The relative contribution of ascending and descending serotonergic pathways in p-chloroamphetamine-induced antinociception

Effects of serotonin depletion by p-chlorophenylalanine, p-chloroamphetamine or 5,7-dihydroxytryptamine on central dopaminergic neurons: focus on tuberoinfundibular dopaminergic neurons and serum prolactin

Three serotonin (5-HT) neurotoxins, p-chlorophenylalanine (PCPA, 125 and 250 mg/kg, i.p.), p-chloroamphetamine (PCA, 10 mg/kg, i.p.) and 5,7-dihydroxytryptamine (5,7-DHT, 200 microg/rat, i.c.v.) were used to examine whether depletion of central 5-HT has an effect on central dopaminergic (DA) neuronal activities or on prolactin (PRL) secretion. Adult ovariectomized Sprague-Dawley rats primed with estrogen (polyestradiol phosphate, 0.1 mg/rat, s.c.) were treated with one of three neurotoxins and then decapitated in the morning after 3-7 days. Blood sample and brain tissues were collected. The acute effect of PCA (from 30 to 180 min) was also determined. The concentrations of 5-HT, DA and their metabolites, 5-hydroxyindoleacetic acid and 3,4-dihydroxyphenylacetic acid, in the median eminence, striatum and nucleus accumbens were determined by HPLC-electrochemical detection. All three toxins significantly depleted central 5-HT stores by 11-20%. Except for PCPA, neither PCA nor 5,7-DHT had any significant effect on basal DA neuronal activities or PRL secretion. PCA also exhibited an acute effect on the release and reuptake of 5-HT and DA. In summary, depletion of central 5-HT stores to a significant extent for 3-7 days did not seem to affect basal DA neuronal activity and PRL secretion.

Sleep/waking effects following intrathecal administration of the 5-HT(1A) Agonist 8-OH-DPAT alone and in combination with the putative 5-HT(1A) antagonist NAN-190 in rats

Sleep, waking, and EEG power spectra were investigated in rats after intrathecal (IT) administration of a 5-HT(1A) agonist and a 5-HT(1A) antagonist. Total slow wave sleep (TSWS) was increased and waking was decreased over the 8-h recording period after the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (38 nmol). Within TSWS, SWS1 was unchanged while SWS-2 tended to be increased. The 5-HT(1A) antagonist 1-[2-Methoxyphenyl)-4-(4-(2-phthalimido)-butyl]piperazine hydrobromide (NAN-190) did not change any sleep/waking stages. Combined treatment with 8-OH-DPAT and NAN-190 increased variance. Following the combination, sleep and waking were not significantly different from control. SWS-2 tended to be reduced compared to the effect of 8-OH-DPAT alone. There were no systematic changes in neither waking nor TSWS fronto-frontal or fronto-parietal EEG power spectrum after any of the treatments, indicating that sleep quality was not changed. The results confirm earlier data suggesting that in the spinal cord, stimulation of 5-HT(1A) receptors have a dampening effect on transmission of sensory information, leading to deactivation and thereby increased sleep tendency. The reason why the 8-OH-DPAT effect was not clearly antagonized by the putative 5-HT1A antagonist NAN-190, may be due to the generally weak antagonistic and also partial agonistic effect of NAN-190 as reported in the literature.

Lesion of descending 5-HT pathways increases zimeldine-induced waking in rats

Sleep, waking, and EEG power spectra were investigated in rats with spinal 5,6-dihydroxytryptamine (5,6-DHT) lesions, following 20 mg/kg zimeldine or vehicle IP injections. 5,6-DHT selectively lesioned the descending serotonergic pathways. Lesion alone did not change sleep and waking stages compared to baseline, except for a reduction in REM sleep. Consistent with earlier findings, zimeldine in nonlesioned rats increased waking the first 2 h of recording. Zimeldine treatment in lesioned rats gave a significant additional 50% increase in waking the first 2 h and a corresponding decrease in total slow wave sleep, suggesting a potentiation of these effects. Zimeldine gave no significant changes in waking EEG power spectral density. Lesion gave a tendency to reduction between 4.0 and 15.5 Hz compared with baseline, and between 10.0 and 16.5 compared to the independent control group. In both comparisons, the combined treatment strengthened this effect, again suggesting a potentiating effect of lesion. In sleep, zimeldine reduced power over the whole spectrum (0.5-20.0 Hz), less in the lower frequencies than in the higher frequencies.

Light and electron microscopic studies of the effects of p-chloroamphetamine on the monoaminergic innervation of the rat spinal cord

A previous report has shown that small diameter serotoninergic (5-HT) axons innervating the forebrain are selectively eliminated by treatment with an amphetamine derivative, (+/-)p-chloroamphetamine (PCA; Mamounas et al., [1991] J. Comp. Neurol. 314:558-586). It is well known that the spinal cord is the target of numerous monoaminergic fibers of different types. We have previously shown that the dorsal and ventral horns and the intermediolateral cell column are innervated by numerous serotonin-, noradrenaline- and dopamine-containing axons, including both thin axons with small varicosities and beaded axons with large varicosities. In all these regions, the large majority of fine indolaminergic fibers do not establish synaptic contacts, contrasting with the beaded axons which mostly exhibit synapses. The present work was conducted to study the effect of PCA on the monoaminergic innervation of the adult rat spinal cord. Animals received two subcutaneous doses of PCA 24 hours apart and were perfused 3 weeks later. Immunocytochemistry was performed to detect 5-HT, noradrenergic and dopaminergic fibers by using light and electron microscopy. Light microscopy revealed that PCA treatment caused a marked and selective elimination of the fine 5-HT-immunoreactive fibers, mainly found in the dorsal horn, but spared all other larger axons. This selective effect on the dorsal horn innervating thin 5-HT fibers was confirmed with the electron microscope by calculating the synaptic incidence(s) of monoaminergic innervation. These results suggest that fine and beaded 5-HT axons correspond to two anatomically, biochemically and pharmacologically different types of fibers, which could arise from two subpopulations of brainstem neurons. In addition, this drug could be used to provide an experimental animal, devoid of 5-HT nonsynaptic fibers, thereby facilitating a study on the role of dorsal horn nonsynaptic system in pain modulation.

The tail-flick and formalin tests in rodents: changes in skin temperature as a confounding factor

In the tail-flick test as well as in the late phase in the formalin test skin temperature may in an important way influence the response. A reduced skin temperature may be misinterpreted as analgesia, and an increased skin temperature as hyperalgesia. These effects and the mechanisms that cause them are discussed. It is of particular importance to be aware of these confounding factors when using drugs or making lesions that influence blood flow or temperature regulation. It is important that all variables influencing the test results are kept constant throughout the experiment. This includes not only the ambient temperature, but also factors that may influence the vasomotor tone of the animals.

Effects of p-chloroamphetamine on brain serotonin neurons

p-Chloroamphetamine (PCA) is a useful pharmacologic tool for selectively increasing brain serotonin function acutely by release of serotonin into the synaptic cleft. PCA produces behavioral, neurochemical and neuroendocrine effects believed due to serotonin release after doses in the range of 0.5-5 mg/kg. At higher doses and at longer times, PCA causes depletion of brain serotonin. The mechanisms of this depletion are not well understood but require the serotonin uptake carrier. Antagonism of PCA-induced depletion of brain serotonin is a useful means of assessing the ability of a compound to block the serotonin uptake carrier on brain serotonin neurons. PCA can also be used as a neurotoxic agent to deplete brain serotonin in functional studies, apparently by destroying some serotonergic nerve terminals. Used in this way, PCA has an advantage over 5,6- and 5,7-dihydroxytryptamines in being effective by systemic injection, and it affects brain serotonergic projections with a different neuroanatomic specificity than the dihydroxytryptamines.

Lesions of bulbo-spinal serotonergic or noradrenergic pathways reduce nociception as measured by the formalin test

Intrathecal administration of the neurotoxins 5,6-dihydroxytryptamine (5,6-DHT) and 6-hydroxydopamine (6-OHDA) in rats selectively lesioned the descending spinal serotonergic and noradrenergic pathways, respectively. Four days after neurotoxin administration the behaviour was evaluated in the formalin test. Several behavioural variables were recorded. The statistical analysis of the results was supplemented using a multivariate statistical method (partial least squares projection to latent variables, PLS) in addition to traditional analysis of variance. The described methods for recording and statistical analysis of behaviour appear to be useful in describing drug-induced differences in behavioural patterns in the formalin test. Both types of lesion reduced the pain-related behaviour in the formalin test (protection of the paw, biting and licking). The results indicate that the descending monoaminergic pathways are parts of a network which maintains adequate nociceptive responses to a chemical stimulus, or to stimuli lasting several minutes, as in the formalin test.

The apparent hyperalgesic effect of a serotonin antagonist in the tail flick test is mainly due to increased tail skin temperature

It has been suggested that reduced activity in raphe-spinal serotonergic systems induces hyperalgesia. In rats, the serotonin antagonist metergoline (0.5 mg/kg intraperitoneally) reduced tail flick latency by 0.92 sec (p less than 0.001) and increased tail skin temperature by 2.4 degrees C (p less than 0.001) when measured 50 min after injection. Multiple regression analysis with tail flick latency as dependent variable and tail skin temperature and metergoline/vehicle as independent variables revealed a highly significant effect of tail temperature on tail flick latency. The increase of tail skin temperature explained a reduction of tail flick latency of 0.64 of the 0.92 sec observed [B = -0.267 +/- 0.034, t(37)= -7.75, p less than 0.0001]. When the effect on tail skin temperature was taken into account, metergoline reduced tail flick latency by 0.28 sec [B = -0.284 +/- 0.114, t(37) = -2.50, p less than 0.05]. Metergoline (0.5 and 2.0 mg/kg) did not significantly alter plantar paw skin temperature or the response temperature in the increasing temperature hot plate test. Thus, the observed effect of metergoline on tail flick latency is primarily due to an effect on tail skin temperature. The possibility exists that the remaining effect of metergoline may be due to inadequate correction for the skin temperature change, and it is concluded that the study provide no clear evidence for a tonic inhibition of nociception by serotonergic systems.

Apparent hyperalgesia in the mouse tail-flick test due to increased tail skin temperature after lesioning of serotonergic pathways

The relationship between tail skin temperature and responsiveness to noxious radiant heat in the tail-flick test was investigated in mice. A significant negative correlation between tail skin temperature and tail-flick latency was found when the tail skin temperature was increased by elevating the ambient temperature. After intracerebroventricular injection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms) tail skin temperatures were increased and tail-flick latencies reduced. In contrast, administration of the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA, 400 mg kg-1 for 5 consecutive days) lead to a slight lowering of tail temperatures and a tendency towards elevation of tail-flick latencies. The results show that factors which affect tail skin temperature also influence the tail-flick test in mice. The divergent effects of 5,7-DHT and PCPA on tail-flick responsiveness may be due to the different effects of these compounds on the tail skin temperature. The results suggest that the reduced tail-flick latency after partial destruction of serotonergic pathways by 5,7-DHT is due primarily to the increased tail skin temperature. The dependence of tail-flick latency on tail skin temperature limits the usefulness of the tail-flick test unless changes in tail skin temperature are controlled for.