Functional magnetic resonance imaging (fMRI) based on blood oxygen level-dependent (BOLD) contrast is widely used for probing brain activity, but its relationship to underlying neural activity remains elusive. Here, we combined fMRI with fiber-optic recordings of fluorescent calcium indicator signals to investigate this relationship in rat somatosensory cortex. Electrical forepaw stimulation (1-10 Hz) evoked fast calcium signals of neuronal origin that showed frequency-dependent adaptation. Additionally, slower calcium signals occurred in astrocyte networks, as verified by astrocyte-specific staining and two-photon microscopy. Without apparent glia activation, we could predict BOLD responses well from simultaneously recorded fiber-optic signals, assuming an impulse response function and taking into account neuronal adaptation. In cases with glia activation, we uncovered additional prolonged BOLD signal components. Our findings highlight the complexity of fMRI BOLD signals, involving both neuronal and glial activity. Combined fMRI and fiber-optic recordings should help to clarify cellular mechanisms underlying BOLD signals.

Little is known about the functional role of axotomized cortical neurons that survive spinal cord injury. Large thoracic spinal cord injuries in adult rats result in impairments of hindlimb function. Using retrograde tracers, we found that axotomized corticospinal axons from the hindlimb sensorimotor cortex sprouted in the cervical spinal cord. Mapping of these neurons revealed the emergence of a new forelimb corticospinal projection from the rostral part of the former hindlimb cortex. Voltage-sensitive dye (VSD) imaging and blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI) revealed a stable expansion of the forelimb sensory map, covering in particular the former hindlimb cortex containing the rewired neurons. Therefore, axotomized hindlimb corticospinal neurons can be incorporated into the sensorimotor circuits of the unaffected forelimb.