Reversible inhibition of medial septum interneurons abolishes initial acquisition of new place-reward associations and reveals a progressive reduction in dependence on medial septal function

Abstract

Hippocampal place cells encode spatial information within a theta-organized temporal framework paced by the medial septum (MS). Lesions or inactivation of the MS impair spatial learning and disrupt hippocampal theta rhythmicity, long-term potentiation, and plasticity-related gene expression, while often sparing spatially selective firing of CA1 neurons (but not CA3 neurons). Although many rodent studies argue against classical systems consolidation by showing persistent hippocampal dependence of spatial memories, other work has suggested that training structure, spaced vs. massed trials, can influence this dependence. To investigate this possibility, we used DREADD-based, reversible, inhibition of MS GABAergic neurons, some of which are known to project to hippocampus. DREADD activation reduced hippocampal theta power and spatial information encoded by hippocampal neurons while largely preserving overall neuronal firing. We then examined the effect of inhibition of medial septal GABAergic neurons on learning the correct goal arm in an 8-arm maze when the correct arm was randomly changed each day. On each day, the mice were given two forced-choice trials indicating the correct arm and then 5 free-choice trials (without correction) and were always started from an opaque cylinder in the maze center, in a random orientation, to eliminate possible use of a simple response strategy. When the correct goal arm varied randomly daily, MS cell inhibition severely and repeatedly impaired acquisition. In contrast, when the correct goal arm was subsequently fixed across days, performance was initially impaired but became resistant to DREADD activation over time. Once performance reached asymptote, the goal arm was switched to a new (random) position. In this case, learning was no longer impaired by DREADD activation. This generalization suggests that repeated fixed-goal training in the same environment induces a broader reorganization of the neural systems supporting spatial behaviour. One possibility is that mice develop a septal-independent task-solving strategy that supports both retrieval of established goal locations and acquisition of new ones within a stable environment. This transition could reflect the gradual adoption of a non-spatial strategy such as visual pattern recognition, or the use of a directional cue. Moving the most salient distal cue in the environment (a single room light) produced a substantial change in the visual configurations seen from the arm entry points, and disrupted performance; however, it did not produce a consistent directional shift relative to the cue, suggesting that the light contributed to an integrated visuo-spatial representation rather than serving as the reference frame origin. We conclude that the mice switched from a hippocampus/MS-dependent spatial strategy to a non-hippocampus-dependent visual pattern matching solution.