Research report Time-dependent involvement of PKA/PKC in contextual memory consolidation Gene V. Wallenstein *, David R. Vago, Anne M. Walberer Departments of Psychology and Neuroscience, Cognitive Neurobiology Institute, University of Utah, Salt Lake City, UT 84112, USA Received 20 September 2001; received in revised form 6 December 2001; accepted 6 December 2001 Abstract Rats were implanted bilaterally with cannulae into the dorsal hippocampus and trained in a Pavlovian fear-conditioning paradigm. Four groups of rats were infused intra-cranially with 1-(5?-isoquinolinesulfonyl)-2-methylpiperazine (H7-dihydrochlor- ide), a potent inhibitor of both protein kinase C (PKC) and cAMP-dependent protein kinase (PKA), at different time intervals in order to examine their involvement in the acquisition and consolidation of contextual fear memory. We demonstrate a significant consolidation deficit of long-term contextual fear-conditioning memory that is maximal when PKA and PKC are inhibited at 90 min post-training. These results suggest the existence of a critical time window, during which these enzymes must be activated for the consolidation of long-term memories. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Memory; Hippocampus; Protein kinase; LTP; Conditioning; Consolidation 1. Introduction Classical fear conditioning is a behavioral paradigm in which animals learn to fear an initially neutral stimulus (conditioned stimulus (CS)) that has been paired with the presentation of an aversive uncondi- tioned stimulus (US), such as a foot shock [4,8]. The learning is extremely fast and enduring [9], and can therefore, be compared with cellular and molecular models of learning and memory that are based on tetanus-induced long-term potentiation (LTP). Although, a cellular model of LTP has emerged in recent years [3], we are still piecing together the molecular/intracellular signaling mechanisms that sup- port the three primary phases (induction, expression, and maintenance) of this form of plasticity. In the hippocampus of the medial temporal lobe, several neurochemical steps have been identified thus far, and it appears they are also important for the development of long-term memories as measured by behavioral tests. For instance, the activation of kinases that phosphor- ylate both receptor proteins and gene transcription factors, is a key step in the eventual production of structural changes to the cell that facilitate LTP main- tenance [1]. Several studies have gone on to demonstrate that these kinases, particularly PKA and PKC, play a time-limited role in LTP since inhibiting their actions only during a critical period following induction (ca. 60  /120 min post tetanus) blocks the maintenance phase of LTP that is usually observed many hours later [15,11]. This critical time window, during which PKA and PKC contribute to the cascade of events underlying LTP maintenance, provides a useful tool for comparing these molecular mechanisms with learning and memory measured behaviorally. Here, we show for the first time that blocking PKA and PKC activity in the hippocampus selectively during this period significantly impairs the consolidation of contextual fear condition- ing memories. These data provide an additional link relating LTP to behavioral learning and memory in that the kinases in question are required for both LTP maintenance and behavioral memory consolidation at approximately the same time. * Corresponding author. Present address: Cognitive Neurobiology Institute, 4372 Elma Lane, La Mesa, CA 91941, USA. Tel.: 1-619- 460-6305 E-mail address: cogneuro_institute@hotmail.com (G.V. Wallenstein). Behavioural Brain Research 133 (2002) 159 /164 www.elsevier.com/locate/bbr 0166-4328/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0166-4328(01)00476-4