Substance P (1-9)

Substance P is known to exert potent effects in peripheral tissues, and is thought to be important for ocular function. The mechanism of action of substance P in the human eye is not known. As a basis for biochemical characterization specific binding of 125I-Bolton-Hunter-substance P was demonstrated in the human eye using autoradiographic methods. Biochemical characterization on slide-mounted tissue preparations showed that binding was saturable with a KD of 0.27 +/- 0.1 nmol/l. Specific binding occurred at comparable autoradiographic densities to both human retina and choroid. Substance P and its carboxyterminal fragment, substance P(3-11), were shown to be highly potent in binding competition experiments against 125I-Bolton-Hunter-substance P. Similar concentrations of substance P(1-9), neurokinin A and neurokinin B failed to significantly alter specific binding of 125I-Bolton-Hunter-substance P. The results indicate expression of high affinity substance P binding sites in human retina and choroid.

L-DOPA administration is the primary treatment for Parkinson's disease (PD) but long-term administration is usually accompanied by hyperkinetic side-effects called L-DOPA-induced dyskinesia (LID). Signaling neuropeptides of the basal ganglia are affected in LID and changes in the expression of neuropeptide precursors have been described, but the final products formed from these precursors have not been well defined and regionally mapped. We therefore used mass spectrometry imaging to visualize and quantify neuropeptides in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine exposed parkinsonian and LID Macaca mulatta brain samples. We found that dyskinesia severity correlated with the levels of some abnormally processed peptides - notably, des-tyrosine dynorphins, substance P (1-7), and substance P (1-9) - in multiple brain regions. Levels of the active neuropeptides; dynorphin B, dynorphin A (1-8), α-neoendorphin, substance P (1-11), and neurokinin A, in the globus pallidus and substantia nigra correlated with putaminal levels of L-DOPA. Our results demonstrate that the abundance of selected active neuropeptides is associated with L-DOPA concentrations in the putamen, emphasizing their sensitivity to L-DOPA. Additionally, levels of truncated neuropeptides (which generally exhibit reduced or altered receptor affinity) correlate with dyskinesia severity, particularly for peptides associated with the direct pathway (i.e., dynorphins and tachykinins). The increases in tone of the tachykinin, enkephalin, and dynorphin neuropeptides in LID result in abnormal processing of neuropeptides with different biological activity and may constitute a functional compensatory mechanism for balancing the increased L-DOPA levels across the whole basal ganglia.

Neurones in the substantia nigra were found to be sensitive to iontophoretically applied substance P, substance P 1-9 methyl ester and substance P 1-9 amide. Substance P 1-2, 4-9 and 5-9 methyl esters, thyrotropin releasing hormone (TRH), Pyroglutamyl-histidyl-2 methyl prolineamide (methyl TRH), Pyroglutamyl-histidyl-2 methyl prolineamide (methyl TRH), histidyl-proline-diketopiperazine (His-Pro) and MSH releasing inhibiting factor (MIF) were without effect on neurones in this area. Thyrotropin releasing hormone (TRH), methyl TRH, His-Pro and MIF were inactive on neurones in the caudate nucleus and nucleus accumbens. Bilateral injections of substance P and substance P 1-9 methyl ester into the ventral tegmental area (VTA) of conscious rats produced locomotor activity, while similar injections of substance P 4-9 and 5-9 methyl esters did not. The locomotor activity produced by amphetamine was prolonged by TRH, while MIF was devoid of such activity. The data suggest that substance P and substance P 1-9 have similar effects in the substantia nigra, although the mechanism of action is unclear. Thyrotropin releasing hormone and MIF probably do not have acute actions in the brain areas tested.