Selective H3 receptor (H3R) blockade: broad efficacy in cognition and schizophrenia.
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Animals↗
Biomedical subjects
L E Rueter
Publications and source records attributed to L E Rueter.
Spinal mechanisms underlying A-85380-induced effects on acute thermal pain.
Systemic administration of nicotinic receptor (nAChR) agonists is antinociceptive in models of acute pain whereas their intrathecal (i. t.) administration has been reported to be antinociceptive, nociceptive or without effect. It has been hypothesized that the action induced is dependent upon the subtype and location of the nAChR activated. In addition, there is considerable evidence that nAChR ligand-induced antinociception is mediated by other neurotransmitter systems via descending pathways from the brainstem to the spinal cord. The present study investigated the effects of i. t. and systemic administration of A-85380, a novel nAChR agonist, in the paw withdrawal model of acute thermal pain in the rat. Given i.t. , A-85380 (1 and 10 nmol/rat) decreased the latency to paw withdrawal by 2-4 s. This pronociception was accompanied by a spontaneous flinching behavior. Both of these effects were differentially blocked by i.t. pretreatment with the nAChR antagonists mecamylamine (10 nmol)>MLA (100 nmol)>DHbetaE (50% with 1000 nmol) but not by alpha-bungarotoxin (0% at 0.63 nmol). Given systemically, A-85380 (0.56 micromol/kg, i.p.) induced antinociception as indicated by an increased latency to paw withdrawal, an effect differentially altered by i.t. pretreatment with monoaminergic antagonists (100 nmol/rat). While mecamylamine and prazosin had no effect, scopolamine, methysergide and MDL 72222 partially antagonized and idazoxan completely antagonized A-85380-induced antinociception. Finally, as measured by in vivo microdialysis, levels of 5-HT, but not NE, in the i.t. space of the lumber region of the spinal cord were significantly increased following the systemic administration of A-85380. Together these data suggest that the nociceptive properties of spinally administered nAChR agents are not mediated by either an alpha(4)beta(2) or an alpha(7) subtype nAChR, whereas the antinociceptive properties of systemically-administered nAChR agents are mediated by descending noradrenergic, serotonergic and muscarinic inhibitory pathways.
Adrenergic alpha-Agonists↗
The identification of novel structural compound classes exhibiting high affinity for neuronal nicotinic acetylcholine receptors and analgesic efficacy in preclinical models of pain.
Neuronal nicotinic acetylcholine receptors represent a new and potentially useful target for the development of novel non-opioid, non-NSAID (nonsteroidal antiinflammatory drug) analgesic agents. A variety of nicotinic acetylcholine receptor agonists such as nicotine, epibatidine and the azetidinyl ether, (R)-5-(2-azetidinylmethoxy-2-chloropyridine (ABT-594) possesses significant efficacy in preclinical models of pain. A preponderance of evidence suggests that nicotinic acetylcholine receptor agonists produce their analgesic effects predominantly via activation of descending inhibitory pain pathways originating in the key brainstem regions of the nucleus raphe magnus, dorsal raphe, and locus coeruleus, and that alpha4-containing nicotinic acetylcholine receptor subunits mediate these effects. Although these studies may provide a pharmacological target for the development of nicotinic acetylcholine receptor analgesics, the rational design of selective ligands based on the protein structure of the binding site is hampered by insufficient structural information. Using an approach based upon homology to known high-affinity ligands for the alpha4beta2 binding site, a four-point model is proposed which defines distance and directionality parameters common to this set of nicotinic acetylcholine receptor ligands.
Analgesics↗
In vivo electrophysiological examination of 5-HT2 responses in 5-HT2C receptor mutant mice.
The present study used 5-HT2C receptor mutant mice and their wild-type littermates to characterize the 5-HT2 receptor using the 5-HT2 agonists (+/-)-2-dimethoxy-4-iodoamphetamine hydrochloride (DOI) and 1-(3-chlorophenyl)piperazine (mCPP) applied locally in the orbitofrontal cortex (OFC) and head of the caudate nucleus. Microiontophoretically-applied 5-HT, DOI and mCPP induced current-dependent inhibition of neuronal firing activity in both brain regions. There was no difference between 5-HT2C receptor mutants and wild-type mice in the ability of 5-HT or DOI to inhibit neuronal firing at any current used. In contrast, there was a reduced ability of mCPP to inhibit firing activity in the OFC when ejected at 10 nA. Unexpectedly, there was a small but significant increase in mCPP-induced inhibition in the caudate nucleus of mutant mice. In the OFC, the 5-HT2A antagonist MDL 100907 (2 mg/kg, i.p.) significantly antagonized the effect of both DOI and mCPP. In contrast, the non-selective 5-HT antagonist clozapine (10 mg/kg, i.p.) significantly antagonized only mCPP in the wild-type mice. However, neither MDL 100907 nor clozapine antagonized DOI or mCPP in the caudate nucleus. Finally, it required significantly less quisqualate to activate neurons in the 5-HT2C receptor mutants than in the wild-type mice, suggesting that 5-HT2C receptors serve a tonic inhibitory role in membrane excitability. The present results indicate that the inhibitory action of DOI is predominantly mediated by the 5-HT2A receptor in the OFC. mCPP, when applied locally, inhibits OFC firing activity by acting on both 5-HT2A and 5-HT2C receptors. However, DOI and mCPP might be acting in the caudate nucleus through an atypical 5-HT2 receptor yet to be characterized.
Analysis of Variance↗
Electrophysiological examination of the effects of sustained flibanserin administration on serotonin receptors in rat brain.
5-HT1A receptor agonists have proven to be effective antidepressant medications, however they suffer from a significant therapeutic lag before depressive symptoms abate. Flibanserin is a 5-HT1A receptor agonist and 5-HT2A receptor antagonist developed to possibly induce a more rapid onset of antidepressant action through its preferential postsynaptic 5-HT1A receptor agonism. Flibanserin antagonized the effect of microiontophoretically-applied DOI in the medial prefrontal cortex (mPFC) following 2 days of administration, indicating antagonism of postsynaptic 5-HT2A receptors. This reduction in the effect of locally-applied DOI was no longer present following 7-day flibanserin administration. Two-day flibanserin administration only marginally reduced the firing activity of dorsal raphe (DRN) 5-HT neurons. Following 7 days of administration, 5-HT neuronal firing activity had returned to normal and the somatodendritic 5-HT1A autoreceptors were desensitized. The responsiveness of postsynaptic 5-HT1A receptors located on CA3 hippocampus pyramidal neurons and mPFC neurons, examined using microiontophoretically-applied 5-HT and gepirone, was unchanged following a 7-day flibanserin treatment. As demonstrated by the ability of the 5-HT1A receptor antagonist WAY 100635 to selectively increase the firing of hippocampal neurons in 2- and 7-day treated rats, flibanserin enhanced the tonic activation of postsynaptic 5-HT1A receptors in this brain region. The results suggest that flibanserin could be a therapeutically useful compound putatively endowed with a more rapid onset of antidepressant action.
Amphetamines↗
In vivo electrophysiological assessment of the agonistic properties of flibanserin at pre- and postsynaptic 5-HT1A receptors in the rat brain.
Flibanserin (BIMT 17) has been described as a 5-HT1A agonist with preferential affinity for postsynaptic 5-HT1A receptors and as a 5-HT2A antagonist. Indeed, using the forskolin-stimulated cAMP accumulation technique, flibanserin but not the 5-HT1A agonists buspirone and 8-OH-DPAT had agonistic activity at postsynaptic 5-HT1A receptors in the cerebral cortex. The present in vivo electrophysiological study investigated the agonistic properties of this novel compound in pre- and postsynaptic areas of the anesthetized rat brain using local microiontophoretic application and systemic administration. The inhibition induced by either local or intravenous administration of flibanserin was current- and dose-dependent. Based on the ability of 5-HT1A antagonists to block or reverse the inhibitory action of the compound, the effect of flibanserin was shown to be mediated via 5-HT1A receptors. In addition, as determined by the concurrent microiontophoretic application of flibanserin and 5-HT, flibanserin behaved as a full agonist in the dorsal raphe nucleus (DRN) and the medial prefrontal cortex (mPFC), but as a partial agonist in the CA3 region of the hippocampus. Based on neuronal responsiveness observed with the local microiontophoretic application of flibanserin, it was found that the agonist was most potent on 5-HT1A receptors in the hippocampus, followed by the mPFC and DRN (I.T50 values: 260, 1,260, and 1,365 nanocoulombs, respectively). However, based on the ED50 values obtained from intravenous administration of the drug, flibanserin was most potent in the DRN followed by the hippocampus and mPFC (ED50 values: 239, 1,414, and 2,984 micrograms/kg, respectively). Therefore, flibanserin presented a marked selectivity for postsynaptic 5-HT1A receptors when applied locally, but not when administered intravenously. It remains to be determined if flibanserin preferentially activates postsynaptic 5-HT1A receptors upon sustained systemic administration.
Animals↗
Effect of long-term administration of duloxetine on the function of serotonin and noradrenaline terminals in the rat brain.
Duloxetine, an inhibitor of both 5-hydroxytryptamine (5-HT) and noradrenaline (NA) reuptake processes, has been developed as a potential antidepressant drug. The present study was initiated to investigate the functioning of multiple components of the 5-HT and NA systems following the long-term administration of duloxetine. In rats treated for 21 days with duloxetine (20 mg/kg/day), the recovery times of dorsal hippocampus CA3 pyramidal neurons from microiontophoretic applications of 5-HT and NA were significantly increased, indicating ongoing reuptake blockade with the minipump in place delivering the drug. The remaining experiments were performed following a 48-h washout. Electrically evoked release of [3H]5-HT from preloaded slices was enhanced in the midbrain, presumably due to a desensitization of the somatodendritic 5-HT1D and 5-HT1A autoreceptors. In addition, evoked release of [3H]5-HT was increased in the hippocampus, which could have been due to the desensitization of the alpha2-adrenergic heteroreceptors located on the 5-HT terminals. In contrast, there was no change in the evoked release of [3H]5-HT in the frontal cortex despite decreased functioning of the 5-HT transporter found in this brain region. Similar to changes in 5-HT release, electrically evoked release of [3H]NA was enhanced in the hippocampus and frontal cortex of rats treated chronically with duloxetine. These increases in [3H]NA release were most likely due to the desensitization of the alpha2-adrenergic autoreceptor in the hippocampus and to the desensitization of the NA transporter in the frontal cortex, respectively. These data suggest that long-term administration of duloxetine is able to induce changes in the 5-HT and NA systems that lead to enhanced release of both 5-HT and NA in some limbic brain areas. Duloxetine, therefore, may be a useful antidepressant compound.
Animals↗
Electrophysiological characterization of the effect of long-term duloxetine administration on the rat serotonergic and noradrenergic systems.
Duloxetine is a dual serotonin (5-HT)/norepinephrine (NE) re-uptake blocker with antidepressant potential. In the present in vivo electrophysiological study, the changes in the function of the rat 5-HT and NE systems after 2- and 21-day administration of duloxetine (20 mg/kg/day) were assessed in the dorsal hippocampus and the dorsal raphe nucleus (DRN). The firing rate of DRN neurons was decreased after 2 days of duloxetine, but returned to the control level after 21-day administration. This recovery of firing rate was presumably due to the desensitization of the DRN somatodendritic 5-HT1A autoreceptors found after long-term duloxetine administration. Overall serotonergic tone was assessed by examining the ability of the 5-HT1A antagonist WAY 100635 to alter hippocampal firing. WAY 100635 increased hippocampal firing rates in 21-day treated rats to a greater extent than in 2-day treated or control rats, suggesting that long-term administration induced an increase in endogenous levels of 5-HT in postsynaptic regions. This increase in 5-HT levels was accompanied by selective changes in the 5-HT and NE systems induced by long-term duloxetine administration, i.e., the desensitization of the alpha-2 adrenergic heteroreceptor on 5-HT terminals and the continued blockade of the 5-HT transporters. In contrast, the sensitivity of the alpha-2 adrenergic and terminal 5-HT1B autoreceptors, as well as that of the postsynaptic 5-HT1A receptor after 21-day treatment was unchanged. Therefore, this study demonstrates that duloxetine increases serotonergic tone in a limbic forebrain structure and may therefore be effective in the treatment of depression.
Adrenergic Uptake Inhibitors↗
A microdialysis examination of serotonin release in the rat forebrain induced by behavioral/environmental manipulations.
Serotonin (5-HT) has been implicated in a variety of behavioral and physiological processes mediated by the central nervous system. However, the exact nature of 5-HT release under naturalistic or physiologic conditions remains unclear. The present study investigated this issue by employing in vivo microdialysis to examine 5-HT release in the hippocampus, corpus striatum, amygdala, and prefrontal cortex of the rat during manipulations that induced varying behavioral and physiological responses. In each rat, two sites were examined simultaneously during two of the following manipulations: tail pinch, tail pinch with food present, feeding, cat exposure, floating, and swimming. Results indicate that 5-HT levels increased 20-65% above baseline levels in response to each manipulation in all forebrain areas examined. The only significant difference found across manipulations was that tail pinch with food present produced a greater increase in 5-HT than cat exposure, which may be attributable to differences in the degree of general activation induced by these manipulations rather than to specific physiological or behavioral aspects. Furthermore, there was a strong positive correlation between 5-HT release and time spent in alert waking. Finally, there was an overall significantly smaller increase in 5-HT release in the corpus striatum compared to the other three sites. These data suggest that the release of 5-HT in the forebrain is closely related to behavioral state, not to specific behaviors, and that the magnitude of 5-HT release may reflect the degree of activation. In addition, this study suggests there can be some degree of differential pattern of release of 5-HT in the forebrain.
Amygdala↗
Changes in forebrain serotonin at the light-dark transition: correlation with behaviour.
In vivo microdialysis in the rat was used to determine whether changes in forebrain serotonin (5-HT) levels seen during the light-dark transition differ by area examined and whether these changes could be accounted for by complementary changes in behaviour. 5-HT levels increased significantly during the first half hour of dark phase in the hippocampus (45.9%), striatum (13.4%), amygdala (19.7%) and prefrontal cortex (18.4%), the increase in the hippocampus being significantly larger than those in the other areas. 5-HT levels co-varied significantly with changes in the levels of alert waking, a behavioural measure of the time spent in active waking, suggesting that this factor might account for the changes in 5-HT across the light-dark transition.
Analysis of Variance↗
Neuronal release of serotonin in the cerebellum of behaving rats: an in vivo microdialysis study.
Release of endogenous serotonin [5-hydroxy-tryptamine (5-HT)] in the cerebellum of awake rats was characterized using in vivo microdialysis. 5-HT output was increased (approximately 70%) by local application of KCI (100 mM) and was reduced (approximately 60%) by both tetrodotoxin (0.5 microM) and omission of Ca2+ from the perfusion fluid. 5-HT release was decreased (approximately 70%) by the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (0.25 mg/kg, s.c.), and this effect was rapidly reversed by a selective 5-HT1A antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo- hexane-carboxamide trihydrochloride (WAY-100635; 0.1 mg/kg, i.p.). These results indicate that a large portion of the measurable 5-HT output in the cerebellum is of neuronal origin, is dependent on impulse flow, and is sensitive to 5-HT1A autoreceptor activation. Further studies examined the relationship between 5-HT levels and general activity of the animals across the light-dark transition and during behavioral manipulations. Both 5-HT levels and behavioral activity were significantly elevated during the dark period, with changes in 5-HT efflux closely paralleling changes in activity. Similar increases (approximately 40%) in 5-HT output were observed during both feeding and feeding in the presence of a stressor (tail pinch). These findings suggest that behavioral state is an important factor determining neuronal 5-HT release in cerebellum under physiological conditions.
Animals↗
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.
Action Potentials↗