precipitates, fluorophores, or chemiluminescence. In the event that neurotransmitters cannot be histochemically identified, an alternative method is to locate them by their neural uptake mechanisms.[1]
pufferfish toxin tetrodotoxin (TTX), a sodium channel blocker, was used to isolate the sodium channel protein by binding it using the column chromatography technique for chemical separation. The amino acid sequence of the protein was analyzed by Edman degradation and then used to construct a cDNA library which could be used to clone the channel protein. Cloning the channel itself allowed for applications such as identifying the same channels in other animals.[1]
Sodium channels are known for working in concert with potassium channels during the development of graded potentials and action potentials. Sodium channels allow an influx of Na+ ions into a neuron, resulting in a
resting membrane potential of a neuron to lead to a graded potential or action potential, depending on the degree of depolarization.[5]
Potassium ion channels
equilibrium potential. As with sodium ions, graded potentials and action potentials are also dependent on potassium channels. While influx of Na+ ions into a neuron induce cellular depolarization, efflux of K+ ions out of a neuron causes a cell to repolarize to resting membrane potential. The activation of potassium ion channels themselves are dependent on the depolarization resulting from Na+ influx during an action potential.[1]
As with sodium channels, the potassium channels have their own toxins that block channel protein action. An example of such a toxin is the large cation, tetraethylammonium (TEA), but it is notable that the toxin does not have the same mechanism of action on all potassium channels, given the variety of channel types across species. The presence of potassium channels was first identified in Drosophila melanogaster mutant flies that shook uncontrollably upon anesthesia due to problems in cellular repolarization that led to abnormal neuron and muscle electrophysiology. Potassium channels were first identified by manipulating molecular genetics (of the flies) instead of performing channel protein purification because there were no known high-affinity ligands for potassium channels (such as TEA) at the time of discovery.[1][6]
Calcium ion channels
axon terminals. A variety of different types of calcium ion channels are found in excitable cells. As with sodium ion channels, calcium ion channels have been isolated and cloned by chromatographic purification techniques. It is notable, as with the case of neurotransmitter release, that calcium channels can interact with intracellular proteins and plays a strong role in signaling, especially in locations such as the sarcoplasmic reticulum of muscle cells.[1]
Receptors
Various types of receptors can be used for cell signaling and communication and can include ionotropic receptors and metabotropic receptors. These cell surface receptor types are differentiated by the mechanism and duration of action with ionotropic receptors being associated with fast signal transmission and metabotropic receptors being associated with slow signal transmission. Metabotropic receptors happen to cover a wide variety of cell-surface receptors with notably different
Prototypical depiction of ionotropic receptor in the case of Ca2+ ion flow
GABA is the brain's main inhibitory neurotransmitter and glutamate is the brain's main excitatory neurotransmitter.[1]
GABA receptors
GABAA and GABAC receptors are known to be ionotropic, while the GABAB receptor is metabotropic. GABAA receptors mediate fast inhibitory responses in the
tranquilizer or anticonvulsant (antiepileptic drugs). On the other hand, GABA receptors can also be targeted by decreasing Cl− cellular influx with the effect of convulsants like picrotoxin
. The antagonistic mechanism of action for this compound is not directly on the GABA receptor, but there are other compounds that are capable of allosteric inactivation, including T-butylbicyclophorothionate (TBPS) and pentylenetetrazole (PZT).
Compared with GABAA, GABAC receptors have a higher affinity for GABA, they are likely to be longer-lasting in activity, and their responses are likely to be generated by lower GABA concentrations.[1]
Glutamate receptors
Ionotropic
excitatory postsynaptic potentials (EPSPs) produced by NMDA receptors, activating Ca2+-based signaling cascades (such as neurotransmitter release). AMPA generate shorter and larger excitatory postsynaptic currents than other ionotropic glutamate receptors.[5]
Nicotinic ACh receptors
Nicotinic receptors bind the acetylcholine (ACh) neurotransmitter to produce non-selective cation channel flow that generates excitatory postsynaptic responses. Receptor activity, which can be influenced by nicotine consumption, produces feelings of euphoria, relaxation, and inevitably addiction in high levels.[5]
Metabotropic receptors
G-protein-linked receptor signaling cascade
receptor tyrosine kinases
.
G protein-linked receptors
The
G protein-linked signaling cascade can significantly amplify the signal of a particular neurotransmitter to produce hundreds to thousands of second messengers in a cell. The mechanism of action by which G protein-linked receptors
cause a signaling cascade is as follows:
Neurotransmitter binds to the receptor
The receptor undergoes a conformational change to allow G-protein complex binding
GDP is exchanged with GTP upon G protein complex binding to the receptor
The α-subunit of the G protein complex is bound to GTP and separates to bind with a target protein such as
adenylate cyclase
The binding to the target protein either increases or decreases the rate of second messenger (such as
cyclic AMP
) production
GTPase hydrolyzes the α-subunit so that is bound to GDP and the α-subunit returns to the G protein complex inactive
Neurotransmitter release
Structure of a synapse where neurotransmitter release and uptake occurs
Neurotransmitters are released in discrete packets known as quanta from the
catecholamines and other peptide neurotransmitters.[7] Neurotransmitters are released from an axon terminal and bind to postsynaptic dendrites in the following procession:[5]
Mobilization/recruitment of synaptic vesicle from cytoskeleton
Docking of vesicle (binding) to presynaptic membrane
Fusion of primed vesicle with presynaptic membrane and exocytosis of the housed neurotransmitter
Uptake of neurotransmitters in receptors of a postsynaptic cell
Initiation or inhibition of action potential in postsynaptic cell depending on whether the neurotransmitters are excitatory or inhibitory (excitatory will result in depolarization of the postsynaptic membrane)
Neurotransmitter release is calcium-dependent
Neurotransmitter release is dependent on an external supply of Ca2+ ions which enter axon terminals via voltage-gated
action potentials. The Ca2+ ions cause the mobilization of newly synthesized vesicles from a reserve pool to undergo this membrane fusion. This mechanism of action was discovered in squid giant axons.[8] Lowering intracellular Ca2+ ions provides a direct inhibitory effect on neurotransmitter release.[1] After release of the neurotransmitter occurs, vesicular membranes are recycled to their origins of production. Calcium ion channels can vary depending on the location of incidence. For example, the channels at an axon terminal differ from the typical calcium channels of a cell body (whether neural or not). Even at axon terminals, calcium ion channel types can vary, as is the case with P-type calcium channels located at the neuromuscular junction.[1]
Differences in sex determination are controlled by
sexual dimorphisms
(phenotypic differentiation of sexual characteristics) of the brain. Recent studies seem to suggest that regulating these dimorphisms has implications for understanding normal and abnormal brain function. Sexual dimorphisms may be significantly influenced by sex-based brain gene expression which varies from species to species.
Animal models such as rodents, Drosophila melanogaster, and Caenorhabditis elegans, have been used to observe the origins and/or extent of sex bias in the brain versus the hormone-producing gonads of an animal. With the rodents, studies on genetic manipulation of sex chromosomes resulted in an effect on one sex that was completely opposite of the effect in the other sex. For example, a knockout of a particular gene only resulted in anxiety-like effects in males. With studies on D. menlanogaster it was found that a large brain sex bias of expression occurred even after the gonads were removed, suggesting that sex bias could be independent of hormonal control in certain aspects.[9]
Observing sex-biased genes has the potential for clinical significance in observing brain physiology and the potential for related (whether directly or indirectly) neurological disorders. Examples of
diseases with sex biases in development include
Many brain functions can be influenced at the cellular and molecular level by variations and changes in gene expression, without altering the sequence of DNA in an organism. This is otherwise known as epigenetic regulation. Examples of epigenetic mechanisms include histone modifications and DNA methylation. Such changes have been found to be strongly influential in the incidence of brain disease, mental illness, and addiction.[10] Epigenetic control has been shown to be involved in high levels of plasticity in early development, thereby defining its importance in the critical period of an organism.[11] Examples of how epigenetic changes can affect the human brain are as follows:
Higher methylation levels in rRNA genes in the hippocampus of the brain results in a lower production of proteins and thus limited hippocampal function can result in learning and memory impairment and resultant suicidal tendencies.[12]
In a study comparing genetic differences between healthy people and psychiatric patients 60 different epigenetic markers associated with brain cell signaling were found.[12]
Environmental factors such as child abuse appears to cause the expression of an epigenetic tag on glucocorticoid receptors (associated with stress responses) that was not found in suicide victims.[12] This is an example of experience-dependent plasticity.
Environmental enrichment in individuals is associated with increased hippocampal gene histone acetylation and thus improved memory consolidation (notably spatial memory).[11]
Molecular mechanisms of neurodegenerative diseases
hippocampal long-term potentiation. It is also possible that a receptor for amyloid-β oligomers could be a prion protein.[14]
Parkinson's disease
reticulata cells, causes inhibition of upper motor neurons similar to the inhibition that occurs in Parkinson's disease.[5]
Huntington's disease
substantia nigra pars reticulata decreases inhibition of upper motor neurons, resulting in ballistic involuntary motor movements, similar to symptoms of Huntington's disease.[5]
