Neto1 is a novel CUB-domain NMDA receptor-interacting protein required for synaptic plasticity and learning.

The N-methyl-D-aspartate receptor (NMDAR), a major excitatory ligand-gated ion channel in the central nervous system (CNS), is a principal mediator of synaptic plasticity. Here we report that neuropilin tolloid-like 1 (Neto1), a complement C1r/C1s, Uegf, Bmp1 (CUB) domain-containing transmembrane protein, is a novel component of the NMDAR complex critical for maintaining the abundance of NR2A-containing NMDARs in the postsynaptic density. Neto1-null mice have depressed long-term potentiation (LTP) at Schaffer collateral-CA1 synapses, with the subunit dependency of LTP induction switching from the normal predominance of NR2A- to NR2B-NMDARs. NMDAR-dependent spatial learning and memory is depressed in Neto1-null mice, indicating that Neto1 regulates NMDA receptor-dependent synaptic plasticity and cognition. Remarkably, we also found that the deficits in LTP, learning, and memory in Neto1-null mice were rescued by the ampakine CX546 at doses without effect in wild-type. Together, our results establish the principle that auxiliary proteins are required for the normal abundance of NMDAR subunits at synapses, and demonstrate that an inherited learning defect can be rescued pharmacologically, a finding with therapeutic implications for humans.

Enhancement of spinal N-methyl-D-aspartate receptor function by remifentanil action at delta-opioid receptors as a mechanism for acute opioid-induced hyperalgesia or tolerance.

BACKGROUND: Intraoperative remifentanil infusions have been associated with postoperative opioid-induced hyperalgesia and tolerance. Using a previously identified subpopulation of spinal neurons that displays an augmentation in N-methyl-D-aspartate (NMDA) receptor current after chronic morphine, investigations were undertaken to determine whether remifentanil induces acute increases in NMDA responses that are concentration dependent and receptor subtype dependent. METHODS: Electrophysiologic recordings of NMDA current were made from cultured rat dorsal horn neurons treated with remifentanil at various concentrations for 60 min. Selective mu- or delta-opioid receptor inhibitors and agonists were used to determine the site of action of remifentanil. RESULTS: Remifentanil at 4, 6, and 8 nM, but not higher or lower concentrations, caused significant mean increases in NMDA peak current amplitude of 37.30% (P < 0.001), 30.19% (P < 0.001), and 23.52% (P = 0.025), respectively, over control conditions. This occurred by 36 min of remifentanil perfusion and persisted throughout its washout. Inhibition by 100 nM naloxone or 1 nM naltrindole attenuated the remifentanil-induced NMDA response increase. Selective delta-opioid agonists [D-Pen(2), D-Pen(5)]enkephalin and deltorphin II displayed a similar bell-shaped concentration-response relation for the enhancement of NMDA responses, and 10 nM deltorphin II occluded the effects of 4 nM remifentanil on NMDA responses. CONCLUSIONS: Clinically relevant concentrations of remifentanil induce rapid, persistent increases in NMDA responses that mirror the development of remifentanil-induced hyperalgesia and tolerance. NMDA enhancement by remifentanil is dependent on the activation of both mu- and delta-opioid receptors and is inducible solely by delta-opioid receptor activation. Therefore, selective delta-opioid inhibition may attenuate acute paradoxical increases in pain and tolerance to opioids.

Subpopulation of dorsal horn neurons displays enhanced N-methyl-D-aspartate receptor function after chronic morphine exposure.

BACKGROUND: Morphine tolerance may be attributed to enhancement of glutamatergic neurotransmission, in particular to increased function of the N-methyl-D-aspartate (NMDA) receptor. The cellular mechanisms responsible for these changes remain poorly defined. The authors identified and characterized a specific subpopulation of dorsal horn neurons, displaying NMDA receptor plasticity in response to chronic morphine administration. METHODS: The authors undertook current clamped and voltage clamped recordings of NMDA receptor-mediated responses from cultured rat dorsal horn neurons that were untreated or treated for 7 days with 1 or 100 microm morphine. RESULTS: Smaller (capacitance < or = 22 pF), tonic firing neurons showed a significantly enhanced NMDA receptor-mediated peak current after prolonged morphine treatment, whereas larger and phasic firing neurons showed no enhancement. With high-concentration but not low-concentration morphine treatment, Mg2+ blockade of NMDA receptors at resting membrane potentials was reduced. Furthermore, the chronic opioid-induced increase in NMDA current was attenuated by pretreatment with either a mu-opioid receptor inhibitor (naloxone) or an NMDA receptor inhibitor (2-amino-5-phosphonovalerate) (low-concentration > high-concentration morphine). CONCLUSIONS: In an electrophysiologically defined subpopulation of dorsal horn neurons, enhanced NMDA receptor function after chronic morphine exposure was shown to be mechanistically dependent on morphine concentration and sensitive to both NMDA and mu-opioid receptor antagonism. Therefore, these changes observed in this population of sensory spinal neurons can be used to study the development and prevention of opioid tolerance described in multiple laboratory and clinical reports.

Desensitization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors facilitates use-dependent inhibition by pentobarbital.

Although the mechanisms underlying the use-dependent inhibition of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) by barbiturates are not well understood, it has generally been assumed to involve open channel block. We examined the properties of the inhibition of AMPARs by the barbiturate pentobarbital (PB) in acutely isolated and cultured hippocampal neurons. PB caused a use- and concentration-dependent inhibition (IC50 = 20.7 microM) of AMPAR-mediated currents evoked by kainate. Contrary to the properties of an open channel blocker, the inhibition by PB developed with double exponential kinetics was reduced under conditions that favor the open channel state of AMPARs and was independent of membrane voltage. In addition, the inhibition was reduced at basic pH, indicating that the uncharged form of PB is active at AMPARs. Preventing AMPAR desensitization with cyclothiazide reduced the potency of inhibition by PB and prevented its trapping after the removal of agonist. PB preferentially reduced the steady-state (IC50 = 92.8 microM), rather than peak (IC50 > 1 mM) component of responses evoked by glutamate and accelerated the onset of desensitization in a concentration-dependent manner. Miniature excitatory postsynaptic currents recorded from cultured hippocampal neurons, the time course of which is minimally influenced by desensitization, are not inhibited by PB. The sensitivity of AMPAR-mediated synaptic responses to inhibition by PB therefore depends on the contribution of desensitization to these events. Our results suggest that PB does not act as an open channel blocker of AMPARs. Rather, the sensitivity, use dependence, and trapping of inhibition by PB are determined by AMPARs desensitization.