Neuroendocrine control

• What does neuroendocrine control mean?

  Neurons secrete hormones (typically peptides) from their nerve endings into the bloodstream, to control the release of hormones from endocrine organs. These are typically hypothalamic neurons, with hormones influencing the anterior pituitary gland via the pituitary stalk and portal vessels, and can be influenced by both positive and negative feedback
• Geoffrey Harris 1950s Hypothalamic control of the pituitary?

  Crude experiment - perfused rabbits with ink, saw that pituitary stalk was filled with ink that was transferred to the anterior pituitary, showing a vascular link

  Then lesioned the portal vessels only of the pituitary stalk (neurons intact), showed atrophy of adrenal and thyroid glands, showing that control lost if disturb vascular connection, pituitary controls these glands
• Draw the hypothalamic-pituitary-thyroid axis?

  Neuroendocrine control via TRH through portal vessels

  90% of thyroid output is T4, rest is T3

  
• Where are TRH neurons distributed in the brain and how was this found?

  Immunocytohistochemistry - use antibodies against TRH, found to be exclusively in the hypothalamus in the paraventricular nucleus (line the 3rd ventricle in the middle of the hypothalamus), axons extend from the cell bodies there to the floor of the hypothalamus (external zone of the median eminence), where there is a plexus containing capillaries that form portal vessels
• Uribe et al 1993 - factors affecting TRH and how this was shown?

  Used northern blotting to measure the quantity of TRH mRNA in different extracts from different conditions, found that cold and suckling increase mRNA, thought to be via ascending adrenergic inputs

  However, hard to find exact region borders (paraventricular nucleus)
• What is most accurate method to measure hormone levels in tissue extracts?

  Radioimmunoassay - mix radioactive antigen on 1st antibody with unlabelled antigen, some of the radioactive antigen displaced based on concentration of unlabelled antigen, radioactivity of the precipitate gives the bound antigen, and the supernatent the free antigen, use to plot a standard curve (radioactivity of pellet decreases as concentration of unlabelled antibody increases), when run samples from experiment can compare radioacitivity to curve to find amount of hormone in sample
• Effects of TSH on thyroid hormone production?

  Stimulates follicular cell production of T4 and T3 in multiple ways- stimulates transport of iodine into follicular cells and then into the follicles (via Na-I symporter and pendrin transporters respectively), TPO activated to incorporate iodine into thyroglobulin, endocytosed into cells, fuse with lysosomes, released from cells

  TSH stimulates every step of pathway, no synthesis if no TSH
• Effect of T3 on TRH mRNA?

  Negative feedback - in situ hybridisation demonstrates, T4 also causes the same effect, assume mirrored changes in protein expression
• Functioning of thyroid receptors?

  Found in the nucleus, bind DNA and activate transcription - transcription factors, bind T3, found in the promoter regions of target genes

  Timescale of hours

  Receptors interact with co-regulator proteins - repressor proteins bind to receptors when hormone not bound, coactivators bind instead when hormones bound, increase transcription

  Beta-2 receptor knockout in mice ablates negative feedback, elevated basal TSH and no inhibition of TSH by T3 caused (radioimmunoassay), reduced concentration of T3 in hypothalamus of KO mice - reduction in local concentration
• Deiodinase expression in the brain?

  iodothyronine deiodinases control the intracellular concentration of T3, type2 and 3 deiodinases in brain, type 2 for T4 to T3, and type 3 inactivates T3 by converting to T2 - expressed in the glia and astrocytes, not in the neurons themselves in the hypothalamus - paracrine effect

  Feedback by T4 thought to signal iodine status, shown using iopanoic acid to block type 1 and 2 deiodinases
• MCT8 role in hypothalamus?

  High affinity transporter of T3 (more than T4), thought to transport from astrocytes into neurons, found in the membranes of TRH neurons