Hypertension is a common outcome associated with obstructive sleep apnea (OSA), a prevalent yet poorly treated cardiovascular disease. Recent studies showed oxytocin (OXT), released from hypothalamic paraventricular nucleus (PVN) neurons, activates cardiac vagal neurons in the dorsal motor nucleus of the vagus (DMNX) and may blunt cardiovascular responses to stress. This study tests whether the release of OXT from PVN fibers in the DMNX is diminished with chronic intermittent hypoxia-hypercapnia (CIH/H) exposure, an animal model of OSA, and whether activation of PVN OXT neurons restores OXT release in the DMNX and prevents the hypertension resulting from CIH/H. To assess OXT release from PVN fibers, Chinese hamster ovarian (CHO) cells were engineered to be highly sensitive to OXT by stable expression of the human recombinant OXT receptor and the calcium indicator R-GECO1. PVN fibers in the DMNX were selectively photoactivated in vitro by expression of channelrhodopsin. The release of OXT onto CHO cells in the DMNX was blunted in rats exposed to 21 days of CIH/H. Chronic activation of PVN OXT neurons in vivo, using designer receptors exclusively activated by designer drugs, restored the release of OXT onto CHO cells in the DMNX. Chronic PVN OXT neuron activation in vivo also prevented the hypertension that occurred in conscious unrestrained telemetry-equipped sham rats exposed to 3 wk of CIH/H. These results demonstrate that chronic activation of OXT neurons restores the release of OXT from PVN fibers in the DMNX and prevents the hypertension that occurs with 3 wk of CIH/H exposure.

Recent advances in optogenetic methods demonstrate the feasibility of selective photoactivation at the soma of neurons that express channelrhodopsin-2 (ChR2), but a comprehensive evaluation of different methods to selectively evoke transmitter release from distant synapses using optogenetic approaches is needed. Here we compared different lentiviral vectors, with sub-population-specific and strong promoters, and transgenic methods to express and photostimulate ChR2 in the long-range projections of paraventricular nucleus of the hypothalamus (PVN) neurons to brain stem cardiac vagal neurons (CVNs). Using PVN subpopulation-specific promoters for vasopressin and oxytocin, we were able to depolarize the soma of these neurons upon photostimulation, but these promoters were not strong enough to drive sufficient expression for optogenetic stimulation and synaptic release from the distal axons. However, utilizing the synapsin promoter photostimulation of distal PVN axons successfully evoked glutamatergic excitatory post-synaptic currents in CVNs. Employing the Cre/loxP system, using the Sim-1 Cre-driver mouse line, we found that the Rosa-CAG-LSL-ChR2-EYFP Cre-responder mice expressed higher levels of ChR2 than the Rosa-CAG-LSL-ChR2-tdTomato line in the PVN, judged by photo-evoked currents at the soma. However, neither was able to drive sufficient expression to observe and photostimulate the long-range projections to brainstem autonomic regions. We conclude that a viral vector approach with a strong promoter is required for successful optogenetic stimulation of distal axons to evoke transmitter release in pre-autonomic PVN neurons. This approach can be very useful to study important hypothalamus-brainstem connections, and can be easily modified to selectively activate other long-range projections within the brain.

Autonomic control of heart rate is mediated by cardioinhibitory parasympathetic cholinergic neurons located in the brainstem and stimulatory sympathetic noradrenergic neurons. During embryonic development the survival and cholinergic phenotype of brainstem autonomic neurons is promoted by brain-derived neurotrophic factor (BDNF). We now provide evidence that BDNF regulates heart rate by a mechanism involving increased brainstem cardioinhibitory parasympathetic activity. Mice with a BDNF haploinsufficiency exhibit elevated resting heart rate, and infusion of BDNF intracerebroventricularly reduces heart rate in both wild-type and BDNF+/− mice. The atropine-induced elevation of heart rate is diminished in BDNF+/− mice and is restored by BDNF infusion, whereas the atenolol-induced decrease in heart rate is unaffected by BDNF levels, suggesting that BDNF signaling enhances parasympathetic tone which is diminished with BDNF haploinsufficiency. Whole-cell recordings from pre-motor cholinergic cardioinhibitory vagal neurons in the nucleus ambiguus indicate that BDNF haploinsufficiency reduces cardioinhibitory vagal neuron activity by increased inhibitory GABAergic and diminished excitatory glutamatergic neurotransmission to these neurons. Our findings reveal a previously unknown role for BDNF in the control of heart rate by a mechanism involving increased activation of brainstem cholinergic parasympathetic neurons.

Stimulation of the nasal mucosa by airborne irritants or water evokes a pronounced bradycardia accompanied by peripheral vasoconstriction and apnea. The dive response, which includes the trigeminocardiac reflex, is among the most powerful autonomic responses. These responses slow the heart rate and reduce myocardial oxygen consumption. Although normally cardioprotective, exaggeration of this reflex can be detrimental and has been implicated in cardiorespiratory diseases, including sudden infant death syndrome (SIDS). An essential component of the diving response and trigeminocardiac reflex is activation of the parasympathetic cardiac vagal neurons (CVNs) in the nucleus ambiguus that control heart rate. This study examined the involvement of cholinergic receptors in trigeminally evoked excitatory postsynaptic currents in CVNs in an in vitro preparation from rats. CVNs were identified using a retrograde tracer injected into the fat pads at the base of the heart. Application of the acetylcholinesterase inhibitor neostigmine significantly decreased the amplitude of glutamatergic neurotransmission to CVNs on stimulation of trigeminal fibers. Whereas nicotine did not have any effect on the glutamatergic responses, the muscarinic acetylcholine receptor (mAChR) agonist bethanechol significantly decreased the excitatory neurotransmission. Atropine, an mAChR antagonist, facilitated these responses indicating this trigeminally evoked brain stem pathway in vitro is endogenously inhibited by mAChRs. Tropicamide, an m4 mAChR antagonist, prevented the inhibitory action of the muscarinic agonist bethanechol. These results indicate that the glutamatergic synaptic neurotransmission in the trigeminally evoked pathway to CVNs is endogenously inhibited in vitro by m4 mAChRs.

In hypertension there is an autonomic imbalance in which sympathetic activity dominates over parasympathetic control. Parasympathetic activity to the heart originates from cardiac vagal neurons located in the nucleus ambiguus. Pre-sympathetic neurons that project to sympathetic neurons in the spinal cord are located in the ventral brainstem in close proximity to cardiac vagal neurons, and many of these pre-sympathetic neurons are catecholaminergic. In addition to their projection to the spinal cord, many of these pre-sympathetic neurons have axon collaterals that arborize into neighboring cardio-respiratory locations and likely release norepinephrine onto nearby neurons. Activation of α 2 adrenergic receptors in the central nervous system evokes a diverse range of physiological effects, including reducing blood pressure. This study tests whether clonidine, an α 2 adrenergic receptor agonist, alters excitatory glutamatergic, and/or inhibitory GABAergic or glycinergic synaptic neurotransmission to cardiac vagal neurons in the nucleus ambiguus. Cardiac vagal neurons were identified in a in-vitro brainstem slice preparation and synaptic events were recording using whole cell voltage clamp methodologies. Clonidine significantly inhibited GABAergic neurotransmission, but had no effect on glycinergic or glutamatergic pathways to cardiac vagal neurons. This diminished inhibitory GABAergic neurotransmission to cardiac vagal neurons would increase parasympathetic activity to the heart, decreasing heart rate and blood pressure. The results presented here provide a cellular substrate for the clinical use of clonidine as a treatment for hypertension as well as a role in alleviating posttraumatic stress disorder by evoking an increase in parasympathetic cardiac vagal activity, and a decrease in heart rate and blood pressure.