Frequency Of Inhibitory Synaptic Events Research Articles

Intrinsically determined cell death of developing cortical interneurons

Cortical inhibitory circuits are formed by GABAergic interneurons, a cell population that originates far from the cerebral cortex in the embryonic ventral forebrain. Given their distant developmental origins, it is intriguing how the number of cortical interneurons is ultimately determined. One possibility, suggested by the neurotrophic hypothesis1-5, is that cortical interneurons are overproduced, and then following their migration into cortex, excess interneurons are eliminated through a competition for extrinsically derived trophic signals. Here we have characterized the developmental cell death of mouse cortical interneurons in vivo, in vitro, and following transplantation. We found that 40% of developing cortical interneurons were eliminated through Bax- (Bcl-2 associated X-) dependent apoptosis during postnatal life. When cultured in vitro or transplanted into the cortex, interneuron precursors died at a cellular age similar to that at which endogenous interneurons died during normal development. Remarkably, over transplant sizes that varied 200-fold, a constant fraction of the transplanted population underwent cell death. The death of transplanted neurons was not affected by the cell-autonomous disruption of TrkB (tropomyosin kinase receptor B), the main neurotrophin receptor expressed by central nervous system (CNS) neurons6-8. Transplantation expanded the cortical interneuron population by up to 35%, but the frequency of inhibitory synaptic events did not scale with the number of transplanted interneurons. Together, our findings indicate that interneuron cell death is intrinsically determined, either cell-autonomously, or through a population-autonomous competition for survival signals derived from other interneurons.

Long-term effects of brain-derived neurotrophic factor on the frequency of inhibitory synaptic events in the rat superficial dorsal horn

Chronic constriction injury (CCI) of rat sciatic nerve produces a specific pattern of electrophysiological changes in the superficial dorsal horn that lead to central sensitization that is associated with neuropathic pain. These changes can be recapitulated in spinal cord organotypic cultures by long term (5–6 days) exposure to brain-derived neurotrophic factor (BDNF) (200 ng/ml). Certain lines of evidence suggest that both CCI and BDNF increase excitatory synaptic drive to putative excitatory neurons while reducing that to putative inhibitory interneurons. Because BDNF slows the rate of discharge of synaptically-driven action potentials in inhibitory neurons, it should also decrease the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) throughout the superficial dorsal horn. To test this possibility, we characterized superficial dorsal horn neurons in organotypic cultures according to five electrophysiological phenotypes that included tonic, delay and irregular firing neurons. Five to 6 days of treatment with 200 ng/ml BDNF decreased sIPSC frequency in tonic and irregular neurons as might be expected if BDNF selectively decreases excitatory synaptic drive to inhibitory interneurons. The frequency of sIPSCs in delay neurons was however increased. Further analysis of the action of BDNF on tetrodotoxin-resistant miniature inhibitory postsynaptic currents (mIPSC) showed that the frequency was increased in delay neurons, unchanged in tonic neurons and decreased in irregular neurons. BDNF may thus reduce action potential frequency in those inhibitory interneurons that project to tonic and irregular neurons but not in those that project to delay neurons.

Serotonergic modulation of neurotransmission in the rat basolateral amygdala.

Whole cell patch-clamp recordings were obtained from projection neurons and interneurons of the rat basolateral amygdala (BLA) to understand local network interactions in morphologically identified neurons and their modulation by serotonin. Projection neurons and interneurons were characterized morphologically and electrophysiologically according to their intrinsic membrane properties and synaptic characteristics. Synaptic activity in projection neurons was dominated by spontaneous inhibitory postsynaptic currents (IPSCs) that were multiphasic, reached 181 +/- 38 pA in amplitude, lasted 296 +/- 27 mS, and were blocked by the GABAA receptor antagonist, bicuculline methiodide (30 microM). In interneurons, spontaneous synaptic activity was characterized by a burst-firing discharge patterns (200 +/- 40 Hz) that correlated with the occurrence of 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive, high-amplitude (260 +/- 42 pA), long-duration (139 +/- 19 mS) inward excitatory postsynaptic currents (EPSCs). The interevent interval of 831 +/- 344 mS for compound inhibitory postsynaptic potentials (IPSPs), and 916 +/- 270 mS for EPSC bursts, suggested that spontaneous IPSP/Cs in projection neurons are driven by burst of action potentials in interneurons. Hence, BLA interneurons may regulate the excitability of projection neurons and thus determine the degree of synchrony within ensembles of BLA neurons. In interneurons 5-hydroxytryptamine oxalate (5-HT) evoked a direct, dose-dependent, membrane depolarization mediated by a 45 +/- 6.9 pA inward current, which had a reversal potential of -90 mV. The effect of 5-HT was mimicked by the 5-HT2 receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-methyl-5-HT), but not by the 5-HT1A receptor agonist, (+/-) 8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT), or the 5-HT1B agonist, CGS 12066A. In projection neurons, 5-HT evoked an indirect membrane hyperpolarization ( approximately 2 mV) that was associated with a 75 +/- 42 pA outward current and had a reversal potential of -70 mV. The response was independent of 5-HT concentration, blocked by TTX, mimicked by alpha-methyl-5-HT but not by 8-OH-DPAT. In interneurons, 5-HT reduced the amplitude of the evoked EPSC and in the presence of TTX (0.6 microM) reduced the frequency of miniature EPSCs but not their quantal content. In projection neurons, 5-HT also caused a dose-dependent reduction in the amplitude of stimulus evoked EPSCs and IPSCs. These results suggest that acute serotonin release would directly activate GABAergic interneurons of the BLA, via an activation of 5-HT2 receptors, and increase the frequency of inhibitory synaptic events in projection neurons. Chronic serotonin release, or high levels of serotonin, would reduce the excitatory drive onto interneurons and may act as a feedback mechanism to prevent excess inhibition within the nucleus.

Serotonin facilitates GABAergic transmission in the CA1 region of rat hippocampus in vitro.

1. The effect of serotonin on inhibitory synaptic transmission was examined in forty-one CA1 pyramidal neurones using intracellular voltage recordings in vitro. 2. Serotonin (20-50 microM) increased the synaptic noise of most (85%) neurones loaded with chloride (n = 33). The duration of this effect was enhanced with increasing concentrations of serotonin and was fully reversible within 5 min. When serotonin was applied at short intervals (less than 10 min), fading of the response was observed. 3. The effect of serotonin on synaptic noise persisted in the presence of the glutamate NMDA and non-NMDA antagonists, APV (100 microM) and CNQX (10 microM), but it was blocked (n = 5) by a GABAA antagonist, bicuculline (10 microM). 4. The increase in inhibitory synaptic events resulted from an enhanced frequency of unitary IPSPs from 4.6 +/- 3.8 Hz in control to 17.2 +/- 12.5 Hz (n = 5) in serotonin, especially of large events. Serotonin caused no change in the amplitude and frequency of miniature synaptic events recorded in the presence of TTX (n = 5). The mean amplitude of unitary inhibitory postsynaptic potentials (IPSPs) increased from 1.37 +/- 0.35 mV in control to 3.67 +/- 1.38 mV in serotonin. The coefficient of variation of unitary IPSPs increased from 0.40 +/- 0.11 in control to 0.74 +/- 0.23 in serotonin when quantal size appeared unchanged. 5. The 5-HT3 agonist 2-methyl-serotonin (52 microM, n = 4) partially mimicked the effect of serotonin, increasing the inhibitory noise without affecting the pyramidal neurone conductance. The serotonin-induced facilitation of unitary IPSPs was blocked by the 5-HT3 antagonists ICS 205-930 (1-90 nM, n = 3) and metoclopramide (30 microM, n = 1). 6. These results suggest that serotonin directly excites GABAergic interneurones acting on a 5-HT3 receptor and consequently increasing the frequency of inhibitory synaptic events recorded in CA1 pyramidal cells.