European Journal tff Pharmacology - Molecuha" Pharmacology., Section, 226 (t992) 279-280 279 '~S, 1992 Elsevier Science Publishers B.V. All rights reserved 0922-4106/92/$05.00

EJPMOL 0114R

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Metabotropic receptor mediated afterdepolarization in neocortical neurons

Char l e s G r e e n e , P e t e r Schwio.dt and W a y n e Cril l

Department of Physiology' and Biophysics, Unil'ersiO, of Washington, Seattle, 1t/.4 98195, USA

Received 6 May 1992, accepte.d 7 May 1992

In rat neocortical slices activation of glutamate metabotropic receptors with glutamate, quisqualate and IS,3R-ACPD caused a spike-evoked afterdepolarization which replaced the normal afterhyperpolarization. In some r, eocortieal neurons the afterde- polarization reached action potential threshold causing prolonged self-sustained firing. These effects of metabotropic receptor activation were similar to the action of muscarinic agonists in the same preparation.

Metabotropic receptors; Glutamate; Afterdepolarization

Glutamate directly opens ionotropic receptor coupled CaCI 2 2, MgCI 2 2, NaH2PO 4 1.25, NaHCO 3 26 and channels and stimulates the formation of diffusable dextrose 10 and was bubbled with 95% 0 2 and 5% second messengers by activating G-protein-coupled COz. metabotropic receptors. Glutamate and quisqualate ac- In 17 neurons from layer V of rat neocortex resting tivate both ionotropie and metabotropic receptors potential was -73 .4 + 9.3 mV S.E.M. and input resis- whereas 1S,3R-ACPD, the active epantiomer of 1- tance varied from 14 to 91 MaQ. Activation of aminocyclopentane-l,3-dicarboxylic acid (trans-ACPD), metabotropic receptors gave the same response in all is a relatively selective agonist for the metabotropic neocortical neurons regardless of their repetitive firing receptors (Irving et al., 1990). Metabotropic receptors pattern. The ionotropic receptors were blocked with arc present on a variety of mammalian neuron types kynurenate (2 mM) or with the selective blockers 6- but we know little about their ttmction. In hippocam- cyano-7-nitroquinoxaline-2,3-dione (CNQX) (50 p.M) pal pyramidal neurons activation of metabotropic re- and 2-amino-5-phosphonovaleric acid (APV) (50 ~zM). ceptors causes a dose-dependent depolarization pre- The bathing solution always contained atropine (5 #M) sumably by blocking the voltage-dependent K + current and in six slices s-propanolol (15 /zM) to eliminate (I m) and blocks the action potential afterhyperpolar- activation of muscarinic receptors or/3-adrenoceptors ization (AHP) by decreasing a CaZ+-activated K + cur- in the impaled neuron by impulses in pre,,,ynaptic cells. rent (IAH P) (Charpak et al., 1990; Stratton et al., 1990), Our study focused on alteration of spike-evoked These changes increase the excitability of hippocampal afterpotentials by metabotropic receptor activation. A neurons. Here we report that metabotropic receptor series of 20 individually evoked spikes (to keep firing activation evokes an additional new type of excitato~' frequency at 199 Hz) was followed by a slow AHP in response in neocortical neurons that only occurs fol- co~trol solution (fig. tA, control). When the bathing lowing a series of actien p~tentials, soiution contained 1S~3R-ACPD (usually at 50 /zM)

Brain slices from rat sensorimotor cortex (400 p.m the spike train was followed by a large afterdepolariza- thick) were placed in a submerged chamber maintained tion (ADP) instead of the AHP (fig. 1A, 1S,3R-ACPD). at 34-35¢'C for recording with microelectrodes filled Peak ADP amplitude was 3.9 + 2.2 mV and the ADP with either 2.7 M KCI or 2 M KMeSO 4. Normal I .sted 7.3_+4.i s. A similar response occurred in cat bathing solution contained (in mM) NaCI 130, ~c.C! 3, Betz cells (unpublished). This response differs qualita-

tively from the AHP reduction observed in neurons from other parts of the brain and constitutes a mecha- nism whereby firing can lead to more firing. For exam-

Correspondence t~: W~3yne E. Crill, M.D., Department of Physiology and Biophysics SJ-40, University of Washiiagton, Seattle, WA 98195, pie, a relatively large spike-evoked ADP caused persis- USA. Tel. (206) 543-0954; Fax (206) 685..0519. tent self-sust~_ined firing following cessation of the

/ t AHP with an ADP whereas tr-amino-3-hydroxy-5- mcthyl-4-isoxazolepropionic acid (AMPA) did not. A

! t - N , ~ ,~. /1S ,3R_ACFD simihtr replacement of the spike-evoked AHP by an ~!:~ - , , ~ . ~ , ADP is caused by muscarinic agonists in cat Betz cells

(Schvdndt et al., 19881 and in rat association cortex .. (Andrade, 19911. Because both muscarine and metab- ~. otropic receptor agonists replace the spike-evoked AHP

~- ~ . . . . . ='-v~ with and ADP in neocortex our findings support the rj Wash hypothesis that metabotropic receptor activation mim- e', L__ ics the effect of muscarinic agonists (Charpak et al., • 1990; Miller. t991).

B

~' , tt m trait ~, m Acknowledgments " i " • L I J ' " $'" ''*1'' -

We thank Gregg | t inz and Paul Newman for their assistance : t ;: ; i ; ; : ; h i : ;~ ; : i ! i~ ;~ ; i during the experiments, Supported by P | tS grants NS 16792 and GM

., ~; ' :~:~;:~.~i~:; , , .~: 07266 and the W,M. Keck Foundation.

~ ' References i-ig. 1. A: Superimposed traces showing the spike-evoked ADP after the application of IS,3R-ACPD ~15 ~zM) and the AI tP before and Andrade. R., 1991, (;ell excitation enhances mu,,,carinic cholinergic after application of ~hc drug. Each trace is the re:,p~,nse flfllowing 20 responses in rat association cortex, Brain Res. 548, 8t. spikes ev~ked at 1110 ltz. Arrowhead marks end of evoked repetitive Cbarpak, S., B.H. G~ihwiler. K.O. Do and T. Knfpfel, 199(I, Potas- firing. During application of ACPD the resting pa)tcntial was main- sium conductances in hippocampal neurons blocked by excitatory rained at c~m!rol level by DC hyperpolarizing c,rrcnt . B: Large ADP amino-acid transmitters, Nature 347. 765. caused by ACPD at 51~ taM evokes repetitive firing in another Irving, A.J., J.G. Schofield, J.C. Watkins, D.C. Sunter and G.L. neuron. Calibration bar is 500 ms in A and B and 2 mV in A and 211 Collingridge, 1990, IS,3R-ACPD stimulates and L-AP3 blocks

mW in B. Ca 2. mobilization in rat cerebellar neurons. Eur. J. Pharmacol. 186, 363.

Miller, R.J., 1991, Mctabotropie excitatory amino acid receptors evoked spike train ill several neurons (fig. 1B). Activa- reveal their true colors, Trends Pharmacol. Sci. 12, 365.

tion of metabotropic receptors could contribute to Schwindt, P.C., W.J. Spain, R.C. Foehring, M.C. Chubb and W.E.

epileptic behavior because the ADP following synapti- Crill, 1988, Slow conductances in neurons from cat sensorimotor

cally evoked repetitive firing could evoke more spikes cortex in vitro and their role in slow excitability changes, J. Neurophysiol. 59, 451,1.

a d d a larger ADP. Stratton, K., P.F. Worley and J.M. Baraban, 1990, Pharmacologic The me1,abotropic receptor agonists glutamate (0.1-1 characterization of phosphoinositide-linked glutamate receptor

hiM) and qt, isqualate (1 /.tM) also replaced tht.' slow =~'citatitm of hippocampal neurons, Eur. J. Pharamcol. 186, 357.