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Anais da Academia Brasileira de Ciências

Print version ISSN 0001-3765On-line version ISSN 1678-2690

An. Acad. Bras. Ciênc. vol.79 no.2 Rio de Janeiro June 2007

http://dx.doi.org/10.1590/S0001-37652007000200009 

BIOMEDICAL AND MEDICAL SCIENCES

 

Role of the serotoninergic system in the sodium appetite control

 

 

Luís C. Reis

Departamento de Ciências Fisiológicas, Instituto de Biologia, Universidade Federal Rural do Rio de Janeiro, BR 465, km 7, 23890-000 Seropédica, RJ, Brasil

 

 


ABSTRACT

The present article reviews the role of the serotoninergic system in the regulation of the sodium appetite. Data from the peripheral and icv administration of serotoninergic (5-HTergic) agents showed the participation of 5-HT2/3 receptors in the modulation of sodium appetite. These observations were extended with the studies carried out after brain serotonin depletion, lesions of DRN and during blockade of 5-HT2A/2C receptors in lateral parabrachial nucleus (LPBN). Brain serotonin depletion and lesions of DRN increased the sodium appetite response, in basal conditions, after sodium depletion and hypovolemia or after beta-adrenergic stimulation as well. These observations raised the hypothesis that the suppression of ascending pathways from the DRN, possibly, 5-HTergic fibers, modifies the angiotensinergic or sodium sensing mechanisms of the subfornical organ involved in the control of the sodium appetite. 5-HTergic blockade in LPBN induced to similar results, particularly those regarded to the natriorexigenic response evoked by volume depletion or increase of the hypertonic saline ingestion induced by brain angiotensinergic stimulation. In conclusion, many evidences lead to acceptation of an integrated participation resulting of an interaction, between DRN and LPBN, for the sodium appetite control.

Key words: hydroelectrolyte balance, sodium appetite, 5-HTergic system, dorsal raphe nucleus, lateral parabrachial nucleus, 5-HTergic agents.


RESUMO

Este artigo revisa o papel do sistema serotoninérgico no controle do apetite ao sódio. Dados derivados da administração periférica e icv de agentes serotoninérgicos demonstraram a participação de receptores 5-HT2/3 na modulação do apetite ao sódio. Estas observações foram estendidas com os estudos realizados após a depleção cerebral de serotonina, lesões do NDR e durante o bloqueio 5-HT2A/2C no núcleo parabraquial lateral (NPBL). A depleção cerebral de serotonina e as lesões do NDR aumentaram o apetite ao sódio, em condições basais, após depleção de sódio, durante a hipovolemia ou após a estimulação beta-adrenérgica. Estas evidências suscitaram a hipótese de que a supressão de vias ascendentes do NDR, possivelmente 5-HT, alteram os mecanismos angiotensinérgicos e a atividade dos sensores de sódio do órgão subfornicial envolvidos no controle do apetite ao sódio. O bloqueio serotoninérgico no NPBL induziu a resultados similares, particularmente aqueles relacionados com a resposta natriorexigênica provocada pela depleção de volume ou o aumento da ingestão de salina hipertônica induzida pela estimulação angiotensinérgica cerebral. Em resumo, as evidências convergem para a admissão de uma participação integrada resultante da interação recíproca entre NDR e NPBL objetivando controlar o apetite ao sódio.

Palavras-chave: equilíbrio hidroeletrolítico, apetite ao sódio, sistema serotoninérgico, núcleo dorsal da rafe, núcleo parabraquial lateral, agentes serotoninérgicos.


 

 

INTRODUCTION

THE ION SODIUM PLAYS CRITICAL ROLE IN THE MAINTENANCE OF THE VOLUME OF EXTRACELLULAR FLUID: INFLUENCE OF THE RENIN-ANGIOTENSINALDOSTERONE SYSTEM

The dynamic balance of the milieu interieur is a sine qua non condition for maintenance of life (Bernard 1878, Andersson 1978, Denton et al. 1996, Voisin and Bourque 2002, Weisinger et al. 2004). The homeostatic prevention of perturbations of the extra cellular fluid (ECF)volume and tonicity is dependent on harmonic efficiency of multiple physiological systems.

It is well known the role of the ion sodium, major ionic component of ECF, as a crucial factor for the cellular excitability and the maintenance of the tonicity and circulating volume (Andersson 1978, Daniels and Fluharty 2004, Weisinger et al. 2004). Experimental evidences have demonstrated that sodium appetite oscillation is monitored by a putative sodium sensing mechanism limited to subfornical organ (SFO). The channels within the SFO, recently described, represent a functional category of sensor neurons distinct from osmosensor neurons (Andersson 1978, Denton et al. 1996, Goldin et al. 2000, Hiyama et al. 2002, 2004, Weisinger et al. 2004) previously theorized from data and conceptions of Andersson (Andersson 1952, 1971, 1978, Andersson and McCann 1954). Another class of sodium sensing neurons is confined to magnocellular sub region of the supra optic nucleus (SON). Local osmosensitivity is operated in vasopressinergic neurons through intrinsic stretch-inactivated cation channels. Sodium detection occurs following acute regulation of the permeabilityand chronic changes in Na+ channels gene expression (Voisin and Bourque 2002).

Terrestrial vertebrates developed physiological systems implicated with the control of the ECF volume from the challenges raised by the environment, e.g., deficit and excessive offer of NaCl (Denton 1984, Schulkin 1991, Fitzsimons 1998, Weisinger et al. 2004). The physiological systems related both to the sodium and the ECF volume homeostasis include the renin-angiotensin-aldosterone (RAAS), atrial natriuretic peptide(ANP) and oxytocin (OT) systems (Fitzsimons 1998, McCann et al. 1997, 2003, Antunes-Rodrigues et al. 2004, Daniels and Fluharty 2004, McKinley and Johnson 2004, Weisinger et al. 2004, Saavedra 2005).

Angiotensin (ANG) is peripherally generated by the catalysis from a circulating alpha2-globulin, theangiotensinogen (Reid et al. 1978, Fitzsimons 1998, Saavedra 2005). Renin, the enzyme secreted by juxtaglomerular cells of the afferent renal arteriole, participates in this reaction in response to hyponatremia, ECF volume shrinkage and hypotension. From this reaction results a decapeptide, ANG I, considered a prohormone. Circulating ANG I undergoes the catalytic action of the angiotensin converting enzyme (ACE) expressed in several territories. However, the lung constitutes the largest surface which produces this enzyme. Following this stage, occurs the formation of ANG II, potent vasoconscrictor agent and also implied on the increase of the renal sodium reabsorption and dipsogenic and sodium appetite expression.

RENIN-ANGIOTENSIN-ALDOSTERONE, ANP AND OXYTOCIN SYSTEMS AND THE RENAL SYMPATHETIC INNERVATION CONSTITUTE INTEGRATED SYSTEMS OF CONTROL OF THE SODIUM RENAL REABSORPTION

The kidney mediates the regulation of the sodium reabsorption in mammals. Thus, the increase of the RAAS activity and subsequent increase of ANG II and aldosterone plasma levels stimulated by the hyponatremia constitute critical variables for the homeostatic restoration of the ECF volume. During hypernatremia condition and therefore higher ECF volume, occurs a depression of the RAAS activity with concomitant increase of ANP and probably oxytocin (OT), culminating with the increase of the renal sodium excretion (Fitzsimons 1998, McCann et al. 2003, Antunes-Rodrigues et al. 2004).

ANP, a natriuretic hormone produced by cardiomiocytes, is released after stretch of the wall of right atrium provoked by blood volume expansion (Antunes-Rodrigues et al. 1991, 2004, DeBold et al. 1996). OT, a hormone produced by magnocellular neurons of the hypothalamic paraventricular nucleus (PVN) is also a natriuretic hormone whose secretion is increased after ECF hypertonic expansion (Soares et al. 1999, Ventura et al. 2002, Antunes-Rodrigues et al. 2004). Interestingly, OT release is dependent on nitrergic activation in the basomedial hypothalamus and the natriuretic response. On the other hand, it is also dependent on nitrergic mediation in the renal tubular sites (Soares et al. 1999, Ventura et al. 2002, McCann et al. 2003, Antunes-Rodrigues et al. 2004).

In addition, it has been suggested that ANPergic neurons in hypothalamus excite the oxytocinergic neurons in the PVN and SON to release oxytocin from the neurohypophysis which by plasma stimulate the release of ANP from the atria (McCann et al. 1997, 2003, Antunes-Rodrigues et al. 2004).

After chronic sodium deprivation and subsequent hyponatremia, renal sensors identify and measure the magnitude of this error. In this context, renal and endocrine mechanisms related to sodium, and consequently water maintenance, are activated. Furthermore, sensors located in the juxtaglomerular apparatus (JGA) recognize reduction in natremia, extra cellular fluid volume, blood pressure and tubular renal sodium content. JGA is an anatomo-functional unit constituted by the juxtaglomerular cells of the afferent arteriole, mesangial cells and macula densa. This system controls tubule-glomerular balance and activates renin angiotensin aldosterone system (RAAS). It subsequently increases the renin release, the systemic ANG II generation and aldosterone secretion (Fitzsimons 1998, Schnermann and Levine 2003). Mediation of renal sodium reabsorption during the ECF volume shrinkage is intensified by RAAS. Simultaneously, it is observed a reduction on cardiac ANP releasing. Furthermore, it is also postulated an OT plasma level decreasing (McCann et al.2003, Antunes-Rodrigues et al. 2004).

The renal autonomic tubular innervation seems to constitute another pathway for sodium excretion control. Renal dennervation induces a transitory increase of the sodium excretion. On the other hand, electric stimulation of renal nerve reduces natriuretic response through increase of the tubular sodium reabsorption (DiBona 2001). These observations were reinforced by evidence that central sympatholytic action induces an increase of the renal sodium excretion (Saad et al. 2002).

Regulation of Na+ reabsorption in the kidney constitutes a critical mechanism for ECF volume and long-term blood pressure control (Harris et al. 1996, Caruso-Neves et al. 2004). In this context, the Na+-ATPase, a transepithelial sodium transporter on the proximal tubule have deserved special attention. This transporter is an important homeostatic target of brain and circulating signals for fine regulation of Na+ reabsorption. For example, it has been demonstrated that ANP inhibits the proximal Na+-ATPase by the NPR-TO/guanilate cyclase/ cGMP pathway (Harris et al. 1996, Caruso-Neves et al. 2004). Similarly, natriuretic effect of OT is dependent on guanilate cyclase/cGMP pathway but through NO synthesis (Soares et al. 1999). Therefore, ANG II and ANP (OT as well) act antagonistically in the proximal tubule as regulators of ECF volume. These evidences have disclosed a wider homeostatic possibility by which both brain and peripheral signals to be integratively acting on the salt appetite and sodium excretion modulation (McCann et al. 2003, Antunes-Rodrigues et al. 2004).

SODIUM APPETITE CONTROL: SIGNALING FOR SODIUM APPETITE EXPRESSION

Following a prolonged sodium deficit elicited by a restriction on dietary sodium access, sensory systems are activated and lead to humoral signaling. By contrast, when sodium offer exceed the physiological set point, there is an increase of the ECF volume involving a depression of the RAAS activity. This response is followed by a concomitant increase of ANP, and probably OT, release and consequent sodium appetite inhibition. In this context, the hypothalamus and the anterior-ventral region of the third ventricle (AV3V) are involved in the ANP release induced by the blood volume expansion and by signals originated in baroreceptors and peripheral volume receptors (Antunes-Rodrigues et al. 1991, 2004, Johnson and Thunhorst 1997).

Visceral sensory signals relayed from the activation of volume receptors and baroreceptors, ascend to the forebrain after synapsing within the brainstem (Denton 1984, Fitzsimons 1998). The nucleus tractus solitarius (NTS) and lateral parabrachial nucleus (LPBN) constitute the main circuits that relay information about the blood pressure and the blood volume elevation in the kidney as well as in vascular endothelium (Denton 1984, Schulkin 1991, Fitzsimons 1998, Daniels and Fluharty 2004). Signals concerning to the tubular sodium concentration along macula densa, can also achieve the brain purposing to activate mechanisms to regulate the sodium excretion and salt ingestion. The perspective of this possibility was subject matter of investigation in which it showed that neurons of SFO receive signaling from renal afferents (Ciriello 1998).

In another experimental approach, electrophysiological records were obtained in oxytocinergic neurons of PVN after electric stimulation of renal afferent nerves in rats (Ciriello 1997). In this article, Ciriello presented circumstantial evidence that the kidney sense variations in the sodium tubular concentration (and therefore of the ECF volume) relaying it to the brain. For this conclusion, the author showed that ANG II excited-neurons of SFO which project toward PVN are responsive to nerve afferent stimulation. Interestingly, central inhibition of ANG II generation or their actions blockade prevented the sensory activation after the volume depletion (Fitch and Weiss 2000). In this context, Thunhorst et al. (1996) have finaly demonstrated that the integrity of the renal nerves is important for the normal elaboration of salt appetite in response to hypovolemia/hypotension.This evidence joined to those achieved by Ciriello (1997, 1998) have undoubtedly corroborated the hypothesis by which the kidney plays a role in the activation of a visceral sensory mechanism implicated in the neuroendocrine and behavioral responses concerning the water and electrolyte control.

Besides being a potent dipsogenic agent, ANG II also increases the expression of the sodium appetite after hyponatremia and concomitant volume depletion. Structures of the lamina terminalis endowed with ANG II receptor are involved on sodium appetite after hypovolemia (Fitzsimons 1998, McKinley et al. 2003). AT1 receptors located in the forebrain, especially in the circumventricular organs (CVO's), have been implicated in the mediation of this homeostatic behavior. Moreover, ANG II stimulates c-fos expression in the OVLT and SFO (Denton et al. 1996, Johnson and Thunhorst1997, Fitzsimons 1998). In turn, aldosterone released during the same physiological context, acts synergistically increasing the expression of ANG II receptors in the SFO (Denton 1984, Schulkin 1991, Fitzsimons 1998, Daniels and Fluharty 2004).

The role of the salt taste in the salt ingestion was recently reassessed (Daniels and Fluharty 2004). In this work, the authors regarded that sodium intake is unique among ingestive behaviors that defines precisely a specific sensory channel responsible for the sensory qualities thereof is being ingested. Sensory neurons arising from gustatory system send fibers through chorda tympani afferents which it directs to dorsal portion of the NTS. The somatotopic organization of the gustatory system in the NTS constitutes the beginning of the central gustatory neuroaxis. Neurophysiological evidence showed that gustatory system plays a crucial role in the detection, acceptability and ingestion of salt in a coordinated way. In turn, chorda tympani axons synapse with the NTS gustatory neurons and receive gabaergic inhibitory innervation whose firing rate is modulated by ANG II-CVO's, thalamic, cortical and limbic inputs.

FOREBRAIN STRUCTURES AND SODIUM APPETITE REGULATION

The central mediation of the sodium appetite is dependent on integrity of structures of the lamina terminalis and of the wall of AV3V region, e.g., subfornical organ (SFO), organum vasculosum laminae terminalis (OVLT) and dorsal and ventral preoptic median nucleus (MnPOd, MnPOv) and hypothalamic structures (Antunes-Rodrigues and Covian 1963, Covian and Antunes-Rodrigues 1963, Miselis 1981, Denton 1984, Schulkin 1991, Johnson and Thunhorst 1997, Fitzsimons 1998, Thunhorst et al. 1999, McCann et al. 2003, McKinley et al. 2003, Antunes-Rodrigues et al. 2004, McKinley and Johnson, 2004, Weisinger et al. 2004). Central integration of the excitatory and inhibitory inputs for the control of sodium appetite requires interaction among neural circuits of the lamina terminalis and, amigdala, septal area and structures of the brainstem (Denton 1984, Fitzsimons 1998, Antunes-Rodrigues et al. 2004). AT1 subtype receptors for ANG II were identified in those structures and icv micro injection of losartan, an AT1 antagonist, has been related to the decrease in the sodium and water ingestion. Moreover, ANG II neurons, located in the lamina terminalis, co-express ACE, especially in OVLT and SFO (Blair-West et al. 1994, Fitzsimons 1998, McKinley et al. 2003, Saavedra 2005). Functional evidence of ANG I-ANG II conversion into the SFO was obtained after icv or intra-SFO administration of captopril. It has abolished the effects produced by systemic low dose of captopril (Thunhorst et al. 1989). Therefore, it was suggested that SFO contains an intrinsic RAS, constituting in this way a relay that connects the brain with peripheral RAS for mediating of the hydro electrolytic balance and to provide a drive mechanism for the sodium appetite regulation (Fitzsimons 1998, Weisinger et al. 2004). Recent evidences clearly showed that SFO constitutes the center of the sodium appetite behavior where sodium concentration is inherently monitored by Na+\text x channel sensing mechanism (Hiyama et al. 2004). The efferent SFO projections are especially directed to two areas: preoptic area of the AV3V region where terminals are distributed, particularly in MnPO, periventricular nucleus, (PeVN) and OVLT. Moreover, endings from the SFO neurons project to hypothalamus where they synapse with magnocellular neurons of the SON and PVN.

Despite of some controversy, it is admitted the involvement of SFO AT1 receptors on the natriorexigenic response induced by volume depletion (Fitzsimons 1998, Menani et al. 1998b, McKinley et al. 2003). Both, SFO and OVLT, are CVOs, therefore they lack blood-brain barrier (BBB). This functional property provides free access for some humoral signals that are recognized and monitored. ANG II icv administration as well as paradigms that evoke volume depletion, stimulate the sodium appetite. In both conditions there is an increase of brain angiotensinergic activity (Fitzsimons 1998).Action of ANG II in SFO also induces enhance of the blood pressure, reaction that would be associated to the dipsogenic and sodium appetite responses in parallel to physiological contexts (Fitzsimons 1998). Systemic or per os administration of low dose of captopril increases circulating ANG I levels. In this condition, the ACE inhibitor just acts peripherally. Therefore, the increase of brain ANG I availability provides the manifestation of the sodium appetite through mechanism that requires central conversion of ANG I to ANG II (El font et al. 1984, Moe et al. 1984, Fitzsimons 1998, Ventura et al. 2001, Badauê-Passos et al. 2003).

Hypothalamus and the lamina terminalis, structures related to the hydro electrolytic regulation, also express receptors and binding sites for ANP and, therefore, its participation has been widely postulated for the control of sodium excretion and salt ingestion (Quirion 1989, Brown and Czarnecki 1990, Saavedra and Kurihara 1991, Gutkowska et al. 1997, McCann et al. 2003, Antunes-Rodrigues et al. 2004). Furthermore, the existence of ANPergic neurons is hypothesized in parallel to the angiotensinergic neurons, which sub serve the water and electrolyte balance (Antunes-Rodrigues et al. 1991, 2004, Reis et al. 1994, McCann et al. 2003). When ANP is administered by icv route, it inhibits the preference for saline in sodium-depleted rats (Antunes-Rodrigues et al. 1986). Additionally, the central injection of ANP attenuates the exaggerated sodium appetite in spontaneously hypertensive rats (Itoh et al. 1986). From these studies, it postulated that inhibitory action of ANP is probably mediated by neurons of the same structures which are excited by ANG II.

Binding sites for OT were also identified in hypothalamus and lamina terminalis structures related to regulation of the sodium appetite (Johnson and Thunhorst 1997, McCann et al. 2003, Antunes-Rodrigues et al. 2004). Furthermore, the evidence that neural O Trelease in SFO alters the excitability of ANG II-sensitive neurons, raises the possibility that the oxytocinergic transmission modulates the ANG II actions (Hosono et al. 1999, Daniels and Fluharty 2004). In this context, icv administration of OT induces a decrease of salt ingestion in sodium-depleted rats (Stricker and Verbalis 1996).Reinforcing this observation was demonstrated by the fact that central administration of OT antagonist attenuates the inhibition of salt ingestion induced by OT in rats submitted to the increase of the plasma osmolality (Blackburn et al. 1995, Stricker and Verbalis 1996, Johnson and Thunhorst 1997, Fitts et al. 2003, Daniels and Fluharty 2004). This comprehension was expanded with the evidence that increased OT gene expression induces a reduced sodium appetite response and, that sodium consumption challenged by volume depletion increases the Fos protein expression in oxytocinergic neurons of PVN in rats (Franchini and Vivas 1995, Franchini et al. 2003). Finally, it was demonstrated that knock-out mice for the OT gene express exaggerated sodium appetite after fluid deprivation or following the hypovolemia induced by sc injection of hyperoncotic substance (Amico et al. 2001, Rigatto et al. 2003).

 

THE MIDBRAIN 5-HTergic SYSTEM

TOPOGRAPHICAL ORGANIZATION OF THE MIDBRAIN DORSAL RAPHE NUCLEUS AND THE CONTROL OF THE 5-HTERGIC NEUROTRANSMISSION

5-HTergic neurons of the midbrain raphe constitute a small group of multi polar cells that are distributed in the midline of the brainstem wherein are particularly located the median (MRN) and dorsal raphe (DRN) nuclei (Azmitia and Segal 1978, Parent et al. 1981, Azmitia 2001, Abrams et al. 2004). For the context of this work we will refer to DRN that is the most prominent of the 5-HTergic nuclei from the midbrain. The DRN is situated in the ventral part of the periaqueductal gray matter of midbrain, extending caudally until the rostral portion of the pons. The 5-HTergic neurons of DRN are organized in clustered cells, in several topographical subdivisions. Among them, the rostral ventromedial area is the one that possesses contingent of cells that project towards forebrain sites implicated with the hydro electrolyte and cardiovascular regulation (Azmitia and Segal 1978, Bosler and Descarries 1988, Jacobs and Azmitia 1992, Azmitia 2001). The ascending projections of DRN are arranged in ubiquitous way in connection to an extensive collaborative distribution in the terminal fields (Azmitia 1987, 2001). According to Azmitia conception, multiple subsets of 5-HTergic neurons participate of the integrated coordination of different control systems, e.g., autonomic, neuroendocrine and behavioral (Azmitia 1987, 2001, Jacobs and Azmitia 1992). This understanding has been resettled in recent report. Electrophysiological study shows that different subtypes of 5-HTergic neurons are implied on exclusive behavioral patterns. This observation allowed to postulate the hypothesis that subpopulation of 5-HTergic neurons topographically organized would contain exclusive functional properties associated with the modulation of specific forebrain systems (Abrams et al. 2004).

5-HTergic neurons synthesize serotonin (5-HT)from the amino acid tryptophan (Azmitia 1987, 2001, Tyce 1990, Boadle-Biber 1993). 5-HTergic transmission and subsequent synaptic 5-HT release take place according to a rhythmic pattern which is spontaneously generated and can be modulated by a somatodendritic auto-feedback (Aghajanian et al. 1987, Azmitia 1987, 2001). This fine system of modulation is mediated through the activation of 5-HT1A somatodendritic auto receptors (SDAR). 5-HT action on this receptor evokes a decrease of the neuronal firing rate and subsequent decrease of the 5-HT turnover. Therefore, the 5-HT concentration in the somatodendritic synapse increases. A proportional decrease on its release in the terminal fields of the ascending 5-HTergic neurons of DRN takes place (Sprouse and Aghajanian 1987, Hutson et al. 1989, Invernizzi et al. 1991).

FOREBRAIN STRUCTURES INVOLVED WITH THE HYDROELECTROLYTE REGULATION ARE INNERVATED BY ASCENDING 5-HTERGIC NEURONS FROM THE DRN

Especially the DRN contains subsets of 5-HTergic neurons which projects toward forebrain areas related to regulation of the electrolyte composition, ECF volume, as well as cardiovascular response (Azmitia and Segal 1978, Bosler and Decarries 1988, Azmitia 2001).Among them, the lateral hypothalamic area (AHL), SFO, OVLT, MnPOd, MnPOv, PVN and SON are the more prominently innervated by 5-HTergic neurons.Neurons that produce 5-HT constitute the first neuronal system to innervate the primordial cortical plate. 5-HTergic neurons of the midbrain represent cellular group among the first neurons to undergo differentiation in the brain and to play a key role in the neurogenesis (Azmitia 2001). 5-HTergic neurons interact with multiple cellular types and subtypes of receptors (Azmitia 1987, 2001, Hoyer et al. 2002). Furthermore, the ubiquitous distribution of terminals and the diversity of forebrain 5-HT receptors has allowed to elaborate the hypothesis that 5-HTergic system would interfere practically in all of the integrative mechanisms of neuronal plasticity, in nature, autonomic, neuroendocrine, behavioral and cognitive (Azmitia and Segal 1978, Azmitia 1987, 2001, Jacobs and Azmitia 1992).

FOREBRAIN STRUCTURES INVOLVED WITH THE HYDROELECTROLYTE REGULATION INTERACT RECIPROCALLY WITH THE MIDBRAIN 5-HTERGIC NEURONS: BIDIRECTIONAL CIRCUITS SFO-DRN-SFO

Forebrain structures involved with the hydro electrolyte and cardiocirculatory control reciprocally innervate the midbrain raphe (Lind 1986, Tanaka et al. 1998, 2001, Celada et al. 2002). Contingent of ANG II-sensitive neurons from SFO project toward 5-HTergic neurons of DRN (Tanaka et al. 1998). This assessment was carried out in study in which neurons of DRN, antidromically identified, were excited by intra-carotid or iontophoretic administration of ANG II into SFO. The authors concluded that ANG II-sensitive neurons from the SFO monitor the ANG II circulating levels and then transmit this information toward DRN. This same group demonstrated that hemorrhage (during which there is an increase of plasma ANG II levels) excites 5-HTergic neurons from the DRN which project fibers toward SFO, where an increase of 5-HT turnover was further detected (Tanaka et al. 2001). As such, increasing plasma ANG II levels also reflect the decrease in the ECF volume raised by the hyponatremia and it is plausible, additionally, to admit that circulating volume restoration is correlated with alterations on the DRN 5-HT turnover. In this context, multiple subtypes of 5-HT receptors, e.g., 5-HT1A, 5-HT2A, and 5-HT2C were pharmacologically identified in SFO (Scrogin et al. 1998). In this study, the authors reasoned that 5-HT release in SFO could be related to the control of vasopressin secretion. However, considering the role of SFO in the sodium appetite expression and AT1 receptors identification over there, the hypothesis of 5-HTergic transmission in this structure to influence the angiotensinergic activity must not be excluded. Tanaka's group has evidenced through the microdialysis that micro injection of ANG II into SFO reduces the extra cellular 5-HT and its metabolites levels (Tanaka et al. 2004).

Moreover, a possible feed-back loop between SFO and raphe is plausible. This assertive is based on observation of ANG II-binding sites and AT1 receptors identified in the DRN (Song et al. 1992, Moulik et al. 2002). It is possible an interaction between blood borne ANP or synaptically ANP released at DRN. Experimental changes in water and salt homeostasis influence the levels of ANP in the DRN (Palkovits et al. 1990). A significant decrease of ANP levels was detected in the DRN after the development of hypertension in SHR rats in comparison to normotensive controls (Bahner et al. 1988).

These demonstrations reinforce the possibility that these forebrain and midbrain structures reciprocally relay information during homeostatic challenges for the regulation of the water and salt ingestion and cardiocirculatory adjustment as glimpsed by Lind (Lind 1986).

Taken together, these observations above constitute circumstantial evidences of the existence of a neuronal circuit SFO-DRN-SFO that must be implicated in the regulation of hydro electrolytic and cardiovascular balance as previously proposed (Lind 1986, Reis et al. 1994, Cavalcante-Lima et al. 2005a, b). Additionally, it should not be ruled out the possibility of other reciprocal interactions between forebrain areas and midbrain raphe, particularly among OVLT, MnPO and AHL and DRN. These speculations from the experimental observations performed by Tanaka, raise the possibility that other humoral signals (e.g., plasma levels of ANP and OT) as well as the neural release of these peptides in structures of the lamina terminalis, integrate monitoring systems of the hydro electrolytic status.

The neuroanatomical relationship mentioned before presuppose the elaboration of complex functional interactions between forebrain and midbrain structures. It may concern the control of autonomic, neuroendocrine and behavioral functions during homeostatic challenges presented by environment.

 

5-HTergic SYSTEM AND RENAL SODIUM EXCRETION REGULATION

EXPERIMENTAL APPROACHES OF THE 5-HTERGIC SYSTEM

To evaluate the role of 5-HTergic system in the renal sodium