In vivo 3D MRI staining of mouse brain after subcutaneous application of MnCl2.
Magn Reson Med. 2002 Nov;48(5):852-9. PMID: 12418000
Watanabe T, Natt O, Boretius S, Frahm J, Michaelis T.

The objective of this paper was to examine the feasibility of using Mn²⁺ as an MRI contrasting agent without the need to disrupt the Blood Brain Barrier as has been done in other studies.  This would allow visualization of brain structures after systemic application of Mn²⁺, without the need for catheterization of the carotid artery to deliver mannitol solution for disruption of the BBB.

Results:

·         Areas with direct access to systemic circulation (anterior pituitary, choroid plexus) show strong increase (200%) at 6hr point.  Areas behind blood brain barrier (olfactory bulb, hippocampal CA3 region) take longer (24hrs) and show a weaker increase (10-25%)
·         Observed tract-like structures in the olfactory bulb – interpreted as evidence for axoplasmic transport of Mn²⁺
·         Saw bright superficial layers in the cerebellum similar to purkinje cell layers and dark deep layers representative of white matter.
·         Saw an increase in CA3 (input region), but not in CA1 (output region)

Conclusions:

·         MRI enhancement of selective areas is by differential uptake of Mn²⁺ through Ca²⁺ channels during action potentials.  Differences in Mn²⁺ content reflect differences in neuron activity during the time after the MnCl₂ injection
·         Differences may also rely on differential expression of enzymes that utilize Mn²⁺ (e.g. glutamine synthetase contains 8 Mn²⁺ ions and contains ~80% of Mn in the brain, SNR was higher in CA3 which is known to have high expression of manganese superoxide dismutase and lower in CA1 that doesn’t)

Methods:  Mice were given subcutaneous injections of MnCl₂.  After IP anesthesia, MRI was performed at 0, 6, 24, and 48hrs in order to compare the Mn²⁺ enhancement in 13 different regions of the brain.  Evaluation of Mn²⁺ enhancement was done by comparing the Signal to Noise ratio, where SNR=(Signal/StdDev)

-DH

PACAP causes PAC1/VPAC2 receptor mediated hypertension and sympathoexcitation in normal and hypertensive rats.

Am J Physiol Heart Circ Physiol. 2012 Oct 1;303(7):H910-7. 
Farnham MM, Lung MS, Tallapragada VJ, Pilowsky PM.

"Pituitary adenylate cyclase-activating polypeptide (PACAP) is an excitatory neuropeptide that plays and important role in hypertension and stress responses". 

The authors proposed an idea that PACAP acts as a sympathoexcitatory agent in the RVLM of normotensive and hypertensive rats. The had 3 specific aims for the experiment and tested them using different mechanisms as in described in the above illustration. The major findings of this study is that injection of PACAP in the RVLM caused an increase in splanchnic SNA, heart rate and MAP in SHR, WKY and SD rats. Pretreatment with PAC1/VPAC2 receptor antagonist PACAP (6-38) attenuated this response but did not produce any changes when injected alone suggesting that PACAP receptors are not tonically active. PACAP (6-38)  in the RVLM produced no changes in sSNA or MAP in the SHR suggesting they may not play a role in maintaining hypertension in SHR.

-Madhan

Exercise training causes sympathoinhibition through antioxidant effect in the rostral ventrolateral medulla of hypertensive rats

Clin Exp Hypertens. 2012;34(4):278-83.
Kishi T, Hirooka Y, Katsuki M, Ogawa K, Shinohara K, Isegawa K, Sunagawa K.

The authors were interested to know whether exercise training affects sympathetic nerve activity through central mechanisms in stroke-prone spontaneously hypertensive rats (SPSHR). They performed a series of experiments to determine the role of oxidative stress and angiotensin II in this pathway. First, they implanted radio-telemetry system to measure mean arterial pressure and heart rate in both SHRSP and Wistar-Kyoto (WKY) rats. The animals were seperated into 4 groups exercise trained (Treadmill) and non exercise trained for 28 days. After 28 days, urinary norepinephrine was measured as an indicator of sympathetic nerve activity. They measured conscious baroreflex sensitivity by spontaneous sequence method. Thiobarbituric acid-reactive substances were measured from RVLM punches as an indicator of oxidative stress. Tempol, a superoxide dismutase mimetic and angiotensin II were microinjected into the RVLM to determine the pathway. The overall findings are that exercise training in SHRSP caused sympathoinhibition and improved baroreflex sensitivity and reduced oxidative stress through blocked AT1R in the RVLM.

-Madhan

Oxidative stress in the rostral ventrolateral medulla modulates excitatory and inhibitory inputs in spontaneously hypertensive rats

J Hypertens. 2012 Jan;30(1):97-106.  

Nishihara M, Hirooka Y, Matsukawa R, Kishi T, Sunagawa K.

This study aims at answering 3 important questions.

1. Determine whether chronic oxidative stress in the RVLM alters synaptic transmission through glutamatergic inputs.

2. Determine whether chronic oxidative stress in the RVLM alters synaptic transmission through GABA-ergic inputs.

3.  Determine whether chronic oxidative stress in the RVLM alters synaptic transmission through excitatory inputs from the PVN.

Adenoviral vectors encoding MnSOD was transferred into the RVLM of male SHRs and WKYs. Microinjection studies using Kynurenic acid (Glutamate receptor antagonist) and Bicuculline (GABA receptor antagonist) showed that reactive oxygen species (ROS) in the RVLM enhanced the excitatory glutamatergic inputs and reduced the GABAergic inputs to the RVLM neurons in SHRs. In addition, activation of the PVN neurons enhanced the pressor and sympathoexcitation through ROS in the RVLM. These findings suggest that ROS in the RVLM increases glutamatergic excitatory inputs and decreases GABAergic inputs in SHRs.

Madhan

SELECTIVE CONTROL OF SYMPATHETIC PATHWAYS TO THE KIDNEY, SPLEEN AND INTESTINE BY THE VENTROLATERAL MEDULLA IN RATS

 

J Physiol. 1990 September; 428: 371–385. K Hayes and L C Weaver

In this study, whether RVLM neurons produce differential responses to postganglionic renal, splenic and mesentric nerves were determined. Earlier studies in cats by other suggested that tonic influences from the RVLM to different sympathetic nerves are not uniform in distribution. Glycine was microinjected unilaterally in the RVLM of  Urethane anaesthetized male rats. Blockade by glycine produced a greater reductions in the renal than splenic nerve activity with no consistent changes in mesentric nerve activity. Interestingly the preganglionic nerves were not blocked to the same extend as the postganglionic nerves. It is possible that loss of activity in a small number of preganglionic axons could cause a greater loss of activity in the postganglionic neurons since many preganglionic inputs must get together to produce an action potential in the ganglionic cell. The findings from this study suggest that the sympathetic pathways to the kidney are more dependent on the excitatory drive from the RVLM than spleen and intestine. Selective effects of different sympathetic nerves can occur because of topographical organization within the medulla.  Questions such as how the brain stem selectively control different sympathetic outflows are yet to be answered.

-Madhan

Cardiovascular Effects Produced By Activation of GABA Receptors In The Rostral Ventrolateral Medulla of Conscious Rats

The major inhibitory neurotransmitter is GABA .As we already know anesthesia can potentiate the effects of GABA. In this article they looked at the role of GABA on GABA _A and GABA _B receptors in conscious animals. They used Wistar rats. They only looked at blood pressure and heart rate. Their finding suggest that the GABA _A receptor is the most important for mediating the inhibitory response under conscious conditions however , GABA _B receptor were not important for the transduction of inhibitory signals. This suggests that anesthesia might alter the GABA B receptor or even potentiate the effects of GABA binding to GABA _B receptors in anesthetized animals.

Neurons of the rostral ventrolateral medulla mediate somatic pressor reflex

Stornetta RL, Morrison SF, Ruggiero DA, Reis DJ.

Department of Neurology, Cornell University Medical College, New York, New York 10021

This article was investigating whether rostral ventral lateral is mediating the somatic pressor reflex. The somatic pressor reflex is activated when the sciatic nerve for example is stimulated, this will cause an increase in arterial pressure along with SNA. The SPR had been identified in cats and dogs however not in rats, so they investigated this response in rats. Where the information from the stimulation travel up afferent nerve was been integrated into a response that would cause changes in Arterial Pressure (AP) was unknown. Since they knew that RVLM neurons played role in tonically and in reflex control of blood pressure along with the fact that the neurons in RVLM had an excitatory effect on sympathetic preganglionic neuron in the IML, this lead them to believe that the SPR may be mediated by the RVLM. So what the concluded from the study was the following:

1.       Stimulation of the sciatic, sural and saphenous caused increases in AP and HR  which is similar dog and cat

2.       Stimulation of muscle nerves such as tibial or femoral nerves resulted in decreases in AP and HR.

3.       After transection of the anterior pons the SPR was still functioning suggesting that the SPR is controlled by a brain region of somewhere below the pons

4.       Lesions of the LRN  reduced the SPR but when kainic(before lesion) was injected into the LRN the SPR was still functional meaning that when the LRN was not the brain region mediating this responses but when the area was lesion there may have been nerve fibers destroyed or because LRN is so close to RVLM some of the neurons in this area could have been damaged

5.       Finally they found that if there is unilateral lesion of an RVLM and the contralateral sciatic is stimulated then the response will be blocked.