3.6 KM ameliorates chronic hyperalgesia and anxiety-like
behavior caused by prolonged activation of BLA astrocytes
Since a single injection of CNO activated BLA astrocytes and instantly
induced hyperalgesia, we next investigated whether prolonged activation
of BLA astrocytes can induce persistent pain, mimicking the development
of DNP. On this basis, we observed whether this pharmacogenetic-induced
chronic hyperalgesia can also be alleviated by KM administration.
To this end, we induced repetitive pharmacogenetic activation of BLA
astrocytes by daily i.p. injections of CNO for a week in naive rats with
bilateral injection of gfaABC1D-M3-EGFP into the BLA and then treated
them with KM (Fig. 8A). We found that daily CNO injections for one week
induced a persistent decrease in the MWT in the rats expressing
gfaABC1D-M3-EGFP, and KM reversed the mechanical allodynia induced by
CNO injection in the rats transfected with gfaABC1D-M3-EGFP in the BLA
within 1 hour, indicating that prolonged activation of BLA astrocytes
was sufficient to induce the development of chronic pain-like
hypersensitivity, which could be blocked by KM (7 mg/kg) (Fig. 8B).
We further determined whether this prolonged pharmacogenetic activation
of BLA astrocytes was sufficient to induce anxiety-like behavior. We
found that in the rats transfected with gfaABC1D-M3-EGFP, CNO injections
made them less active in the central region in the OF and significantly
reduced the number of entries and time spent in the open arms in the
EPM; KM reversed these effects of CNO injections in the rats transfected
with gfaABC1D-M3-EGFP (Fig. 8C-8H). The immunohistochemical results
showed in the BLA of the rats that had BLA astrocytes continuously and
specifically activated, the fluorescence intensity of GFAP was
significantly enhanced, and the number of GFAP-positive cells was
significantly increased. When combined with the KM intervention, BLA
astrocytes were still, GFAP fluorescence intensity became weak, and the
number of positive cells decreased (Fig. 8I-8K).
Collectively, these results indicated that prolonged activation of BLA
astrocytes was sufficient to induce chronic hyperalgesia and
anxiety-like behaviors that mimic STZ-induced DNP. After administration
of KM, prolonged activation of BLA astrocytes induced a fully developed
chronic hyperalgesia, and anxiety-like behaviors were significantly
improved, which indicated that BLA astrocytes are one of the important
targets of the anti-DNP effects of KM.