Intraplantar injection of anandamide inhibits mechanically-evoked responses of spinal neurones via activation of CB2 receptors in anaesthetised rats.

Anti-nociceptive effects of the endocannabinoid anandamide are well established. Anandamide has, however, also been shown to activate pro-nociceptive vanilloid 1 (VR1) receptors present on primary afferent nociceptors. The aim of the present study was to determine the effect of intraplantar injection of anandamide on dorsal spinal neuronal responses in control rats and rats with hindpaw carrageenan-induced inflammation. Effects of intraplantar administration of anandamide (50 microg in 50 microl) on peripheral mechanically-evoked responses of spinal neurones were studied in halothane-anaesthetised rats in vivo. Responses of spinal neurones to mechanical punctate stimulation (von Frey filaments, 8-80 g) of the peripheral receptive field were similar in non-inflamed rats and rats with hindpaw carrageenan-induced inflammation. Intraplantar injection of anandamide, but not vehicle, significantly (P<0.05) inhibited innocuous and noxious mechanically-evoked responses of spinal neurones in rats with hindpaw inflammation, but not in non-inflamed rats. Co-administration of the cannabinoid (2) (CB(2)) receptor antagonist, SR144528 (10 microg in 50 microl), but not the cannabinoid (1) (CB(1)) receptor antagonist, SR141716A (10 microg in 50 microl), significantly blocked inhibitory effects of anandamide on peripheral evoked neuronal responses in rats with hindpaw inflammation. This study demonstrates inhibitory effects of exogenous anandamide on mechanically-evoked responses under inflammatory conditions in vivo, which are mediated by peripheral CB(2) receptors.

Spinal GABA(B)-receptor antagonism increases nociceptive transmission in vivo.

GABA(B) receptors modulate primary afferent fibre evoked responses of spinal neurones. Here effects of the selective GABA(B) receptor antagonist, CGP-35348, on electrically-evoked responses of spinal neurones in control and carrageenan-inflamed rats were studied. Spinal CGP-35348 (0.1-10 microg/50 microl) did not alter Abeta- or Adelta-fibre evoked neuronal responses in control rats, although C-fibre evoked responses and post discharge responses of spinal neurones were significantly facilitated by 3.0 and 10.0 microg/50 microl CGP-35348 (p < 0.05). In carrageenan-treated animals, spinal CGP-35348 did not alter electrically evoked responses of spinal neurones at any dose. Our data suggest that following acute peripheral inflammation there is loss of endogenous GABA(B) receptor mediated inhibition of C-fibre transmission at the level of the spinal cord.

The effects of GABA(B) receptor activation on spontaneous and evoked activity in the dentate gyrus of kainic acid-treated rats.

Enhancement of GABAergic inhibition is central to the treatment of epilepsy. The role of the GABA(B) receptor, however, is poorly understood. The current study investigates the effects of r-baclofen (a GABA(B) receptor agonist) on spontaneous and evoked electrophysiological activity in the dentate gyrus of normal and epileptic rats in vivo. Administration of kainic acid (KA), which causes similar pathology to that seen in human temporal lobe epilepsy, was used to prepare chronically epileptic rats. Bursts of spontaneous high-amplitude field potentials (spiking) were observed in isoflurane-anaesthetised control and KA-treated rats in vivo; however, this activity was significantly more frequent in KA-treated rats (223+/-26.1 spikes min(-1)) than in control rats (124+/-17.4 spikes min(-1)). Feedback inhibition, measured using paired-pulsed stimulation, was also greater in KA-treated rats; 50% inhibition of the second response was observed at 43.05+/-4.46 ms in KA-treated animals, as opposed to 26.27+/-2.39 ms in control animals. r-Baclofen (10 mg kg(-1) i.v.) abolished spontaneous spiking and also reduced feedback inhibition in both control and KA-treated rats. These effects of r-baclofen may be due to inhibition of GABA release, through activation of pre-synaptic GABA(B) receptors on terminals of interneurones in the inhibitory feedback pathway. These results suggest a link between feedback inhibition and spontaneous spiking, and provide support for the hypothesis that mechanisms of synchronisation may give rise to seizure activity in human temporal lobe epilepsy.

Multi-neuronal recordings reveal a differential effect of thapsigargin on bicuculline- or gabazine-induced epileptiform excitability in rat hippocampal neuronal networks.

The present study was performed to investigate the effects of depleting intracellular Ca(2+) stores on bicuculline- or gabazine-induced epileptiform excitability. Studies were performed on monolayer rat hippocampal neuronal networks utilising a system that allowed simultaneous multiple extracellular single-unit recordings of neuronal activity. Hippocampal neuronal networks were prepared from enzymatically dissociated hippocampi from 18-day-old fetal Wistar rats. The cells were cultured in Neurobasal medium with B27 serum-free supplements directly onto the surface of planar multiple microelectrode arrays with a central recording array of 64 (4 x 16) indium-tin thin-film recording electrodes. All cells recorded at 21 days-in-vitro exhibited spontaneous discharge activity with firing rates between 0.3-30.7 Hz. gamma-aminobutyric acid (GABA) produced a concentration-dependent decrease in firing (EC(50)=9.1 microM) which could be blocked by pre-application of bicuculline methobromide (10 microM). Addition of the GABA(A)-receptor antagonists gabazine (10 microM) or bicuculline (10 microM) resulted in the rapid generation of synchronised bursting within all the cells recorded. Bicuculline exhibited heterogeneity of action on firing rate, whereas gabazine always increased firing. Pre-incubation with thapsigargin, which depletes intracellular calcium stores, resulted in a decrease in the amount of neuronal excitation produced by bicuculline, but not by gabazine, suggesting that bicuculline-induced neuronal excitation requires release of Ca(2+) from intracellular stores.