Excitatory and inhibitory synaptic currents and receptors in rat medial septal neurones.

R. Schneggenburger, J. López‐Barneo, A. Konnerth

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Abstract

1. A thin‐slice preparation was used to study the postsynaptic potentials and the underlying currents of visually identified rat medial septal (MS) neurones under tight‐seal voltage‐ and current‐clamp conditions. 2. Upon stimulation of the afferent fibres, all MS neurones exhibited a sequence of excitatory‐inhibitory postsynaptic potentials (EPSP‐IPSP). Under voltage clamp, with potassium glutamate as internal solution and at negative holding potentials (Vh), this synaptic pattern appeared as an initial inward current followed by a longer lasting outward current. 3. The inward postsynaptic current was completely abolished by 5 microM‐6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) whereas the outward current disappeared in the presence of 10 microM‐bicuculline. Thus the major excitatory and inhibitory synaptic inputs were identified as being due to activation of quisqualate/kainate glutamatergic and gamma‐aminobutyric acid (GABAA) receptors, respectively. 4. At positive Vh a CNQX‐resistant component of the excitatory postsynaptic current (EPSC) was revealed. This component was slower than the one mediated by the quisqualate receptor and was abolished by 3‐3(2‐carboxypiperazine‐4‐yl)propyl‐1‐phosphonate (CPP), indicating that N‐methyl‐D‐aspartate (NMDA) receptors are involved in excitatory synaptic transmission in MS cells. The existence of the two main subtypes (NMDA and non‐NMDA) of glutamatergic receptors in MS neurones was also confirmed by the responses of the neurones to bath application of the different agonists (glutamate, quisqualate, kainate and NMDA). 5. The CNQX‐sensitive EPSC had a reversal potential near 0 mV. The fast rise time (approximately 0.7 ms) indicates a somatic location of the excitatory synapses. The relaxation kinetics of the fast EPSC were fitted by a single exponential function with a time constant of 1.13 +/‐ 0.1 ms. This parameter was independent of Vh. Fast EPSCs were blocked by CNQX in a dose‐dependent manner (dissociation constant, KD = 0.2 microM). 6. Inhibitory postsynaptic currents (IPSCs) were studied in symmetrical chloride solutions after blockade of the excitatory receptors. The current‐voltage relation was linear and reversed at 0 mV. The IPSCs had a fast rise time and their decay was best fitted by the sum of two exponentials with time constant of approximately 20 and 50 ms (Vh = ‐60 mV). The IPSCs were abolished by bicuculline (KD = 1 microM), a selective antagonist of GABAA receptors. As expected, bath application of GABA produced large whole‐cell currents. 7. In many cells, in addition to the usual EPSP‐IPSP sequence, failures of either the EPSP or the IPSP were frequently observed during the experimental protocol.(ABSTRACT TRUNCATED AT 400 WORDS)

Original languageEnglish
Pages (from-to)261-276
Number of pages16
JournalJournal of Physiology
Volume445
Issue number1
DOIs
StatePublished - 1 Jan 1992
Externally publishedYes

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