Noradrenergic Input from Nucleus of the Solitary Tract Regulates Parabrachial Activity in Mice

Laterality of noxious heat stimulus has little effect on NE transients. Noxious thermal stimuli applied to the face (A) or hindpaw (B) produce NE transients that are similar in magnitude and duration whether applied contralaterally or ipsilaterally to the fiber optic probe in PB. In each graph, the light gray traces represent the mean response of an individual mouse to approximately five stimuli. The black trace and shaded region indicate the group mean with 95% confidence intervals. The approximate duration of stimulation across all mice is indicated by the colored bar below each set of traces. The magnitude of transients evoked by contralateral or ipsilateral were not different from each other when measured as the area under the curve (C) or peak ΔF/F (D). Paired t test, p > 0.05.

Contralateral mechanical stimuli produce larger magnitude NE transients in PB. A, Responses of individual mice (light traces) and mean with 95% confidence intervals (dark trace with shaded region) to noxious pinch applied to the hindpaws (green bar). (B) Contralateral stimuli produced transients that were greater in magnitude measured as the area under the curve and peak ΔF/F. C, Responses of individual mice (light traces) and mean with 95% confidence intervals (dark trace with shaded region) to noxious pinch applied to the tail (green bar) and corresponding area under the curve and peak ΔF/F values in D. Paired t test, *p < 0.05, **p < 0.01.

NE transient onset latencies vary by stimulus modality. Expanded views of NE transients evoked by mechanical (A) or thermal (C) stimulation (group mean and 95% CIs). Latencies from the beginning of the stimulus to onset of the NE2h response were shorter for mechanical stimuli (B) and varied by stimulus location for thermal stimuli (D), likely reflecting the rate at which each stimulus became noxious. There were no differences in latencies between contralateral and ipsilateral stimuli of the same modality and location. Paired t test, p > 0.05.

Electrical stimulation of cNTS generates long lasting NE transients in PB. A, Example heat maps from one mouse showing the change in NE2h fluorescence evoked by electrically stimulating cNTS at different frequencies but fixed intensity. The letters “C” and “I” indicate trials where the stimulus was applied to the contralateral and ipsilateral cNTS relative to the fiber photometry probe. B, Mean responses for each stimulus frequency for the data shown in A. We observed a similar frequency-dependent response in three mice (C). There was no effect of stimulation side at any frequency, so data from contralateral and ipsilateral stimuli were combined for analysis (paired t test, p > 0.05).

cNTS neurons respond to noxious stimulation. Raster plots and peristimulus time histograms of single unit recordings from unidentified cNTS neurons responding to noxious mechanical (A) and thermal (B) stimuli. C, Group data from 8 neurons (3 mechanical and 5 thermal) reveal a significant response to noxious stimuli by cNTS neurons (Wilcoxon matched-pairs test, *p < 0.05), with onset latencies that may reflect the slow rate at which the stimulus became noxious (D).

cNTS_cat afferent stimulation potentiates excitatory synaptic transmission in PB. A, Representative voltage clamp recording from a PB neuron showing the sustained increase in synaptic activity after cNTS_cat afferent stimulation (10 Hz, 10 s, red bars). B, Synaptic activity increased to a mean of 370% of prestimulation values in eight neurons. There was no effect of cNTS_cat stimulation on event amplitude (C), despite a small reduction in membrane resistance (D). The cNTS_cat-evoked increase in sEPSC frequency was prolonged in five neurons and decayed back to prestimulus values with a time constant of 100 s (E).