**1.3. Ghrelin**

112 Neuroendocrinology and Behavior

bloodstream of the posterior pituitary.

The effects of AVP are mediated mainly via V1and V2 receptors.

Vasopressin (arginine vasopressin, AVP) is the first identified neuropeptide. AVP is a nonapeptide that is synthesized in magnocellular and parvocellular neurons, located in the paraventricular and supraoptic nuclei of the hypothalamus (Swaab et al., 1975). Most of vasopressin is released from the axonal terminals of magnocellular neurons directly iinto the

V1 receptors are located on the vascular smooth muscle membranes. They are also found in myometrium and urinary bladder smooth muscle cell membranes. V1-receptor activation mediates vasoconstriction by receptor-coupled activation of phospholipase C and release of Ca2+ from intracellular stores via the phosphoinositide cascade (Thibonnier, 1992, Briley et

V2 renal receptors are present in the renal collecting duct system and endothelial cells. Kidney V2 receptors interact (by the G protein complex) with adenylyl cyclase to increase intracellular cyclic adenosine monophosphate (cAMP) and cause retention of water (Orloff

V3 pituitary receptors (formerly known as V1b or AVPr1B), have central neural system effects, such as increasing adrenocorticotropic hormone production, activating different G proteins, and act via increasing intracellular cAMP (Thibonnier et al, 1997, Holmes et al,

The classical effects of vasopressin are mainly related to maintenance of water-electrolyte homeostasis and blood pressure. During the last years the data about the effects of this neuropeptide on brain function and behavioral reactions increase. The brain effects of vasopressin can be divided into two main types: those related to its peripheral effects such as hormone and focused on the maintenance of water balance. Others are associated with higher brain functions as learning, memory, emotion and they are independent of its hormonal effects (Frank & Landgraf, 2008). Vasopressinergic axons propelled from hypothalamus to many brain regions as hipocampus, septum, amygdala and brainstem, secrete AVP that acts as neurotransmitter. This extrahypothalamic vasopressin network is an anatomical basis of involvement of limbic-midbrain structure in processes of learning and memory. AVP facilitates consolidation and retrieval of memory (Kovacs et al., 1979)

Vasopressin participates in formation of circadian rhythms and regulation of biological clock. The suprachiasmatic nucleus (SCN) is responsible for generation of circadian rhythmicity in mammalian brain and is an obvious source for a vasopressin innervation of GABAergic neurons located in this area (Hermes et al., 2000). A significant diurnal variation in vasopressin release in the SCN was detected, with the highest levels occurring during midday and a trough around midnight (Kalsbeek et al., 1995). It was demonstrated that melatonin synthesis was stimulated after local injection in pineal gland of vasopressin. Also the night melatonin plasma concentration was increased after prolonged period of water deprivation. These results show that vasopressin can modulate melatonin synthesis in the

**1.2. Vasopressin** 

al., 1994).

& Handler, 1967).

2001, Kam et al, 2007).

Ghrelin is a multifunctional peptide hormone (28 amino acids) secreted from the cells of the diffuse neuro-endocrine system. Ghrelin-secreting cells are found from the stomach to the colon (in the oxyntic glands of the fundus and less in the small and large intestine) (Broglio et al., 2002; Lee et al., 2002; Inui et al., 2004). Ghrelin has been detected in the central nervous system, e.g. in arcuate nucleus and hypothalamus (Lu et al., 2002), in pancreas (Date et al. 2002), in some cells of the immune system (lymphocytes and monocytes) (Mager et al., 2008) and also in human ovaries and testes (García et al., 2007). There is a hypothesis that ghrelin might have not only endocrine but also autocrine and paracrine mechanism of action.

The presence of ghrelin receptor subtype GHS-R1a is detected in hypothalamus (n.arcuatus) and the pituitary gland, in multiple organs with nonendocrine and endocrine function (heart, lung, liver, kidney, pancreas, stomach, small and large intestines, adipose tissue,

immune cells, gonads, thyroid gland, adrenal gland) (Broglio et al., 2003; Inui et al., 2004; Van Der Lely et al., 2004) and in gastrointestinal vasculature (Mladenov et al., 2006). They are also expressed by lumbosacral autonomic preganglionic neurons of the micturition reflex pathways (Ferens et al., 2010)

The Effects of Some Neuropeptides on Motor Activity of Smooth Muscle Organs in Abdominal and Pelvic Cavities 115

and the area postrema, which all take part in the modulation of appetite control (Lim et al., 2010; Vartiainen, 2009). Ghrelin-secreting hypothalamic neurons send efferent fibers onto key circuits involved in the central regulation of energy homeostasis. They balance the activity of orexigenic neuropeptide Y/agouti-related peptide neurons in the arcuate nucleus and the activity of anorexigenic pro-opiomelanocortin (POMC) neurons that secrete alpha melanocyte stimulating hormone (α-MSH) and thus modulate the resultant effect (Van Der

Several new intracellular targets/mediators of the appetite-inducing effect of ghrelin in the hypothalamus have recently been identified, including the AMP-activated protein kinase, its upstream kinase calmodulin kinase kinase 2, components of the fatty acid pathway and the

Recently, it has been demonstrated that ghrelin plays an important role in the regulation of central and peripheral lipid metabolism through specific control of hypothalamic AMPactivated protein kinase (AMPK), a critical metabolic regulator of both cellular and whole-

Centrally administered ghrelin has various effects such as arousal, increasing gastric acid secretion and gastrointestinal motility, inhibition of water intake and release of some

Ghrelin may be synthesized in the hypothalamus. Ghrelin have hypothalamic actions on growth hormone-releasing hormone neurons (Sun et al., 2007). Ghrelin acts centrally to exert a global stimulatory effect on the hypothalamo-pituitary-adrenal axis. Ghrelin increases absolute whole adrenal gland weight and whole adrenal gland volume and elevates blood concentrations of ACTH, aldosterone and corticosterone (Milosević VLj et al., 2010). Ghrelin may function as a metabolic modulator of the gonadotropic axis, with inhibitory effects in line with its role as signal of energy deficit. These effects likely include inhibition of luteinizing hormone secretion, as well as partial suppression of normal puberty onset (Tena-

Ghrelin-immunoreactive neurons are present in the paraventricular, dorsomedial, ventromedial and arcuate nuclei, areas important for circadian output. Contrary to the effects of ghrelin on appetite, growth hormone release and the sleep–wake cycle, little is

Central ghrelin is also a gastroprotective factor in gastric mucosa. The gastric protection elicited by central ghrelin requires integrity of capsaicin-sensitive sensory neurons, which play an important role in gastric cytoprotection. Growing evidence indicates that the mechanisms triggered by peptides to increase resistance of the gastric mucosa involve changes in the release of gastric protective factors. Endogenous prostaglandins are involved

The short-term activation of AMPK in turn results in decreased hypothalamic levels of malonyl-CoA and increased carnitine palmitoyltransferase 1 (CPT1) activity. Ghrelin deficiency induces reductions in both de novo lipogenesis and beta-oxidation pathways in

known about the effects of ghrelin on circadian rhythms (Yannielli et al., 2007).

hormones from the pituitary, mainly growth hormone (Hashimoto et al., 2011).

Lely et al., 2004).

Sempere, 2008).

uncoupling protein 2 (Lim et al., 2010).

body energy homeostasis (Kola et al., 2005).

in ghrelin gastroprotection (Sibilia et al., 2008).

The activation of the receptor causes the stimulation of the G-protein subunit Gα11. This leads to the activation of intracellular signaling cascades via the phospholipase C (PLC) (Vartiainen, 2009).

The principal physiological action of ghrelin is the stimulation of secretion of growth hormone. Therefore ghrelin is a hormone with anabolic effect. It participates in the regulation of metabolism, energy homeostasis and feeding behaviors which are mediated via a complex neuroendocrine network (Van Der Lely et al., 2004). Ghrelin increases appetite and food intake and decreases fat utilisation as a metabolic fuel and increases fat storage in the adipose tissue. Ghrelin modulates gastic motility and emptying and gastric acid secretion and stimulates ileum peristalsis, most of these effects being vagally mediated (Ghigo et al., 2005). It induces fasted motor activity in the duodenum (Fujino et al., 2003). Ghrelin activity is mediated via enteric nervous system (Tack et al., 2006).

Ghrelin and its receptors are present not only in the peripheral tissues but also in the central nervous system (Kang et al., 2011). Ghrelin functions as a peripheral hormone that is released mainly from the stomach and affects the hypothalamus, but also as a neuropeptide in hypothalamus (Kojima and Kangawa, 2005). Like other neuropeptides, Ghrelin is widely distributed in the brain in key areas of emotional regulation, and plays role as modulators of behavioural states (Kang et al., 2011). Ghrelin plays an important role in the regulation of energy balance by regulating food intake, body weight, glucose homeostasis and feeding behaviour which are mediated by a complex neuroendocrine network (Kalra et al., 1999; Van Der Lely et al., 2004). The regulation of energy balance is related to somatic growth and instinctive behaviour, including feeding, reproduction and emotion, and is a complex phenomenon involving interaction of the central and peripheral nervous systems, neuroendocrine system and gastrointestinal system (Matsuda et al., 2011). Ghrelin induces in the brain an orexigenic effect, modifies locomotor activity and also is involved in the control of psychophysiological functions and regulation of metabolism (Kang et al., 2011). The hypothalamic region of the brain plays very important role in the regulation of feeding and neuroendocrine functions (Kalra et al., 1999). Many types of neurons in the hypothalamus and related regions express ghrelin and some neuropeptides, such as, orexin, NPY, agouti-related peptide (AGRP), melanin-concentrating hormone (MCH) and other, which are implicated in the regulation of feeding behaviour and also in energy homeostasis in mammals (Eva et al., 2006; Kalra et al., 1999; Pickar et al., 1993). Ghrelin increases orexigenic effect and food intake but decreases energy expenditure thus inducing weight gain. (Kojima & Kangawa, 2005; Van Der Lely et al., 2004). Ghrelin exerts its central orexigenic effect through activation of hypothalamic neurons in the arcuate nucleus, important area involved in the regulation of energy balance and in addition it stimulates the neurons of other areas of the central nervous system, for example nucleus paraventricularis, dorsomedial parts of hypothalamus, and areas in the brain stem nucleus tractus solitarius and the area postrema, which all take part in the modulation of appetite control (Lim et al., 2010; Vartiainen, 2009). Ghrelin-secreting hypothalamic neurons send efferent fibers onto key circuits involved in the central regulation of energy homeostasis. They balance the activity of orexigenic neuropeptide Y/agouti-related peptide neurons in the arcuate nucleus and the activity of anorexigenic pro-opiomelanocortin (POMC) neurons that secrete alpha melanocyte stimulating hormone (α-MSH) and thus modulate the resultant effect (Van Der Lely et al., 2004).

114 Neuroendocrinology and Behavior

(Vartiainen, 2009).

reflex pathways (Ferens et al., 2010)

immune cells, gonads, thyroid gland, adrenal gland) (Broglio et al., 2003; Inui et al., 2004; Van Der Lely et al., 2004) and in gastrointestinal vasculature (Mladenov et al., 2006). They are also expressed by lumbosacral autonomic preganglionic neurons of the micturition

The activation of the receptor causes the stimulation of the G-protein subunit Gα11. This leads to the activation of intracellular signaling cascades via the phospholipase C (PLC)

The principal physiological action of ghrelin is the stimulation of secretion of growth hormone. Therefore ghrelin is a hormone with anabolic effect. It participates in the regulation of metabolism, energy homeostasis and feeding behaviors which are mediated via a complex neuroendocrine network (Van Der Lely et al., 2004). Ghrelin increases appetite and food intake and decreases fat utilisation as a metabolic fuel and increases fat storage in the adipose tissue. Ghrelin modulates gastic motility and emptying and gastric acid secretion and stimulates ileum peristalsis, most of these effects being vagally mediated (Ghigo et al., 2005). It induces fasted motor activity in the duodenum (Fujino et al., 2003).

Ghrelin and its receptors are present not only in the peripheral tissues but also in the central nervous system (Kang et al., 2011). Ghrelin functions as a peripheral hormone that is released mainly from the stomach and affects the hypothalamus, but also as a neuropeptide in hypothalamus (Kojima and Kangawa, 2005). Like other neuropeptides, Ghrelin is widely distributed in the brain in key areas of emotional regulation, and plays role as modulators of behavioural states (Kang et al., 2011). Ghrelin plays an important role in the regulation of energy balance by regulating food intake, body weight, glucose homeostasis and feeding behaviour which are mediated by a complex neuroendocrine network (Kalra et al., 1999; Van Der Lely et al., 2004). The regulation of energy balance is related to somatic growth and instinctive behaviour, including feeding, reproduction and emotion, and is a complex phenomenon involving interaction of the central and peripheral nervous systems, neuroendocrine system and gastrointestinal system (Matsuda et al., 2011). Ghrelin induces in the brain an orexigenic effect, modifies locomotor activity and also is involved in the control of psychophysiological functions and regulation of metabolism (Kang et al., 2011). The hypothalamic region of the brain plays very important role in the regulation of feeding and neuroendocrine functions (Kalra et al., 1999). Many types of neurons in the hypothalamus and related regions express ghrelin and some neuropeptides, such as, orexin, NPY, agouti-related peptide (AGRP), melanin-concentrating hormone (MCH) and other, which are implicated in the regulation of feeding behaviour and also in energy homeostasis in mammals (Eva et al., 2006; Kalra et al., 1999; Pickar et al., 1993). Ghrelin increases orexigenic effect and food intake but decreases energy expenditure thus inducing weight gain. (Kojima & Kangawa, 2005; Van Der Lely et al., 2004). Ghrelin exerts its central orexigenic effect through activation of hypothalamic neurons in the arcuate nucleus, important area involved in the regulation of energy balance and in addition it stimulates the neurons of other areas of the central nervous system, for example nucleus paraventricularis, dorsomedial parts of hypothalamus, and areas in the brain stem nucleus tractus solitarius

Ghrelin activity is mediated via enteric nervous system (Tack et al., 2006).

Several new intracellular targets/mediators of the appetite-inducing effect of ghrelin in the hypothalamus have recently been identified, including the AMP-activated protein kinase, its upstream kinase calmodulin kinase kinase 2, components of the fatty acid pathway and the uncoupling protein 2 (Lim et al., 2010).

Recently, it has been demonstrated that ghrelin plays an important role in the regulation of central and peripheral lipid metabolism through specific control of hypothalamic AMPactivated protein kinase (AMPK), a critical metabolic regulator of both cellular and wholebody energy homeostasis (Kola et al., 2005).

Centrally administered ghrelin has various effects such as arousal, increasing gastric acid secretion and gastrointestinal motility, inhibition of water intake and release of some hormones from the pituitary, mainly growth hormone (Hashimoto et al., 2011).

Ghrelin may be synthesized in the hypothalamus. Ghrelin have hypothalamic actions on growth hormone-releasing hormone neurons (Sun et al., 2007). Ghrelin acts centrally to exert a global stimulatory effect on the hypothalamo-pituitary-adrenal axis. Ghrelin increases absolute whole adrenal gland weight and whole adrenal gland volume and elevates blood concentrations of ACTH, aldosterone and corticosterone (Milosević VLj et al., 2010). Ghrelin may function as a metabolic modulator of the gonadotropic axis, with inhibitory effects in line with its role as signal of energy deficit. These effects likely include inhibition of luteinizing hormone secretion, as well as partial suppression of normal puberty onset (Tena-Sempere, 2008).

Ghrelin-immunoreactive neurons are present in the paraventricular, dorsomedial, ventromedial and arcuate nuclei, areas important for circadian output. Contrary to the effects of ghrelin on appetite, growth hormone release and the sleep–wake cycle, little is known about the effects of ghrelin on circadian rhythms (Yannielli et al., 2007).

Central ghrelin is also a gastroprotective factor in gastric mucosa. The gastric protection elicited by central ghrelin requires integrity of capsaicin-sensitive sensory neurons, which play an important role in gastric cytoprotection. Growing evidence indicates that the mechanisms triggered by peptides to increase resistance of the gastric mucosa involve changes in the release of gastric protective factors. Endogenous prostaglandins are involved in ghrelin gastroprotection (Sibilia et al., 2008).

The short-term activation of AMPK in turn results in decreased hypothalamic levels of malonyl-CoA and increased carnitine palmitoyltransferase 1 (CPT1) activity. Ghrelin deficiency induces reductions in both de novo lipogenesis and beta-oxidation pathways in

the hypothalamus. There are reductions in fatty acid synthase (FAS) mRNA expression both in the ventromedial nucleus of the hypothalamus and whole hypothalamus, as well as in FAS protein and activity. CPT1 activity is also reduced. Chronic ghrelin treatment does not promote AMPK-induced changes in the overall fluxes of hypothalamic fatty acid metabolism in normal rats and this effect is independent of ghrelin status. In addition, ghrelin plays a dual time-dependent role in modulating hypothalamic lipid metabolism. (Diéguez C et al., 2010; Sangiao-Alvarellos et al., 2010)

The Effects of Some Neuropeptides on Motor Activity of Smooth Muscle Organs in Abdominal and Pelvic Cavities 117

neurons of the hypothalamic nuclei in rats (McKinley et al., 2003; Von Bohlen et al., 2006). AT1 receptor binding sites with higher density were localized in the lamina terminalis, hypothalamic paraventricular nucleus and the nucleus tractus solitaries, lamina terminalis and the subfornical organ. The median preoptic nucleus also contains membranes rich in AT1 receptors. All regions that are included in the regulation of cardiovascular functions as caudal ventrolateral medulla, and the midline raphe, also have AT1receptors (Allen et al., 1999; Lenkei et al., 1997). In the midbrain -in the lateral parabrachial nucleus, substantia nigra and periaqueductal gray AT1 receptors are presented from moderate to high densities (Lenkei et al., 1997). These localizations of RAS brain components show that Ang II is involved in the regulations of thirst, drinking, facilitating vasopressor effects and secretion of vasopressin, adrenocorticotrophic and luteinizing hormones. Ang II also stimulates secretion of neurotransmitters such as noradrenaline and 5-hydroxytryptamine (5-HT) and inhibits acetylcholine release. The brain RAS appears to be also an important modulator of

Recent findings demonstrate that central effects of Ang II contribute to facilitated learning and enhance associative memory and learning possibly with differential effects on acquisition, storage and recall. Brain RAS is involved in the development of affective disorders and Ang II has a modulating effect of on anxiety (Georgiev & Yonkov, 1985).

RAS receptors alterations have been found in some neurodegenerative disorders - Parkinson's and Huntington's disease (Ge & Barnes, 1996). The number of AT1 receptors in caudate nucleus, putamen and substantia nigra was significantly decreased in Parkinson's disease patients in comparison to controls. In Huntington's disease patients, AT1receptors was found to be slightly decreased in putamen (Ge & Barnes, 1996). AT2 receptors that are localized in caudate nucleus was decreased in Parkinson's and increased in Huntington's disease patients. The receptor alterations were considerable; therefore the authors have concluded that brain RAS seems to decisively contribute to the pathology of the dopaminergic nigrostriatal pathway in these patients and may be a novel therapeutic target

In 1928 Ivy and Oldberg extracted from dog duodenal mucosis a substance which injected intravenously contracted the gallbladder. The authors named this substance cholecystokinin (CCK). Later, Harper & Raper (1943) found a compound in this extract that stimulated the pancreatic secretion and called it pancreozymin. Purifying both hormones and determing their amino-acid sequention, Mutt (1980) proved them to be the same linear polypeptide, containing 33 amino-acid residues, and proposed the hybrid name "cholecystokininpancreozymin". Different CCK forms have been shown to exist: CCK-58, CCK-39, CCK-33, CCK-27, CCK-12, CCK-8, CCK-4, all of them containing a bioactive C-terminal. CCK-58 and CCK-39 are precursors of CCK-33 and by the degradation of the latter the shorter forms are obtained. Cholecystokinin octapeptide (CCK-8) is the most active one and is most widely spread in the gastro-intestinal tract and in the central nervous system. A cholecystokinin

analogue, named caerulein, has been isolated from the skin of the frog *Hyla caerulea*.

the blood pressure circadian rhythm and it influences renal renin release.

for neurodegenerative disorders (Savaskan, 2005).

**1.5. Cholecystokinin** 

A reciprocal relationship exists between ghrelin and insulin, suggesting that ghrelin regulates glucose homeostasis (Sun et al., 2007).
