Women are twice as likely as men to suffer from depression and anxiety disorders and have an increased risk of onset during periods associated with hormonal changes, such as the postpartum period and the menopausal transition. Furthermore, some women seem more sensitive to normal hormone fluctuations across the menstrual cycle, since approximately 3-5% suffers from premenstrual dysphoric disorder (PMDD). Why these disorders are so common in women has not been established but there is a probable involvement of the ovarian hormones. The aim of this thesis was to investigate the effect of the ovarian hormones on the female brain during different reproductive states using psychological tests known to affect brain activity in different ways. Paper one examined the effect of the ovarian hormones on prepulse inhibition (PPI) on the acoustic startle response (ASR) and comprised cycling women and postmenopausal women. The cycling women had lower levels of PPI compared to postmenopausal women and postmenopausal women with moderate estradiol levels had lower PPI compared to postmenopausal women with low estradiol levels. Paper two examined the effect of anticipation and affective modulation on the ASR in women with PMDD and healthy controls. Women with PMDD have an increased modulation during anticipation of affective pictures compared to healthy controls during the luteal phase of the menstrual cycle. Paper three examined brain activity during response inhibition among women with PMDD and healthy controls by the use of a Go/NoGo task and fMRI. Women with PMDD displayed a decreased activity in the left insula during follicular phase and an increased activity during the luteal phase compared to controls. Paper four comprised women in the postpartum period and non-pregnant controls to examine brain activity during response inhibition. While this study revealed decreased activity at 4 weeks postpartum compared to 48 hours postpartum we cannot ascertain the role of the ovarian steroids, since none of the significant brain areas correlated with ovarian steroid or neurosteroid serum concentrations. The results of this thesis demonstrate that the ovarian hormones, or at least various hormonal states, have a probable impact on how the female brain works.

Background: The learning perspective of panic disorder distinguishes between acute panic and anxious apprehension as distinct emotional states. Following animal models, these clinical entities reflect different stages of defensive reactivity depending upon the imminence of interoceptive or exteroceptive threat cues. The current study tested this model by investigating the dynamics of defensive reactivity in a large group of patients with panic disorder and agoraphobia (PD/AG). Methods: Three hundred forty-five PD/AG patients participated in a standardized behavioral avoidance test (being entrapped in a small, dark chamber for 10 minutes). Defense reactivity was assessed measuring avoidance and escape behavior, self-reports of anxiety and panic symptoms, autonomic arousal (heart rate and skin conductance), and potentiation of the startle reflex before and during exposure of the behavioral avoidance test. Results: Panic disorder and agoraphobia patients differed substantially in their defensive reactivity. While 31.6% of the patients showed strong anxious apprehension during this task (as indexed by increased reports of anxiety, elevated physiological arousal, and startle potentiation), 20.9% of the patients escaped from the test chamber. Active escape was initiated at the peak of the autonomic surge accompanied by an inhibition of the startle response as predicted by the animal model. These physiological responses resembled the pattern observed during the 34 reported panic attacks. Conclusions: We found evidence that defensive reactivity in PD/AG patients is dynamically organized ranging from anxious apprehension to panic with increasing proximity of interoceptive threat. These data support the learning perspective of panic disorder.

Alcohol abuse disorders (AUD) represent a serious worldwide health problem with far reaching social, financial, and interpersonal implications. One of the most devastating facets of these disorders is the propensity to relapse following periods of abstinence. Ethanol withdrawal (EWD) is believed to promote relapse by increasing anxiety and craving, and may contribute to the development of cognitive decline associated with long-term dependence. Clinical data suggest that stress also plays a main role in both the development of AUD as well as relapse to drinking. As a physiological stressor, EtOH elevates levels of stress hormones (cortisol in humans, corticosterone (CORT) in the rat). Both CORT and EtOH have been shown to alter the composition, function, and activity of the N-methyl-D-aspartate (NMDA) receptor, and in particular, the NR2B subunit of this receptor. These alterations have been suggested to mediate EWD, which may negatively impact abstinence rates. This synergistic interaction between EtOH and CORT may present a therapeutic target for the treatment of EWD. In fact, data suggest that blocking the glucocorticoid receptor, which is a main target for CORT, with RU-486 could promote abstinence, as treatment with the drug has been shown to reduce consumption and the development dependence, as well as the severity of EWD and the cognitive deficits following EWD. However, these latter effects have not been validated in models of voluntary EtOH consumption. As there is considerable evidence that active versus passive intake can significantly impact neuroadaptations to ethanol this is an important consideration. These studies sought to characterize consumption and evaluate the development of dependence in a chronic voluntary model of intermittent access (IA) to EtOH. CORT plasma levels and protein expression of the glucocorticoid and NR2B receptors were measured during and/or following exposure. Finally, to assess the role of CORT in EtOH consumption and the development of dependence, the glucocorticoid receptor antagonist ORG-34517 was administered during access to EtOH. IA access to 20% EtOH produced varying levels of consumption (2.0-6.7g/kg/24hr exposure) and blood EtOH levels (6.3-116.9 mg/dl), but did not significantly affect food consumption or weight gain. Baseline CORT levels were found to be predictive of subsequent EtOH consumption and levels of consumption were sufficient to elevate CORT levels following one hour of EtOH exposure. Further, IA to EtOH was sufficient to produce dependence, as measured by elevations in the acoustic startle reflex following 26 hours and five days of withdrawal. No alteration in protein expression was observed regarding either the NR2B or glucocorticoid receptors and exposure to ORG-34517 had no effect on consumption or withdrawal.

Background. Various studies show that cannabis use alters attention and cognitive functioning in healthy humans and may contribute to development of schizophrenia or worsening of pre-existing psychosis. However, the impact of cannabis use on brain function in humans is not well understood. Schizophrenia is associated with a deficit in prepulse inhibition (PPI), the normal inhibition of the startle reflex by a non-startling stimulus (prepulse), presented before the startle stimulus at short time intervals (lead-time intervals). Such PPI deficit is thought to reflect a sensorimotor gating dysfunction in schizophrenia. PPI is also modulated by attention and PPI reduction in schizophrenia is observed when patients are asked to attend to, not ignore, the stimuli producing PPI. The aim of the current study was to investigate the association between self-reported chronic cannabis use and attentional modulation of PPI in healthy controls and in patients with schizophrenia. Furthermore, the association between cannabis use and other startle reflex modulators, including prepulse facilitation (PPF) of the startle reflex magnitude at long lead-time intervals, prepulse facilitation of the startle reflex onset latency and habituation of the startle reflex magnitude, were examined. Method. Auditory-evoked electromyographic signals were recorded from orbicularis oculi muscles in chronic cannabis users (29 healthy controls and 5 schizophrenia patients) and non-users (22 controls and 14 patients). The data for 36 participants (12 non-user controls, 16 healthy cannabis users, and eight non-user patients) were used in the final analyses and the patient data were used as a pilot study, because relatively few participants met the rigorous exclusionary criteria. Participants were instructed to attend to or to ignore either the startle stimuli alone (70 100 dB) or prepulse (70 dB) and startle stimuli (100 dB) separated by short lead-time intervals (20 200 ms) and long lead-time intervals (1600 ms). In order to ignore the auditory stimuli the participants played a visually guided hand-held computer game. A pilot study showed that the response component of playing the game had no effects on attentional modulation of the startle reflex magnitude and onset latency. Results. Relative to controls, cannabis use in healthy humans was associated with a reduction in PPI similar to that observed in schizophrenia while attending to stimuli, and with an attention-dependent dysfunction in the startle reflex magnitude habituation. While ignoring the stimuli there were no statistical differences in PPI between cannabis users and controls, although PPI in cannabis users tended to differ from that of the patients. The reduction in PPI in cannabis users was correlated with the increased duration of cannabis use, in years, but not with the concentration of cannabinoid metabolites in urine or with the recency of cannabis use in the preceding 24 hours. Furthermore, cannabis use was not associated with any differences in PPF, onset latency facilitation, and startle reflex magnitude in the absence of prepulses. The accuracy of self-reports of substance use was also investigated in this study and was found to be excellent. In addition, the study examined the validity of the substance use module of the diagnostic interview, CIDI-Auto 2.1, which was found to be acceptable for cannabis misuse diagnoses (abuse and/or dependence). Finally, cannabis dependence was found to be associated with more diagnoses of mental illness other than schizophrenia (mainly depression). Conclusions. The results of the current study suggest that chronic cannabis use is associated with schizophrenia-like deficit in PPI in otherwise healthy humans. This PPI reduction is associated with attentional impairment rather than a global sensorimotor gating deficit in healthy cannabis users.

A ketamina, antagonista não competitivo de receptores NMDA, apresenta potentes efeitos psicomiméticos, sendo capaz de acentuar o estado psicótico de pacientes esquizofrênicos. Uma das áreas cerebrais afetadas por seu uso é o córtex pré-frontal, já que o desempenho em tarefas que dependem de sua atividade é profundamente alterado pela administração aguda de ketamina. Assim como na esquizofrenia, estas alterações podem sofrer influência de fatores hormonais, alterações estas que podem ser explicadas pelos efeitos dos hormônios sexuais femininos, como o estrogênio, os quais apresentam um papel regulador sobre os sistemas dopaminérgicos, serotonérgicos, glutamatérgicos e GABAérgicos, todos afetados pelos efeitos agudos e crônicos do uso de ketamina. Este projeto, portanto, teve como meta avaliar os possíveis efeitos da administração crônica e retirada de ketamina sobre a expressão de comportamentos relacionados à ansiedade humana em ratas da linhagem Wistar, assim como a influência dos hormônios estradiol e a progesterona sobre esta variável. As possíveis alterações farmacológicas induzidas pela administração crônica de ketamina sobre os sistemas dopaminérgicos e serotoninérgicos da divisão pré-límbica (PrL) do córtex pré-frontal medial serão avaliadas através da injeção local de antagonista/agonista específicos. Nossos dados reforçam a ideia de que a ketamina demonstra de forma significativa a expressão da resposta aprendida de medo. E também, os dados mostram que a abstinência da droga altera este comportamento, particularmente a capacidade cognitiva relacionada ao encadeamento de estímulos. Da mesma forma que outras drogas de abuso, estas alterações parecem envolver tanto o sistema dopaminérgico quanto serotoninérgico do CPFm. / Ketamine, a non-competitive antagonist of NMDA receptors, has potent psychomimetic effects, being able to accentuate the psychotic state of schizophrenic patients. One of the brain areas affected by its use is the prefrontal cortex, since performance in tasks that depend on its activity is profoundly altered by the acute administration of ketamine. As in schizophrenia, these changes may be influenced by hormonal factors, which can be explained by the effects of female sex hormones, such as estrogen, which play a role in the dopaminergic, serotonergic, glutamatergic and GABAergic systems, all affected acute and chronic effects of ketamine use. This project therefore aimed to evaluate the possible effects of chronic administration and withdrawal of ketamine on the expression of behaviors related to human anxiety in Wistar rats, as well as the influence of the hormones estradiol and progesterone on this variable. The possible pharmacological changes induced by chronic ketamine administration on the dopaminergic and serotonergic systems of the prelambial (PrL) division of the medial prefrontal cortex will be assessed by specific local antagonist / agonist injection. Our data reinforce the idea that ketamine demonstrates significantly the expression of the learned response of fear. Also, the data show that drug abstinence alters this behavior, particularly the cognitive capacity related to the chaining of stimuli. Like other drugs of abuse, these changes appear to involve both the dopaminergic and serotonergic system of CPFm.

O desequilíbrio da homeostase emocional tem sido considerado como um mecanismo subjacente aos transtornos de ansiedade e humor. Em fêmeas, as alterações na secreção hormonal durante as diferentes fases do ciclo estral podem ser a base das alterações na reatividade emocional a eventos estressantes. Estudos comportamentais sobre diferenças sexuais no processamento das emoções mostram resultados conflitantes em fêmeas devido às dificuldades na seleção dos melhores modelos animais para testar as diferenças associadas ao ciclo estral. Uma vez que os testes comportamentais foram desenvolvidos em animais do sexo masculino, eles podem não ser apropriados para fêmeas. O presente estudo foi desenvolvido para contribuir nessa linha de pesquisa usando diferentes modelos de animais de medo incondicionado e condicionado, considerando as diferentes fases do ciclo estral das ratas. Comparou-se o desempenho de machos e fêmeas nas quatro fases do ciclo estral em dois testes de medo incondicionado: o switch-off, em que ratos cruzam uma caixa vai-e-vem para desligar uma luz aversiva, e o registro de vocalizações ultrassônicas (VUSs) a 22 kHz emitidos por animais sob o estresse agudo de restrição. Nos testes de medo condicionado, registrou-se o sobressalto potencializado pelo medo e a resposta decongelamento a um contexto aversivo. Em ambos os testes de medo condicionado, a reatividade emocional não se mostrou diferente entre os sexos. No entanto, no que diz respeito ao medo incondicionado, ratas em diestro tardio apresentaram maior reatividade emocional em desligar a luz intensa e maior emissão de VUSs em resposta à restrição em relação a outras fases do ciclo. Estes achados sugerem que o perfil hormonal durante a fase do diestro 2 pode aumentar a reatividade emocional de ratas frente a estímulos inatos, porém não àqueles aprendidos. / Dysfunctional emotional regulation has been implicated as a potential mechanism underlying anxiety and mood disorders. Changes in hormonal secretion during the different phases of the estrous cycle may underlie changes in emotional reactivity to stressful events in female animals. Previous behavioral studies of sex differences in emotion processing in females have yielded conflicting results. This may be due to the range of different behavioral tests used and difficulties in selecting the best animal models to test for estrous cycle-linked differences in responsiveness. Furthermore, the commonly used behavioral tests were developed in male animals and it may not be appropriate to translate directly the protocols from males to females. In the present study we have attempted to address these problems by using different animal models of anxiety based on tests for unconditioned or conditioned fear. We compared the performance of male rats and female rats at four stages of the estrous cycle defined by differences in vaginal cytology. To test for unconditioned fear, we used two tests: a light switch- off test, in which rats escape to the other compartment of a shuttle-box to turn off an aversive light and recordings of 22 kHz ultrasound vocalizations (USVs) during acute restraint stress. For the conditioned fear paradigm, we used fear potentiated startle in an aversive context and conditioned freezing using an aversive context as the conditioned stimulus. In both tests of conditioned fear there were no gender or estrous cycle-linked differences in emotional reactivity. However, with respect to unconditioned fear, female rats in late diestrus showed greater emotional reactivity expressed as switch-off responses to a light environment and USVs in response to restraint compared to other phases of the cycle. These findings suggest that the hormonal profile during the late diestrous phase may predispose to up-regulated emotional reactivity in rats facing emotional challenges to unconditioned, but not conditioned fear- inducing stimuli.

Rationale and objective: The adenosine A(2A) receptor forms a mutually inhibitory heteromer with the dopamine D2 receptor, and A(2A) agonists decrease D2 signaling. This study analyzed whether an adenosine A(2A) agonist would alleviate deficits in sensorimotor gating and increases in cyclic-AMP response element binding protein (CREB) in the nucleus accumbens (NAc) in the neonatal quinpirole model of schizophrenia (SZ). Methods: Male and female Sprague-Dawley rats were neonatally treated with saline (NS) or quinpirole HCl (NQ; 1 mg/kg) from postnatal days (P) 1–21. Animals were raised to P44 and behaviorally tested on auditory sensorimotor gating as measured through prepulse inhibition (PPI) from P44 to P48. Approximately 15 min before each session, animals were given an ip administration of saline or the adenosine A(2A) agonist CGS 21680 (0.03 or 0.09 mg/kg). One day after PPI was complete on P49, animals were administered a locomotor activity test in the open field after saline or CGS 21680 treatment, respectively. On P50, the nucleus accumbens (NAc) was evaluated for CREB protein. Results: NQ-treated rats demonstrated a deficit in PPI that was alleviated to control levels by either dose of CGS 21680. The 0.03 mg/kg dose of CGS 21680 increased startle amplitude in males. The 0.09 mg/kg dose of CGS 21680 resulted in an overall decrease in locomotor activity. NQ treatment significantly increased NAc CREB that was attenuated to control levels by either dose of CGS 21680. Conclusions: This study revealed that an adenosine A(2A) receptor agonist was effective to alleviate PPI deficits in the NQ model of SZ in both male and female rats.

Posttraumatic stress disorder (PTSD) is an anxiety disorder that may follow exposure to severe emotional trauma and presents with various symptoms of anxiety, hyperarousal and cognitive anomalies. Interestingly, only 10-30% of an exposed population will go on to develop full-blown PTSD. Cholinergic neurotransmission is implicated in anxiety as well as other typical manifestations of PTSD, particularly cognitive changes. The frontal cortex and hippocampus regulate and in turn are affected by stress, and have also been implicated in the underlying neuropathology of PTSD. These areas are densely innervated by cholinergic neurons originating from the basal forebrain. In this study, the time dependent sensitization (TDS) model was used to induce symptoms of PTSD in animals. The study was designed to determine the long-term effects of an intense, prolonged aversive procedure on central muscarinic acetylcholine receptor (mAChR) characteristics and the correlation if any of those findings to cognitive aspects and general arousal as characteristics associated with PTSD. In order to achieve this goal, male Sprague-Dawley rats were exposed to the TDS stress paradigm with behavioral/neuro-receptor assessments performed on day 7 post re-stress (duration of each experiment in whole is 14 days). Acoustic startle reflex (ASR) was used to determine emotional state (hyperarousal), while the conditioned taste aversion (CTA) paradigm was implemented in order to assess aversive memory. Muscarinic receptor binding studies were performed in the frontal cortex and hippocampus. Moreover, both the stress-exposed and control animals were pre-tested in the acoustic startle chamber in order to attempt to separate stress sensitive from stress-resilient animals based on predetermined ASR criteria. The ASR niodel was previously validated in our laboratory, while the CTA model was validated in this project before application. In the CTA model, an i.p. injection with lithium chloride (LiCl) (associated with digestive malaise), was used as unconditioned stimulus (US) and was paired with a saccharinlcyclamate drinking solution as conditioned stimulus (CS) to induce aversion to the novel taste (CS) when presented in the absence of the US. Population data of animals tested in the ASR experiment indicated no statistical significant difference between stressed and control animals. However, when each animal was assessed individually, 22.5 % of the exposed population displayed all increase above the predetermined criteria of 35 % in startle response, indicating a state of heightened arousal. In contrast, only 4.2 O h of control animals (no stress) displayed an increase in arousal based on the above mentioned criteria. Muscarinic receptor densities (Bm,) in the total population of animals exposed to stress showed a statistical significant increase in both the hippocampus and frontal cortex when compared to controls, with no changes in & values observed in either one of the areas. In the CTA experiment, TDS stress was implemented as US paired with a saccharinlcyclamate drinking solution as CS. An acute session of prolonged stress (as used in the TDS model) effectively induced aversion to a novel taste and a subsequent reminder of the stress (restress) paired with the CS sustained the acquire adversive memory. Furthermore, LiCl was reintroduced as US in order to assess the effect of prior exposure to two types of stress (acute and TDS) on subsequently acquired CTA memory. Prior exposure to acute stress had no significant effect on subsequently acquired aversive memory when measured either 3- or 7 days post-conditioning (CS-US). Stress-restress (TDS) exposure, however, indicated a significant decrease in aversive memory from 3- to 7 days post-conditioning (CS-US) as well as a significant decrease in aversive memory between the control- and the TDS group 7 days post-conditioning. The mAChR density (B,,) in the frontal cortex; but not in the hippocampus, was elevated at the same point in time (7 days post CS-US pairing) that CTA memory was impaired following TDS stress (stress-restress). Ultimately, these data support an association between altered cholinergic receptors and hyperarousallanxiety in an animal model of PTSD. The data also support the phenomenon of individual susceptibility to stress in animals that parallels that observed in humans exposed to severe trauma. Impaired aversive memory (CTA) is a consequence of prior exposure to TDS stress, but not acute stress, and is likewise mediated by an altered central cholinergic transmission displayed as an increase in mAChRs in the frontal cortex. The lack of studies regarding the influence of the cholinergic system in PTSD related behavior earns ,this project value as inimitable PTSD research. / Thesis (M.Sc. (Pharmacology))--North-West University, Potchefstroom Campus, 2007.

OLIVEIRA, A.R. Envolvimento de receptores dopaminérgicos da área tegmental ventral e do complexo basolateral da amígdala na aquisição e na expressão do medo condicionado. 2010. 93 f. Tese (Doutorado) Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo. O condicionamento Pavloviano é um dos paradigmas mais utilizados para estudar as bases biológicas das emoções, assim como da aprendizagem e memória. A dopamina (DA) é um dos principais neurotransmissores envolvidos na mediação de estados de medo e ansiedade. Um conjunto crescente de evidências dá suporte à hipótese de que a ativação da via mesocorticolímbica, proveniente de neurônios dopaminérgicos da área tegmental ventral (ATV), é particularmente sensível à estimulação aversiva. Entre as regiões inervadas por esta via, o complexo basolateral da amígdala (BLA) é um componente essencial dos circuitos neurais do medo condicionado. Assim, o presente estudo explorou o envolvimento de mecanismos DA da ATV e do BLA, através do uso de agonistas e antagonistas de receptores DA, na aquisição e expressão do medo condicionado à luz. Não houve efeito das drogas DA no sobressalto potencializado pelo medo (SPM), quando injetadas na ATV antes do condicionamento, indicando que os receptores DA da ATV não participam da aquisição do medo condicionado à luz. Ao contrário, quando injetado na ATV antes do teste, quimpirole (agonista D2) reduziu o SPM, enquanto as demais drogas não tiveram efeito. A administração de SCH 23390 (antagonista D1) no BLA não produziu efeitos no SPM, indicando que os receptores D1 do BLA não parecem envolvidos na expressão do SPM. Já a administração de sulpirida (antagonista D2) no BLA inibiu o SPM produzido pela luz. Além disso, a expressão do medo condicionado foi associada a um aumento do congelamento e dos níveis extracelulares de DA no BLA, ambos inibidos com a administração de quimpirole na ATV. A capacidade do quimpirole em diminuir o SPM e o congelamento condicionado parece ser resultado de sua ação em auto-receptores D2 da ATV. A ativação desses receptores diminui os níveis de dopamina em áreas que recebem terminações da via mesocorticolímbica. Os resultados com a sulpirida realçam a importância dos receptores D2 do BLA na expressão do medo condicionado Pavloviano. / OLIVEIRA, A.R. Involvement of dopaminergic receptors of ventral tegmental area and basolateral amygdala in the acquisition and expression of conditioned fear. 2010. 93 p. Thesis (Doctoral) Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo. The Pavlovian fear conditioning is one of the most used paradigms to study the biological basis of emotion, as well as of learning and memory. Dopamine (DA) is one of the most important neurotransmitters involved in mechanisms underlying states of fear and anxiety. A growing body of evidence supports the hypothesis that excitation of the mesocorticolimbic pathway, originating from DA neurons in the ventral tegmental area (VTA), is particularly sensitive to fear-arousing stimuli. Among the forebrain regions innervated by this pathway, the basolateral amygdala (BLA) is an essential component of the neural circuitry of conditioned fear. The present study explored the involvement of VTA and BLA DA receptors, using DA agonists and antagonists, in the acquisition and expression of conditioned fear to a light conditioned stimulus (CS). None of the drugs used produced significant effects on fear-potentiated startle (FPS) when injected in VTA before conditioning, indicating that VTA DA receptors are not involved in the acquisition of conditioned fear to a light-CS. In contrast, when injected before the test session, intra-VTA quinpirole (D2 agonist) significantly reduced FPS, whereas the other drugs had no effect. Intra-BLA SCH 23390 (D1 antagonist) did not produce significant effects on FPS, indicating that BLA D1 receptors do not appear to be involved in the expression of FPS. On the other hand, intra-BLA sulpiride (D2 antagonist) inhibited FPS produced by light-CS previously paired with footshocks. Also, conditioned fear was associated with increased freezing and DA levels in the BLA, both inhibited by intra-VTA quinpirole. Quinpirole\'s ability to decrease FPS and conditioned freezing may be the result of an action on VTA D2 presynaptic autoreceptors. The activation of those receptors decreases dopamine levels in terminal fields of the mesocorticolimbic pathway. Sulpirides results stress the importance of BLA D2 receptors in the fear-activating effects of the Pavlovian conditioning.

O aumento do reflexo de sobressalto na presença de um estímulo que tenha sido previamente pareado a choques nas patas é tomado como índice de medo e nomeado sobressalto potencializado pelo medo (SPM). O congelamento, interrupção de todos os movimentos observáveis, exceto aqueles associados com a respiração, também tem sido utilizado como índice de medo em ratos. Um crescente número de evidências sugere o envolvimento de mecanismos dopaminérgicos em diferentes aspectos da memória afetiva, como sua formação, evocação e expressão. No entanto, resultados sobre como e por meio de quais receptores os mecanismos dopaminérgicos influenciam o medo têm sido inconsistentes. O presente estudo examina o envolvimento dos receptores dopaminérgicos na aquisição e na expressão do medo condicionado à luz. Para isso, foram analisados os efeitos do antagonista D1, SCH 23390, do agonista D1, SKF 38393, do antagonista D2, sulpirida, e do agonista D2, quimpirole, no SPM e no congelamento. A atividade motora dos animais também foi avaliada no teste do campo aberto. SCH 23390, SKF 38393, sulpirida e quimpirole, administrados antes do condicionamento, não produziram efeitos no SPM, mas SCH 23390 diminuiu o congelamento. As administrações de SCH 23390, SKF 38393 e sulpirida antes do teste também não produziram efeitos no SPM e no congelamento. Quimpirole, em doses que agem em receptores pré-sinápticos, causou uma redução significativa no SPM e no congelamento, quando administrado antes do teste. A ação das drogas não foi devida a efeitos não-específicos uma vez que elas não produziram efeitos no teste do campo aberto. Os resultados sugerem que mecanismos dopaminérgicos devem estar envolvidos tanto na aquisição, quanto na expressão do medo condicionado à luz. Receptores D1 pós-sinápticos parecem participar da aquisição do congelamento condicionado à luz-CS, mas não do SPM. Por outro lado, receptores D2 pré-sinápticos parecem estar envolvidos na expressão do medo condicionado à luz-CS. / The increase in the startle reflex in the presence of a stimulus that has been previously paired to footshock is taken as an index of fear and named fear potentiated startle (FPS). Freezing behavior, a cessation of all observable movements, except those associated with respiration, has also been used as an index of fear in rats. A growing body of evidence has suggested that dopaminergic mechanisms are implicated in different aspects of affective memory, namely its formation, expression or retrieval. However, the results of studies that have examined how, and through which receptors, dopaminergic mechanisms influence fear have been inconsistent. This work is aimed at examining the involvement of dopaminergic receptors in the acquisition and expression of conditioned fear to ligth-CS. We evaluated the effects of systemic administration of the D1 antagonist, SCH 23390, the D1 agonist, SKF 38393, the D2 antagonist, sulpiride, and the D2 agonist, quinpirole before and after conditioning on FPS and freezing. The motor activity of the animals was also evaluated in an open field test. SCH 23390, SKF 38393, sulpiride and quinpirole, injected before conditioning sessions, did not produce any effect on FPS, but SCH 23390 decreased freezing. Injections of SCH 23390, SKF 38393 and sulpiride before testing session did not produce any effect on FPS or freezing. Quinpirole, injected at doses acting at presynaptic level, caused significant reduction in FPS and freezing, when injected before testing. Drugs’ action was not due to nonspecific effects since they had no effect in the open field test. Our findings indicate that DA mechanisms are involved in the acquisition and expression of conditioned fear using light-CS. Dopaminergic mechanisms mediated by postsynaptic D1 receptors seem to be involved in the acquisition of conditioned freezing to light-CS, but not in FPS. On the other hand, dopaminergic mechanisms mediated by presynaptic D2 receptors seem to be involved in the expression of conditioned fear to light-CS.