Research reportInvolvement of cannabinoid receptors in the amygdala and prefrontal cortex of rats in fear learning, consolidation, retrieval and extinction
Introduction
Endocannabinoid signalling regulates transmitter release via cannabinoid receptors 1 (CB1R) that are located presynaptically on several transmitter systems in the brain [1], [2], [3]. CB1R inhibit transmitter release as heteroreceptors mainly on glutamatergic and GABAergic nerve terminals in the cerebellum and in the forebrain. Hence, CB1R and endocannabinoid signalling are in strategic positions to regulate a variety of cognitive functions, such as learning and memory. For example, enhancement of endocannabinoid signalling by the inhibition of its metabolizing enzyme modulates acquisition and extinction of spatial memory [4], the selective CB1R-antagonist AM251 has an amnestic effect in the inhibitory avoidance paradigm [5], and the synthetic CB1R agonist WIN 55,212-2 (WIN) impairs spatial memory retrieval [6]. In addition the genetic deletion of CB1R in mice impaired the extinction of conditioned fear [7]. Many recent studies support the important role of endocannabinoid signalling in mnemonic processes [8], [9], [10], [11], [12], [13], [14]. However, there are still some open questions concerning the involvement of CB1R with respect to different brain systems and different phases of learning and memory.
We here focus on fear conditioning and fear memory. Classical fear conditioning comprises different phases of learning and memory processes such as acquisition, consolidation, retrieval and extinction [15], [16]. Recently, different paradigms of fear conditioning were used to study the role of endocannabinoid signalling in fear conditioning. But the differences of for example olfactory fear conditioning [12], auditory fear conditioning [11], contextual fear conditioning [10], visual fear conditioning [17] and trace fear conditioning [13] make direct comparisons between these studies difficult. Furthermore, different species (mice or rats), knockout and wildtype animals and various drugs in different doses were used in these studies. Therefore, we here investigated in a systematic approach in rats the role of CB1R in the basolateral complex of the amygdala (BLA) and the medial prefrontal cortex (mPFC) in different phases of fear learning. We used the fear-potentiated startle (FPS) paradigm and tested the effects of the CB1R agonist WIN and the antagonist AM251 on acquisition, consolidation, retrieval and extinction of fear after local microinfusion of the drugs.
Section snippets
Subjects
A total of 265 adult male Wistar rats (Hannover strain, Harlan-Winkelmann, Borchen, Germany) weighing between 250 and 300 g were used. Animals were housed in groups of five in Macrolon cages (type IV) under standard conditions on a 12 h light-dark cycle with lights on at 7 o’clock. All experimental procedures were performed during the rats’ light cycle. They received free access to tap water and were maintained on their experimental body weight by controlled feeding of 12 g rodent chow per rat per
Histology
A total of 265 animals underwent stereotactic surgery. The target areas were missed in nine animals. Data from these rats were not included in the analysis. Three animals died during surgery. Fig. 1A–D shows the localization of the injection cannulae in the BLA. In 127 rats evaluation of thionine-stained brain sections (Fig. 2A and B) using light microscopy indicated that the target area of the IC microinjection was accurately hit. Fig. 3A–D illustrates the localization of the injection
Discussion
Cannabinoid 1 receptors (CB1R) in the amygdala and the prefrontal cortex play important roles in associative learning [12], [22], [23], [24], [25]. The results of the present fear-conditioning study support this notion and suggest that the various phases of learning and memory in the rat can be influenced differentially in the mPFC and BLA by the synthetic CB1R ligands WIN and AM251.
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