Protein kinase M (PKM), an atypical isoform of protein kinase C, has been suggested to be necessary and sufficient for the maintenance of long-term potentiation (LTP) and long-term memory (LTM). This evidence is heavily based on the use of inhibitory peptide (ZIP), a supposed specific inhibitor of PKM that interferes with both LTP and LTM. Problematically, both LTP and LTM are unaffected in both constitutive and conditional PKM knock-out mice, yet both are still impaired by ZIP application, suggesting a nonspecific mechanism of action. Because translational interference can disrupt neural activity, we assessed network activity after a unilateral intrahippocampal infusion of ZIP in anesthetized rats. ZIP profoundly reduced spontaneous hippocampal local field potentials, comparable in magnitude to infusions of lidocaine, but with a slower onset and longer duration. Our results highlight a serious confound in interpreting the behavioral effects of ZIP. We suggest that future molecular approaches in neuroscience consider the intervening level of cellular and systems neurophysiology before claiming influences on behavior.

Key points Persistent inward currents (PICs) in spinal motoneurons are long‐lasting, voltage‐dependent currents that increase excitability; they are dramatically potentiated by serotonin, muscarine, and noradrenaline (norepinephrine). Loss of these modulators (and the PIC) during sleep is hypothesized as a major contributor to REM sleep atonia. Reduced excitability of XII motoneurons that drive airway muscles and maintain airway patency is causally implicated in obstructive sleep apnoea (OSA), but whether XII motoneurons possess a modulator‐sensitive PIC that could be a factor in the reduced airway tone of sleep is unknown. Whole‐cell recordings from rat XII motoneurons in brain slices indicate that PIC amplitude increases ∼50% between 1 and 23 days of age, when potentiation of the PIC by 5HT2, muscarinic, or α1 noradrenergic agonists peaks at <50%, manyfold lower than the potentiation observed in spinal motoneurons. α1 noradrenergic receptor activation produced changes in XII motoneuron firing behaviour consistent with PIC involvement, but indicators of strong PIC activation were never observed; in vivo experiments are needed to determine the role of the modulator‐sensitive PIC in sleep‐dependent reductions in airway tone. Abstract Hypoglossal (XII) motoneurons play a key role in maintaining airway patency; reductions in their excitability during sleep through inhibition and disfacilitation, i.e. loss of excitatory modulation, is implicated in obstructive sleep apnoea. In spinal motoneurons, 5HT2, muscarinic and α1 noradrenergic modulatory systems potentiate persistent inward currents (PICs) severalfold, dramatically increasing excitability. If the PICs in XII and spinal motoneurons are equally sensitive to modulation, loss of the PIC secondary to reduced modulatory tone during sleep could contribute to airway atonia. Modulatory systems also change developmentally. We therefore characterized developmental changes in magnitude of the XII motoneuron PIC and its sensitivity to modulation by comparing, in neonatal (P1–4) and juvenile (P14–23) rat brainstem slices, the PIC elicited by slow voltage ramps in the absence and presence of agonists for 5HT2, muscarinic, and α1 noradrenergic receptors. XII motoneuron PIC amplitude increased developmentally (from −195 ± 12 to −304 ± 19 pA). In neonatal XII motoneurons, the PIC was only potentiated by α1 receptor activation (5 ± 4%). In contrast, all modulators potentiated the juvenile XII motoneurons PIC (5HT2, 5 ± 5%; muscarine, 22 ± 11%; α1, 18 ± 5%). These data suggest that the influence of the PIC and its modulation on XII motoneuron excitability will increase with postnatal development. Notably, the modulator‐induced potentiation of the PIC in XII motoneurons was dramatically smaller than the 2‐ to 6‐fold potentiation reported for spinal motoneurons. In vivo measurements are required to determine if the modulator‐sensitive, XII motoneuron PIC is an important factor in sleep‐state dependent reductions in airway tone.

The nearly axiomatic idea that de novo protein synthesis is necessary for long-term memory consolidation is based heavily on behavioral studies using translational inhibitors such as anisomycin. Although inhibiting protein synthesis has been shown to disrupt the expression of memory, translational inhibitors also have been found to profoundly disrupt basic neurobiological functions, including the suppression of ongoing neural activity in vivo. In the present study, using transverse hippocampal brain slices, we monitored the passive and active membrane properties of hippocampal CA1 pyramidal neurons using intracellular whole cell recordings during a brief ~30-min exposure to fast-bath-perfused anisomycin. Anisomycin suppressed protein synthesis to 46% of control levels as measured using incorporation of radiolabeled amino acids and autoradiography. During its application, anisomycin caused a significant depolarization of the membrane potential, without any changes in apparent input resistance or membrane time constant. Anisomycin-treated neurons also showed significant decreases in firing frequencies and spike amplitudes, and showed increases in spike width across spike trains, without changes in spike threshold. Because these changes indicated a loss of cellular energetics contributing to maintenance of ionic gradients across the membrane, we confirmed that anisomycin impaired mitochondrial function by reduced staining with 2,3,5-triphenyltetrazolium chloride and also impaired cytochrome c oxidase (complex IV) activity as indicated through high-resolution respirometry. These findings emphasize that anisomycin-induced alterations in neural activity and metabolism are a likely consequence of cell-wide translational inhibition. Critical reevaluation of studies using translational inhibitors to promote the protein synthesis dependent idea of long-term memory is absolutely necessary. NEW & NOTEWORTHY Memory consolidation is thought to be dependent on the synthesis of new proteins because translational inhibitors produce amnesia when administered just after learning. However, these agents also disrupt basic neurobiological functions. We show that blocking protein synthesis disrupts basic membrane properties of hippocampal neurons that correspond to induced disruptions of mitochondrial function. It is likely that translational inhibitors cause amnesia through their disruption of neural activity as a result of dysfunction of intracellular energetics.