Superficial neurokinin 1 (NK1)-expressing neurones drive a descending serotonergic pathway from the brainstem back onto spinal dorsal horn neurones to enhance nociceptive transmission via a facilitatory action at spinal 5HT3 receptors. Ablation of these cells attenuates allodynia and hyperalgesia which is mirrored by marked deficits in the evoked responses of deep (lamina V-VI) wide dynamic range (WDR) cells to noxious mechanical, thermal and chemical stimuli. To assess whether removing the origin of this facilatatory spino-bulbo-spinal loop results in any potential alterations in GABAergic spinal inhibitory systems we investigated the effects of spinal bicuculline, a selective GABA_A receptor antagonist, on the evoked activity of deep dorsal horn WDR neurones following selective ablation of superficial NK1-expressing dorsal horn neurones. Ketamine (1.5 mg/kg I.P.)-anaesthetised Sprague Dawley rats (120-130g) were given an intrathecal injection of substance P (SP) conjugated to a cytotoxin, saporin (SAP) delivered to the L4-5 region to ablate NK-1 neurons. SAP alone was used as control. Animals were allowed to recover. Twenty-eight days later in vivo electrophysiology experiments under halothane (1%) anaesthesia were performed. The effects of cumulative doses of bicuculline (5, 50, 250 μg/50μl) on the evoked responses of lamina V WDR neurons to electrical (Aβ- Aδ- and C-fibre) and innocuous and noxious mechanical (brush, prod and von Frey (vF) 8 and 26g) stimuli were measured. Statistical analysis was performed using repeated measures ANOVA. In the SAP control group (n=9), bicuculline significantly facilitated the Aδ- and C-fibre-evoked responses as well as the post-discharge and non-potentiated response of the neurones in a dose-dependent manner (257±45, 162±18, 259±64 and 264± 67% of control, respectively; P<0.05). The evoked neuronal responses to prod and mechanical punctate stimuli (vf8 and 26g) were also significantly increased (247±57, 550±215 and 305±130% of control, respectively; P<0.05). Remarkably, the marked excitatory effect on lamina V neuronal responses by blocking spinal GABA_A receptors was lost in the SP-SAP-treated group (n=9) on all neuronal responses measured. The effect of bicuculline on the electrically and naturally evoked responses in these animals ranged from 96±37 to 148±55% of control. These data suggest an intact facilitatory spino-bulbo-spinal loop is essential for GABA-mediated inhibitory controls mediated via spinal GABA_A receptors since ablation of superficial NK1 neurons blocked any GABA-mediated enhancement of neuronal responses. Moreover these findings provide further evidence to support that activation of NK1-expressing cells predominantly drives a facilitation to enhance deep dorsal horn neuronal responses since the overall excitability of the spinal cord is reduced after ablation of NK1 neurones, yet intrinsic GABA inhibitory controls do not increase.