BOLD response to direct thalamic stimulation reveals a functional connection between the medial thalamus and the anterior cingulate cortex in the rat.

Recent functional neuroimaging studies in humans and rodents have shown that the anterior cingulate cortex (ACC) is activated by painful stimuli, and plays an important role in the affective aspect of pain sensation. The aim of the present study was to develop a suitable stimulation method for direct activation of the brain in fMRI studies and to investigate the functional connectivity in the thalamo-cingulate pathway. In the first part of the study, tungsten, stainless steel, or glass-coated carbon fiber microelectrodes were implanted in the left medial thalamus (MT) of anesthetized rats, and T2*-weighted gradient-echo (GE) images were obtained in the sagittal plane on a 4.7 T system (Biospec BMT 47/40). Only the images obtained with the carbon fiber electrode were acceptable without a reduction of the signal-to-noise ratio (SNR) and image distortion. In the second part of the study, a series of two-slice GE images were acquired during electrical stimulation of the MT with the use of a carbon fiber electrode. A cross-correlation analysis showed that the signal intensities of activated areas in the ipsilateral ACC were significantly increased by about 4.5% during MT stimulation. Functional activation, as assessed by the distribution of c-Fos immunoreactivity, showed strong c-Fos expression in neurons in the ipsilateral ACC. The present study shows that glass-coated carbon fiber electrodes are suitable for fMRI studies and can be used to investigate functional thalamocortical activation.