Apparent dissociation between hemodynamic and neurochemical responses during chemogenetic modulation of cortical...

Chemogenetic tools such as Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) provide a powerful means to causally manipulate defined neuronal populations in vivo. While chemogenetic fMRI studies have consistently demonstrated robust hemodynamic responses following circuit perturbation, considerably less is known about the accompanying metabolic consequences. Functional magnetic resonance spectroscopy (fMRS) offers the potential to probe these neurochemical processes, yet the relationship between hemodynamic and metabolic responses remains poorly understood. Here, we combined chemogenetics, pharmacological fMRI (Ph-fMRI), and proton magnetic resonance spectroscopy (¹H-MRS/fMRS) at 7 T to investigate the temporal evolution of metabolic and hemodynamic responses in the rat motor cortex.

Female Fischer rats received viral injections in the motor cortex to express either a pan-neuronal hM3D(Gq) DREADD construct (hSyn-hM3D(Gq)) or an interneuron-targeted construct (hDlx-hM3D(Gq)). Ph-fMRI, fMRS, and ¹H-MRS measurements were performed before, during, and following systemic administration of clozapine-N-oxide (CNO, 1 mg/kg). Functional MRS was acquired during the acute response phase (0-60 min post-injection), while conventional ¹H-MRS measurements were obtained at a delayed time point (70 min post-injection).

Chemogenetic modulation produced robust and opposing hemodynamic responses. Pan-neuronal activation elicited focal positive BOLD responses (+3.5 ± 1.5%), whereas interneuron-targeted activation generated significant negative BOLD responses (-3.3 ± 0.8%). In contrast, acute fMRS measurements revealed no significant changes in Glx or GABA concentrations during the first hour following CNO administration, despite the presence of strong hemodynamic effects. However, delayed metabolic differences were detected at the static ¹H-MRS time point acquired 70 min after CNO administration. Relative to Sham animals, the ipsilateral motor cortex of pan-neuronal DREADD animals showed higher GABA (+14.4%) and total choline compound concentrations (+57.8%), whereas interneuron-targeted DREADD animals exhibited lower Glx (-15.6%), total NAA (-16.9%), glucose (-25.9%), and total creatine (-25.4%). These delayed effects emerged after the period of peak hemodynamic responses and were not accompanied by significant acute fMRS changes.

Chemogenetic perturbation of cortical circuits produced robust hemodynamic responses but more subtle and temporally complex metabolic effects. The absence of detectable acute changes in Glx and GABA despite strong BOLD responses, together with the emergence of delayed neurochemical alterations, highlights the challenges of interpreting metabolic signals in relation to circuit activity.