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Neuroendocrinology is the study of interactions between the nervous and endocrine systems. The nervous system is composed of the brain, spinal cord, ganglia, and nerves. Neural cells communicate directly with one another (and with cells of sensory and effector tissues) by means of neurotransmitters. The endocrine system is composed of ductless glands that release hormones that act systemically. Neuroendocrinology therefore investigates reciprocal inﬂuences of local and widespread systems for signaling in animals. This research paper will outline the progression of research in neuroendocrinology from the analysis of regulation of the pituitary as an output of the nervous system to various inﬂuences of hormones on the nervous system.
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1. Deﬁnition Of ‘Neuroendocrinology’
The term ‘neuroendocrinology’ is composed of several morphemes. The preﬁx ‘neuro-’ is derived from words for ‘nerve’ or ‘sinew,’ and so refers to a nervous system with physical connections between the parts. The term ‘endocrine’ is a combination of ‘endo-,’ meaning ‘within’ and ‘-crine,’ from a Greek word for ‘separate.’ Therefore, the endocrine system is composed of glands that are separate from the tissues they inﬂuence within the body. The suffix ‘-ology,’ of course, signiﬁes a body of knowledge. Therefore, the most literal meaning of neuroendocrinology is the study of the interactions between the nervous and the endocrine systems.
It should be emphasized that the nervous and endocrine systems share common signaling functions, in that the cells of both systems release chemical messengers that bind to sensitive cells. The binding of the chemical messengers initiates a response only in target cells having the appropriate receptor. In the case of the nervous system, the chemical messenger is a neurotransmitter that is released and acts primarily at an area of close cellular contact, a synapse. The cells of the nervous system are localized in centralized structures such as the brain and spinal cord and make numerous synapses with one another and with other cells by means of extended cellular processes (axons or dendrites). In the case of the endocrine system, hormones are released from the cells of a gland and are carried through the circulatory system to act on cells throughout the body. It has become increasingly clear that the ﬁelds of neuroscience and endocrinology are not so distinct as once thought, and there are now known to be many overlapping characteristics and functions of neural and endocrine systems.
2. Historical Development Of Research In Neuroendocrinology
Neuroendocrine research has progressed from an original focus on the endocrine system as a specialized output of the nervous system to a consideration of the complex feedback inﬂuences of endocrine glands on the nervous system. A primary emphasis of neuroendocrinology has been an analysis of the regulatory interplay between the hypothalamus and pituitary, and glands in other places of the body. Additionally, neuroendocrinology has investigated the direct neuronal control of glands other than the pituitary. The ﬁeld has further expanded to include hormonal inﬂuences on brain behavioral systems.
For many years, studies on invertebrates have suggested that neuronal cells secrete substances systemically (Speidel 1919). The hormonal function of neural cells was termed ‘neurosecretion’ (Scharrer and Scharrer 1954), and the idea of neurosecretory cells acting as an interface between the nervous and endocrine systems gave rise to the ﬁeld of neuroendocrinology. Lesions of the median eminence of the hypothalamus were found to inﬂuence the activity of the pituitary gland profoundly, and the effects of the lesions were mimicked by blocking the blood ﬂow from the hypothalamus to the pituitary (see Harris 1972). These results suggested that hypothalamic neurons secrete factors directly into the blood, and are therefore neurosecretory.
Many hormonal factors have now been isolated from the hypothalamus and it has become clear that most of these factors are short peptides (see Akil et al. 1999, Frohman et al. 1999). Characterization of the release of neuropeptides became an early major focus of neuroendocrinology (see Table 1). There are two distinct systems for neuropeptide secretion related to pituitary function. The posterior lobe of the pituitary (or neurohypophysis) consists of terminals of neurosecretory cells with somas in the hypothalamus (see Akil et al. 1999, Frohman et al. 1999). The secretions of the posterior pituitary are neuropeptides that enter the circulation. The anterior lobe of the pituitary (or adenohypophysis) is an endocrine gland. Neuropeptides from the median eminence are carried through the portal blood supply to the anterior pituitary and control the release of anterior pituitary hormones. In turn, several of the anterior pituitary hormones regulate the functions of glands in other parts of the body. Such glands are said to form part of a hypothalamo-pituitary axis of hormonal regulation, as in, for example, the ‘hypothalamo-pituitary-thyroid axis.’
2.2 Sympathetic Nervous System
The output of the nervous system is typically described as having a somatic portion, controlling voluntary muscle contractions, and an autonomic division, regulating activity of the viscera (see Dodd and Role 1991). Activation of the sympathetic nervous system due to injury or perceived threat causes effects classiﬁed as a ‘ﬁght-or-ﬂight’ response. These effects include an increased heart rate, respiration, blood pressure, and glucose mobilization. Axons from the sympathetic division of the autonomic nervous system exit the spinal cord and synapse in a chain of ganglia alongside the vertebrae or in prevertebral ganglia closer to the tissue innervated. The postganglionic cells release norepinephrine at synapses in smooth muscle, cardiac muscle, or exocrine glands. Additionally, some ﬁbers from the spinal cord directly innervate the adrenal medulla through the greater splanchnic nerve. The endocrine cells of the adrenal medulla secrete epinephrine and norepinephrine systemically, much as the postganglionic sympathetic neurons release norepinephrine at synapses.
2.3 Temporal Aspects Of Neural Regulation Of The Endocrine System
A common feature of neurosecretion is the striking rhythmicity of the release of hormone (see Akil et al. 1999, Frohman et al. 1999). Levels of circulating hormones controlled by a hypothalamo-pituitary regulation typically ﬂuctuate with a daily, or ‘circadian,’ rhythm of approximately 24 hours. Superimposed on the circadian rhythm is a more rapid ‘ultradian’ pattern of discrete pulses of hormone, which is also regular in rhythm. The period of the ultradian secretion of hormone varies by species. In humans, the typical pattern of secretion of anterior pituitary hormones is a series of spikes at 90–120 minute intervals, with the maximal concentration of the spikes varying systematically according to the time of day. The temporal pattern of pulsatile and circadian secretions of hormones is thought to be the result of the pattern of activity of hypothalamic neurosecretory cells. A major function of the brain in regulating the endocrine system may therefore be in the timing of hormonal ﬂuctuations. The suprachiasmatic nucleus of the hypothalamus has a clock-like function in the brain, and there are probably other circadian and ultradian clocks in the brain.
The pineal gland has the unique role of integrating sensory and endocrine functions. In lower vertebrates the pineal is actually a photosensitive third eye (Dodt and Meiss 1982). In mammals, the photoreceptive capacity seems to have been lost, but the role of the pineal is nonetheless linked to photic input. The innervation of the mammalian pineal includes multisynaptic inputs from the retina, especially by way of the suprachiasmatic nucleus of the hypothalamus, and the hormone of the pineal, melatonin, is secreted predominantly at night.
2.4 Feedback Regulation Of The Nervous System By Peripheral Hormones
Neuroendocrinology is not only the study of endocrine outputs from the nervous system, but also examines the effects of peripheral hormones on the brain. A central theme of neuroendocrinology is the importance of negative feedback processes, wherein an output of a system inhibits the controller of the system. In most of the hypothalamo-pituitary axes, there is a demonstrated effect of the hormone of the peripheral gland on the neurosecretory cells in the hypothalamus (see Akil et al. 1999, Frohman et al. 1999). Growth hormone (GH), for example, inhibits the release of growth hormone-releasing hormone (GHRH) and increases release of somatostatin. Because of the feedback of GH on the hypothalamus, secretion of GH decreases as levels of the hormone become elevated, and the circulating levels of GH are controlled. In an analogous fashion, thyrotrophinreleasing hormone (TRH) release is very sensitive to circulating unbound levels of thyroid hormone. The feedback of sex steroids on the hypothalamic secretion of gonadotrophin-releasing hormone (GnRH) is extraordinarily complex, and shifts from inhibitory to stimulatory just before ovulation in females (see Fink 1979).
2.5 Effects Of Hormones On The Developing Brain
A major effect of the endocrine system on the brain is seen during development. Thyroid hormones have long been known to have an essential role in brain development, and hypothyroidism during development can lead to serious disturbances, such as cretinism. These effects are reﬂected in structural deﬁcits in certain types of brain neurons (see Oppenheimer and Schwartz 1997). Perinatal steroids, primarily androgens, play a key role in the determination of gender differences in brain structure and function (see Cooke et al. 1998).
2.6 Regulation Of Behavior By Hormones
It has become increasingly clear that hormones inﬂuence a variety of brain functions in adulthood, and behavioral neuroendocrinology has become a fertile area of research. Reproductive behaviors of both genders are inﬂuenced by gonadal steroids, due to direct effects on the brain (see Beach 1967). Thyroid hormone effects in adult brain are illustrated by a variety of clinical signs and symptoms of thyroid disease, and there are a variety of indications that thyroid hormones are important in the etiology of depression (see Henley and Koehnle 1997). Peptide hormones including corticotrophin-releasing factor (CRF), GHRH, SS, GnRH, arginine vasopressin, oxytocin, vasotocin, and adrenocorticotropic hormone (ACTH) can inﬂuence behaviors including feeding, reproductive behavior, and behavioral responses to stress (see Koob 1992, Le Moal et al. 1992).
3. Recent Trends Of Neuroendocrine Research
3.1 Novel Mechanisms Of Action Of Hormones In The Brain
In peripheral tissues and in the brain, a well-deﬁned mechanism of action of steroid and thyroid hormones is through a family of nuclear receptors that alter gene expression. Recent studies have indicated that several steroids can affect binding and activity membrane receptors for neurotransmitters such as γaminobutyric acid (GABA) (see McEwen 1998). Thyroid hormones have also been shown to have effects at membrane receptors for GABA (Martin et al. 1996). A current focus of neuroendocrine research is the functional signiﬁcance of multiple distinct mechanisms of brain actions of a given hormone in the brain.
3.2 Convergence Of Research On Molecular Signals
Gradually, distinctions between hormones and neurotransmitters have eroded. Some neuroactive steroids are known to be synthesized from cholesterol in the brain and may have important direct signaling functions within brain tissue (see McEwen 1998). Several lines of evidence suggest that thyroid hormones may also have neurotransmitter-like functions in the brain (Dratman and Gordon 1996, Martin et al. 1996).
Neuropeptides that had been originally studied in relation to pituitary function are now known to be present in a variety of brain areas (Mains and Eipper 1998). Many other neuropeptides originally classiﬁed as gastric hormones are also found in brain areas other than the hypothalamus. These neuropeptides probably do not enter the circulation as hormones but may be released locally and act within brain tissue, like neurotransmitters. These neuropeptides appear to modulate the actions of other, more typical, neurotransmitters acting on the same cell and are sometimes termed ‘neuromodulators.’
Cytokines are peptides, including interleukins and tumor necrosis factors, which have important roles in immune response. These molecules are also found in the brain and may have neuromodulatory roles (Vitkovic et al. 2000). A rapidly developing area of research is the role of cytokines in the regulation of sleep (Krueger et al. 1999). It seems likely that other signaling factors with actions that have been characterized extensively in one system will turn out to have wide-ranging effects in neural tissue.
4. Future Directions Of Neuroendocrine Research
During the history of neuroendocrinology, a host of factors have been found to carry signals from the brain to the endocrine system and from the endocrine system to the brain. The process of identifying and cataloging factors and their receptors has been facilitated by molecular biological techniques, and seems likely to accelerate rapidly when data from human and other genome sequences becomes widely available. As the list of signaling factors and their receptors approaches completion, neuroendocrine research should turn to the complex issue of how the inﬂuences of these factors could be integrated within various tissues. For example, one probable direction of future research is the analysis of the mechanisms and functional roles of the pulsatile and circadian rhythms of hormonal secretions. A surprising number of the signaling factors have similar roles in nervous, endocrine, immune, and other systems. Future researchers may eventually study chemical signals in biological systems as a single uniﬁed discipline, classifying signals and receptors by their structure, gene family, or presumed evolutionary origins, instead of according to the secretory or target tissues.
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