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  • br Materials and methods br Results


    Materials and methods
    Discussion Drug of abuse induces widespread dexamethasone acetate changes in the molecular and cellular functions in circuits between VTA and its neighbored regions such as NAC, and amygdala. It is belived that these changes result behavioral phenotype that characterize addiction [10,41]. In fact, the driver of consumption is to alleviate the negative or aversive symptoms in opioids abusers (OA) [16]. It should be considered that dopamine adjusts reward value, creates incentive salience for reward and improves reward-related memory [11,14]. Increasing in dopamine level stimulates DRD1s to induced reinfforcing effect and inhibits aversive signal via DRD2s. Although maximum reinforcement occurred when both DRD1 and DRD2 stimulate [49]. It was found that D1-like DRs (DRD1 and DRD5) levels increased in the VTA of OA. As D1-like DRs do not express in dopaminergic neurons [44], the enhancement of the receptors may occur in VTA glutamatergic neurons. Glutamate neurons in VTA are a part of VTA micro-circuitry which can excite dopaminergic neurons and may play a major role in reward [50]. Since, D1-like DRs are coupled with Gs [3], increase in these receptors, leads to enhanced dopaminergic neurons firing. There are studies revealed that strengthening of glutamatergic input into VTA dopaminergic neurons is associated with consolidation of responses to the reinforcement-predictive cues [12,23]. Data revealed that chronic morphine abuse reduce the soma size of VTA dopaminergic neurons [29] and increase dopaminergic neurons activity [9,30]. However; the dopamine releases in to the NAC decreases [29] and output of these adaptation changes is reward tolerance [39]. The results of the current study also indicated that D1-like DRs increased in NAC of OA. This elevation in D1-like DRs may be related to acute dopamine depletion in the presence of a high dose of drugs or supersensitivity following decreased receptor occupancy by dopamine [51]. There is a report that DRD1 increased in NAC of chronic methamphetamine but not heroin abusers [52]. Chronic opiate exposure led to growth dendritic tree and spins in the NAC which is adaptive changes to long-lived sensitized responses to opioids consumption [32].With regards to the amygdala, results of the current study revealed that D1-like DRs were up-regulated in OA. It has been indicated that DRD1 activation in the basolateral amygdala (BLA), potentiate opiate reward salience only when opiate is used acutely [20]. However, during chronic morphine administration, activation of DRD2 potentiates opiate reward salience [20]. Amygdala also receives dopamine input from VTA which modulates glutamatergic transmission in BLA-pyramidal neurons [19]. An experimental study showed that DRD1 in the amygdala has an essential role in morphine-induced conditioned place preference [54]. Li et al. demonstrated that presynaptic DRD1 increased during chronic morphine treatment and subsequently enhanced glutamate release [19]. Projection of amygdala to the NAC is responsible for opiate-seeking behavior [37]. The results of the present study indicated that DRD2 increased in VTA. Some investigations showed that DRD2 stimulation in VTA led to excition of dopaminergic neurons in the amygdala and medial prefrontal cortex (mPFC) [17,26]. There is also suggestion that some VTA GABA neurons expressed DRD2 [27]. Therefore, increase in DRD2 expression level in VTA might be related to GABA neurons which lead to increase in the firing rate of dopaminergic neurons. In the current investigation, the DRD2 expression level increased in the NAC of OA. An animal study showed that overexpression of postsynaptic DRD2 in the NAC increased motivation [45]. Evidence also showed that morphine dependence and withdrawal induced DRD2 enhancing level in medium spiny neurons (MSN) in the NAC [8] which is similar to the current results. In this survey, DRD2 increased in the amygdala of OA. In morphine-dependent/withdrawn state in the BLA, motivational saliency induced by morphine switched from D1-mediated ERK-dependent mechanisms to D2-mediated CamKinaseII-dependent mechanisms [22]. Thus, an increase of DRD2 in the amygdala of morphine consumers is predictable. Vice versa, PET imaging study results revelaed that DRD2/3 binding was decreased in the striatum of heroin abusers compared with the control group [28].