The effect of air puff stress on c-Fos expression in rat hypothalamus and brainstem: central circuitry mediating sympathoexcitation and baroreflex resetting.

Furlong TM, McDowall LM, Horiuchi J, Polson JW, Dampney RA. Eur J Neurosci. 2014 Mar 12. [Epub ahead of print] This was a pretty neat paper in showing the complexity of sympathetic control of blood pressure. They took rats, set them up for telemetry recording of heart rate and arterial pressure, and gave they psychological stressors in the form of repeated air puffs in the face. They then looked at what regions of the brain would come up as c-Fos positive. I was pretty surprised to see that the RVLM was pretty much not involved at all. They were able to show c-Fos expression in the RVLM after prolonged hypotension (via nitroprusside), just not after air puffs. They did see expression in NTS-projecting neurons in the VPAG and PVN though (CTB costain). So is it just me, or is it really cool how physical and psychological stressors can tie in to the same system, but still be completely separate. -DH

Activity-dependent regulation of NMDA receptors in substantia nigra dopaminergic neurones.

Wild AR, Jones S, Gibb AJ. J Physiol. 2014 Feb 15;592(Pt 4):653-68. doi: 10.1113/jphysiol.2013.267310. Epub 2013 Dec 16. Because Madhan is investigating the possibility of physical activity causing a downregulation in NMDA-Receptors, this paper caught my eye. In this study, the authors looked how dopaminergic cells show acute rundown in NMDAR currents in brain slices containing the substantia nigra (the area known to suffer massive cell loss in Parkinson’s Disease) after repeated stimulation. They used whole-cell patch clamp to recorded NMDAR currents after repeated doping of the perfusion medium with NMDA. They found that rundown occurred in a couple of different ways. Some of it was dependent on calcium influx (NMDAR allows calcium ions to pass through in addition to sodium ions), which they could show by voltage clamping the cell near the reversal of calcium. However, even after replacing extracellular Ca2+ with Ba2+, significant rundown occurred, suggesting there are calcium-independent mechanisms in effect too. They suspected that the Glu2NB receptor subunit was involved, but were not able to block current rundown after using a Glu2NB-preferring antagonist. They were, however, able to block current rundown with a dynamin antagonist, suggesting that receptor regulation occurs through clathrin-mediated endocytosis. One thing that was pretty cool was how they also patched on to GABAergic neurons in the same region and found that they had a higher rate of rundown, suggesting that they have a built in mechanism for protecting themselves against excitotoxicity that dopaminergic neurons lack. -DH

Autonomic regulation of brown adipose tissue thermogenesis in health and disease: potential clinical applications for altering BAT thermogenesis

Autonomic regulation of brown adipose tissue thermogenesis in health and disease: potential clinical applications for altering BAT thermogenesis. Domenico Tupone, Christopher J. Madden and Shaun F. Morrison. Frontiers In Neuroscience 9:1-14, 2014 (open access) doi: 10.3389/fnins.2014.00014. This is an excellent and timely review from the standpoint of the field and in terms of our collaboration with Dr. Granneman's laboratory. Shaun Morrison has been studying brown adipose tissue or BAT for a number of years. His and his postdoc's (Chris Madden) work have really come into clinical significance with the discovery of BAT in humans and its ties to obesity. Basically BAT is a source of thermogenesis and therefore can be important in terms of burning calories to stay warm in a cold environment. Interestingly for our laboratory is the fact the BAT is innervated by the sympathetic nerves which release norepinephrine onto beta 3 receptors to activate BAT. Dr. Morrision's laboratory has worked out a lot of the central circuitry or brainstem pathways by which BAT and BAT SNA is regulated under a variety of conditions, mostly associated with cold temperature exposure and the turning on of BAT. Interestingly, BAT is one set of sympathetic nerves that are not controlled by the RVLM but by a neighboring structure in the ventral medulla, the midline raphe. There are some really beautiful figures in this review and is worth reading for anyone interested in BAT, BAT SNA, and pathwways involved in thermogenesis. In the next few weeks in fact, Madhan and Priya hope to have BAT SNA recordings up and going in the laboratory, comparing runners versus sedentary rats. Should be exciting to see it develop and be sure to keep an eye out at Experimental Biology for both Shaun Morrison's and Chris Madden's work. ~PJM

Central Command Neurons of the Sympathetic Nervous System: Basis of the Fight-or-Flight Response.

Central Command Neurons of the Sympathetic Nervous System: Basis of the Fight-or-Flight Response. Arthur S. P. Jansen, Xay Van Nguyen, Vladimir Karpitskiy,Thomas C. Mettenleiter, Arthur D. Loewy. Science 270:644-646, 1995. This is a classic paper by Arthur Loewy's group at Washington University in St. Louis in which they propose the existence of "central command" neurons. These central command neurons are hypothesized to innervate multiple sympathetic targets and be responsible for the all or none, fight or flight response that has been traditionally associated with activation of the sympathetic nervous system. One thing to keep in mind here was that this paper was published in the mid-1990s when it was still controversial whether there were neurons that controlled some or all of the sympathetic outputs, whether there existed individual neurons that controlled individual sympathetic outputs, or if both types existed (likely the reality). Unlike the Australians (i.e. McAllen and colleagues), who had just the year prior shown differential control of sympathetic outputs with very small (5ul) microinjections in the RVLM of cats, Loewy's group was trying to demonstrate the reverse idea; that is, individual neurons have the anatomical connections to control multiple sympathetic outputs. To do so, they injected rats with two different viruses to produce retrograde and transynaptic tract tracing. They put one virus in the stellate ganglion which contains the axons of sympathetic preganglionics to the heart and they put a different virus in the adrenal gland which contains the sympathetic preganglionics controlling epinephrine release. While they did show that both viruses wound up within cells of the RVLM, there were several caveats pointed out in the paper and additional ones not pointed out. First, the number of cells that did show double labeling in the RVLM were very small and if you look in the Methods sections you will see it took them hundreds of rats to get this to work out. Second, there are several technical issues in using viruses that likely preclude definitive conclusions about the double-labeled cells. In any case, it was published in Science and is often quoted as the paper that demonstrated the existence of these neurons. It also propagates the still pervasive idea that sympathetics are an all or none phenomenon. We know now that this is not the case in several instances of physiology and pathophysiology. ~PJM

C1 Neurons: The Body's EMTs

Guyenet, Patrice G., et al. "C1 neurons: the body's EMTs." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 305.3 (2013): R187-R204. Definition: C1 neurons commonly thought of as the body’s emergency medical technicians, are defined as neurons containing phenylethanolamine N-Methyl transferase (PNMT). The enzyme PNMT is allows for the production of the catecholamine epinephrine from norepinephrine. The majority of C1 neurons also have the enzymes tyrosine hydroxylase and dopamine β-hydroxylase as well which allows for the synthesis for norepinephrine and dopamine. Presently, it is suspected that most C1 neurons are clustered in the ventrolateral medulla (VLM), however the VLM also contains A1 neurons (catecholaminergic neurons that do not make PNMT). C1 neurons have projections to a variety of areas that largely play a role in autonomic responses or stress behaviors. Although C1 neurons are classified as catecholaminergic, there is no direct evidence of catecholamine release to intended targets. Instead, most C1’s are thought to use glutamatergic signaling transmission mainly through the use of vesicular glutamate transporter 2. There is also evidence to show the release of certain neuropeptides along with glutamate including TRH, Substance P, NPY and Enkephalin to produce a greater variety of excitatory and or inhibitory effects. About one third of all C1 neurons are located specifically in the rostral ventrolateral medulla (RVLM) and target sympathetic pre-ganglionic neurons. This region is referred to as the “pressor” region due to the fact that excitation of these neurons with glutamate leads to an increase in arterial blood pressure via vasoconstriction and cardimotor output. It is thought then that the responsibility of these C1 neurons within the RVLM is to maintain arterial blood pressure on a second to second basis. Regulation of these tonic neurons comes from another region in the VLM called the intermediate VLM (CVLM) that contains GABAergic neurons, and is meditated by a mechanism known as the baroreflex. It was also found that non-C1 neurons also reside in the RVLM, but their exact function is unknown. Interestingly, it was found that if the majority of C1 neurons are killed off using anti-DβH-saporin only a 10 mmHg change in arterial blood pressure is witnessed. Factors that could potentially account for this are compensation with non-C1 neurons, volume expansion of the kidney, baroreflex compensation, or peripheral catecholaminergic receptor supersensitivity. Other autonomic/stressor responses C1 neurons have been linked to include hypoxia, the CRH-ACTH-Corticosterone Cascade, the glucoprivic responses, as well as some parasympathetic sympathetic responses. Although it is thought that C1 neurons contribute to homeostatic regulation under a variety of physiological conditions, there are still many questions and functions yet to be discovered and understood. ~JI

Exercise training attenuates hypertension and cardiac hypertrophy by modulating neurotransmitters and cytokines in hypothalamic paraventricular nucleus.

PLoS One. 2014 Jan 17;9(1):e85481. Jia LL, Kang YM, Wang FX, Li HB, Zhang Y, Yu XJ, Qi J, Suo YP, Tian ZJ, Zhu Z, Zhu GQ, Qin DN. “Regular exercise as an effective non-pharmacological antihypertensive therapy is beneficial for prevention and control of hypertension, but the central mechanisms are unclear”. The authors investigated whether exercise training in spontaneously hypertensive rats (SHRs) could delay the progression of hypertension and reduce cardiac hypertrophy in them by balancing the excitatory and inhibitory neurotransmitters and pro and anti-inflammatory cytokines in the paraventricular nucleus (PVN). The rats were treadmill trained from 7 to 16 weeks. The authors observed that sedentary SHRs had higher mean arterial pressure and cardiac hypertrophy and these factors were significantly attenuated in the exercise trained SHRs. They authors found that sedentary SHRs had greater concentration of glutamate and norepinephrine and lower concentration of GABA in the PVN compared to their exercise trained counterparts. The authors also measured a number of pro and anti-inflammatory cytokines in the PVN and plasma to suggest that these factors could be involved in mediating reduced sympathetic nerve activity and blood pressure observed in the exercise trained SHRs compared to sedentary SHRs. More translational approach would take these findings along with other similar findings to the next level.-Madhan

Exercise training lowers the enhanced tonically active glutamatergic input to the rostral ventrolateral medulla inhypertensive rats.

CNS Neurosci Ther. 2013 Apr;19(4):244-51. Zha YP, Wang YK, Deng Y, Zhang RW, Tan X, Yuan WJ, Deng XM, Wang WZ. “It is well known that low-intensity exercise training (ExT) is beneficial to cardiovascular dysfunction in hypertension”. The authors investigated the effects of exercise training on glutamatergic inputs in the RVLM of a spontaneously hypertensive rats. The animals were treadmill trained for about 12 weeks. The authors observed that exercise trained SHR rats had significantly blunted responses of arterial pressure, heart rate and renal sympathetic nerve activity to bilateral microinjection of Kynurenic acid in the RVLM compared to sedentary SHRs. The authors found that exercise trained SHRs had reduced glutamate concentration (measured by HPLC) and the lower protein expression of vesicular glutamate transporter 2 (that packs the glutamate in the presynaptic terminal). The authors suggest that exercise training lowered the enhanced glutamatergic input in the RVLM in SHRs and this could be the possible mechanism by which it reduced blood pressure and sympathetic nerve activity in them.-Madhan