Question

explain the following Neurotransmitters:
i. Serotonin
ii. Histamine
iii. Dopamine ( 5 marks each )
iv. Glycine
v. Glutamate
vi. Gama amino butyric acid (GABA)
vii. Nitric oxide (NO)

Answer 1

1. SEROTONIN

Serotonin (5-HT) is synthesized from tryptophan (an essential amino acid). It is in activated by monoamine oxidase to form 5-Hydroxy indole acetic acid which is excreted in urine.

5-HT is present in the brain and non-neural cells. In the brain, serotonergic neurons have cell bodies in the brain stem and they project to portions of Hypothalamus, limbic system, neocortex and spinal cord. Non neural cells that contain serotonin are blood platelets, mast cells and GIT enterochromaffin cells and Myenteric plexus.

There are 7 group of Receptors of serotonin (5HT-1 to 5-HT7), each group has further subgroups from A to F. They are mostly coupled with G protein coupled receptors and affect adenylyl cyclase and Phospholipase C.

Effects:- Regulation of carbohydrate intake and hypothalamic releasing hormones.

Pain inhibition

Hallucinations and depression of mood.

2) HISTAMINE

Histamine is formed by decarboxylation of histidine. It is secreted in brain and non neural cells. In the brain, histaminergic neurons have their cell bodies mainly in the posterior hypothalamus and their axons project to all parts of brain including the cerebral cortex and spinal cord. It's receptors are of three types: H1 H2, H3. All three types are found in brain and peripheral tissues. H1 receptor activates phospholipase C. H2 increase Intracellular cAMP. H3 Receptors are presynaptic and they mediate inhibition of the release of histamine via G-protein. Histamine is an excitatory neurotransmitter. It is believed that histamine plays a role in arousal and sexual behaviour, regulation of secretion of some anterior pituitary hormones, drinking, pain threshold and sensation of itch.

3) DOPAMINE

Dopamine is naturally acting precursor of norepinephrine. It acts on three receptors. D1 receptor activate adenylyl cyclase via Gs protein. D2 receptor inhibit adenylyl cyclase via Gi protein. The brain contains more of D2 receptors. D3 receptors are localized to nucleus accumbens.

Dopaminergic neurons have their cell bodies in the midbrain. They project from the substantia nigra to striatum, olfactory tubercle, nucleus accumbens, limbic system. It doesn't cross Blood Brain Barrier.

Dopamine is metabolized by monoamine oxidase and catecnol-o-methyl transferase.

It controls movements. It induces vomiting. It inhibits prolactin secretion and stimulates GnRH. Schizophrenia also has increased levels of Dopamine.

4) GLYCINE

It is the simplest amino acid with its alkyl group being Hydrogen atom. It is an inhibitory neurotransmitter from the primarily in the grey matter of spinal cord and brainstem. It produces direct inhibition (postsynaptic inhibition) in the spinal cord and acts by increasing chloride conductance by acting on the glycine receptor. Agents such as strychnine anti tetanus toxin antagonise the postsynaptic inhibitory action of glycine and produce convulsions and muscular hyperactivity.