Neurotensin inhibition of GABAergic transmission via mGluR-induced endocannabinoid signalling in rat periaqueductal grey

Abstract

Neurotensin modulates pain via its actions within descending analgesic pathways which include brain regions such as the midbrain periaqueductal grey (PAG). The aim of this study was to examine the cellular actions of neurotensin on PAG neurons. Whole cell patch clamp recordings were made from rat midbrain PAG slices in vitro to examine the postsynaptic effects of neurotensin and its effects on GABA A mediated inhibitory postsynaptic currents (IPSCs). Neurotensin (100-300 nm) produced an inward current in subpopulations of opioid sensitive and insensitive PAG neurons which did not reverse over membrane potentials between -50 and -130 mV. The neurotensin induced current was abolished by the NTS1 and NTS1/2 antagonists SR48692 (300 nm) and SR142948A (300 nm). Neurotensin also produced a reduction in the amplitude of evoked IPSCs, but had no effect on the rate and amplitude of TTX-resistant miniature IPSCs. The neurotensin induced inhibition of evoked IPSCs was reduced by the mGluR5 antagonist MPEP (5μm) and abolished by the cannabinoid CB 1 receptor antagonist AM251 (3μm). These results suggest that neurotensin produces direct neuronal depolarisation via NTS1 receptors and inhibits GABAergic synaptic transmission within the PAG. The inhibition of synaptic transmission is mediated by neuronal excitation and action potential dependent release of glutamate, leading to mGluR5 mediated production of endocannabinoids which activate presynaptic CB 1 receptors. Thus, neurotensin has cellular actions within the PAG which are consistent with both algesic and analgesic activity, some of which are mediated via the endocannabinoid system.

Cholecystokinin Exerts an Effect via the Endocannabinoid System to Inhibit GABAergic Transmission in Midbrain Periaqueductal Gray

“…The CCK-S-induced inward current was likely to be mediated by activation of a non-selective cation conductance and inhibition of an inwardly rectifying potassium conductance, because it was associated with a near parallel shift in the current-voltage relationship. Although this differs to a previous study, in which it was found that CCK-S exerted an effect solely by inhibiting an inwardly rectifying potassium conductance in dorsolateral PAG neurons (Yang et al, 2006(Yang et al, , 2007, it is similar to our previous findings for neurotensin and substance P (Drew et al, 2005;Mitchell et al, 2009).…”

“…This is similar to the proportion of neurons that respond to the G qcoupled excitatory neurotransmitters neurotensin and substance P within the PAG (Drew et al, 2005;Mitchell et al, 2009). Thus, like substance P and neurotensin, cholecystokinin has a direct excitatory effect on PAG neurons, which opposes the postsynaptic inhibitory functions of m-opioids.…”

“…The lack of effect of cholecystokinin on TTX-resistant miniature IPSCs suggests that the cholecystokinin-induced reduction in presynaptic GABA release involved additional mechanisms external to the synapse, rather than direct postsynaptic CCK1 receptorinduced retrograde endocannabinoid signaling, as observed for mGluR5 and M1 mAChR receptor signaling in PAG (Drew et al, 2008;Lau and Vaughan, 2008). We have previously shown that activation of G q -coupled NK1 and NTS1/2 receptors indirectly activates the mGluR5-endocannabinoid system by enhancing endogenous glutamate release (Drew et al, 2009;Mitchell et al, 2009). In this study, CCK-S produced neuronal depolarization and an increase in spontaneous action potential-dependent glutamatergic EPSCs, however, we cannot rule out a direct CCK receptor-mediated enhancement of presynaptic glutamate release (Yang et al, 2006).…”

“…which engages mGluR5-induced endocannabinoid signaling (Drew et al, 2009;Mitchell et al, 2009). To address this, we first examined whether CCK-S increased action potential-dependent release of glutamate.…”

“…Although functional studies have shown that the GABAergic system has a key role in the modulation of pain and anxiety within the PAG (Fields et al, 2006;Graeff et al, 1993), the effect of cholecystokinin on GABAergic synaptic transmission within this brain structure is unknown. We have recently demonstrated that activation of other G q -coupled receptors, including metabotropic glutamate (mGluR5), muscarinic (M1 mAChR), neurokinin (NK1/2), and neurotensin (NTS1/2) receptors, indirectly inhibits GABAergic synaptic transmission via the endocannabinoid system within the PAG (Drew et al, 2008(Drew et al, , 2009Lau and Vaughan, 2008;Mitchell et al, 2009). In this regard, cannabinoid agonists and endocannabinoids modulate analgesic and anxiolytic activity within the PAG, including a component of stress-induced analgesia (Finn et al, 2003;Hohmann et al, 2005;Lichtman et al, 1996;Moreira et al, 2007).…”