By L. Jensgar. Bluefield College. 2018.
It is easy to speculate that in an active neuron with a rapid firing pattern generic 100 mg kamagra gold fast delivery, the continued release of a peptide may eventually lead to depletion of the peptide occurring best 100mg kamagra gold. If this also happens in the CNS it would provide a mechanism whereby the release and resultant receptor effects of a transmitter no longer match the firing pattern and demands of the neuron and so could contribute to long-term adaptations of neurons by a reduction in the time over which a peptide is effective buy discount kamagra gold 100mg on line. The release of some peptides may differ from that of other transmitters purchase kamagra gold 100mg amex, depending on the firing rate of the neurons 100 mg kamagra gold overnight delivery. The large vesicles needed to store a peptide may need a greater rate of depolarisation for membrane fusion and release of the contents. In the salivary gland the release of vasoactive intestinal polypeptide requires high-frequency stimulation whereas acetylcholine is released by all stimuli. Due to the complexities and problems of access to CNS synapses it is not known if the same occurs here but there is no reason why this should not. In sensory C-fibres a prolonged stimulus appears to be a prerequisite for the release of substance P. BREAKDOWN A peptide, once released, is not subject to reuptake like most transmitters, but is broken down by membrane peptidases. There are no known peptide transporters so that reuptake and re-use are not likely. The peptidases are predominantly membrane bound at the synapse and many are metalloproteases in that they have a metal moiety, most often zinc, near the active site. These enzymes are generally selective for particular 254 NEUROTRANSMITTERS, DRUGS AND BRAIN FUNCTION amino-acid sequences so that one peptidase may cleave a number of peptides if the amino-acid sequences overlap. A number of peptidases are found in the vasculature, including aminopeptidases and angiotensin-converting enzyme and any peptide with an acidic amino acid near the amino-terminal end of the peptide will be degraded after systemic administration. At a central synapse, the termination of action of a peptide relies on these peptidases. Thus, if there is considerable release at any one time, the peptide may saturate the enzyme(s) and so metabolism will not keep pace with release. Thus the peptide could escape the synapse where it was released and then diffuse through the tissue. The peptide may then act at sites distant from the neuron that released it, and these sites will be determined simply by receptors for the particular peptide. Consequently, volume transmission or non-synaptic effects may be important. This has been shown for lutenising hormone releasing factor (LHRH) in sympathetic ganglion cells where the peptide can act on neurons over distances of many hundreds of microns. In the CNS, the areas of spinal cord where neurokinins can be detected increase markedly when a prolonged intense peripheral stimulus is applied, suggesting saturation of peptidases allowing the intact transmitter to move through the tissue. Finally, one of the neurokinins, substance P, is found with calcitonin gene-related peptide (CGRP) in sensory neurons that terminate in the spinal cord. When CGRP occupies the active site the actions of substance P are greater since there is less degradation. MANIPULATION OF ENDOGENOUS PEPTIDES The only approach so far has been centred on the opioids. The synthesis of peptidase inhibitors has been a successful strategy so that kelatorphan, a mixed peptidase inhibitor, inhibiting at least two of the important breakdown enzymes (aminopeptidase N/M and neutral endopeptidase) affords almost complete protection to the enkephalins. The spinal application of the inhibitor produces a reduction of nociceptive responses of cells with the pool of enkephalins protected by the inhibitor likely to be derived from both a segmental release and from descending pathways activated by the stimulus. The recent reports of a systemically active mixed peptidase inhibitor, RB101, is the next stage towards the clinical application of this novel approach to pain relief. Interestingly, the side-effect profile of RB101 appears to be unlike that of morphine in that it does not cause physical and psychological dependence, suggesting that the receptor activation produced by endogenous opioids is unlike that of exogenous drugs. This is to be expected as the peptide transmitter release and consequent receptor activation will only occur following physiological events Ð the inhibitors only act when the peptide is released. The potential problems with this approach are that of the two enzymes involved (aminopeptidase N/M and neutral endopeptidase), neither is selective for the enkephalins and inhibition of the former could also increase levels of angiotensin, endothelin, CCK and substance P, among others. PLASTICITY IN PEPTIDES The processing of the peptide en route from the cell body to the terminal can result in different products being released from the same gene product. Nerve growth factor appears important in this, as does NMDA- induced calcium influx. There is much evidence for an induction of early onset protoncogenes in neurons elicited by neuronal activity and c-fos and c-jun are protein markers of these events. When a gene is switched on or off after neuronal activity then some peptides will always be present in neuronal systems and others appear as a result of damage and/or dysfunction to neurons. Thus the pharmacology of a neuron will change as a consequence of pathological changes.
In a similar manner to the AMPA receptor cheap 100mg kamagra gold with visa,RNA editing in the pore region at the Q/R site controls the Ca2 permeability of the kainate receptor subunits GluR5 and GluR6 cheap kamagra gold 100 mg on-line,with significant levels of the unedited (Ca2 permeable) version of these receptors present in the adult CNS purchase 100mg kamagra gold with visa. The AMPA receptor subunits are all found within many regions of the CNS but in differing numbers generic kamagra gold 100 mg with visa,and generic kamagra gold 100mg without a prescription,in the spinal cord,have differing lamina distributions. GluR1 and GluR2 are generally the most abundant AMPA receptor subunits with lower levels of GluR3 and GluR4. The majority of AMPA receptors allow Na to enter neurons and thus in most areas of the CNS studied,the initial stage in excitatory synaptic transmission is a fast-depolarising response due to the release of glutamate and subsequent activation of the AMPA receptor. The receptor has a complicated structure and this is highlighted by the presence of many pharmacologically distinct binding sites through which the receptor activity can be modulated. The channel associated with the receptor is blocked by Mg2 at resting potential (770 mV). Receptor activation requires the removal of this Mg2 block (voltage-gated) as well as the binding of glutamate and the co-agonist,glycine (ligand-gated). The different binding sites (glutamate,phencyclidine,polyamine,glycine) are illustrated,and together with antagonists which act at the various sites (in parentheses). The polyamine site is an intracellular site which modulates the affinity of other agonists and antagonists mRNA coding for the kainate receptor subunits GluR5 and GluR7 is also found in isolated neurons in the CNS although many kainate GluR5 receptors are thought to be located presynaptically on terminals of neurons that release glutamate. Kainate receptors are therefore thought to be excitatory autoreceptors that enhance the release of glutamate. It could be predicted that the widespread distribution of AMPA receptors precludes the use of antagonists at this receptor in therapy since adverse effects are highly likely. By contrast,the kainate receptor might be an interesting target since its functional role will be linked to the level of glutamate release. Thus,antagonists at this receptor should reduce excessive glutamate release while having less effect on more normal functional synapses. The role of the kainate receptor system in the brain is at an early stage since there are as yet few pharmacological tools to study its function. However,mutations in the kainate receptor genes have been made in mice and there is a GluR6 kainate receptor knock-out mouse. Kainate binding is absent in areas of the brain which normally have high levels such as the hippocampus. Here,in normal animals kainate receptors mediate a postsynaptic current which is absent in the GluR6 knock-out mouse. The mice have 216 NEUROTRANSMITTERS,DRUGS AND BRAIN FUNCTION reduced motor activity but can learn maze tasks. Studies have shown that neurons expressing high levels of GluR1 mRNA but lacking GluR2 are found in the superficial laminae of the spinal cord,an area where nociceptive primary afferents terminate,suggesting that a subpopulation of AMPA receptors in this region may have significant Ca2 permeability. Calcium-permeable non-NMDA receptors have been demonstrated in spinal cord slices using kainate-induced cobalt loading. Studies performed using cultured neurons in vitro have suggested that Ca2 entry through Ca2-permeable AMPA receptors in the spinal cord may provide a mechanism for the strengthening of transmission at synapses and enhancement of nociceptive transmission. Other studies have suggested a link between Ca2-permeable AMPA receptors and inhibitory systems since in the dorsal horn of the spinal cord many of these receptors are found on GABA neurons. Clearly,the functional role of Ca2-permeable non-NMDA receptors in vivo will depend on their location in the integrated circuitry of the CNS. Joro Spider Toxin (JSTx) has been reported to be a selective blocker of Ca2-permeable non-NMDA responses evoked by AMPA/kainate rather than those evoked by NMDA and so will be a useful tool for studying the roles of these receptors. NMDA RECEPTORS Much attention has been focused on the role of the N-methyl-D-aspartate (NMDA) receptor for glutamate,activation of which produces slow prolonged neuronal depolarisation. Thus unlike the AMPA receptor,it is not responsible for the fast transmission of excitation nor the initiation of impulses but has been shown to be critical for maintaining excitatory responses such as the manifestation of wind-up in spinal cord,long-term potentiation in the hippocampus,epileptiform activity and in neuroexcitotoxicity. Mechanisms of central amplification of a nociceptive input have been suggested to underlie aspects of the enhanced spinal transmission of nociceptive messages in protracted pain states,and in this case there is good clinical evidence to support the concepts that have arisen from animal studies. The NMDA receptor has a heteromeric structure composed of two subunit types; NR1 and NR2,the latter having four subunits (NR2A±NR2D) (Fig. Molecular genetic techniques have demonstrated that native NMDA receptors are likely to be composed of a combination of the NR1 subunit (which can exist in eight different splice variants) and one or more of the four NR2 subunits which are the main determinants of functional diversity among the NMDA receptors (see Chapter 3 for further details). It has been shown that there are distinct developmental and spatial expression patterns of NMDA receptor NR1 subunit splice variants and NR2 receptor subunits in the CNS. Although the exact subunit stoichiometry is not yet known for any NMDA receptor, heterologous expression studies suggest that they are likely to be tetramers composed of two NR1 subunits and two NR2 subunits providing the possibility for considerable structural diversity of NMDA receptors. The subtypes have been partially mapped in the CNS and show differing regional distributions.
The load is increased at regular in- val between R waves is reduced cheap kamagra gold 100 mg visa, and the ECG segment between tervals buy kamagra gold 100 mg without prescription, and the test ends when the patient becomes ex- the S and T waves is depressed purchase kamagra gold 100 mg overnight delivery. With proper supervision kamagra gold 100 mg without prescription, the stress test is a safe method for detecting coronary artery disease order 100 mg kamagra gold free shipping. Because the exer- cise load is gradually increased, the test can be stopped at the first sign of problems. Training also improves en- involving isometric contraction and greatly elevated arte- dothelium-mediated regulation, responsiveness to adeno- rial pressure, such as lifting weights. Preserving endothelial vasodilator func- active in dynamic exercise leads directly to larger resting tion may be the primary benefit of chronic physical activ- and exercise stroke volume. Nonetheless, resting bradycardia is a poor index of Chronic, dynamic exercise is associated with increased cir- endurance fitness because genetic factors explain a much culating levels of high-density lipoproteins (HDLs) and re- larger proportion of the individual variation in resting heart duced low-density lipoproteins (LDLs), such that the ratio rate than does training. These changes in The effects of endurance training on coronary blood cholesterol fractions occur at any age if exercise is regular. Since myocardial oxygen consumption is cally accompany increased chronic physical activity in roughly proportional to the rate-pressure product (heart sedentary individuals, undoubtedly contribute to these rate mean arterial pressure), and since heart rate falls af- changes in plasma lipoproteins. Nonetheless, in people ter training at any absolute exercise intensity, coronary with lipoprotein levels that place them at high risk for coro- flow at a fixed submaximal workload is reduced in parallel. Because exercise acutely and chron- cise on insulin sensitivity and central obesity can restore ically enhances fat metabolism and cellular metabolic ca- ovulation in anovulatory obese women suffering from pacities for -oxidation of free fatty acids, it is not surpris- polycystic ovary disease. Continued exer- pase activity, in concert with increased lecithin-cholesterol cise throughout pregnancy characteristically results in nor- acyltransferase activity and apo A-I synthesis, enhance the mal-term infants after relatively brief labor. The risk of large infant size for gestational age, in- creased in diabetic mothers, is reduced by maternal exercise Exercise Has a Role in Preventing and Recovering through improved glucose tolerance. The incidence of um- From Several Cardiovascular Diseases bilical cord entanglement, abnormal fetal heart rate during Changes in the ratio of HDL to total cholesterol that take labor, stained amniotic fluid, and low fetal responsiveness place with regular physical activity reduce the risk of scores may all be reduced in women who are active through- atherogenesis and coronary artery disease in active people, out pregnancy. Further, when examined 5 days after birth, as compared with those who are sedentary. A lack of exer- newborns of exercising women perform better in their abil- cise is now established as a risk factor for coronary heart ity to orient to environmental stimuli and their ability to disease similar in magnitude to hypercholesterolemia, hy- quiet themselves after sound and light stimuli than weight- pertension, and smoking. A reduced risk grows out of the matched children of nonexercising mothers. When coronary ischemia does occur, increased vagal tone may reduce the risk of fibrillation. Increased breathing is perhaps the single most obvious Regular exercise often, but not always, reduces resting physiological response to acute dynamic exercise. Responders typically show diminished rest- work intensity and then supralinearly beyond that point. In obesity-linked hypertension, declining insulin secre- goals of oxygen intake and carbon dioxide removal. Nonetheless, because some obese people who exercise and lose weight show no blood pressure changes, exercise Metabolic Demands, but the Exact remains adjunctive therapy for hypertension. Control Mechanisms Is Unknown Exercise increases oxygen consumption and carbon dioxide Pregnancy Shares Many Cardiovascular production by working muscles, and the pulmonary re- Characteristics With the Trained State sponse is precisely calibrated to maintain homeostasis of these gases in arterial blood. In mild or moderate work, ar- The physiological demands and adaptations of pregnancy in some ways are similar to those of chronic exercise. Both of them increase blood volume, cardiac output, skin blood flow, and caloric expenditure. Acutely, it increases body core temperature, causes splanchnic (hence, uterine and umbilical) vasoconstriction, and alters the endocrinological milieu; chronically, it increases caloric requirements. This last de- mand may be devastating if food shortages exist: the super- imposed caloric demands of successful pregnancy and lacta- tion are estimated at 80,000 kcal. Given adequate nutritional resources, however, there is little evidence of other damaging effects of maternal exercise on fetal development. The failure of exercise to harm well-nourished pregnant women may re- late in part to the increased maternal and fetal mass and blood volume, which reduces specific heat loads, moderates vaso- constriction in the uterine and umbilical circulations, and di- minishes the maternal exercise capacity. At least in previously active women, even the most in- tense concurrent exercise regimen (unless associated with The dependence of minute ventilation on FIGURE 30. Ventilation or embryogenesis, although the combined effects of exer- rises linearly with intensity until exercise nears maximal levels. CHAPTER 30 Exercise Physiology 557 terial PO2 (and, hence, oxygen content), PCO2, and pH all The ventilatory control mechanisms in exercise remain remain unchanged from resting levels (Table 30. Where there are stimuli—such as in mixed ve- respiratory muscles accomplish this severalfold increase in nous blood, which is hypercapnic and hypoxic in propor- ventilation primarily by increasing tidal volume, without tion to exercise intensity—there are seemingly no recep- provoking sensations of dyspnea.
Gonadal sex determination is regulated by a testis- The primordial external genitalia include the genital tu- determining gene designated SRY (sex-determining region 100 mg kamagra gold mastercard, bercle order kamagra gold 100 mg with visa, genital swellings safe kamagra gold 100mg, urethral folds purchase 100mg kamagra gold free shipping, and urogenital si- Y chromosome) buy kamagra gold 100mg fast delivery. Differentiation of the external genitalia also occurs mosome, SRY encodes a DNA-binding protein, which between weeks 8 and 12 and is determined by the presence binds to the target DNA in a sequence-specific manner. Differentiation along The presence or absence of SRY in the genome determines the male line requires active 5 -reductase, the enzyme whether male or female gonadal differentiation takes place. Without DHT, re- Thus, in normal XX (female) fetuses, which lack a Y chro- gardless of the genetic, gonadal, or hormonal sex, the ex- mosome, ovaries, rather than testes, develop. The Whether possessing the XX or the XY karyotype, every structures that develop from the primordial structures are embryo goes initially through an ambisexual stage and has illustrated in Figure 39. A 4- to 6-week-old human embryo possesses in- gen-dependent differentiation occurs only during fetal life different gonads, and undifferentiated pituitary, hypothal- and is thereafter irreversible. Testicular descent into the rived from coelomic epithelium and underlying mes- scrotum, which occurs during the third trimester, is also enchyme, and primordial germ cells, which migrate from controlled by androgens. In many species, a sharp decline in the circulating levels of progesterone and The duration of pregnancy in women averages 270 14 a concomitant rise in estrogen precede birth. Parturition or the onset progesterone does not fall significantly before delivery. Uncoordinated uterine contractions start about rise in placental progesterone-binding protein or by a de- 1 month before the end of gestation. The termination of cline in the number of myometrial progesterone receptors. They increase intracellular calcium ine muscle is regulated by hormones and by mechanical concentrations of myometrial cells and activate the actin- factors. The hormones include progesterone, estrogen, myosin contractile apparatus. The mechanical fac- parturition, the concentration of prostaglandins in amni- tors include distension of the uterine muscle and stretching otic fluid rises abruptly. Aspirin and in- Progesterone hyperpolarizes myometrial cells, lowers domethacin, inhibitors of prostaglandin synthesis, delay or their excitability, and suppresses uterine contractions. Estrogen, in gen- tions, and its release from both maternal and fetal pitu- eral, has the opposite effects. Oxytocin is used clinically to CHAPTER 39 Fertilization, Pregnancy, and Fetal Development 697 Indifferent drogen precursors. Injections of ACTH and cortisol in late stages pregnancy do not induce labor. Interestingly, the adminis- tration of estrogens to the cervix causes ripening, probably by increasing the secretion of prostaglandins. Genital fold Genital swelling POSTPARTUM AND PREPUBERTAL PERIODS Genital tubercle Lactation is controlled by pituitary and ovarian hormones, requires suckling for continued milk production, and is the major source of nutrition for the newborn. As the child grows, puberty will occur around age 10 to 11 because the hypothalamus activates secretion of pituitary hormones Male Female that cause secretion of estrogens and androgens from the Glans gonads and adrenals during that time. Alterations in hor- Fused Glans mone secretion lead to abnormal onset of puberty and go- urogenital Urethral groove nadal development. Several hormones participate in Prepuce mammogenesis, the differentiation and growth of the mam- Body of mary glands, and in the production and delivery of milk. Galac- Urethral orifice Scrotal topoiesis, the maintenance of lactation, is regulated by PRL. The mammary glands begin to differentiate in the pectoral region as an ectodermal thickening on the epidermal ridge during weeks 7 to 8 of induce labor (see Clinical Focus Box 39. The prospective mammary glands lie along bilat- significance of oxytocin is that it helps expel the fetus from eral mammary ridges or milk lines extending from axilla to the uterus, and by contracting uterine muscles, it reduces groin on the ventral side of the fetus. Most of the ridge dis- uterine bleeding when bleeding may be significant after de- integrates except in the axillary region. Interestingly, oxytocin levels do not rise at the time mals with serially repeated nipples, a distinct milk line with of parturition. Mam- corpus luteum and the decidua, assists parturition by soft- mary buds are derived from surface epithelium, which in- ening the cervix, permitting the eventual passage of the vades the underlying mesenchyme.
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