Hindbrain / Brainstem Study Notes

Hindbrain / Brainstem Overview

  • The hindbrain includes critical areas that manage a variety of physiological and behavioral functions, particularly those related to satiation and feeding behaviors.

C-Fos as a Window into Brain Structure and Function Relationships

  • Immediate Early Genes: Genes activated rapidly in response to relevant stimuli or events.

  • C-Fos: An indirect marker of recent neural activity; expressed soon after neurons fire action potentials.

    • Timing: C-Fos expression occurs approximately 90 minutes after an event.

    • Visualization: Can be visualized in postmortem brain sections using antibody-based immunohistochemistry.

Visualization of Neurons

  • Procedures to visualize neurons include:

    • Coronal Sections: Rat brain coronal sections processed with antibodies to target specific proteins (like c-Fos).

    • Fluorescent Markers: Antibodies tagged with fluorescent markers (secondary antibodies) for visualization.

    • Methods: Brains are frozen and sectioned using a microtome or cryostat and then viewed under a fluorescent microscope with a camera.

Cell Visualization Examples

  • Types of Neurons:

    • Can observe various neuron types including pyramidal neurons.

  • Co-localization:

    • Different markers can be visualized simultaneously; for example:

    • CTB: Green mark

    • Orexin: Red mark

    • Merged visualization shows co-localization of these proteins.

RNA Interference (RNAi) Overview

  • Discoverers: Andrew Fire and Craig Mello awarded the 2006 Nobel Prize in Physiology for discovering RNAi in C. elegans (1998).

  • Function: Small RNA molecules (miRNA, siRNA, shRNA) bind to endogenous mRNA molecules to modulate their activity (increase or decrease).

    • Role in Protection: Serves as an endogenous cellular process to protect against viral infections.

  • Applications in Neuroscience:

    • Used to knock down gene expression of specific targets (like receptors) to understand feeding behavior

    • Achieved via the stereotaxic delivery of viral vectors (Adeno-associated virus (AAV), Lentivirus).

Satiation and Meal Size Control

  • Hindbrain Functions:

    • Satiation signals processed in the hindbrain include:

    • Cholecystokinin (CCK)

    • Serotonin (5-HT)

    • Gastric distension

    • Glutamate

    • Bombesin

    • Peptide YY

    • Enterostatin

    • Glucagon-like Peptide 1 (GLP-1)

  • Controversy: Some argue social and habitual factors control meal frequency while meal size determines total food intake.

Energy Intake Trends

  • Statistics:

    • Increase in total energy intake from 1977 to 2006 of 570 kcal/day.

    • Breakdown of portion size contribution to energy intake:

    • 1977-1991: Portion size contributed to a rise of 15 kcal/day/year; 4 kcal/day/year from Eating Occasion.

    • 1994-2006: Portion size's contribution decreased to 1 kcal/day/year, while Eating Occasion increased by 39 kcal/day/year.

Decerebrate Rat Studies

  • Definition: A decerebrate rat has a complete transection between the midbrain and the forebrain (essentially a forebrain lesion).

  • Hindbrain Satiation Responses:

    • While the hindbrain adequately controls satiation, it does not manage conditioned flavor avoidance, thirst due to dehydration, or caloric intake moderation.

    • Decerebrate rats still exhibit normal rejection of nutrients when full.

Experimental Findings on Caloric Intake

  • Study on chronic decerebrate rats indicated:

    • Rats were challenged to maintain three daily intraorally delivered meals, but decerebrate rats did not increase meal size when meals were reduced.

    • Suggests the hindbrain alone is insufficient for daily caloric intake regulation and points to the need for forebrain-hindbrain interaction for short- and long-term intake control processes.

Nucleus of the Solitary Tract (NTS)

  • Function: First CNS site to receive taste information and visceral information from the GI tract, relaying signals to the rest of the brain.

  • Medial NTS (mNTS): A critical site for meal size control, being the first to receive gut-derived sensory signals via the vagus nerve.

    • Involves a vago-vagal reflex pathway: afferent vagus → NTS → dorsal motor nucleus (DMX) → efferent vagus.

Gastric Distension and c-Fos Expression

  • Gastric distension activates c-Fos in the medial NTS, which can be observed through specific paradigms (e.g., balloon inflation studies):

    • Three conditions illustrated varying activation through different volumes of distension.

  • Activation of catecholaminergic neurons indicated by specific markers and post-mortem assessments.

Leptin and mNTS Interaction

  • Leptin's Role: Interacts with the mNTS to control food intake particularly affecting meal size:

    • Mechanism 1: POMC neurons project to mNTS, releasing alpha MSH to reduce meal size.

    • Mechanism 2: Leptin acts directly on mNTS receptors to reduce food intake.

  • Neuropharmacology Outcomes:

    • Knockdown of leptin receptor signaling in mNTS increases food intake and body weight gain in rat models.

Conclusion and Significance of Loss vs. Gain of Function

  • Overview of Findings:

    • Immunohistochemistry and RNA interference studies support the notion that the hindbrain can process satiation signals independent of the forebrain.

    • Medial NTS remains paramount for meal size control, receiving vagal sensory input and responding to satiation signals via c-Fos expression.

    • The role of leptin in feeding behavior is mediated through both direct and indirect pathways involving the mNTS and hypothalamus.

    • Questions to Ponder: Evaluate the significance of loss of function approaches compared to gain of function strategies in neuropharmacological settings (e.g., agonists vs. antagonists).