Differential effects on delayed non-matching-to-position in rats of microinjections of muscarinic receptor antagonist scopolamine or NMDA receptor antagonist MK-801 into the dorsal or ventral extent of the hippocampus

Abstract

Glutamatergic NMDA and cholinergic muscarinic receptors are thought to contribute to cognitive processes mediated by the hippocampus. Evidence from lesion studies suggests that, despite cytoarchitectural uniformity within the hippocampus, information processing may not be uniform along the septo-temporal axis. The present study examined whether blockade of NMDA or muscarinic receptors in hippocampal subregions produced regional dissociations in the disruption of performance on an operant, spatial delayed non-matching-to-position (DNMTP) paradigm that also assessed vigilance. Rats were extensively pretrained on DNMTP, then bilaterally cannulated into either the dorsal or ventral hippocampus. Following retraining, scopolamine or MK-801 were administered prior to sessions. MK-801 administered into dorsal hippocampus produced delay-independent deficits in DNMTP delayed choice. Neither scopolamine administered into the dorsal or ventral hippocampus, nor MK-801 administered into the ventral hippocampus, produced significant disruption of DNMTP delayed choice. However, some dissociations were evident in other measures of vigilance. Scopolamine into the dorsal and ventral hippocampus increased errors of omission, scopolamine into the ventral hippocampus decreased sample response accuracy, and MK-801 into the dorsal hippocampus decreased sample response accuracy and increased response bias. These results are consistent with the suggestion that subregions of the hippocampus may be involved in different aspects of information processing and also suggest that the cholinergic inputs to the hippocampus may be functionally independent of NMDA receptor-mediated neurotransmission.