L47. Pharmacology- Intro To Neuropharmacology & Drug Targeting

• Drugs used in the clinic target neurons, making them neuropharmacological agents
• Target neuronal biology, especially membrane excitability and synaptic transmission
• Neurons use excitatory synapses to interconnect and form neuronal networks that conduct a compuation

The Central Nervous System

• The central nervous system (CNS) has around 100 billion neurons and even more glia
• Neurons create discrete anatomical nuclei/functional subsystems connected mostly by chemical synapses
• Functional neuronal networks are made through excitatory synapses
• Excitatory synapses rely mainly on glutamate as a neurotransmitter

Excitatory Synapses

• Notable exceptions include motor and pre-ganglionic autonomic neurons which rely on acetylcholine
• Other synapses use serotonin and purines as excitatory neurotransmitters
• Neurotransmitter release at excitatory synapses activates ionotropic receptors on the postsynaptic cell
• This leads to the generation of an excitatory post-synaptic potential (EPSP)

Neuromuscular Junctions and EPSP

• Motor neurons create large synapses with multiple active zones on muscle fibers
• This process is easiest to visualize at the neuromuscular junction
• This allows for the coordinated fusion of many synaptic vesicles during an action potential
• The invasion of the presynaptic terminal by an action potential results in a suprathreshold EPSP/endplate potential
• A high safety factor is present at the neuromuscular junction synapse
• Administering a blocker like curare reduces EPSP below threshold
• This reveals the underlying EPSP/end-plate potential

CNS Synapses Sizes

• Most CNS neurons make smaller synapses with one or a small number of active zones
• This probabilistically release few vesicles to produce subthreshold EPSPs
• Triggering an action potential mostly requires intense presynaptic activity as well as the coordinated activity of multiple neurons firing simultaneously
• This process scaled to millions of neurons and synapses, is thought to underlie the ability to carry on neuronal processing

Ionotropic Receptors and EPSPs

• Ionotropic receptors are ion channels directly opened by neurotransmitters
• They are also known as ligand-gated ion channels
• These receptors/channels are typically closed but open upon binding to a neurotransmitter
• These receptors mediate excitatory synaptic transmission, permeate cations like sodium and potassium.
• Sodium's driving force is larger than potassium, resulting in a net inward current that depolarizes the postsynaptic cell
• Neurotransmitter released by the presynaptic terminal lingers briefly in the synaptic cleft which opens receptors for a short time
• Due to this process, a transient depolarization of the postsynaptic membrane, or EPSP, occurs.
• Nicotinic receptors are composed of five subunits from the cys-loop protein family
• Ionotropic glutamate receptors are composed of four subunits
• ATP receptors are made of three subunits

Genome Diversity

• Genomes code for multiple subunits for each receptor type
• Heterogeneity is important for pharmacology because small changes in amino acid composition lead to different binding sites
• These sites can be targeted via synthetic ligands with different degrees of selectivity

Ionotropic Glutamate Receptors and Classes

• Ionotropic glutamate receptors include GluA (AMPA), GluN (NMDA), and GluK (Kainate) receptors
• GluA receptors in the brain permeate monovalent cations which mediate EPSCs
• GluN receptors permeate sodium, potassium, and calcium
• Ambient magnesium blocks ion permeation through the GluN receptor at the resting membrane potential
• The magnesium effect is relieved at depolarized potentials, allowing GluN receptor conduction

Long Term Potentiation

• GluN receptors underlie long term potentiation (LTP)
• These allow for ion fluxes, including calcium, at synapses experiencing strong postsynaptic depolarizations
• Calcium influx can be a result of ongoing isostypic gradients i.e. during an action potential
• Calcium influx through GluN occurs through synapses as part of a synaptic barrage
• The resulting rise in postsynaptic calcium activates Calcium Calmodulin Protein Kinase II (CamKII)
• CamKII signals the insertion of GluA receptors into the postsynaptic density, which potentiates the synapse

Synaptic Strength Underlying Learning

• Changes in synaptic strength underlie memory and learning in the brain
• It is possible to model simple neuronal networks of interconnected excitatory neurons
• Synapses on units that "fired" during correct trials are strengthened
• Networks "learn" through these iterations
• This underlies much of modern artificial intelligence (AI) like ChatGPT

Inhibitory Synapses in the CNS

• Neuronal networks consist of both excitatory and inhibitory neurons
• Inhibitory neurons shape the function and reduce runaway excitation of synaptic networks
• Inhibitory neurons project their axons locally to shape neuronal circuits
• They are also known as “interneurons”

GABA and Synaptic Transmission

• Inhibitory synapses rely on the neurotransmitter GABA
• Fast GABAergic synaptic transmission is mediated via ionotropic GABAĀ receptors
• These receptors belong to the cys-loop family and are made from five subunits, like the nicotinic receptor
• Mammalian genomes express many GABAA receptor subunits, leading to diverse heteropentameric GABAA receptors
• This large number of subunits provides support for function diversity in GABA signaling Chloride flows into the cell down its electrochemical gradient when GABAA receptors are activated, generating transient hyperpolarization

Inhibitor Postsynaptic Potential

• This hyperpolarization is known as an inhibitory postsynaptic potential (IPSP)
• The hyperpolarization and increased conductance inhibit the neuron, countering excitatory inputs

Neuromodulation

• Neuronal communication relies on a mix of neurotransmitter that signal through metabotropic receptors
• Neurons also communicate using variety of neurotransmitters that signal through metabotropic receptors
• Neuromodulators modulate neuronal properties, modifying neuron function
• This allows neuronal networks to function in different states
• These receptors are often heptahelical or G-protein coupled receptors.
• Best-known neuromodulators are norepinephrine, dopamine, serotonin, histamine, acetylcholine, and cannabinoids
• Peptide neurotransmitters/neurohormones such as opioid peptides, substance P, VIP, and leptin function as neuromodulators in the brain
• Cells that release these transmitters cluster in small cell groups in many brain regions and diffusely innervate other regions
• Effects of these are to regulate their fuctioning in relation to excitation

Glutamate or GABA and Co-transmission

• A neuromodulator is co-released with either glutamate or GABA in co-transmission
• Genomes encode metabotropic receptors that sense either glutamate or GABA
• Synapses signal through ligand-gated ion channels and metabotropic receptors using the same or two co-released neurotransmitters
• Heptahelical metabotropic receptor activation catalyzes GDP/GTP exchange at heterotrimeric proteins inside the cell
• G protein then dissociates into activated alpha and beta-gamma subunits and signal intracellularly

Heterotrimeric G Proteins

• Heterotrimeric G proteins signal through four main intracellular signaling cascades
• Heterotrimeric G proteins act on different receptors which allows neuromodulators access to signaling cascades.
• Norepinephrine acts on three adrenergic receptor classes to target Gq-11, Gio, and Gs

Autonomic Ganglionic Synapses

• Figure 17 illustrates the integration of fast, ionotropic receptor-mediated and slow, metabotropic receptor-mediated synaptic transmission at an autonomic ganglionic synapse

Acetylcholine and Ganglionic Neurons

• Preganglionic neuron activation releases acetylcholine onto the ganglionic neuron.
• Nicotinic receptor activation results in a fast EPSP that is followed by a slower membrane depolarization
• Acetylcholine activates metabotropic G protein-coupled receptors (muscarinic receptors)
• These receptors activate the Gq/11/PLC signaling cascade and signal the closing of “M” (Kv7, KVNQ) channels
• "M" channels maintain the resting membrane potential with their closure leading to membrane depolarization

Iontropic, Metabotropic

• Combined iontropic and metabotropic synaptic responses result not only a transient excitation of the postsynaptic cell but also enhanced excitability following fast excitation
• Neuromodulators and receptors serve as targets for neuropharmacological therapeutic agents as well as abuse drugs

Principles of Drugs

• Paul Ehrlich popularized the concept of “magic bullet”, where drugs selectively act on therapeutic interest with limited effect on other processes

Pharmacological Targeting

• You can selectively block sympathetic effects on the heart by targeting B1-adrenergic drugs like atenolol
• This is because sympathetic actions are mediated by genetically and molecularly structured receptors
• Following the turn of the 21st century and decoding the human genome, a new era of selective pharmacological targeting has arrived