Neuropharmacology and Analgesia Presynaptic kainate receptors increase GABAergic neurotransmission in rat periaqueductal gray neurons Michiko Nakamura a , Kyu-Hyung Choi a , Sung-Keun Choi a , Chung-Sik Do a , Ju-Hye Jun a , Hyung-Kook Kwon a , So-Min Lee a , Ryu-Jin Moon a , Ki-Joung Yi a , Il-Sung Jang a,b, ⁎ a Department of Pharmacology, School of Dentistry, Kyungpook National University, Daegu 700-412, Republic of Korea b Brain Science & Engineering Institute, Kyungpook National University, Daegu 700-412, Republic of Korea abstract article info Article history: Received 28 September 2009 Received in revised form 13 February 2010 Accepted 3 March 2010 Available online 19 March 2010 Keywords: Kainate receptors Presynaptic facilitation GABAergic mIPSCs PAG neurons Patch clamp Neurons within the periaqueductal gray (PAG) have been implicated in the central regulation of pain signals by affecting the descending inhibitory pathway. Here we report on the functional role of presynaptic kainate receptors within the PAG. Using a conventional whole-cell patch clamp technique, we recorded GABAergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) from mechanically isolated rat PAG neurons in the presence of 300 nM tetrodotoxin and 20 μM DL-2-amino-5-phosphonovaleric acid under voltage-clamp conditions. Kainic acid at a 10 μM concentration significantly increased the frequency of GABAergic mIPSCs without affecting their amplitude, suggesting that kainic acid acts presynaptically to enhance spontaneous GABA release. The kainic acid-induced increase in mIPSC frequency was completely blocked by CNQX, a selective AMPA/kainate receptor antagonist. While neither AMPA nor NMDA affected GABAergic mIPSC frequency, ATPA, a selective agonist of GluR5-containing kainate receptors, increased GABAergic mIPSC frequency in a concentration-dependent manner. The kainic acid-induced increase in mIPSC frequency was completely suppressed either in the presence of 100 μM Cd 2+ , a general voltage- dependent Ca 2+ channel (VDCC) blocker, or in the Na + -free external solution. These results suggest that presynaptic kainate receptors have a low permeability to Ca 2+ , and that their activation elicits a presynaptic depolarization large enough to activate presynaptic VDCCs. Presynaptic kainate receptors on GABAergic nerve terminals appear to modulate GABAergic transmission, and in doing so may play an important role in the regulation of PAG neuron excitability. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The midbrain periaqueductal gray (PAG) is involved in the various functions including pain, fear and anxiety, vocalization, lordosis and cardiovascular control (for reviews, Behbehani, 1995; Millan, 2002). The PAG is a major component of a descending pain inhibitory pathway that promotes central analgesia and is one of the target sites for analgesics like opioids and cannnabinoids (Yaksh, 1997; Lichtman et al., 1996; Finn et al., 2003). Electrical stimulation of the PAG region is also known to activate a descending inhibitory pathway that reduces pain, including neuropathic pain (Reynolds, 1969; Monhemius et al., 2001). The primary intrinsic neural circuitry of the PAG is a tonically active spontaneous GABAergic network. Inhibition of this network might change the intrinsic excitability of PAG neurons, which is an important mechanism for modulating the output of the PAG (Behbehani et al., 1990; Ogawa et al., 1994). The modulation of spontaneous GABAergic activity within the PAG would thus play a crucial role in the regulating the various functions in which the PAG is involved, including pain control. Kainate receptors, in addition to AMPA and NMDA receptors, are nonselective ligand-gated ionotropic receptors that mediate fast excit- atory synaptic transmission within the central nervous system. However, it has been suggested that kainate receptors also have metabotropic actions, such as the activation of G proteins and intracellular signaling cascades linked to protein kinase C (PKC) and protein kinase A (PKA) (Lerma, 2003; Rodríguez-Moreno and Sihra, 2007). Since kainate receptor-mediated excitatory postsynaptic currents (EPSCs) have much smaller amplitudes and much slower decay kinetics than AMPA receptor- mediated EPSCs, it has been suggested that postsynaptic kainate receptors contribute to the temporal integration of synaptic transmission (Castillo et al., 1997; Frerking and Ohliger-Frerking, 2002). In addition, kainate receptor-mediated EPSCs contribute to synaptic plasticity at the hippo- campal mossy fiber synapses (Bortolotto et al., 1999; Contractor et al., 2001; Park et al., 2006). While postsynaptic kainate receptors play important roles in synaptic transmission, kainate receptors also exist on presynaptic terminals and modulate neurotransmitter release at various synapses. For example, in the spinal cord dorsal horn, the activation of European Journal of Pharmacology 635 (2010) 72–78 ⁎ Corresponding author. Department of Pharmacology, School of Dentistry, Kyung- pook National University, 188-1, Samduk 2 ga-dong, Jung-gu, Daegu 700-412, Republic of Korea. Tel.: +82 53 660 6887; fax: +82 53 424 5130. E-mail address: jis7619@mail.knu.ac.kr (I.-S. Jang). 0014-2999/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ejphar.2010.03.004 Contents lists available at ScienceDirect European Journal of Pharmacology journal homepage: www.elsevier.com/locate/ejphar