34
📖 Lesson

Basic Neurochemistry-V

PSYP610 - Neurological Bases of Behavior

🎯 Objectives

To familiarize the students with the:

  • Various NT and their role in the modulation of behaviors 🧠
  • Classification of Neurotransmitters 📊: Monoamines: Catecholamines 🧪 and Indoleamine 💜, acetylcholine 🎯, amino acid 🧬, and Peptide 🔗
  • Neurotransmitter's role in modulation of behaviors and Aberration ⚠️
  • Drugs and Behavior 💊🎭
  • Classification of Psychopharmacological substances 🔬
  • Behavioral correlates, Treatment 🩺
  • Mechanism of synaptic transmission 📡

🎭 NE and Behaviors (continued)

😰 Stress

There are various ways in which stress can be induced in the laboratory 🔬. One of these methods is to give continuous and inescapable shocks ⚡. Rats are placed in a cage with a steel wire grid on the floor 🐀. Shock is passed through these to the rat's feet 🦶 (the paw and feet are the only part apart from their nose which does not have fur protection 🧥).

Stress induced by foot shocks 😰 (stress) lead to increased NA levels and turnover in the hindbrain ⬆️📈. The turnover rates increase means that more and more NA is being used and being metabolized 🔄. This has been measured using the push pull cannulae 💉. Various pharmacological procedures have also shown that only NA increases after foot shock ✅.

⚡ Electroconvulsive Therapy

Similarly, Electroconvulsive therapy 🩺 also leads to increased NA levels in the forebrain 📈. Trauma of all kinds also increased NA activity in the brain ⬆️.

🍽️ NA and Feeding

Feeding is one of the basic motivations of animals 🐁🍴. The NA systems also are involved in the control of feeding behaviors 🧠. If NA is administered directly in Lateral Hypothalamus 💉 leads to increased eating in animals which have already eaten to the point of satiation 🍽️➕ (they are full and they stop eating in the normal state 🛑). How do we know it is NA only? ❓ When we inject drugs which specifically for block NA 💊, the NA induced feeding is also blocked (no NA, no feeding!) 🚫🍴.

🔬 Research Evidence

This is demonstrated by injections of Phentolamine 💊 (A-adrenergic blocker), which leads to a blockade of NA induced feeding 🛑.

Liebowitz (1971) 👩‍🔬, a well-known researcher has shown through her experiments that NA may be acting to reduce the inhibition of the normal inhibition of lateral hypothalamic feeding center by the Ventromedial hypothalamus (VMH) 🏥. So, the LH starts the feeding 🍴⚡ and the VMH stops it 🛑 (by telling LH to stop sending signals for eating). When NA is injected 💉, it stops the inhibition of VMH 🚫🛑, so that messages of feeding can continue 🍽️➕.

🎁 NE and Self-Stimulation: ICS

Positive Reinforcement or "reward" 🎁 is linked with NA. Learning and conditioning using positive reinforcers or rewards are linked to intercranial self-stimulation or the self-stimulation ⚡. Positive ICS areas are areas in the brain where implanted electrodes would get maximal response of self-stimulation by the animals 🐀. The animals would repeatedly press levers for electrical stimulation to these areas in the brain 🔘⚡. These areas closely correspond to distributions of NA and DA systems 🧪, indicating that these neurotransmitters are modulating the reward behaviors 🎭.

🧪 Experimental Evidence

If we inject alpha methyl paratyrosine 💊, we reduce the amounts of tyrosine, DA and NA ⬇️. This injection also blocks the self-stimulation response in animals which were stimulating before the injection of AMPT 🚫.

❓ Identifying the Specific NT

How do we know which one of these two neurotransmitters is involved? ❓ Logically following it we would use a drug which would block only DA 🛑, or only NA 🛑, or 5HT 🛑 one by one after AMPT?

When we do so, we see that the AMPT blockade is reversed by the Alpha receptor agonists of NA ✅, not by B-receptor agonists or DA agonists or 5HT agonists ❌. Thus, showing that NA is involved ⚡.

🔬 Push-Pull Cannulae Research

Researchers have also used the Push pull cannuleas 💉 in Ventricle to pull out the metabolites after self-stimulation. It was reported that with the self-stimulation leads to an increase in the release of NA ⬆️. Further it has been shown that the NA Dorsal bundle is more than the Ventral bundle 📊.

😢 NA and Depression

The effectiveness of Monoamine Oxidase Inhibitors 💊 in treatment of depression has provided support to the Catecholamine hypothesis of Depression 🧠, and in particular the involvement of NA 💫.

This involvement is indicated by the fact that long term anti depression treatment in animals leads to a reduction in NA stimulated cyclic AMP 📉 (Beta receptors involved). This indicates that more NE stays available ⬆️ (as it is not degraded) therefore less needs to be released ⬇️.

💊 Antidepressant Effects

Antidepressant also increases the synaptic availability of NE (more NA becomes available) ⬆️.

⚠️ Reserpine and Depression

Reserpine 💊 (which has been used for treating mental illness in the Indian subcontinent since ages) when injected into the brain leads to depression like syndrome 😢 (remember, it destroys the storage vesicles 💥 and depletes NE, DA and 5HT from the presynaptic membrane ⬇️⬇️⬇️).

✅ Iproniazid Treatment

Iproniazid 💊 (which is an MAOI and an effective antidepressant) when administered increased brain concentrations of NE and 5HT ⬆️. Thus, showing that NA is involved in depression, as decreases in NA lead to depression 😢, and reward behaviors as depletion of NE reduced self-stimulation 🎁❌.

💜 Major Neurotransmitters: Monoamines: Indolamines

😊 Serotonin: Indolamine

Serotonin (also known as 5-hydroxytryptamine or 5-HT) 💜 is one of the major neurotransmitters of the brain with an important role in several behaviors (ranging from sleep 😴 to depression 😢). The neurons are known as serotonergic neurons 🧬 and the pathways as serotonergic pathways 🛤️.

📜 Historical Discovery

Scientists had known since the mid-19th century 📅 that there is a substance involved in powerful contraction of the smooth muscles 💪. Later, this was also found in the Ohio research labs 🇺🇸 to be the possible cause of high blood pressure 🩺, in American labs, this was called serotonin around the same time Italian scientists 🇮🇹 were trying to identify the substance in the intestinal mucosa, and also of the gut which led to powerful contractions of smooth muscle of the intestinal tract 🫀. This substance was called Enteramine by the Italian scientists. This substance is also found in clotted blood 🩸. These two groups of scientists eventually found that this substance was identical to 5-hydroxytryptamine (5HT) 🧪. This has a strong resemblance to Lysergic acid diethylamide (LSD) molecule 🌀.

🧬 Synthesis and Distribution

In the brain 5HT is synthesized in the same way as NE and DA from the precursor which is taken from the circulating blood 🩸. The precursor for 5HT is Tryptophan 🧬, which varies according to the daily intake of the Tryptophan rich foods 🍽️ (milk 🥛, red meat 🥩, fruits such as bananas 🍌 pineapples 🍍 etc.).

The body and the brain both have a high concentration of 5HT 📊, where it is synthesized independently 🏭. About 90% of 5HT is found in the gastrointestinal area (in the enterochromaffin cells of the intestine) 🫀 and only about 1-2% in the brain 🧠.

🔆 The Pineal Gland

The highest concentration of 5HT in the brain is found in the pineal gland 🔆. The Pineal gland is a very small organ lying on the dorsal surface of the thalamus 🧠. The pineal contains all the enzymes for the use of serotonin in addition to two other enzymes for transformation of serotonin ⚗️. The pineal contains about 50% more serotonin per gram of the brain that the rest of the brain areas 📈. (Wonder why?) ❓

🌙 Melatonin

The extension of pineal serotonin is Melatonin 🌙. In the pineal, Tryptophan is transformed into N-Acetyl Serotonin, which is then transformed into Melatonin 🔄. Melatonin is the substance which you see when your skin gets darkened by the sun (more melatonin, more pigmentation) 🌞🎨.

Melatonin secretion is enhanced by light ☀️ and suppressed by darkness 🌙. Thus, Melatonin content is affected by the Light Dark (L/D cycles) 🌍 and this bring daily and seasonal changes in the 5HT content in the pineal and the brain 📅.

We will talk more about serotonin's involvement in day night cycles (sleep) 😴🌙, in the later part of this lesson 📚.

⚗️ Serotonin Synthesis

We will discuss the synthesis of serotonin and where it begins and the enzymatic actions which occur 🧪. We must be again very clear that the brain synthesizes its own serotonin from the amino acid 🏭.

1️⃣ Tryptophan

This is the first step in the synthesis pathway 🔬. Tryptophan enters the cells but in competition with phenylalanine (daily variation depends upon the consumption of tryptophan rich foods 🍽️).

2️⃣ Hydroxylation of Tryptophan

This is the rate limiting step ⚠️. The hydroxylation of tryptophan takes place immediately at the 5th position of the molecule to form 5-Hydroxytryptophan or 5HTP 🧬. The enzyme involved in this action is Tryptophan hydroxylase ⚗️.

This step can be blocked by a drug called Parachlorophenylalanine (PCPA) 💊🛑. PCPA competes with tryptophan for this enzyme and binds irreversibly with this enzyme 🔗. In rats 🐀 one injection of PCPA of 200 mg/kg depletes brain 5HT drastically (to about 20%) ⬇️ and recovery to normal levels can take weeks ⏰.

3️⃣ Decarboxylation

5HTP is immediately decarboxylated to form 5HT ⚡. The decarboxylating agent is the one similar to Dopa (the same protein is used in the Catecholamines and Serotonin neurons for decarboxylation) 🧪. The enzyme L-Amino Acid Decarboxylase is involved in this action ⚗️.

4️⃣ Deactivation

Serotonin is deactivated or deaminated by Monoamine Oxidase (MAO) 🧪 as in the other monoamines. The metabolite of this action is 5-Hydroxy Indole Acetic Acid (5-HIAA) 🔬.

🗺️ Serotonergic Anatomical Location and Pathways

Though attempts had been made to identify the pathways of 5HT 🔍, it became possible only after Falck and Hillarp's 👨‍🔬 formaldehyde induced fluorescence histochemcial procedures became well known 🌟, and through the immunocytochemcial methods (through retrograde transport) 🔬, and procedures using radiolabelled amino acids taken up by the orthograde axoplasmic system when injected into the neurons 💉.

Dahlstrom and Fuxe (1964) 📅 and other researchers identified about nine clusters of 5HT neurons in the nuclei of the raphe system 🧠 located in the midline of the pons and upper brain stem. These are spread out like islands (or a bunch of grapes 🍇).

⬆️ Ascending 5HT Bundles

Ascending 5HT bundles travel through the Medial Forebrain Bundle 🛤️ with terminals in:

  • Reticular formation 🧠 (You will find out later why this connection is important ⭐)
  • Hypothalamus 🏥
  • Lateral geniculate nuclei 👁️
  • Preoptic area 🧠
  • Hippocampus 🌊
  • Cortex 🧠 (crucial role in sleeping and awakening 😴⏰)

These also project into the telencephalon and the diencephalon 🧠, and descend into the spinal cord 🦴 (Cooper, Bloom and Roth, 7th edition) 📚.

🛤️ Pathways

There are several serotonergic pathways each with their own connection and receptors 🔗. Nuclei Raphe comprise of several different groups. The Dorsalis, Superior and Magnus nuclei pass through the MFB 🧠.

🔵 Nuclei Raphe Dorsalis

The serotonergic receptors here are B7 🔵. It sends projections to:

  • Neocortex 🧠
  • Olfactory bulb 👃
  • Thalamus 🎯
  • Amygdala 💚
  • Hippocampus 🌊
  • Substantia Nigra 🖤
  • Locus Cerelleus 🧠

🟢 Nuclei Centralis Superior

The serotonergic receptors are known as B8 🟢. They project to:

  • Cerebral cortex 🧠
  • Hippocampus 🌊
  • Superchiasmatic nuclei (SCN) ☀️
  • Anterior hypothalamus 🏥
  • Medial preoptic area 🧠
  • Raphe dorsalis 🔵

🔴 Nuclei Raphe Magnus

The serotonergic receptors here are B3 🔴. They extend to medulla and the anterior hypothalamic area 🧠.

🟣 Nuclei Raphe Obscurus

This is an interesting pathway as the powerful hallucinogen LSD 🌀 acts here. The receptors here are known as B2 🟣.

🟡 Raphe Pallidus

Has B1 receptors 🟡, it contains substance P 🔗 (a peptide) involved in Pain ⚠️, goes down into spinal cord 🦴.

💊 Steps in 5HT Synthesis Where Drugs Can Modulate Action

Step 1️⃣: Synthesis by Enzyme

Tryptophan is converted into 5 hydroxytryptophan by tryptophan hydroxylase in the serotonergic neuron ⚗️. This process can be blocked by the action of PCPA 💊 which uses up the hydroxylating enzyme 🛑.

Step 2️⃣: Storage

Reserpine 💊, a major tranquilizing agent affects DA, NE and 5HT storage vesicles by irreversibly damaging the storage vesicles 💥. It is not clear whether DA, NE and 5HT responsible for behavioral depression 🤔.

However, researchers have found that when Reserpine is administered along with 5HTP or DOPA 💉, there are increases in sedation (induced by reserpine) 😴. Injections of PCPA (removed 90% of the brain serotonin) ⬇️ before reserpine, no behavioral effects of reserpine were seen 🚫.

Step 3️⃣: Release

There are no drugs which are specific serotonin blocking agents 🚫, but a major hallucinogenic drug Lysergic Acid Diethylamide (LSD) 🌀 potentiates serotonin effects in low doses ⚡.

LSD inhibits the release of serotonin by blocking the firing of serotonergic neurons (indirect blocking) 🛑.

LSD in high doses ⬆️- led to an increase in 5HT levels by reducing break down of serotonin (measured through reduced metabolites i.e., 5HIAA) 📉.

⬆️ Increased LSD Dosages

Increase LSD dosages reduced 5HT turnover rates, how? 🤔

  1. Serotonin receptor sites occupied 🔐
  2. Inhibit serotonin production by blocking action of 5HT 🛑
  3. LSD appears to decrease the release of 5HT ⬇️

Step 4️⃣: Receptor Interaction

LSD acts as a partial agonist at the receptor sites of post-receptor membrane 🎯.

Step 5️⃣: Reuptake

Serotonin action can be terminated by reuptake in the presynaptic area ♻️. Tricyclics such as Imipramine 💊 also increase 5HT levels by inhibiting reuptake ⬆️. The Selective serotonin Reuptake inhibitors (SSRI's) 💊 are effective for treatment of anxiety 😰✅.

Step 6️⃣: Degradation

Degradation by MAO can be inhibited by MAOI 💊. Iproniazid 💊 blocks MAO action in the presynaptic area 🛑.

Thus, we have seen that in a manner to other neurotransmitters 🧪, drugs interact with 5HT at various sites and can modulate levels of 5HT 🔄, and these drugs are also effective in treating psychopathology 🩺✅.

📚 References

  • Kalat, J.W. (1998). Biological Psychology. Brooks/ Cole Publishing
  • Carlson, N. R. (2005). Foundations of physiological psychology. Pearson Education New Zealand.
  • Pinel, J. P. (2003). Biopsychology. (5th ed). Allyn & Bacon Singapore.
  • Bloom, F., Nelson., & Lazerson. (2001), Behavioral Neuroscience: Brain, Mind and Behaviors. (3rd ed). Worth Publishers New York
  • Bridgeman, B. (1988). The Biology of Behavior and Mind. John Wiley & Sons, New York
  • Brown, T.S. & Wallace, P.S. (1980). Physiological Psychology. Academic Press, New York
  • Seigel, G. J., Agranoff, B.W, Albers W.R. & Molinoff, P.B. (1989). Basic Neurochemistry: Molecular, Cellular and Medical Aspects
  • Cooper, J.R., F.E Bloom, F. E., & Roth, R. H. (1970). The Biochemical basis of neuropharmacology (5th Ed.). New York, Oxford Univ. Press.