Smelling someone else's alarm bells

Deric Bownds Mindblog has a post, Do our noses sniff danger in the air? on a recently investigated cluster of cells in the tips of mammalian noses (including humans, it is posited) called the Grueneberg ganglion.
From Science, Aug 2008:

"Grueneberg Ganglion Cells Mediate Alarm Pheromone Detection in Mice
Julien Brechbühl, Magali Klaey, Marie-Christine Broillet*

Alarm pheromones (APs) are widely used throughout the plant and animal kingdoms. Species such as fish, insects, and mammals signal danger to conspecifics by releasing volatile alarm molecules. Thus far, neither the chemicals, their bodily source, nor the sensory system involved in their detection have been isolated or identified in mammals. We found that APs are recognized by the Grueneberg ganglion (GG), a recently discovered olfactory subsystem. We showed with electron microscopy that GG neurons bear primary cilia, with cell bodies ensheathed by glial cells. APs evoked calcium responses in GG neurons in vitro and induced freezing behavior in vivo, which completely disappeared when the GG degenerated after axotomy. We conclude that mice detect APs through the activation of olfactory GG neurons.

Department of Pharmacology and Toxicology, University of Lausanne, Bugnon 27, CH-1005 Lausanne, Switzerland."

See also:

Sensing Alarm

Peter Stern

Science, AAAS, Cambridge CB2 1LQ, UK

In 1973, Hans Grueneberg observed the presence of a structure at the tip of the rodent nose that, he thought, belonged to the Nervus terminalis. Recently, using transgenic techniques, several groups reported the rediscovery of this structure. They named this structure the Grueneberg ganglion in memory of the original work. However, the function of these cells remains a matter of controversy. Despite the lack of typical olfactory neuronal features, the ganglion was suggested to have some olfactory function, based on the expression of olfactory marker protein and on its neural connections to the olfactory bulb of the brain. Brechbühl et al. have now identified a function for the Grueneberg ganglion cells. A combination of anatomical, surgical, and behavioral techniques was used to suggest that the Grueneberg ganglion is involved in alarm pheromone detection.

J. Brechbühl, M. Klaey, M.-C. Broillet, Grueneberg ganglion cells mediate alarm pheromone detection in mice. Science 321, 1092-1095 (2008). [Abstract] [Full Text]

Citation: P. Stern, Sensing Alarm. Sci. Signal. 1, ec302 (2008).

It doesn't surprise me anymore that recent neuroscience that I happen to be interested in usually turns out to have come from Lausanne, Switzerland. It does surprise me a little that there have been some papers about this from earlier, so it isn't exactly fresh news.

What's interesting about this is that it used to be thought that there were only two systems, the vomernasal organ (vestigial in humans) and the regular olfactory pathways, or sense of smell. It seems there are actually these three, and that in humans, the Grueneberg ganglion is still with us. It makes sense to me that as humans evolved close social ties and the capacity to relate via language and visual cues, smell (in terms of signaling pheromones) would begin to be less essential to survival. Yet there is this system still existing in humans (it would seem) that can still "smell" and respond to "alarm". I can hardly wait to hear how this turns out, how it might tie in with those whose information base is the pondering of human relations. I can already see applications in my own work - maybe those with persistent pain have alarm bells in a constant uproar - maybe the olfactory bulbs at the other end of their Greuneberg ganglia are dysregulated, or not downregulable by them for some reason. Just a stray thought. In any case it behooves us as therapists to be able to downregulate our own alarm bells, as always, no matter which part of the brain they are to be found or which exteroceptive sense might contribute to them..

Here is some additional information:
1. Mammals emit smell to signal danger. Excerpt: "Cells in the Grueneberg ganglion use their own calcium to transmit the danger warning to the brain... Only warning pheromones could trigger the warning signal."
2. Short video entitled, Smelling Fear
3. Breer H. Fleischer J. Strotmann J; The sense of smell: multiple olfactory subsystems. Cellular & Molecular Life Sciences. 63(13):1465-75, 2006 Jul.

"The mammalian olfactory system is not uniformly organized but consists of several subsystems each of which probably serves distinct functions. Not only are the two major nasal chemosensory systems, the vomeronasal organ and the main olfactory epithelium, structurally and functionally separate entities, but the latter is further subcompartimentalized into overlapping expression zones and projection-related subzones. Moreover, the populations of 'OR37' neurons not only express a unique type of olfactory receptors but also are segregated in a cluster-like manner and generally project to only one receptor-specific glomerulus. The septal organ is an island of sensory epithelium on the nasal septum positioned at the nasoplatine duct; it is considered as a 'mini-nose' with dual function. A specific chemosensory function of the most recently discovered subsystem, the so-called Grueneberg ganglion, is based on the expression of olfactory marker protein and the axonal projections to defined glomeruli within the olfactory bulb. This complexity of distinct olfactory subsystems may be one of the features determining the enormous chemosensory capacity of the sense of smell."

4. Ma M. Encoding olfactory signals via multiple chemosensory systems. Critical Reviews in Biochemistry & Molecular Biology. 42(6):463-80, 2007 Nov-Dec.

"Most animals have evolved multiple olfactory systems to detect general odors as well as social cues. The sophistication and interaction of these systems permit precise detection of food, danger, and mates, all crucial elements for survival. In most mammals, the nose contains two well described chemosensory apparatuses (the main olfactory epithelium and the vomeronasal organ), each of which comprises several subtypes of sensory neurons expressing distinct receptors and signal transduction machineries. In many species (e.g., rodents), the nasal cavity also includes two spatially segregated clusters of neurons forming the septal organ of Masera and the Grueneberg ganglion. Results of recent studies suggest that these chemosensory systems perceive diverse but overlapping olfactory cues and that some neurons may even detect the pressure changes carried by the airflow. This review provides an update on how chemosensory neurons transduce chemical (and possibly mechanical) stimuli into electrical signals, and what information each system brings into the brain. Future investigation will focus on the specific ligands that each system detects with a behavioral context and the processing networks that each system involves in the brain. Such studies will lead to a better understanding of how the multiple olfactory systems, acting in concert, offer a complete representation of the chemical world."

Breer, H., Fleischer, J., Strotmann, J. (2006). Signaling in the Chemosensory Systems. Cellular and Molecular Life Sciences, 63(13), 1465-1475. DOI: 10.1007/s00018-006-6108-5

"Dialogues in Clinical Neuroscience" online

A reader, Kent Schnake, sent me a link that looks very good - it's to Dialogues in Clinical Neuroscience, which appears to be open access, for past issues, anyway.

Here is a link to the 2004 issue on neuroplasticity. In it is an article by Fred Gage, downloadable.


Here are some posts that have included discussion of Fred Gage's work:

1. Nervous System Basics VIII: PLASTICITY
2. History of Neuroplasticity

Microglia and Pain: A Manual Therapy Perspective IV

In reference to Microglia and Pain: A Manual Therapy Perspective:
Part I
Part II
Part III

REFERENCES:
1. Rock BR et al; Role of Microglia in Central Nervous System Infections. (open access) Clinical Microbiology Reviews, October 2004, p. 942-964, Vol. 17, No. 4
2. McMahon s and Koltenburg M; Wall and Melzack’s Textbook of Pain 5th Ed. Churchill Livingstone (September 21, 2005)
3. Ramachandran VS (ed); Encyclopedia of the Human Brain. Academic Press; 1st edition (June 2002): Stoll G et al; Microglia Vol. 3. pp 29-41; Angevine JB; Organization of the Nervous System Vol 3 pp 313- 371; Brown AM and Ransom BR; Neuroglia Overview Vol 3 pp 479- 491
4. Verkhratsky A and Butt A; Glial Neurobiology. Wiley 1 edition (Sept. 2007)
5. Kettenmann H; The brain’s garbage men. Nature Vol 446 Apr. 2007
6. . Piao ZG et al; Activation of glia and microglial p38 MAPK in medullary dorsal horn contributes to tactile hypersensitivity following trigeminal sensory nerve injury. PAIN 121 (2006)
7. Echeverry S et al; Characterization of cell proliferation in rat spinal cord following peripheral nerve injury and the relationship with neuropathic pain. PAIN 135 (2008)
8. Coull JAM et al; BDNF from microglia causes the shift in neuronal anion gradient underlying neuropathic pain. Nature 438 (Dec. 2005)
9. Tsuda M et al; P2X4 receptors induced in spinal microglia gate tactile allodynia after nerve injury. Nature 424 (Aug 2003)
Piao, Z.G. (2006). Activation of glia and microglial p38 MAPK in medullary dorsal horn contributes to tactile hypersensitivity following trigeminal sensory nerve injury.. Pain, 121(3), 219-231.

Microglia and Pain: A Manual Therapy Perspective III

In reference to Microglia and Pain: A Manual Therapy Perspective I, and Microglia and Pain: A Manual Therapy Perspective II:


The Dorsal Horn and Microglia

What role do microglia play in pain? Dorsal horns are the laminated posterior areas of the spinal cord where incoming sensory info is handled. Secondary neurons within the cord, actual CNS neurons, deal with it from then on. At the junction between the incoming sensory neurons and the secondary ascending neurons, there are, yes, you guessed it, microglia hanging around, waiting for a chance to “activate” and move along novel chemo-attractive gradients, substances released into the parenchyma by the presence of inflammation(6) and hypoxia(3), among other things(1), including nerve compression.



Once “activated” they “feed,” increase their populations, leave behind chemical “litter.” These chemicals “inhibit” the secondary ascending fibers. Inhibition? That’s good, isn’t it? Well, maybe not - if your job as a secondary ascending neuron is to be a bottle-neck, and your “bottleneck” function becomes inhibited by microglial activation, the brain is more likely to be confronted by too much nociception, too rapidly, and will need to allocate new resources to learn to downregulate it somehow.

What stimulates microglia at a spinal cord level? The Textbook of Pain chapter states, nerve “damage” such as spinal nerve ligation, chronic constriction injury, or rhizotomy. These are factors associated with neuropathic pain definitions, moreso than neurogenic. But remember the overlap. McMahon et al go on to say:
1. “..synaptic connections between neurons are in a continual state of change highly dependent on the activity not only of the pre- and postsynaptic neurons but also of the surrounding glia.
2. ..continual interplay of various modulatory processes serves to produce synaptic modifications (plasticity) that underlie physiological processes such as learning and memory.
3...common molecular pathways that produce these normal forms of plasticity also lead to pathological processes characterized by excessive excitation, including …pain.
4. In the dorsal horn, central sensitization is a form of excessive excitatory synaptic response in nociceptive transmission neurons, which leads to an increased gain of the pain transmission system and pain hypersensitivity.
5. Our knowledge of the molecular mechanisms of pain plasticity in the dorsal horn is rapidly growing.
6. Future advances will provide new insights into the neurobiological basis of pain, and we anticipate that these will provide the basis for novel types of analgesics and of new diagnostic and management strategies beyond what is presently envisaged.”

The story isn’t over yet, but it’s probably safe to say that whatever makes a synapse in the dorsal horn behave in a manner outside the norm is likely to gain the attention of microglia in the cord. They are thought to be responsible for mechanical allodynia that comes along with central sensitization, based on studies that carefully manipulated the P2X4 receptor they express9. Furthermore, whatever helps a synapse in the dorsal horn recover its ability to conduct business as usual, will likely help decrease central sensitization. It may be that the future of pain control will be in the hands of whoever can find the means to keep the microglial population in check and not allow them to gain the upper hand.

<<<<<<<<<<<<<<<<< >>>>>>>>>>>>>>>>>>>>>>

The fourth post will contain references and links to these three content posts.
All pictures/links have been added for the blog and did not appear in the article.
(Picture of dorsal horn was adapted from Nature Neuroscience)

Microglia and Pain: A Manual Therapy Perspective: Part II

In reference to Microglia and Pain: A Manual Therapy Perspective: Part I:


Glial functions

In general, glia keep things running smoothly. Astrocytes make sure the synapses are working properly - duties include the regulation of ions to maintain optimal chemical stability in the extracellular fluid, sopping up and storing excess neurotransmitters such as glutamate, buffering K+, and storing glycogen, feeding the neurons with it when a burst is suddenly needed in a given region. They occupy physical space between neurons and capillaries, keep the blood supply and the neurons apart from each other. Oligodendrocytes in the CNS, and their cousins, the Schwann cells in the periphery, manufacture and maintain myelin coverage of their respective neurons. There are other kinds that do other jobs. (See image below and to right, modified from an article, The Dark Side of Glia (Science May 2005).)


Microglia are the smallest of the glia, and (in my opinion) the most mysterious, not just because of their unconventional origins, but also their behaviour; mostly they just sit there, inactive, sessile, not bothering anything, making up 5-15% of the total glial population. Yet they sense everything, and are capable of “activation,” generating a big response to altered environment, which sometimes works well (to take care of invaders) and sometimes not so well (from a pain point of view). They have many kinds of ion receptors - when their environment provides them with sufficient chemical provocation, they change their morphology and begin to mobilize, amoeba-style, drawn to sites of infection or where damage has occurred, where the blood-brain barrier has been breached by injury or vascular failure.

In the brain they act the way macrophages do in the outer body, absorb invaders and corpses, but there is a not-so-good side to all this: the nervous system is generally not used to having these little creatures moving around in it, and sometimes seems to have trouble adapting when they become active. Why should this be?

It is easier to understand if we consider what happens in an ecosystem when scavengers find sustenance in it:

1. First, they will reproduce rapidly. Think of flies on a carcass - soon there are many more flies buzzing around. In the brain and spinal cord, microglia, like the single-cell “creatures” they are, reproduce enormously when times are good - from their perspective. But the CNS, the spinal cord, is an enclosed space, without a lot of room for a burgeoning population of microglia, no matter how small they may be.

2. Microglia and their population explosion alter the chemical environment the nervous system has been used to. Think of excretion (flyspecks) left in their wake - the nervous system is already injured, and now it must adapt to a new chemical environment (polluted in a sense) on top of everything else. Substances released include cytokines, chemokines, trophic factors etc., some of which the neurons can use like “fertilizer” to grow with, others of which merely irritate them.

3. When this sort of “plasticity” occurs in the spinal cord, nociception becomes upregulated.

4. When the brain is exposed to such upregulation, depending on context, it might not succeed in successfully downregulating it back to normal.

The rest of this piece will concern itself with points 3 and 4.


Neurogenic and Neuropathic Pain

In Wall and Melzack’s Textbook of Pain (5th ed.) is a discussion about defining neuropathic as opposed to neurogenic pain. Microglial activation is associated with neuropathic pain which is in turn associated with neuronal damage. In general, from our perspective, we could consider neurogenic pain as more easily downregulated with manual therapy, because there is no irreversible nerve damage - the neurons and dorsal horn can recover. What we do with our contact, both verbal and manual, likely assists nervous systems to increase descending modulation to help decrease perceived pain. Possibly this is sufficient for a system with mere neurogenic pain to right itself, and microglial populations presumably go back to normal levels eventually.


(Image modified from Textbook of Pain 5th Ed.)

Neuropathic pain, or pain felt in the context of actual neuronal or dorsal horn damage due to injury, infection, or ongoing metabolic insult, is less likely to right itself with manual therapy - in fact, manual therapy may worsen matters instead. Luckily for us, at a glance it would seem that there is more neurogenic pain in the population than there is frank neuropathic pain. In the book, the authors conclude that there is no real dividing line yet - that there is a big area of overlap. This is frontier land. We must learn optimal ways to sort out the two main kinds of pain, in the clinic, based on close listening to a patient’s pain history, or risk adding to pain felt by some.

Microglia and Pain: A Manual Therapy Perspective: Part I

I am going to post, in digestible blogpost-sized chunks, a piece I've written for an ortho newsletter, which I'm pleased to report has been accepted by the editor for inclusion sometime in the fall.

MICROGLIA AND PAIN: A MANUAL THERAPY PERSPECTIVE




"The human nervous system is a hierarchy, culminating in the brain, of 100 billion or more neurons of 10,000 types, 1-10 trillion neuroglial cells, 100 trillion chemical synapses, 160,000 km of neuronal processes, thousands of neuronal clusters and fibers tracts, hundreds of functional regions, dozens of functional subsystems, 7 central regions, and 3 main divisions. All of these parts form a coherent, bodily pervasive, diversified, complex epithelium with interdependent connectivity of neurons, mostly neither sensory nor motor but anatomically and functionally intermediate. The key organizing principles of the system are centralization and integration. The nervous system performs two roles: regulation and initiation. In the first, it counteracts: responsively and homeostatically, gathering stimuli from outside and inside the body (including the brain), assessing their short-term and long-range significance, generating activity from faster breathing to stock trading, even to functional plasticity in learning or after brain damage. In the other, it acts: endogenously… replacing one state of neural activity with another, generating activity from doing nothing at all to creative thinking and extraordinary achievement... Although the divisions and regions of the nervous system are identical in all normally developed humans, their genetic specification and personal history are unique, as are the permutations and combinations of their unified function. Each human nervous system is unprecedented. The work of each… is unpredictable, ever-different, surprising, startling, at times horrifying, but not infrequently magnificent." - Jay B. Angevine, Nervous System Organization, Vol.3 of Encyclopedia of the Human Brain

Introduction

Learning about pain can take a manual therapist down strange new paths.

Were manual therapy a city, one would find oneself on a comfortably broad avenue that constitutes all the accumulated wisdom of manual therapy - one would see a large population of peers moving around, or camped along both sides of the street. One can live one’s whole life here, and never venture beyond the edge of town.

In a very short paper I want to take you not just past the edge of town, but way out into the country-side, some of it still wild frontier. I want to try to convey, as Jay Angevine’s quote above conveyed to me, a sense of the vastness of what we must begin to learn to map in our own minds, if we are to ever understand what it is we deal with every day of our lives as we confront pain in our patients. For pain certainly stems from processes within the system described above.


Glia are not all of a kind

From a neuroresearch perspective, the brain/CNS gets most of the attention; spinal cord and peripheral nerves are considered as tentacles out from it. The body itself (the 98% of our physicality that is non-neural, non-neuronal) is mostly ignored - it is not part of the nervous system - instead it viewed as that which is acted upon by the nervous system. This seems backwards to us, at first, but in time this perspective starts to make sense. (After awhile, from a pain standpoint, it is the only perspective that makes sense.)

What is in the brain? Neurons and glia – lots of glia, lots of blood vessels. Neurons, even at 100 billion strong, are outnumbered at least ten to one by various kinds of glia. Neurons are huge compared to glia. Because glia are much smaller, it takes many more of them to make up a good half of brain volume. And microglia are the smallest of all, equal in numbers to neurons.

Where do glia come from? They form from the same precursor cells as neurons do, for the most part. The origins of microglia, however, are still a bit murky. Conventional thought has them as being from the early embryonic hemopoietic system, invading the brain early on before the blood-brain barrier is properly in place, then kept at bay via chemically controlled conditions by the other glia, just waiting to “activate.” Other researchers (a minority) think that they come from the same precursor cells as neurons and the other glia. This debate is still not quite settled, but it is agreed that they function as the nervous system’s “immune system.”

More on proteins

In reference to System Proteomics:
On Carl Zimmer's blog, The Loom, I found Your Yeasty Network. His post references this open access paper:Protein Complex Evolution Does Not Involve Extensive Network Rewiring, by van Dam and Snel in the Netherlands. They are trying to determine relationships within something they term an "interactome." Zimmer loves the illustration (see his link).

Although the paper doesn't refer to protein complexes at synapses specifically, I should think anything that has to do with evolution of protein complexes will have to do with protein complexes at synapses.

Inside his post Zimmer refers to another of his posts, Carrying ancient history in the gut - here, he examines the differences between the protein complexes of yeast (more complex) and giardia (simpler).