Descending adrenergic input to the primate spinal cord and its possible role in modulation of spinothalamic cells.

The present study focuses on 3 different aspects of the descending adrenergic system in the primate: (1) the distribution of adrenergic fibers and terminals in the spinal cord, (2) the source of this input and (3) the possible physiological effects of this system on spinal nociceptive processing. Antibodies to the enzyme phenylethanolamine-N-methyltransferase (PNMT) were employed to map the distribution of epinephrine-containing axonal profiles in the primate spinal cord. Smooth longitudinally oriented fibers were localized to the outer edge of the lateral funiculus. PNMT-containing axonal enlargements were distributed to the superficial dorsal horn, intermediate gray matter and the region surrounding the central canal at all spinal cord levels. PNMT-immunostained profiles were also observed in the intermediolateral cell column. A double labeling study employing retrograde transport of HRP from the spinal cord and PNMT immunohistochemistry identified a small population of HRP-PNMT-labeled neurons in the 'C1' region at the levels of the medulla and ponto-medullary junction. Thus, these cells are a probable source of adrenergic input to the spinal cord. Electrophysiological studies demonstrated that iontophoresis of epinephrine onto identified primate spinothalamic tract neurons in the lumbar dorsal horn resulted in inhibition of the glutamate-induced firing of these cells. The data from these studies support the hypothesis that adrenergic (PNMT-containing) cells in the caudal brainstem project to all levels of the cord and may contribute to descending modulation of nociceptive processing at these levels.

Actions of opioids on primate spinothalamic tract neurons.

The effects of iontophoretically applied opiates were tested on 24 spinothalamic tract cells in 12 anesthetized monkeys. The drugs used were chosen because of their agonist actions on different classes of opiate receptors (mu, morphine; kappa, dynorphin; delta, methionine enkephalinamide; sigma, N-allylnormetazocine or SKF 10047 and phencyclidine). The actions of the opiate drugs were generally inhibitory, although excitatory or mixed effects were sometimes seen, especially with morphine and dynorphin. Drug effects could change, depending on the position of the iontophoretic electrode array or on the current employed. Naloxone sometimes antagonized the action of the opiate drugs used, but naloxone did not seem to be a drug suited for iontophoretic application. A number of explanations are discussed to explain the variable actions of the opiate drugs.

Effects of iontophoretically released amino acids and amines on primate spinothalamic tract cells.

The effects of glutamate (Glu), gamma-aminobutyric acid (GABA), glycine (Gly), serotonin (5-HT), norepinephrine (NE), dopamine (DA), and acetylcholine (ACh) were examined in this study by iontophoretic application onto primate spinothalamic tract (STT) neurons identified antidromically by stimulation in the contralateral thalamus. Drugs were tested for effects on background activity, Glu-induced firing, and activity evoked by pinching of the skin. Whereas Glu excited STT cells and was thus used for tests of the other compounds, the amino acids GABA and Gly inhibited Glu- and pinch-induced activity in all STT cells examined. STT cells were also inhibited by 5-HT, NE, and DA. Only two cases of excitation by 5-HT were seen (of 58 cells tested). ACh also had inhibitory actions on STT cells, although 3 of 21 cells exhibited some enhancement of activity. The effects of these compounds on identified STT cells resemble previous demonstrations of the effects of these drugs on dorsal horn interneurons. The results suggest that GABA, Gly, 5-HT, NE, and DA may be inhibitory neurotransmitters on nociceptive STT cells.

Effects of iontophoretically released peptides on primate spinothalamic tract cells.

The peptides substance P (SP), methionine-enkephalin (M-ENK), leucine-enkephalin (L-ENK), and cholecystokinin (CCK) were released iontophoretically near spinothalamic tract (STT) cells in anesthetized monkeys and STT-like cells in decorticate, spinalized monkeys. Peptide effects were observed on background discharges, firing induced by release of glutamate, and activity evoked by pinching the skin. SP could have any of several actions on STT cells, including excitation, inhibition, or biphasic effects. Multiple effects often resulted while recording from an individual cell when the dose or the electrode position was changed. M-ENK and L-ENK generally inhibited STT cells, and in some cases it was possible to demonstrate a reversal of the inhibition by naloxone. CCK also caused an inhibition that was additive with that produced by L-ENK. The multiple actions of SP on STT cells suggests the possibility that there may be more than one type of SP receptor on STT cells, although alternative explanations should be considered. Inhibition of STT cells by M- and L-ENK is consistent with the known analgesic action of opiates through spinal cord mechanisms. CCK has an action on STT cells similar to that of the enkephalins.

Primate raphe- and reticulospinal neurons: effects of stimulation in periaqueductal gray or VPLc thalamic nucleus.

Recordings were made from 132 raphe- and reticulospinal tract neurons in the medial part of the lower brain stem in 32 anesthetized monkeys. Recording sites were in the nucleus raphe magnus, the rostral nucleus raphe obscurus, and the reticular formation adjacent to the raphe. The neurons were identified by antidromic activation from the upper lumbar spinal cord. Of the population sampled, 83 cells were activated antidromically from the left dorsal lateral funiculus (DLF), 32 from the right DLF, and 17 from both sides. The mean latency for antidromic activation was 8.2 +/- 7.1 ms, corresponding to a mean conduction velocity of 22.8 m/s. No conduction velocities characteristic of unmyelinated axons were observed. Collision tests indicated that raphe-spinal axons that bifurcated to descend in both DLFs branched within the spinal cord. The effects of stimulation in the periaqueductal gray (PAG) or adjacent midbrain reticular formation were tested on 102 spinally projecting neurons in the medial medulla. Of these, 60 cells were excited, 9 cells were inhibited, 8 showed mixed excitation and inhibition, and 25 cells were unaffected. The mean latency for excitation was 11.6 ms and for inhibition, 17.8 ms. Threshold for excitation of raphe- and reticulospinal neurons ranged from 50 to 400 microA. Raphe- and reticulospinal tract cells could often (31/46 cells tested) be excited following stimulation in the ventral posterior lateral nucleus of the thalamus. The mean latency of excitation was 35.6 ms (range, 6-112 ms). Receptive fields could be demonstrated for 80 raphe- and reticulospinal cells, while 48 neurons possessed no demonstrable cutaneous receptive field. Most cells had large excitatory receptive fields, often encompassing the surface of the entire body and face. Some neurons had complex excitatory and inhibitory receptive fields, whereas other cells had large inhibitory receptive fields over much of the surface of the body and face. For most cells (52/55) with excitatory receptive fields, the only effective stimuli were noxious mechanical or noxious heat stimuli. Nonnoxious mechanical stimuli, such as brushing the skin, were capable of activating only a few (3/55) raphe- and reticulospinal neurons, and these were more effectively excited by noxious stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Drug effects on the 5-HT response of Schistosoma mansoni.

Several vertebrate 5-HT antagonists at concentrations around 0.1 mM reduced 5-HT-induced increases in the motor activity of the parasitic blood fluke Schistosoma mansoni. The order of potency for 5-HT response antagonism was haloperidol greater than cyproheptadine greater than mianserin greater than trazodone greater than spiperone greater than methysergide. Nisoxetine, a 5-HT uptake inhibitor in vertebrate preparations, was also a potent antagonist of the 5-HT response in schistosomes. The potent antischistosomal praziquantel reduced the 5-HT response similarly to the other antiserotonergic drugs, but at much lower concentrations, beginning around 0.1 microM. The 5-HT agonist quipazine stimulated worm activity at 1-0.1 mM when applied alone, but reduced the 1 mM 5-HT response when quipazine and 5-HT were administered concurrently. Dopamine (DA) alone had no effect on the overall activity of S. mansoni. Although no drug was found to have absolute species specificity, quantitative differences were observed between the relative activity of drugs in schistosomes and vertebrates.

Schistosoma mansoni: species-selective response to N-chloroethyl-acetylcholine derivatives.

Three new acetylcholine mustard analogs were tested on schistosome and vertebrate neuromuscular preparations. These compounds extend a previously reported structure-activity series. The new compounds investigated include isopropyl-, cyclohexyl-, and benzyl-2-acetoxyethyl-2'-chloroethylamine (PrM, ChM, and BzM). In schistosome motor activity studies, all of the compounds caused irreversible reductions in the activity of Schistosoma mansoni after 1 hr exposure followed by 19 hr in drug-free medium. Under nonlethal conditions of dosage and exposure time, all compounds blocked carbachol-induced paralysis, indicating a possible action at schistosome cholinergic sites. All the compounds also reduced the labeling of schistosomes by dimethylaminonapthalene-5-sulfonamidoethyl dimethylamine hydrochloride (DDNS), a fluorescent ACh analog. In isolated vertebrate tissue experiments. PrM and ChM had slight agonist activity in the guinea pig ileum, and only PrM had agonist activity in the frog rectus abdominis preparation. PrM and BzM were found to antagonize ACh responses in the guinea pig ileum. Exposure of vertebrate tissues for 1 hr to high concentrations of ChM caused no long-lasting inhibition of ACh, pilocarpine, and serotonin (5HT) responses. Histamine responses were slightly reduced from control. Following 1 hr exposure of vertebrate tissues to PrM, initial reductions in all responses were seen, followed by recoveries to near control. BzM treatment, however, reduced all responses far below control; the tissues did not recover even after washing for 2 hr. It is concluded that ChM and, to a lesser extent, PrM, had a greater effect on schistosomes than on vertebrate tissues. BzM did not display species selectivity in this favorable direction.

Effects of biogenic amines on raphe-spinal tract cells.

Stimulation in the nucleus raphe magnus produces analgesia in behavioral paradigms and inhibits spinal cord nociceptive neurons. Similar effects result from stimulation of the periaqueductal gray (PAG). Such actions may be mediated via a synaptic link between PAG and nucleus raphe magnus or the adjacent reticular formation. In this study we have examined the effects of biogenic amines applied iontophoretically in the vicinity of nucleus raphe magnus neurons that project to the spinal cord in monkeys. Raphe-spinal tract (RST) neurons were identified by antidromic activation after stimulation of the dorsolateral funiculi at an upper lumbar level. The actions of serotonin, quipazine, norepinephrine, dopamine and acetylcholine (ACh) were tested against the background activity, the activity evoked by glutamate pulses or the excitation of RST cells by stimulation in the PAG. Serotonin, quipazine, norepinephrine and dopamine produced a current-dependent inhibition of background activity and the responses to glutamate pulses in all RST cells tested. No cases of excitation were found. By contrast, ACh enhanced activity produced by glutamate pulses in all RST cells observed. ACh also enhanced the background activity of all but one of the RST cells; however, ACh did not activate cells with little or no background discharge. Serotonin and norepinephrine often inhibited PAG excitation of RST cells. No facilitation of PAG excitation was observed. We conclude that the actions of serotonergic and catecholaminergic agonists on raphe-spinal cells are inhibitory whereas the effect of ACh is facilitatory.

Effects of acetylcholine mustard analogs on schistosome and vertebrate neuromuscular preparations.

Two acetylcholine mustard analogs were synthesized for studies of the structural requirements for drugs having selective neurotoxic effects in schistosomes. Drugs investigated in the present study include methyl- and butyl-2-acetoxyethyl-2'-chloroethylamine (MeM and BuM). In worm activity monitor experiments, both MeM and BuM irreversibly paralyzed Schistosoma mansoni after 1-hr exposure followed by 19 hr in drug-free medium. The concentrations required for parasite paralysis were similar to effective concentrations of known antischistosomal drugs. An immediate effect of both compounds was blockage of carbachol-induced paralysis, suggesting that they may bind at schistosome cholinergic sites. Furthermore, fluorescent labeling of schistosomes with dansyl choline was reduced by both compounds. In vertebrate studies, while MeM was a potent agonist at both muscarinic and nicotinic receptors, BuM had only a slight effect. MeM and BuM had no major irreversible cholinergic effect in vertebrate tissues. BuM, therefore, has distinctly different pharmacological actions in schistosomes and in vertebrates.