A substância cinzenta periaquedutal dorsal (dPAG) e o hipotálamo medial (MH) são duas estruturas encefálicas que estão envolvidas na elaboração de estados aversivos e expressão de respostas defensivas. A estimulação elétrica da dPAG ou do MH produz uma série de respostas comportamentais que se assemelham às respostas defensivas induzidas pela presença de um predador. Esses mesmos comportamentos podem ser eliciados com a microinjeção local de agonistas glutamatérgicos nessas estruturas, indicando o envolvimento de aminoácidos excitatórios na expressão das respostas defensivas incondicionadas. Apesar disso, a participação destas estruturas no medo condicionado ainda é pouco conhecida. Assim, o objetivo deste estudo foi avaliar o envolvimento da mediação glutamatérgica da dPAG e de núcleos do MH núcleo anterior (AH) e núcleo pré-mamilar dorsal (PMd) na expressão do medo condicionado à luz. Para isso, foram avaliados os efeitos de agonistas e antagonistas glutamatérgicos (AMPA/Cainato e NMDA) administrados nessas estruturas no teste do sobressalto potencializado pelo medo (SPM) e na medida de congelamento condicionado. Ratos Wistar machos com cânulas-guias implantadas na dPAG, AH ou PMd foram submetidos ao condicionamento aversivo (pareamentos luz+choque). Vinte e quatro horas depois, esses animais receberam injeções intra-dPAG, AH ou PMd de NMDA ou ácido caínico (agonistas NMDA e AMPA/Cainato, respectivamente) ou AP7 ou NBQX (antagonistas NMDA e AMPA/Cainato, respectivamente) e foram submetidos ao teste do SPM. A resposta de congelamento condicionado foi avaliada na mesma sessão. Eventuais alterações motoras foram avaliadas no teste do campo aberto. A administração dos agonistas glutamatérgicos na dPAG promoveu efeitos pró-aversivos no SPM e congelamento condicionado. NBQX sozinho não produziu nenhum efeito significativo, ao passo que o AP7 diminuiu somente o congelamento condicionado. Entretanto, ambos os antagonistas bloquearam os efeitos dos respectivos agonistas. Já a administração dos agonistas e antagonistas glutamatérgicos no AH e PMd, em doses que não afetaram a atividade motora, não produziu efeitos significativos na resposta de congelamento condicionado e SPM. Os presentes resultados sugerem a participação de aminoácidos excitatórios da dPAG, mas não do MH, na expressão do medo condicionado à luz. / The dorsal periaqueductal gray matter (dPAG) and the medial hypothalamus (MH) are two brain structures that are involved in the elaboration of aversive states and expression of defensive responses. Electrical stimulation of the dPAG or MH produces a series of behavioral responses that resemble those defensive responses triggered in the presence of a predator. These same behaviors can be elicited with the local microinjection of glutamate agonists into these structures, indicating the involvement of excitatory amino acids in the expression of unconditioned fear responses. Nevertheless, the involvement of these structures in fear conditioning is still unknown. The objective of this study was to evaluate the involvement of glutamatergic mediation of the dPAG and MH nuclei anterior nucleus (AH) and dorsal pre-mammillary nucleus (PMd) in the expression of conditioned fear to the light. Thus, we evaluated the effects of glutamatergic agonists and antagonists (AMPA/Kainate and NMDA) administered into these structures in fear potentiated startle (FPS) and conditioned freezing responses to the light. Male Wistar rats with guide-cannulae implanted in the dPAG, AH or PMd were subjected to aversive conditioning (light+shock pairings). Twenty-four hours later, the animals were injected intra-dPAG, AH or PMd with NMDA or kainic acid (NMDA and AMPA/Kainate agonists, respectively) or AP7 or NBQX (NMDA and AMPA/Kainate antagonists, respectively) and were subjected to the FPS test. The conditioned freezing response was measured in the same session. Potential motor effects were evaluated with the open-field test. The administration of glutamate agonists into the dPAG promoted pro-aversive effects in the FPS and conditioned freezing. NBQX produced no significant effect per se, whereas AP7 only decreased conditioned freezing. Both antagonists blocked the effects of the respective agonist. On the other hand, the administration of glutamatergic agonists and antagonists into AH and PMd, in doses that did not affect motor activity, produced no significant effects on conditioned fear responses. The present results suggest the involvement of mechanisms mediated by excitatory amino acids of the dPAG, but not of the MH, in the expression of conditioned fear responses to light.

Sabe-se que o medo condicionado contextual (MCC) pode ativar diversas áreas mesencefálicas, como a Substância Cinzenta Periaquedutal Dorsal (SCPd). Diversos estudos avaliaram como mecanismos serotoninérgicos modulam o comportamento defensivo induzido pela estimulação elétrica da SCPd. Uma função proeminente dos receptores 5-HT2A é regular estados aversivos induzidos pela ativação da SCPd e, apesar de saber que experiências aversivas prévias exercem um importante papel em determinados tipos de ansiedade, somente estudos com estimulação da SCPd de ratos sem experiência aversiva prévia foram conduzidos até o momento. O objetivo do presente estudo foi investigar o papel funcional dos receptores 5-HT2A localizados nos substratos neurais da aversão na SCPd de ratos previamente submetidos ao condicionamento contextual aversivo. Foram avaliadas as respostas de congelamento e fuga obtidas a partir do procedimento de estimulação elétrica da SCPd na sessão teste, realizada 24 horas após o condicionamento, quando os animais foram recolocados no contexto previamente pareado com choques nas patas ou no contexto diferente. A função dos receptores 5-HT2A da SCPd foi avaliada pela injeção local de -metil-serotonina ou cetanserina, agonista e antagonista seletivos de receptores do tipo 5-HT2A, respectivamente. De acordo com estudos anteriores, a -metil-serotonina aumentou os limiares aversivos (efeito antiaversivo) determinados pela estimulação da SCPd em animais ingênuos, enquanto que a injeção de cetanserina não produziu qualquer efeito significativo. No presente estudo, a cetanserina reduziu o limiar de congelamento (efeito proaversivo), enquanto que a -metil-serotonina continuou apresentando efeito antiaversivo em animais com experiência aversiva prévia (choques nas patas). Estes resultados sugerem que experiências aversivas anteriores podem produzir importantes alterações na função sináptica dos receptores 5-HT2A da SCPd, podendo refletir na reatividade da SCPd à sua estimulação aversiva. / It has been shown that contextual fear conditioning (CFC) may activate brainstem regions such as the dorsal periaqueductal gray (dPAG). Several studies have been carried out to disclose how 5-HT2 mechanisms modulate the aversive stimulation of the dPAG. One prominent function of 5-HT2 receptors is to regulate the aversive states induced by activation of the dPAG. However, in spite of the notion that past stressful experiences play a crucial role in certain types of anxiety only studies with stimulation of the dPAG of rats without previous aversive experience have been conducted so far. We investigated the mediation of 5-HT2 receptors located in the neural substrates of aversion of the dPAG in rats previously submitted to CFC. Defensive behaviors induced by activation of the dPAG were assessed by measuring the lowest intensity of electric current applied to this structure (threshold) able to produce freezing and escape responses during testing sessions of CFC, in which animals were placed in a context previously paired to footshocks. The 5-HT2 function of the dPAG in this condition was evaluated by local injections of -methyl-5-HT and ketanserin, selective agonist and antagonist of 5-HT2 receptors, respectively. In accordance with previous studies, -methyl-5-HT increased the aversive thresholds (antiaversive effects) determined by stimulation of the dPAG in naive rats and injection of ketanserin into the dPAG did not produce any significant effects. On the other hand, ketanserin decreased the freezing threshold (proaversive effect) determined by the dPAG electrical stimulation while -methyl-5-HT continued to show antiaversive effects in animals with prior experience with foot shocks. The present results suggest that a past stressful experience can produce changes in the synaptic function of 5-HT2 receptors within the dPAG with important impact on the defensive behaviors induced by electrical stimulation of this area.

A substância cinzenta periaquedutal dorsal (dPAG) e o hipotálamo medial (MH) são duas estruturas encefálicas que estão envolvidas na elaboração de estados aversivos e expressão de respostas defensivas. A estimulação elétrica da dPAG ou do MH produz uma série de respostas comportamentais que se assemelham às respostas defensivas induzidas pela presença de um predador. Esses mesmos comportamentos podem ser eliciados com a microinjeção local de agonistas glutamatérgicos nessas estruturas, indicando o envolvimento de aminoácidos excitatórios na expressão das respostas defensivas incondicionadas. Apesar disso, a participação destas estruturas no medo condicionado ainda é pouco conhecida. Assim, o objetivo deste estudo foi avaliar o envolvimento da mediação glutamatérgica da dPAG e de núcleos do MH núcleo anterior (AH) e núcleo pré-mamilar dorsal (PMd) na expressão do medo condicionado à luz. Para isso, foram avaliados os efeitos de agonistas e antagonistas glutamatérgicos (AMPA/Cainato e NMDA) administrados nessas estruturas no teste do sobressalto potencializado pelo medo (SPM) e na medida de congelamento condicionado. Ratos Wistar machos com cânulas-guias implantadas na dPAG, AH ou PMd foram submetidos ao condicionamento aversivo (pareamentos luz+choque). Vinte e quatro horas depois, esses animais receberam injeções intra-dPAG, AH ou PMd de NMDA ou ácido caínico (agonistas NMDA e AMPA/Cainato, respectivamente) ou AP7 ou NBQX (antagonistas NMDA e AMPA/Cainato, respectivamente) e foram submetidos ao teste do SPM. A resposta de congelamento condicionado foi avaliada na mesma sessão. Eventuais alterações motoras foram avaliadas no teste do campo aberto. A administração dos agonistas glutamatérgicos na dPAG promoveu efeitos pró-aversivos no SPM e congelamento condicionado. NBQX sozinho não produziu nenhum efeito significativo, ao passo que o AP7 diminuiu somente o congelamento condicionado. Entretanto, ambos os antagonistas bloquearam os efeitos dos respectivos agonistas. Já a administração dos agonistas e antagonistas glutamatérgicos no AH e PMd, em doses que não afetaram a atividade motora, não produziu efeitos significativos na resposta de congelamento condicionado e SPM. Os presentes resultados sugerem a participação de aminoácidos excitatórios da dPAG, mas não do MH, na expressão do medo condicionado à luz. / The dorsal periaqueductal gray matter (dPAG) and the medial hypothalamus (MH) are two brain structures that are involved in the elaboration of aversive states and expression of defensive responses. Electrical stimulation of the dPAG or MH produces a series of behavioral responses that resemble those defensive responses triggered in the presence of a predator. These same behaviors can be elicited with the local microinjection of glutamate agonists into these structures, indicating the involvement of excitatory amino acids in the expression of unconditioned fear responses. Nevertheless, the involvement of these structures in fear conditioning is still unknown. The objective of this study was to evaluate the involvement of glutamatergic mediation of the dPAG and MH nuclei anterior nucleus (AH) and dorsal pre-mammillary nucleus (PMd) in the expression of conditioned fear to the light. Thus, we evaluated the effects of glutamatergic agonists and antagonists (AMPA/Kainate and NMDA) administered into these structures in fear potentiated startle (FPS) and conditioned freezing responses to the light. Male Wistar rats with guide-cannulae implanted in the dPAG, AH or PMd were subjected to aversive conditioning (light+shock pairings). Twenty-four hours later, the animals were injected intra-dPAG, AH or PMd with NMDA or kainic acid (NMDA and AMPA/Kainate agonists, respectively) or AP7 or NBQX (NMDA and AMPA/Kainate antagonists, respectively) and were subjected to the FPS test. The conditioned freezing response was measured in the same session. Potential motor effects were evaluated with the open-field test. The administration of glutamate agonists into the dPAG promoted pro-aversive effects in the FPS and conditioned freezing. NBQX produced no significant effect per se, whereas AP7 only decreased conditioned freezing. Both antagonists blocked the effects of the respective agonist. On the other hand, the administration of glutamatergic agonists and antagonists into AH and PMd, in doses that did not affect motor activity, produced no significant effects on conditioned fear responses. The present results suggest the involvement of mechanisms mediated by excitatory amino acids of the dPAG, but not of the MH, in the expression of conditioned fear responses to light.

O comportamento emocional tem sido considerado fundamental para a sobrevivência dos animais, sendo o medo uma se suas mais primitivas e importantes formas. A substância cinzenta periaquedutal dorsal (SCPD) tem-se destacado como uma estrutura importante na organização das respostas defensivas. Estudos usando estimulação elétrica e química da SCPD e microinjeções de drogas agonistas e antagonistas de receptores serotoninérgicos mostraram uma mediação serotoninérgica através dos subtipos de receptores 5-HT1A e 5-HT2A na regulação do comportamento defensivo organizado nesta estrutura. O objetivo deste trabalho foi examinar a mediação serotoninérgica na SCPD através de receptores 5-HT1A nas respostas defensivas de animais sem e com experi~encia aversiva prévia. Para isto, os limiares de congelamento e fuga foram determinados em ratos implantados com uma cânula acoplada a um eletrodo na SCPD, antes e após microinjeção local do agonista 8-OH-DPAT (4 e 8 nmol) e do antagonista WAY100635 (10 nmol). Os efeitos destas drogas injetadas na SCPD foram avaliados também sobre o tempo de congelamento pós-estimulação em animais ingênuos e também em animais colocados em um contexto no qual receberam choques inescapáveis nas patas 24h antes (medo condiconado contextual). O 8-OH-DPAT, de maneira dose dependente, produziu um efeito anti-aversivo sobre os limiares de congelamento e fuga em ratos sem experiência aversiva prévia, mas não nos animais com experiência aversiva prévia quando comparado com seus controles. Por outro lado, este agonista 5-HT1A reduziu o tempo de congelamento contextual. Já o WAY100635 não produziu alterações significativas sobre os limiares aversivos em ratos ingênuos ou com experiência aversiva prévia, mas elevou o tempo de congelamento contextual nestes aniamis (efeito pró-aversivo). Estes resultados estão em concordância com a idéia de uma modulação fásica exercida pela 5-HT sobre os substratos neurais do medo organizado na SCPD. Por outro lado, mecanismos mediados pelos receptores 5-HT1A não são alterados em animais com experiência aversiva prévia. Acreditamos que estes resultados trazem uma contribuição importante para a nossa compreensão sobre a integração de estados aversivos no SNC e, particularmente sobre o funcionamento destes substratos neurais de defesa na SCPD de animais com experiência aversiva prévia. / It is well established that 5-HT1 mechanisms modulate the defensive behavior produced by stimulation of the dorsal periaqueductal gray (dPAG). However, in spite of the notion that past stressful experiences play a role in certain types of anxiety only few studies with stimulation of the dPAG of rats without previous aversive experience have been conducted so far. In this study, we examined the mediation of 5-HT1 receptors of the dPAG in rats naive and in rats previously submitted to contextual fear conditionong (CFC). Defensive behaviors induced by activation of the dPAG were assessed by measuring the lowest intensity of electric current applied to this structure (threshold) able to produce freezing and escape responses during testing sessions of CFC, in which animals were placed in a context previously paired to footshocks. The persistence of the freezing behavior after the interruption of the dPAG electrical stimulation was also evaluated. The 5-HT1 function of the dPAG in this condition was evaluated by local injectinos of 8-OH-DPAT (4 and 8 nmol/) and WAY100635 (10nmol), selective agonist and antagonist of 5-HT1 receptors, respectively. In accordance with previous studies, 8-OH-DPAT increased the aversive thresholds (antiaversive effects) and injection of WAY100635 into the dPAG did not produce significant effects in naive rats. On the contrary, both serotonergic drugs 8-OH-DPAT and WAY100635 did not produce any significant effects on the aversive thresholds. Post-stimulation freezing was not affected by any treatment given to animals before or after CFC. However, the contextual conditioned freezing was attenuated or enhanced by intra-dPAG of 8-OH-DPAT and WAY100635, respectively. The present results suggest that 5-HT1 receptor-mediated mechanisms exert a phasic inhibition on the neural substrates of fear in the dPAG in naive rats whereas past stressful experience does not produce significant changes in the synaptic function of 5-HT1 receptors within the dPAG.

Posttraumatic stress disorder is a debilitating anxiety disorder that can develop in the aftermath of a traumatic or life–threatening event involving extreme horror, intense fear or bodily harm. The disorder is typified by a symptom triad consisting of re–experiencing, hyperarousal and avoidance symptoms. Approximately 15–25% of trauma–exposed individuals go on to develop PTSD, depending on the nature and severity of the trauma. Although dysfunctional adaptive responses exist in multiple neurobiological pathways in the disorder, e.g. glutamate, GABA, glucocortocoids and serotonin, the noradrenergic system is particularly prominent and represents an important pharmacological target in attempts at preventing the development of PTSD posttrauma. However, current literature shows opposing and conflicting results regarding the effect of selective noradrenergic agents in memory processing, and the effect of modulation of selective noradrenergic receptors are spread over diverse protocols and paradigms of learning and fear also employing different strains of animals. Fear conditioning is a behavioural paradigm that uses associative learning to study the neural mechanisms underlying learning, memory and fear. It is useful in investigating the underpinnings of disorders associated with maladaptive fear responses. Performing fear conditioning experiments with the aim of applying it to an animal model of PTSD, and relating these behavioural responses to a defined neural mechanism, will assist both in the elucidation of the underlying pathology of the disease, as well as the development of more effective treatment. This project has set about to re–examine the diverse and complex role of noradrenergic receptors in the conditioned fear response with relevance to PTSD. To the best of my knowledge, this study represents the first attempt at studying a range of noradrenergic compounds with diverse actions and their ability to modify conditioned fear in a single animal model. This work thus introduces greater consistency and comparative relevance not currently available in the literature, and will also provide much needed pre–clinical evidence in support of treatment strategies targeting the noradrenergic system in the prevention of PTSD posttrauma. The first objective of this study was to set up and validate a passive avoidance fear conditioning protocol under our laboratory conditions using the Gemini Avoidance System. The noradrenergic system plays a prominent role in memory consolidation and fear conditioning, while administration of –adrenergic blockers, such as propranolol, have been shown to abolish learning and fear conditioning in both humans and animals. Propranolol has also demonstrated clinical value in preventing the progression of acute traumatic stress syndrome immediately posttrauma to full–blown PTSD. To confer predictive validity to our model, the centrally active –adrenergic antagonist, propranolol, and the non–centrally acting –adrenergic antagonist, nadolol, were administered to Wistar rats after passive avoidance fear conditioning training in the Gemini Avoidance System. Wistar rats were used because of their recognised enhanced sensitivity to stress. Evidence from this pilot study confirmed that propranolol 10 mg/kg significantly inhibits the consolidation of learned fear in rats, whereas nadolol is ineffective. This effectively validated our protocol and the apparatus for further application in this study and also confirmed the importance of a central mechanism of action for –adrenoceptor blockade in the possible application of these drugs in preventing the development of PTSD posttrauma. The second objective of this study was to investigate the role of 1–, 2–, 1–, and 2–receptors in a conditioned fear passive avoidance paradigm. This was done in order to investigate how selective pharmacological modulation of these receptors may modify the conditioned fear response, and whether any of these receptor systems might exert opposing effects in passive fear conditioning. Various centrally active noradrenergic agents were employed over a 3–tiered dose response design, including the 1–antagonist, prazosin, the 2–agonist, guanfacine, the 2–antagonist, yohimbine, the 1–antagonist, betaxolol and the 2–antagonist ICI 118551. The effect of post–exposure administration of these drugs on conditioned fear was compared to that of propranolol 10 mg/kg. Selected doses of betaxolol (10 mg/kg) and ICI 118551 (1 mg/kg) attenuated fear conditioning to an extent comparable to propranolol, as did prazosin (0.1 mg/kg). Yohimbine tended to boster fear learning at all doses tested, albeit not significantly, while guanfacine did not produce any significant effect on memory retention at any of the doses studied. This latter observation was surprising since yohimbine tended to bolster fear conditioning while earlier studies indicate that 2–agonism impairs conditioned fear. Concluding, this study has conferred validity to our passive avoidance model and has provided greater insight into the separate roles of noradrenergic receptors in contextual conditioned fear learning. The study has provided supportive evidence for a key role for both 1– and 2–antagonism, as well as 1–antagonism, in inhibiting fear memory consolidation and hence as viable secondary treatment options to prevent the development of PTSD posttrauma. However, further study is required to delineate the precise role of the 2–receptor in this regard. / Thesis (M.Sc. (Pharmacology))--North-West University, Potchefstroom Campus, 2012.

Posttraumatic stress disorder is a debilitating anxiety disorder that can develop in the aftermath of a traumatic or life–threatening event involving extreme horror, intense fear or bodily harm. The disorder is typified by a symptom triad consisting of re–experiencing, hyperarousal and avoidance symptoms. Approximately 15–25% of trauma–exposed individuals go on to develop PTSD, depending on the nature and severity of the trauma. Although dysfunctional adaptive responses exist in multiple neurobiological pathways in the disorder, e.g. glutamate, GABA, glucocortocoids and serotonin, the noradrenergic system is particularly prominent and represents an important pharmacological target in attempts at preventing the development of PTSD posttrauma. However, current literature shows opposing and conflicting results regarding the effect of selective noradrenergic agents in memory processing, and the effect of modulation of selective noradrenergic receptors are spread over diverse protocols and paradigms of learning and fear also employing different strains of animals. Fear conditioning is a behavioural paradigm that uses associative learning to study the neural mechanisms underlying learning, memory and fear. It is useful in investigating the underpinnings of disorders associated with maladaptive fear responses. Performing fear conditioning experiments with the aim of applying it to an animal model of PTSD, and relating these behavioural responses to a defined neural mechanism, will assist both in the elucidation of the underlying pathology of the disease, as well as the development of more effective treatment. This project has set about to re–examine the diverse and complex role of noradrenergic receptors in the conditioned fear response with relevance to PTSD. To the best of my knowledge, this study represents the first attempt at studying a range of noradrenergic compounds with diverse actions and their ability to modify conditioned fear in a single animal model. This work thus introduces greater consistency and comparative relevance not currently available in the literature, and will also provide much needed pre–clinical evidence in support of treatment strategies targeting the noradrenergic system in the prevention of PTSD posttrauma. The first objective of this study was to set up and validate a passive avoidance fear conditioning protocol under our laboratory conditions using the Gemini Avoidance System. The noradrenergic system plays a prominent role in memory consolidation and fear conditioning, while administration of –adrenergic blockers, such as propranolol, have been shown to abolish learning and fear conditioning in both humans and animals. Propranolol has also demonstrated clinical value in preventing the progression of acute traumatic stress syndrome immediately posttrauma to full–blown PTSD. To confer predictive validity to our model, the centrally active –adrenergic antagonist, propranolol, and the non–centrally acting –adrenergic antagonist, nadolol, were administered to Wistar rats after passive avoidance fear conditioning training in the Gemini Avoidance System. Wistar rats were used because of their recognised enhanced sensitivity to stress. Evidence from this pilot study confirmed that propranolol 10 mg/kg significantly inhibits the consolidation of learned fear in rats, whereas nadolol is ineffective. This effectively validated our protocol and the apparatus for further application in this study and also confirmed the importance of a central mechanism of action for –adrenoceptor blockade in the possible application of these drugs in preventing the development of PTSD posttrauma. The second objective of this study was to investigate the role of 1–, 2–, 1–, and 2–receptors in a conditioned fear passive avoidance paradigm. This was done in order to investigate how selective pharmacological modulation of these receptors may modify the conditioned fear response, and whether any of these receptor systems might exert opposing effects in passive fear conditioning. Various centrally active noradrenergic agents were employed over a 3–tiered dose response design, including the 1–antagonist, prazosin, the 2–agonist, guanfacine, the 2–antagonist, yohimbine, the 1–antagonist, betaxolol and the 2–antagonist ICI 118551. The effect of post–exposure administration of these drugs on conditioned fear was compared to that of propranolol 10 mg/kg. Selected doses of betaxolol (10 mg/kg) and ICI 118551 (1 mg/kg) attenuated fear conditioning to an extent comparable to propranolol, as did prazosin (0.1 mg/kg). Yohimbine tended to boster fear learning at all doses tested, albeit not significantly, while guanfacine did not produce any significant effect on memory retention at any of the doses studied. This latter observation was surprising since yohimbine tended to bolster fear conditioning while earlier studies indicate that 2–agonism impairs conditioned fear. Concluding, this study has conferred validity to our passive avoidance model and has provided greater insight into the separate roles of noradrenergic receptors in contextual conditioned fear learning. The study has provided supportive evidence for a key role for both 1– and 2–antagonism, as well as 1–antagonism, in inhibiting fear memory consolidation and hence as viable secondary treatment options to prevent the development of PTSD posttrauma. However, further study is required to delineate the precise role of the 2–receptor in this regard. / Thesis (M.Sc. (Pharmacology))--North-West University, Potchefstroom Campus, 2012.

No description available.

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A substância cinzenta periaquedutal dorsal (SCPd) é considerada uma das principais estruturas do teto mesencefálico envolvida no substrato neural da aversão a estímulos proximais. GABA e 5-HT são apontados como neurotransmissores envolvidos na modulação das respostas defensivas elaboradas na SCPd. Recentemente, mecanismos neurocininérgicos também têm sido propostos como mediadores das reações de defesa organizadas nessa estrutura. O objetivo do presente estudo foi avaliar o envolvimento dos receptores NK-1 e NK-3 da SCPd no comportamento defensivo induzido pela estimulação elétrica dessa região em ratos com e sem experiência prévia ao condicionamento contextual aversivo. Para isso, os limiares aversivos de congelamento e fuga foram medidos durante a estimulação elétrica da SCPd em ratos ingênuos e em animais submetidos previamente ao procedimento de condicionamento contextual aversivo. A mediação destas repostas defensivas pelos receptores NK-1 e NK-3 foi avaliada pela injeção local de spantide (100 pmol/0,2 L) e SB 222200 (50 e 100 pmol/0,2 L), antagonistas seletivos de receptores NK-1 e NK-3, respectivamente. Os resultados mostraram que a injeção intra-SCPd de spantide aumentou os limiares aversivos determinados pela estimulação elétrica da SCPd em animais ingênuos e com experiência aversiva prévia. Injeções similares de 100 pmol de SB 222200 na SCPd também causaram um aumento nos limiares de congelamento e fuga. Entretanto, esses efeitos devem ser atribuídos a um déficit motor causado por essa dose da droga, uma vez que SB 222200 na dose de 100 pmol, reduziu significativamente a atividade motora dos animais submetidos ao teste do campo aberto. Estes resultados sugerem que receptores NK-1, mas não NK-3, estão envolvidos na mediação do comportamento defensivo induzido pela estimulação elétrica da SCPd. / The dorsal periaqueductal gray (dPAG) constitutes the main structure of the neural substrate for the defensive response to proximal aversive stimuli. It is well established that GABA and 5-HT are important neuromediators of the defense reaction at the dPAG level but neurokinin-mediated mechanisms have also been recently implicated in these processes. The aim of the present investigation was to examine the involvement of NK-1 and NK-3 receptors of the dPAG in the unconditioned defensive behaviors induced by electrical stimulation of the dPAG of rats with and without previous experience with contextual fear conditioning. For this, aversive thresholds were measured as the lowest intensity of electric current to produce freezing and escape behaviors using a procedure of gradual increases of the electrical stimulation of the dPAG. These thresholds were also measured in rats previously submitted to a contextual fear conditioning procedure. The mediation of these defensive responses by NK-1 and NK-3 receptors of the dPAG was evaluated by local injections of spantide (100 pmol/0.2 L) and SB 222200 (50 and 100 pmol/0.2 L), selective antagonists of NK-1 and NK-3 receptors, respectively. The present results showed that spantide injections into the dPAG increased the aversive thresholds (anti-aversive effects) determined by stimulation of the dPAG in naive and in animals subjected previously to the contextual fear conditioning. Similar injections of SB 222200 100 pmol into the dPAG increased the freezing and escape thresholds. However, this effect can be attributable to a motor deficit since this dose of SB 222200 decreased the exploratory activity of the animals subjected to the open field test. These results suggest that NK-1 receptors, but not NK-3 receptors, are involved in the mediation of the defensive behaviors induced by electrical stimulation of the dPAG.

Sabe-se que o medo condicionado contextual (MCC) pode ativar diversas áreas mesencefálicas, como a Substância Cinzenta Periaquedutal Dorsal (SCPd). Diversos estudos avaliaram como mecanismos serotoninérgicos modulam o comportamento defensivo induzido pela estimulação elétrica da SCPd. Uma função proeminente dos receptores 5-HT2A é regular estados aversivos induzidos pela ativação da SCPd e, apesar de saber que experiências aversivas prévias exercem um importante papel em determinados tipos de ansiedade, somente estudos com estimulação da SCPd de ratos sem experiência aversiva prévia foram conduzidos até o momento. O objetivo do presente estudo foi investigar o papel funcional dos receptores 5-HT2A localizados nos substratos neurais da aversão na SCPd de ratos previamente submetidos ao condicionamento contextual aversivo. Foram avaliadas as respostas de congelamento e fuga obtidas a partir do procedimento de estimulação elétrica da SCPd na sessão teste, realizada 24 horas após o condicionamento, quando os animais foram recolocados no contexto previamente pareado com choques nas patas ou no contexto diferente. A função dos receptores 5-HT2A da SCPd foi avaliada pela injeção local de -metil-serotonina ou cetanserina, agonista e antagonista seletivos de receptores do tipo 5-HT2A, respectivamente. De acordo com estudos anteriores, a -metil-serotonina aumentou os limiares aversivos (efeito antiaversivo) determinados pela estimulação da SCPd em animais ingênuos, enquanto que a injeção de cetanserina não produziu qualquer efeito significativo. No presente estudo, a cetanserina reduziu o limiar de congelamento (efeito proaversivo), enquanto que a -metil-serotonina continuou apresentando efeito antiaversivo em animais com experiência aversiva prévia (choques nas patas). Estes resultados sugerem que experiências aversivas anteriores podem produzir importantes alterações na função sináptica dos receptores 5-HT2A da SCPd, podendo refletir na reatividade da SCPd à sua estimulação aversiva. / It has been shown that contextual fear conditioning (CFC) may activate brainstem regions such as the dorsal periaqueductal gray (dPAG). Several studies have been carried out to disclose how 5-HT2 mechanisms modulate the aversive stimulation of the dPAG. One prominent function of 5-HT2 receptors is to regulate the aversive states induced by activation of the dPAG. However, in spite of the notion that past stressful experiences play a crucial role in certain types of anxiety only studies with stimulation of the dPAG of rats without previous aversive experience have been conducted so far. We investigated the mediation of 5-HT2 receptors located in the neural substrates of aversion of the dPAG in rats previously submitted to CFC. Defensive behaviors induced by activation of the dPAG were assessed by measuring the lowest intensity of electric current applied to this structure (threshold) able to produce freezing and escape responses during testing sessions of CFC, in which animals were placed in a context previously paired to footshocks. The 5-HT2 function of the dPAG in this condition was evaluated by local injections of -methyl-5-HT and ketanserin, selective agonist and antagonist of 5-HT2 receptors, respectively. In accordance with previous studies, -methyl-5-HT increased the aversive thresholds (antiaversive effects) determined by stimulation of the dPAG in naive rats and injection of ketanserin into the dPAG did not produce any significant effects. On the other hand, ketanserin decreased the freezing threshold (proaversive effect) determined by the dPAG electrical stimulation while -methyl-5-HT continued to show antiaversive effects in animals with prior experience with foot shocks. The present results suggest that a past stressful experience can produce changes in the synaptic function of 5-HT2 receptors within the dPAG with important impact on the defensive behaviors induced by electrical stimulation of this area.