Hormones and Their Functions in Reproduction (AGSC310) PDF

Lecture 8 Hormones and their functions in reproduction AGSC310 –Physiology of Reproduction Luis Fernando Schutz, Ph.D. 1  Hormones Learning objectives: Ø Understand what are hormones and end...

Lecture 8 Hormones and their functions in reproduction AGSC310 –Physiology of Reproduction Luis Fernando Schutz, Ph.D. 1  Hormones Learning objectives: Ø Understand what are hormones and endocrine glands Ø Know the biochemical classification of hormones Ø Know factors that influence the target cell biological response to a hormone Ø Understand the neuroendocrine control of reproduction Ø Know hormones that regulate reproduction Ø Know how hormones control reproduction through feedback 2  Hormones Hormones and endocrine glands Ø hormones: v are signaling molecules secreted by endocrine glands into the blood and delivered to target tissues to regulate physiology and behavior the Tiniest leave Ø endocrine glands: they 8S cell a differenfy v glands that are ductless and secrete their products, the hormones, directly into the blood ü reproductive hormones originate from various endocrine glands, including the hypothalamus, pituitary, gonads (ovaries and testes), uterus, and placenta Tpophesus 3  Classification of hormones Biochemical classification of hormones: popeptides Ø peptides and proteins combo of amino acids Ø glycoproteins carbohydrates glucose glycolysis Ø steroids Ø prostaglandins 4  Classification of hormones Biochemical classification of hormones: Ø peptides and proteins v peptides: relatively small molecules with only a few amino acids (< 50) joined by peptide bonds v proteins: polypeptide chain(s) with more than 50 amino acids Ø glycoproteins of v polypeptide hormones that contain carbohydrate moieties nucleus carbohydrates Ø steroids v synthesized from cholesterol Ø prostaglandins v lipids consisting of 20-carbon unsaturated hydroxy fatty acids derived from arachidonic acid 8  Target tissues Target tissues Ø contain specific receptors for hormones v receptors bind to a specific hormone that stimulates a specific cellular response v hormone actions require the presence of specific receptors on target cells ex FSH in the ovary ble of the presence of the right receptor Hormones can only elicit an action in a specific tissue if this is a target tissue for that specific hormone. This specificity of the target tissues is determined by the presence of the specific receptors. 9  Target tissues Target tissues Ø receptors for specific hormones can be located at distinct sites in or on the cells of target tissues v receptors for peptide, protein and prostaglandin are located on the plasma membrane v receptors for steroid hormones can be located on the plasma membrane or can be intracellular receptors located either in the nucleus (intranuclear receptors) or in the cytoplasm ü to bind to intracellular receptors, steroid hormone diffuses through the plasma membrane because it is lipid soluble ü after the steroid hormone enters the cell, it diffuses through the cytoplasm and into the nucleus 10  Target tissues Factors that influence the target cell biological response to a hormone it can be stimulated to increase decrease receptors Ø Receptor density: number of receptors on or in the target cell Ø Receptor-hormone affinity due to the shape size of the receptor Ø Half-life of hormone: affects the presence of the hormone in the circulation and the duration of the cell response how long it takes 12 the molecule for it fine how long Ø Pattern and duration of hormone secretion will last 11  Receptors Receptors influence target cells response to hormones Ø the receptor affinity for hormones and the numbers of receptors on or in the cell regulate the degree of cell stimulation and cellular response to the hormone v the greater affinity of the hormone for the receptor, the higher potential for target cell responses v hormones can regulate whether there is an increase or decrease in the number of receptors ü for example, FSH promotes the synthesis of LH receptors by follicular cells v nutritional factors can influence receptors numbers 12 ex cholesterol  Hormonal half-life Hormonal half-life a good balance to Ø is relatively short provides avoid further problems Ø refers to the time required for one-half of hormone levels to disappear Ø determined by the clearance rate (metabolism rate) Ø short half-life is important to regulate the duration of the hormonal response ü once the hormone is secreted and released into the blood and causes a response, it is degraded so that further response does not occur ü when a hormone is continuously produced (such as progesterone during pregnancy), its action continues for as long as the hormone is present ü compared to neural control, hormonal control is slower and has durations of minutes, hours or even days ü the longer the half life, the higher potential for target cell responses 13  Receptors Pattern and duration of hormones secretion Ø hormones are usually secreted in three types of patterns: v Episodic: releases then stops quickly ü fast ü hormones under nervous control (ex: neuropeptides from Hypothalamus) v Basal (tonic): low levels but at a consistent rate ü the hormone level stays low and fluctuates with low amplitude pulses ü ex: GnRH secretion from the tonic center of hypothalamus v Sustained: constant high levels of production release ü hormone level remains elevated in a steady way for a long period ü ex: progesterone during pregnancy 14  Neuroendocrinology Neuroendocrine control of reproduction Ø the interplay between the nervous system and the endocrine system is critical for control of reproduction v in a neuroendocrine reflex, neurons called neurosecretory cells release a neurohormone that enters the blood and acts on a remote target tissue § neurohormones are hormones released by neurons v the gonadotropin-releasing hormone (GnRH), released by the hypothalamus, is an example of neurohormone § the GnRH is a low-molecular weight peptide hormone (a polypeptide of 10 aa) that is also classified as a neuropeptide 15  Neuroendocrinology Neuroendocrine control of reproduction Ø the neuroendocrine reflex: v starts with sensory neurons in that synapse with interneurons in the spinal cord v efferent neurons traveling from the spinal cord synapse with other neurons in the hypothalamus v the hypothalamic neurons release neurohormones from their terminals into the blood v neurohormones released into the blood travel to the target tissue 16  Neuroendocrinology Neuroendocrine control of reproduction Ø the suckling reflex is an example of the neuroendocrine reflex v when suckling occurs, sensory nerves in the teat or nipple of the lactating female detect the tactile stimulus v these sensory signals travel to the spinal cord and then to the hypothalamus where they synapse with other nerves v the hypothalamic neurons then depolarize (“fire”), causing release of oxytocin directly from the nerve terminals in the posterior pituitary (where it was stored) into the blood muscle cells v oxytocin travels through the blood to the myoepithelial cells of the mammary gland and causes these cells to contract, which results in milk ejection 17  Neuroendocrinology The hypothalamus Ø is the neural control center for reproductive hormones Ø is a complex portion of the brain consisting of clusters of nerve cell bodies Ø the clusters of nerve cell bodies are called hypothalamic nuclei v each hypothalamic nucleus has a specific name, a different function, and is stimulated by different sets of conditions 18  Neuroendocrinology The hypothalamus Ø the hypothalamic nuclei that have direct effect on reproduction are: PVN Surge v Tonic center center Tonic center v Surge center v Paraventricular nucleus (PVN) Posterior Anterior Pituitary Pituitary 19 vascular connection  Neuroendocrinology The hypothalamus PVN Ø the neurons in the paraventricular nucleus (PVN) secrete oxytocin Surge that is transported down the axon center to the terminals in the posterior Tonic center pituitary, where oxytocin is stored Ø oxytocin will be released into the Posterior Pituitary blood when the neuron is stimulated Anterior Pituitary The hypothalamus and the posterior pituitary have a neural connection To target tissue 20  Neuroendocrinology The hypothalamus Ø the axons from neurons in the Surge center surge center and the tonic Tonic center center extend to the stalk Stalk region and terminate upon blood vessels of the Ant. Post. Pit. Pit. hypothalamo-hypophyseal portal system Portal system SHA = Superior Hypophyseal Artery PPP = Primary Portal Plexus The hypothalamus and the anterior pituitary PV = Portal Vessels have a vascular connection SPP = Secondary Portal Plexus 21  Neuroendocrinology The hypothalamus Ø Hypothalamo-hypophyseal portal system v the axons from neurons in the surge center and the tonic center release neuropeptides (such as the GnRH) that enter the hypothalamo-hypophyseal portal system at the stalk of the pituitary v the neuropeptides are then transferred to a second capillary plexus in the anterior pituitary where they cause pituitary cells to release other hormones (such as gonadotropins) GnRH moves from Toniccenterfsurge center to the stack of the the anterior pituitary then transferred 22 to pituitary  Neuroendocrinology The hypothalamus Ø Hypothalamo-hypophyseal portal system v the hypothalamo-hypophyseal portal system is important because allows very small quantities of neuropeptides to act directly on the cells of the anterior pituitary before the neuropetide becomes diluted by the systemic circulation limited number of peptides allows for dilution 23  PVN Neuroendocrinology Surge center Tonic center Posterior Pituitary Anterior Pituitary To target tissue The hypothalamus and the anterior The hypothalamus and the posterior pituitary have a vascular connection pituitary have a neural connection 24  Neuroendocrinology Differences between neural and endocrine systems Ø the endocrine system is generally slower, but longer lasting than the neural system v the endocrine system relies on hormones v extremely small quantities of a hormone can cause dramatic physiologic responses ü hormones act at blood levels ranging from nanograms to picograms per ml of blood 25  Hormones Hormones that regulate reproduction Source Hormone Chemical class Main actions Hypothalamus Gonadotropin Neuropeptide Release of gonadotropins (FSH and LH) from (Surge and tonic Releasing H. (decapeptide) the anterior pituitary centers) (GnRH) in stimulates something the gonads Hypothalamus Oxytocin Neuropeptide Stimulates uterine motility (Paraventricular (synthesized by the Stimulates milk ejection nucleus) hypothalamus, stored Stimulates PGF2⍺ synthesis by the posterior pituitary; also synthesized by the C.L.) 26  Hormones Hormones that regulate reproduction Source Hormone Chemical class Main actions Ant. Pituitary Follicle- Stimulating Glycoprotein Stimulates follicular development (female) Hormone (FSH) Stimulates estradiol synthesis (female) Stimulates Sertoli cells function (male) follicles Luteinizing Hormone Glycoprotein Stimulates ovulation (LH) Stimulates formation of the corpus luteum Stimulates progesterone production (female) corpus Stimulates testosterone production (male) luteum Prolactin Protein Lactation Maternal behavior lactation Stimulates corpus luteum function (in some species) Post. Pituitary Oxytocin is stored Neuropeptide Stimulates uterine motility here (not synthesized) Stimulates milk ejection Stimulates PGF2⍺ synthesis 27  Hormones Hormones that regulate reproduction Source Hormone Chemical class Main actions Ovaries Estradiol (E2) Steroid Mating behavior (estrus) (granulosa Secondary sex characteristics cells) Iéˢ Regulates GnRH release Enhance uterine motility Stimulates luteolysis Inhibin Glycoprotein Inhibits FSH secretion Ovaries (theca Testosterone Steroid Substrate for E2 synthesis cells) estrodial synthesis Ovaries Progesterone (P4) Steroid Maintenance of gestation (corpus Stimulates endometrial secretion luteum) gestation Inhibits GnRH release Relaxin (also Protein Softening of pelvic ligaments and cervix synthesized by the placenta) in order for parturition to be successful 28  Hormones Hormones that regulate reproduction Source Hormone Chemical class Main actions Uterus Prostaglandin F2⍺ Fatty acid Luteolysis (degradation of C.L.) (endometrium) (PGF2⍺) (prostaglandin) Promotes uterine contraction Ovulation Placenta Relaxin (also Protein Softening of pelvic ligaments and cervix synthesized by the corpus luteum) Placental lactogen Protein Mammary stimulation of the dam Human chorionic Glycoprotein Facilitates production of P4 by the ovary gonadotropin (hCG) (woman) prfgesterone Equine chorionic Glycoprotein Causes formation of accessory corpora gonadotropin (eCG) lutea in the mare (mare) 29  Hormones Hormones that regulate reproduction Source Hormone Chemical class Main actions Testis (Leydig Testosterone Steroid Anabolic growth cells) Promotes spermatogenesis Promotes secretion of accessory sex glands Secondary sex characteristics Testis (Sertoli Inhibin Glycoprotein Inhibits FSH secretion cells) has carbohydrate nucleus in it 30  Feedback Positive and Negative feedback are major controllers of reproductive hormones Ø positive and negative feedback control secretion of GnRH, which in turn regulates the secretion of gonadotropins (FSH and LH) 31  Feedback Positive and Negative feedback are major controllers of reproductive hormones Ø An example of negative feedback: v Progesterone (P4) inhibits GnRH neurons and, therefore, when P4 is high, GnRH neurons secrete only basal levels of GnRH ü These basal levels (low amplitude pulses) of GnRH released by the hypothalamus will only stimulate the release of low levels of gonadotropins (FSH and LH) ü Therefore, in the presence of P4, the surge center of the hypothalamus will not be able to release high amplitude pulses of GnRH, which stimulate the anterior pituitary to release high levels of LH (called LH surge) that is important for ovulation O 32  Feedback Positive and Negative feedback are major controllers of reproductive hormones Ø An example of positive feedback: v the surge center of the hypothalamus secretes only basal levels (low amplitude pulses) of GnRH until there is a positive feedback from estradiol (E2) in absence of P4 progesterone v when E2 becomes sufficiently high, in the absence of P4, it will stimulate the surge center to release large quantities of GnRH that cause the release of large quantities of LH that will stimulate ovulation 33  Feedback Feedback control of reproduction Hypothalamus Surge center Tonic center Steroids produced by the ovary (E2 and P4) regulate the secretions of Anterior GnRH via feedback Pituitary Ovary CL 34  Feedback Feedback control of reproduction Ø Negative feedback Surge v High P4 only allows secretion of basal levels center of GnRH from the tonic center and inhibits Tonic the surge of LH from the surge center (FSH center and LH are being released at basal levels, no GnRH surge of LH) FSH feedback on LH surge E2 CL P4 35  Feedback Feedback control of reproduction feedback on surge center Ø Positive feedback Surge center v Following luteolysis during ovarian follicular GnRH development, the source of P4 no longer Tonic exists center v In the absence of P4, E2 is able to stimulate high amplitude pulses of GnRH from the surge center v high amplitude pulses of GnRH stimulate the anterior pituitary to release large LH quantities of LH (LH surge) E2 Anterior v LH surge will stimulate ovulation and the Pituitary formation of a new corpus luteum after ovulation 36  Discussion Discussion in groups (2-3): Ø select three hormones that regulate reproduction Ø characterize these hormones according to their source, chemical class, and main action 37