C1 neurons excite locus coeruleus and A5 noradrenergic neurons along with sympathetic outflow in rats

Abstract

C1 neurons activate sympathetic tone and stimulate the hypothalamic–pituitary–adrenal axis in circumstances such as pain, hypoxia or hypotension. They also innervatepontine noradrenergic cell groups, including the locus coeruleus (LC) and A5. Activation ofC1 neurons reportedly inhibits LC neurons; however, because these neurons are glutamatergicand have excitatory effects elsewhere, we re-examined the effect of C1 activation on pontinenoradrenergic neurons (LC and A5) using a more selective method. Using a lentivir us thatexpresses channelrhodopsin2 (ChR2) under the control of the artificial promoter PRSx8, werestricted ChR2 expression to C1 neurons (67%), retrotrapezoid nucleus neurons (20%) andcholinergic neurons (13%). The LC contained ChR2-positive terminals that formed asymmetricsynapses and were immunoreactive for vesicular glutamate transporter type 2. Low-frequencyphotostimulation of ChR2-expressing neurons ac tivated LC (38 of 65; 58%) and A5 neurons (11 of16; 69%) and sympathetic ner ve discharge. Locus coeruleus and A5 inhibition was not seen unlesspreceded by excitation. Locus coeruleus activation was eliminated by intracerebroventricularkynurenic acid. Stimulation of ChR2-expressing neurons at 20 Hz produced modest increasesin LC and A5 neuronal discharge. In additional rats, the retrotrap ezoid nucleus region wasdestroyed with substance P–saporin prior to lentivirus injection into the rostral ventrolateralmedulla, increasing the propor tion of C1 ChR2-expressing neurons (83%). Photostimulation inthese rats activated the same proportion of LC and A5 neurons as in control rats but producedno effect on sympathetic nerve discharge owing to the destruction of bulbospinal C1 neurons. Inconclusion, low-frequency stimulation of C1 n eurons activates pontine noradrenergic neurons.