Cholinergic modulation of cortical sensory-evoked responses

Abstract

Cholinergic inputs to the cortex from the basal forebrain are essential for performing complex behaviours that require precise integration of sensory and motor information. However, the role of the basal forebrain cholinergic system in cortical sensory processing in awake behaving animals is unknown. Acetylcholine modulates the cortical microcircuitry in complex ways, with opposing effects on different neuronal types leading to both inhibitory and disinhibitory actions for excitatory pyramidal neuron circuitry. Therefore, it is crucial to understand how cholinergic modulation influences anatomically registered cell types. I focused on Layer 2/3 pyramidal neurons since they are the primary source of cortico-cortical connections responsible for broadcasting information across cortical areas, areal connections, presumably to integrate sensory and motor behaviours. To assess the role of cholinergic modulation of this system, I examined how a chronic depletion of basal forebrain cholinergic fibres alters Layer 2/3 network activity across cortical areas in response to brief tactile stimulations. First, I determined how recruitment of cortical areal connections occurs in mice with an intact cholinergic system. To do this, I used mice expressing the voltage indicator VSFP Butterfly 1.2 specifically in cortical Layer 2/3 pyramidal neurons and widefield fluorescence imaging through a thinned skull window in head-fixed awake mice as they received single or double brief stimulation to the forepaw or air puff to the whiskers. I also generated real-time maps of sensory-evoked changes in areal Layer 2/3 activity. Following these experiments, I standardised a chronic but selective cholinergic lesion in the cortex using the neurotoxin mu p75SAP both at the anatomical and behavioural levels. Mice with this lesion only had difficulty performing complex behavioural tasks. Finally, I obtained and analysed real-time maps of sensory, paw-evoked changes in areal Layer 2/3 activity in mice with a chronic cholinergic lesion. The areal Layer 2/3 activity maps showed fast depolarisation of initiated within distinct areas of the sensory cortex in response to whisker and paw stimulation. Sensory evoked responses after paw and whisker stimulation responses exhibited a similar fast depolarisation, but their time scale and broadcast patterns differed markedly. Notably, we observed different patterns of slower hyperpolarisation (presumed inhibition). Double stimulation to the forepaw revealed a second smaller depolarisation consistent with recruitment of inhibition and also with the well-known adaptation of sensory-evoked responses. In mice with the unilateral lesion, the sensory-evoked responses in the forelimb area exhibited an increased initial depolarisation and significantly prolonged and widespread hyperpolarisation that disrupted the areal Layer 2/3 pyramidal neuron activity maps and adaptation after paired stimulation. The results indicate that cholinergic inputs from the basal forebrain are essential for cortical sensory processing and sensory adaptation by the Layer 2/3 cortical microcircuitry. Disruption of this sensory processing may underlie the degraded performance of complex behaviours in disorders where cholinergic transmission is altered, as in Alzheimer’s disease.