Introduction
In the study of the neural bases of affective activity, modules can be represented by a single neuron or set of neurons in a population, discrete areas in a brain region, or by synapses mediated by a neurotransmitter in a given nervous center.
In this sense, the limbic system is made up of different neural modules that attribute a positive or negative connotation to various stimuli in the environment, which include the hedonic component of affective valence, that is, whether the individual, when faced with a specific environmental element, will experience , respectively, satisfaction or displeasure.
These affective states are accompanied by a motivational component, which allows, upon perception of a situation, brain processes to be activated tending to enhance activities aimed at obtaining the reward of what is perceived as pleasant or to avoid pressing situations (that involve pain or discontent). ) through passive avoidance or experiencing fear.
It should be noted that affective expressions can be objective, without being influenced by the hedonic component, which is verified in the manifestations of pleasure in the orofacial region.
Furthermore, in response to changes in the environment or internal environment (physiological or psychological changes), a given neural module can assign a positive or negative valence in the generation of emotions, that is, its ability to represent different affective modes of expression. according to a characteristic context: the existence of differential gene expression, the activity of the nervous circuit of which it is part, the neuronal excitability profile or the type of neurotransmission, among other conditions.
However, it is important to evaluate whether the neural modules present in structures of the limbic system, such as the amygdala and the nucleus accumbens , regulate different affective states in response to a continuously changing environment or whether, despite such changes, they correspond to a determined affective mode. This analysis constitutes the objective of the present review.
Nucleus accumbens and amygdala: modules and affective modes
Based on what was observed in animal models, it was proposed that blockade of the mesolimbic dopaminergic system caused anhedonia ; However, in additional studies carried out in rats, it was postulated that the integrity of this circuit is not required to experience pleasure in the face of a positive stimulus (presence of food, among others), but it is required to desire to obtain what will produce satisfaction (proceed to consume it). ).
In these studies, the inhibition of this system was achieved by generating lesions, caused by 6-hydroxydopamine, in the dopaminergic outputs from the midbrain to the nucleus accumbens or the use of a monoamine antagonist.
It should be noted that in the nuclei accumbens and ventral pallidum, the brain stem and the orbitofrontal and insular cortices there are centers that regulate pleasurable sensations or hedonic hot spots , which are inferred to be areas of reduced size, based on observations made in brains of rats, and integrate a circuit involved in increasing the satisfaction that a pleasant situation produces, which responds to the stimulus of opioids.
In particular, this increase is recorded by causing the activation of kappa opioid receptors, located in one of these centers, which is found in the rostrodorsal portion of the medial shell of the nucleus accumbens.
According to the author, it is important to mention that the enhancement of the pleasant sensation does not occur by activating the kappa receptors located in the neurons of the areas of the rostrodorsal portion of the medial cover, different from the center that promotes the pleasant sensation, and even, when such activation is induced in other brain structures, it leads to the aversion response.
Likewise, it is possible to distinguish neural modules located in different areas of the nucleus accumbens , which generate contrasting responses to a given neurochemical stimulus, which has been confirmed in murine models.
In this sense, the induction of inhibitory synapses by activating GABA A receptors or blocking glutamate, AMPA receptors (application in the form of a microinjection of muscimol or DNQX [6,7-dinitroquinoxaline-2,3-dione] , respectively), can provoke the desire to consume food or defensive behavior, if it occurs, respectively, in areas of the rostral or caudal region of the medial cover of the nucleus accumbens .
However, the same neural module can generate different affective states when faced with different neurochemical signals and environmental conditions.
In this way, in the presence of inhibition of AMPA receptors, certain areas of the nucleus accumbens can trigger defensive behaviors in an adverse context or the desire to consume food in a pleasant environment, if, respectively, the endogenous activation of dopaminergic neurotransmission, mediated by D1 and D2 receptors (DRD1 and DRD2, respectively), or sole excitation of DRD1.
However, it is not possible to rule out that these different affective modes respond to different neural modules, made up of neurons that express DRD1 or DRD2, since these receptors are not usually located in the medium spiny neurons of the nucleus accumbens .
The author highlights that neuronal hyperpolarization caused by the activation of GABA A receptors in areas of the rostral or caudal region of the medial shell of the nucleus accumbens leads to the enhancement of the hedonic component of affective valence, that is, responses are promoted. of pleasure or repulsion, respectively (external stimulus: taste of sucrose [murine model]).
Furthermore, the inhibition of the glutamatergic signal caused by the blockade of AMPA receptors induces an increase in the desire to consume food or in defensive behavior, as it occurs, respectively, in areas of the rostral or caudal region of the medial cover. of the nucleus accumbens , without affecting the hedonic component.
With regard to the involvement of the amygdala structures, in the determination of affective valences it is currently considered that both the central and basolateral nuclei of the amygdala (CeA and BLA, respectively) participate in the generation of the defensive behavior and fear (negative affective states) and the desire to consume food and other emotions that carry pleasant feelings.
Although it is not possible to establish the unequivocal correspondence of a neural module with an affective mode in the amygdala, due to the inconsistency of the information when contrasting different studies, it is feasible to evaluate the functioning of the amygdala in the genesis of affective valences if limits it to certain experimental designs and laboratory conditions.
In this sense, the information regarding the unique implication of the neurons that secrete corticotropin-releasing factor in the CeA, in the generation of aversion to stressful situations or unpleasant sensations during the withdrawal syndrome and, in particular, the neurons that have efferences to the nucleus of the bed of the stria terminalis, in the manifestation of avoidance and fear behaviors, must be contrasted with the observations made in transgenic mice.
In these animal models, the excitation of the different neuronal types present in the CeA is achieved by applying a laser that induces the expression of channelrhodopsin, which is controlled by the synapsin promoter. Said activation is carried out in coordination with the ingestion of only one of the two sugar solutions offered, so that the rodent consumes the solution associated with the application of the stimulus.
These observations allow us to postulate that the different neuronal populations of the CeA could be involved in the generation of positive or negative emotions according to internal conditioning elements (activation of other nervous structures, physiological and psychological states) and external conditions (stressful or adverse situations or attractive elements). , such as the presence of food).
In this sense, it is essential to carry out additional studies that, with a similar experimental design, evaluate the effect of different conditions on the generation of affective modes, in order to arrive at reproducible results, which allow inferring the implication of a specific neural module in the generation of specific affective states.
These studies will allow us to understand the diversity of affective modes controlled by the neural modules of the limbic system and the consequences of alterations in said regulation, with respect to the appearance of diseases.
Conclusion Although the mechanisms underlying the alternation in the functioning of the neural modules of the limbic system, with respect to the generation of positive and negative affective modes, are unknown, the consideration of this alternation will allow the structuring of successive studies, in which the information necessary to answer this question. |
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