Chemical signals steroids

The hippocampus has innumerable afferent and efferent connections to other brain structures both within the limbic system and beyond. There are receptors for many different chemical signals ranging from the "classical neurotransmitters such as acetylcholine to steroid hormones and neurotrophic factors. Some of these receptors are located in the synapses that form the intrinsic hippocampal circuits and others are the targets of specific projection pathways from other brain areas. A comprehensive review of all neurotransmitter interactions relevant to function is not within the scope of this chapter. There are detailed reviews of modulation of neurochemical systems on place learning in the watermaze (McNamara and Skelton, 1993) or other limbic-system dependent tasks (Izquierdo and Medina, 1995) in animals. The effects of key neurochemical, other than NMDA channel-mediated, and environmental influences are discussed below. 

Lipids have a variety of roles in living systems. They are used in animals for energy storage as fats. Cell membranes are made up of lipids called phospholipids. Steroids—such as cholesterol, shown in Figure 4— are lipids used for chemical signaling. Waxes, such as those found in candles and beeswax are also lipids. 

The hormonal-signaling process is summarized in Figure 7-1. Hormones are released from secretory tissues in response to metabolic signals as well as electrical or chemical signals from the nervous system. The released hormone binds to a receptor, which can either be on the cell surface or, as in the case of steroid and similar hormones, within the cell. The hormone-receptor complex starts a series of events in which the signal is converted to other chemical forms that bring about changes in the biochemical reactions within the cell. 

The chemical natures of hormones play a predictably important role in their roles in cell signaling. Steroid hormones, for example, can enter the cell directly through the plasma membrane or can bind to plasma membrane receptors. Nonsteroid hormones enter the cell exclusively as a result of binding to plasma membrane receptors (Figure 24.8). 

The above discussion shows that both urine and vaginal discharges are closely associated with steroidal hormones as will be discussed in the next section. However, vertebrate semiochemistry is not restricted to reproductive behavior nor to mammals. Territoriality, defense and odor recognition may also depend on emission of chemical signals. 

Hunter and coworkers used chemical reactivity to signal through the bilayer. An internal chemical signal was produced from an external chemical reaction in phospholipid vesicles, without physical transport of a chemical species through the membrane. A mixture of membrane-spanning bis-steroids was added to the membranes of vesicles one bis-steroid had free external and internal cysteine residues, while the other had these cysteine residues protected with a disulfide link. The system was then activated by the addition of a membrane-impermeable reductant, which selectively deprotected the external disulfides. The external signal was then added, in the form of the membrane-impermeable oxidant potassium feiricyanide, which dimerized two trans-membrane receptors (6 and 7). The internal residues then... 

This capacity to amplify these minute chemical signals comes with a cost, for the steroids associated with reproduction come with consequences. Even as far back as the 1930s, it was known that rodents exposed to elevated levels of estrogen, the primary feminizing sex steroid, develop mammary cancer. However, until recently, the mechanism by which steroids were linked to cancer was unclear. Elucidation of estrogen as a cancer promoter would have to wait for the development of the immortal cell lines, as explained previously. 

When an ovum implants into the uterine lining, a placenta—da intricate capillary network—begins to develop between the mother and the embryo. Capillaries from the mother and fetus anastomose, or mingle, closely together. While the mother s blood does not come into direct contact with the fetus s, oxygen, nutrients, and other chemicals, such as cell signaling steroids can be passed from the mother s system to the developing fetus. 

Plant terpenoids have strong odors and are commonly used for chemical signaling and defense against predators. Animal terpenoids, more commonly known as steroids and sterols, are used for cellular function and are precursors to vitamins and hormones. 

Cellular functions are controlled by extracellular signals such as hormones, neurotransmitters, odorants, light and other chemical or physical stimuli. Only a few of these signal molecules, e.g., the highly lipid-soluble steroids or thyroid hormones, can diffiise across the... 

Some information (e.g. reproductive state) appears to be contained solely within the chemical matrix of certain marks, particularly within the stable genital secretions. Thus, longer-lasting signals may be broadcast to any animal that comes in contact with labial or scrotal marks. Such a scenario seems particularly applicable to females that scent mark most frequently prior to the onset of estrus cycles, but nonetheless when their sex steroids are on the rise (Drea 2007). Such advertisement may encourage male immigration at a time that would maximize the opportunity of female mate choice, even if the mechanism of, or criteria for, selection remain obscure. 

In terms of their chemical structures, signalling molecules fall into five main categories (i) peptides, (ii) steroids, (iii) amino acids and their derivatives, (iv) fatty acid derivatives, and (v) nucleotides. 

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