Impairment of conditioned contextual fear of C57BL/6J mice by intracerebral injections of the NMDA receptor antagonist APV

The effects of acute injections of the NMDA receptor antagonist APV on one-trial fear conditioning of C57BL/6J mice were investigated in a time, dose (0.4-3.2 microg) and region-specific manner. Conditioned fear was determined by the assessment of freezing and the computer-controlled measurement of inactivity. Additionally, conditioned heart rate responses were evaluated in the tone-dependent memory test performed in the home cage with the help of implanted ECG transmitters. Injections of APV into the dorsal hippocampus (i.h.) 15 min before training, impaired dose-dependent contextual fear conditioning without discrimination between contextual foreground (unsignaled shock) and background (signaled shock) conditioning as tested 24 h after training. Short-term memory analyzed 1 h after training was also impaired. The observation that a smaller dose of APV was required for intracerebroventricular than for i.h. injection to achieve a similar memory impairment, pointed to the involvement of extrahippocampal NMDA receptors. APV treatment did not affect conditioned tone-dependent fear as indicated by unchanged freezing and heart rate responses of the freely moving mice. The results indicated the contribution of hippocampal and extrahippocampal NMDA receptors to multisensory contextual information processing during acquisition.

Strain and substrain differences in context- and tone-dependent fear conditioning of inbred mice

The performance of C57BL/6J (6J), C57BL/6N (6N), DBA/2J (2J) and DBA/2N (2N) mice in context- and tone-dependent fear conditioning was determined 24 h after fear conditioning to evaluate and compare different behavioral measures as indices of emotional learning. Freezing, the change in activity and the size of the explored area were evaluated as behavioral parameters indicating fear. Additionally, the heart rate (HR) increase elicited by tone presentation was evaluated as an autonomic indicator of fear. During the context-dependent memory test, freezing was high only in 6J and 6N mice, whereas a drop of activity and a reduced exploratory area was measured in all strains. During the tone-dependent memory test, high freezing, low activity, reduced exploratory area and a strong HR increase were demonstrated only in 6N and 6J mice, whereas behavioral and HR changes of 2J and 2N mice were always low. In extinction tests, context- and tone-dependent freezing of 6J mice decayed significantly faster than the freezing of 6N mice, whereas in both substrains the conditioned tachycardia to tone extinguished similarly in the home cage. The data demonstrate that monitoring of additional behavioral measures besides freezing and autonomic measures is necessary to interpret differences in associative learning performance of mouse strains that could be related to a differential expression of fear.

Differential impairment of auditory and contextual fear conditioning by protein synthesis inhibition in C57BL/6N mice

A 1-trial fear conditioning was used to investigate the temporal development of fear responses expressed as increase of freezing or heart rate and its impairment by the protein synthesis inhibitor cycloheximide (CHX) in male C57BL/6N mice. Heart rate was measured with an implanted transmitter. In the memory tests, mice were exposed to tone and context provided either as foreground or background stimulus during training. The fear responses developed differently from 0 to 24 hr after training under these 3 conditions. A single pretraining CHX injection impaired both memory forms, whereas a single posttraining CHX injection impaired tone- but not context-dependent memory, with the context provided as background stimulus. It was concluded that consolidation of tone-, foreground context-, and background context-dependent fear conditioning may be mediated by partly different neuronal or partly different biochemical pathways, or both.

Differential impairment of auditory and contextual fear conditioning by protein synthesis inhibition in C57BL/6N mice

A 1-trial fear conditioning was used to investigate the temporal development of fear responses expressed as increase of freezing or heart rate and its impairment by the protein synthesis inhibitor cycloheximide (CHX) in male C57BL/6N mice. Heart rate was measured with an implanted transmitter. In the memory tests, mice were exposed to tone and context provided either as foreground or background stimulus during training. The fear responses developed differently from 0 to 24 hr after training under these 3 conditions. A single pretraining CHX injection impaired both memory forms, whereas a single posttraining CHX injection impaired tone- but not context-dependent memory, with the context provided as background stimulus. It was concluded that consolidation of tone-, foreground context-, and background context-dependent fear conditioning may be mediated by partly different neuronal or partly different biochemical pathways, or both.