Characterization of monoamine release in the lateral hypothalamus of awake, freely moving rats using in vivo microdialysis

Serotonin and brain function: a tale of two receptors

Previous attempts to identify a unified theory of brain serotonin function have largely failed to achieve consensus. In this present synthesis, we integrate previous perspectives with new and older data to create a novel bipartite model centred on the view that serotonin neurotransmission enhances two distinct adaptive responses to adversity, mediated in large part by its two most prevalent and researched brain receptors: the 5-HT1A and 5-HT2A receptors. We propose that passive coping (i.e. tolerating a source of stress) is mediated by postsynaptic 5-HT1AR signalling and characterised by stress moderation. Conversely, we argue that active coping (i.e. actively addressing a source of stress) is mediated by 5-HT2AR signalling and characterised by enhanced plasticity (defined as capacity for change). We propose that 5-HT1AR-mediated stress moderation may be the brain's default response to adversity but that an improved ability to change one's situation and/or relationship to it via 5-HT2AR-mediated plasticity may also be important - and increasingly so as the level of adversity reaches a critical point. We propose that the 5-HT1AR pathway is enhanced by conventional 5-HT reuptake blocking antidepressants such as the selective serotonin reuptake inhibitors (SSRIs), whereas the 5-HT2AR pathway is enhanced by 5-HT2AR-agonist psychedelics. This bipartite model purports to explain how different drugs (SSRIs and psychedelics) that modulate the serotonergic system in different ways, can achieve complementary adaptive and potentially therapeutic outcomes.

Myocardial interstitial serotonin and its major metabolite, 5-hydroxyindole acetic acid levels determined by microdialysis technique in rat heart

AIMS

The aim of this study was to elucidate myocardial interstitial serotonin (5-HT) kinetics in the heart, including 5-HT reuptake and enzymatic degradation to 5-hydroxyindole acetic acid (5-HIAA) via monoamine oxidase (MAO).

MAIN METHODS

Using microdialysis technique in anesthetized rats, we simultaneously monitored myocardial interstitial levels of 5-HT and its major metabolite, 5-HIAA, in the left ventricle and examined the effects of local administration of a MAO inhibitor, pargyline, or a 5-HT uptake inhibitor, fluoxetine.

KEY FINDINGS

Pargyline increased dialysate 5-HT concentration from 1.8±0.3 at baseline to 3.9±0.5nM but decreased dialysate 5-HIAA concentration from 20.7±1.0 at baseline to 15.8±1.4nM at 60-80min of administration. Fluoxetine increased dialysate 5-HT concentration from 1.9±0.4 at baseline to 6.5±0.9nM at 60-80min of administration, but did not change dialysate 5-HIAA concentration. Local administration of ADP (100mM) increased dialysate 5-HT and 5-HIAA concentrations. Pargyline did not affect ADP-induced increase in dialysate 5-HT concentration but suppressed ADP-induced increase in dialysate 5-HIAA concentration during 60min of ADP administration. Fluoxetine increased dialysate 5-HT concentration at 40-60min of ADP administration, but did not affect ADP-induced increase in dialysate 5-HIAA concentration.

SIGNIFICANCE

Simultaneous monitoring of myocardial interstitial 5-HT and 5-HIAA levels provides valuable information on 5-HT kinetics including reuptake and enzymatic degradation by MAO, which play a role in the regulation of myocardial interstitial 5-HT levels at baseline and when 5-HT levels are elevated.

Chronic effects of antidepressants on serotonin release in rat raphe slice cultures: high potency of milnacipran in the augmentation of serotonin release

Most clinically-used antidepressants acutely increase monoamine levels in synaptic clefts, while their therapeutic effects often require several weeks of administration. Slow neuroadaptive changes in serotonergic neurons are considered to underlie this delayed onset of beneficial actions. Recently, we reported that sustained exposure of rat organotypic raphe slice cultures containing abundant serotonergic neurons to selective serotonin (5-HT) reuptake inhibitors (citalopram, fluoxetine and paroxetine) caused the augmentation of exocytotic serotonin release. However, the ability of other classes of antidepressants to evoke a similar outcome has not been clarified. In this study, we investigated the sustained actions of two tricyclic antidepressants (imipramine and desipramine), one tetracyclic antidepressant (mianserin), three 5-HT and noradrenaline reuptake inhibitors (milnacipran, duloxetine and venlafaxine) and one noradrenergic and specific serotonergic antidepressant (mirtazapine) on serotonin release in the slice cultures. For seven of nine antidepressants, sustained exposure to the agents at concentrations of 0.1-100 μ m augmented the level of increase in extracellular serotonin. The rank order of their potency was as follows: milnacipran>duloxetine>citalopram>venlafaxine>imipramine>fluoxetine>desipramine. Neither mirtazapine nor mianserin caused any augmentation. The highest augmentation by sustained exposure to milnacipran was partially attenuated by an α 1-adrenoceptor antagonist, benoxathian, while the duloxetine-, venlafaxine- and citalopram-mediated increases were not affected. These results suggest that inhibition of the 5-HT transporter is required for the enhancement of serotonin release. Furthermore, the potent augmentation by milnacipran is apparently due to the accompanied activation of the α 1-adrenoceptor.

Involvement of dopamine D1 receptors and α1-adrenoceptors in the antidepressant-like effect of chlorpheniramine in the mouse tail suspension test

It has been reported that chlorpheniramine, a classical antihistamine, has antidepressant-like effects in animal models of depression. In this study, we examined the involvement of dopaminergic (dopamine D(1) and dopamine D(2) receptors), noradrenergic (alpha(1)- and beta-adrenoceptors) and serotonergic (5-HT(1A) and 5-HT(2) receptors) receptors in the antidepressant-like effect of chlorpheniramine in the mouse tail suspension test. We also investigated the involvement of these monoamine receptors in the antidepressant-like effect of imipramine for comparison with the mechanisms of the effect of chlorpheniramine. Both imipramine and chlorpheniramine significantly reduced the duration of immobility in the tail suspension test without affecting spontaneous locomotor activity in mice. The anti-immobility effect of imipramine (30 mg/kg, i.p.) was significantly antagonized by the selective dopamine D(1) receptor antagonist SCH23390 but not by the other receptor antagonists. In contrast, the anti-immobility effect of chlorpheniramine was significantly inhibited by SCH23390 and the selective alpha(1)-adrenoceptor antagonist prazosin, but not by the other receptor antagonists. In conclusion, these results suggest that chlorpheniramine exerts an antidepressant-like effect in the mouse tail suspension test that is mediated by at least the activation of dopamine D(1) receptors and alpha(1)-adrenoceptors. In addition, the antidepressant-like effect of chlorpheniramine may be induced by several mechanisms that are different from those involved in the antidepressant-like effect of imipramine.

Systemic morphine-induced release of serotonin in the rostroventral medulla is not mimicked by morphine microinjection into the periaqueductal gray

We used in vivo microdialysis in awake rats to test the hypothesis that intravenous morphine increases serotonin (5-HT) release within the rostral ventromedial medulla (RVM). We also injected morphine into various sites along the rostrocaudal extent of the periaqueductal gray (PAG), and examined the extent of its diffusion to the RVM. Intravenous morphine (3.0 mg/kg) produced thermal antinociception and increased RVM dialysate 5-HT, 5-hydroxyindole acetic acid (5-HIAA), and homovanillic acid (HVA) in a naloxone-reversible manner. As neither PAG microinjection of morphine (5 micro g/0.5 micro L) nor RVM administration of fentanyl or d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) increased RVM 5-HT, we were unable to determine the precise site of action of morphine. Surprisingly, peak morphine levels in the RVM were higher after microinjection into the caudal PAG as compared to either intravenous injection or microinjection into more rostral sites within the PAG. Naloxone-precipitated withdrawal in morphine-tolerant rats not only increased extracellular 5-HT in the RVM, but also dopamine (DA) and HVA. We conclude that substantial amounts of morphine diffuse from the PAG to the RVM, and speculate that opioid receptor interactions at multiple brain sites mediate the analgesic effects of PAG morphine. Further studies will be required to elucidate the contribution of 5-HT and DA release in the RVM to opioid analgesia and opioid withdrawal.

Peripheral inflammatory response in Alzheimer's disease and multiinfarct dementia

Whether peripheral inflammatory molecules can be considered markers of dementia is still an open issue. We have investigated the presence of circulating cytokines and the ability of blood cells to release them in response to an inflammatory stimulus in patients with different types of dementia and in age-matched controls. A significant increase in circulating interleukin-1beta in moderate Alzheimer and in multiinfarct (145 and 224 times control concentration, respectively) dementia and in circulating tumor necrosis factor-alpha concentration in multiinfarct dementia patient group (156%) were found. Tumor necrosis factor-alpha and interleukin-6 released from blood cells after exposure to lipopolysaccharide were significantly reduced in moderate Alzheimer (60%, both cytokines) and multiinfarct patients (71 and 50%, respectively), while interleukin-10 was decreased only in multiinfarct patients (61%). The results show that patients with Alzheimer disease or multiinfarct dementia have an upregulation of circulating cytokines and a downregulation of cytokines released by blood cells.

Additive effects on rat brain 5HT release of combining phentermine with dexfenfluramine

OBJECTIVE AND DESIGN

This study examined the effects of the anti-obesity agents, phentermine and dexfenfluramine given alone or in combination, on in vitro and in vivo 5HT release from rat brain tissue.

RESULTS

In vitro, phentermine was without effect on basal [3H]5HT efflux from hypothalamic slices whereas dexfenfluramine (10 microM) evoked a 131% increase in [3H]5HT release. In combination, the two drugs did not alter [3H]5HT release beyond that caused by dexfenfluramine alone. At pharmacologically equivalent doses, phentermine (5.7 mg/kg, i.p.) caused a rapid, modest elevation, and dexfenfluramine (3 mg/kg, i.p.) a larger but equally rapid elevation of extracellular 5HT in the microdialysates from the rat anterior hypothalamus. In combination, the increase in extracellular 5HT evoked by these drugs was not significantly greater than the sum of their individual effects.

CONCLUSIONS

This study provides evidence that phentermine's actions are not restricted to catecholamine systems and indicates that combining phentermine with dexfenfluramine results in an additive increase in neuronal 5HT release.

Effects of phentermine and fenfluramine on extracellular dopamine and serotonin in rat nucleus accumbens: therapeutic implications

Combined administration of the amphetamine analogs phentermine and fenfluramine (PHEN/FEN) has been used in the treatment of obesity. While these medications are thought to modulate monoamine transmission, the precise neurochemical effects of the PHEN/FEN mixture have not been extensively studied. To assess the mechanism of PHEN/FEN action, in vivo microdialysis studies were performed in the nucleus accumbens of conscious freely moving rats. A series of amphetamine derivatives including phentermine, chlorphentermine, fenfluramine, and PHEN/FEN (1:1 ratio), were infused locally into the accumbens via reverse-dialysis (1, 10, 100 microM) or injected systemically (1 mg/kg, ip). Dialysate samples were assayed for dopamine (DA) and serotonin (5-HT) by high-performance liquid chromatography with electrochemical detection. When infused locally, phentermine preferentially increased extracellular DA, whereas fenfluramine selectively increased extracellular 5-HT. Local administration of chlorphentermine or the PHEN/FEN mixture caused parallel elevations of both transmitters. Analogous results were obtained when the drugs were injected systemically. Phentermine stimulated robust locomotor activity in mice, whereas chlorphentermine and fenfluramine did not. PHEN/FEN caused modest locomotor stimulation after a low dose, but had no effect at the highest dose. Accumulating evidence suggests that chronic drug and alcohol abuse is associated with deficits in both DA and 5-HT neuronal function. Thus, dual activation of DA and 5-HT neurotransmission with monoamine releasing agents may be an effective treatment strategy for substance use disorders, as well as for obesity. Synapse 36:102-113, 2000. Published 2000 Wiley-Liss, Inc.

Is phentermine an inhibitor of monoamine oxidase? A critical appraisal

Phentermine produces a spectrum of concentration-dependent biochemical effects. It interacts with NE transporters at 0.1 microM, DA transporters at about 1 microM, 5-HT transporters at 15 microM and MAO-A at about 100 microM. When administered at typical anorectic doses, phentermine primarily interacts with DA and NE transporters and does not produce biochemical or neurochemical effects which would occur if it were inhibiting MAO-A. Some other explanation other than MAO inhibition must be sought to explain how oral phentermine increases platelet 5-HT, since platelet MAO-B does not metabolize platelet 5-HT, and since amphetamine-type drugs are even weaker inhibitors of MAO-B than MAO-A. Clinical studies in humans have shown that amphetamine, which is a more potent inhibitor of MAO-A than phentermine, does not inhibit MAO-A at therapeutic doses. Neither phentermine alone, fluoxetine alone or their combined use have been associated with cardiac valvulopathy, and clinical experience has shown their combined use to be free of significant adverse effects. Viewed collectively, there appears to be no data to support the hypothesis that phentermine inhibits MAO at typical therapeutic doses.

Bromocriptine reduces obesity, glucose intolerance and extracellular monoamine metabolite levels in the ventromedial hypothalamus of Syrian hamsters

We examined whether reductions in body fat stores and insulin resistance in Syrian hamsters induced by bromocriptine are associated with reductions in daily norepinephrine (NE) and serotonin activities as indicated by their extracellular metabolite levels in the ventromedial hypothalamus (VMH). High levels of these monoamines within the VMH have been suspected to induce obesity and insulin resistance. Microdialysate samples from the VMH of freely moving obese male hamsters (BW: 208 +/- 5 g) were collected hourly over a 25-hour period before bromocriptine treatment, during the first day of and after 2 weeks of bromocriptine treatment (800 microg/animal daily, i.p.), and body composition and glucose tolerance analyses were conducted before and after 2 weeks of treatments. The microdialysate samples were analyzed by HPLC for metabolites of serotonin: 5-hydroxy-indoleacetic acid (5-HIAA), NE: 3-methoxy-4-hydroxy-phenylglycol (MHPG), and dopamine: homovanillic acid (HVA). Bromocriptine treatment for 14 days significantly reduced body fat by 60% and areas under the glucose and insulin curves during a glucose tolerance test by 50 and 46%, respectively. Concurrently, extracellular VMH contents of 5-HIAA, MHPG, and HVA were reduced by 50, 29 and 66%, respectively (p < 0.05). Similarly, VMH 5-HIAA and MHPG contents were 48 and 44% less, respectively (p < 0.05), in naturally glucose-tolerant hamsters compared with naturally glucose-intolerant hamsters. Bromocriptine induced reductions of body fat, and improvements in glucose intolerance may result in part from its ability to decrease serotonin and NE activities in the VMH.

A critical review of 5-HT brain microdialysis and behavior

Serotonin (5-HT) has been implicated in many central nervous system-mediated functions including sleep, arousal, feeding, motor activity and the stress response. In order to help establish the precise role of 5-HT in physiology and behavior, in vivo microdialysis studies have sought to identify the conditions under which the release of 5-HT is altered. Extracellular 5-HT levels have been monitored in more than fifteen regions of the brain during a variety of spontaneous behaviors, and in response to several physiological, environmental, and behavioral manipulations. The vast majority of these studies found increases (30-100%) in 5-HT release in almost all brain regions studied. Since electrophysiological studies have shown that behavioral arousal is the primary determinant of brain serotonergic neuronal activity, we suggest that the increase in 5-HT release seen during a wide variety of experimental conditions is largely due to one factor, namely an increase in behavioral arousal/motor activity associated with the manipulation.

Evoked GABA release is not mediated by N-type VDCC in the frontal cortex of awake rats: effects of neomycin

The purpose of the present study was to analyze the Ca2+ channel involved in GABA release under resting and K(+)-evoked conditions in vivo. We used microdialysis to investigate the effects of the voltage-dependent calcium channel (VDCC) blockers neomycin, kanamycin, and omega-conotoxin GVIA, and the voltage-dependent Na+ channel blocker tetrodotoxin, in the frontal cortex of awake rats. The GABA content in frontal dialysates was analyzed by high performance liquid chromatography coupled to fluorescence detection. Basal GABA release was kanamycin, omega-conotoxin, and tetrodotoxin resistant, whereas neomycin induced a significant increase from the basal level. The K(+)-evoked release of GABA was kanamycin and omega-conotoxin resistant, but tetrodotoxin sensitive. The effects of neomycin were masked by the action of this drug on basal release. These results suggest that neomycin may affect GABA release in the frontal cortex through a mechanism independent of VDCC. In addition, the K(+)-evoked release of GABA in this cortical area was not mediated by the N-type voltage-dependent calcium channels, but was dependent on neural activity or TTX sensitive.

Hippocampal type I and type II corticosteroid receptors are differentially regulated by chronic prazosin treatment

Two types of hippocampal corticosteroid receptors play an important role in regulating the secretion of corticosterone: type I receptors are thought to regulate both the basal and stress induced release of corticosterone whereas type II receptors seem to be involved only in the stress response. Although these receptors are known to be regulated by circulating levels of corticosterone, there is also evidence for a direct neural control independent of hormonal influences. Furthermore, several studies suggest differential regulation of type I and type II corticosteroid receptors, with greater hormonal control of type II and greater neural control of type I. In order to investigate this theory of differential regulation of type I and type II corticosteroid receptors, we studied the effect of chronic treatment with either vehicle or the alpha 1 noradrenergic antagonist prazosin (0.5 mg/kg, i.p), on hippocampal corticosteroid receptors. Rats in one group had intact adrenal glands, whereas rats in a second group were adrenalectomized, their plasma corticosterone levels being maintained in the physiological range by implantation of corticosterone pellets. Thus, in the first group, the effects of drug-induced changes in both noradrenergic transmission and corticosterone secretion on corticosteroid receptors were investigated, whereas in the second group, the influence of altered noradrenergic transmission was effectively isolated. The results of this experiment show that, in comparison to the vehicle treatment, chronic treatment with the alpha 1 receptor antagonist prazosin decreased the number of type I corticosteroid receptors in adrenalectomized animals with corticosterone substitutive therapy. This effect on type I was not evident in adrenal-intact animals. In contrast, the prazosin treatment reduced the number of type II corticosteroid receptors in adrenal-intact animals, but not in adrenalectomized animals with corticosterone substitutive therapy. It has also been demonstrated here that, in the adrenal-intact animals, chronic prazosin induces hypersecretion of corticosterone after stress, which may account for the reduction of type II corticosteroid receptors noted in this group. Taken together, these results support the theory that type I and type II are differentially regulated: type I receptors can be regulated by noradrenaline independently of corticosterone, whereas type II receptors seem to be adjusted by circulating levels of corticosterone. These results may also suggest possible pharmacotherapies of hypothalamo-pituitary-adrenal axis dysregulation, such as that occurring during depression, Alzheimer's disease and Cushing syndrome, by targeting type I corticosteroid receptors.

Functional interaction between serotonin and other neuronal systems: focus on in vivo microdialysis studies

In this review, the functional interactions between serotonin (5-HT) and other neuronal systems are discussed with the focus on microdialysis studies in the mammalian brain (mainly rats). 5-HT release is negatively regulated not only by somatodendritic 5-HT1A and terminal 5-HT1B (5-HT1D) autoreceptors but also by alpha 2-adrenergic and mu-opioid heteroreceptors that are located on serotonergic nerve terminals. 5-HT by itself is involved in the inhibitory effects of noradrenaline release and the facilitatory regulation of dopamine release via multiple 5-HT receptors. Acetylcholine release appears to be regulated by inhibitory 5-HT1B heteroreceptors located on cholinergic nerve terminals. Long-term treatment with 5-HT-uptake inhibitors and noradrenaline-uptake inhibitor produces desensitization of 5-HT1A autoreceptors and alpha 2-heteroreceptors, respectively, which may be related therapeutically to the delayed onset of the effects of antidepressants. Some microdialysis studies have predicted that the combination of a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist might produce much greater availability of 5-HT in the synaptic cleft in terms of much faster induction of subsensitivity of 5-HT1A autoreceptors. Clinical trials based on this hypothesis have revealed that combination therapy with a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist ameliorated the therapeutic efficacy in depressive patients. Taken together, neurochemical approaches using microdialysis can contribute not only to clarification of the physiological role of the serotonergic neuronal systems but also might be a powerful pharmacological approach for the development of therapeutic strategies.

Effects of acute and repeated administration of citalopram on extracellular levels of serotonin in rat brain

The effects of acute (2 days) and repeated (21 days) administration (50 mg/kg in the diet) of the selective serotonin (5-HT, 5-hydroxytryptamine) reuptake inhibitor, citalopram, on extracellular levels of 5-HT and their modulation by terminal autoreceptors in the hypothalamus of freely moving rats were compared in vivo by microdialysis. When studied without washout, extracellular levels of 5-HT were increased by both acute and repeated citalopram administration. In rats treated repeatedly, extracellular 5-HT levels were 43% (but not significantly) greater than in those treated acutely. Extracellular levels of 5-HT in control and citalopram-treated rats were similar when measured after 24 h washout. The enhancing effect of non-selective serotonergic autoreceptor antagonists, methiothepin (100 microM) or 1-(1-naphthyl)piperazine (NP) (10 microM), administered through the microdialysis probe, after 24 h washout, was similar in both control and chronically treated groups. These results suggest that repeated administration of citalopram followed by a washout of 24 h does not lead to desensitization of the terminal autoreceptor as measured in vivo in contrast to the effects we have shown previously in vitro. In rats treated chronically with citalopram without washout, methiothepin had a greater maximal effect on 5-HT outflow in comparison to rats receiving acute citalopram treatment. This finding suggests that a 5-HT autoreceptor antagonist or a combination of such a drug with a 5-HT uptake inhibitor would produce a greater increase of extracellular levels of 5-HT in hyposerotonergic states such as depression.

Serotonin (5-HT) release in the dorsal raphé and ventral hippocampus: raphé control of somatodendritic and terminal 5-HT release

Somatodendritic and terminal release of serotonin (5-HT) was investigated by simultaneously measuring extracellular concentrations of 5-HT, 5-hydroxyindole-3-acetic acid (5-HIAA) and homovanillic acid (HVA) in the dorsal raphé and ventral hippocampus in freely moving rats. Perfusion of tetrodotoxin (TTX, 1 microM and 10 microM) into the dorsal raphé simultaneously decreased dorsal raphé and hippocampal 5-HT release. However, following TTX perfusion into the hippocampus (10 microM), hippocampal 5-HT release was profoundly reduced but dorsal raphé 5-HT remained unchanged. Systemic injections with 5-HT1A agonist, buspirone (1.0-5.0 mg/kg, i.p.) decreased 5-HT and 5-HIAA and increased HVA concentrations in the dorsal raphé and in the hippocampus. The decreases in the raphé and hippocampal 5-HT induced by systemic buspirone were antagonized in rats pretreated with 1.0 mM (-) pindolol, locally perfused into the dorsal raphé. Local dorsal raphé perfusion of (-) pindolol alone (0.01-1.0 mM) increased dorsal raphé 5-HT and concomitantly induced a small increase in hippocampal 5-HT. Buspirone perfusion into the dorsal raphé did not change (10 nM, 100 nM), or produced a small increase (1.0 mM) in raphé 5-HT, without changing hippocampal 5-HT. These data provide evidence that 5-HT release in the dorsal raphé is dependent on the opening of fast activated sodium channels and that dorsal raphé 5-HT1A receptors control somatodendritic and hippocampal 5-HT release

Serotonin outflow in the hypothalamus of conscious rats: origin and possible involvement in cardiovascular control

The push-pull technique was used to investigate the effects of neuroactive compounds and experimentally induced blood pressure changes on the release of endogenous serotonin in the posterior hypothalamic area of the rat. Hypothalamic superfusion with artificial cerebrospinal fluid which contained 80 mM K+ or 1 microM veratridine enhanced the rate of serotonin release. Superfusion with tetrodotoxin (5 microM) led to a pronounced decrease in the serotonin release rate. Increases in blood pressure elicited by intravenous infusions of noradrenaline (3-4 micro g/kg/min) or phenylephrine (10 microg/kg/min) enhanced the release of serotonin in the hypothalamus. Similarly, the serotonin release rate was enhanced by hypervolaemia. Decreases in blood pressure elicited by intravenous administration of nitroprusside (30-40 microg/kg/min) or chlorisondamine (3 mg/kg) reduced the release of serotonin. Likewise, the serotonin release rate was decreased by hypovolaemia. With one exception (hypothalamic superfusion with tetrodotoxin) neither neuroactive drugs, nor experimentally elicited blood pressure changes modified the release rate of the metabolite 5-hydroxyindoleacetic acid (5-HIAA). These findings show that changes in blood pressure lead to counteractive alterations in the release of serotonin. Thus, serotoninergic neurons of the posterior hypothalamus seem to be involved in the homeostasis of blood pressure by exerting a hypotensive function. At least in the hypothalamus, the concentration of 5-HIAA in the superfusate does not seem to be a reliable marker for the activity of serotoninergic neurons.

Inhibition of catecholamine synthesis with alpha-methyl-p-tyrosine apparently increases brain serotoninergic activity in the rat: no influence of previous chronic immobilization stress

The functional relationship between brain catecholamines and serotoninergic function was studied in stress-naive and chronically immobilized rats after blockade of catecholamine synthesis with alpha-methyl-p-tyrosine (alpha MpT). The levels of noradrenaline (NA), serotonin, and 5-hydroxyindole acetic acid (5-HIAA) in pons plus medulla, brainstem, hypothalamus, hippocampus, and frontal cortex, and those of 3-methoxy, 4-hydroxyphenile-tileneglicol sulphate (MHPG-SO4) in the hypothalamus were measured by HPLC. Chronic immobilization (IMO) resulted in higher NA levels in pons plus medulla and hypothalamus, the latter area (the only one in which the NA metabolite was determined) also showing slightly elevated MHPG-SO4 levels as compared to stress-naive rats. Chronic IMO did not alter either serotonin or 5-HIAA levels, but acute stress consistently increased 5-HIAA levels in all areas, independently of previous chronic stress. Administration of alpha-MpT drastically reduced NA and increased 5-HIAA levels in all brain regions excepting the frontal cortex. The effect of the drug on serotoninergic function was not altered by previous chronic exposure to IMO. These data suggest that the noradrenergic system appears to exert a tonic inhibitory effect on serotoninergic activity in the brain, with the intensity of the effect depending on the brain area studied. In addition, chronic stress does not appear to alter the functional relationship between noradrenergic and serotoninergic activities, although interactions might exist in more restricted brain areas; this deserves further study.

Catecholamine release in the rat hypothalamic paraventricular nucleus in response to haemorrhage, desipramine and potassium

In vivo microdialysis and HPLC were used to measure catecholamine release in the rat hypothalamic paraventricular nucleus (PVN) during haemorrhage. The effects of noradrenaline uptake blockade with 1 microM desipramine (DMI) and a depolarising concentration of potassium (100 mM) through the probe were also examined. Dialysis probes implanted in the PVN of urethane anesthetised rats were perfused with modified Ringer solution at 1.1 microliter/min. Thirty minute collections were analysed for DOPA, noradrenaline, DOPAC, HVA and 5-HIAA. Basal concentrations, in the absence of DMI, were: DOPA 203.6 +/- 44.0 pg/ml, noradrenaline 128.0 +/- 20.4 pg/ml; DOPAC 5.6 +/- 0.7, HVA 5.1 +/- 2.2 and 5-HIAA 87.2 +/- 17.8 ng/ml. Basal noradrenaline was doubled in the presence of DMI while basal and stimulated DOPA, DOPAC, HVA and 5-HIAA were not affected by DMI. Haemorrhage resulted in a significant noradrenaline release (48% over resting levels) in the presence of DMI (n = 10, P < 0.05); in the absence of DMI, a smaller and non-significant increase (30% over basal levels) was observed. Potassium-induced depolarisation caused a significant two- and four-fold increase in noradrenaline release (P < 0.001), with decreases in the dopamine metabolites DOPAC (31%, 44%) and HVA (35%, 28%), and the serotonin metabolite, 5-HIAA (41%, 33%), in the presence and absence of DMI, respectively. The catecholamine precursor DOPA did not vary throughout either experiment. The results indicate that haemorrhage induces a 48% increase in noradrenaline release in the rat PVN which provides evidence for a role of noradrenergic projections to the PVN in cardiovascular control.

Neuropharmacology of 5-hydroxytryptamine1B/D receptor ligands

In spite of a lack of compounds acting selectively at the 5-hydroxytryptamine (5-HT)1B and 5-HT1D receptor subtypes, by cross-relating the available data, this review attempts to tentatively assign behavioural and other in vivo correlates of these receptor subtypes. In addition, a summary of data from microdialysis studies is included to develop an integrated view. Finally, a suggestion is made as to the possible pathophysiological consequences of 5-HT1D receptor dysfunction in man.

Changes in extracellular PVN monoamines and macronutrient intake after idazoxan or fluoxetine injection

Norepinephrine (NE) and serotonin (5-HT) in the paraventricular nucleus (PVN) have opposite effects on feeding, with NE stimulating carbohydrate intake through alpha 2 noradrenergic receptors and 5-HT inhibiting carbohydrate intake. This study examined the action of drugs that affect brain monoaminergic systems, in terms of their impact on nutrient intake and on PVN monoamines measured using microdialysis. The drugs studied were idazoxan, a blocker of alpha 2 receptors, or fluoxetine, a 5-HT reuptake blocker. In rats maintained on pure macronutrient diets, idazoxan (1 mg/kg) and fluoxetine (10 mg/kg), 120 min after injection both reduced total food intake, and specifically carbohydrate intake. In dialysis experiments, successive 20-min dialysate samples were taken, three samples before and seven samples after intraperitoneal injection of idazoxan (5 and 20 mg/kg), fluoxetine (10 mg/kg), or vehicle. Idazoxan increased NE, homovanillic acid, and dihydroxyphenylacetic acid in the PVN. Fluoxetine induced a significant increment of 5-HT in PVN, while producing a smaller increase in NE, dopamine, and homovanillic acid. These results support the conclusion that the impact of these drugs on macronutrient intake may be a consequence of their action on endogenous monoamine systems in the PVN. Thus, in this nucleus, the blockade of alpha 2-noradrenergic receptors, like stimulation of 5-HT receptors, attenuates normal ingestion of carbohydrate.

Pharmacological modulation of endogenous dopamine and DOPAC outflow from nucleus accumbens

The release of endogenous DA and DOPAC from nucleus accumbens slices were studied measuring net outflow of DA and DOPAC in the superfusate of static chambers, to analyze the correlation between DA and DOPAC outflows and identify which DA stores may serve as possible sources for DOPAC formation. Under resting conditions, or following stimulation with low (< 15 mM) KCl concentration, DOPAC outflow was greater than DA. When DA release was stimulated by higher (> 25 mM) KCl concentrations, DA outflow increased, proportionally more than DOPAC. In the virtual absence of Ca2+ in the Krebs solution DA outflow, induced by 25 mM KCl, was reduced to about 10%, while DOPAC outflow was only reduced to 45%. When the synthesis of DA was inhibited with alpha-MPT, DA and DOPAC outflow were unchanged during the first stimulation period. During a second stimulation period, however, their outflow were significantly reduced. Nomifensine, a DA uptake inhibitor, increased the basal DA outflow by about 100%, but only blocked DOPAC basal outflow by about 25%. The 25 mM KCl stimulated DA outflow was not affected by Nomifensine, while the stimulated DOPAC outflow was reduced by about 50%. These results demonstrate that there is a weak correlation between the outflows of DA and DOPAC, suggesting a complex relationship between the mobilization of the different DA pools and DOPAC outflow. The formation of DOPAC from some of these pools, appear to be dependent on the stimulation levels and on the pharmacological manipulation of the tissue.

From synapse to vesicle: the reuptake and storage of biogenic amine neurotransmitters

Biogenic amine transport systems in the presynaptic plasma membrane and the synaptic vesicle provide a mechanism for rapidly terminating the action of released transmitters and for recycling neurotransmitters. Alterations in the activity of these transporters, either by endogenous regulatory mechanisms or by drugs, affect the regulation of synaptic transmitter levels. For drugs such as antidepressants and stimulants that interact with these transport systems, the therapeutic and behavioral consequences are profound. Now that the cDNAs encoding the transporters have been isolated, we can expect rapid progress in understanding how the individual proteins work at the molecular level to couple ion gradients to the reuptake and storage of biogenic amine neurotransmitters.

Striatal extracellular dopamine in conscious vs. anesthetized rats: effects of chloral hydrate anesthetic on responses to drugs of different classes

Many investigations using the microdialysis technique have been performed in anesthetized animals, both in this laboratory and elsewhere. Concern arises with this preparation that the anesthetic may compromise neuronal function, or that it may interact with test drugs affecting neurotransmitter overflow. In addition, in these studies the microdialysis probe typically is introduced into the brain on the day of testing, and data collection commences within an hour or two following probe insertion. It has been suggested that transmitter recovered in the perfusate probably represents leakage due to tissue damage as well as exocytotic release, and may not accurately reflect neuronal responses to the manipulations of interest. Such potential confounds present important implications for the interpretation of data from these studies. The present investigation examined the effects of chloral hydrate anesthetic on (1) basal dopamine (DA) overflow in the anterior striatum, and (2) DA responses to systemically delivered drugs of two different classes known to influence DA activity. Three putative indices of impulse-dependent release were measured: (a) the time course and stability of basal DA overflow over several hours; (b) sodium channel involvement by adding tetrodotoxin (TTX) to the artificial CSF; and (c) calcium channel involvement using magnesium (Mg) in a calcium-free perfusate. Basal DA levels became stable in both conscious and anesthetized preparations by the second hour after probe insertion. Levels of recovered DA overflow in the anterior striata of conscious rats were approximately double those in chloral hydrate-anesthetized rats. Consistent with other findings, this suggests a general depression of CNS function by chloral hydrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of clonidine on the release of serotonin from the rat hippocampus as measured by microdialysis

The purpose of the present study is to clarify the effect of clonidine on the release of serotonin from the rat hippocampus in vivo. For this purpose, endogenous serotonin release was measured by brain microdialysis. Potassium-evoked serotonin release from the hippocampus of freely moving rats was significantly inhibited when clonidine (10(-5) M) was added to the perfusion solution, while the 5-hydroxyindoleacetic acid output remained unchanged. In catecholaminergically denervated rats, clonidine (10(-5) M) also inhibited the potassium-evoked serotonin release from the hippocampus and the 5-hydroxyindoleacetic acid output was unaffected by clonidine. These results suggest that the inhibitory effect of clonidine on serotonin release from the hippocampus might reflect the activation of alpha 2-adrenoceptors which are localized on the serotonergic nerve terminals.

Simultaneous measurement of extracellular morphine and serotonin in brain tissue and CSF by microdialysis in awake rats

In this report, we describe an HPLC with electrochemical detection assay for the simultaneous measurement of levels of morphine, serotonin, 5-hydroxyindole-3-acetic acid, and homovanillic acid in dialysates of various brain areas and CSF in the awake rat. Morphine could be detected in the dialysates after a single intraperitoneal injection, with doses as low as 1.0 mg/kg. The time course of extracellular morphine content in the lateral hypothalamus, striatum, cerebellum, periaqueductal gray, and dorsal horn of the spinal cord and in CSF, from the ventricles and cisterna magna, was similar. We detected morphine in the first 15-min sample, and levels peaked 45-60 min after injection. Maximal dialysate levels, however, varied with the type of dialysis probe used and the area sampled. The most efficient in vivo recovery was in CSF dialysates from the cisterna magna, presumably because of minimal tissue interference with the dialysis probe. For this reason, the cisterna is an ideal region for sampling CSF. Morphine had no significant effect on the extracellular concentrations of serotonin in any of the areas studied and did not modify or only slightly increased levels of tissue metabolites; however, morphine markedly increased the CSF levels of 5-hydroxyindole-3-acetic acid and homovanillic acid. Because microdialysis in freely moving animals permits assessment of the behavioral effects of morphine while continuously monitoring the drug levels in discrete brain regions, this approach will greatly facilitate future studies of the neurochemical basis of morphine's effects in the brain.

In Vivo Neurochemical Evaluation of Striatal Serotonergic Hyperinnervation in Rats Depleted of Dopamine at Infancy

Destruction of nigrostriatal dopamine (DA) neurons with 6-hydroxydopamine (6-OHDA) early in development results in hyperinnervation of striatum by the serotonergic afferents deriving from the dorsal raphe nucleus. We have used in vivo microdialysis to investigate the degree to which serotonergic neurotransmission in striatum is altered by this increase in the density of serotonin (5-HT) terminals. The effects of several manipulations known to influence 5-HT function on extracellular 5-HT and 5-hydroxyindoleacetic acid in striatum were compared in adult rats treated neonatally with 6-OHDA and in intact adult rats. Basal levels of 5-HT in extracellular fluid (ECF) of striatum were similar in neonatally DA-depleted rats and in intact rats. Perfusion with the 5-HT reuptake blocker, fluoxetine (100 microM), increased 5-HT in striatal ECF of neonatally DA-depleted rats to levels that were threefold greater than those achieved in intact rats. Likewise, K(+)-depolarization of the 5-HT terminals (100 mM in perfusate) or systemic administration of the 5-HT releaser, (+/-)-fenfluramine (10 mg/kg i.p.), increased the concentration of 5-HT in striatal ECF of neonatally DA-depleted rats to levels approximately threefold greater than those observed in striatum of intact rats. These findings indicate that the 5-HT hyperinnervation of striatum that takes place in rats depleted of DA at infancy is associated with an increased capacity for neurotransmitter release in this system. Concomitant increased in high-affinity 5-HT uptake may prevent the occurrence of any measurable changes in the resting concentration of 5-HT in striatal ECF.

Do opioids evoke the release of serotonin in the spinal cord? An in vivo microdialysis study of the regulation of extracellular serotonin in the rat

This study investigated the regulation of serotonin (5-HT) and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the dorsal spinal cord of awake, freely moving rats, using microdialysis coupled to HPLC with electrochemical detection and tested the hypothesis that opioids exert their analgesic effect in part through the increased release of 5-HT in the dorsal horn. A dialysis tube was placed transversely at the L4 segment of the dorsal spinal cord and the basal concentration of 5-HT in the dialysate was characterized by infusion of a variety of substances through the dialysis probe: tetrodotoxin (TTX), KCl, imipramine, fluoxetine and amphetamine (AMPH). To evaluate the contribution of opioids, we also studied the effects of either systemic or intracerebroventricular (i.c.v.) injection of morphine or DAMGO. Extracellular concentrations of 5-HT and 5-HIAA were partially and reversibly reduced by TTX. In the presence of KCl, imipramine, fluoxetine or AMPH, 5-HT levels significantly increased. Under these conditions, extracellular 5-HIAA levels usually decreased. By contrast, the effects of opioids on 5-HT concentrations were highly variable. Low doses of morphine administered systemically increased 5-HT concentrations in only 3 of 6 rats. This was paralleled by a decrease in 5-HIAA. Higher doses of morphine, alone or in the presence of fluoxetine, did not change 5-HT concentrations. Intracerebroventricular injection of morphine or DAMGO increased the extracellular concentrations of 5-HT in only about one third of the animals. After intracerebroventricular opioid injection, extracellular concentrations of 5-HIAA either decreased by about 20% or did not change.(ABSTRACT TRUNCATED AT 250 WORDS)