Chemical or electrical stimulation of basal forebrain neurons activates specific subsets of cortical GABA-interneurons in parallel with increases in cortical cerebral blood flow

Introduction

Basal forebrain (BF) neurons project to cortical microvessels and GABA-interneurons and, upon stimulation, induce increases in cortical cerebral blood flow (CBF). The identity of the different subsets of GABA-interneurons activated by BF stimulation under conditions of increased cortical CBF was determined by double-immunocytochemistry of c-Fos (a marker of cell activation) and peptides (somatostatin (SOM), neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP)) or enzymes (nitric oxide synthase (NOS), choline acetyltransferase (ChAT)) co-localized with glutamate decarboxylase (GAD67) in GABA interneurons.

Methods

Cortical CBF was measured bilaterally using Laser Doppler flowmetry during and 20 min after a unilateral BF stimulation. This was achieved by BF microinjection of sodium glutamate (NaGlut, 100 nmol/100 nl over 1 min), NMDA (5 nmol/100 nl over 1 min), or saline (control, 100 nl over 1 min), or by electrical stimulation (100 Hz 1 sec, 50 μA, on/1 sec off, 5 min) in Sprague Dawley rats (3–4/group) previously implanted in the right BF (4–6 days earlier) with guide cannula or stimulating electrode. Activated cells (c-Fos-positive) were identified in different cortical areas or layers in brains fixed 60 min after the end of the stimulation.

Results

NaGlut, NMDA and electrical BF stimulations elicited ipsilateral cortical CBF increases (p<0. 05, t-test) corresponding to 62.6, 65.4 and 96.1% from baseline, respectively. All paradigms of BF stimulation elicited similar patterns of interneurons activation, although the extent of activation differed between them. Overall, BF stimulation induced a significant increase in the number of NPY, SOM and GAD67 interneurons immunolabelled for c-Fos in the ipsilateral as compared to the contralateral parietal cortex, without activation of VIP, NOS or ChAT interneurons. A larger proportion of NPY, SOM and GAD67 activated neurons (26.9, 51.6, 42.4%, respectively, p<0.001) was obtained with NMDA (Figure 1), while electrical stimulation resulted in activation of a smaller proportion of these same interneurons (10.5, 10.9, 6.6%, p<0.05). All c-Fos-positive neurons in layer I of the rat cerebral cortex co-stained only for GAD67, and these were activated in the ipsilateral parietal cortex after NMDA, NaGlut and electrical BF stimulation (40, 26 and 8.6%, respectively, p<0.001). Saline injection had no effect on either CBF or c-Fos.

Figure 1

Conclusions

These results show that increased cortical perfusion following BF stimulation is accompanied by activation of GABA interneurons and, specifically, those containing SOM and/or NPY, two peptides with vasocontractile properties. We suggest that these subsets of GABA interneurons might regulate cortical perfusion, possibly by limiting the extent of the vasodilatation induced by cortical release of acetylcholine and nitric oxide from activated basalocortical fibres.