A comparison of the effects of morphine on 5-HT metabolism in the periaqueductal gray, ventromedial medulla and medullary dorsal horn: in vivo electrochemical studies in freely moving rats

The effect of systemic morphine on serotonin (5-HT) metabolism within the dorsal raphe nucleus (DRN) has been investigated by in vivo 5-hydroxyindole electrochemical (peak ‘3’) detection in freely moving rats. Morphine caused a weak and delayed, but naloxone-reversible, increase in peak ‘3’. This increase was poorly, if at all, correlated with the morphine-induced analgesia. Finally, stress and/or noxious stimulation had no effect on this signal. These results are compared with our previous studies using the same methodological approaches and show that morphine caused a significant and specific increase in 5-HT metabolism at the levels of nucleus raphe magnus (NRM) and medullary dorsal horn. Furthermore, as shown in the present paper, there was also a good correlation between the time course of such increases and the analgesic effect of morphine. These findings are discussed with reference to the involvement of 5-HT mechanisms in the so-called DRN-NRM-dorsal horn ‘intrinsic analgesic system’.     

Effect of subcutaneous administration of the chemical algogen formalin, on 5-HT metabolism in the nucleus raphe magnus and the medullary dorsal horn: a voltammetric study in freely moving rats

The effect of subcutaneous administration of the chemical algogen formalin, on serotonin (5-HT) metabolism in the nucleus raphe magnus (NRM) and the medullary dorsal horn (MDH) has been investigated using in vivo 5-hydroxyindole electrochemical (peak ‘3’) detection with treated, multi-carbon fiber electrodes and differential pulse, or normal pulse, voltammetry in freely moving rats. The subcutaneous (s.c.) injection of 50 μl of 10% formalin in the left forepaw was followed, at the NRM level, by a significant increase in the voltammograms as compared to controls (50 μl of saline 0.9% s.c. in left forepaw) for about 70 min after the injection, before a return to control values. At the MDH level, the formalin injection induced no significant effect on peak 3, as compared to controls, during the first 70 min. After that, the voltammograms significantly increased and remained above controls for up to 180 min. Thus, the time-courses of NRM and MDH effects appear markedly different. These findings suggest that, depending on the anatomical level (NRM or MDH) and/or the period of observation, one can measure differences in the time-course of the increase in 5-HT metabolism in the NRM-dorsal horn serotonergic system by tonic noxious stimuli, such as the formalin test.     

In vivo electrochemical detection of 5-hydroxyindole within the trigeminal nucleus caudalis of freely moving rats: the effect of morphine

The trigeminal nucleus caudalis is considered the equivalent of the orofacial nociceptive system of the dorsal horn of the spinal cord. At the level of this trigeminal area (i.e. medullary dorsal horn) the transmission of noxious inputs is strongly modulated by a descending, serotonergic system mainly originating from the nucleus raphe magnus (NRM). The present study in freely moving animals reports the effect of morphine on the 5-hydroxyindole oxidation current recorded in the medullary dorsal horn. Complementary data from recordings in spinal dorsal horn in acutely anesthetized rats are also presented. A current recorded at 270–290 mV (peak ‘3’), characteristic of 5-hydroxyindoleacetic acid (5-HIAA), was measured with treated multi-fiber carbon electrodes, using differential pulse voltammetry (DPV) or differential normal pulse voltammetry (DNPV). In control rats, the amplitude of the peak remained constant for many hours. Morphine (10 mg/kg i.p.) caused a significant increase which plateaued between 35 and 80 min (mean increase: 127 ± 5 % of control values); recovery was complete by about 3 h. Simultaneous injection of naloxone (1 mg/kg i.p.) totally abolished the effect of morphine. By contrast, morphine was without effect on peak 3 recorded in the spinal dorsal horn of chloral hydrate (450 mg/kg i.p.) anesthetized rats. It is concluded that in non-anesthetized freely moving animals morphine clearly increases the metabolism of serotonin (5-HT) in the medullary dorsal horn. This finding confirms previous neurochemical data showing an increased synthesis or release of 5-HT in the spinal cord after systemic morphine or its microinjection into either the periaqueductal gray matter or the NRM, and underlines the value of in vivo electrochemistry in monitoring changes in 5-HT metabolism directly and continuously during various physiological and pharmacological procedures.     

In vivo electrochemical evidence that the tricyclic antidepressant femoxetine potentiates the morphine-induced increase in 5-HT metabolism in the medullary dorsal horn of freely moving rats

Acute administration of tricyclic antidepressants (TCAs) is known to potentiate morphine antinociception. At the medullary dorsal horn (MDH) level systemic morphine has been shown to increase serotonin (5-HT) metabolism as measured by in vivo electrochemistry in freely moving rats. Using similar electrochemical detection of 5-hydroxyindole (peak '3') within the MDH, the present study investigated the effect of the specific 5-HT uptake inhibitor femoxetine on peak 3 and the effects of this TCA on changes in 5-HT metabolism induced by morphine. Acutely administered femoxetine (40 mg/kg i.p.) (i) induced a small but significant increase in peak 3 and (ii) strongly potentiated the effect of morphine (10 mg/kg i.p.) on 5-HT metabolism, this potentiation being opiate specific since simultaneous injection of naloxone (1 mg/kg i.p.) abolished the effect of morphine. These findings provide an in vivo neurochemical basis for the potentiation of morphine antinociception by TCAs. They further emphasize the importance of 5-HT bulbospinal descending pathways in morphine antinociception.     

In vivo electrochemical detection of 5-hydroxyindoles in the dorsal horn of the spinal cord: the contribution of uric acid to the voltammograms

Treated carbon fiber electrodes were used with differential normal pulse voltammetry (DNPV) for in vivo determination of the relative participation of uric acid (UA) to peak 3 derived between 250-300 mV in the dorsal horn of the spinal cord of anesthetized rats. In vitro, treated carbon fiber electrodes respond linearly over a large range of concentrations of UA (oxidation potential around 250 mV) and 5-hydroxyindoleacetic acid (5-HIAA, oxidation potential around 280-290 mV), but are 3 to 4 times more sensitive to 5-HIAA than to UA. In vivo the question remains as to the exact nature of peak 3 because the difference between oxidation potentials of UA and 5-HIAA is not great enough to permit a separate monitoring of the two compounds. In normal rats, administration of the xanthine oxidase inhibitor allopurinol, produced a progressive decrease of the signal, which reached 64.3% of controls at 120 min (35.6% diminution) after injection, and then plateaued around this value for up to 2 h. The administration of the monoamine oxidase inhibitor (MAOI) clorgyline, produced a classical decay in the voltammograms due to a diminution of endogenous 5-HIAA; however, allopurinol injected 3 h after MAOI gave an additional decrease of peak 3 of about 28%. Finally, in rats pretreated with parachlorophenylalanine (pCPA), the residual peak (32.48% as compared to peak 3 of normal rats taken as 100%), the potential of which is shifted to near that of UA, could be decreased by allopurinol to a level of 9.6% of the peak in control animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional meaning of tryptophan-induced increase of 5-HT metabolism as clarified by in vivo voltammetry

Differential pulse voltammetry with carbon fiber electrodes was used to study serotonin (5-HT) metabolism in freely moving rats. The electrodes implanted in the striatum recorded the extracellular 5-hydroxyindoleacetic acid (5-HIAA) oxidation peak after oral tryptophan (150 mg/kg). This 5-HT precursor did not modify the 5-HIAA peak in any rat tested, but it raised 5-HIAA levels determined in total tissue by a classical biochemical method (HPLC). The administration of 5-hydroxytryptophan (5-HTP) (25 mg/kg i.p.) induced an increase of 5-HIAA detectable both in the extracellular medium by voltammetry and in tissue samples. As previously shown, dorsal raphe electrical stimulation raises extracellular 5-HIAA in the striatum and this effect is enhanced by pretreatment with tryptophan. The results suggest that tryptophan in 'normal' conditions enhances 5-HT metabolism without affecting 5-HT release unless such release is stimulated. 5-HTP increases 5-HT metabolism and release.     

Altered responsiveness to substance P and 5-hydroxytryptamine in cat dorsal horn neurons after 5-HT depletion with p-chlorophenylalanine

The responsiveness of functionally identified cat spinal dorsal horn neurons to iontophoretically applied substance P (SP) and 5-hydroxytryptamine (5-HT) has been investigated by means of extracellular recording after 5-HT depletion with p-chlorophenylalanine (p-CPA). In addition, the spinal levels of 5-HT, SP, cholecystokinin octapeptide, neurotensin, and vasoactive intestinal polypeptide have been measured in intact and p-CPA-pretreated cats. In the present study we have demonstrated an altered responsiveness of dorsal horn neurons to locally applied SP and 5-HT. We found in p-CPA-pretreated cats that the proportion of neurons responding with excitation to SP and 5-HT was significantly increased. At the same time, depression induced by 5-HT in the dorsal horn cells was virtually absent in p-CPA-pretreated animals. Our finding that spinal level of 5-HT was significantly decreased in p-CPA-treated animals is consistent with previous studies. No convincing alteration in the spinal levels of 4 analyzed peptides was found in p-CPA-treated animals. The present study has shown that pharmacological depletion of 5-HT has two major effects: (1) it increases significantly the proportion of dorsal horn neurons excited by SP and 5-HT; and (2) it is ineffective in inducing 5-HT supersensitivity. Further work is needed to explain mechanisms involved in these effects.     

GABA mimetics decrease extracellular concentrations of 5-HIAA (as measured by in vivo voltammetry) in the dorsal raphe of the rat

The effect of gamma-aminobutyric acid (GABA) mimetics on extracellular concentrations of 5-hydroxyindoleacetic acid (5-HIAA) (as measured by differential pulse voltammetry with carbon fiber electrodes) in the dorsal raphé has been investigated in the rat. Systemic administration of dipropylacetamide decreased extracellular 5-HIAA to a similar extent, and within a comparable time course, in the dorsal raphé and striatum. Similar results were obtained after intradorsal raphé infusion of muscimol (100 ng). In contrast, local infusion of tetrodotoxin into the dorsal raphé failed to alter serotonin metabolism in this area. It is concluded that GABA depresses serotonin metabolism not only in nerve endings, but also in dendrites (and/or cell bodies) of serotonergic neurons.     

Effect of amygdala kindling on the in vivo release of GABA and 5-HT in the dorsal raphe nucleus in freely moving rats

Our laboratory has previously reported a significant subsensitivity to iontophoretically applied GABA (gamma-aminobutyric acid) in dorsal raphe neurons of amygdala-kindled rats. This subsensitivity was selective for GABA and persisted at least 3 months after the last kindled seizure. In the present series of experiments, we explored mechanisms by which kindling could result in persistent GABA sensitivity changes, using in vivo microdialysis to quantitate neurotransmitter [including GABA and 5-hydroxytryptamine (5-HT)] release in the dorsal raphe nucleus of awake, unrestrained amygdala-kindled rats. Depolarization-induced release of GABA is markedly increased in the dorsal raphe nucleus in amygdala-kindled animals. This change in depolarization-induced GABA release appeared to be graded, dependent upon the stage to which the animal is kindled. Thus GABA release is increased in animals kindled to Stage 2 and even greater in animals kindled to Stage 5 seizures. The change in GABA release is also selective, since no consistent change in the release of other putative amino acid neurotransmitters or 5-HT was observed in these same animals. We hypothesize that this increase in depolarization-induced release of GABA in the amygdala-kindled animal underlies the development of subsensitivity to GABA in dorsal raphe neurons.     

Effect of naloxone on morphine-induced changes in striatal dopamine metabolism and glutamate, ascorbic acid and uric acid release in freely moving rats

Recent findings have shown that systemic morphine increases extracellular dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), ascorbic acid (AA) and uric acid concentrations in the striatum of freely moving rats. The morphine-induced increase in DA oxidative metabolism is highly correlated with that of xanthine. In the present study, we evaluated the effects of subcutaneous (s.c.) naloxone (1 mg/kg) on morphine-induced changes in DA, DOPAC, HVA, 5-hydroxyindoleacetic acid (5-HIAA), AA, uric acid and glutamate in the striatum of freely moving rats using microdialysis. Dialysates were assayed by high performance liquid chromatography with electrochemical detection or (glutamate) ultraviolet detection. Morphine (5–20 mg/kg) given s.c. increased DA, DOPAC+HVA, 5-HIAA, AA and uric acid and decreased glutamate dialysate concentrations over a 3 h period after morphine. Morphine (1 mM), given intrastriatally, did not affect all the above parameters, with the exception of an early short-lasting decrease in AA concentration. Naloxone antagonised all morphine-induced changes with the exception of AA increase and glutamate decrease in dialysate concentrations. Systemic or intrastrial (0.2–2 mM) naloxone increased AA and decreased glutamate dialysate concentrations. When given intranigrally, morphine (1 mM) increased DOPAC+HVA, AA and uric acid and decreased glutamate dialysate concentrations over a 2 h period after morphine; DA and 5-HIAA concentrations were unaffected. These results suggest that: (i) morphine increases striatal DA release and 5-hydroxytryptamine oxidative metabolism by a μ-opioid receptor-mediated mechanism mainly at extranigrostriatal sites; (ii) morphine increases DA and xanthine oxidative metabolism and affects glutamate and AA release by a μ-opioid receptor mediated mechanism acting also at nigral sites; and (iii) a μ-opioid receptor-mediated mechanism tonically controls at striatal sites extracellular AA and glutamate concentrations.     

Expression of 5-HT2A receptor mRNA in rat spinal dorsal horn and some nuclei of brainstem after peripheral inflammation

The expression of 5-hydroxytryptamine 5-HT2A receptor mRNA was studied in the lumbar spinal dorsal horn, nucleus of raphe magnus (NRM), ventrolateral periaqueductal gray (vlPAG) and dorsal raphe nucleus (DRN) following carrageenan inflammation using in situ hybridization technique. The findings of this study demonstrated that 5-HT2A receptor mRNA was expressed with low to moderate levels in lumbar spinal dorsal horn, NRM, vlPAG and DRN. Following carrageenan inflammation, the expression of 5-HT2A receptor mRNA in ipsilateral dorsal horn, bilateral NRM, vlPAG and DRN was significantly increased. The peak occurred at 3 h and then there was a clear decrease but still a substantial number of labeled cells at 24 h after injection of carrageenan. This result suggested that the synthesis of 5-HT2A receptor is enhanced in spinal dorsal horn, NRM, vlPAG and DRN during inflammatory pain.     

Long-term synaptic plasticity in the spinal dorsal horn and its modulation by electroacupuncture in rats with neuropathic pain

Our previous study has reported that electroacupuncture (EA) at low frequency of 2 Hz had greater and more prolonged analgesic effects on mechanical allodynia and thermal hyperalgesia than that EA at high frequency of 100 Hz in rats with neuropathic pain. However, how EA at different frequencies produces distinct analgesic effects on neuropathic pain is unclear. Neuronal plastic changes in spinal cord might contribute to the development and maintenance of neuropathic pain. In the present study, we investigated changes of spinal synaptic plasticity in the development of neuropathic pain and its modulation by EA in rats with neuropathic pain. Field potentials of spinal dorsal horn neurons were recorded extracellularly in sham-operated rats and in rats with spinal nerve ligation (SNL). We found for the first time that the threshold for inducing long-term potentiation (LTP) of C-fiber-evoked potentials in dorsal horn was significantly lower in SNL rats than that in sham-operated rats. The threshold for evoking the C-fiber-evoked field potentials was also significantly lower, and the amplitude of the field potentials was higher in SNL rats as compared with those in the control rats. EA at low frequency of 2 Hz applied on acupoints ST 36 and SP 6, which was effective in treatment of neuropathic pain, induced long-term depression (LTD) of the C-fiber-evoked potentials in SNL rats. This effect could be blocked by N-methyl-d-aspartic acid (NMDA) receptor antagonist MK-801 and by opioid receptor antagonist naloxone. In contrast, EA at high frequency of 100 Hz, which was not effective in treatment of neuropathic pain, induced LTP in SNL rats but LTD in sham-operated rats. Unlike the 2 Hz EA-induced LTD in SNL rats, the 100 Hz EA-induced LTD in sham-operated rats was dependent on the endogenous GABAergic and serotonergic inhibitory system. Results from our present study suggest that (1) hyperexcitability in the spinal nociceptive synaptic transmission may occur after nerve injury, which may contribute to the development of neuropathic pain; (2) EA at low or high frequency has a different effect on modulating spinal synaptic plasticities in rats with neuropathic pain. The different modulation on spinal LTD or LTP by low- or high-frequency EA may be a potential mechanism of different analgesic effects of EA on neuropathic pain. LTD of synaptic strength in the spinal dorsal horn in SNL rats may contribute to the long-lasting analgesic effects of EA at 2 Hz.     

Inhibitory GABAergic influence on striatal serotonergic transmission exerted in the dorsal raphe as revealed by in vivo voltammetry

In vivo differential pulse voltammetry with electrochemically treated carbon fiber microelectrodes has been used to investigate the anatomical nature of the GABAergic influence on striatal serotonergic transmission in the rat. Lesion studies and pharmacological treatments demonstrated that the electrochemical signal recorded at 300 mV in the striatum probably corresponds to the oxidation of extracellularly released 5-hydroxyindoleacetic acid. Thus, dorsal raphé lesions or systemic administration of alpha-propyldopacetamide, NSD 1015, pargyline and MK212 decreased, whereas reserpine injection increased the amplitude of the signal. Moreover, L-5-hydroxytryptophan administration caused an increase in the signal which was almost completely prevented by pargyline pretreatment. Acute administration of dipropylacetamide (150 mg/kg i.p.) reduced the amplitude of the signal from the striatum, while injection of gamma-acetylenic GABA (200 mg/kg i.p.) was without effect. Repeated (but not acute) treatment with the GABA receptor agonist, progabide (400 mg/kg i.p.b.i.d. for 14 days), led to a pronounced decrease in the amplitude of the signal from the striatum. A similar effect was observed after intradorsal raphé infusion of GABA (10 and 100 micrograms), gamma-vinyl GABA (100 micrograms) and SL 75102 (10 micrograms), a principal metabolite of progabide. In contrast, local injection of the GABA receptor antagonists, bicuculline (1 and 10 micrograms) or R5135 (0.05 microgram), failed to affect the peak amplitude in the striatum. When infused into the dorsal raphé, R5135 (0.05-0.1 microgram) antagonized the diminution of the signal induced by intradorsal raphé infusion of GABA (100 micrograms) or SL 75102 (10 micrograms). Finally, electrolytic lesion of the habenular nuclei completely blocked the diminution of the signal from striatum induced by an intradorsal raphé infusion of GABA (100 micrograms). These results indicate that the inhibitory GABAergic control of striatal serotonergic transmission is exerted at the level of the dorsal raphé cells and depends upon the integrity of the habenulo-dorsal pathway.     

Stimulation of spinal 5-HT(2A/2C) receptors potentiates the capsaicin-induced in vivo release of substance P-like immunoreactivity in the rat dorsal horn

Stimulation of spinal serotonin (5-HT)_2A/2C receptors has previously been reported to lead to either a pro-nociceptive or an anti-nociceptive response. Behavioral data have indicated that the pro-nociceptive effect is related to the release of substance P (SP). The aim of this in vivo microdialysis study was to investigate if stimulation of spinal 5-HT_2A/2C receptors by the selective agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) induces spontaneous or capsaicin-evoked increase in the release of SP-like immunoreactivity (SP-LI) in the rat dorsal horn. A dose of capsaicin (25 μM in the perfusion medium administered for 30 min), which did not lead to a significant release of SP-LI on its own, induced a significant increase of greater than 4-fold of the SP-LI level following spinal application of 50 nmol DOI. Higher (500 nmol) or lower (5 nmol) doses of DOI failed to induce a similar effect. In rats with a peripheral inflammation, induced by carrageenan, capsaicin (25 μM) induced a non-significant increase of SP-LI. A significant 8-fold increase of the SP-LI level was detected following administration of 50 nmol DOI in combination with capsaicin. The effect of DOI, which was completely prevented by co-administration of the 5-HT_2A receptor antagonist ketanserin in control animals without peripheral inflammation, was only partly blocked in animals with carrageenan induced peripheral inflammation. In conclusion, stimulation of 5-HT_2A/2C receptors facilitates the capsaicin-evoked release of SP-LI in the dorsal horn in both animals with and without carrageenan-induced unilateral inflammation. The observation that the highest dose of DOI failed to induce SP-LI release may be due to an inhibitory postsynaptic action at this dose.     

Differential pulse voltammetry in the dorsal horn of the spinal cord of the anesthetized rat: Are the voltammograms related to 5-HT and/or to 5-HIAA?

Treated carbon fiber microelectrodes were used with the differential pulse voltammetry method for in vitro and in vivo determination of indoleamines. Under these conditions a peak of oxidation current which is characteristic of 5-hydroxyindoles is recorded at 280-300 mV. Treated carbon fiber microelectrodes respond in vitro linearly over a large range of concentrations of 5-hydroxytryptamine (5-HT) and of 5-hydroxyindoleacetic acid (5-HIAA), but are 5-8 times more sensitive to 5-HT than to 5-HIAA. In vivo, the question remains as to the exact nature of the peak because the oxidation potentials of 5-HT and 5-HIAA are close together and cannot be monitored separately. Pharmacological investigations were hence carried out in order to characterize the electrochemical signal detected at 300 mV in the dorsal horn of the lumbar spinal cord of chloral hydrate-anesthetized rats. Using 250 micron long carbon fiber microelectrodes, the electrochemical signal stabilizes at 30-90 min and the peak remains constant for up to 210 min. Administration of the monoamine oxidase inhibitor (MAOI) clorgyline produced a progressive decrease of the signal which reached a decrease of 33% of control at 180 min after injection. At this time biochemical measures demonstrated a 117% increase in 5-HT and a 32% decrease in 5-HIAA in the dorsal half of the spinal cord. Reserpine provoked an increase of 20% in the electrochemical peak and the 5-HIAA outflow blocker probenecid gave rise to a sustained plateau of about 60% above control values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of electrical stimulation of thalamic nucleus submedius and periaqueductal gray on the visceral nociceptive responses of spinal dorsal horn neurons in the rat

Electrical stimulation of the nucleus submedius (Sm) has been shown to suppress the viscerosomatic reflex (VSR), which is evoked by colorectal distension (CRD). We have examined the effects of focal electrical stimulation (0.3 ms, 50 Hz, 100 microA, 10 s) of the Sm and the periaqueductal gray (PAG) on the excitatory responses evoked by CRD in spinal dorsal horn neurons within the L6-S1 region in the urethane-anesthetized Wistar rats. Extracellular recordings were made from 32 spinal excitatory CRD responses. All of these neurons were convergent neurons with cutaneous receptive fields. The majority of the neurons (27/32) were wide dynamic range (WDR) neurons (responding to noxious and non-noxious cutaneous stimuli) while the remaining five neurons were nociceptive specific (NS) neurons (responding only to noxious cutaneous stimuli). The effects of electrical stimulation applied to 28 sites within the Sm were assessed for spinal neurons. Electrical stimulation in seven sites within the Sm (25%) inhibited the CRD excitatory response of dorsal horn neurons, while in two sites (7%) the same stimulation yielded facilitation. Electrical stimulation in the majority of the sites in the Sm (19/28, 68%) did not affect spinal excitatory CRD responses. On the other hand, electrical stimulation of the PAG clearly inhibited 20 of 22 (90%) CRD excitatory responses. These results suggest that the majority of Sm neurons may suppress VSR activity at a supraspinal reflex center rather than via a descending inhibition of spinal visceral nociceptive transmission, as is the case for the PAG.     

In vivo electrochemical detection of extraneuronal 5-hydroxyindole acetic acid and norepinephrine in the dorsal raphe nucleus of urethane-anesthetized rats

In vivo electrochemical detection of endogenous neurotransmitters was done in the dorsal raphe nucleus of urethane-anesthetized male Sprague-Dawley rats. Stereotaxically implanted carbon paste electrodes were scanned using a cyclic voltammetry amplifier with semiderivative signal processing over the potential range—0.2 to +0.5 V at the rate of 10 mV/s. Two distinct peaks were observed at +0.15 V (peak 1) and +0.25 V (peak 2), respectively. Peak identification was assessed by comparing the oxidation potential observed in vivo with those observed in in vitro experiments using pure catecholamines, indoleamines and their metabolites as well as ascorbic acid. Further characterization of in vivo peaks was done by observing changes in electrochemical peaks as well as tissue neurotransmitter concentrations after pharmacological manipulations. p-Chlorophenylalanine, m-hydroxybenzylhydrazine (NSD-1015), pargyline, a-methyl-p-tyrosine and fusaric acid were administered in an effort to block catecholamine or serotonin synthesis or degradation. Results of these experiments revealed that peak 1 primarily represents extracellular norepinephrine, while peak 2 is primarily produced by extracellular 5-hydroxyindoleacetic acid (5-HIAA).     

Differential effects of psychological stress on activation of the 5-hydroxytryptamine- and dopamine-containing neurons in the brain of freely moving rats

We investigated the effects of psychological stress, lacking direct physical stimulus, on the release of 5-hydroxytryptamine (5-HT) and dopamine (DA) in the basolateral nucleus of the amygdala (BLA) and the dorsal raphe nuclei (DRN) in the rat using the in vivo microdialysis technique with dual probes, one in each region of the same animals. Psychological stress was employed using the communication box paradigm. Psychological stress for 1 h significantly increased dialysate 5-HT levels in the DRN and the BLA. Psychological stress-induced 5-HT release in the BLA was significantly greater than those in the DRN, indicating that modifications of the serotonergic neurons in the BLA are more sensitive to psychological stress than are those of the DRN. Psychological stress also increased DA release in the BLA, while the dialysate DA levels in the DRN were unchanged. These results suggest that psychological stress preferentially activates ascending serotonergic neurons from the DRN to the BLA but not those of dopaminergic neurons. Furthermore, our findings indicate that both the serotonergic neurons and the dopaminergic neurons in the BLA may have a distinct role to play in the neuronal responses to psychological stress.     

Crucial role of the 5-HT2C receptor, but not of the 5-HT2A receptor, in the down regulation of stimulated dopamine release produced by pressure exposure in freely moving rats

Helium pressure of more than 2 MPa is a well known factor underlying pressure-dependent central neuroexcitatory disorders, referred to as the high-pressure neurological syndrome. This includes an increase in both serotonin (5-HT) and dopamine (DA) release. The relationship between the increase in 5-HT transmission produced by helium pressure and its effect on DA release has been clarified in a recent study, which have first demonstrated that the helium pressure-induced increase in DA release was dependent on some 5-HT receptor activation. In the present study, we examined in freely moving rats the role of 5-HT_2A and 5-HT_2C receptors in the increase in DA release induced by 8 MPa helium pressure. We used the 5-HT_2A receptor antagonist ketanserin and the 5-HT_2C receptor agonist m-CPP which have been demonstrated to reduce DA function. Because neither ketanserin is an ideal 5-HT_2A receptor antagonist nor m-CPP an ideal 5-HT_2C receptor agonist, additional experiments were made at normal pressure to check up on the selectivity of ketanserin and m-CPP for 5-HT_2A and 5-HT_2C receptors, respectively. Administration of m-CPP reduced both DA basal level and the helium pressure-induced increase in DA release, whereas administration of ketanserin only showed a little effect on the increase in DA release produced by high helium pressure. These results suggest that the 5-HT_2C receptor, but not the 5-HT_2A receptor, would play a crucial role in the helium pressure-induced increase in DA release. This further suggests that helium pressure may simultaneously induce an increase in 5-HT transmission at the level of 5-HT_2A receptors and a decrease in 5-HT transmission at the level of 5-HT_2C receptors.