs solution at a temperature of 25-26°C.…”

The respiratory control mechanisms in the brainstem and spinal cord: integrative views of the neuroanatomy and neurophysiology

“…Thus, this region is also referred to as the pFRG/RTN. The caudal portion of the pFRG overlaps the most rostral portion of the ventral respiratory group (the Bötzinger complex), which is the ventral part of the retrofacial nucleus near the caudal end of the facial nucleus [4] and is thought to play an important role in the respiratory rhythm generation, particularly of the adult in vivo preparation [5, 6]. This caudal portion of the pFRG corresponds to so-called rostral ventrolateral medulla (RVL) [7–9], where most Pre-I, inspiratory, and expiratory neurons have been recorded in previous electrophysiological studies.…”