Increased numbers of substance P-containing sensory neurons in a rat strain with a genetic neurotrophic defect

The role of calcium channels in substance P-induced contractile response in the rat iris

This study was undertaken to assess the role of calcium channels in the contractile response induced by substance P in the isolated rat iris. Substance P produced graded and sustained contraction in the rat iris. Pre-incubation of preparations with thapsigargin (1 microM), verapamil (1 microM), isradipine (1 microM) or with omega-conotoxin MCIIA (0.1 microM) did not significantly inhibit substance P-mediated contraction in the isolated rat iris. However, pre-incubation of the preparations with nicardipine (1 microM) or ruthenium red (1 mM) caused parallel displacement to the right of the substance P concentration-response curve without affecting its maximal response. In contrast, amiloride (1 microM), markedly inhibited substance P-mediated contraction (73 +/- 5%), while econazole (1 mM) also significantly inhibited (44 +/- 11%) substance P-mediated contraction in the isolated rat iris. Collectively, these results suggest that substance P-mediated contractile response in the isolated rat iris depends largely on the influx of external Ca2+, by a mechanism which might involve the T-type calcium channels.

Reduced inflammation in genetically hypertensive rat airways is associated with reduced tachykinin NK(1) receptor numbers

The airways of the genetically hypertensive rat (GH) are hyperinnervated by substance P-containing sensory nerves and exhibit reduced inflammatory responsiveness to substance P and to capsaicin. The present study measured tracheal inflammation to resiniferatoxin (1.0 microgram/kg i.v.), a capsaicin analogue, which lacks the hypotensive action of capsaicin itself, alone or after the neuronal nitric oxide synthase inhibitor 1-(2-trifluoromethylphenyl)imidazole (TRIM) (50 mg/kg i.p.). The inflammatory response to resiniferatoxin alone was 50% lower in untreated GH than in control rats, a similar strain difference to that seen previously with capsaicin. Pre-treatment with TRIM had no effect on inflammation in either strain. Binding kinetics of the tachykinin NK(1) receptor antagonist [3H](S)-1-(2-[3-(3, 4-dichlorophenyl)-1-(3-isopropoxyphenylacetyl)piperidin-3-yl]ethyl)-4- phenyl-l-azoniabicyclo[2,2,2,]octane chloride ([3H]SR140333)(0.125-16.0 nM) showed 50% reduction of B(max) in GH versus control tracheae (74+/-13 cf.165+/-26 fmol/mg protein). Our results indicate that the reduced neurogenic inflammatory responsiveness in GH rats can be attributed entirely to reduced tachykinin NK(1) receptor numbers.

Decreased vascular permeability response to substance P in airways of genetically hypertensive rats

1. The inbred genetically hypertensive strain (GH) of the Otago Wistar rat possesses more sensory neurons containing the neuropeptide substance P (SP) than does its genetically related control normotensive strain. 2. As SP contributes to airway inflammation by increasing microvascular permeability, we assessed the extravasation of Evans Blue dye in trachea and main bronchus of anaesthetized GH and control rats, in the presence of endogenous (capsaicin-liberated) or exogenous SP. 3. Following intravenous administration of either capsaicin (75 microg kg(-1)) or SP (3.3 nmol kg(-1)), extravasation of Evans Blue in airways from GH rats was only about 60% of that in airways of control rats. This difference was not gender-specific and responses to capsaicin were abolished by pretreatment with a selective NK1 receptor antagonist SR 140333 (360 nmol kg(-1)). 4. By contrast, the extravasation of dye caused by intravenous 5-hydroxytryptamine (0.5 micromol kg(-1)) was similar in magnitude in both GH and control strains. 5. Falls in systemic arterial blood pressure in response to exogenous SP (0.1-3 nmol kg(-1)) or acetylcholine (0.2-2 nmol kg(-1)) were also very similar between strains, but those in response to capsaicin (75 microg kg(-1)) in the GH rats were about double those in control rats. The hypotensive response to SP was abolished by SR 140333, but that to capsaicin was unaffected. 6. Our results indicate that the increased peripheral innervation density by SP-nerves in GH rats is accompanied by reduced inflammatory responses to SP. This does not involve decreased vasodilator potency of SP and is therefore probably related to altered endothelial responsiveness.

Neurokinin A and substance P vary independently in different regions of rat sensory neurons

Neurokinin A (NKA) and substance P (SP) are often co-localized in small sensory neurons and have been suggested to subserve similar roles. We have now compared tissue levels of NKA and SP in rat cervical dorsal root ganglia with those in the central and peripheral terminations of the same neurons. We found that NKA content was less than that of SP in dorsal root neuron perikarya and in two peripheral tissues (superior cervical ganglion and ear skin) containing SP axon terminals from cervical spinal ganglia; in a third peripheral tissue, trachea, equal amounts of NKA and SP were present. By contrast, in the spinal cord containing the central terminals of these sensory neurons there was almost twice as much NKA as SP. Our results indicate that, although NKA and SP are co-localized in sensory neurons, their levels vary independently, suggesting distinct functional roles.