Effect of intranasal substance P on the Parkinsonian syndrome

What substance P might tell us about the prognosis and mechanism of Parkinson's disease?

The neuropeptide substance P (SP) plays an important role in neurodegenerative disorders, among which Parkinson's disease (PD). In the present work we have reviewed the involvement of SP and its preferred receptor (NK1-R) in motor and non-motor PD symptoms, in both PD animal models and patients. Despite PD is primarily a motor disorder, non-motor abnormalities, including olfactory deficits and gastrointestinal dysfunctions, can represent diagnostic PD predictors, according to the hypothesis that the olfactory and the enteric nervous system represent starting points of neurodegeneration, ascending to the brain via the sympathetic fibers and the vagus nerve. In PD patients, the α-synuclein aggregates in the olfactory bulb and the gastrointestinal tract, as well as in the dorsal motor nucleus of the vagus nerve often co-localize with SP, indicating SP-positive neurons as highly vulnerable sites of degeneration. Considering the involvement of the SP/NK1-R in both the periphery and specific brain areas, this system might represent a neuronal substrate for the symptom and disease progression, as well as a therapeutic target for PD.

Effect of glutamate and antagonists of N-methyl-D-aspartate receptors on experimental parkinsonian syndrome in rats

Intranigral administration of glutamate to rats with parkinsonian syndrome induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine augmented the development of parkinsonian symptoms (oligokinesia and muscular rigidity), but did not affect motor activity of intact animals. Memantine administered intraperitoneally in parallel with induction of parkinsonian syndrome weakened the development of oligokinesia and muscular rigidity in a dose-dependent manner starting from 5 mg/kg and abolished toxic effect of glutamate. Ketamine (15 mg/kg) under the same conditions less potently prevented the development of oligokinesia, did not prevent the development of muscular rigidity, and did not antagonize glutamate toxicity. The data attest to an important role of glutamate and activation of N-methyl-D-aspartate receptors in the induction and development of parkinsonian syndrome.

Intracerebroventricular administration of substance P increases dopamine content in the brain of 6-hydroxydopamine-lesioned rats

The interactions existing between substance P- and dopamine-positive neurons, notably in the basal ganglia, suggest that substance P may have therapeutic use in treatment of Parkinson's disease characterized by impaired dopaminergic transmission. The effects of intracerebroventricularly administered substance P were tested on the levels of dopamine and its metabolites in the striatum, nucleus accumbens and frontal cortex of 6-hydroxydopamine-lesioned rats. Intracerebroventricular injection of 6-hydroxydopamine decreased the levels of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the brain structures under investigation. Administration of substance P in low dose (0.35 nmol/kg) had no effect on the 6-hydroxydopamine-induced reduction of the dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid contents in the brain. However, treatment with substance P in higher dose (3.5 nmol/kg) increased the concentrations of dopamine and its metabolites in the striatum, nucleus accumbens and frontal cortex relative to saline-treated group. Additionally, 6-hydroxydopamine lesions significantly increased 3,4-dihydroxyphenylacetic acid/dopamine and homovanillic acid/dopamine ratios in the striatum and nucleus accumbens. Substance P (3.5 nmol/kg) partially reversed lesion-induced increases in 3,4-dihydroxyphenylacetic acid/dopamine and homovanillic acid/dopamine ratios in the striatum, but did not alter these ratios in nucleus accumbens. To test whether substance P fragmentation is responsible for this phenomenon, substance P(5-11), which is one of the main substance P fragments in rat CNS, was administered in equimolar dose. Substance P(5-11) was found to have no effect on the content of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the striatum and nucleus accumbens. In the frontal cortex, substance P(5-11) produced decreases in dopamine levels and increases in homovanillic acid/dopamine ratio. The results of this study suggest that substance P helps to restore dopamine deficit in the brain in an animal model of Parkinson's disease, with the positive effects being more prominent on the nigrostriatal than on the mesocorticolimbic dopaminergic system, but substance P(5-11) is not responsible for this effect.