Neurotransmission Product Guide | Edition 1 Delphinium Delphinium A source of Methyllycaconitine Contents by Research Area: • Dopaminergic Transmission • Glutamatergic Transmission • Opioid Peptide Transmission • Serotonergic Transmission • Chemogenetics Tocris Product Guide Series Neurotransmission Research Contents Page Principles of Neurotransmission 3 Dopaminergic Transmission 5 Glutamatergic Transmission 6 Opioid Peptide Transmission 8 Serotonergic Transmission 10 Chemogenetics in Neurotransmission Research 12 Depression 14 Addiction 18 Epilepsy 20 List of Acronyms 22 Neurotransmission Research Products 23 Featured Publications and Further Reading 34 Introduction Neurotransmission, or synaptic transmission, refers to the passage of signals from one neuron to another, allowing the spread of information via the propagation of action potentials. This process is the basis of communication between neurons within, and between, the peripheral and central nervous systems, and is vital for memory and cognition, muscle contraction and co-ordination of organ function. The following guide outlines the principles of dopaminergic, opioid, glutamatergic and serotonergic transmission, as well as providing a brief outline of how neurotransmission can be investigated in a range of neurological disorders. Included in this guide are key products for the study of neurotransmission, targeting different neurotransmitter systems. The use of small molecules to interrogate neuronal circuits has led to a better understanding of the under- lying mechanisms of disease states associated with neurotransmission, and has highlighted new avenues for treat- ment. Tocris provides an innovative range of high performance life science reagents for use in neurotransmission research, equipping researchers with the latest tools to investigate neuronal network signaling in health and disease. A selection of relevant products can be found on pages 23-33. Key Neurotransmission Research Products Box Number Title Page Box Number Title Page Box 1 Dopaminergic Transmission 5 Box 6 Antidepressants 15 Box 2 Glutamatergic Transmission 7 Box 7 Ketamine and its Metabolites 17 Box 3 Opioid Transmission 9 Box 8 Addiction 19 Box 4 Serotonergic Transmission 11 Box 9 Epilepsy 20 Box 5 Chemogenetic Compounds: DREADD ligands 13 and PSEMs 2 | NEUROTRANSMISSION RESEARCH Principles of Neurotransmission The majority of neurotransmission occurs across chemical syn- Neurotransmitters cross the synaptic cleft and bind to their spe- apses, where an endogenous neurotransmitter is released by cific receptors. These maybe ligand-gated ion channels (LGICs) the presynaptic neuron and detected by receptors on the post- or G protein-coupled receptors (GPCRs), with some neuro- synaptic neuron (Figure 1). Neurotransmitters can be broadly transmitters having receptors in both categories. Binding of split into three categories; amino acids including glutamate and a neurotransmitter to a LGIC causes a conformational change glycine, amines including dopamine (DA), serotonin (5-HT) in the structure of the protein, allowing the passage of ions and norepinephrine (NE), and peptides such as dynorphin, the through the channel. Passage of ions through channels that are enkephalins and neuropeptide Y. selective for positively-charged cations results in depolariza- tion of the postsynaptic membrane and initiation of an action While the amino acids glutamate and glycine are found in all potential in the postsynaptic neuron. In contrast, passage of cells of the body, other neurotransmitters are only synthe- ions through negatively-charged, anion selective channels sized by neurons. Following synthesis, neurotransmitters are results in hyperpolarization of the postsynaptic membrane, taken up and stored in synaptic vesicles, ready for release. The so inhibiting action potential initiation. Binding of a neuro- release of a neurotransmitter is triggered by the arrival of action transmitter to a GPCR results in the activation of G proteins, potentials in the axon terminal of the presynaptic neuron, open- which are then able to act on enzymes to modulate intracellular ing voltage-gated Ca2+ channels and allowing influx of ions. signaling pathways. The end result of this is the modulation of The resulting elevation in intracellular Ca2+ concentration activity of other proteins, including ion channels and enzymes. causes synaptic vesicles to merge with the presynaptic mem- brane, releasing the neurotransmitter into the synaptic cleft by Once a neurotransmitter has bound to its receptor, it is cleared exocytosis. from the synaptic cleft to allow another wave of synaptic Figure 1 | Principles of Neurotransmission Presynaptic neuron 5-HT1,5 Postsynaptic neuron DAT D2,3,4 Opioid receptors G Group II and III mGluRs i/0 SERT (–) AC ATP 5-HT Glu 4,6,7 D Opioid 1.5 G (–) proteins s cAMP PDE Precursor proteins PKA Ca2+ DA 5-HT2 Group I mGluRs IP3 CaMK Gq/11 5-HT PLC DAG PKC (–) Mg2+ CaMK Glu NMDARs Ca2+ Increased 5-HT neuronal 3 excitability Na+ AMPARs and Kainate receptors This simplified schematic shows the main events during dopaminergic, glutamatergic, opioid peptide and serotonergic neurotransmission. DA and 5-HT are both biogenic amines that are derived from amino acids, while glutamate itself is an amino acid and opioid peptides are cleaved from precursor proteins. All neurotransmitters undergo exocytosis from the presynaptic membrane and cross the synaptic cleft where they bind to their specific receptors. These receptors may be ligand gated ion channels, such as ionotropic glutamate receptors, or G protein-coupled receptors, such as all subtypes of opioid receptor. Passage of ions through a ligand gated ion channel alters the excitability of a neuron. The action of neurotransmitters at GPCRs alters intracellular signaling pathways, with the specific pathway being dependent on the G protein-coupled to the receptor. www.tocris.com | 3 Tocris Product Guide Series transmission. Neurotransmitter molecules are taken up by the concentration gradient. This depolarizes the cell membrane presynaptic neuron, or by other cell types such as astrocytes, past the threshold for action potential initiation. As Nav chan- via specific reuptake transporters. Neurotransmitters may then nels become inactivated, preventing the flow of Na+, voltage- be metabolized to be reused for further production, or they can gated potassium channels (Kv channels) open allowing the be recycled into synaptic vesicles. efflux of+ K . This causes repolarization of the cell membrane, as the balance of ion movement across the membrane leads to The strength of the synaptic connection between two neurons the cytosol being more negatively charged than extracellular depends on a range of factors. These include the number of fluid. When K channels are open the cell membrane is highly individual synapses between two neurons, the probability of v permeable to K+, but permeability to Na+ is low as Na channels neurotransmitter release at the presynaptic membrane and the v are still inactivated. This leads to a period of hyperpolarization size of the post-synaptic potential induced by binding of the until K channel close, and the resting membrane potential is neurotransmitter to its receptor. The presence of neurotrans- v re-established. mitter receptors on the pre-synaptic membrane also regulates the release of neurotransmitters, through both positive and Propagation of an action potential occurs as a wave of depolar- negative feedback loops. Synaptic connection strength is a key ization that spreads along an axon. When an area of the mem- factor in cognitive processes including memory formation. brane becomes depolarized, it opens neighboring Nav channels, which then allow depolarization of that section of the mem- Action Potentials brane. The inactivation of Nav channels in the preceding sec- An action potential is the signal that conveys information along tion of membrane ensures that an action potential travels in a neuron and is also the trigger for release of a neurotrans- only one direction alone an axon. Some axons in the central mitter at a synapse. Physically, an action potential is the rapid nervous system have a sheath around them, composed of mye- reversal of the resting membrane potential, caused by opening lin, which acts as an electrical insulator. Nav and Kv channels and closing of voltage-gated ion channels. At rest, the cytosol are localized on gaps in the myelin sheath, known as nodes of of a neuron is negatively charged (polarized) with respect to Ranvier. Myelination increases that speed of action potential extracellular fluid, due to the distribution of ions across the propagation as the action potential effectively ‘hops’ along an cell membrane. axon, occurring only at nodes of Ranvier in a process known as saltatory conduction. An action potential is initiated by the opening of voltage-gated + + Na channels (Nav channels) allowing influx of Na down its Figure 2 | DDC in Human Brain Figure 3 | D1R in Human Brain Figure 4 | DAT1 in Human Brain Detected in immersion-fixed paraffin- D1R detected in immersion-fixed paraffin- DAT1 detected in immersion-fixed sections embedded sections of human brain embedded sections of human brain (caudate of human brain (substantia nigra) using (substantia nigra) using a Goat Anti-Human/ nucleus) using a Mouse Anti-Human a Mouse Anti-Human/Mouse/Rat DAT1 Mouse/Rat DDC Antigen Affinity-Purified Dopamine D1R Monoclonal Antibody (R&D Monoclonal Antibody (Novus Biologicals, Polyclonal Antibody (R&D Systems, Cat.