Genetic differences in ethanol drinking of the rat following injection of 6-OHDA, 5,6-DHT or 5,7-DHT into the cerebral ventricles

The preference characteristics for ethanol of four different strains of rats were determined in a two-choice situation by offering water and ethanol in a concentration which was increased from 3 to 30% over a 12-day test sequence. Using stereotaxic procedures, 50 mug 5,6-dihydroxytryptamine (5,6-DHT), 200 mug 6-hydroxydopamine (6-OHDA) or 100 mug 5,7-dihydroxytryptamine (5,7-DHT) were then injected acutely into the lateral cerebral ventricle in a 20 mul volume. Rats of the Sprague-Dawley strain increased their ethanol preference following the lesioning of the serotonergic system by 5,6-DHT, whereas similar destruction of catecholaminergic neurons by 6-OHDA markedly suppressed ethanol intake; Long-Evans rats displayed a similar trend in ethanol drinking patterns. However, animals of the Holtzman strain manifested the increased drinking after 5,6-DHT, but showed no suppression of drinking following 6-OHDA. The preference of rats of the Wistar strain was unaffected by 5,6-DHT but attenuated by 6-OHDA. 5,7-DHT had little or no effect on ethanol consumption in any of these strains. These findings thus suggest that genetic factors are an important determinant in an animal's response to a drug that affects 5-HT or NE systems in the brain, particularly when ethanol selection is investigated.     

In vivo chronoamperometric measures of extracellular serotonin clearance in rat dorsal hippocampus: contribution of serotonin and norepinephrine transporters

The effects of blockade of serotonin (5-HT) and norepinephrine (NE) transporters (SERT and NET, respectively) on the removal of locally applied 5-HT from extracellular fluid (ECF) were examined using in vivo chronoamperometry. Male Sprague-Dawley rats were anesthetized with chloralose/urethane, and a Nafion-coated, carbon fiber electrode attached to a multibarrel micropipette was positioned into either the dentate gyrus or CA3 region of the dorsal hippocampus. Pressure ejection of 5-HT elicited reproducible electrochemical signals of similar peak amplitude and time course in both structures. Local application of the selective serotonin reuptake inhibitors (SSRI) fluvoxamine and citalopram prolonged the clearance of 5-HT in both brain regions and also increased signal amplitude in the CA3 region. These effects were abolished in rats pretreated with 5, 7-dihydroxytryptamine (5,7-DHT), a selective 5-HT neurotoxin. The NE uptake inhibitors desipramine (DMI) and protriptyline did not alter the 5-HT signal in the CA3 region but prolonged the clearance of 5-HT in the dentate gyrus; this effect was absent in rats pretreated with 6-hydroxydopamine (6-OHDA), a selective catecholamine neurotoxin. The prolongation of the removal of 5-HT from the ECF in the dentate gyrus caused by fluvoxamine or desipramine was of comparable magnitude and was dose dependent. Furthermore, per picomole of 5-HT applied, the signal amplitude and clearance time were significantly increased in the dentate gyrus of rats lesioned with either 5,7-DHT or 6-OHDA. Only 5,7-DHT treatment caused this effect in the CA3 region. From these data, it is inferred that in certain regions of brain (dentate gyrus), both the SERT and NET contribute to the active clearance of exogenously applied 5-HT.     

Comparative analysis of ribonuclease P RNA structure in Archaea

The relationship between central serotonergic activities and voluntary alcohol consumption was studied in Sprague-Dawley rats, which normally have low alcohol preference. After initial screening for an evenly matched baseline alcohol preference, selective central serotonergic lesioning was induced by intracisternal injection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). Control rats received injections of vehicle only. Both 5,7-DHT and vehicle-treated rats were further divided into two subgroups, which either had continued free access to ethanol (alcohol-drinking) or were deprived of it (alcohol-free). All rats were then tested again for alcohol preference. All rats were then killed, and the levels of monoamines in the brains were determined by high performance liquid chromatography with electrochemical detection. Behavioral results indicated that all 5,7-DHT-treated rats had significantly higher alcohol preference and consumption than the corresponding sham controls. Except in the cerebellum, the 5,7-DHT-treated rats had significantly lower levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in most brain regions compared with those in the corresponding sham controls. Treatment with 5,7-DHT also resulted in a decrease in serotonin turnover in all brain regions in the alcohol-free rats, except in the cerebellum. In alcohol-drinking rats, however, 5,7-DHT treatment only reduced serotonin turnover in the pons. The levels of norepinephrine and dopamine in several brain regions were not significantly different. Thus, it appeared that in the Sprague Dawley rats, 5,7-DHT treatment depleted 5-HT and 5-HIAA levels in most brain regions while increasing alcohol consumption. Chronic alcohol-drinking attenuated the increase in alcohol consumption associated with serotonergic lesions. Voluntary alcohol consumption seemed more associated with 5-HT turnover than with tissue 5-HT levels. Our data also suggested that tolerance to alcohol-induced hypothermia was primarily attributable to long-term alcohol drinking rather than serotonergic lesioning.     

Monoamine neurotoxins: selective and delayed effects on behavior in colonies of laboratory rats

Male laboratory rats were raised in two colonies, each of 27 rats, and then given intraventricular injections of the norepinephrine neurotoxin 6-hydroxydopamine (6-OHDA) or the serotonin neurotoxin 5,6-dihydroxytryptamine (5,6-DHT) or saline. They were then returned to their enclosure and behavior during the next 50 days was observed. Shortly after neurotoxin injections the 6-OHDA rats spent more time in the burrows than controls and when out were inactive. The 5,6-DHT rats in contrast spent more time in the open than controls, ran more in activity wheels, approached humans, and fought more. Fighting, mounting, and hoarding in the colony gradually increased for 25 days; during this time the status of the 6-OHDA animals fell progressively whereas the 5,6-DHT animals increased in dominance. Social behavior returned to more normal levels after 50 days. Several successive stages of behavioral alterations occur following neurotoxin injections.     

Polydipsic and home-cage fluid choices: the effects of sweetening

MAO inhibitors of diverse chemical structures were found to inhibit the uptake of 3H-NE into chopped rat cerebral cortex in vitro. The following molar (M) IC50 values to inhibit 3H-NE uptake were obtained: iproniazid, 7.9 X 10(-4)M; pargyline, 3.1 X 10(-4)M; pheniprazine, 6.3 X 10(-6)M; phenelzine, 3.9 X 10(-6)M; tranylcypromine, 2.5 X 10(-6)M; amphetamine, 2.5 X10(-7)M. In addition to decreasing deaminative catabolism, 5 X 10(-5)M amphetamine, tranylcypromine and pheniprazine plus 10(-3)M phenelzine produced a release of 3H-NE from tissue stores into incubation media. Similar concentrations of pargyline and iproniazid were ineffective to release 3H-NE from brain tissue.     

Effect of serotonin axon injury on the somatostatinergic system in rat frontoparietal cortex

In order to investigate the possibility that, in the rat, some cerebral cortex somatostatin (SS) receptors may be localized presynaptically on the terminals of serotonergic neurons, serotonin [5-hydroxytryptamine, (5-HT)] neurons in the central nervous system were damaged with a local intracerebral injection of the serotonergic neurotoxin, 5,7-dihydroxytryptamine(5,7-DHT). The injection of 5,7-DHT (11 micrograms free base dissolved in 10 microliters of isotonic saline containing 0.01% ascorbic acid) in rats produced an reduction by about 74% in frontoparietal cortical 5-HT content at 1 and 3 weeks after injection. These changes were associated with a significant decrease by about 30% in the total number of specific SS receptors in the frontoparietal cortex at both times studied without influencing the apparent affinity of the receptors. Together, these results suggest that a portion of the frontoparietal cortex SS receptors may be localized presynaptically on the serotonergic nerve terminals. The 5,7-DHT did not affect SS-like immunoreactivity (SSLI) levels suggesting that SS and 5-HT are not colocalized within the same neuronal elements in the rat frontoparietal cortex.     

The role of cortical serotonin in anxiety and locomotor activity in Wistar rats.

The brain's serotonergic system is known to play an important role in the modulation of anxiety. While the role of serotonin (5-HT) in subcortical structures is well investigated, little is known about the function of cortical 5-HT. The present series of studies used local injections of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the medial prefrontal cortex (mPFC), entorhinal cortex (EC), or occipital cortex (OccC) of rats to chronically reduce 5-HT neurotransmission in these brain areas. The animals were tested for anxiety-like behavior on the elevated plus-maze and open field. An 82% depletion of 5-HT from the mPFC increased anxiety-like behavior, while no general motor effects were evident. In contrast, a 63% 5-HT-depletion of the EC or a 78% 5-HT-depletion of the OccC did not have any effects on emotional or exploratory behaviors. These findings are in line with a proposed role of 5-HT in the mPFC in the modulation of anxiety- and stress-mediated behavior and demonstrate a functional differentiation between different cortical 5-HT projections. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Acute central effects of 5,6-dihydroxytryptamine in fowl

In adult hens (Gallus domesticus) infusion into the IIIrd cerebral ventricle of 5,6-DHT (50, 75 and 100 mug) produced, after 5-10 min behavioural and electrocortical sedation and sleep lasting about 6-8 hr,and a monophasic or biphasic increase in body temperature for about 8 hr, Two and a half hr after 5,6-DHT administration, the histochemical picture showed a sustained decrease in yellow and green fluorescence in areas adjacent to the IIIrd ventricle, in the anterior hypothalamus and in the preoptic area, whereas no significant changes were detected in fluorescence of the tegmental cell-bodies. The present experiments suggest that behavioural and electrocortical effects evoked by 5,6-DHT seem to be due to a synergistic action of 5-HT and catecholamines displaced and released by this compound, whereas hyperthermic effects seem to be due to a more sustained release of 5-HT and/or to a direct action on 5-HT receptors and/or inhibition of 5-HT reuptake.     

Increased 5-HT1A receptor immunoreactivity in the rat hippocampus following 5,7-dihydroxytryptamine lesions in the cingulum bundle and fimbria-fornix

Serotonin (5-HT) projections from the ascending raphe nuclei reach the dorsal hippocampus via the cingulum bundle (CB) and fimbria-fornix (FF). Microinjection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the CB and FF produces a significant decrease in the density of 5-HT immunoreactive fibers in the hippocampus as early as 3 days postlesion (Zhou, F.C. and Azmitia, E.C. (1983) Brain Res. Bull., 373, 337-348). In the present study we used an anti-peptide antibody against the second extracellular loop of the 5-HT1A receptor and employed immunocytochemistry to examine changes in the expression and distribution of the 5-HT1A receptor in the hippocampus 14 days following administration of 5,7-DHT into the CB and FF. The density of 5-HT immunoreactive fibers was greatly reduced 14 days following the lesions. 5-HT1A immunoreactivity (IR) was localized to the proximal axon near the axon hillock of cells in the pyramidal cell layer of the cornu Ammonus and in the granule cell layer of the dentate gyrus. The intensity of 5-HT1A-IR was increased in the CA1 and dentate gyrus following 5,7-DHT lesions. Intensity in the CA3 also increased but not to a significant level. These findings demonstrate that 5-HT denervation in the hippocampus is followed by increased expression of the 5-HT1A receptor protein. These changes in receptor expression 14 days postlesion may represent adaptive changes by postsynaptic cells following reduced 5-HT innervation and may be the molecular basis for 5-HT1A receptor supersensitivity.     

Inhaled corticosteroids increase interleukin-10 but reduce macrophage inflammatory protein-1alpha, granulocyte-macrophage colony-stimulating factor, and interferon-gamma release from alveolar macrophages in asthma

In the present study, we examined denervation-induced changes in the sensitivity of hypothalamic postsynaptic serotonin1A (5-HT1A) receptor function with respect to changes in the dose-dependent elevation in plasma hormones [adrenocorticotropic hormone (ACTH), corticosterone, prolactin, oxytocin, prolactin, renin and vasopressin] by the 5-HT1A agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT). Rats received intracerebroventricular (i.c.v.) injections of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or vehicle (0.1% ascorbate in saline) 3 weeks before challenge with increasing doses of 8-OH-DPAT (0, 10, 50 or 200 micrograms/kg s.c.). The effectiveness of 5,7-DHT-induced destruction of serotonergic neurons was confirmed by a 93% reduction in [3H]paroxetine-labeled 5-HT uptake sites in the hypothalamus. No changes in basal levels of ACTH, corticosterone, oxytocin, prolactin, renin and vasopressin were observed in rats that received i.c.v. 5,7-DHT injections. The dose-response curves for 8-OH-DPAT-induced elevations of plasma corticosterone and prolactin levels were shifted to the left in rats treated with 5,7-DHT, whereas no significant difference in the ACTH dose-response curve was observed between rats treated with vehicle and rats treated with 5,7-DHT. In contrast, the maximal oxytocin response to 8-OH-DPAT was attenuated in rats treated with 5,7-DHT. A 5,7-DHT-induced decline in the synthesis of oxytocin could explain this phenomenon. Although 8-OH-DPAT did not increase plasma levels of renin or vasopressin in rats treated with vehicle, 8-OH-DPAT produced an elevation (75%) in plasma renin concentration but not in vasopressin levels in rats that received i.c.v. injections of 5,7-DHT. No change was observed in [3H]8-OH-DPAT labeled 5-HT1A receptors in the hypothalamus. In summary, denervation of hypothalamic serotonergic nerve terminals produces supersensitivity of some neuroendocrine responses to 8-OH-DPAT independent of changes in the density of hypothalamic 5-HT1A receptors.     

Facilitation of acetylcholine release in rat frontal cortex by indeloxazine hydrochloride: involvement of endogenous serotonin and 5-HT4 receptors

Effects of indeloxazine hydrochloride, an inhibitor of serotonin (5-HT) and norepinephrine (NE) reuptake with a facilitatory effect on 5-HT release, on acetylcholine (ACh) output in frontal cortex of conscious rats were characterized using an in vivo microdialysis technique. Systemic administration of indeloxazine (3 and 10 mg/kg, i.p.) increased ACh and 5-HT output in a dose-dependent manner. Depletion of endogenous monoamines by reserpine and of 5-HT by p-chlorophenylalanine, but not that of catecholamines by alpha-methyl-p-tyrosine, significantly attenuated the facilitatory effect of indeloxazine on ACh release. When applied locally by reverse dialysis, indeloxazine (10 and 30 microM) and the selective 5-HT reuptake inhibitor citalopram (10 microM), but not the NE reuptake inhibitor maprotiline (30 microM), increased cortical ACh output. Indeloxazine (10 mg/kg)-induced increase in ACh release was significantly inhibited by local application of the 5-HT4 receptor antagonists RS23597 (50 microM) and GR113803 (1 microM), while the 5-HT1A antagonist WAY-100135 (100 microM), 5-HT1A/1B/beta-adrenoceptor antagonist (-)propranolol (150 microM), 5-HT2A/2C antagonist ritanserin (10 microM) and 5-HT3 antagonist ondansetron (10 microM) failed to significantly modify this effect. Neither depletion of monoamines nor treatment with serotonergic antagonists significantly changed the basal ACh level, indicating that endogenous monoamines do not tonically activate ACh release. These results suggest that indeloxazine-induced facilitation of ACh release in rat frontal cortex is mediated by endogenous 5-HT and involves at least in part cortical 5-HT4 receptors.     

Involvement of brain serotonergic function in lidocaine-induced convulsions in mice

Influences of drug-induced manipulations of central serotonergic function on lidocaine- and pentylenetetrazol (PTZ)-induced convulsions were examined in mice. Agents that suppressed serotonergic transmission increased, whereas drugs that facilitated serotonin (5-HT) function decreased the incidence of lidocaine-induced convulsions. These treatments had similar influences on the incidence of PTZ-induced convulsions. Lidocaine (10(-5)-10(-3) M) reduced the stimulation evoked [3H]5-HT release from cortical slices, followed with an increased spontaneous [3H] overflow at higher concentrations. These results may suggest that brain 5-HT neurons are causally involved as inhibitory neurons in lidocaine-induced convulsions as in the case of PTZ-induced convulsions.     

Effect of acetylsalicylic acid on formalin test and on serotonin system in the rat brain

1. Acetylsalicylic acid (ASA; 400 mg/kg, i.p.) increased serotonin (5-HT) content in rat brain but did not modify the number or the affinity of 5-HT1A receptors in the pons and the cerebral cortex, whereas the number of cortical 5-HT2 receptors decreased significantly. 2. Pretreatment with parachlorophenylaline (100 mg/kg/day for 4 days) depleted 5-HT brain content but modified neither the serum levels of salicylates nor the 5-HT2 cortical receptor characteristics, and it abolished the antinociceptive effect of ASA, 400 mg/kg, in the first phase of the formalin test. 3. These data support the involvement of the central serotonergic system in the antinociceptive activity of ASA.     

Naloxone-reversible antinociception by paracetamol in the rat

Paracetamol at the dose of 400 mg/kg i.p. displayed antinociceptive activity in the hot-plate test and the formalin test. Moreover, it induced a significant increase in brain serotonin (5-HT) concentration and a reduction in the number of 5-HT2 receptors in cortical membranes. Pretreatment with naloxone abolished this antinociceptive activity both in the hot-plate test and in the first phase of the formalin test without affecting the serum concentration of paracetamol. At the same time, naloxone prevented the increase in 5-HT concentration in the central nervous system and the reduction in 5-HT2 receptors in cortical membranes. Competition experiments demonstrated that paracetamol possesses affinity for [3H]naloxone binding sites. The action of morphine on nociception and on the serotonergic system was similar to that of paracetamol; all morphine-induced effects were blocked by naloxone. These data provide further evidence for a central antinociceptive effect of paracetamol and support the hypothesis that paracetamol exerts its antinociceptive activity through the serotonergic system. Moreover, our results point to the relationship between serotonergic and opiatergic systems in the antinociceptive activity of paracetamol.     

A method for the determination of 5,6-EET using the lactone as an intermediate in the formation of the diol

The 5,6 epoxyeicosatrienoic acid (5,6-EET) exhibits a range of biological activities but the functional significance of this labile eicosanoid is unknown due, in part, to difficulties of quantitation in biological samples. We have developed a sensitive and specific method to measure 5,6-EET utilizing its selective capacity to form a lactone. The initial conversion of 5,6-EET and 5,6-dihydroxyeicosatrienoic acid (5,6-DHT) to 5,6-delta-lactone is followed by selective purification using reverse phase high performance liquid chromatography (HPLC), reconversion to 5,6-DHT and quantitation by gas chromatography-mass spectrometry (GCMS). In oxygenated Krebs' buffer, 5,6-EET degrades to 5,6-delta-lactone and 5,6-DHT with a t1/2 approximately 8 min. In the presence of camphorsulfonic acid, 5,6-EET and 5,6-DHT convert to a single HPLC peak (lambda = 205) comigrating with 5,6-delta-lactone. Incubation of 5,6-delta-lactone with triethylamine resulted in a single HPLC peak with the retention time of 5,6-DHT. In the perfusate from the isolated kidney, release of 5,6-EET (20 +/- 5 pg/ml), measured indirectly via conversion to 5,6-DHT, was approx. 6-fold less than that reported for prostaglandin E2 (PGE2) and 20-HETE. The coronary perfusate concentration of 5,6 EET was 9 +/- 2 pg/ml. 5,6-EET recovered from renal and coronary perfusates was increased 2-fold to 45.5 +/- 5.5 pg/ml and 21.6 +/- 6.3 pg/ml, respectively, by arachidonic acid.     

Involvement of 5-HT6 receptors in nigro-striatal function in rodents

4-Amino-N-(2,4 bis-methylamino-pyrimidin-4-yl) benzene sulphonamide (Ro 04-6790) is a potent, selective and competitive antagonist for the 5-HT6 receptor which can be detected in the cerebro-spinal fluid (CSF) of rats following intraperitoneal administration. Since 5-HT6 receptor mRNA and 5-HT6 receptor-like immunoreactivity have been shown to be present in the striatum, the purpose of the present study was to evaluate the effect of 5-HT6 receptor antagonism on haloperidol- and SCH 23390-induced catalepsy in mice and on the turning behaviour of rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle. Ro 04-6790 (3, 10 and 30 mg kg(-1) i.p.) did not induce catalepsy and had no effect on catalepsy induced by either haloperidol or SCH 23390. Ro 04-6790 (3, 10 and 30 mg kg(-1) i.p.) did not itself induce rotational behaviour in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle nor did it affect the rotational behaviour induced by either L-Dopa or amphetamine. 5-HT6 receptor antagonism inhibited the rotational behaviour of 6-OHDA lesioned rats induced by treatment with the muscarinic antagonists scopolamine and atropine. The data support earlier conclusions from experiments with antisense oligonucleotides that the 5-HT6 receptor is involved in the control of acetylcholine neurotransmission in the rat brain.     

Met-enkephalin inhibits 5-hydroxytryptamine release from the rat ventral spinal cord via delta opioid receptors

The effect of opioid receptor agonists and antagonists on the electrically evoked release of endogenous serotonin (5-hydroxytryptamine, 5-HT) was studied in superfused slices of the rat ventral lumbar spinal cord. Met-ENK (1 x 10(-8)M-1 x 10(-6)M) and DPDPE (1 x 10(-8)M-1 x 10(-6)M) reduced the evoked 5-Ht release in a concentration dependent fashion. DAMGO (1 x 10(-8)-1 x 10(-6)) and (-)-trans-(1S,2S)-U-50488 (1 x 10(-6)M) had no effect on the 5-HT release. The inhibitory effect of met-ENK was completely abolished by ICI-174,864, but neither by naloxonazine nor nor-binaltorphimine. Following i.c.v. treatment with 5,7-dihydroxytryptamine (5,7-DHT), the tissue concentration of 5-HT was reduced by 97%, whereas the concentration of noradrenaline was reduced by only 5%. The tissue concentration of met-ENK, as measured by radioimmunoassay, was not significantly altered. The results suggest that met-ENK is present in the rat ventral spinal cord mainly in non-serotonergic nerve terminals and exerts an inhibitory action on 5-HT release via delta opioid receptors.     

Interactions of histaminergic and serotonergic neurons in the hypothalamic regulation of prolactin and ACTH secretion

Serotonergic and histaminergic neuronal systems are both involved in mediation of the stress-induced release of the pituitary hormones prolactin (PRL) and ACTH. We investigated the possibility of an interaction between serotonin (5-HT) and histamine (HA) in regulation of PRL and ACTH secretion in conscious male rats. Animals were pretreated systemically with antagonists to 5-HT1, 5-HT2 or 5-HT3 receptors prior to intracerebroventricular (icv) administration of HA. The 5-HT1 + 2 receptor antagonist methysergide prevented and the 5-HT2 receptor antagonist LY 53857 attenuated the HA-induced PRL release while the 5-HT3 receptor antagonist ondansetron had no effect on this response. None of the three 5-HT receptor antagonists affected the ACTH response to HA. Specific blockade of HA synthesis by alpha-fluoromethylhistidine or blockade of postsynaptic HA receptors by icv infusion of the H1 receptor antagonist mepyramine or the H2 receptor antagonist cimetidine inhibited the PRL response to 5-HT or to the 5-HT precursor 5-hydroxytryptophan (5- HTP) given in combination with the 5-HT reuptake inhibitor fluoxetine (Flx). Blockade of the histaminergic system had no effect on the ACTH response to serotonergic stimulation. The H3 receptors are inhibitory HA receptors. Systemic pretreatment with the H3 receptor agonist R(alpha)methylhistamine, or the H3 receptor antagonist thioperamide had no effect on the hormone response to activation of the serotonergic system by 5-HTP plus Flx. We conclude that the serotonergic and histaminergic neuronal systems interact in their stimulation of PRL secretion, but not in their stimulation of ACTH secretion. This interaction involves serotonergic 5-HT1 and 5-HT2 receptors and histaminergic H1 and H2 receptors. Furthermore, the previously observed inhibitory effect of the H3 receptor agonist R(alpha)methylhistamine on stress-induced PRL and ACTH release seems not to be exerted by activation of presynaptic H3 receptors located on serotonergic neurons but rather on histaminergic neurons.     

Ascending afferent regulation of rat midbrain dopamine neurons

Standard, extracellular single-unit recording techniques were used to examine the electrophysiological and pharmacological responsiveness of midbrain dopamine (DA) neurons to selected, ascending afferent inputs. Sciatic nerve stimulation-induced inhibition of nigrostriatal DA (NSDA) neurons was blocked by both PCPA (5-HT synthesis inhibitor) and 5,7-DHT (5-HT neurotoxin), suggesting mediation by a serotonergic (5-HT) system. Direct stimulation of the dorsal raphe (which utilizes 5-HT as a neurotransmitter and inhibits slowly firing NSDA neurons) inhibited all mesoaccumbens DA (MADA) neurons tested. Paradoxically, DPAT, a 5-HT1A agonist which inhibits 5-HT cell firing, enhanced sciatic nerve stimulation-induced inhibition of NSDA neurons. MADA neurons were not inhibited by sciatic nerve stimulation and, therefore, could not be tested in this paradigm. In contrast to the dorsal raphe, electrical stimulation of the pedunculopontine tegmental nucleus preferentially excited slowly firing NSDA and MADA neurons. Thus, both excitatory and inhibitory ascending afferents influence the activity of midbrain DA neurons, and intact 5-HT systems are necessary for sciatic nerve stimulation to alter DA cell activity. However, the role that 5-HT plays in mediating peripheral sensory input remains unclear.