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Selective neuromodulation using Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) has become an increasingly important research tool, as well as an emerging therapeutic approach. However, the safety profile of DREADD expression is unknown. Here, different titers of adeno-associated viral (AAV) vector were administered in an attempt to vary total expression levels of the inhibitory DREADD hM4D(Gi) in excitatory hippocampal neurons. Male Sprague-Dawley rats were injected with AAV2/7 encoding DREADD-mCherry, DREADD or mCherry. Pronounced neuronal loss and neuroinflammatory reactions were observed after transduction with the high titer DREADD AAV, which also resulted in the highest DREADD expression levels. No such effects were observed in the mCherry control group, despite an equally high titer, nor in conditions where lower viral vector titers were injected. In the high titer DREADD conditions, dentate gyrus evoked potentials were inhibited upon clozapine-induced activation of hM4D(Gi), while in low titer conditions dentate gyrus evoked potentials were enhanced. Recordings of single neuronal activity nevertheless indicated a reduction in spontaneous firing of granule cell layer neurons. Our results indicate that prolonged, high levels of DREADD expression can have neurotoxic effects and that chemogenetic suppression of excitatory hippocampal neurons can paradoxically enhance dentate gyrus evoked potentials. Significance statementDesigner receptors exclusively activated by designer drugs (DREADDs) are engineered receptors that can be used to selectively modulate specific groups of cells. Especially in neuroscience, DREADDs are widely adopted. However, there is not much known on their safety profile. Here, we assess the effect of different expression levels of the DREADD hM4D(Gi) by varying the titer of the adeno-associated viral (AAV) vector used to transduce specific neurons in the rat's brain. We found that high expression levels result in strong neuromodulatory effects, but also induce neuronal loss and tissue damage. Less pronounced, non-toxic expression levels paradoxically seem to display opposite neuromodulatory effects at network level.

Adenosine acts as an endogenous anticonvulsant and seizure terminator in the brain. Many of its anticonvulsive effects are mediated through the activation of the adenosine A1 receptor, a G protein-coupled receptor with a wide array of targets. Activating A1 receptors is an effective approach to suppress seizures. This review gives an overview of the neuronal targets of the adenosine A1 receptor focusing in particular on signaling pathways resulting in neuronal inhibition. These include direct interactions of G protein subunits, the adenyl cyclase pathway and the phospholipase C pathway, which all mediate neuronal hyperpolarization and suppression of synaptic transmission. Additionally, the contribution of the guanyl cyclase and mitogen-activated protein kinase cascades to the seizure-suppressing effects of A1 receptor activation are discussed. This review ends with the cautionary note that chronic activation of the A1 receptor might have detrimental effects, which will need to be avoided when pursuing A1 receptor-based epilepsy therapies.

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