Organizaçäo neural de diferentes tipos de medo e suas implicaçöes na ansiedade / Neural organization of different types of fear: implications for the understanding of anxiety

A natureza das respostas de medo em animais expostos a situaçöes ameaçadoras depende da intensidade e da distância do estímulo aversivo. Esses estímulos podem ser potencialmente perigosos, distais ou proximais ao animal. Esforços têm sido feitos no sentido de identificar os circuitos neurais recrutados na organizaçäo das reaçöes defensivas a estas condiçöes aversivas. Neste artigo, sumarizamos evidências que associam os sistemas cerebrais de defesa ao conceito de medo-stress-ansiedade. Respostas de orientaçäo ao estímulo de perigo, à esquiva e à preparaçäo para o enfrentamento do perigo parecem estar associados à ansiedade. O giro do cíngulo e o córtex pré-frontal de um lado; o núcleo mediano da rafe, septo e o hipocampo de outro fazem parte dos circuitos cerebrais que integram essas respostas emocionais. No outro extremo, estímulos de medo que induzem formas ativas de defesa, mas pouco elaboradas, determinam estados emocionais de natureza diferente e parecem associadas a manifestaçöes elementares de medo. A substância cinzenta periaquedutal dorsal constitui o principal substrato neural para a integraçäo desses estados aversivos no cérebro. Comportamentos defensivos desse tipo säo produzidos pela estimulaçäo elétrica e química desta estrutura. A medida que os estímulos ameaçadores, potenciais e distais däo lugar a estímulos de perigo muito intensos ou säo substituídos por estímulos proximais de medo, ocorre uma comutaçäo (switch) dos circuitos neurais usualmente responsáveis pela produçäo de respostas condicionadas de medo para reaçöes defensivas com baixo nível de regulaçäo e organizaçäo que se assemelham aos ataques de pânico. Portanto, dependendo da natureza do evento estressor ou do estímulo incondicionado, o padräo de respostas defensivas orientadas e organizadas cede lugar a respostas motoras incoordenadas e incompletas. A amígdala e o hipotálamo medial podem funcionar como uma espécie de interface comutando os estímulos para os substratos neurais apropriados para elaboraçäo das respostas defensivas condicionadas ou incondicionadas

Interaction between NO and oxytocin: influence on LHRH release

Nitric oxide synthase (NOS)-containing neurons have been localized in various parts of the CNS. These neurons occur in the hypothalamus, mostly in the paraventricular and supraoptic nuclei and their axons project to the neural lobe of the pituitary gland. We have found that nitric oxide (NO) controls luteinizing hormone-releasing hormone (LHRH) release from the hypothalamus acting as a signal transducer in norepinephrine (NE)-induced LHRH release. LHRH not only releases LH from the pituitary but also induces sexual behavior.On the other hand, it is known that oxytocin also stimulates mating behavior and there is some evidence that oxytocin can increase NE release. Therefore, it occurred to us that oxytocin may also stimulate LHRH releave via NE and NO. To test this hypothesis, we incubated medial basal hypothalamic (MBH) explants from adult male rats in vitro. Following a preincubation period of 30 min, MBH fragments were incubated in Krebs-Ringer bicarbonate buffer in the presence of various concentrations of oxytocin. Oxytocin relesed LHRH at concentrations ranging from 0.1 nM to 1muM with a maximal stimulatory effect (P<0.001) at 0.1 muM, but with no stimulatory effect at 10 muM. That these effects were mediated by NO was shown by the fact that incubation of the tissues with NG-monomethyl-L-arginine (NMMA), a competitive inhibitor of NOS, blocked the stimulatory effects. Furthermore, the release of LHRH by oxytocin was also blocked by prazocin, an alpha1-adrenergic receptor antagonist, indicating that NE mediated this effect. Oxytocin at the same concentrations also increased the activity of NOS (P<0.01) as measured by the conversion of [14C]arginine to citrulline, which is produced in equimolar amounts with NO by the action of NOS. The release of LHRH induced by oxytocin was also accompanied by a significant (P<0.02) increase in the release of prostaglandin E2 (PGE2), a mediator of LHRH release that is released by NO. On the other hand, incubation of neural lobes with vaious concentrations of sodium nitroprusside (NP) (300 or 600 muM), a releaser of NO, revealed that NO acts to suppres (P<0.01) the release of oxytoxin. Therefore, our results indicate that oxytocin releases LHRH by stimulating NOS via NE, resulting in an increased release of NO, which increases PGE2 release that in turn induces LHRH release. Furthermore, the released NO can act back on oxytocinergic terminals to suppress the release of oxytocin in an ultrashort-loop negative feedback.

Regional distribution of Fos-like immunoreactivity in the rat brain after exposure to fear-inducing stimuli

Fos protein immunohistochemistry was used to identify the neural substrate of fear/anxiety. The structures activated by exposure of Long Evans male rats (280-300 g) to the elevated plus-maze, a widely used animal model of anxiety, were compared with those activated by chemical stimulation of two aversive areas of the brain, the dorsal periaqueductal gray matter and the medial hypothalamus. Three different patterns of activation were obtained: Pattern 1 resulted from microinjection of the excitatory amino acid kainate (60 pmol; N = 5) or of the GABA(A) receptor antagonist SR-95531 (16 pmol; N = 3) into the dorsal periaqueductal gray matter and consisted mainly of caudal structures; Pattern 2 was observed after kainate injection (60 pmol; N = 4) into the medial hypothalamus and had a predominantly prosencephalic distribution; Pattern 3 extended from rostral to caudal brain regions and was induced by microinjection of either SR-95531 (16 pmol; N = 1) or kainate (120 pmol; N = 3) into the medial hypothalamus, as well as by 15-min exposure to the plus-maze (N = 3). Control animals were either injected with saline into the MH (N = 3) or the PAG (N = 3) or were exposed for 15 s to the elevated plus maze (N = 3) and exhibited no significant labeling. These results further support the participation of periventricular structures in the regulation of fear and aversion.

Effect of third ventricular injection of beta-endorphin on the electrophysiological responses of some regions of endocrine hypothalamus.

Multiunit activity (MUA) of arcuate nucleus and cortical EEG were recorded in the regularly cycling female rats on the day of proestrous under urethane anaesthesia. The MUA was compared before and after injection of beta-endorphin in third ventricle. In some animals MUA increased after 30-40 min and persisted for 3-4 hr, in others MUA got inhibited within 5-10 min of injection of beta-Endorphin and effect lasted for 5-6 hr. There was no change in frontoparietal EEG activity. In another group of animals medial pre-optic responses (MPO) to stimulation of medial amygdala were tested before and after ventricular infusion of beta-endorphin. Most of the facilitatory MPO responses got blocked. These observations suggest the involvement of opioid receptors in the mediation of neuroendocrine control of preovulatory events through the amygdalo-preoptico-medial basal hypothalamic axis. There seems to be heterogeneity of beta-endorphin receptors in the arcuate nucleus.

Effect of glucose on the activity of feeding centres before and after parasagittal cuts between medial and lateral hypothalamus.

Intracarotid administration of isotonic glucose (0.5 ml of 5.4%) in the starving albino rats produced an increase in the multiunit activity (MUA) of ventromedial hypothalamus (satiety centre) and a decrease in the MUA of the lateral hypothalamic area (feeding centre). Intracarotid infusion of normal saline did not change the MUA of any of these centres. Parasagittal knife cuts placed in between the satiety and feeding centres did not affect the responsiveness of these centres to glucose administration as indicated by the recorded multiunit activity.