| The biological effects of cannabinoids are mainly mediated by two members of the G protein-coupled receptor family: cannabinoid type 1 receptor(CB1R)and cannabinoid type2 receptor(CB2R).CB1 R is highly expressed in the central nervous system(CNS)and is involved in the regulation of various brain functions,including executive function,emotion,reward,and memory processing.Unlike CB1 R,CB2R is considered to be the "peripheral" cannabinoid receptor.However,CB2R is widely expressed in CNS and is involved in behavioral regulation related to dopamine(DA),including feeding behavior,body weight regulation,depression,anxiety and schizophrenic-like behavior,etc.Recent studies have shown that CB2R is expressed in cytoplasm and axons of dopaminergic neurons in the substantia nigra(SN),however its function is unclear.Previous studies in my laboratory showed that activating CB2R on dopaminergic neurons in the ventral tegmental area(VTA)can regulate the addictive behavior of animals by inhibiting the excitability of neurons.In this study,15-20-day-old C57BL/6N mice were selected to investigate the effect of CB2R activation on neuronal excitability of SN dopaminergic neurons and its possible mechanism by whole-cell patch clamp recording,electrochemical and behavioral detection.The results were as follows:1.Immunofluorescence staining shows that tyrosine hydroxylase(TH)positive neurons in the SN co-localized with CB2R positive cells,indicating the presence of CB2R expression on dopamine neurons in the SN.2.Most dopaminergic neurons were located in the ventral midbrain,the cell bodies were generally triangular,findle-shaped or multipolar.The neurons had low or no spontaneous discharge frequency,and showed inward rectification characteristics after the stimulation of-100 p A hyperpolarization current.The half-height width of the action potential was about 3 ms.3.CB2R agonist JWH133 dose-dependently inhibits the spontaneous firing frequency of dopaminergic neurons in the SN,in which 10 μmol/L of JWH133 can reduce the firing frequency of neurons by 34 %,and the results showed a significant difference(P<0.01).10μmol/L of JWH133 was used for the subsequent experiments.4.We observed the effect of JWH133 on the evoked action potential of dopaminergic neurons.The numbers of evoked action potential decreased,the action potential interval,the initiation time of action potential and the after-hyperpolarization potential amplitude increased after 10 μmol/L JWH133 treatment compared with the basal discharge.The difference was significant(P<0.05).5.Compared with the control,10 μmol/L JWH133 induced a significant decrease in the frequency of miniature excitatory postsynaptic currents(m EPSCs)(P<0.05).The half decay time and the current amplitude was not significantly changed.Mini-inhibitory postsynaptic currents(m IPSCs)had no significant effect on their frequency,amplitude and half-decay time after JWH133 application.6.Compared with the control,the release of DA in the striatum was significantly increased after the stereotaxic injection of 10 μmol/L JWH133 3.2 μL in the SN of mice(P<0.05).7.Compared with the control group,unilateral injection of 10 μmol/L JWH133(3.2 μL)into SN area could not induced the contralateral dystonic posturing behavior of haloperidol treated rats,but the descent latency was significantly prolonged at 50 min(P<0.05).These results suggest that CB2R was expressed in SN region of mice.Activation of CB2 receptors on the SN dopaminergic neurons inhibits neuronal excitability,which may partly be attributed to the inhibition of presynaptic glutamate release.Although activation of CB2R on SN dopaminergic neurons inhibited neuronal excitability,microdialysis results showed that CB2R on SN dopaminergic neurons increased DA release in striatum.This study provides a new experimental basis for further understanding the role of CB2R on SN dopaminergic neurons. |
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