Brain-pituitary axis

There is no specific information on the effects of ecotoxicants on the fish brain-pituitary axis. Assays for fish TSH are not yet routine. The only current practical measure of thyrotrope activity is from their histological appearance but this is difficult to interpret since it may represent adjustments in negative feedback to maintain plasma free T4 levels. 

The sympathetic nervous system (SNS) and the hypothalamic-pituitary axis work together as important modulators of the immune system after exposure to stressors. Norepinephrine (NE) and epinephrine (EPI) (catecholamines from the SNS) and neuroendocrine hormones modulate a range of immune cell activities, including cell proliferation, cytokine and antibody production, lytic activity, and migration. This chapter will focus on these two major pathways of brain-immune signaling, briefly summarizing the evidence for SNS and hypothalamic-pituitary-adrenal (HPA) modulation of immune function, their influence on immune-mediated diseases, immune modulation in aging, and early life influences on these pathways. 

Although it is the dominant organ of the neural system, the brain also has an endocrine function, enabling the all-important overlap between neural and endocrine control systems. The most obvious and classically recognized hormonal function of the brain arises from the peptide hormones of the hypothalamus. The hypothalamus is intimately connected with the pituitary, producing the hypothalamic-pituitary axis. The hypothalamus is part of the brain the pituitary, although located within the skull, is not part of the brain but is part of the endocrine system. Peptide hormones from the hypothalamus influence pituitary function and thus endocrine function throughout the body. 

Figure 5.2 The Hypothalamic Pituitary Axis The hypothalamus is part of the diencephalon within the brain. The pituitary, although located within the skull, is more correctly a part of the endocrine system than the nervous system. Together, the hypothalamus and pituitary form the interface between the nervous system and endocrine system and exert control over the majority of other hormone secreting organs. Releasing and inhibiting factors permit the hypothalamus to control the pituitary. Pituitary hormones are released into the general circulation, affecting metabolic function throughout the thorax and abdomen.

Webster EL, Grigoriadis DE, De Souza EB. Corticotropin-releasing factor receptors in the brain-pituitary-immune axis. In McCubbin JA, Kaufmann PJ, Nemeroff... 

Among the catecholamines, dopamine has long been of interest to both chemists and neuroscientists. It is one of the most important neurotransmitters and is ubiquitous in the mammalian central nervous system[5]. It modulates many aspects of brain circuitry in a major system of the brain including the extra pyramidal and mesolimbic system, as well as the hypothalamic pituitary axis[6]. It also plays a crucial role in the functioning of the central nervous, cardiovascular, renal and hormonal systems[4], A loss of dopamine containing neurons or its transmission is also related to a number of illnesses and conditions including Parkinson s disease, schizophrenia, motivational habit, reward mechanisms and the regulation of motor functions and in the function of the central nervous, hormonal and cardiovascular system[5,18,19]. It is therefore of interest to measure dopamine in the extracellular fluid in animals to order to monitor neurotransmission processes and correlate neurochemistry with behavior[19]. 

Figure 50-9 The regulatory feedback loop of the hypothalamic-pituitary-gonadal axis. Neural and sensory input from the brain elicits the release of Gn-RH. Gn-RH in turn stimulates the synthesis and release of the gonadotropins FSH and LH, which act on the gonads (ovary and testes) to elicit the ripening and ovulation of the ovary and steroidogenesis (estradiol and progesterone) in the female and spermatogenesis and testosterone production in the male. Inhibin formed by the ovaries and testes along with estradiol and testosterone negatively feeds back to the hypothalamic-pituitary axis to modulate Gn-RH, FSH, and LH release.

The platyfish, a small freshwater teleost, has been studied intensively for more than 60 years (Kallman, 1975). Sufficient information has been amassed on its neuroendocrine system and on the hereditary mechanisms that control its age at sexual maturation, to make the platyfish invaluable for the study of the ontogeny, functioning and aging of the brain-pituitary-gonadal (BPG) axis. 

We have begun to explore the nature of olfactory-brain-pituitary-gonad (OBPG) axis function by performing experimental manipulations which, although in their initial phases, further support our contention that a fundamental structural and functional association exists among the components of the OBPG axis. 

Engelmann M, Landgraf R, Wotjak CT (2004) Interaction between the hypothalamic-neurohypophysial system (HNS) and the hypothalamic-pituitary-adrenal (HPA) axis under stress—an old concept revisited. Front Neiu oendocrinol 25 132-149 Ermisch A, Riihle HJ, Landgraf R, Hefi J (1985) Blood-brain barrier and peptides. J Cereb Blood Flow Metab 5 350-357... 

Jacobsen, L.K., Giedd, J.N., Kreek, M.J., Gottschalk, C., and Kosten, T.R (2001 b) Quantitative medial temporal lobe brain morphology and hypothalamic-pituitary-adrenal axis function in cocaine dependence a preliminary report. Drug Alcohol Depend 62 49-56. 

NPY produces a variety of central nervous system effects, including increased feeding (it is one of the most potent orexigenic molecules in the brain), hypotension, hypothermia, respiratory depression, and activation of the hypothalamic-pituitary-adrenal axis. Other effects include vasoconstriction of cerebral blood vessels, positive chronotropic and inotropic actions on the heart, and hypertension. The peptide is a potent renal vasoconstrictor and suppresses renin secretion, but can cause diuresis and natriuresis. Prejunctional neuronal actions include inhibition of transmitter release from sympathetic and parasympathetic nerves. Vascular actions include direct vasoconstriction, potentiation of the action of vasoconstrictors, and inhibition of the action of vasodilators. 

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