Memories are vulnerable to disruption following a learning experience until they become stabilized through processes beginning during the learning phase and extending for a few hours afterward. That is, memory is labile until it undergoes the consolidation process (Lechner et al., 1999; McGaugh, 2000). Memory can be characterized in a number of different ways, one being for how long the memory persists. Thus memory can be divided into short (STM), intermediate (ITM) and long-term (LTM) forms. STM persists for only a few minutes while the longer lasting ITM and LTM persist for hours, days, weeks and years. ITM endures for only a few hours, while LTM survives for at least 1 day. The cellular, biochemical and molecular differences underlying these two forms of long-lasting memory are not completely understood (Squire and Kandel, 1999; Kandel and Pittenger, 1999). Most of the recent studies on formation and persistence of memory have focused on neural analogues of STM and LTM (Lechner et al., 1999; Martin et al., 2000). At both the behavioural and neuronal levels, however, far less attention has been paid to the shorter-lasting form of LTM, which was termed ITM in 1993 by Rosenzweig et al. (1993) (Lukowiak et al., 2000; Sutton et al., 2001). The variation in the length of perseverance between ITM and LTM is most likely due to important molecular dissimilarities that underlie their encodement. Chief among these differences is the necessity for the transcription process. While both ITM and LTM require new protein synthesis, only LTM requires the transcription process. That is, LTM requires both altered gene activity and protein synthesis, while ITM requires only the translation process (Davis and Squire, 1984; Rosenzweig et al., 1993; McGaugh, 2000). Data obtained so far strongly support the idea that there are evolutionarily conserved mechanisms underlying LTM formation. They appear to involve both a cAMP-dependent MAP kinase signal transduction cascade culminating in the activation of CREB transcription factors (Tully, 1998; Mayford and Kandel, 1999; Silva et al., 1998) and formation of the CCAAT enhancer binding protein (C/EBP; Alberini et al., 1994; Taubenfeld et al., 2001). Far less is known about the molecular basis underlying ITM. Prior to the discovery of a memory component of intermediate duration dependent upon different classes of protein kinase activities than those required for LTM (Rosenzweig, 1993), it was widely believed that ITM was indistinguishable from LTM. These shorter-lasting forms of memory have since been distinguished at a behavioural level through classical conditioning of Aplysia feeding behaviour (Botzer et al., 1998) and sensitization of the siphon withdrawal response (a form of non-associative learning; Sutton et al., 2001), as well as through operant conditioning of aerial respiration in Lymnaea (Lukowiak et al., 2000). At the neuronal level, analogues of ITM have been demonstrated at both Aplysia and Hermissenda CNS synapses where synaptic transmission is facilitated (Ghirardi et al., 1995; Crow et al., 1999; Sutton et al., 2001). 1605 The Journal of Experimental Biology 206, 1605-1613 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00301 Aerial respiratory behaviour in the pond snail, Lymnaea stagnalis, can be operantly conditioned. This associative learning then undergoes consolidation into a long-lasting memory which, depending on the training procedure used, causes intermediate-term memory (ITM; lasting 3·h) or long-term memory (LTM; lasting >6·h) to be formed. We determined the differential susceptibility of these two forms of memory to translation and transcription blockers. The injection of a translation blocker, Anisomycin, 2.5·h before training prevents the establishment of both ITM and LTM. On the other hand, injection of the transcription blocker Actinomycin D, 2.5·h before training, did not prevent the establishment of ITM, but did, however, prevent LTM formation. Thus in Lymnaea, following associative learning, both ITM and LTM are dependent on new protein synthesis. ITM appears to be dependent on protein synthesis from pre- existing transcription factors, whilst LTM is dependent on protein synthesis from new transcription messages. Key words: Lymnaea stagnalis, intermediate memory, long-term memory, protein synthesis, associative learning. Summary Introduction Intermediate and long-term memories of associative learning are differentially affected by transcription versus translation blockers in Lymnaea Susan Sangha, Andi Scheibenstock, Chloe McComb and Ken Lukowiak* Department of Medical Physiology and Biophysics, Neuroscience Research Group, 3330 Hospital Drive NW, University of Calgary, Calgary, Alberta T2N 4N1, Canada *Author for correspondence (e-mail: lukowiak@ucalgary.ca) Accepted 18 February 2003