Sunday, June 1, 2008

"Sky-blue place" IV: Descending modulation

In reference to:
Locus Ceruleus: "Sky-blue place"
"Sky-blue place" II: Projections
"Sky-blue place" III: Input

This will be the last post in this series.
I think I've turned over most of the stones I could find learning about this cool little brain spot that seems to know just when to wake up the brain and when to be quiet.

I want to bring forward a few more juicy tidbits here, however, from Textbook of Pain 5th Ed.

1. The PAG (periaqueductal grey) and locus ceruleus seem to enhance one another's function: (p. 394:)
"Concurrent delivery".. (of "ethylketocyclazocine,""reported to have μ-agonist properties"), "at doses that together were less than injected in either site alone, produced a significant, naloxone-reversible increase in response latency. These observations were argued to reflect a synergic interaction between these two anatomically distinct systems (Bodnar et al 1991)."
If something is naloxone-reversible it means it has an opioid effect of some kind.

2. Anterior insular cortex projects to LC: (p. 127:)
"Dorsolateral pontine systems may also contribute to cortical control of spinal nociceptive transmission. Increasing GABA levels in the anterior insular cortex produces an analgesic effect that is blocked by intrathecal administration of α-adrenergic antagonists. Because this cortical region projects to the locus coeruleus as well as the RVM, it was suggested that inhibition of the insular outflow disinhibits noradrenergic neurons of the locus coeruleus (Jasmin et al 2003b). This could be through an action in the pons or via the RVM."

I missed this when I did the projections post.
(Note: RVM = rostral ventromedial medulla)

3. Linkage to affective states: (p. 234:)
"Chapman (2004) described how processing of nociceptive signals produces affect in multiple neurotransmitter pathways that project to the cortex. Noradrenergic, serotonergic, dopaminergic and acetylcholinergic fibres and pathways are involved. Drawing on an extensive literature on the biology of emotions (e.g. Gray 1987), noradrenergic pathways are recognized as linked most closely to negative emotional states. Of particular importance are nociceptive afferent systems operating and transmitting through the limbic brain-in particular the locus coeruleus, the dorsal noradrenergic bundle, the ventral noradrenergic bundle, and the hypothalamo-pituitary-adrenocortical axis-to all of the neocortex. These are not specific in their activation to nociception, but are also responsive to non-nociceptive, aversive emotional states. These systems are recognized as fostering survival by allowing biological vigilance to threatening and harmful stimuli, both external and internal. Chapman proposes that the affective dimensions of pain can best be conceptualized as involving a two-stage mechanism. The immediate experience would be akin to hypervigilance or fear, with this rapid response giving rise through efferent messages to visceral and other event-related, autonomic activity that creates a strong negative subjective experience and an affective response involving images and symbols."

4. Supraspinal analgesia: (p. 431:)
"Fibres descending from the RVM to the dorsal horn of the spinal cord are mostly serotonergic, enkephalinergic, glycinergic and GABAergic. The nucleus raphe magnus contained within the RVM and the noradrenergic nuclei (locus coeruleus, subcoeruleus, A5 and A7 cell groups) are major PAG relays for noradrenergic and serotonergic descending pathways, respectively, to the dorsal horn (Kwiat & Basbaum 1992). Rather than the RVM being a homogeneous population of serotonergic neurons, GABA- (and glycine-) releasing neurons are now thought to constitute a significant proportion of spinally projecting RVM fibres (Antal et al 1996). The pharmacology of noradrenergic and serotonergic modulation in the dorsal horn is complex but opioids can also interact with noradrenergic mechanisms and there are many studies showing that the effector mechanism and location for the major noradrenaline target receptor-the α2 adrenoceptor-is very similar to that of opioid receptors."

Here is a picture of where LC is to be found in the brain (see red arrow, image from Atlas of Functional Neuroanatomy and modified).

Look at how tiny it is. (I think if you click on the picture you can enlarge it some more.)

Here is a link to a set of notes I made on this little brain part.

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