Differences in kainate receptor involvement in hippocampal mossy fibre long-term potentiation depending on slice orientation John L. Sherwood a,1,3,⇑ , Mascia Amici a,3 , Sheila L. Dargan a,2 , Georgia R. Culley a , Stephen M. Fitzjohn a,1 , David E. Jane a , Graham L. Collingridge a,b , David Lodge a , Zuner A. Bortolotto a a MRC Centre for Synaptic Plasticity, School of Physiology & Pharmacology, University of Bristol, BS8 1TD, UK b Department of Brain & Cognitive Sciences, Seoul National University, Republic of Korea article info Article history: Available online 27 April 2012 Keywords: Hippocampus Plasticity Granule cell Kainate abstract Long-term potentiation (LTP) is a well-established experimental model used to investigate the synaptic basis of learning and memory. LTP at mossy fibre – CA3 synapses in the hippocampus is unusual because it is normally N-methyl-D-aspartate (NMDA) receptor-independent. Instead it seems that the trigger for mossy fibre LTP involves kainate receptors (KARs). Although it is generally accepted that pre-synaptic KARs play an essential role in frequency facilitation and LTP, their subunit composition remains a matter of significant controversy. We have reported previously that both frequency facilitation and LTP can be blocked by selective antagonism of GluK1 (formerly GluR5/Glu K5 )-containing KARs, but other groups have failed to reproduce this effect. Moreover, data from receptor knockout and mRNA expression studies argue against a major role of GluK1, supporting a more central role for GluK2 (formerly GluR6/Glu K6 ). A potential reason underlying the controversy in the pharmacological experiments may reside in differ- ences in the preparations used. Here we show differences in pharmacological sensitivity of synaptic plas- ticity at mossy fibre – CA3 synapses depend critically on slice orientation. In transverse slices, LTP of fEPSPs was invariably resistant to GluK1-selective antagonists whereas in parasagittal slices LTP was con- sistently blocked by GluK1-selective antagonists. In addition, there were pronounced differences in the magnitude of frequency facilitation and the sensitivity to the mGlu2/3 receptor agonist DCG-IV. Using anterograde labelling of granule cells we show that slices of both orientations possess intact mossy fibres and both large and small presynaptic boutons. Transverse slices have denser fibre tracts but a smaller proportion of giant mossy fibre boutons. These results further demonstrate a considerable heterogeneity in the functional properties of the mossy fibre projection. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The hippocampal mossy fibre (MF) pathway displays two unique properties, a pronounced frequency facilitation that is effective over large temporal windows (Salin et al., 1996), and N-methyl-D-aspartate (NMDA) receptor-independent long-term potentiation (LTP) of 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4- yl)propanoic acid (AMPA) receptor-mediated synaptic transmis- sion (Harris and Cotman, 1986) that is generally believed to be expressed pre-synaptically (Kawamura et al., 2004; Lopez-Garcia et al., 1996; Maeda et al., 1997; Reid et al., 2004; Xiang et al., 1994; Zalutsky and Nicoll, 1990). While it is widely accepted that presynaptic kainate receptors (KARs) play an important role in fre- quency facilitation and LTP (Bortolotto et al., 1999; Breustedt and Schmitz, 2004; Contractor et al., 2001; Lauri et al., 2001a,b; Pinhe- iro et al., 2007; Schmitz et al., 2003; but see Kwon and Castillo, 2008), there remains disagreement over the locus and subunit stoichiometry of these KARs. 0197-0186/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.neuint.2012.04.021 Abbreviations: ACET/UBP316, (S)-1-(2-amino-2-carboxyethyl)-3-(2-carboxy-5- phenylthiophene-3-yl-methyl)-5-methylpyrimidine-2,4-dione; AMPA, 2-amino-3- (5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid; aCSF, artificial cerebrospinal fluid; ATPA, (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid; CA3, cornu ammonis region 3; D-AP5, D-(À)-2-amino-5-phosphonopentanoic acid; DCG- IV, (2S,2 0 R,3 0 R)-2-(2 0 ,3 0 -dicarboxycyclopropyl)glycine; DMSO, dimethyl sulfoxide; fEPSP, field excitatory postsynaptic potential; GluK1, kainate receptor subunit 1 (formerly GluR5/GLU K5 ); GluK2, kainate receptor subunit 2 (formerly GluR6/GLU K6 ); GluK3, kainate receptor subunit 3 (formerly GluR7/GLU K7 ); KAR, kainate receptor; LTP, long-term potentiation; LY382884, (3S, 4aR, 6S, 8aR)-6-(4-carboxy- phenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-deca-hydroisoquinoline-3-carboxylic acid; MF, mossy fibre; mGlu, metabotropic glutamate; NMDA, N-methyl-D-aspartate; P, postnatal day (for example, P35 is postnatal day 35). ⇑ Corresponding author. Tel.: +44 (0)1276 483280. E-mail address: sherwoodjo@lilly.com (J.L. Sherwood). 1 Present address: Eli Lilly and Co., Erl Wood Manor, Windlesham GU20 6PH, UK. 2 Present address: School of Biosciences, Cardiff University, Biomedical Sciences Building, Museum Avenue, Cardiff CF10 3AX, UK. 3 These authors contributed equally to the work. Neurochemistry International 61 (2012) 482–489 Contents lists available at SciVerse ScienceDirect Neurochemistry International journal homepage: www.elsevier.com/locate/nci