Concurrent measurement of substance P and serotonin in spinal superfusates: failure of capsaicin and p-chloroamphetamine to co-release

Emerging relationships between cytochemical properties and sensory modality transmission in primary sensory neurons

Primary sensory neurons have been categorized according to a variety of characteristics, including modality responsiveness, somal size, cytology, cytochemistry, and the organization of their central axon collateral arborizations. A major aim in the study of primary afferents has been to determine the relationships between dorsal root ganglia neuronal physiology, anatomy, and chemistry that could provide a basis for a classification scheme more directly relevant to function. Here we briefly review these relationships and examine the utility of specific histochemical and immunohistochemical markers representative of distinct populations of neurons that may transmit particular sensory modalities. In addition, we discuss some of our observations suggesting that one population of dorsal root ganglia neurons contains high levels of cytochrome oxidase, carbonic anhydrase, parvalbumin, and calbindin D28k, while a separate population contains fluoride-resistant acid phosphatase, calcitonin gene-related peptide, and displays immunoreactivity with an antibody that labels the central arborization of a specific class of unmyelinated afferents in the dorsal horn. This may have implications for the combinations of substances contained within neurons with distinct sensory functions.

Tachykinin release from rat spinal cord in vitro and in vivo in response to various stimuli

The aim of the present study was to establish an experimental model, previously used in cat, for studying tachykinin release from the rat spinal cord in vivo and to compare the results with those obtained in vitro. Stimulation with pulses of 40 mM potassium or 10 microM capsaicin in the spinal cord superfusion fluid increased the release of substance P (SP)- and neurokinin A (NKA)-like immunoreactivity (LI) both in vivo and in vitro. The amounts of SP-LI and NKA-LI released by potassium in vitro were 1.02 +/- 0.12 and 1.17 +/- 0.22 fmol/mg tissue, respectively. Also the ratio between the amounts released by two consecutive potassium stimulations were similar for SP-LI and NKA-LI. Reversed-phase high performance liquid chromatography of the NKA-LI released in vitro by potassium or capsaicin revealed a major immunoreactive component coeluting with synthetic NKA. Despite the use of highly sensitive radioimmunoassays, basal release of SP-LI and NKA-LI was found only in 9 of 31 in vivo experiments. In these, peripheral electrical stimulation of the sciatic nerves (50 Hz, 50 V and 0.05 ms or 10 Hz, 10 V and 5 ms) induced an increase of the SP-LI and NKA-LI levels in the superfusates. This increase persisted for more than 40 min after a 2 min stimulation. In most experiments, however, no SP-LI or NKA-LI could be detected in the superfusates, neither at basal conditions nor following electrical nerve stimulation. Similarly, no release of SP-LI could be detected in response to various noxious mechanical, thermal or chemical stimuli applied to the skin. The present results demonstrate that the superfused rat spinal cord may be used to study in vivo release of tachykinins in response to intense chemical stimulation of the entire spinal cord. However, the method seems to be less suitable for studies of tachykinin release in response to electrical activation engaging only a few spinal segments or in response to natural noxious stimuli. The results obtained in vitro suggest that SP and NKA are released in equimolar amounts from the spinal cord upon stimulation with potassium.

Release of endogenous monoamines into spinal cord superfusates following the microinjection of phentolamine into the nucleus raphe magnus

Previous studies have suggested that raphe-spinal neurons located in the nucleus raphe magnus (NRM) are tonically inhibited by noradrenergic neurons. Furthermore, blockade of the inhibitory noradrenergic input to the NRM induces antinociception which appears to be mediated by the release of both serotonin and norepinephrine in the spinal cord. The present experiments were designed to directly measure the release of endogenous serotonin and norepinephrine into spinal cord superfusates before and after the microinjection of the alpha-adrenergic antagonist phentolamine into the NRM. High-performance liquid chromatography with electrochemical detection was used to quantitate the monoamines. The injection of phentolamine into the NRM induced a significant increase in the amount of both norepinephrine and serotonin released in the spinal cord. This enhanced release was not observed following either the injection of phentolamine into sites outside the NRM or the injection of saline vehicle into the NRM. These results support the proposal that the antinociception induced by the blockade of the inhibitory noradrenergic input to the NRM is mediated by the activation of spinally-projecting serotonergic and noradrenergic neurons.

Renal hyperemia in portal hypertension is not mediated by gastrointestinal peptides

The objectives of this study were to characterize the time course of development of the renal hyperemia induced by chronic portal vein stenosis (PVS) in the rat, and to assess the possibility that vasoactive blood-borne gastrointestinal peptides mediate the renal hyperemia in established portal hypertension. Blood flow to the kidneys was measured with radioactive microspheres over a ten day time course. On day 2, no difference in renal blood flow (RBF) was observed in PVS rats as compared with controls. However, by day 4, RBF significantly increased by 35% in PVS vs. control animals. On day 6, the renal hyperemia in PVS rats reached a maximal value that was 42% higher than controls. A steady state hyperemia (approximately 40%) was maintained thereafter. Radioimmunoassay of plasma from control and established portal hypertensive rats (10 days samples) revealed that vasoactive intestinal polypeptide, substance P, cholecystokinin, gastrin, neurotensin, pancreatic polypeptide, beta-endorphin and peptide histidine-isoleucine amide are not elevated in arterial plasma of portal hypertensive rats. These data suggest that the renal hyperemia induced by chronic portal vein stenosis is apparent within 4 days of the onset of a hypertensive state and attains a steady state by day 8. Furthermore, at least eight blood-borne gastrointestinal peptides are not directly involved in the renal hyperemia associated with chronic portal hypertension.

Substance P distribution and effects in the canine epicardial coronary arteries

Substance P (SP), a vasoactive neuropeptide detected in animal and human hearts has been reported to increase coronary blood flow in animals. However, no data are available on SP effects on epicardial coronary arteries, the site of coronary disease. To determine the amount and distribution of SP and its action in the large coronary vessels, we studied two groups of dogs. One group was anesthetized for collecting three 1 cm segments of the circumflex coronary artery (CX) and left anterior descending artery (LAD) through a left thoracotomy. These segments represented proximal (I), middle (II), and distal (III) portions of the two arteries. Concentrations (ng/g) of SP-like immunoreactivity (SP-LI) were determined by radioimmunoassay. SP-LI was present in LAD (I: 1.17 +/- 0.20, II: 1.08 +/- 0.36, III: 1.14 +/- 0.25) and CX (I: 1.44 +/- 0.38, II: 1.51 +/- 0.47, III: 0.70 +/- 0.20). SP differences among segments of LAD and segments I and II of CX were not significant, but there was a significant difference between segment III of CX and the others. In the second group of closed chest anesthetized dogs, we examined the effects of intracoronary SP infusion before and during administration of serotonin (5HT). LAD and CX artery responses (% area change) to SP and to SP plus 5HT were examined using quantitative coronary angiography. Intracoronary 133Xe in saline provided coronary flow data. SP infusion produced significant vasodilation in segment II (15% area increase) and III (17%) during the highest dose (1 microgram/min). The three SP doses infused with 5HT (0.05 mg/min) did not produce vasodilation, although LAD segment III constriction from 5HT was abolished during the highest dose of SP infusion. The presence of SP, and its dilatory effect on the coronary arteries, suggests a role in maintaining vasodilator tone in the coronary arteries.

Amphetamine-induced changes in immunoreactive NPY in rat brain, pineal gland and plasma

Acute injection of d-amphetamine (10 mg/kg), administered to rats 60 minutes prior to sacrifice, induced a doubling of immunoreactive NPY (NPY-IR) in pineal gland. No changes, however, could be detected in levels of NPY-IR in grossly dissected or microdissected regions of rat brain, nor were changes evident in plasma level concentrations of NPY-IR following acute amphetamine pretreatment. When amphetamine was injected twice daily for six days and once more 60 minutes prior to sacrifice, levels of NPY-IR were decreased in caudate putamen and the paraventricular and dorsomedial nuclei of the hypothalamus, while concentrations of NPY-IR were increased in medial preoptic nucleus, pineal gland, and plasma. These data indicate that levels of NPY-IR are susceptible to manipulation by amphetamine, where the extent and direction of change (increase or decrease) depends on both the frequency of drug administration and the nature of the sampled tissue. Based on the effects of amphetamine on central and peripheral norepinephrine and epinephrine disposition observed in other studies, the data also suggest that NPY-IR and catecholamine dispositions are not directly correlated and may be inversely related in some tissue.

The acute and chronic effects of capsaicin on slow excitatory transmission in rat dorsal horn

The acute and chronic effects of capsaicin on rat spinal dorsal horn neurons and the excitatory transmission in the dorsal horn were investigated by means of intracellular recording techniques in the spinal cord slice preparation. Bath application of capsaicin (1-2 X 10(-5) M) produced in a majority of cells a prolonged depolarization associated with an increase in synaptic activity and intense neuronal discharges. During and immediately following the capsaicin depolarization, repetitive stimulation of a dorsal root failed to elicit the slow depolarization. After neonatal capsaicin treatment the proportion of dorsal horn neurons exhibiting the slow excitatory transmission was markedly reduced, however, the fast excitatory postsynaptic potentials were present in all examined cells. In addition, the proportion and sensitivity of the cells responding with a slow depolarization to substance P increased.

Action of intrathecal capsaicin and its structural analogues on the content and release of spinal substance P: selectivity of action and relationship to analgesia

Intrathecal injections of capsaicin (CAP) and 4 other homovanillic acid (HMV) derivatives related to the structure of CAP were carried out. Capsaicin, 1-nonenoylvanillylamide (NVA), HMV-dodecylamide (DCA) (but not HMV-cyclohexylamide (CHA) or HMV-hexadecylamide (HDC] reduced the spinal content of substance P (SP), as measured by radioimmunoassay (RIA), and increased the tail-flick latency. Similar injection of kainic acid and piperine reduced levels of SP but failed to affect the tail-flick latency. None of the agents used affected spinal levels of cholecystokinin (CCK) or vasoactive intestinal peptide (VIP) as measured by RIA. In experiments using in vivo superfusion of the rat spinal cord, CAP, DCA and NVA were found to stimulate release of SP. Capsaicin had no effect on the levels of CCK or VIP immunoreactivity in the spinal superfusate. A tachyphylaxis to the effect of CAP and DCA on spinal SP release was demonstrated. Pretreatment with either agent blocked the releasing effect of the second. Pretreatment with an inactive analogue (HDC) had no effect on the subsequent activity of CAP. Kainic acid and piperine did not induce release of SP from the spinal cord. The relative selectivity of spinally administered capsaicinoids with regard to their effects on the content and release of peptides known to be contained in primary afferents and the presence of a similar structure-activity relationship for depletion and release of SP, desensitization and antinociception suggest the presence of a specific receptor site associated with a specific population of primary afferents through which pain information may pass. Whether SP is an 'afferent pain transmitter' is not clear, but at the least, it appears to serve as a marker for a population of afferents acted upon by spinally administered capsaicinoids.