Glutamate is the fast excitatory neurotransmitter of Small Cardioactive Peptide ± containing Aplysia radula mechanoafferent neuron B21 Avniel N. Klein a, * , Klaudiusz R. Weiss a,b , Elizabeth C. Cropper a,b a Department of Physiology and Biophysics, Box 1218, Mount Sinai School of Medicine, 1 Gustave L. Levy Place, New York, NY 10029, USA b Fishberg Research Center for Neurobiology, Mount Sinai School of Medicine, 1 Gustave L. Levy Place, New York, NY 10029, USA Received 11 April 2000; received in revised form 3 June 2000; accepted 3 June 2000 Abstract B21 is a radula mechanoafferent neuron in the mollusc Aplysia which likely plays a crucial role in integrating environ- mental cues into the feeding motor program. To facilitate understanding B21's interactions with its postsynaptic followers, we sought to identify its neurotransmitter. We ®nd that B21 makes a chemical synapse onto the follower neuron B8. Although B21-induced excitatory postsynaptic potentials (EPSPs) in B8 paradoxically diminish in amplitude with B8 hyperpolarization, we show that an inwardly rectifying current is responsible. We conclude that these B21- induced EPSPs are likely glutamatergic as they are blocked by the glutamate antagonist DNQX. Furthermore, B8 exhibits a depolarizing response to exogenous glutamate, which is antagonized by DNQX. Finally, exogenous glutamate occludes B21-evoked EPSPs in B8. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Aplysia; Sensory; Transmitter; Small Cardioactive Peptide; Glutamate; Anomalous recti®er Even relatively simple behaviors that consist of repetitive motions respond to environmental cues without a loss of coordination or ef®ciency. Simple, (e.g. invertebrate) preparations have proven to be particularly experimentally advantageous for studies that seek to determine how afferent activity can recon®gure ongoing motor programs [11]. Our laboratory studies changes in feeding that occur when an Aplysia that has been making biting responses successfully grasps food and converts a bite into a hybrid response, a bite-swallow [8]. Data suggest that bite to bite-swallow conversions are likely to be mediated in part by increased activity in radula mechanoafferents, including the identi®ed neuron B21 [9,13,14]. Previous studies have shown that radula mechanoaffer- ents are peptide-containing, e.g. they contain the inverte- brate neuropeptides, small cardioactive peptides A and B (SCPa and SCPb) [9,14]. Exogenous application of the SCPs does not, however, completely mimic effects of B21 stimulation on postsynaptic followers. For example, although B21 elicits fast excitatory postsynaptic potentials (EPSPs) in the radula closer motor neuron B8 [14], SCP has not been observed to produce fast depolarization in any of the feeding neurons that have been studied [12,15,17]. Thus, previous data suggested that B21 contains a classical trans- mitter in addition to the SCPs. In the current study, we present data indicating that this is in fact the case. Experiments were performed on isolated buccal ganglia removed from 100±150 g Aplysia californica maintained in 14±168C arti®cial seawater (ASW). Animals were anesthe- tized by injection of isotonic MgCl 2 . Unless otherwise indi- cated, B21 and B8 were impaled with single-barreled electrodes (5±10 MV), and current was injected via an Axoclamp 2B. Spiking in B21 was triggered by short (20± 30 ms) depolarizing current pulses. Initially, we sought to con®rm that the B21±B8 connec- tion does in fact have a prominent chemical component. It has been shown that many neurons in the buccal ganglion of Aplysia are electrically coupled [3,4,7,9,14,16]. Conse- quently, a response that is apparently a chemical EPSP may actually be a coupling potential. Although B21- elicited EPSPs in B8 appear to facilitate, nonsynaptic potentials can have a similar appearance [19]. A previous study has shown that B21-elicited EPSPs in B8 are rever- sibly blocked in 4£ Mg 21 , 0.5£ Ca 21 ASW [14], which Neuroscience Letters 289 (2000) 37±40 0304-3940/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(00)01262-3 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 11-212-241-8051; fax: 11-212- 860-3369. E-mail address: ak1@doc.mssm.edu (A.N. Klein).