Firing Of Dorsal Raphe Neurons Research Articles

Peri-ictal activation of dorsomedial dorsal raphe serotonin neurons reduces mortality associated with maximal electroshock seizures.

Over one-third of patients with epilepsy will develop refractory epilepsy and continue to experience seizures despite medical treatment. These patients are at the greatest risk for sudden unexpected death in epilepsy. The precise mechanisms underlying sudden unexpected death in epilepsy are unknown, but cardiorespiratory dysfunction and arousal impairment have been implicated. Substantial circumstantial evidence suggests serotonin is relevant to sudden unexpected death in epilepsy as it modulates sleep/wake regulation, breathing and arousal. The dorsal raphe nucleus is a major serotonergic center and a component of the ascending arousal system. Seizures disrupt the firing of dorsal raphe neurons, which may contribute to reduced responsiveness. However, the relevance of the dorsal raphe nucleus and its subnuclei to sudden unexpected death in epilepsy remains unclear. The dorsomedial dorsal raphe may be a salient target due to its role in stress and its connections with structures implicated in sudden unexpected death in epilepsy. We hypothesized that optogenetic activation of dorsomedial dorsal raphe serotonin neurons in TPH2-ChR2-YFP (n = 26) mice and wild-type (n = 27) littermates before induction of a maximal electroshock seizure would reduce mortality. In this study, pre-seizure activation of dorsal raphe nucleus serotonin neurons reduced mortality in TPH2-ChR2-YFP mice with implants aimed at the dorsomedial dorsal raphe. These results implicate the dorsomedial dorsal raphe in this novel circuit influencing seizure-induced mortality. It is our hope that these results and future experiments will define circuit mechanisms that could ultimately reduce sudden unexpected death in epilepsy.

Delta glutamate receptor conductance drives excitation of mouse dorsal raphe neurons.

The dorsal raphe nucleus is the predominant source of central serotonin, where neuronal activity regulates complex emotional behaviors. Action potential firing of serotonin dorsal raphe neurons is driven via α1-adrenergic receptors (α1-AR) activation. Despite this crucial role, the ion channels responsible for α1-AR-mediated depolarization are unknown. Here, we show in mouse brain slices that α1-AR-mediated excitatory synaptic transmission is mediated by the ionotropic glutamate receptor homolog cation channel, delta glutamate receptor 1 (GluD1). GluD1R-channels are constitutively active under basal conditions carrying tonic inward current and synaptic activation of α1-ARs augments tonic GluD1R-channel current. Further, loss of dorsal raphe GluD1R-channels produces an anxiogenic phenotype. Thus, GluD1R-channels are responsible for α1-AR-dependent induction of persistent pacemaker-type firing of dorsal raphe neurons and regulate dorsal raphe-related behavior. Given the widespread distribution of these channels, ion channel function of GluD1R as a regulator of neuronal excitability is proposed to be widespread in the nervous system.

Transient Firing of Dorsal Raphe Neurons Encodes Diverse and Specific Sensory, Motor, and Reward Events

Serotonin (5-hydroxytryptamine [5-HT]) is known to influence a wide range of behaviors and physiological processes, but relatively little is known about events that trigger 5-HT release. To address this issue, we recorded from neurons in the dorsal raphe nucleus (DRN) in rats performing an odor-guided spatial decision task. A large fraction of DRN neurons showed transient firing time locked to behavioral events on timescales as little as 20 ms. DRN transients were sometimes correlated with reward parameters, but also encoded specific sensorimotor events, including stimulus identity and response direction. These behavioral correlates were diverse but showed no apparent relationship with waveform or other firing properties indicative of neurochemical identity. These results suggest that the 5-HT system does not encode a unitary signal and that it will broadcast specific information to the forebrain with speed and precision sufficient not only to modulate but also to dynamically sculpt ongoing information processing.

Do 5-HT1B/1D Autoreceptors Modulate Dorsal Raphe Cell Firing? In Vivo Electrophysiological Studies in Guinea Pigs with GR127935

GR127935 is a selective antagonist of release-modulating 5-HT1B/1D autoreceptors on serotonergic terminals and, as such, would be expected to produce increases in extracellular 5-HT. The changes in 5-HT observed are mixed, however, possibly due to the presence of somatodendritic 5-HT1B/1D autoreceptors. Theoretically, blockade of these autoreceptors would elevate intra-raphe 5-HT which, in turn, would activate somatodendritic 5-HT1A autoreceptors and slow firing rate. As recorded in anesthetized guinea pigs, dorsal raphe cell firing was unaffected by doses of GR127935 ranging from 20 to 20 000 μg/kg i.v. Lower doses of GR127935 (0.002–2 μg/kg i.v.) yielded highly variable responses, although these were not significantly different from baseline. 8-OH-DPAT in these and similar neurons produced the robust dose-dependent inhibitory response expected of a 5-HT1A agonist; increases in extracellular 5-HT resulting from re-uptake blockade by fluoxetine also suppressed unit activity. Doses of CP-135,807, a centrally-acting 5-HT1B/1D agonist, to increase tone on the somatodendritic 5-HT1B/1D autoreceptor produced only a trend toward decreases in dorsal raphe neuronal firing. The overall weak effect of GR127935 on raphe unit activity suggests that the mechanism described previously must take into account factors such as the degree of intra-raphe 5-HT release, the endogenous tone on the autoreceptors, receptor selectivity and intrinsic activity of GR127935 and/or heterogeneity within the subtype. © 1997 Elsevier Science Ltd.

A pharmacological profile of the selective silent 5-HT 1A receptor antagonist, WAY-100635

WAY-100635 ( N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride) is an achiral phenylpiperazine derivative that binds with high affinity and selectivity to the 5-HT 1A receptor. WAY-100635 displaced specific binding of the 5-HT 1A radioligand, [ 3H]8-OH-DPAT (8-hydroxy-2-(di- n-propylamino)tetralin), to rat hippocampal membranes with a pIC 50 of 8.87. This represented a greater than 100-fold selectivity relative to binding at other 5-HT receptor subtypes and major neurotransmitter receptor, reuptake and ion channel sites. In functional assays, WAY-100635 was a potent 5-HT 1A receptor antagonist, with no evidence of any 5-HT 1A receptor agonist or partial agonist activity. In the isolated guinea-pig ileum WAY-100635 was a potent and, at high concentrations, an insurmountable antagonist of the 5-HT 1A receptor agonist action of 5-carboxamidotryptamine, with an apparent pA 2 value (at 0.3 nM) of 9.71. WAY-100635 blocked the inhibitory action of 8-OH-DPAT on dorsal raphe neuronal firing in the anaesthetised rat at doses which had no inhibitory action per se. In behavioural models, WAY-100635 itself induced no overt behavioural changes but potently antagonised the behavioural syndrome induced by 8-OH-DPAT in the rat and guinea-pig (minimum effective dose = 0.003 mg/kg s.c. and ID 50 = 0.01 mg/kg s.c., respectively). WAY-100635 also blocked the hypothermia induced by 8-OH-DPAT in the mouse and rat with ID 50 values of 0.01 mg/kg s.c. These data indicate that WAY-100635 will be used as a standard antagonist in further studies of 5-HT 1A receptor function.

The effect of clonidine on the activity of neurons in the rat dorsal raphe nucleus in vitro.

The dorsal raphe (DR) nucleus is a system of nuclei in the midline of the lower brainstem, which is considered one of the most important nuclei in the modulation of pain in the central nervous system. Central noradrenergic systems play an important role in the control of cardiovascular regulation and pain transmission. Clonidine, an alpha 2-adrenergic agonist is used extensively in anesthesia research. In this study, we evaluated the involvement of clonidine in the activity of DR nucleus and its possible role in pain modulation. Seventy-four neurons within the DR nucleus in the rat brainstem slice preparation were tested using extracellular recording techniques. Application of noradrenaline (NA), 50 mumol/L, induced firing activity in 68 neurons tested (92%). NA produced a regular long-lasting firing activity on the DR neurons. Fifty-six neurons (88%) previously excited by NA were inhibited by clonidine, 20 mumol/L. Clonidine suppressed the firing activity of neurons. The results indicate that the firing of DR neurons was under noradrenergic influence and was inhibited by clonidine, which in turn alters nociception by modifying the central serotonergic system.

Electrophysiological evidence for a large receptor reserve for inhibition of dorsal raphe neuronal firing by 5-HT1A agonists.

Previous studies [Meller et al. (1990) Mol. Pharmacol., 37:231-237] have shown that a large receptor reserve exists for the inhibition of serotonin synthesis in rat cortex and hippocampus by the 5-HT1A agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), whereas little or no reserve exists for the lower efficacy agonists ipsapirone and BMY 7378. The current studies were undertaken to determine if the above drugs exhibit similar relative efficacies and receptor reserves in an electrophysiological model of 5-HT1A receptor activation, i.e., the inhibition of dorsal raphe cell firing. Intravenous dose-response curves were constructed in untreated control rats, or in rats which received an injection of the irreversible receptor inactivator N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 6 mg/kg, s.c.) 24 hours before recording. All three drugs fully inhibited dorsal raphe cell firing in control rats (ED50's: 1.5 micrograms/kg, 8-OH-DPAT; 30.0 micrograms/kg, ipsapirone; 17.5 micrograms/kg, BMY 7378). However, unlike effects on serotonin synthesis, EEDQ treatments caused no depression of the maximal inhibitory response for any of the agonists, although all dose-response curves were shifted to the right (ED50's: 10.1 micrograms/kg, 6.7-fold shift, 8-OH-DPAT; 139.9 micrograms/kg, 4.7-fold shift, ipsapirone; 53.8 micrograms/kg, 3.1-fold shift, BMY 7378). Although the order of agonist efficacies was similar for both inhibition of serotonin synthesis and dorsal raphe cell firing (8-OH-DPAT > ipsapirone > BMY 7378), a large (> 50%) receptor reserve was estimated for all three drugs in this electrophysiological system.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and function of the brain serotonin system

Structure and function of the brain serotonin system

Serotonin uptake blockers inhibit the firing of presumed serotonergic dorsal raphe neurons in vitro

AbstractSome antidepressant drugs are potent inhibitors of neuronal uptake of serotonin. In vivo, these compounds inhibit serotonergic dorsal raphe neuronal firing rates, presumably through increased stimulation of 5‐HT1a autoreceptors. We recorded from electrophysiologically identified serotonergic dorsal raphe neurons in rat brain slices and determined the effects of five serotonin uptake blockers on the firing rates of these units in vitro. Each drug decreased the neuronal firing rates in a concentration‐dependent manner. IC50 values derived from concentration‐response curves are: fluvoxamine, 0.8 μM; sertraline, 1.1 μM; imipramine, 2.7 μM; chlorimipramine, 2.8 μM; and fluoxetine, 4.2 μM. Exposure of brain slices to 10 μM tetrabenazine, a serotonin depleting agent, prior to treatment with serotonin uptake blockers resulted in a rightward shift of the concentration‐response curve. In vitro single unit recording allows: (1) direct comparison with neurochemical data obtained in vitro; (2) access to tissue bypassing blood brain barrier and liver enzymes; (3) quick wash out of drug from tissue; and (4) ability to record from single unitsover long periods (hours). This in vitro test provides a fast, simple means of determining neurophysiological effects of potential antidepressant drugs on the serotonin system.

3,4-methylenedioxymethamphetamine-induced release of serotonin and inhibition of dorsal raphe cell firing: potentiation by L-tryptophan

The effects of the serotonin (5-HT) precursor L-tryptophan on MDMA (3,4-methylenedioxymethamphetamine)-induced inhibition of dorsal raphe neuronal firing were characterized using extracellular single-unit recording and microdialysis techniques in the in vitro midbrain slice preparation. Pretreatment with L-tryptophan (100 microM) lowered the doses of MDMA required to inhibit unit activity. Based upon IC50 values, L-tryptophan increased the potency of MDMA by approximately 3-fold. In a parallel series of experiments, microdialysis probes resting on the brain slice surface provided a means to estimate 5-HT release from the dorsal raphe nucleus. Pretreatment with L-tryptophan increased MDMA-induced 5-HT release in a manner consistent with the suppression of dorsal raphe cell firing: compared to untreated preparations, peak 5-HT release, total release and the duration of release were all increased. Taken together, these data suggest that the enhancement by L-tryptophan of MDMA-induced 5-HT release and inhibition of dorsal raphe neuronal firing is due to an increase in the amount of 5-HT available for release. The question is raised as to what effect L-tryptophan may have on the psychotropic and neurotoxic actions of MDMA.

Effects of a selective 5‐HT2 agonist, DOI, on 5‐HT neuronal firing in the dorsal raphe nucleus and 5‐HT release and metabolism in the frontal cortex

Systemic administration of the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (50 and 100 micrograms kg-1, i.v.) inhibited dorsal raphe neuronal firing. DOI (100 micrograms kg-1, i.v.) also produced a decrease in extracellular 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the frontal cortex measured by microdialysis. However, local administration of DOI into the frontal cortex produced no change in extracellular 5-HT and 5-HIAA at any dose given (1, 10 and 100ng). The results demonstrate that DOI is a potent inhibitor of 5-HT neuronal firing and terminal release and that the effects on release are not mediated by an action within the terminal region. The site of action and the receptor involved in the inhibition remains to be determined.

MDMA (3,4-methylenedioxymethamphetamine) inhibits the firing of dorsal raphe neurons in brain slices via release of serotonin

The effects of MDMA (3,4-methylenedioxymethamphetamine) on the activity of serotonin (5-HT)-containing dorsal raphe neurons were characterized using extracellular single-unit recording and microdialysis techniques in the in vitro midbrain slice preparation. Addition of (+)-MDMA, (−)-MDMA or p-chloroamphetamine (PCA) to the superfusate (final concentration 3–100 μM) produced a concentration-dependent inhibition of 5-HT cell firing which was reversible and reproducible. Based upon IC 50 values, (+)-MDMA was 2- to 3-fold more potent than (−)-MDMA. Pretreatment with the selective 5-HT uptake inhibitor fluoxetine, at a concentration which had no effect on baselin firing (20 μM), blocked the inhibitory effect of (+)-MDMA and PCA on dorsal raphe neurons. The selective norepinephrine uptake inhibitor desipramine (20 μM) was ineffective. In a parallel series of experiments, microdialysis probes resting on the brain slice surface provided a means to estimate 5-HT release from the dorsal raphe nucleus. (+)-MDMA (100 μM) caused the release of measurable quantities of 5-HT with a time course which corresponded to the change in dorsal raphe cell firing rate. Taken together, these data suggest that MDMA acts indirectly to inhibit dorsal raphe neurons through release of endogenous 5-HT.

Inhibition of serotonergic dorsal raphe neurons by systemic and iontophoretic administration of buspirone, a non-benzodiazepine anxiolytic drug

Extracellular single-unit recordings were made from serotonergic dorsal raphe neurons in chloral hydrate anesthetized male Sprague-Dawley rats. Buspirone, a clinically effective non-benzodiazepine anxiolytic drug, caused inhibition of firing of these neurons when given by intravenous (ED 50 = 0.011 mg/kg, i.v.), intraperitoneal (ED 50 = 0.088 mg/kg, i.p.), and intragastric (effective dose = 1.0–20.0 mg/kg, i.g.) injection. Buspirone also inhibited these cells when it was administered to the outside of recorded neurons by microiontophoresis ( srmeffective currents = 2–15 nA). Iontophoretically applied buspirone did not potentiate nor block the effects of iontophoretically applied GABA. Systemic administration of two putative buspirone metabolites (1,2-pyrimidinyl piperazine and 5-hydroxy buspirone) in relatively high doses had a weak effect and no effect, respectively, on dorsal raphe neuronal firing. It is concluded that buspirone potently and directly inhibits the firing of serotonergic dorsal raphe neurons in the rat. Since buspirone inhibits the firing of serotonergic dorsal raphe neurons and binds to 5-HT 1A receptors, the present study supports the notion that central serotonergic systems may be involved in the therapeutic effects of anxiolytic drugs.

Reserpine suppression of dorsal raphe neuronal firing: mediation by adrenergic system

Reserpine, which induces a depletion of both serotonin (5-HT) and norepinephrine (NE) in brain, suppresses the firing of 5-HT cells in the dorsal raphe. Since these cells are known to be inhibited by either increased 5-HT “availability” or a reduction of adrenergic tone, studies were aimed at defining the role of these transmitter systems in mediating reserpine's suppression of 5-HT cell firing. Intraventricular injections of 6-hydroxydopamine, which destroyed NE terminals in the raphe region, prevented reserpine's effect on 5-HT cell activity. Both 1-amphetamine, which releases endogenous NE, and high doses of clonidine (200–400 μg/kg), a post-synaptic α-agonist, reversed the reserpine-induced depression of firing. Pretreatment with parachlorophenylalanine, which inhibits 5-HT synthesis, did not block either reserpine's depressant effect or amphetamine's ability to restore activity. Taken together, these results suggests that reserpine suppresses 5-HT cell firing by interfering with tonic adrenergic transmission, a normally present pre-requisite for 5-HT cell firing. Local iontophoresis of small amounts of NE dramatically reversed reserpine's depressant effect, but not that produced by systemically administered LSD. Since the dorsal raphe receives an adrenergic innervation, catecholamine terminals there could be responsible for providing the adrenergic tone upon which the firing of 5-HT cells depends. he possible involvement of local GABAergic interneurons in mediating an indirect NE disinhibition of 5-HT cell activity is discussed.