Exam 1

Criteria and Classification of Neurotransmitters

• Definition and Criteria for Neurotransmitters:

  • The chemical must be released from a presynaptic terminal onto a postsynaptic target cell.

  • The chemical must be synthesized within the same presynaptic cell (neuron) that releases it. Synthesis can occur in either the soma (cell body) or the axon terminal, but the releasing nerve cell must synthesize the chemical itself.

  • The chemical must be released as a direct result of action potentials reaching the axon terminal, which triggers exocytosis.

• Major Chemical Classes of Neurotransmitters:

  • Amino Acid-Derived Neurotransmitters: Synthesized from basic dietary amino acids. Examples include glutamate, GABA, dopamine, serotonin, norepinephrine, and acetylcholine.

  • Neuropeptides: Small protein molecules that are molecularly significantly larger and bulkier than standard amino acid-derived neurotransmitters.

  • Steroids: Lipid-based regulatory molecules.

  • Gases: Soluble gas signaling molecules such as nitric oxide ($\text{NO}$) and carbon monoxide ($\text{CO}$).

  • Endocannabinoids: Lipid-derived retrograde signaling molecules.

Glutamate and GABA: Primary Excitatory and Inhibitory Systems

• Gamma-Aminobutyric Acid (GABA):

  • GABA is the primary inhibitory neurotransmitter in the central nervous system.

  • Mechanism of Inhibition: GABA binds to its receptors to open anion channels, allowing chloride ($\text{Cl}^-$) ions to flow into the cell. The influx of negative ions hyperpolarizes the membrane potential (making it more negative), moving it further from the threshold potential and preventing the neuron from firing an action potential.

• Glutamate:

  • Glutamate is the primary excitatory neurotransmitter in the central nervous system.

  • Mechanism of Excitation: Glutamate binds to its receptors to directly or indirectly open cation channels, permitting positive ions—specifically sodium ($\text{Na}^+$) and calcium ($\text{Ca}^{2+}$)—to enter the cell. The influx of positively charged ions depolarizes the membrane potential (bringing it closer to $0\,\text{mV}$), increasing the probability that the cell reaches threshold potential and fires an action potential.

• Ubiquity and Divergent Synthesis:

  • Glutamate and GABA are the most abundant neurotransmitters in the brain and are present across every single brain region.

  • GABA is synthesized locally in every brain region without exception.

  • Glutamate is transmitted to every brain region, though certain specific regions do not synthesize it locally.

  • Both GABA and glutamate are synthesized from the same amino acid precursor (glutamine). Their synthesis diverges because specific cell types express distinct enzymes required to generate either GABA or glutamate.

• Seizures, Sedation, and Clinical Applications:

  • Seizure Physiology: Seizures represent pathological overexcitation characterized by massive, synchronous, continuous firing of populations of neurons without adequate inhibition. This occurs when GABAergic neurons malfunction or shut down, leaving glutamatergic excitation uninhibited.

  • Seizure Treatment: Administering a GABA A receptor agonist (such as Valium) activates GABA A receptors, mimicking natural GABA inhibition to suppress overexcited glutamatergic neurons and arrest seizure activity.

  • Excessive Inhibition and Alcohol Mechanism: Excessive GABAergic inhibition does not cause seizures; instead, it causes neural sedation and motor/cognitive impairment. Alcohol acts as a positive allosteric modulator and agonist at GABA A receptors. Excessive alcohol consumption overactivates GABA A receptors, leading to massive $\text{Cl}^-$ influx, widespread neuronal inhibition, and loss of consciousness.

Ionotropic and Metabotropic Glutamate Receptors

• Ionotropic Glutamate Receptors:

  • AMPA Receptors: Purely ligand-gated cation channels. Activated solely by glutamate binding to open and allow sodium ($\text{Na}^+$) influx, driving postsynaptic depolarization.

  • Kainate Receptors: Purely ligand-gated cation channels activated by glutamate.

  • NMDA (N-Methyl-D-Aspartate) Receptors:

  • Bimodal Regulation: NMDA receptors are both ligand-gated and voltage-gated.

  • Subunit Structure: Heteromeric structures composed of multiple subunits (e.g., NR1 and NR2 combinations). The glutamate binding domain resides on the NR2 subunit; a separate binding site exists for glycine.

  • Magnesium ($\text{Mg}^{2+}$) Block: At resting or negative membrane potentials, a magnesium ion ($\text{Mg}^{2+}$) physically plugs the pore of the receptor. Glutamate binding alone is insufficient to open the channel while $\text{Mg}^{2+}$ remains bound.

  • Removal of $\text{Mg}^{2+}$ Block: For the NMDA receptor to open, initial depolarization must occur via nearby AMPA receptors. When sufficient AMPA receptors open to permit $\text{Na}^+$ influx, the membrane potential depolarizes past threshold (shifting from negative values toward $-40\,\text{mV}$ or $0\,\text{mV}$). This intracellular positivity electrostatically expels the $\text{Mg}^{2+}$ ion from the channel pore.

  • Dual Cation Permeability: Once unblocked, the open NMDA receptor allows influx of both sodium ($\text{Na}^+$) and calcium ($\text{Ca}^{2+}$) ions.

  • Role in Learning and Memory: NMDA receptors are the only neurotransmitter-gated receptors at the dendritic and somatic level that permit $\text{Ca}^{2+}$ influx. Entry of $\text{Ca}^{2+}$ triggers an intracellular signaling cascade that drives the physical insertion of additional AMPA receptors into the postsynaptic membrane, strengthening synaptic efficacy.

• Metabotropic Glutamate Receptors (mGluRs):

  • Seven-transmembrane domain G-protein coupled receptors (GPCRs).

  • Activated by glutamate to initiate intracellular second messenger cascades that indirectly modulate ion channels; act over slower timescales than ionotropic receptors.

Structure and Physiology of GABA Receptors

• GABA A Receptor:

  • Heteropentameric ionotropic receptor channel composed of five distinct subunits forming a central pore.

  • Function: Selectively conducts chloride ($\text{Cl}^-$) ions into the cell to cause rapid hyperpolarization.

  • Pharmacological Binding Sites: Contains distinct binding sites for GABA, neurosteroids, barbiturates, benzodiazepines, and picrotoxin.

• GABA B Receptor:

  • Seven-transmembrane domain metabotropic GPCR with an extracellular N-terminus and an intracellular C-terminus.

  • Function: Binding of GABA to the extracellular domain activates intracellular second messenger systems, which indirectly open separate $\text{Cl}^-$ ion channels.

Major Aminergic and Cholinergic Neurotransmitter Systems

• Norepinephrine (NE):

  • Anatomic Origin: Synthesized in the locus coeruleus, a small nucleus located in the brainstem.

  • Projection Targets: Sends expansive axonal projections across the entire brain, including the prefrontal cortex, frontal lobe, midbrain structures, hippocampus, basal ganglia, and cerebral cortex.

  • Biosynthesis: Derived from the amino acid tyrosine.

  • Functional Roles: Regulates central arousal, alertness, and the sleep-wake cycle.

  • Electrophysiological Profile: Locus coeruleus neurons fire continuous action potentials during alert, active waking behaviors, but become completely silent during sleep.

  • Pathophysiological Manipulation: Destruction of locus coeruleus neurons abolishes wakefulness and increases sleep duration; overactivation of these neurons produces anxiety, hyperarousal, and hypervigilance.

  • Synaptic Clearance: Removed from the synaptic cleft primarily via presynaptic norepinephrine transporters (NET) alongside enzymatic degradation.

• Serotonin (5-HT):

  • Anatomic Origin: Synthesized in the raphe nuclei, a family of nuclei distributed throughout the brainstem.

  • Projection Targets: Axons project diffusely across the entire cerebral cortex and midbrain structures.

  • Biosynthesis: Derived from the amino acid tryptophan.

  • Functional Roles: Regulates sleep-wake cycles, feeding behaviors and hunger, sexual function, mood, anxiety, and depression.

  • Synaptic Clearance: Cleared from the synaptic cleft by serotonin transporters (SERT), which serve as the primary target for Selective Serotonin Reuptake Inhibitor (SSRI) antidepressants.

• Dopamine (DA):

  • Biosynthesis: Derived from tyrosine. Shares initial enzymatic steps with norepinephrine, but dopaminergic neurons lack the final converting enzyme present in locus coeruleus neurons.

  • Pathways and Functions:

  • Mesolimbocortical Pathway: Originates in the Ventral Tegmental Area (VTA) and projects to the nucleus accumbens, insula, temporal cortex, and broader limbic cortex. Mediates emotional processing, motivation, reward reinforcement, and reinforcement learning. Implicated in addiction and schizophrenia.

  • Nigrostriatal Pathway: Originates in the Substantia Nigra and projects specifically to the striatum (caudate nucleus and putamen). Dominantly controls movement execution.

  • Clinical Correlations: Degeneration of substantia nigra dopaminergic neurons drives the dyskinesia and motor deficits of Parkinson's disease. Dysfunctional VTA dopaminergic transmission underlies addictive behaviors and motivation deficits.

  • Synaptic Clearance: Reuptake from the synaptic cleft via dopamine transporters (DAT).

• Acetylcholine (ACh):

  • Biosynthesis: Synthesized from acetyl-CoA and choline by choline acetyltransferase.

  • Pathways and Functions:

  • Basal Forebrain System: Cholinergic neurons originating in the basal forebrain (including the nucleus basalis) project to the cortex and hippocampus to mediate memory formation and cognitive learning. These neurons degenerate in Alzheimer's disease.

  • Motor System: Alpha motor neurons in the brainstem and spinal cord project long axons directly to peripheral skeletal muscle fibers to execute muscle contractions. Because these axons span extreme distances down the spinal column, they form peripheral nerves.

  • Nutritional and Alcohol-Related Pathology: Severe Alcohol Use Disorder (AUD) leads patients to derive daily caloric intake primarily from alcohol, causing severe systemic nutritional deficiencies—specifically Vitamin B12 deficiency. Chronic B12 deficiency damages cholinergic neurons in the nucleus basalis and basal forebrain, resulting in profound memory impairments.

Principles of Hormonal vs. Neurotransmitter Communication

• Functional Comparison:

  • Spatial Scale: Neurotransmitters act over extremely short distances across a narrow synaptic cleft ($\approx 20\text{--}40\,\text{nm}$) onto a specific postsynaptic target cell. Hormones are secreted into the vascular system and travel long distances throughout the entire body to act on target tissues expressing matching receptors.

  • Temporal Scale: Neurotransmitter transmission occurs rapidly on a sub-second/millisecond threshold to directly initiate actions or responses. Hormonal communication acts slowly over seconds, minutes, hours, days, weeks, or months to exert long-term structural or behavioral changes.

  • Behavioral Control: Neurotransmitters initiate discrete motor actions and immediate behaviors. Hormones modulate and shape behavior gradually over time; they do not directly initiate acute movements.

• Communication Modes:

  • Endocrine Communication: Endocrine cells secrete hormones directly into the bloodstream, where blood circulation distributes them to distant target cells or target organs.

  • Exocrine Communication: Exocrine glands secrete substances directly onto epithelial surfaces outside the internal milieu of the body (e.g., tears, sweat).

  • Paracrine Communication: A cell secretes a chemical signal that diffuses through local extracellular fluid to act on adjacent or neighboring target cells without entering blood vessels.

  • Autocrine Communication: A cell secretes a hormone that binds to surface receptors on the exact same cell that released it, providing direct self-regulation and feedback.

• Bidirectional Modulation and Biological Rhythms:

  • The relationship between hormones and behavior is bidirectional: hormones alter behavior, and behavioral activities (exercise, diet, stress, alcohol consumption) alter hormone levels.

  • Hormonal secretion displays cyclical biological rhythms, ranging from daily/circadian cycles (e.g., melatonin driving sleepiness and wakefulness) to weekly, monthly, or annual cycles.

• Neuroendocrine Integration and Dual-Function Signaling:

  • Neuroendocrine Cells: Specialized brain neurons (e.g., in the hypothalamus) that receive synaptic neurotransmitter inputs, fire action potentials, and secrete hormones directly into blood capillaries.

  • Dual-Function Chemicals: Certain molecules function as central neurotransmitters and peripheral hormones. For example, norepinephrine acts as a neurotransmitter when released by locus coeruleus terminals in the brain, but acts as a systemic hormone when secreted into the blood by the adrenal glands. Dopamine functions as a central neurotransmitter in the brain and as a local signaling molecule in the gastrointestinal system (without crossing the blood-brain barrier).

Chemical Classes and Receptor Mechanisms of Hormones

• Structural Classes of Hormones:

  • Peptide Hormones: Large, bulky molecules composed of long linked chains of amino acids (e.g., oxytocin).

  • Amine Hormones: Smaller modified single amino acids retaining an amine group (e.g., norepinephrine).

  • Steroid Hormones: Lipid-soluble molecules derived from cholesterol, characterized by a core structure of four fused carbon rings (e.g., cortisol, neurosteroids).

• Hormone Receptor Mechanisms:

  • Surface Metabotropic Receptors:

  • Targeted primarily by peptide hormones and amine hormones.

  • Composed of seven-transmembrane G-protein coupled receptors located on the cell membrane surface.

  • Binding activates intracellular second messenger cascades (e.g., cyclic AMP, $\text{IP}_3$) to alter cell function within seconds to minutes.

  • Intracellular Nuclear Receptors:

  • Targeted by lipid-soluble steroid hormones and neurosteroids that readily cross cell membranes.

  • Receptors reside on or within the cell nucleus. Upon binding, the hormone-receptor complex acts as a transcription factor to modulate gene transcription and protein synthesis, producing cellular changes over hours or days.

  • Absolute Receptor Rules:

  • Hormones NEVER bind to ionotropic receptors; ionotropic receptors are exclusive to fast neurotransmitter signaling.

  • Neurotransmitters NEVER bind to nuclear receptors; they remain in the extracellular/synaptic space and do not enter postsynaptic cells to bind genomic targets.

Homeostatic Feedback and Hypothalamic-Pituitary Function

• Negative Feedback Architecture:

  • Negative feedback is critical to prevent pathological overactivity (e.g., chronic elevated cortisol causing tissue damage, anxiety, and depression).

  • Autocrine Feedback: The secreted hormone binds back to receptors on the secreting endocrine cell itself to shut down further release.

  • Multi-Level Target-Driven Feedback: Systemic circulating hormones travel through the bloodstream back to the brain to bind inhibitory receptors on neurons in the pituitary gland and hypothalamus, shutting down upstream hormone synthesis and release.

• Hypothalamic-Pituitary Subsystems:

  • Hypothalamus: Contains specialized neuroendocrine cells that synthesize and secrete releasing hormones.

  • Posterior Pituitary Gland:

  • Axons from neuroendocrine cell bodies in the hypothalamus extend directly down into the posterior pituitary lobe.

  • Action potentials in these hypothalamic neurons trigger the direct secretion of hormones into local capillaries.

  • Key Hormones Secreted: Oxytocin (mediates social bonding, parent-infant attachment, romantic attachment, and reproductive behaviors) and Vasopressin (regulates thirst, fluid retention, and blood pressure).

  • Anterior Pituitary Gland:

  • Hypothalamic neuroendocrine cells release releasing hormones into the median eminence, a specialized capillary bed known as the hypophyseal portal system.

  • Releasing hormones travel through portal blood vessels directly to the anterior pituitary, stimulating anterior pituitary cells to synthesize and secrete tropic hormones (e.g., prolactin, adrenocorticotropic hormone [ACTH], gonadotropic hormones).

  • Tropic hormones enter systemic circulation to activate peripheral endocrine target glands (such as the adrenal glands, thyroid gland, and gonads), driving the release of final peripheral hormones.