ELSEVIER Brain Research 659 (1994) 75-81 BRAIN RESEARCH Selective suppression of intrinsic but not afferent fiber synaptic transmission by baclofen in the piriform (olfactory) cortex Akaysha C. Tang, Michael E. Hasselmo * Department of Psychology, Harvard University, 33 Kirkland St., Cambridge, MA 02138, USA Accepted 14 June 1994 Abstract The GABA B agonist baclofen has been shown to suppress synaptic transmission in subregions of the hippocampus and in the piriform (olfactory) cortex. Here we report a laminar selectivity of suppression of synaptic potentials in the olfactory cortex. In brain slice preparations, baclofen suppresses extracellularly recorded field potentials at the intrinsic fiber synapses proximal to the superficial pyramidal cell bodies (layer Ib) while leaving the afferent fiber synaptic potentials recorded at the distal dendrites (layer Ia) little affected. This dose-dependent selective suppression of intrinsic fiber synaptic transmission is also correlated with an increase of paired-pulse facilitation. These results suggest that afferent and intrinsic synaptic inputs may be differentially modulated by the activation of GABA 8 receptors and that this selective suppression is at least partially mediated via a presynaptic mechanism. Keywords: Baclofen; Piriform cortex; Laminar selectivity; Synaptic transmission; GABA B receptor; Intrinsic fiber 1. Introduction Given the complexity and diversity of the synaptic organization of the cerebral cortex, it is useful to classify excitatory synaptic connections broadly into three groups: afferent fiber projections arising from outside the cortex, intrinsic fiber projections arising from within a cortical region, and association fibers arising from other cortical regions [16,33]. In the pri- mary olfactory cortex (piriform cortex) the afferent and intrinsic fibers are segregated in distinct layers, Ia and Ib, where they synapse at the distal and promixal dendrites of the pyramidal neurons [31] (Fig. la). A similar laminar segregation of synapses from different fiber pathways also appears in the dentate gyrus, CA3 and CA1 regions of the hippocampus [34,38] (Fig. lb). This laminar segregation allows separate experimental investigation of the modulation of synapses made by fibers of different origins. Physiological and pharmacological studies reveal that various neurotransmitter systems exert differential in- * Corresponding author. 0006-8993/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0006-8993(94)00758-5 fluences over synaptic transmission in the layers proxi- mal and distal from the cell bodies in both the piriform cortex and hippocampus. In piriform cortex, choliner- gic agonists selectively suppress synaptic transmission at intrinsic fiber synapses [16] and glutamatergic modu- lation via AP4 receptors has a complementary depres- sant effect on afferent fiber synapses [17]. In the hip- pocampus, suppression of synaptic transmission by the cholinergic agonist carbachot is significantly stronger at the Schaffer collateral synapses in stratum radiatum than at the synapses of the perforant path in stratum lacunosum-moleculare [18]. In addition, the GABA 8 agonist baclofen has been reported to selectively decrease the magnitude of EP- SPs triggered by Schaffer collateral stimulation, but leave unaffected EPSPs triggered by the perforant path [1,5,24]. The similarity between the piriform cortex and hippocampus with regard to laminar segregation and cholinergic modulation suggests the possibility that ba- clofen may demonstrate a similar laminar selectivity in the piriform cortex. This study focused on the possible differential effect of the GABA B agonist baclofen on synaptic transmis- sion in the afferent and intrinsic fiber layers of the piriform cortex. In the experiments reported here, ba-