Applied Pharmacology of the Nervous and Endocrine System Drug Targets

UNIVERSITYOF PORTSMOUTH

M33319 - Applied Pharmacology of the Nervous and Endocrine System Drug Targets: Neurotransmitter and Receptor Systems Ngan Pan Bennett Au, PhD Lecturer in Pharmacology (Neuroscience) School of Pharmacy and Biomedical Science, University of Portsmouth

Focus of APNES

• Primarily pharmacology-focused (mode of actions, molecular/cellular mechanisms), not focusing on medical condition (neuropharmacology module)
• How a drug works on the central nervous system (CNS)
• In-depth understanding of the mechanisms of action of drugs
• Textbook: Rang and Dale's Pharmacology (9th edition, eBook)

Learning Objectives

• Identify and define the functions of the major neurotransmitter systems in the central nervous system.
• Identify and define the functions and signalling mechanisms of the major neurotransmitter receptor systems in the central nervous system.
• Define the effects of these neurotransmitter-receptor systems in terms of their effects on synaptic transmission and neuronal excitability
• Name and explain mechanisms of drugs that alter the functions of these receptors.

Chemical Signalling in the Nervous System

Timescale Process Chemical mediators Molecular mechanisms ms Impulse conduction None Voltage-gated ion channels (Ch. 4) Transmitter release [Ca2+], Exocytosis (Ch. 4) Fast synaptic transmission Fast transmitters (e.g. glutamate, GABA, ACh) Ligand-gated ion channels (Ch. 3) S Slow synaptic transmission Slow transmitters (e.g. monoamines, peptides, ACh) Neuromodulation Slow transmitters + others (e.g. NO, arachidonic acid metabolites) G protein-coupled receptors (Ch. 3) linked to ion channels, [Ca2+]i, second messengers Soluble guanylyl cyclase (Ch. 21) min Synaptic plasticity Delayed pharmacological effects Many neuroactive drugs (e.g. antidepressants, Ch. 48) Receptor up-/down-regulation ? Altered gene expression h Pharmacological tolerance Many neuroactive drugs (Ch. 50) (e.g. opioids, benzodiazepines) day Structural remodelling Chemokines Cytokines Growth factors Kinase-linked receptors controlling gene expression ? Adhesion molecules ? Steroids month/year Degeneration, regeneration and repair (very limited in CNS) ?

Targets for Drug Action

• The protein targets for drug action can be broadly divided into:
• receptors
• ion channels
• enzymes
• transporters

A RECEPTORS Direct lon channel opening/closing Enzyme activation/inhibition Agonist/ inverse agonist Transduction mechanisms E lon channel modulation DNA transcription Antagonist No effect Endogenous mediators blocked B ION CHANNELS Blockers Permeation blocked E Modulators Increased or decreased opening probabilityUNIVERSITYOF PORTSMOUTH

Receptors

• Receptors are sensing elements
• Responsible for chemical communications
• Coordinate the functions and responses for the endogenous mediators
• Agonists v.s. antagonist
• e.g. Cannabinoids, opioid

A RECEPTORS Direct lon channel opening/closing Enzyme activation/inhibition Agonist/ inverse agonist Transduction mechanisms E lon channel modulation DNA transcription Antagonist No effect Endogenous mediators blocked B ION CHANNELS Blockers Permeation blocked E Modulators Increased or decreased opening probabilityUNIVERSITYOF PORTSMOUTH

Ion Channels

• lon channels are gateways in the cell membranes
• control ins and outs of particular ions
• Direct or indirect interactions (intermediate)
• Benzodiazepines:
• facilitate the opening of GABAA receptors by neurotransmitter GABA
• Alternation of expression of ion channels (e.g. gabapentin)

A RECEPTORS Direct lon channel opening/closing Enzyme activation/inhibition Agonist/ inverse agonist Transduction mechanisms E lon channel modulation DNA transcription Antagonist No effect Endogenous mediators blocked B ION CHANNELS Blockers Permeation blocked E Modulators Increased or decreased opening probabilityUNIVERSITYOF PORTSMOUTH

Enzymes

• Drug molecules are substrate analogues that act as competitive inhibitors
• Drug molecules are false substrates: Formation of abnormal/non- functional products that affect normal metabolic pathway
• Prodrug: inactive drug

c ENZYMES Inhibitor Normal reaction inhibited False substrate Abnormal metabolite produced Prodrug Active drug produced 8UNIVERSITYOF PORTSMOUTH

Transporters

• Transporter protein controls the movement of ions or polar organic molecules across the cell membrane
• e.g. neurotransmitter transporters at nerve terminals
• e.g. cocaine blocks the uptake of monoamine (dopamine, norepinephrine and serotonin) neurotransmitters

D TRANSPORTERS Normal transport Inhibitor or Transport blocked False substrate Abnormal compound accumulated 9UNIVERSITYOF PORTSMOUTH

Different Chemical Mediators in the CNS

Table 38.1 Types of chemical mediators in the central nervous system Mediator typeª Examples Targets Main functional role Conventional small-molecule mediators Glutamate, GABA, acetylcholine, dopamine, 5-hydroxytryptamine, etc. Ligand-gated ion channels G protein-coupled receptors Fast and slow synaptic neurotransmission Neuromodulation Neuropeptides Substance P, neuropeptide Y, endorphins, orexins, corticotrophin-releasing factor, etc. G protein-coupled receptors Neuromodulation Lipid mediators Prostaglandins, endocannabinoids G protein-coupled receptors Neuromodulation 'Gaseous' mediators Nitric oxide, carbon monoxide, hydrogen sulfide, etc Guanylyl cyclase Neuromodulation Neurotrophins, cytokines Nerve growth factor, brain-derived neurotrophic factor, interleukin-1 Kinase-linked receptors Neuronal growth, survival and functional plasticity Steroids Androgens, oestrogens Nuclear and membrane receptors Functional plasticity ªMost central nervous system pharmacology has been centred on small-molecule mediators and, less commonly, neuropeptides. Other mediator types are now being targeted for therapeutic purposes.UNIVERSITYOF PORTSMOUTH

Chemical Transmission in the CNS

• Neurotransmitter:
• Released by the pre-synaptic vesicles
• Produced rapid excitatory or inhibitory responses in the postsynaptic neurons
• Fast neurotransmitters (e.g. glutamate and GABA) operate through ligand- gated ion channels
• Slow neurotransmitters and neuromodulators (e.g. dopamine, neuropeptides and prostanoids) operate through G protein-coupled receptors

Neuromodulator and Neurotrophic Factors

• Neuromodulator:
• Released by neurons and astrocytes
• Produced slower pre- or post-synaptic responses
• Neurotrophic factors:
• Mainly produced in non-neuronal cells
• Act on tyrosine kinase-linked receptors (for neurotrophins and growth factors)
• Regulate gene expression, control neuronal growth and phenotypic changes.

Glial Cells and Chemical Signalling

• Glial cells (astrocytes, oligodendrocytes, ependymal cells, microglia ... )
• particularly astrocytes - actively involved in chemical signalling
• often regarded as "inexcitable neurons"
• Some agents (e.g. glutamate, 5-hydroxytryptamine, acetylcholine) can act through both ligand-gated ion channels and G protein-coupled receptors
• function as both neurotransmitter and neuromodulator

Mediators and Long-Term Changes

• Many chemical mediators (e.g. Glutamate, NO) are produced by neurons and glial cells
• Some mediators (cytokines, chemokine, growth factors and steroids) control long-term changes in the brain
• Regulate gene expression
• Synaptic plasticity and structure remodelling

Drug Action in the CNS - General Principles

• Most of these targets have multiple molecular isoforms
• Results in subtle differences in functions and pharmacology
• Current neuroactive drugs are relatively 'non-specific
• bind to different targets (multiple receptors, transporters and ion channels)
• The relationship between pharmacological profile and its therapeutic effect remains largely unclear
• Slowly developing secondary responses to the primary drug-target interactions (drug addiction, drug tolerance/dependence)

Class of Receptors

1. Ligand-gated ion channels (ionotropic receptors) 2. G protein-coupled receptors (metabotropic) 3. Kinase-linked receptors 4. Nuclear receptors lons lons E R/E NUCLEUS -> Hyperpolarisation or depolarisation Change in excitability Protein phosphorylation R Gene transcription Gene transcription Ca2+ release Protein phosphorylation Other Cellular effects Cellular effects Cellular effects Cellular effects Time scale Milliseconds Seconds Hours Hours Examples Nicotinic ACh receptor Muscarinic ACh receptor Cytokine receptors Oestrogen receptor R R R G G or O + or O Second messengers Protein synthesis Protein synthesis

Amino Acid Neurotransmitters

• Three major amino acid neurotransmitters
• Excitatory: Glutamate
• Inhibitory: GABA
• Mixed (either excitatory or inhibitory, depending on receptor targets): Glycine
• NMDA (glutamate) receptors: excitatory
• Strychnine-sensitive glycine receptors: inhibitory
• Receptors are widely expressed throughout the CNS
• Primary mechanism to regulate neuronal activity
• Drugs targeting these receptors have widespread actions on the brain: likely to have more side-effects

-Glutamine GAD Glutamine synthase Glutaminase GABA Glutamate Glyoxylate GABA-T Transaminase Succinic semialdehyde a-Ketoglutarate Glycine TRICARBOXYLIC ACID CYCLE Succinate - Aspartate -Oxaloacetate Transaminase Fig. 39.1 Metabolism of transmitter amino acids in the brain. Transmitter substances are marked with green boxes. GABA-T, GABA transaminase; GAD, glutamic acid decarboxylase.UNIVERSITYOF PORTSMOUTH

Glutamate Receptor Subtypes

• Glutamate and related excitatory amino acids (aspartate and homocysteate) activate both:
• lonotropic (ligand-gated ion channels) glutamate receptors (16 different subunits)
• NMDA (GluN1, GluN2A, GluN2B, GluN2C, GluN2D, GluN3A, GluN3B)
• AMPA (GluA1, GluA2, GluA3, GluA4)
• Kainate (GluK1, GluK2, GluK3, GluK4, GluK5)
• Metabotropic (G protein-coupled) receptors