Systemic and intrathecal baclofen produce bladder antinociception in rats

BACKGROUND: Baclofen, a clinically available GABA(B) receptor agonist, produces non-opioid analgesia in multiple models of pain but has not been tested for effects on bladder nociception. METHODS: A series of experiments examined the effects of systemic and spinally administered baclofen on bladder nociception in female anesthetized rats. Models of bladder nociception included those which employed neonatal and adult bladder inflammation to produce bladder hypersensitivity. RESULTS: Cumulative intraperitoneal dosing (1–8 mg/kg IP) and cumulative intrathecal dosing (10–160 ng IT) of baclofen led to dose-dependent inhibition of visceromotor responses (VMRs) to urinary bladder distension (UBD) in all tested models. There were no differences in the magnitude of the analgesic effects of baclofen as a function of inflammation versus no inflammation treatments. Hemodynamic (pressor) responses to UBD were similarly inhibited by IT baclofen as well as UBD-evoked excitatory responses of spinal dorsal horn neurons. The GABA(B) receptor antagonist, CGP 35,348, antagonized the antinociceptive effects of IT baclofen on VMRs in all tested models but did not affect the magnitude of the VMRs by itself suggesting no tonic GABA(B) activity was present in this preparation. Tolerance to a seven day continuous IT infusion of baclofen was not observed. CONCLUSIONS: These data provide support for a clinical trial of baclofen as a non-opioid treatment of human bladder pain.