What would 5-HT do? Regional diversity of 5-HT1 receptor modulation of primary afferent neurotransmission

5-HT (serotonin) is a significant modulator of sensory input to the CNS, but the only analgesics that selectively target G-protein-coupled 5-HT receptors are highly specific for treatment of headache. Two recent papers in BJP shed light on this puzzling situation by showing that primary afferent neurotransmission to the superficial layers of the spinal and trigeminal dorsal is inhibited by different subtypes of the 5-HT₁ receptor – 5-HT_{1B(and 1D)} in the trigeminal dorsal horn and 5-HT_1A in the spinal dorsal horn. The inputs being studied probably include nociceptive afferents, and the similarities of the methods employed in the two studies minimize the possibility that the different findings are an experimental artefact. Rather, the findings raise interesting questions about the possible anatomical or functional basis for the apparent regional selectivity of 5-HT₁ receptor actions, and whether these differences could be exploited for therapy. The results also emphasize the relative lack of information we have about the molecular details of the pro- or anti-nociceptive actions of 5-HT itself on primary afferent neurotransmission.

LINKED ARTICLE

This article is a commentary on Choi et al., pp. 356–367 of this issue. To view this paper visit http://dx.doi.org/10.1111/j.1476-5381.2012.01964.x     

GABA(A) receptor-mediated presynaptic inhibition on glutamatergic transmission

We investigated the functional roles of presynaptic GABA_A receptors on excitatory nerve terminals in contributing to spontaneous and action potential-evoked glutamatergic transmission to rat hippocampal CA3 pyramidal neurons. Single CA3 neurons were mechanically isolated with adherent nerve terminals, namely the ‘synaptic bouton preparation’, and spontaneous glutamatergic excitatory synaptic potentials (sEPSCs) and EPSCs evoked by focal electrical stimuli of a single presynaptic glutamatergic boutons (eEPSCs) were recorded using conventional whole-cell patch recordings. Selective activation of presynaptic GABA_A receptors on these excitatory nerve terminals by muscimol, markedly facilitated sEPSCs frequency but inhibited eEPSC amplitude. The facilitation of sEPSC frequency was completely occluded by GABA_A receptor-Cl⁻ channel blockers bicuculline or penicillin (PN). PN itself concentration-dependently inhibited the GABA_A receptor response induced by bath application of muscimol, but had no effect on the glutamate receptor response. In addition, pretreatment with a blocker of the Na⁺, K⁺, 2Cl⁻ co-transporter type 1 (NKCC-1), bumetanide, prevented the muscimol-induced inhibition of eEPSCs. The results indicate that activation of presynaptic GABA_A receptors directly depolarizes glutamatergic excitatory nerve terminals and thereby differentially modulates sEPSCs and eEPSCs.     

Correlated species differences in the effects of cannabinoid ligands on anxiety and on GABAergic and glutamatergic synaptic transmission

Cannabinoid ligands show therapeutic potential in a variety of disorders including anxiety. However, the anxiety-related effects of cannabinoids remain controversial as agonists show opposite effects in mice and rats. Here we compared the effects of the cannabinoid agonist WIN-55,212 and the CB1 antagonist AM-251 in CD1 mice and Wistar rats. Special attention was paid to antagonist-agonist interactions, which had not yet been studied in rats. In mice, WIN-55,212 decreased whereas AM-251 increased anxiety. The antagonist abolished the effects of the agonist. In contrast, WIN-55,212 increased anxiety in rats. Surprisingly, the antagonist potentiated this effect. Cannabinoids affect both GABAergic and glutamatergic functions, which play opposite roles in anxiety. We hypothesized that discrepant findings resulted from species differences in the relative responsiveness of the two transmitter systems to cannabinoids. We investigated this hypothesis by studying the effects of WIN-55,212 on evoked hippocampal inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs). IPSCs were one order of magnitude more sensitive to WIN-55,212 in mice than in rats. In mice, IPSCs were more sensitive than EPSCs to WIN-55,212. This is the first study showing that the relative cannabinoid sensitivity of GABA and glutamate neurotransmission is species-dependent. Based on behavioural and electrophysiological findings, we hypothesize that WIN-55,212 reduced anxiety in mice by affecting GABA neurotransmission whereas it increased anxiety in rats via glutamatergic mechanisms. In rats, AM-251 potentiated this anxiogenic effect by inhibiting the anxiolytic GABAergic mechanism. We suggest that the anxiety-related effects of cannabinoids depend on the relative cannabinoid responsiveness of GABAergic and glutamatergic neurotransmission.     

Effects of steroids on NMDA receptors and excitatory synaptic transmission in neonatal motoneurons in rat spinal cord slices

The effect of steroids on NMDA receptors and excitatory postsynaptic transmission was studied in fluorescence-labelled motoneurons in thin spinal cord slices. In outside-out patches, NMDA-induced responses were potentiated by 79% in the presence of 20-oxopregn-5-en-3beta-yl sulfate (PS), while in the presence of 20-oxo-5alpha-pregnan-3alpha-yl sulfate (3alpha5alphaS) and 20-oxo-5beta-pregnan-3alpha-yl sulfate (3alpha5betaS) they were diminished by 57% and 66%, respectively. PS and 3alpha5betaS had no effect on the amplitude of single NMDA receptor channel openings, however, both compounds altered relative distribution of the openings to individual conductance levels. In control cases, the most frequent openings of the NMDA receptor channels were at the 70 pS conductance level, while in the presence of PS or 3alpha5betaS, the most frequent openings were at the 55 pS conductance level. Analysis of the mean current transferred by NMDA receptor channel openings at individual conductance levels indicated that in the presence of PS, the mean current induced by 55 pS conductance openings was significantly increased. In the presence of 3alpha5betaS, the mean currents induced by 55 pS and 70 pS conductance openings were significantly decreased. The amplitude of NMDA receptor-mediated EPSCs was potentiated by 54% in the presence of PS and the deactivation kinetics slowed. Neither the amplitude nor the kinetics of NMDA receptor-mediated EPSCs was significantly changed in the presence of 3alpha5betaS. The results of our experiments indicate that neurosteroids affect NMDA receptors in motoneurons. The effect appears to be influenced by the receptor subunit composition.     

Levetiracetam inhibits glutamate transmission through presynaptic P/Q-type calcium channels on the granule cells of the dentate gyrus

Background and purpose:

Levetiracetam is an effective anti-epileptic drug in the treatment of partial and generalized seizure. The purpose of the present study was to investigate whether levetiracetam regulates AMPA and NMDA receptor-mediated excitatory synaptic transmission and to determine its site of action in the dentate gyrus (DG), the area of the hippocampus that regulates seizure activities.

Experimental approach:

Whole-cell patch-clamp method was used to record the AMPA and NMDA receptor-mediated excitatory postsynaptic currents (EPSC_AMPA and EPSC_NMDA) in the presence of specific antagonists, from the granule cells in the DG in brain slice preparations from young Wistar rats (60–120 g).

Key results:

Levetiracetam (100 µM) inhibited both evoked EPSC_AMPA and EPSC_NMDA to an equal extent (80%), altered the paired-pulse ratio (from 1.39 to 1.25), decreased the frequency of asynchronous EPSC and prolonged the inter-event interval of miniature EPSC_AMPA (from 2.7 to 4.6 s) without changing the amplitude, suggesting a presynaptic action of levetiracetam. The inhibitory effect of levetiracetam on evoked EPSC_AMPA was blocked by ω-agatoxin TK (100 nM), a selective P/Q-type voltage-dependent calcium channel blocker, but not by nimodipine (10 µM) or ω-conotoxin (400 nM).

Conclusions and implications:

These results suggest that levetiracetam modulated the presynaptic P/Q-type voltage-dependent calcium channel to reduce glutamate release and inhibited the amplitude of EPSC in DG. This effect is most likely to contribute to the anti-epileptic action of levetiracetam in patients.     

Inhibition of spontaneous excitatory transmission induced by codeine is independent on presynaptic K channels and novel voltage-dependent Ca channels in the guinea-pig nucleus tractus solitarius

The nucleus tractus solitarius (NTS) constitutes the cough reflex arc and is thought to be one of the main sites of codeine's action. We have previously demonstrated using the guinea-pig brainstem slice that codeine inhibits the solitary tract-evoked excitatory postsynaptic currents (EPSCs) in the second-order NTS neurons through activating the presynaptic K⁺ channels. For further understanding of modulation of synaptic transmission by the antitussive, the effects of codeine (0.3–3.0 mM) on spontaneous EPSCs (sEPSCs) and miniature EPSCs (mEPSCs) were investigated in the NTS neurons of guinea-pigs. Codeine decreased the frequency and amplitude of sEPSCs. This action of codeine was mimicked by specific μ and κ receptor agonists, and blocked by μ and κ receptor antagonists. An agonist of δ receptors was ineffective on sEPSCs. The inhibitory effect of codeine on sEPSCs persisted under perfusion of a K⁺ channel blocker, 4-aminopyridine. In the presence of tetrodotoxin or Cd²⁺ which blocks, respectively, the action potential-induced or voltage-dependent Ca²⁺ entry-induced transmitter release, codeine still had an inhibitory effect on the frequency of mEPSCs without any considerable effect on their amplitude. The present study demonstrates that codeine depresses spontaneous excitatory synaptic transmission in the NTS neurons via presynaptic μ and κ receptors that do not couple with K⁺ and Ca²⁺ channels. These results suggest inhibitory modulation of the local circuit activity of NTS neurons by codeine, resulting in suppression of cough reflex.     

Effects of A2 type botulinum toxin on spontaneous miniature and evoked transmitter release from the rat spinal excitatory and inhibitory synapses

We observed effects of newly developed A2 type botulinum toxin (A2NTX) on spontaneous miniature and evoked transmitter release from inhibitory (glycinergic or GABAergic), or excitatory (glutamatergic) nerve terminals in rat spinal cord, by use of ‘synaptic bouton’ preparations, under voltage-clamp condition. A2NTX (0.1–1 pM) initially augmented and then decreased amplitude and frequency of spontaneous miniature release of glycine or GABA (mIPSCs) concentration-dependently. At an increased concentration (1–10 pM), A2NTX suppressed the amplitude of glutamatergic mEPSCs. The rank order of the inhibitory effects was glycinergic > GABAergic >> glutamatergic synapses. Focal electrical stimulation of ‘synaptic boutons’ elicited eIPSC or eEPSC with larger amplitude and low failure rate (Rf). A2NTX (0.01–1 pM) initially enhanced the amplitude or decreased the failure rate of eIPSC or eEPSC, and then almost completely abolished the generation of eIPSC or eEPSC. The action of A2NTX on the evoked transmitter release was partially reversible. The rank order of the inhibitory effects on the amplitude or Rf were glycinergic eIPSC ≥ GABAergic eIPSC > glutamatergic eEPSCs. Excess extracellular K⁺ or Ca²⁺ (excess [K⁺]_o or [Ca²⁺]_o), and 4-AP restored spontaneous miniature glycinergic, GABAergic or glutamatergic postsynaptic currents suppressed by A2NTX. We conclude that A2NTX inhibits spontaneous miniature release at 0.1–10 pM and evoked release at 0.01–1 pM in rat spinal cord, and the inhibition was much efficient in the evoked rather than the spontaneous miniature release. Excess [K⁺]_o, 4-AP and excess [Ca²⁺]_o, which can raise the intracellular Ca²⁺ concentration via the activation of voltage-dependent Ca²⁺ channels, rescue the transmission suppressed by A2NTX poisoning, suggesting the transmitter release machinery became less sensitive to intracellular Ca²⁺ in A2NTX poisoned ‘synaptic boutons’.     

Multiple roles of chemokine CXCL12 in the central nervous system: a migration from immunology to neurobiology

Chemotactic cytokines (chemokines) have been traditionally defined as small (10-14kDa) secreted leukocyte chemoattractants. However, chemokines and their cognate receptors are constitutively expressed in the central nervous system (CNS) where immune activities are under stringent control. Why and how the CNS uses the chemokine system to carry out its complex physiological functions has intrigued neurobiologists. Here, we focus on chemokine CXCL12 and its receptor CXCR4 that have been widely characterized in peripheral tissues and delineate their main functions in the CNS. Extensive evidence supports CXCL12 as a key regulator for early development of the CNS. CXCR4 signaling is required for the migration of neuronal precursors, axon guidance/pathfinding and maintenance of neural progenitor cells (NPCs). In the mature CNS, CXCL12 modulates neurotransmission, neurotoxicity and neuroglial interactions. Thus, chemokines represent an inherent system that helps establish and maintain CNS homeostasis. In addition, growing evidence implicates altered expression of CXCL12 and CXCR4 in the pathogenesis of CNS disorders such as HIV-associated encephalopathy, brain tumor, stroke and multiple sclerosis (MS), making them the plausible targets for future pharmacological intervention.     

The antidepressant tianeptine persistently modulates glutamate receptor currents of the hippocampal CA3 commissural associational synapse in chronically stressed rats

Recent hypotheses on the action of antidepressants imply a modulation of excitatory amino acid transmission. Here, the effects of long-term antidepressant application in rats with the drug tianeptine were examined at hippocampal CA3 commissural associational (c/a) glutamate receptor ion channels, employing the whole-cell patch-clamp technique. The drug's impact was tested by subjecting rats to daily restraint stress for three weeks in combination with tianeptine treatment (10 mg/kg/day). Whereas stress increased the deactivation time-constant and amplitude of the N-methyl-d-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs), it did not affect the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor-mediated EPSCs. Concomitant pharmacological treatment of stressed animals with tianeptine resulted in a normalized scaling of the amplitude ratio of NMDA receptor to AMPA/kainate receptor-mediated currents and prevented the stress-induced attenuation of NMDA-EPSCs deactivation. Both paired-pulse-facilitation and frequency-dependent plasticity remained unchanged. Both in control and stressed animals, however, tianeptine treatment strengthened the slope of the input-output relation of EPSCs. The latter was mimicked by exposing hippocampal slices in vitro with 10 micro m tianeptine, which rapidly increased the amplitudes of NMDA- and AMPA/kainate EPSCs. The enhancement of EPSCs could be blocked by the intracellular presence of the kinase inhibitor staurosporine (1 micro m), suggesting the involvement of a postsynaptic phosphorylation cascade rather then presynaptic release mechanisms at CA3 c/a synapses. These results indicate that tianeptine targets the phosphorylation-state of glutamate receptors at the CA3 c/a synapse. This novel signal transduction mechanism for tianeptine may provide a mechanistic resolution for its neuroprotective properties and, moreover, a pharmacological trajectory for its memory enhancing and/or antidepressant activity.