vphy week 4 part 1
Neurotransmitter signaling basics: EPSP, IPSP, and receptor types
When a G protein-coupled receptor opens a potassium channel, K^+ exits the cell, causing hyperpolarization; this is an inhibitory postsynaptic potential (IPSP).
Example: G protein opening a K^+ channel -> hyperpolarization → IPSP.
When a receptor opens a sodium channel, Na^+ enters the cell, causing depolarization; this is an excitatory postsynaptic potential (EPSP).
Example: Opening a Na^+ channel → depolarization → EPSP.
In many synapses, the same presynaptic input can produce either EPSP or IPSP depending on the postsynaptic receptor subtype.
Some receptors are GPCRs, some are ionotropic; for instance, the 5-HT3 receptor is an ionotropic receptor that opens a Na^+ channel and produces EPSP.
Dendrites and the soma are sites of integration for these gradient potentials, and their summation determines whether the membrane potential reaches the threshold to fire an action potential at the axon initial segment.
So, dendrites and soma are key integration zones for EPSPs and IPSPs.
Descent motor pathways: pyramidal vs extrapyramidal
The primary motor pathway in humans starts in the motor cortex (precentral gyrus) in the frontal lobe.
It is often called the pyramidal tract because the axons project from the cortex down the spinal cord.
Extrapyramidal systems include all other motor systems outside the direct corticospinal tract; they are distributed in the midbrain, pons, and medulla and are important for motor control regulation beyond the direct corticospinal influence.
A full motor control framework includes multiple levels of control, including cortical, subcortical, brainstem, and spinal circuits.
Visual and limbic involvement in motor behavior
Visual cortex and other cortical regions influence motor outputs because vision heavily modulates behavior in humans (vision is a dominant sense).
The limbic system integrates emotion, memory, and motivation with motor output; it lies at the limb (edge) between the cerebral cortex and the diencephalon.
Key limbic structures include prefrontal cortex, cingulate cortex, amygdala, hippocampus, hypothalamus, and parts of the insula.
Limbic system is richly innervated by biogenic amines (norepinephrine, serotonin, dopamine) and modulates mood, arousal, appetite, and emotional behavior.
The limbic system’s role in mood has clinical implications for mood disorders and their treatment.
Biogenic amines: catecholamines and serotonin
The major monoaminergic systems studied are:
Catecholamines: dopamine (DA), norepinephrine (NE), and epinephrine (EPI) — collectively called catecholamines because of the catechol group in their structures.
Serotonin (5-HT) — an indoleamine.
Synthesis: all three catecholamines are derived from the amino acid tyrosine; serotonin is derived from tryptophan.
Catecholamine biosynthesis (tyrosine-derived)
Tyrosine → DOPA via tyrosine hydroxylase:
DOPA → Dopamine via dopa decarboxylase:
Dopamine → Norepinephrine via dopamine beta-hydroxylase:
Norepinephrine → Epinephrine via phenylethanolamine N-methyltransferase (PNMT):
Serotonin biosynthesis (tryptophan-derived)
Tryptophan → 5-HTP via tryptophan hydroxylase, then 5-HTP → serotonin (5-HT) via decarboxylase:
Serotonin is an indoleamine because of the indole ring structure.
Receptors and signaling modalities
Catecholamines (DA, NE, EPI) primarily act via G protein-coupled receptors (GPCRs), which can produce EPSPs or IPSPs depending on receptor subtype and signaling pathways.
Example: NE and epinephrine can increase heart rate and modulate smooth muscle tone via different adrenergic receptor subtypes (alpha and beta).
Serotonin receptors are mostly GPCRs as well, with the notable exception of 5-HT3, which is an ionotropic receptor that opens a cation channel (Na^+ entry) and can produce EPSP effects.
The main point: GPCRs can produce either EPSPs or IPSPs; the 5-HT3 receptor is one of the few ionotropic serotonin receptors.
Termination of monoaminergic signaling
For biogenic amines in the CNS, the primary mechanism to terminate signaling is reuptake into the presynaptic terminal via specific transporters (DAT for dopamine, NET for norepinephrine, SERT for serotonin).
These transporters use the Na^+ (and Cl^−) gradients to drive reuptake (secondary active transport).
Once inside the presynaptic terminal, monoamines can be repackaged into vesicles or degraded by enzymes such as monoamine oxidase (MAO) inside the terminal.
Vesicular monoamine transporter (VMAT) moves monoamines into vesicles; VMAT function is proton-gradient-driven and requires a vesicular ATPase to establish the proton (H^+) gradient.
The vesicular ATPase uses ATP to pump protons into the vesicle, creating a proton gradient that VMAT uses to transport monoamines into the vesicle.
The enzyme responsible for breakdown of monoamines in the synaptic cleft is monoamine oxidase (MAO); another mechanism is reuptake followed by intracellular degradation or vesicular reuse.
Acetylcholine (ACh) termination is via acetylcholinesterase, which rapidly hydrolyzes ACh in the synaptic cleft; this is a different terminating mechanism from monoamines.
In summary, three main termination routes for monoamines are:
Reuptake into presynaptic neuron (DAT, NET, SERT)
Enzymatic breakdown by MAO (inside neuron)
Vesicular sequestration and/or degradation after reuptake
The transporters and enzymes are common drug targets (see pharmacology below).
Functional roles of biogenic amines in mood, emotion, and cognition
Mood regulation and affect are strongly linked to biogenic amines, especially norepinephrine, dopamine, and serotonin.
Early neuropsychiatric treatments, including tricyclic antidepressants, targeted monoaminergic signaling and improved mood by increasing monoamine transmission.
Imaging studies (PET, fMRI) and historical data show that limbic structures and the prefrontal cortex are key sites where biogenic amines modulate mood, affect, and cognitive control.
The limbic system, including the prefrontal cortex, cingulate cortex, amygdala, hippocampus, hypothalamus, and parts of the insula, is densely innervated by NE, dopamine, and serotonin and is central to emotion, memory, learning, and mood regulation.
Mood disorders (depression, anxiety) are associated with dysregulated biogenic amine signaling in limbic and cortical circuits; increasing monoaminergic signaling can alleviate depressive symptoms in many patients.
Ketamine represents an emerging treatment approach for depression that acts via glutamatergic signaling (NMDA receptor antagonism) and may not primarily target biogenic amines, illustrating alternative mechanisms beyond monoamines.
Biogenic amines and depression: hypotheses and therapeutic targets
Classic hypothesis: decreased activity of biogenic amine pathways in limbic systems contributes to depression; restoring monoaminergic signaling can alleviate symptoms.
Therapeutic strategies that increase synaptic monoamines include:
MAO inhibitors (inhibit breakdown of monoamines, increasing their levels)
Tricyclic antidepressants (TCAs) often block reuptake transporters (DAT, NET, SERT), increasing synaptic monoamines
Selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine (increase serotonin in the synapse)
SNRIs (serotonin-norepinephrine reuptake inhibitors) and other transporter inhibitors that target multiple monoamines
Reuptake inhibitors raise synaptic monoamine levels, enhancing receptor activation and downstream signaling.
Reserpine: an old drug that inhibits VMAT, decreasing vesicular monoamine storage and release, which can cause depressive symptoms; historically used for hypertension (vasoconstriction via peripheral NE) but with depressive side effects.
Mechanism: inhibits vesicular monoamine transporter, reducing packaging and release, leading to decreased monoamine availability.
The same neurotransmitter can have different effects depending on the neural pathway:
Nigrostriatal pathway (substantia nigra to striatum) is motor-related; loss of dopamine here leads to motor deficits in Parkinson's disease.
Mesolimbic pathway (VTA to limbic areas) is involved in mood and reward; dysregulation can contribute to mood disorders and schizophrenia.
Dopamine pathways: nigrostriatal and mesolimbic systems
Nigrostriatal pathway:
Origin: substantia nigra
Projection: to the striatum (basal ganglia)
Key role: control of voluntary movement and motor learning; loss of dopamine in this pathway is a hallmark of Parkinson's disease, causing bradykinesia and other motor impairments.
Therapeutic note: MAO inhibitors and other dopaminergic drugs can improve motor symptoms by increasing dopamine availability.
Mesolimbic pathway:
Origin: ventral tegmental area (VTA)
Projection: to limbic structures (e.g., nucleus accumbens, prefrontal cortex)
Key role: mood, reward, motivation; implicated in mood disorders and schizophrenia.
Overactivation of mesolimbic pathway is associated with positive symptoms of schizophrenia; many antipsychotics block dopamine receptors to mitigate these symptoms.
Parkinson's disease: basal ganglia and motor control
Basal ganglia are a collection of subcortical nuclei (including caudate, putamen, globus pallidus, and others) that regulate motor control and suppression of involuntary movements.
A major pathophysiology in Parkinson's disease is the loss of dopamine input to the basal ganglia, particularly from the substantia nigra pars compacta, leading to motor symptoms such as bradykinesia, rigidity, tremor, and reduced spontaneous movement.
Treatments often target boosting dopamine signaling in the basal ganglia circuit, for example with MAO inhibitors or precursors to dopamine, to improve motor function.
Cortical and subcortical networks: mood and cognition
The mesolimbic and mesocortical pathways link dopaminergic signaling to mood and executive function in the frontal cortex.
Prefrontal cortex: critical in controlling affect and higher-order cognitive processes; receives dense noradrenergic and dopaminergic input and participates in mood regulation.
The limbic system modulates emotion, memory, learning, and mood; disruption in biogenic amine signaling within limbic circuits contributes to mood disorders.
Pharmacological targets and clinical implications
Termination mechanisms provide multiple drug targets:
Reuptake inhibitors (DAT, NET, SERT) raise synaptic monoamines and are used in depression and anxiety treatment (e.g., SSRIs like fluoxetine).
VMAT inhibitors (e.g., reserpine) decrease monoamine storage and release, used historically as antihypertensives but with depressive side effects.
MAO inhibitors reduce breakdown of monoamines, increasing their synaptic availability.
Drugs that influence the vesicular ATPase or the proton gradient can indirectly affect VMAT function and monoamine storage.
ACh system: termination via acetylcholinesterase; acetylcholine dynamics are separate from monoamines but important for neuromuscular and autonomic signaling.
Therapeutic theme: adjusting the balance of biogenic amines in specific brain circuits can modulate mood, arousal, and motor function; the same neurotransmitter can have different effects depending on the circuit and receptor subtypes involved.
Summary of core concepts to remember
EPSPs vs IPSPs depend on receptor type and ion flow; dendrites and soma integrate these signals to determine action potential initiation.
The primary motor pathway (pyramidal tract) originates in the motor cortex; extrapyramidal pathways include basal ganglia circuits critical for movement control.
The basal ganglia and nigrostriatal dopamine pathway are central to motor control and are profoundly affected in Parkinson's disease; the mesolimbic pathway is central to mood, reward, and psychiatric conditions such as schizophrenia.
Biogenic amines (DA, NE, EPI, 5-HT) are synthesized from tyrosine and tryptophan, respectively, and act primarily through GPCRs (except 5-HT3, which is ionotropic).
Termination of monoaminergic signaling is achieved mainly by reuptake (DAT/NET/SERT) and MAO-mediated breakdown; VMAT moves monoamines into vesicles using a proton gradient established by a vesicular ATPase.
Pharmacological approaches to mood disorders include reuptake inhibitors (SSRIs, SNRIs), MAO inhibitors, and VMAT-targeted strategies; ketamine represents a glutamatergic approach to depression beyond monoamines.
Clinically, manipulating dopaminergic signaling can improve motor symptoms in Parkinson's, influence mood and affect in depression, and affect positive symptoms in schizophrenia depending on the circuit targeted.
Key equations and gradients to remember
Catecholamine biosynthesis (tyrosine-derived):
Serotonin synthesis (tryptophan-derived):
Reuptake and degradation (summary): DAT/NET/SERT mediate reuptake; MAO degrades monoamines; VMAT sequesters into vesicles using the proton gradient; Vesicular ATPase maintains the gradient (drives VMAT indirectly).