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Now the serotonin system in the brain arises along the midline and under the nuclei that run down the stripe through the middle of the midbrain and brainstem called the raphe and the raphe system includes a number of clusters of nerve cells. The two most important are the dorsal and median raphe nuclei. Serotonin producing cells are located there. The median or the more posterior group sends motor neurons down to interact with the spinal cord anterior horn cells to facilitate and stimulate peripheral transmission to the motor end plate and the skeletal muscle system. Again, when this system gets revved up you can expect twitching and abnormal movements that are, again, seen in excessive serotonergic activity in the serotonin syndrome.

On the ascending group, the dorsal raphe system sends axons that go up into the hypothalamus limbic system and cortex, in fact, share some of the same pathways. For example, the median forebrain bundle is one of the hottest spots in the brain where if you stick a self stimulating electrode in the right brain in that area you can get animals to bar press for electrical stimulation in these systems that will go on until the cows come home. Rats will give up eating, drinking, sex, sleep, anything to be able to get more stimulation themselves in that forebrain bundle in this system. Not only the serotonin and noradrenergic systems going up to the limbic system but also the ascending dopaminergic systems are located there and all of these probably contribute in complicated ways to mediate affective states, drive states, behavioral reinforcements, and may underlie some addictive behaviors and probably are messed up in a very fundamental way, an important way in contributing to the biology of melancholic type of depressions.
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One of the other intriguing insights that I’ve alluded to already in passing is that many of these systems talk to each other. They’re not just working all by themselves but the adrenergic and serotonergic systems are actually crossing. One of the theories about the biology of the sleep/wake cycle and phases of sleep and circadian rhythms that make it difficult for people to lecture more than 60-90 minutes without a coffee break because attention flags and all these rhythmic activities of arousal and sleep and attention and many endocrine rhythms, temperature rhythms, other things that happen are regulated by hot point mechanisms that probably have to do with the circuitry down in the brainstem of these interactions.
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From a pharmacologic point of view these interactions have allowed some very interesting games to be played with drugs. For example, there are alpha-2 receptors sitting on the neuroterminals for the adrenergic system. They are among the most important of receptors to inhibit release of norepinephrine and alpha-2 antagonists tend to be mood elevating or stimulant. Drugs that stimulate these receptors tend to be antiadrenergic, lower blood pressure. Tend to be quieting behaviorally. May have mild antimanic effects and so on. There are also inhibitory agents that sit as heteroceptors on the serotonin axons and terminals. Again, if you block this effect, you’ll increase the release of serotonin and that may be one of the ways in which mirtazapine has its indirect effects on both adrenergic and serotonergic activity.
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Then the serotonin system has its own autoreceptors. Their nomenclature will keep shifting but there are autoreceptors and they tend to be modified with long term exposure to serotonin reuptake inhibitors. The alpha receptors up here, autoreceptors in the adrenergic system become modified, down-regulate, desensitize after long term exposure to the tricyclics. I’ll say more about that in a minute but these phenomena probably have a lot to do with the later equilibrium that’s established after treating with antidepressant drugs over many weeks when their clinical effects begin to come into play. There are also serotonin receptors, heteroceptors that sit on the adrenergic terminals. Again, drugs that block or facilitate these will have indirect effects on the adrenergic system. To give you a closeup view of a central, or peripheral for that matter, adrenergic terminal and this model actually serves very well for the serotonin system. There are enzymatic mechanisms that produce both serotonin and norepinephrine from precursor amino acids, tyrosine to dopa and dopamine to norepinephrine and the adrenergic system and tryptophan serotonin in the serotonin system.

Among these drugs one of the great no-nos in contemporary pharmacology is thou shalt never, ever mix any of these drugs, and I mean any of them, including selegiline and maglobamide with a serotonin reuptake inhibitor. The reason for that is that you can get catastrophic central neurotoxic reactions that include confusion, delirium, fever, collapse, coma and some fatalities have occurred.
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This is the so-called serotonin syndrome and the descending serotonergic circuits that go to the lower anterior motor system of the spinal cord stimulate motor output. One of the early indications that you’re moving into a serotonin reaction is restlessness, agitation, fear, anxiety and twitchings. Some patients will actually start having chronic, modified jerking at times reactions and that can be an early tipoff that you’re moving into that kind of potentially lethal process and you need to back off.
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The other class of drugs that account for this are the analgesics that are phenylpiperazine category, particularly meperidine or Demerol. Again, a second rule of the ten commandments with respect to pharmacology is thou shalt never give an MAO inhibitor and Demerol at the same time. God forbidding we have someone coming into the Emergency Room with a splitting headache who’s just had a pepperoni pizza and they’re on Parnate and you give them Demerol, not a nice thing to do so. Again you can induce this kind of catastrophic central toxic state called serotonin syndrome.

Now, the mechanism of action of the atrophics is subtle and complicated and I think because of time I don’t want to get overly detailed. You can read about it in the handout. But just to give you a kind of a bird’s eye view of the story.

Most of the attention is drawn toward the central systems that involve serotonin and norepinephrine as their major neurotransmitters and the noradrenergic system arises mainly from small pigmented nuclei found on the brainstem called the locus caeruleus which is a paired set of nuclei on each side of the brainstem. These neurons come out and bifurcate and some of the axons go upstream into the hypothalamus-pituitary system and the cortex and some of them descend and go to the central part of the peripheral adrenergic sympathetic system.

In the inter mediolateral parts of the spinal cord you have a descending adrenergic system regulating the output of a nicotinic cholinergic system that goes to the peripheral sympathetic ganglia where you stimulate the post ganglionic sympathetic adrenergic fibers that go to the viscera, blood vessels, glands and so on. That’s why it’s not surprising that drugs that interact with these systems often have adrenergic side effect consequences. It’s also not surprising when I tell you that the biology of classic melancholy of major depression has a lot of autonomic symptomatology – dysfunction of gut and other visceral activities – and it’s probably mediated by underactivity through this descending adrenergic system.
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Some people like to think of the locus caeruleus as something like the head ganglion of the sympathetic nervous system where the main target rather than blood vessels and viscera and so on is the brain itself. These ascending systems permeate and percolate, extensively into hypothalamus limbic system and cortex where they produce a kind of a background regulatory, indirect slow time base kind of effect, regulate the activity of other neuronal systems that are involved in thinking and feeling, eating, sex drive, sleeping, and all sorts of biological phenomena that are, again, often deranged in biologically based severe melancholy depression and hopefully reversed by drugs that facilitate the process of adrenergic transmission.

Then down at the bottom is mirtazapine, another newer, again unusual drug with a subtle complex pharmacology that involves interactions with, again, both the serotonin and noradrenergic systems. Mirtazapine is a pretty good alpha-2 antagonist and that facilitates the release of norepinephrine and it also has properties of blocking some serotonin receptors, particularly type 2. Some of the type 2 sit on noradrenergic nerve terminals to facilitate the release of norepinephrine and that may be yet another means by which mirtazapine may facilitate adrenergic transmission. It also has properties in the serotonin system and, again, has a complicated mixture of subtle serotonergic and appreciable adrenergic properties.
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It would be remiss not to mention some of these old timers and even though they’re not commonly used these days, most physicians are afraid of them because of their interactions with other drugs and substances, particularly tricyclics, and that can release norepinephrine and produce catastrophically acute elevations of blood pressure.
Nevertheless, the MAO inhibitors are very useful drugs. They have powerful antidepressant and other properties and they are drugs that are worth keeping in mind when other standard agents are not working. There are numerous studies now returning to MAOIs. Particularly well studied is phenelzine or Nardil, but much less with the others to date showing that these drugs will rescue 50,60% of treatment resistant people who’ve had vigorous trials of tricyclics with serotonin reuptake type antidepressants. So they’re worth knowing about.

Selegiline is a drug that’s been used mainly in neurology as a mild early antiparkinson medicine and it has some subtle psychotropic properties. Probably mildly anxiolytic and may be mood elevating especially in bigger doses or recently introduced is a skin patch delivery system to trickle tiny amounts of selegiline in the body indirectly over a 24 hour period. That seems to enhance the tolerability, safety and perhaps even the efficacy of selegiline or deprenyl as an antidepressant. Again, that’s still experimental but the work going on in this area with that drug looks very encouraging.
There are several other experimental selective MAOA inhibitors. MAOA system is the one that’s particularly relevant to serotonin/norepinephrine nerve terminals and MAOA inhibitors of this kind which are short acting and reversible unlike all the other antidepressants which are long acting irreversible. Drugs like maglobamide seem to be a bit safer in terms of interacting. You get much less risk of hypertensive crises and that sort of thing.
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