Several studies suggest that axonal projections from the hippocampal formation (HF) to the basolateral amygdala (BLA) play a role in Pavlovian fear conditioning to contextual conditional stimuli. We have used electrophysiological techniques to characterize neuronal transmission in these projections in urethane-anesthetized rats. Single-pulse electrical stimulation of the ventral angular bundle (VAB), which carries projections from the HF to the BLA, reliably evoked a biphasic extracellular field potential in the BLA that consisted of an early, negative and a late, positive component. The negative component of the field potential occurred at a short latency (3-8 msec), was both temporally and spatially correlated with VAB-evoked multiple-unit discharges in the BLA, and exhibited properties typical of a monosynaptic response. Infusion of lidocaine or glutamate receptor antagonists into the BLA attenuated VAB-evoked field potentials, indicating that they are generated by local synaptic glutamatergic transmission. Both paired-pulse stimulation and brief trains of high-frequency stimulation (HFS) induced a short-lasting facilitation of BLA field potentials, whereas longer and more numerous trains of HFS produced an enduring, NMDA receptor-dependent long-term potentiation (LTP) of the potentials. The induction of LTP was accompanied by a decrease in paired-pulse facilitation (PPF), suggesting a presynpatic modification underlying its expression. Electrolytic lesions placed in regions of the HF that project to the BLA or excitotoxic lesions placed in the BLA eliminated Pavlovian fear conditioning to a contextual conditional stimulus. The critical role of both structures in context conditioning implicates plasticity at HF-BLA synapses in this form of learning.

In this study, we determined the contribution of juvenile neurogenesis to the performance of mice on a remote memory for temporally based association task and in a novelty based spatial pattern separation task. This was accomplished by mating homozygous DNMT1‐loxP mice with heterozygous GFAP‐Cre mice and comparing Cre+ (no postnatal neurogenesis) to Cre− (wild type) littermate offspring. The results indicate that Cre+ mice are impaired relative to Cre− mice in the remote memory for a temporal based association task and in a novelty based spatial pattern separation task. These results support the temporal integration model of Aimone et al., [(2006) Nat Neurosci 9:723–727] and provide further support for an important role for postnatally born neurons in spatial pattern separation. In contrast, Cre+ mice are not impaired relative to Cre− mice in an object‐context recognition task and a spatial location recognition task. These latter data suggest that postnatally derived neurons in the dentate gyrus (DG) do not support all spatial and object recognition functions of the DG. © 2014 Wiley Periodicals, Inc.