Paired-Pulse Depression of Excitatory Postsynaptic Current Sinks in Hippocampal CA1 In Vivo L. Stan Leung, 1,2 * Pascal Peloquin, 1,2 and Kevin J. Canning 1,2 ABSTRACT: Paired-pulse depression (PPD), a short-term neural plas- ticity, was studied in hippocampal CA1 of urethane-anesthetized rats in vivo, using field potential recordings and current source density analy- sis. PPD was robust when an ipsilateral CA3 (iCA3) conditioning pulse of moderate stimulus intensity was followed 30–200 ms later by a con- tralateral CA3 (cCA3) test pulse; the ratio of the conditioned (C) to the nonconditioned (NC) response, as measured by the peak excitatory sink at the apical dendrites, ranged from 0.6 to 0.8. An alveus conditioning pulse evoked a large antidromic population spike in CA1 and a modest depression of the CA3-evoked excitatory sink (C/NC ratio of 0.85). High-intensity paired pulses, both delivered to iCA3, also showed PPD of the proximal excitatory sinks; however, paired-pulse facilitation of the dendritic sinks was found at the mid-apical dendrites, >250 lm from the soma. Local injection of GABA A antagonist picrotoxin or bicu- culline increased the C/NC ratio at IPIs of <150 ms, as well as the ratio of the amplitude of the population spikes (P2/P1; where P2 and P1 are the population spikes evoked by the second and first pulse, respec- tively). GABA B receptor antagonists, CGP35348 given intracerebroven- tricularly or CGP56999A administered locally, increased C/NC and P2/ P1 at IPIs of 150–400 ms. It is concluded that conditioned depression of the excitatory sinks was caused by mainly feedforward and some feed- back inhibition at the apical dendrites. GABA A -mediated postsynaptic inhibition dominated at early latencies, while GABA B -mediated inhibi- tion prevailed at long latencies, probably at both presynaptic and post- synaptic sites. PPD of the excitatory sinks provides a measure of popula- tion dendritic inhibition in vivo. V V C 2008 Wiley-Liss, Inc. KEY WORDS: paired-pulse depression; paired-pulse facilitation; GABA B receptors; GABA A inhibition; population excitatory postsynaptic potentials; population spikes; proximal dendrites INTRODUCTION When two pulses are delivered to the same synapse, paired-pulse depression (PPD) of excitatory postsynaptic potentials (EPSPs) in hippo- campal pyramidal cells has been reported (Debanne et al., 1996; Chen et al., 2004). More commonly, paired-pulse facilitation (PPF) of the EPSPs was observed when many afferents were stimulated (Creager et al., 1980; Leung and Fu, 1994). PPD of population EPSPs (pEPSPs) in CA1 was reported after high-in- tensity afferent stimulation in behaving rats (Kam- phuis et al., 1988; Gorter et al., 2002) and in anesthe- tized rats (Wu and Leung, 2003). In contrast, PPD of hippocampal EPSPs was not typically observed in vitro (Leung and Fu, 1994), and even PPD of hippo- campal population spikes (PopSpikes) was not often observed at interpulse intervals (IPIs) of less than 30 ms (Schwartzkroin, 1977; Buckmaster and Schwartz- kroin, 1985; Nathan and Lambert, 1991; Zhao and Leung, 1991; Hirota and Roth, 1997). This study was motivated by the lack of experimen- tal studies on the mechanisms underlying PPD of the pEPSPs. We hypothesize that the excitatory synaptic currents among a population of hippocampal neurons (i.e., the pEPSPs) are decreased by feedforward and feedback inhibition in vivo. Excitatory currents may be shunted by the high GABA A -receptor mediated conductance that predominates at the soma and proxi- mal dendrites of pyramidal cells (Fig. 1B; Andersen et al., 1964; Ben-Ari et al., 1981). In addition, postsy- naptic GABA B receptors on the dendrites of pyramidal cells (Fig. 1B; Bowery et al., 1987; Misgeld et al., 1995; Sloviter et al., 1999) may also shunt the EPSPs, and presynaptic GABA B receptors suppress glutamate release (Lanthorn and Cotman, 1981; Olpe et al., 1982; Isaacson et al., 1993; Molyneaux and Hasselmo, 2002; Poon et al., 2006). In this study, extracellular evoked potentials were recorded by a multichannel electrode and analyzed as current source density (CSD) in urethane-anesthetized rats in vivo. Paired pulse responses in CA1 were recorded in response to a conditioning ipsilateral CA3 (iCA3) pulse followed by a test pulse delivered to the ipsilateral or the contralat- eral CA3 (cCA3). METHODS AND MATERIALS Forty long Evans rats (220–450 g) were used. A rat was anesthetized with urethane (1.2–1.5 g/kg, i.p.). A silicon recording probe (NeuroNexus Technologies, Ann Arbor, MI) was positioned in CA1 area at P3.6– 4.5, L2.4–3 (with respect to bregma); the probe used had 16 recording sites spaced 50 lm apart on a vertical shank. A stimulating electrode was placed in 1 Department of Physiology and Pharmacology, University of Western Ontario, London, Ontario, Canada; 2 Department Clinical Neurological Sciences, University of Western Ontario, London, Canada K.J. Canning is currently at R&D Alliances, Medical Division, Glaxo- SmithKline Inc. (Canada), 7333 Mississauga Road, Mississauga Ontario, Canada L5N6L4. *Correspondence to: L. Stan Leung, Department of Physiology and Phar- macology, University of Western Ontario, London, Ontario, Canada N6A5C1. E-mail: sleung@uwo.ca Grant sponsor: Canadian Institutes of Health Research; Grant numbers: 15685, 64433. Accepted for publication 26 April 2008 DOI 10.1002/hipo.20458 Published online 11 June 2008 in Wiley InterScience (www.interscience. wiley.com). HIPPOCAMPUS 18:1008–1020 (2008) V V C 2008 WILEY-LISS, INC.