Putting objects in context: A prefrontal-hippocampal-perirhinal cortex network

When we encounter an object, we spontaneously form associations between the object and the environment in which it was encountered. These associations can take a number of different forms, which include location and context. A neural circuit between the hippocampus, medial prefrontal cortex and perirhinal cortex is critical for object-location and object-sequence associations; however, how this neural circuit contributes to the formation of object-context associations has not been established. Bilateral lesions were made in the hippocampus, medial prefrontal cortex or perirhinal cortex to examine each region contribution to object-context memory formation. Next, a disconnection lesion approach was used to examine the necessity of functional interactions between the hippocampus and medial prefrontal cortex or perirhinal cortex. Spontaneous tests of preferential exploration were used to assess memory for different types of object-context associations. Bilateral lesion in the hippocampus, medial prefrontal cortex or perirhinal cortex impaired performance in both an object-place-context and an object-context task. Disconnection of the hippocampus from either the medial prefrontal cortex or perirhinal cortex impaired performance in both the object-place-context and object-context task. Interestingly, when object recognition memory was tested with a context switch between encoding and test, performance in the hippocampal and medial prefrontal cortex lesion groups was disrupted and performance in each disconnection group (i.e. hippocampus + medial prefrontal cortex, hippocampus + perirhinal cortex) was significantly impaired. Overall, these experiments establish the importance of the hippocampal-medial prefrontal-perirhinal cortex circuit for the formation of object-context associations.

Remembering the order of serially presented objects: A matter of time?

Remembering the sequence, in which stimuli are encountered or events have occurred, is a key process in episodic memory and can also facilitate recognition memory. Rodents, when presented with a sequence of objects, will explore the object encountered first; yet, whether this behaviour is because the rodents spontaneously encode the order of stimuli presentation or because of relative familiarity or temporal decay is unknown. Here, we tested sequence memory in rats using a series of spontaneous preference tasks. Experiment 1 demonstrated that when rats are presented with a sequence of four objects, with an inter-sample interval of 5 min or 1 h, they preferentially explored the object presented earlier in the list irrespective of the inter-sample interval. We then demonstrated that such memory for order was not affected by increasing or decreasing the inter-sample interval between the middle objects (Experiment 2). Finally, we showed that memory for order is not a function of absolute object familiarity, as animals showed clear discrimination between the objects presented in the sample phases and a novel object, independent of the sample objects' position in the sequence (Experiment 3). These results show that animals are able to encode the order of objects presented in a sequence, and as such temporal order memory is not achieved using the process of relative or absolute familiarity or temporal decay.

Disruption of hippocampal-prefrontal cortex activity by dopamine D2R-dependent LTD of NMDAR transmission

Functional connectivity between the hippocampus and prefrontal cortex (PFC) is essential for associative recognition memory and working memory. Disruption of hippocampal-PFC synchrony occurs in schizophrenia, which is characterized by hypofunction of NMDA receptor (NMDAR)-mediated transmission. We demonstrate that activity of dopamine D2-like receptors (D2Rs) leads selectively to long-term depression (LTD) of hippocampal-PFC NMDAR-mediated synaptic transmission. We show that dopamine-dependent LTD of NMDAR-mediated transmission profoundly disrupts normal synaptic transmission between hippocampus and PFC. These results show how dopaminergic activation induces long-term hypofunction of NMDARs, which can contribute to disordered functional connectivity, a characteristic that is a hallmark of psychiatric disorders such as schizophrenia.

A temporally distinct role for group I and group II metabotropic glutamate receptors in object recognition memory

Recognition memory, involving the ability to discriminate between a novel and familiar object, depends on the integrity of the perirhinal cortex (PRH). Glutamate, the main excitatory neurotransmitter in the cortex, is essential for many types of memory processes. Of the subtypes of glutamate receptor, metabotropic receptors (mGluRs) have received less study than NMDA receptors; thus, the reported experiments examined the role of mGluRs in familiarity discrimination in the rat PRH. Experiments 1 and 2 assessed the effects of systemic administration of MPEP, a group I mGluR (specifically mGluR5) antagonist, and/or LY341495, a group II mGluR antagonist, on a spontaneous object novelty preference task. Simultaneous antagonism of both group I and II mGluRs impaired familiarity discrimination following a 24-h but not a 15-min delay, while antagonism of either mGluR subtype alone had no effect at either delay. The impairment was in acquisition, as in Experiment 3 coadministration of MPEP and LY341495 did not affect recognition memory performance when administered either after the sample phase or prior to test. The impairment in long-term recognition memory was mediated by mGluRs in the PRH, as localized intracortical antagonism of group I and II mGluRs also produced a deficit (Experiment 4). No evidence was found for an involvement of group III mGluRs in the acquisition of long-term familiarity discrimination (Experiment 5). These findings establish that glutamatergic neurotransmission in the PRH via group I and II mGluRs is crucial for the acquisition, but not for the consolidation or retrieval of long-term object recognition memory.