Manganese-Enhanced Magnetic Resonance Imaging as a Diagnostic and Dispositional Tool after Mild-Moderate Blast Traumatic Brain Injury

Rodriguez et al.
Journal of Neurotrauma. April 2016, 33(7): 662-671.

   In this paper, they used a form of manganese-enhanced MRI (MEMRI) to study the effects of blast traumatic brain injury, a severe form of TBI which sometimes leads to long term neuropsychiatric problems. They were looking for markers in mouse brains which might lead to markers to identify injury in humans. In order to do this, they worked with a specially designed chamber that used compressed helium to send blast waves of different strengths at anesthetized mice.  Mice were then given IP injections of 40mg/kg manganese. Mice brains were imaged using T2 scans to be sure there were no major structural changes or damage, and T1 weighted scans to examine manganese uptake at 6, 24, 48, 72hrs, and 14 and 28 days after the helium blast.
   Blast-exposed mice showed greater transmission of manganese across the blood-csf barrier at 6 hours post-injection compared to controls. By 24 hours, this increased uptake caused greater signal throughout the brain in blast mice. The differences lasted for at least a week in all regions, in some regions after two weeks, and disappeared after a month. They found that in a few tissues, signal intensity kept increasing for 72 hours before going back down.
   One very interesting thing in this study was that they tried to use a 0.01M Mn phantom in their scans as a control, but found that its image values were more variable than expected from scan to scan (presumably due to microscopic air bubbles and changes in position of the phantoms), but the brain ROIs they studied didn't show the same variability, so they resorted to using non-blast-exposed saline-vehicle-injected mice as image quality controls for normalizing T1 values.
   There was also one other finding that I have a hard time understanding. They found that if mice underwent the blast treatment while wearing little mousey body armor, the manganese enhancement in the brain was attenuated and happened at levels similar to those in a mouse not exposed to a blast. -DH

Lateral tegmental field neurons of cat medulla: a source of basal activity of ventrolateral medullospinal sympathoexcitatory neurons.

Barman SM, Gebber GL.
J Neurophysiol. 1987 May;57(5):1410-24.

   The RVLM does not exist all by itself, no matter how much I focus my attention on it, that is not going to change. This goal of this paper was to look at another region involved in control of SNA, the lateral tegmental field (LTF) of the cat medulla, and how it interacted with the RVLM. Both the RVLM and the LTF  contain neurons with activity correlated with cardiac SNA, slow or stop their rate of firing with increases in blood pressure, and glutamate or electrical stimulation of both areas increases SNA. The hypothesis was that the LTF drives activity in the RVLM, which then drives SNA.
   They located neurons in the LTF, were able to antidromically activated them from the ipsilateral or contralateral RVLM (conduction velocity ~0.5m/s), and check them for barosensitivity and cardiac related rhythmic activity. They also found a few neurons that had 2 different conduction velocities when the stimulating electrode was placed at different depths of the RVLM, suggesting multiple axon branches within the RVLM region from the same LTF neuron that either took a short path and a long path, or had very different properties between collaterals. The long/slow path was always more ventral than the short/fast path, with the ventral sites being located outside the RVLM.
   Then things got more complicated!  They found that if they recorded spinally projecting RVLM neurons and stimulated in the LTF, they could find cases where some RVLM neurons could be antidromically activated, some neurons could be synaptically activated (variable latency action potentials that could not follow at high frequencies), and some neurons showed both characteristics, suggesting reciprocal communication between the LTF and the RVLM. They also found a number of RVLM neurons which could be antidromically activated by stimulating either in the T2 IML or the LTF, which indicates the presence of collateralizing axons with both ascending and descending projections.
   There was a lot more to this paper that I did not cover in this blog entry, including where the axons of RVLM neurons likely branched, latencies of efferent signals from the different regions, discusson on potentially sympathoinhibitory cells in the LTF, and more. There's a lot of info in this paper and it's worth a couple of reads to try to get it all. -DH

Morphology of rostral medullary neurons with intrinsic pacemaker activity in the rat.

Sun MK, Stornetta RL, Guyenet PG.
Brain Res. 1991 Aug 9;556(1):61-70.

   In slices, and in some in vivo preparations, some presympathetic RVLM neurons fire spontaneously, even when glutamate receptors are blocked. They had previously shown that spontaneously active spinally projecting RVLM neurons were non-C1 type cells, so this study followed up on that by looking at these "pacemaker" neurons and get phentotypic information about them, including cell type and morphology. To do this, they used rat medulla slices and intracellular recording with peroxidase- or lucifer yellow-filled electrodes. They located and recorded from spontaneously active cells before injecting the labeling compound.
   They found that the pacemaker cells had fusiform or triangular bodies that were significantly larger compared to 46 PNMT positive cells they measured, though they note that there may have been some bias toward recording larger cells. They found that the cells had fairly simple dendrites which mostly spread toward the ventral surface of the slice and then out to the ventromedial and dorsolateral edges, which didn't seem to be unique to the pacemaker cells (C1 cells may also fit this pattern). This made a lot of sense since previous experiments had been done by some groups simply by dripping drugs onto the ventral surface of the RVLM.
   They identified what they believed to be axons (though this was not confirmed by electron microscopy) through their thin, smoother appearance which was different from the light branching seen on most dendrites. The axons projected dorsally and medially, similar to C1 cells. This was in contrast to the lateral course seen by another group, though they suggest that might have been a different subpopulation of cells. They also did not find axonal branching (ascending and descending) seen by others in C1 neurons and say this may be more evidence of different cell types. I admit to being a little cofused about this interpretation since they say the branching has been shown by others to be ~2.6mm away from the body, and their slices were only 500um thick. They had to use cells towards the middle of their slices to limit cutting off dendrites, so that means they could only really observe ~250um. Maybe I'm just misinterpreting the text, because in the conclusions they state that their prior electrophysiological studies DO support axonal branching. Either way, it is nice to learn more about the types of cells I can expect to see in my reconstruction study. -DH

Identification of C1 presympathetic neurons in rat rostral ventrolateral medulla by juxtacellular labeling in vivo.

Schreihofer AM, Guyenet PG.
J Comp Neurol. 1997 Nov 3;387(4):524-36.

   Before this paper, it was known that some RVLM neurons were barosensitive, but showing the phenotypes of which ones were linked to SNA had been difficult. There were multiple lines of evidence stating that C1 neurons were involved, but the presympathetic non-C1 cells complicated the issue. In fact, experiments that measured conduction velocities of spinally projecting neurons suggested that C1 neurons might not be a major player, despite making up 50-70% of the cells projecting to the IML.

   In this study, they recorded 96 cells in 41 rats, and 87% of the cells were spinally projecting (tested via antidromic activation). Many times the blood pressure needed to be increase to stop spontaneous action potentials in order to induce an antidromic one. The distance between recording and stimulating electrodes was estimated at 35mm, which allowed them use constant antidromic latencies to estimate conduction velocity. They attemped to label 67 of the 87 cells and recovered 49 of the ones they attempted. They found that faster conducting (and more spontaneously active) cells were harder to label than slower ones (presumed to be C1). When they looked for TH immunoreactivity, they were able to recover 39 labeled, processed, spinally projecting cells.

   The cells broke roughly into thirds as non-TH-ir cells, slow TH-ir cells, and fast TH-ir cells. They applied a metric of TH immunoreactivity densitometry to find that slow TH-ir cells had much greater signal than fast TH-ir cells. All slow firing cells were TH-ir. They confirmed this by labeling analyzing another set of 5 labeled cells (3 slow, 2 fast) and staining for PNMT and got the same results - slow cells showed stain while fast cells didn't. They also reconstructed the neurons similar to how our lab has, but they didn't find any differences between the 3 types of neurons in terms of size or shape, which is something I would have been interested in seeing.

   The main findings here were that ~70% of the spinally projecting cells they found were indeed C1 cells, and that ALL of the slow firing spinally projecting cells were C1. This was also evidence that C1 are inhibited by increases in blood pressure, and their function was most likely sympathoexcitatory. It also helps explain why previous papers did not report C1 spinally projecting neurons - because they only recorded fast neurons, which selected against ~50% of the C1 neurons. -DH

A five-parameter logistic equation for investigating asymmetry of curvature in baroreflex studies.

Ricketts JH, Head GA.
Am J Physiol. 1999 Aug;277(2 Pt 2):R441-54.

   This paper started off with the first two pages as a review of different techniques which had previously been used to mathematically describe baroreflex curves measuring arterial pressure against nerve activity and heart rate. It then went on to describe a new method, a 5-parameter equation, that was predicted to be able to account for asymmetry sometimes seen in baroreflex curves. The argument in favor of this new method was that it would be able to fit a curve better when the curve showed asymmetry between upper and lower limits, or differences in slope on either side of the midpoint. If no such asymmetry existed, the 5th parameter was averaged out and the formula was able to approximate the curve the same way a traditional 4-parameter equation would.

   The paper then went on to analyze pre-existing baroreflex data of MAP vs heart rate and renal nerve sympathetic activity in rabbits and dogs via 4- and 5- parameter curves - in both cases, they tested by forcing the curves to fit to the resting points and tested again without forcing them. They found that when there was an asymmetry in the curves of some subjects, the 5-parameter equation could account for it, though for the most part the 5-parameter method did not report values that were significantly different than the 4-parameter method. The biggest changes seemed to be caused by forcing the curve through the resting point, and according to this paper, doing that seems like a questionable technique. The paper did make a good point that even though this technique may not result in significantly different results over the 4-parameter technique in most cases, in cases where asymmetry is present, it will yield a more accurate result - and if you need it for one parameter, you should use the same technique for all parameters within a study. I think that's a pretty fair argument. -DH

Physical Activity and Cardiorespiratory Fitness Are Beneficial for White Matter in Low-Fit Older Adults

Agnieszka Zofia Burzynska, Laura Chaddock-Heyman, Michelle W. Voss, Chelsea N. Wong, Neha P. Gothe, Erin A. Olson, Anya Knecht, Andrew Lewis, Jim M. Monti, Gillian E. Cooke, Thomas R. Wojcicki, Jason Fanning, Hyondo David Chung, Elisabeth Awick, Edward McAuley, and Arthur F. Kramer

PLoS One. 2014; 9(9): e107413.

Our lab studies effects of sedentary vs physical activity on the brain and have shown an active lifestyle to be overall beneficial to cardiovascular health. I found this paper exciting because it looks at not only how an active lifestyle and good cardiovascular health can have positive effects on white matter later in life but also the neural correlates for these relationships. Using 88 healthy low-fit adults they examined the relationship between cardiorespiratoy fitness (CRF) with three different levels of physical activity (PA) and how it affected the levels of white matter (WM). Adults used for this study were between the ages of 60-79 years old, had no history of stroke or neurological illness, and had similar scores thresholds many other test. Participants PA was monitored for a week with a accelerometer worn on the hip and measurements collected placed them in one of the three levels of PA: sedentary behavior, light PA, and moderate to vigorous PA (MV-PA). T2 weighted MRI images were used to examine the integrity of WM and presence of white matter hyperintensities (WMH); lesions that appear in WM during old age that damage WM integrity. Age-related degeneration of WM micro-structures can be captured as decreased fractional anisotropy (FA) measured with diffusion tensor imaging (DTI). The study concluded that higher levels of MV-PA were linked to lower WMH volume, PA and CRF are related but not equivalent in their relationships with WM health in aging, and PA is associated with WM health in aging in an intensity- and region-specific manner. 

It’s interesting to look at the additional effects an active lifestyle can have on the body past the cardiovascular level. This paper shows it’s never too late to start doing any level of PA and that its has many of its benefits are still unknown to us. 

Zachery