Dopaminergic Regulation of Cone Retinomotor Movement in Isolated Teleost Retinas: II. Modulation by γ-Aminobutyric Acid and Serotonin

In the accompanying paper we reported that 3,4-dihydroxyphenylethylamine (dopamine) induced light-adaptive retinomotor movements in teleost photoreceptors and that this effect was mediated by D2 dopamine receptors located on the photoreceptors themselves. In this study, we investigated the effects on cone retinomotor movement of three agents that have been reported by others to modulate retinal dopamine release: gamma-aminobutyric acid (GABA), 5-hydroxytryptamine (5-HT, serotonin), and melatonin. We report here that the GABA antagonists bicuculline and picrotoxin induced light-adaptive cone contraction in dark-adapted green sunfish retinas cultured in constant darkness; thus they mimic the effect of light or exogenously applied dopamine. Since their effects were blocked by either the D2 dopamine antagonist sulpiride or by Co2+, it seems likely that these agents act by enhancing retinal dopamine release. The GABA agonist muscimol produced effects opposite to those of GABA antagonists. Muscimol inhibited light-induced cone contraction in previously dark-adapted retinas and induced dark-adaptive cone elongation in light-adapted retinas. These results suggest that in green sunfish retinas, as has been reported for other retinas, GABA inhibits dopamine release. 5-HT induced light-adaptive cone contraction in dark-adapted retinas; thus 5-HT also mimics the effect of light or exogenously applied dopamine. The effect of 5-HT was blocked by sulpiride, Co2+, or the 5-HT antagonist mianserin. These results suggest that 5-HT induces cone contraction by stimulating dopamine release. Melatonin neither inhibited dopamine-induced cone contraction in retinas cultured in the dark nor induced cone elongation in retinas cultured in the light. Our results suggest that both GABA and 5-HT (but not melatonin) affect cone retinomotor movements in green sunfish by modulating dopamine release: GABA by inhibiting and 5-HT by stimulating dopamine release. We report in the companion paper that dopamine induced contraction in isolated cone fragments. Together these observations strongly suggest that dopamine serves as the final extracellular messenger directly inducing light-adaptive cone retinomotor movement, and that GABA and 5-HT affect these movements by modulating dopamine release.     

GABA modulation of cholinergic transmission in rat oviduct

The effects of electrical stimulation, γ-aminobutyric acid (GABA), acetylcholine (ACh), norepinephrine (NE), 5-hydroxytryptamine (5-HT), GABA agonists and bicuculline were studied on spontaneous movements of isolated rat oviduct. The tissue did not respond to electrical stimulation or to GABA, NE and 5-HT when added to the incubation medium. ACh produced contractions related to its concentration which were maximal at the diestrous-1 phase when GABA caused a 20% rise in the ACh contraction. This effect was mimicked by GABA agonists whereas it was suppressed by bicuculline. β-Estradiol benzoate (EB) increased ACh contractions in diestrous-1 and in the late proestrous phases. GABA did not modify the EB effect. Progesterone did not modify ACh contractions in any of the studied phases. These findings suggest a possible modulatory role for GABA on ACh responses in the isolated rat oviduct.     

Serotonin-1B receptor-mediated inhibition of [3H]GABA release from rat ventral tegmental area slices

In order to assess a role of 5-HT(1B) receptors for regulation of GABA transmission in the ventral tegmental area (VTA), VTA slices from the rat were incubated with [(3)H]GABA and beta-alanine, and superfused in the presence of nipecotic acid and aminooxyacetic acid. [(3)H]GABA release was induced by exposures to the medium containing 30 mM potassium for 2 min. The results showed that high potassium-evoked [(3)H]GABA release was sensitive to calcium withdrawal or blockade of sodium channels by tetrodotoxin, suggesting that tritium overflow induced by high potassium derived largely from neuronal stores. Administration of CP 93129 (0.15 and 0.45 microM), a 5-HT(1B) receptor agonist, or RU 24969 (0.15 and 0.45 microM), a 5-HT(1B/1A) receptor agonist, but not 8-OH-DPAT (0.45 microM), a 5-HT(1A) receptor agonist, inhibited high potassium-evoked [(3)H]GABA release in a concentration-related manner. The RU 24969-induced inhibition of [(3)H]GABA release was antagonized by either SB 216641, a 5-H(1B) receptor antagonist, or cyanopindolol, a 5-HT(1B/1A) receptor antagonist, but not by WAY 100635, a 5-HT(1A) receptor antagonist. Pre-treatment with SB 216641 also antagonized CP 93129-induced inhibition of [(3)H]GABA release. The results support the hypothesis that 5-HT(1B) receptors within the VTA can function as heteroreceptors to inhibit GABA release.     

Serotonergic influences on pituitary gland and hypothalamic induction of prolactin and luteinizing hormone release in the young turkey (Meleagris gallopavo)

Primary anterior pituitary cell cultures were utilized to study the influence of serotonin (5-HT) directly on the pituitary. Cells incubated with 10(-5) and 10(-4) M 5-HT exhibited a significant prolactin (Prl) release, whereas cells incubated with 10(-10) to 10(-6) M 5-HT did not. Cells incubated with 10(-10) to 10(-4) M quipazine (5-HT agonist) or methysergide (MES; 5-HT antagonist) did not release Prl in amounts greater/less (P greater than 0.01) than spontaneous release. Luteinizing hormone (LH) release from cells incubated in the presence of 5-HT, quipazine, or MES was similar to spontaneous release. The hypothalamic extract-induced Prl and LH release from cells was not influenced by quipazine, but Prl release was diminished in a dose-related fashion by MES. The influence of 5-HT on hypothalamic induction of Prl and LH release was investigated utilizing in vitro culture of hypothalamic fragments (HF). Media samples from HF incubated with 10(-6) and 10(-4) M 5-HT induced a release of Prl. Media samples from HF incubated with 10(-4) M MES induced less Prl release than media samples from control fragments. When HF were incubated with both 10(-4) M 5-HT and 10(-4) M MES, the expected 5-HT-mediated Prl release was not evident. These culturing situations had no influence on LH release. In vitro Prl release from pituitary cells of the young turkey was stimulated through 5-HT activity at the hypothalamus, but not by direct 5-HT action on the pituitary cells.     

A-type GABA receptor as a central target of TRPM8 agonist menthol

Menthol is a widely-used cooling and flavoring agent derived from mint leaves. In the peripheral nervous system, menthol regulates sensory transduction by activating TRPM8 channels residing specifically in primary sensory neurons. Although behavioral studies have implicated menthol actions in the brain, no direct central target of menthol has been identified. Here we show that menthol reduces the excitation of rat hippocampal neurons in culture and suppresses the epileptic activity induced by pentylenetetrazole injection and electrical kindling in vivo. We found menthol not only enhanced the currents induced by low concentrations of GABA but also directly activated GABA(A) receptor (GABA(A)R) in hippocampal neurons in culture. Furthermore, in the CA1 region of rat hippocampal slices, menthol enhanced tonic GABAergic inhibition although phasic GABAergic inhibition was unaffected. Finally, the structure-effect relationship of menthol indicated that hydroxyl plays a critical role in menthol enhancement of tonic GABA(A)R. Our results thus reveal a novel cellular mechanism that may underlie the ambivalent perception and psychophysical effects of menthol and underscore the importance of tonic inhibition by GABA(A)Rs in regulating neuronal activity.     

Antimicrobial action spectrum of triterpene and steroid glycosides]

Antimicrobial activity of 9 triterpene glycosides of the plant and animal origin was studied. It was found that saponins inhibited the fungal growth to different extents and had no activity against grampositive and gramnegative bacteria. Triterpene glycosides of the animal origin, i.e. holoturins A and B, stichoposids A and C had the most pronounced antifungal activity as compared to the saponins isolated from plants. Changes in the length of the carbon chain from 2 to 6 monosugars had no significant effect on the activity of the triterpeneglycosides studied.     

Effect of Liraglutide on Corneal Kindling Epilepsy Induced Depression and Cognitive Impairment in Mice

GLP-1 play important role in neuroprotection and GLP-1 receptor deficit mice showed decreased seizure threshold and increased cognitive impairment. Therefore, study was premeditated to investigate the effect of liraglutide (GLP-1 analogue) on cornel kindling epilepsy induced co-morbidities in mice. Corneal kindling was induced by electrical stimulation (6 mA, 50 Hz, 3 s); twice daily for 13 days. Liraglutide (75 and 150 µg/kg) and phenytoin (20 mg/kg) were administered in corneal kindled groups. On day 14, elevated plus maze, passive shock avoidance paradigms were performed, and on day 15, retention was taken. On day 16 tail suspension test were performed. On 20th day challenge test was performed with same electrical stimulation and retention was observed on elevated plus maze and passive avoidance paradigm. Animal were sacrificed on 21st day for biochemical (LPO, GSH, and nitrite) and neurochemical (GABA, glutamate, DA, NE, 5-HT and their metabolites) estimation. Electrical stimulation by corneal electrode for 13 days developed generalized clonic seizures, increased cognitive impairment, oxidative stress and neurochemical alteration in mice brain. Co-treatment with liraglutide (75 and 150 μg/kg) significantly prevented the seizure severity, restored behavioural activity, oxidative stress and restored the altered level of neurotransmitters observed in corneal kindled mouse.

     

Unique ionotropic receptors for D-aspartate are a target for serotonin-induced synaptic plasticity in Aplysia californica

The non-L-glutamate (L-Glu) receptor component of D-aspartate (D-Asp) currents in Aplysia californica buccal S cluster (BSC) neurons was studied with whole cell voltage clamp to differentiate it from receptors activated by other well-known agonists of the Aplysia nervous system and investigate modulatory mechanisms of D-Asp currents associated with synaptic plasticity. Acetylcholine (ACh) and serotonin (5-HT) activated whole cell excitatory currents with similar current voltage relationships to D-Asp. These currents, however, were pharmacologically distinct from D-Asp. ACh currents were blocked by hexamethonium (C6) and tubocurarine (D-TC), while D-Asp currents were unaffected. 5-HT currents were blocked by granisetron and methysergide (MES), while D-Asp currents were unaffected. Conversely, while (2S,3R)-1-(Phenanthren-2-carbonyl)piperazine-2,3-dicarboxylic acid(PPDA) blocked D-Asp currents, it had no effect on ACh or 5-HT currents. Comparison of the charge area described by currents induced by ACh or 5-HT separately from, or with, D-Asp suggests activation of distinct receptors by all 3 agonists. Charge area comparisons with L-Glu, however, suggested some overlap between L-Glu and D-Asp receptors. Ten minute exposure to 5-HT induced facilitation of D-Asp-evoked responses in BSC neurons. This effect was mimicked by phorbol ester, suggesting that protein kinase C (PKC) was involved.     

GABA receptor modulation of 5-HT neuronal firing: characterization and effect of moderate in vivo variations in glucocorticoid levels

Evidence from electrophysiological studies suggests that 5-HT neuronal firing in the dorsal raphe nucleus (DRN) may be regulated by both GABA(A) and GABA(B) receptors. Here, we addressed the question of whether the activity of individual 5-HT neurons is regulated by both GABA(A) and GABA(B) receptors. In addition, we examined the concentration-response relationships of GABA(A) and GABA(B) receptor activation and determined if GABA receptor regulation of 5-HT neuronal firing is altered by moderate alterations in circulating corticosterone. The activity of 5-HT neurons in the DRN of the rat was examined using in vitro extracellular electrophysiology. The firing of all individual neurons tested was inhibited by both the GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo-[5,4-c]-pyridin-3-ol hydrochloride (THIP) (25 microM) and the GABA(B) receptor agonist baclofen (1 microM). Responses to THIP (5, 10, 25 microM) and baclofen (1, 3, 10 microM) were concentration dependent and attenuated by the GABA(A) and GABA(B) receptor antagonists, bicuculline (50 microM) and phaclofen (200 microM), respectively. To examine the effects of corticosterone on the sensitivity of 5-HT neurons to GABA receptor activation, experiments were conducted on adrenalectomized animals with corticosterone maintained for two weeks at either a low or moderate level within the normal diurnal range. These changes in corticosterone levels had no significant effects on the 5-HT neuronal response to either GABA(A) or GABA(B) receptor activation. The data indicate that the control of 5-HT neuronal activity by GABA is mediated by both GABA(A) and GABA(B) receptors and that this control is insensitive to moderate changes in circulating glucocorticoid levels.     

Intracellular recording of the frog dorsal root single fibres' responses mediated by the GABAA and 5-HT-receptors

The effects of GABA, bicuculline and 5-HT on primary afferents in the isolated spinal cord of the frog Rana ridibunda were studied. Bath application of GABA (1 mM) reduced the primary afferent depolarisation (PAD) in IX segment of the spinal cord evoked by X dorsal root stimulation (57 +/- 8% of initial level, n = 5, p < 0.05). The action potentials (AP) recorded in dorsal root afferents was also suppressed under the GABA action (74 +/- 9%, p < 0.05). Bath application of bicuculline (50 microM) reduced the PAD (21 +/- 7%), n = 6, p < 0.05), meanwhile the AP in dorsal root afferents was resistant against the bicuculline action. Bath application of 5-HT (25 microM) depressed the PAD (34 +/- 7%, n = 7, p < 0.05) and the amplitude of the AP recorded from the single afferent fibre in dorsal column (76 +/- 6%, n = 7, p < 0.05). In contrast to GABA, 5-HT more effectively suppressed the late phase of the PAD evoked by X dorsal root stimulation and caused (76 +/- 6%, n = 7, p < 0.05) an alteration of the AP shape. All effects induced by these drugs were reversible. The mechanisms of GABA and 5-HT modulation of spinal cord afferent income are discussed.     

Acid stimulation (sour taste) elicits GABA and serotonin release from mouse taste cells

Several transmitter candidates including serotonin (5-HT), ATP, and norepinephrine (NE) have been identified in taste buds. Recently, γ-aminobutyric acid (GABA) as well as the associated synthetic enzymes and receptors have also been identified in taste cells. GABA reduces taste-evoked ATP secretion from Receptor cells and is considered to be an inhibitory transmitter in taste buds. However, to date, the identity of GABAergic taste cells and the specific stimulus for GABA release are not well understood. In the present study, we used genetically-engineered Chinese hamster ovary (CHO) cells stably co-expressing GABA(B) receptors and Gαqo5 proteins to measure GABA release from isolated taste buds. We recorded robust responses from GABA biosensors when they were positioned against taste buds isolated from mouse circumvallate papillae and the buds were depolarized with KCl or a stimulated with an acid (sour) taste. In contrast, a mixture of sweet and bitter taste stimuli did not trigger GABA release. KCl- or acid-evoked GABA secretion from taste buds was Ca(2+)-dependent; removing Ca(2+) from the bathing medium eliminated GABA secretion. Finally, we isolated individual taste cells to identify the origin of GABA secretion. GABA was released only from Presynaptic (Type III) cells and not from Receptor (Type II) cells. Previously, we reported that 5-HT released from Presynaptic cells inhibits taste-evoked ATP secretion. Combined with the recent findings that GABA depresses taste-evoked ATP secretion, the present results indicate that GABA and 5-HT are inhibitory transmitters in mouse taste buds and both likely play an important role in modulating taste responses.     

Serotonergic modulation of murine fundic tone

Fundic tone is maintained through a balance of excitatory and inhibitory input to fundic smooth muscle. The aim of this study was to determine the role of serotonin (5-HT) and 5-HT receptors in modulating murine fundic tone. Muscle strips were prepared from the murine fundus. Intracellular recordings were made from circular smooth muscle cells, and the effects of 5-HT on tone and excitatory and inhibitory junction potentials evoked by electrical field stimulation (EFS) were determined. 5-HT induced a concentration-dependent contraction and smooth muscle depolarization that was tetrodotoxin resistant. The 5-HT(1B/D) receptor antagonists GR-127935 and BRL-155172 significantly inhibited 5-HT-induced contractions. The 5-HT(1B/D) agonist sumatriptan contracted murine fundic muscle. The 5-HT(1A) receptor agonist buspirone relaxed fundic smooth muscle, and the relaxation was inhibited by WAY-100135 but not by N(omega)-nitro-l-arginine or tetrodotoxin. 5-HT enhanced both the excitatory and inhibitory responses to EFS. The 5-HT(3) receptor antagonist MDL-72222 partly inhibited both the excitatory and inhibitory response elicited by EFS, whereas the 5-HT(4) receptor antagonist GR-113808 partly inhibited the EFS-evoked inhibitory response. The 5-HT reuptake inhibitor fluoxetine contracted smooth muscle strips, a contraction that was partially inhibited by GR-127935 and abolished by tetrodotoxin. In conclusion, the data suggest that 5-HT modulates murine fundic contractile activity through several different receptor subtypes. Sustained release of 5-HT maintains fundic tone through postjunctional 5-HT(1B/D) receptors. 5-HT(3) receptors modulate excitatory neural input to murine fundic smooth muscle, and both 5-HT(3) and 5-HT(4) receptors modulate inhibitory neural input to murine fundic smooth muscle.     

Evidence for GABA control of serotonin metabolism in the rat suprachiasmatic area

Changes in endogenous serotonin (5-HT) metabolism after in vivo stimulation of GABAergic transmission were investigated in the rat suprachiasmatic area (SCA). Activation of GABA transmission was performed by systemic administration of either amino-oxyacetic acid: AOAA, a GABA-transaminase inhibitor or RS baclofen, a GABA B agonist. After drugs administration, the amounts of endogenous 5-HT and 5-HIAA were measured. The release and synthesis of 5-HT were investigated in vitro, using a static incubation of tissue fragments. AOAA or RS baclofen induced an increase in endogenous 5-HT content but did not affect 5-hydroxyindole-acetic acid (5-HIAA). Both drugs induced an increase in the release and synthesis of 5-HT. Detailed study of the effects of AOAA over time on 5-HT metabolism showed that the increase in 5-HT release preceded the increase in amine synthesis. These results suggest that the in vivo stimulation of GABA transmission induces an increase in metabolic activity of the 5-HT neuronal system in the SCA. This effect may likely be mediated via activation of GABA B receptors.     

Role of Rubia cordifolia Linn. in radiation protection

The radioprotective potential of alcoholic extract of root of R. cordifolia, was studied by survival, hemopoietic cell protection and micronucleus assay. The LD50 value for the alcoholic root extract was found to be 1200 mg/kg body weight at 72 hr post irradiation. A significant radiation protection (67%) as assessed by increased animal survival was observed when R. cordifolia (RC) extract was administered intraperitoneally, 90 min. before the radiation exposure. Besides, the extract also inhibited radiation induced lipid peroxidation measured by the inhibition of thiobarbituric acid reactive substance (TBARS). The RC extract at a selected dose of 460 mg/kg body weight was effective in protecting the radiation induced suppression of endogenous colony forming units in spleen. A significant inhibition of radiation (2 Gy) induced micronuclei formation was observed when RC extract was administered 90 min prior to irradiation. Thus, it appears that the alcoholic root extract of R. cordifolia provides significant protection against radiation induced lipid peroxidation, hemopoietic injury and genotoxicity. The mechanism of action of RC extract appears to be through its anti-oxidant, metal chelation and anti-inflammatory property.     

The stimulation of GABAB receptors increases serotonin release in the rat suprachiasmatic area

The action of γ-aminobutyric acid (GABA) and related compounds on the spontaneous release of newly synthesized [³H]5-hydroxytryptamine ([³H]5-HT) was studied in the suprachiasmatic area (SCA) using a superfusion system. GABA (10 μM) increased [³H]5-HT release from SCA by up to 190%. Bicuculline or picrotoxin (10 μM) failed to inhibit the stimulatory effect of GABA. Muscimol (10 μM), a GABA_A agonist, was ineffective, however β-p-chlorophenyl GABA, R(−)baclofen, enhanced over 200% the release of the indoleamine; this latter effect was stereospecific. RS baclofen was twice less potent than the R(−)isomer in increasing the [³H]5-HT release. S(+)baclofen failed to affect the release of the indoleamine, whereas it attenuated the effect of its enantiomer. The effect of R(−)baclofen was Ca²⁺ dependent and was abolished by tetrodotoxin (TTX).Taken together these results suggest that in the SCA, [³H]5-HT release is facilitated by the stimulation of GABA_B receptors. The possible localization of these receptors is discussed in the light of morphological data recently reported by Bosler et al. (1985) and results obtained after TTX application.     

5-HT(2C) receptors localize to dopamine and GABA neurons in the rat mesoaccumbens pathway

The serotonin 5-HT(2C) receptor (5-HT(2C)R) is localized to the limbic-corticostriatal circuit, which plays an integral role in mediating attention, motivation, cognition, and reward processes. The 5-HT(2C)R is linked to modulation of mesoaccumbens dopamine neurotransmission via an activation of γ-aminobutyric acid (GABA) neurons in the ventral tegmental area (VTA). However, we recently demonstrated the expression of the 5-HT(2C)R within dopamine VTA neurons suggesting the possibility of a direct influence of the 5-HT(2C)R upon mesoaccumbens dopamine output. Here, we employed double-label fluorescence immunochemistry with the synthetic enzymes for dopamine (tyrosine hydroxylase; TH) and GABA (glutamic acid decarboxylase isoform 67; GAD-67) and retrograde tract tracing with FluoroGold (FG) to uncover whether dopamine and GABA VTA neurons that possess 5-HT(2C)R innervate the nucleus accumbens (NAc). The highest numbers of FG-labeled cells were detected in the middle versus rostral and caudal levels of the VTA, and included a subset of TH- and GAD-67 immunoreactive cells, of which >50% also contained 5-HT(2C)R immunoreactivity. Thus, we demonstrate for the first time that the 5-HT(2C)R colocalizes in DA and GABA VTA neurons which project to the NAc, describe in detail the distribution of NAc-projecting GABA VTA neurons, and identify the colocalization of TH and GAD-67 in the same NAc-projecting VTA neurons. These data suggest that the 5-HT(2C)R may exert direct influence upon both dopamine and GABA VTA output to the NAc. Further, the indication that a proportion of NAc-projecting VTA neurons synthesize and potentially release both dopamine and GABA adds intriguing complexity to the framework of the VTA and its postulated neuroanatomical roles.     

Serotonin sensitive adenylate cyclase in horse brain synaptosomal membranes.

In different membranal preparations isolated from horse brain stritum we have shown the existence of an adenylate cyclase system sensitive to serotonin (5-HT). Activation of the adenylate cyclase was determined by measuring cAMP using a radioimmunoassay. This serotoninergic sensitive enzyme is characterized by a high apparent affinity constant (in the nanomolar range), located on synaptosomal membranes. It is inhibited by antiserotoninergic drugs (cyproheptadine, cinanserin, methysergide, LSD), and synergistically activated by GTP. This serotoninergic activation is clearly additive to the activation induced by dopamine. It appears different from the adenylate cyclase system previously described in the literature which is also activated by 5-HT, but which has a low apparent affinity constant (in the micromolar range); the latter is apparently located in non-synaptosomal membranes, and its activation by 5-HT is non-additive to the activation induced by dopamine.The serotoninergic sensitive adenylate cyclase reported in this study, might be related to the serotoninergic binding system which we have previously described which has similar affinity constant, a similar subcellular distribution and which is inhibited in the same concentration ranges by antiserotoninergic drugs. These two systems might represent a synaptosomal serotoninergic receptor complex.     

Effects of transmitters on pyramidal and nonpyramidal neurons in hippocampal slices

Investigations were performed on the effects of acetylcholine (ACh), norepinephrine (NE), 5-hydroxytryptamine (5-HT), and -aminobutyric acid (GABA) on the background firing of the three following groups of field CA₃ neurons in guinea pig hippocampal slices: nonpyramidal neurons of the stratum radiatum moleculare (NSR), stratum pyramidale cells with single spike discharges (SD units), and those with complex discharge patterns (CD units) within the same layer. The action of ACh and NE on presumed interneurons of the pyramidal layer (IPL) was also investigated; CD units were found to differ from the remaining groups, which reacted similarly to the transmitters tested. It was shown that NE, 5-HT, and GABA inhibited the activity of CD cells, while ACh produced inhibitory-activating response in 50% of these units. Both NE and ACh exerted a monophasic activating effect on NSR, ISP, and SD, however, while 5-HT and GABA induced activation in a proportion of NSR and SD cells, as well as inhibitory response. The excitatory effects produced by ACh, NE, and 5-HT on NSR persisted during blockade of synaptic transmission, indicating that associated afferent fibers may be acting directly on these cells.Institute of Biological Physics, Academy of Sciences of the USSR, Pushchino-on-Oka. Translated from Neirofiziologiya, Vol. 20, No. 1, pp. 64–74, January–February, 1988.     

The peristaltic reflex induced by short-chain fatty acids is mediated by sequential release of 5-HT and neuronal CGRP but not BDNF

Short-chain fatty acids (SCFAs) accelerate colonic transit. This study examined whether this action was mediated by activation of the peristaltic reflex. SCFAs (acetate, butyrate, or propionate) were applied to the central compartment of a three-compartment flat-sheet preparation of the rat middle to distal colon. The release of serotonin (5-HT), brain-derived neurotrophic factor (BDNF), and CGRP was measured in all three compartments. Ascending contraction and descending relaxation were measured in the orad and caudad compartments. The addition of SCFAs at physiological to supraphysiological concentrations (0.5-100 mM) to the central compartment elicited concentration-dependent ascending contraction and descending relaxation (EC50 approximately 5 mM). At this concentration, SCFAs induced an 8- to 11-fold increase in 5-HT release and a 2- to 3-fold increase in CGRP release in the central compartment only. They had no effect on BDNF release. CGRP release was inhibited by a 5-HT4 but not a 5-HT3 receptor antagonist. Ascending contraction and descending relaxation were also inhibited by 5-HT4 and by CGRP receptor antagonists added to the central compartment. 5-HT and CGRP release, as well as ascending contraction and descending relaxation induced by mechanical stimulation of the mucosa (2-8 strokes), were significantly augmented by 1 mM acetate. Acetate (1 mM) also doubled propulsive velocity in isolated whole segments of the guinea pig colon. In conclusion, chemical stimulation of the mucosa by SCFAs triggers a peristaltic reflex mediated by the release of 5-HT from mucosal cells and activation of 5-HT4 receptors on sensory CGRP-containing nerve terminals. This SCFA-induced peristaltic pathway augments the peristaltic reflex elicited by mechanical stimulation of the mucosa.     

Constitutive histamine H2 receptor activity regulates serotonin release in the substantia nigra

The substantia nigra pars reticulata (SNr) forms a principal output from the basal ganglia. It also receives significant histamine (HA) input from the tuberomammillary nucleus whose functions in SNr remain poorly understood. One identified role is the regulation of serotonin (5-HT) neurotransmission via the HA-H(3) receptor. Here we have explored regulation by another HA receptor expressed in SNr, the H(2)-receptor (H(2)R), by monitoring electrically evoked 5-HT release with fast-scan cyclic voltammetry at carbon-fiber microelectrodes in SNr in rat brain slices. Selective H(2)R antagonists (inverse agonists) ranitidine and tiotidine enhanced 5-HT release while the agonist amthamine suppressed release. The 'neutral' competitive antagonist burimamide alone was without effect but prevented ranitidine actions indicating that inverse agonist effects result from constitutive H(2)R activity independent of HA tone. H(2)R control of 5-HT release was most apparent (from inverse agonist effects) at lower frequencies of depolarization (< or = 20 Hz), and prevailed in the presence of antagonists of GABA, glutamate or H(3)-HA receptors. These data reveal that H(2)Rs in SNr are constitutively active and inhibit 5-HT release through H(2)Rs on 5-HT axons. These data may have therapeutic implications for Parkinson's disease, when SNr HA levels increase, and for neuropsychiatric disorders in which 5-HT is pivotal.