Morphological, Electrophysiological And Behavioral Studies Of Substance P System In Rat Globus Pallidus

The globus pallidus occupies a critical position in the indirect pathway of the basal ganglia.By innervating all the basal ganglia nuclei,the globus pallidus plays an important role in normal movement regulation and in the pathophysiology of basal ganglia disorders. Being an undecapeptide and a member of the neurokinin family,substance P plays an important role in the central nervous system by acting either as a neurotransmitter or a neuromodulator.Previous studies have shown that substance P involves in the etiology of many central nervous system disorders.Morphological studies have revealed the existence of substance P and its high-affinity receptor,neurokinin-1 receptor,in globus pallidus.The expression of neurokinin-1 receptor in external globus pallidus has been reported to be decreased or unchanged in parkinsonian patients.Object:To study the effects of substance P on the firing rates of globus pallidus neurons in normal and parkinsonian rats,the expression of substance P receptors in globus pallidus of normal and parkinsonian rats,and behavioral effects of substance P in awake rats.Methods:In vivo extracellular single unit recordings,immunohistochemistry,and cannulae embedding were used in the present study.Results:1.Micro-pressure ejection of the selective neurokinin-1 receptor agonist,[Sar9,Met(O2)11]substance P,increased the spontaneous firing rate of pallidal neurons.In 53 pallidal neurons,0.1 mM[Sar9,Met(O2)11] substance P increased the firing rate from 18.23±1.58 Hz to 21.92±1.80 Hz(P＜0.001). The average increase was 29.83±4.25%.Within the range of concentration from 0.01 mM to 1 mM,[Sar9,Met(O2)11]substance P produced an approximate bell-shaped concentration-dependent effects in increasing the firing rate of pallidal neurons.After lesioning pallidal cholinergic neurons,0.1 mM[Sar9,Met(O2)11]substance P still increased the firing rate in 23 pallidal neurons.The average increase was 25.32±5.13%, which was not statistically different compared to that without 192 IgG-saporin treatment (P＞0.05).2.Local administration of the selective neurokinin-1 receptor antagonist, SR140333B,prevented the excitatory effects induced by[Sar9,Met(O2)11]substance P.3. In 6-hydroxydopamine lesioned rats,micro-pressure ejection of[Sar9,Met(O2)11] substance P(0.1 mM) increased the spontaneous firing rate of pallidal neurons by 9.09±2.03%(n=36) on the lesioned side,which was significantly weaker than that on the unlesioned side(20.69±3.27%,n=38,P＜0.01),and that in normal rats(P＜0.01).Besides, there is no significant difference between the SMSP-induced increase of firing rate on unlesioned side of 6-hydroxydopamine rats and that in intact rats(P＞0.05). Micro-pressure ejection of SR140333B prevented the excitatory effects induced by [Sar9,Met(O2)11]substance P.4.In 6-hydroxydopamine parkinsonian rats,the number of substance P receptor immunoreactive neurons in globus pallidus on the lesioned side (16.78±3.72,n=6) was significantly less than that on the unlesioned side(24.59±3.67, n=6,P＜0.01),and that in normal rats(28.32±5.10,n=6,P＜0.01).The difference between number of substance P receptor immunoreactive neurons on unlesioned side of 6-hydroxydopamine rats and that in intact rats was no statistically significant(P＞0.05).5. In behaving rats,we further observed the postural effects of substance P in the globus pallidus.Consistent with electrophysiological results,unilateral microinjection of [Sar9,Met(O2)11]substance P(0.1 mM) into the globus pallidus induced a SR140333B sensitive contralateral dystonic posturing in the presence of systemic haloperidol administration.Conclusion:Our in vivo electrophysiological and behavioral results suggest that substance P produces excitatory effects on globus pallidus neurons via neurokinin-1 receptors,and that the activity of neurokinin-1 receptors in globus pallidus may be decreased under parkinsonian state.Morphological findings further indicate that substance P receptors of globus pallidus decreased in Parkinson's disease.The present findings may provide a rationale for further investigations into the potential of pallidal substance P system in the treatment of Parkinson's disease.