Let's look anatomically for just a moment, and consider the source of some of these biogenic amine transmitters, beginning with dopamine. Well, I mentioned some of the places where dopamine is released. Dopamine is derived from cells that have their cell bodies here in the midbrain, which is at the very top of the brain stem in a collection of compact cells known as the substantia nigra pars compacta. And right adjacent to that is a region called the ventral tegmental area. Here we have cells that have their somata here and they send axons to important parts of the brain, such as the basal ganglia, specifically the striatum which is part of the basal ganglia. And from the ventral tegmental areas projections into medial temporal lobe structures such as the amygdala, as well as to this ventral and medial aspect of the prefrontal cortex. So dopamine is a very important neuromodulator in all of these places. Here, just to highlight a little bit of neuroanatomy, we have a cross-section through the forebrain as illustrated in the upper-right of this slide, shows you about where we're looking. And I want to draw your attention to the center of this slice. You'll notice label number 11, here we have this dark substance, which sits in the midbrain. And that dark substance is in fact the substantia nigra, which of course, is Latin for dark substance. So here we've got the cell bodies of the neurons that synthesize and release dopamine at some distance of their axon terminals up in the basal ganglion and in the cortex. And in the medial part of this region where we have dark substance, that's our ventral tegmental area. And that too, is a place where the cell bodies of these neurons reside that send their axons up into structures associated with the limbic forebrain and release dopamine there. Well, norepinephrine is another important small molecule neurotransmitter, and norepinephrine is derived from cells who have their home in the brain stem but now in the dorsal part of the pons in a structure called the locus coeruleus. The locus coeruleus is another place where we find black substance, or actually the locus coeruleus means blue spot. If we are lucky enough to cut a section through the tegmentum of the pons just right, we might see these two little spots that look sort of a deepish grayish blue color. Those are the locations of where we find our cell bodies of neurons that are producing norepinephrine and releasing it in various places in the brain. Really just about everywhere in the brain and even down into the spinal cord do these cells grow their axons. So all of these biogenic means have in common a limited number of cells whose cell bodies are in the brain stem that grow axons that branch and grow profusely throughout various parts of the forebrain, hindbrain, and even the spinal cord. One last example is the neurotransmitter, serotonin. So serotonin comes form a group of cells that are found in a couple of different places in the brain stem. They tend to be found right along the midline in a group of nuclei that we call the seam nuclei, or the Raphe nuclei. Raphe means seam, so right along a seam where the two sides of the brian stem come together. That's where we find our cell bodies that release serotonin and as I mentioned, serotonin's a very important neuromodulator. Serotonin appears to be a molecule that when out of proper physiological balance can lead to various affective disorders. Many of our drugs that we use to restore normal health with respect to affective disorder act by selectively blocking the re-uptake of serotonin, thereby increasing the synaptic concentrations of that drug in important brain structures. Okay well, there are more animations to view. If you care to see some summary, some of the biochemistry, and some of the molecular biology regarding the function of these biogenic synaptic systems. I would encourage you to view animation 6.3 and 6.5 that review the organization of dopamine and serotonin serotonin synapses. Okay, at this point I think we're ready to switch gears just a bit and talk about the neuropeptide neurotransmitters. So as I mentioned, the neuropeptide neurotransmitters are larger molecules. They're made of a number of amino acids that are covalently bonded to small peptides. Sometimes they can be rather short or sometimes they can be of considerable size. So, as you might imagine, these molecules must be handled somewhat differently in the presynaptic terminal, and, indeed, they are, and we'll come back to that in just a minute. But before we talk more about how they're handled in the presynaptic terminal, let's just make a few points about their physiology. So, the neuropeptides mediates slower and more long lasting effects than most of our small molecule transmitters and that's because the neuropeptides seem to interact with metabotropic types of receptors. And as we'll discuss these are receptors that require cellular metabolism in order to mediate their function, and all that takes time. So the neuropeptides mediate slower and more long lasting synaptic effects, some of which can even go on for hours to days, depending upon the downstream effects of the metabolic pathways that are activated. Some of the important neurotransmitters in this category include what's shown here in this slide. The enkephalins, so there are a number of different enkephalins that are important. The enkephalins are opioids that have henogesic effects in the brain and so these bind to receptors in various places including the spinal cord. Other important peptide transmitters are substance P, which is very important in modulating the transmission of pain signals in the dorsal horn of the spinal cord. And many of our hypotholamic releasing hormones that are secreted into the porter circulation from the hypothalamus to the anterior pituitary are neuropeptides. As are the hormones that are released directly in the posterior pituitary into the blood stream from hypothomlamic neurons that send their axons into the neurohypothesis. So these hypothalamic hormones are other examples of neuropeptides. And lastly, let's consider the activities of some neurotransmitters that are still relatively poorly understood, and for that reason, we call them unconventional neurotransmitters. Not because they are necessarily so different from our small molecules and our peptides. In some cases they are, but mainly because we don't yet have a full understanding of their mechanisms of action. And so let's begin by talking about the purinergic neurotransmitters. So these include our purines, such as adenosine triphosphate. So you're all familiar with molecule from your studies of cellular metabolism. You know that ATP is the energy currency for cells, and what you may not have known is that it actually has some biological activity at synapses in the nervous system. So ATP can be co released with other small molecule neurotransmitters. Once it's in the extracellular space, ATP is typically metabolized into adenosine. And adenosine itself can have it's own biological activity. Adenosine tends to bind to various receptors, and these receptors have various functions in different kinds of brain circuits. One important circuit where adenosine is important is in the hypothalamus. And adenosine appears to build up throughout our day and appears to be an important trigger that leads to drowsiness. So, the more our cells in the hypothalamus are consuming energy, the more adenosine builds up in the extracellular fluids. That seems to be something like an index of our state of fatigue, and the degree of which we require sleep. Well, one thing many of us do, I myself am guilty of this, is that we fight this system by consuming caffeinated beverages. And what these caffeinated beverages do is they block adenosine receptors. So the xanthines including caffeine and theophylline, they are blocking the receptors that are mediating this natural sense of drowsiness that will, we think, in some way have a restorative function on brain circuits. Another of the so called unconventional neurotransmitters that's fairly recently been discovered are a series of molecules that are derived from membrane lipids called endocannabinoids. These molecules are hydrophobic, so when synthesized they can freely diffuse across plasma membranes, and they bind to receptors that are also bound by the psychoactive components of cannabis, hence their their name endocannabinoids. So these include some very interesting receptors, such as the CB1 receptor which is present in high concentrations in structures associated with the brain's reward system. It's also present throughout the cerebral cortex and in the cerebellum, in the basal ganglia. So lots of places where we think that the drug of abuse, the cannabinoids derived from cannabis, have their effects on cognition and sensory and motor function. So these endocannabinoids seem to have some interesting physiological roles in modulating plasticity. Especially plasticity at inhibitory synapses. And you can read more about that in the textbook if you like and figure 6.19. We'll come back to this when we talk about mechanisms of synaptic plasticity in a future tutorial. One final neurotransmitter to discuss is nitric oxide. So this truly is an unconventional neurotransmitter in that it's a gas. It's not a solute that can be dissolved in an aqueous medium. It's not a peptide. But rather it's a gas. And nitric oxide is produced from the metabolism of the amino acid arginine, as is illustrated here in this pathway. So arginine Is processed by an enzyme called nitric oxide synthase, and nitric oxide in a gaseous form is produced as is citrulline. And that nitric oxide then is free to diffuse across the plasma membrane, or it can interact with target structures maybe within the cell, where the gas was synthesized. Here in this particular illustration we're showing the activation of a guanylyl cyclase enzyme, which results in the production of cyclic GMP, one of the important second messenger systems we have within the brain. Now, one of the interesting activities of nitric oxide involves its ability to diffuse freely from, lets say a postsynaptic cell to a presynaptic cell. So, there is ongoing interest in the idea that nitric oxide might be some kind of a retrograde messenger that allows from signals to travel back from a post synaptic cell to a pre synaptic site. Of some interest is the activities of nitric oxide as a vasal dialator. And so, this is a function that is performed in a variety of of tissues, and that activity is mediated by a guanylyl cyclase pathway. And perhaps it's of interest to some of you that Viagra and other related drugs that are used to affect erectile dysfunction function by affecting nitrous oxide mediated vasodialation. Similar pathways, however, are found in a variety of structures within the brain and elsewhere in the nervous system. Hence these drugs can often have side effects, often involve sensory processing and we'll see why when we talk about the mechanisms of phototransduction. And we'll discover an important role for the guanylyl cyclase pathway within the photo receptors of our retinas. OK, well let's look now at a side by side comparison the processing of small molecule transmitters and peptide transmitters. So I would encourage you to have a look at the table that I've prepared for you, in page 3 of your handout, which talks about the synthesis, the packaging release, and the removal of neurotransmitter, at synapses that use small molecules and peptide transmitters. So with respect to the synthesis, small molecules are synthesized typically in the pre synaptic terminal where there are enzymes that are present that can create the active neurotransmitter right there in the presynaptic terminal. For peptide neurotransmitters, the synthetic pathway is primarily in the cell body where the peptide is processed like all other peptides. Through the endoplasmic reticulum, thegGolgi apparatus, where various kinds of post translational modifications may be performed. Meanwhile, this peptide can be packaged into a vesicle of some sort and then transported down the axon. And along the way further modifications, or perhaps cleavages of that peptide can occur. As the initial product of the transcript is then processed and prepared for its active function in the presynaptic terminal. Couple of other differences to note. Small molecule transmitters typically are packaged into vesicles that then are concentrated right along the active zones at the presynaptic membrane. Whereas peptide transmitters are often found in vesicles that are diffusing around the presynaptic terminal further away from that active zone. So consequently, the vesicles that contain the neuropeptides, which typically are much larger Then those that contain the small molecules are further removed from those active sites and therefore require a larger influx of calcium in order to trigger calcium mediated fusion events. So with a relatively modest depolarization of a presynaptic terminal we're likely only to see the calcium mediated fusion of small vesicles that release the small molecule transmitters. It's only with a very large depolarization that the presynaptic terminal is likely to release much peptide neurotransmitter because in order for that to happen, sites well away from our typical high efficiency active site are engaged and those larger vesicles then fuse and release peptide neurotransmitters. But only when that pre-synaptic terminal is flooded with calcium. So this might be one physiological way of holding in reserve the peptide neurotransmitters, only when there is intense depolarization of the pre-synaptic terminal. And lastly, considering the removal of the neurotransmitter once it's been released the small molecule transmitters have a variety of systems. The transmitter itself might be degraded, and pre-cursor substances might be then taken back up into the [INAUDIBLE] terminal or they may run through a [INAUDIBLE] cell where they may be in some storage form before being returned from a [INAUDIBLE] to the [INAUDIBLE] terminal. Of course, the transmitter can simply diffuse away as well. For a peptide neurotransmitter, these substances, likewise, are free to diffuse away, but there are also proteases that are found here in the synaptic cleft that can cleave these peptide neurotransmitters. And degrade them right at their region of release in and around the synaptic cleft. So let's also take a few minutes to consider a typical glutamatergic synapse in the brain. So as I've already mentioned glutamate's one of the most important small molecule in neurotransmitters that we have it's widely considered to be the most important small molecule for mediating fast excitatory events in the central nervous system. So in a typical pre-synaptic terminal, we have an important enzyme called glutaminase. This enzyme converts glutamine into glutamate, and glutamate Is then concentrated in synaptic vesicles, where it's taken into that vesicle via a transporter called vesicular glutamate uptake transporter. So, VGLUT, for short. So, this synaptic vesicle then, of course, can fuse with the presynaptic terminal when calcium floods that active zone. The glutamate is free then to diffuse into the synaptic cleft where where it can react with postsynaptic glutamate receptors. Now some of this glutamate, of course, diffuses away from the synaptic cleft where there are excitatory amino acid transporters in the glial cells that surround the synaptic compartment. Of course, the important glial cell that we're talking about here would be the astrocyte. So the excitatory amino acid transporter takes up the glutamate where it's converted back to glutamine. So glutamine is in essence a storage form of glutamate that can sit in reserve within astrocytes before being recycled as needed back to the presynaptic terminal. So there are other kinds of cotransporters that allowed glutamine to cycle back to this presynaptic terminal where it can once again be hydrolyzed by glutaminase in the glutamate ready for another round of synaptic release. Okay, lastly let's just make a couple of points here in conclusion. So we've talked about our two classes of neurotransmitters, or small molecules. Our peptide transmitters, we've also talked about what we've called some unconventional neurotransmitters, and these neurotransmitters interact with two classes of receptors. Our ionotropic receptors, which mediate rapid synaptic events since the receptor for the ligand as part of an ion channel complex. And there are metabotropic receptors that require the activation of second messenger systems. So they mediate more slowly evolving and potentially longer lasting synaptic effects. Our small molecule transmitters can interact with both kinds of receptors are peptide neurotransmitters as far as we know only interact with metabotropic family of receptors. Now I think from this simple schematic, it should be apparent to you that if we want to understand the post synaptic activity of a neurotransmitter, we need to know more than simply the identity of that neurotransmitter. It's not enough to know if it's a small molecule or if it's a peptide. So knowing the chemical structure itself is not sufficient to understand the physiology of the effect mediated by that transmitter. One needs to understand something about the receptors that interact with that neurotransmitter in question. So in order to account for the physiological activity of the chemical neurotransmitter you need to know the receptor. So in our next tutorial, we're going to talk more about the structure and function of both types of receptors, the ionotrophic receptors and the metabotropic receptors. And I think you'll be impressed as I am with the varsity of effects that can be activated, even by the very same chemical signal. So, next time we'll talk about neuro transmitter receptors. I'll see you then.
