Key pointsr Parvalbumin-containing (PV) neurons from mouse CA1 hippocampus (HC) and prefrontal cortex exhibit a fast spiking phenotype in vitro. Within CA1, HC PV cells are mainly comprised of basket and bistratified cell types.r Direct activation of muscarinic acetylcholine receptors (mAChRs) enhances excitability more in CA1 HC than in prefrontal cortex PV cells. r In vivo activation of M 1 mAChRs in PV cells is important in recognition and working memory but not spatial memory.Abstract Parvalbumin-containing (PV) neurons, a major class of GABAergic interneurons, are essential circuit elements of learning networks. As levels of acetylcholine rise during active learning tasks, PV neurons become increasingly engaged in network dynamics. Conversely, impairment of either cholinergic or PV interneuron function induces learning deficits. Here, we examined PV interneurons in hippocampus (HC) and prefrontal cortex (PFC) and their modulation by muscarinic acetylcholine receptors (mAChRs). HC PV cells, visualized by crossing PV-CRE mice with Rosa26YFP mice, were anatomically identified as basket cells and PV bistratified cells in the stratum pyramidale; in stratum oriens, HC PV cells were electrophysiologically distinct from somatostatin-containing cells. With glutamatergic transmission pharmacologically blocked, mAChR activation enhanced PV cell excitability in both CA1 HC and PFC; however, CA1 HC PV cells exhibited a stronger postsynaptic depolarization than PFC PV cells. To delete M 1 mAChRs genetically from PV interneurons, we created PV- Finally, relative to wild-type controls, PV-M 1 knockout mice exhibited impaired novel object recognition and, to a lesser extent, impaired spatial working memory, but reference memory remained intact. Therefore, the direct activation of M 1 mAChRs on PV cells contributes to some forms of learning and memory.

The activity of background potassium and sodium channels determines neuronal excitability, but physiological roles for "leak" Na ϩ channels in specific mammalian neurons have not been established. Here, we show that a leak Na ϩ channel, Nalcn, is expressed in the CO 2 /H ϩ -sensitive neurons of the mouse retrotrapezoid nucleus (RTN) that regulate breathing. In RTN neurons, Nalcn expression correlated with higher action potential discharge over a more alkalized range of activity; shRNA-mediated depletion of Nalcn hyperpolarized RTN neurons, and reduced leak Na ϩ current and firing rate. Nalcn depletion also decreased RTN neuron activation by the neuropeptide, substance P, without affecting pH-sensitive background K ϩ currents or activation by a cotransmitter, serotonin. In vivo, RTN-specific knockdown of Nalcn reduced CO 2 -evoked neuronal activation and breathing; hypoxic hyperventilation was unchanged. Thus, Nalcn regulates RTN neuronal excitability and stimulation by CO 2 , independent of direct pH sensing, potentially contributing to respiratory effects of Nalcn mutations; transmitter modulation of Nalcn may underlie state-dependent changes in breathing and respiratory chemosensitivity.

We discuss recent evidence which suggests that the principal central respiratory chemoreceptors are located within the retrotrapezoid nucleus (RTN) and that RTN neurons are directly sensitive to [H(+) ]. RTN neurons are glutamatergic. In vitro, their activation by [H(+) ] requires expression of a proton-activated G protein-coupled receptor (GPR4) and a proton-modulated potassium channel (TASK-2) whose transcripts are undetectable in astrocytes and the rest of the lower brainstem respiratory network. The pH response of RTN neurons is modulated by surrounding astrocytes but genetic deletion of RTN neurons or deletion of both GPR4 and TASK-2 virtually eliminates the central respiratory chemoreflex. Thus, although this reflex is regulated by innumerable brain pathways, it seems to operate predominantly by modulating the discharge rate of RTN neurons, and the activation of RTN neurons by hypercapnia may ultimately derive from their intrinsic pH sensitivity. RTN neurons increase lung ventilation by stimulating multiple aspects of breathing simultaneously. They stimulate breathing about equally during quiet wake and non-rapid eye movement (REM) sleep, and to a lesser degree during REM sleep. The activity of RTN neurons is regulated by inhibitory feedback and by excitatory inputs, notably from the carotid bodies. The latter input operates during normo- or hypercapnia but fails to activate RTN neurons under hypocapnic conditions. RTN inhibition probably limits the degree of hyperventilation produced by hypocapnic hypoxia. RTN neurons are also activated by inputs from serotonergic neurons and hypothalamic neurons. The absence of RTN neurons probably underlies the sleep apnoea and lack of chemoreflex that characterize congenital central hypoventilation syndrome.

Catecholaminergic neurons of the rostral ventrolateral medulla (RVLM-CA neurons; C1 neurons) contribute to the sympathetic, parasympathetic and neuroendocrine responses elicited by physical stressors such as hypotension, hypoxia, hypoglycemia, and infection. Most RVLM-CA neurons express vesicular glutamate transporter (VGLUT)2, and may use glutamate as a ionotropic transmitter, but the importance of this mode of transmission in vivo is uncertain. To address this question, we genetically deleted VGLUT2 from dopamine-β-hydroxylase-expressing neurons in mice [DβHCre/0;VGLUT2flox/flox mice (cKO mice)]. We compared the in vivo effects of selectively stimulating RVLM-CA neurons in cKO vs. control mice (DβHCre/0), using channelrhodopsin-2 (ChR2– mCherry) optogenetics. ChR2–mCherry was expressed by similar numbers of rostral ventrolateral medulla (RVLM) neurons in each strain (~400 neurons), with identical selectivity for catecholaminergic neurons (90–99% colocalisation with tyrosine hydroxy-lase). RVLM-CA neurons had similar morphology and axonal projections in DβHCre/0 and cKO mice. Under urethane anesthesia, photostimulation produced a similar pattern of activation of presumptive ChR2-positive RVLM-CA neurons in DβHCre/0 and cKO mice. Photostimulation in conscious mice produced frequency-dependent respiratory activation in DβHCre/0 mice but no effect in cKO mice. Similarly, photostimulation under urethane anesthesia strongly activated efferent vagal nerve activity in DβHCre/0 mice only. Vagal responses were unaffected by α1-adrenoreceptor blockade. In conclusion, two responses evoked by RVLM-CA neuron stimulation in vivo require the expression of VGLUT2 by these neurons, suggesting that the acute autonomic responses driven by RVLM-CA neurons are mediated by glutamate.

The C1 neurons, located in the rostral ventrolateral medulla (VLM), are activated by pain, hypotension, hypoglycemia, hypoxia, and infection, as well as by psychological stress. Prior work has highlighted the ability of these neurons to increase sympathetic tone, hence peripheral catecholamine release, probably via their direct excitatory projections to sympathetic preganglionic neurons. In this study, we use channelrhodopsin-2 (ChR2) optogenetics to test whether the C1 cells are also capable of broadly activating the brain's noradrenergic system. We selectively expressed ChR2(H134R) in rostral VLM catecholaminergic neurons by injecting Cre-dependent adeno-associated viral vectors into the brain of adult dopamine-␤-hydroxylase (D␤H)Cre/0 mice. Most ChR2-expressing VLM neurons (75%) were immunoreactive for phenylethanolamine N-methyl transferease, thus were C1 cells, and most of the ChR2-positive axonal varicosities were immunoreactive for vesicular glutamate transporter-2 (78%). We produced light microscopic evidence that the axons of rostral VLM (RVLM) catecholaminergic neurons contact locus coeruleus, A1, and A2 noradrenergic neurons, and ultrastructural evidence that these contacts represent asymmetric synapses. Using optogenetics in tissue slices, we show that RVLM catecholaminergic neurons activate the locus coeruleus as well as A1 and A2 noradrenergic neurons monosynaptically by releasing glutamate. In conclusion, activation of RVLM catecholaminergic neurons, predominantly C1 cells, by somatic or psychological stresses has the potential to increase the firing of both peripheral and central noradrenergic neurons.