Mammalian Neuroendocrinology

Paul V. Malven

• Editorial: CRC Press
• Año: 2019
• ISBN:
  9781000723014
• Edición: 1
• Categoría: Medicina
• Categoría: Endocrinologia e metabolismo

Mammalian Neuroendocrinology presents a concise examination of how the nervous and endocrine systems interact to regulate physiological processes. Selected experimental results are presented to illustrate the current understanding of neuroendocrine processes. Neural influences and endocrine feedback mechanisms related to the secretion of adenohypophysial hormones are summarized for each hormone. The book's concise nature and readable style are highly suited for use as a graduate textbook.
There are also chapters devoted to each of the following topics: neurohypophysis, pineal gland, adrenal medulla, hormones and behavior, and neuroendocrine immunology. Comparisons are made among species of mammals, including laboratory rodents, domesticated ungulates, and primates. Agricultural and/or clinical aspects relevant to these topics are covered when appropriate. Mammalian Neuroendocrinology is excellent for students and professionals in neuroendocrinology, neuroscience, endocrinology, reproduction studies, and animal sciences.

Chapter 1

PRINCIPLES OF NEUROENDOCRINOLOGY

The discipline of Neuroendocrinology examines the interactions between the nervous system and the endocrine system. During the development of Endocrine Physiology and Neurophysiology as scientific disciplines, the distinction between neural and endocrine systems was very clear, but in the last 30 years the clearcut differences have become less apparent. The scientific discipline of Neuroendocrinology has developed in the interface between strictly endocrine and strictly neural mechanisms. The origin of the field was the discovery over 30 years ago that certain neurons secreted chemical messengers into blood (Scharrer and Scharrer, 1963). This characteristic was previously reserved for hormones secreted into blood by endocrine glands. The term neurohormone was coined to describe a hormone produced by a neuron. Restrictions on the use of that term have become less stringent in recent years. In current usage, the proof that a chemical messenger produced by neurons acts as a true hormone (i.e., is secreted into blood) has not always been rigorously enforced. In the opinion of this author, to qualify as a hormone or neurohormone a chemical messenger must have an endocrine mode of action. With such a strict definition, the chemical messengers produced by neurons can be described as having one or more of the following types of action on other cells:

1. Endocrine action: Enter the blood stream to reach and alter activity of distant target cells.

2. Paracrine action: Diffuse locally through interstitial spaces to reach and influence neighboring cells.

3. Neurocrine action: Cross a synaptic junction to either activate or inhibit the postsynaptic cell.

Chemical messengers that act in an endocrine manner are generally known as hormones or neurohormones. Chemical messengers that act in a neurocrine manner are generally known as neurotransmitters. Descriptive terms for chemical messengers produced by neurons and that act in a paracrine manner include (1)neuromodulator and (2) localized hormone, but there is no generally accepted term. Moreover, it is not always known whether a particular chemical messenger acts in a paracrine or neurocrine manner, and some compounds can act in more than one manner.

The chemical structures of neuronal products known as chemical messengers can vary considerably. Peptides constitute a large class of chemical messengers produced by neurons. The size of these neuropeptides is almost always much smaller than peptides produced by the endocrine system. Some of the greatest advances in Neuroendocrinology have involved these neuropeptides, including their discovery as secretory products of neurons and emerging knowledge about their physiological functions within the nervous system and elsewhere in the body. The other major type of chemical messenger produced by neurons consists of modified amino acids and includes many of the aminergic neurotransmitters that were discovered during the early days of Neurophysiology. Examples of modified amino acids produced by neural elements are catecholamines (norepinephrine, dopamine), indolamines (serotonin), acetylcholine, and others. Some neuronal amino acids do not seem to require modification to function as chemical messengers (e.g., gamma aminobutyric acid, glycine, glutamate, and aspartate).

It is not possible to make generalizations about the type of action that a chemical messenger may exert based on its chemical structure. For example, both the neuropeptide somatostatin and the catecholamine dopamine appear to exert all three types of actions depending on the target tissue. Somatostatin acts in an endocrine manner to inhibit secretion of somatotropin by the pituitary gland and in a paracrine manner to inhibit secretion of insulin and glucagon by cells of the pancreatic islets. Somatostatin also acts in the central nervous system (CNS) on adjacent neurons (either paracrine or neurocrine action). Dopamine acts in an endocrine way to inhibit secretion of prolactin by the pituitary gland. It also acts within the CNS in a neurocrine manner, and within the mediobasal hypothalamus, dopamine may act in paracrine manner on adjacent neural elements.

Neuroendocrine Transduction

One critical process in the discipline of Neuroendocrinology is called neuroendocrine transduction. This process transforms neural information (i.e., action potentials) into chemical messengers secreted into blood (i.e., hormones) where they exert endocrine effects. The small number of identified neuroendocrine transducers have been studied in detail. The diagrams in Figure 1-1 illustrate the two general categories and four specific types of neuroendocrine transducers. Neuron A in Figure 1-1 typifies the simplest form of secretomotor innervation in which a single CNS neuron innervates a secretory cell. One example is found in the adrenal medulla where neurally derived chromaffin cells are innervated by axons of the sympathetic nervous system. In response to synaptic release of the neurotransmitter acetylcholine, the chromaffin cells discharge epinephrine and norepinephrine into blood. Another less well-known example represented by neuron A in Figure 1-1 involves hypothalamic neurons sending axons to innervate non-neural cells of the pars intermedia of the hypophysis. The chemical messenger released by these axons (probably dopamine) inhibits the release of pars intermedia products. Neuron B of Figure 1-1 represents a modified secretomotor innervation involving a two-neuron chain in which the axon of the second neuron innervates the secretory cell. Innervation of the pineal gland by the sympathetic neurons typifies this situation. Postganglionic neurons originating in the superior cervical ganglion release norepinephrine at their secretomotor terminals adjacent to the pinealocyte and this activates the release of melatonin into blood and cerebrospinal fluid (CSF).

Neurosecretory neurons depicted as C and D in Figure 1-1 release their neuronal products into blood. The two types differ only in the type of blood vessel into which they secrete. Hypothalamic neurons, which send axons into the pars nervosa of the hypophysis, release chemical messengers (e.g., vasopressin, oxytocin and others) into the general circulation (neuron C). Other neurons in the hypothalamus and adjacent regions have axons that extend to the median eminence where they discharge their chemical messengers into the capillaries of the hypophysial portal veins. These chemical messengers travel in the portal blood a few millimeters to the capillaries of the hypophysis where most of them probably act in an endocrine manner to stimulate or inhibit the secretion of adenohypophysial hormones into the general circulation. Of course, the hypophysial portal blood then enters the general circulation and any neurohormones that remain also enter that circulation. There is no clear evidence that neurohormones secreted into hypophysial portal blood reach the general circulation in physiologically relevant concentrations, but the possibility remains. In summary, neuroendocrine transduction can involve either secretomotor innervation or neurosecretory neurons, but in both types the transduction from neural to hormonal signal involves substantial amplification of that signal as well as more sustained generalized actions than are possible within the nervous system.

Figure 1-1. Types of neuroendocrine transducers.

Neuroendocrine transduction is depicted in diagrams which illustrate the secretory principles for two types of secretomotor innervation (A and B) and two types of neurosecretion (C and D).

Neuroendocrine Integration

In addition to the transduction of neural signals into endocrine signals as just described, the neuroendocrine system mediates the cooperation between the nervous and endocrine systems to regulate in an optimum manner the physiological functions of the organism. This function can be described as neuroendocrine integration and is illustrated diagrammatically in Figure 1-2. In addition to a neurosecretory neuron that transduces the information, other ordinary (i.e.,non-neurosecretory) neurons play important roles in the integration of information. Two such neurons represented in Figure 1-2 are (1) integrative neuron and (2) hormone-sensitive neuron that each have direct input to neurosecretory neurons. Figure 1-2 also illustrates that integrative neurons of the neuroendocrine system receive a variety of inputs. These may include (1) information about the ambient environment obtained through the special senses, (2) integration of current inputs with the learned or conditioned information stored in higher cortical centers, (3) endogenous free-running rhythms (e.g., circadian or ultradian), (4) neurally mediated sensory information from internal organs (e.g., reproductive tract) and sensors (e.g., blood osmolarity, pH and pressure), and (5) neural signals from specific hormone-sensitive neurons (e.g., feedback from endocrine glands).

Figure 1-2. Neuroendocrine integration.

Schematic representation of the various elements involved in neuroendocrine integration.

Hormonal Products of Neurosecretory Neurons

The number of chemical messengers of neurosecretory neurons for which endocrine actions are proved is relatively low (Table 1-1). Small peptides secreted by the neurohypophysis following their synthesis in the hypothalamus represent a significant proportion of known neurohormones. The chemical structures of oxytocin and vasopressin were identified many years ago and in recent years details of their biosynthesis and gene structures have been forthcoming. Oxytocin, with some structural...