Antisense Oligodeoxynucleotide Perfusion Blocks Gene Expression of Synaptic Plasticity-related Proteins without Inducing Compensation in Hippocampal Slices.

The elucidation of the molecular mechanisms of long-term synaptic plasticity has been hindered by both the compensation that can occur after chronic loss of the core plasticity molecules and by ex vivo conditions that may not reproduce in vivo plasticity. Here we describe a novel method to rapidly suppress gene expression by antisense oligodeoxynucleotides (ODNs) applied to rodent brain slices in an "Oslo-type" interface chamber. The method has three advantageous features: 1) rapid blockade of new synthesis of the targeted proteins that avoids genetic compensation, 2) efficient oxygenation of the brain slice, which is critical for reproducing in vivo conditions of long-term synaptic plasticity, and 3) a recirculation system that uses only small volumes of bath solution (< 5 ml), reducing the amount of reagents required for long-term experiments lasting many hours. The method employs a custom-made recirculation system involving piezoelectric micropumps and was first used for the acute translational blockade of protein kinase Mζ (PKMζ) synthesis during long-term potentiation (LTP) by Tsokas et al., 2016. In that study, applying antisense-ODN rapidly prevents the synthesis of PKMζ and blocks late-LTP without inducing the compensation by other protein kinase C (PKC) isoforms that occurs in PKCζ/PKMζ knockout mice. In addition, we show that in a low-oxygenation submersion-type chamber, applications of the atypical PKC inhibitor, zeta inhibitory peptide (ZIP), can result in unstable baseline synaptic transmission, but in the high-oxygenation, "Oslo-type" interface electrophysiology chamber, the drug reverses late-LTP without affecting baseline synaptic transmission. This comparison reveals that the interface chamber, but not the submersion chamber, reproduces the effects of ZIP in vivo. Therefore, the protocol combines the ability to acutely block new synthesis of specific proteins for the study of long-term synaptic plasticity, while maintaining properties of synaptic transmission that reproduce in vivo conditions relevant for long-term memory.

Compensation for PKMζ in long-term potentiation and spatial long-term memory in mutant mice.

PKMζ is a persistently active PKC isoform proposed to maintain late-LTP and long-term memory. But late-LTP and memory are maintained without PKMζ in PKMζ-null mice. Two hypotheses can account for these findings. First, PKMζ is unimportant for LTP or memory. Second, PKMζ is essential for late-LTP and long-term memory in wild-type mice, and PKMζ-null mice recruit compensatory mechanisms. We find that whereas PKMζ persistently increases in LTP maintenance in wild-type mice, PKCι/λ, a gene-product closely related to PKMζ, persistently increases in LTP maintenance in PKMζ-null mice. Using a pharmacogenetic approach, we find PKMζ-antisense in hippocampus blocks late-LTP and spatial long-term memory in wild-type mice, but not in PKMζ-null mice without the target mRNA. Conversely, a PKCι/λ-antagonist disrupts late-LTP and spatial memory in PKMζ-null mice but not in wild-type mice. Thus, whereas PKMζ is essential for wild-type LTP and long-term memory, persistent PKCι/λ activation compensates for PKMζ loss in PKMζ-null mice.

Inhibition of protein kinase Mζ disrupts the stable spatial discharge of hippocampal place cells in a familiar environment.

It is widely held that spatial computations in the rodent hippocampus require the location-specific discharge of place cells that together form a stable cognitive map used to solve and perform spatial tasks. It is not known, however, if map stability requires persistent hippocampal synaptic strength changes that are vulnerable to blockade of protein kinase Mζ (PKMζ) phosphorylation activity, a manipulation that reverses hippocampal LTP and disrupts multiple forms of long-term memory. Here we report that acute intrahippocampal inhibition of PKMζ disrupts place cell activity in a familiar environment, where the map is expected to be stable. After this disruption, new, stable spatial firing patterns can later form, but the new and original maps are unrelated even though the rat is exposed to a constant environment. We therefore propose that the previously demonstrated erasure of stored spatial memory and the disruption of place cell firing are parallel effects of PKMζ blockade. We similarly propose that the known sparing of new spatial memory formation depends on the sparing of new map formation. On these bases, we argue that the loss of the map used to perform a practiced spatial task leads to behavioral performance deficits, and that synaptic plasticity maintained by PKMζ, which stabilizes the map, is essential for the proper expression of spatial memory.

Representation of objects in space by two classes of hippocampal pyramidal cells.

Humans can recognize and navigate in a room when its contents have been rearranged. Rats also adapt rapidly to movements of objects in a familiar environment. We therefore set out to investigate the neural machinery that underlies this capacity by further investigating the place cell-based map of the surroundings found in the rat hippocampus. We recorded from single CA1 pyramidal cells as rats foraged for food in a cylindrical arena (the room) containing a tall barrier (the furniture). Our main finding is a new class of cells that signal proximity to the barrier. If the barrier is fixed in position, these cells appear to be ordinary place cells. When, however, the barrier is moved, their activity moves equally and thereby conveys information about the barrier's position relative to the arena. When the barrier is removed, such cells stop firing, further suggesting they represent the barrier. Finally, if the barrier is put into a different arena where place cell activity is changed beyond recognition ("remapping"), these cells continue to discharge at the barrier. We also saw, in addition to barrier cells and place cells, a small number of cells whose activity seemed to require the barrier to be in a specific place in the environment. We conclude that barrier cells represent the location of the barrier in an environment-specific, place cell framework. The combined place + barrier cell activity thus mimics the current arrangement of the environment in an unexpectedly realistic fashion.

Seizure-induced changes in place cell physiology: relationship to spatial memory.

Status epilepticus (SE) is a frequent neurological emergency associated with a significant risk of morbidity in survivors. Impairment of hippocampal-specific memory is a common and serious deficit occurring in many of the survivors. However, the pathophysiological basis of cognitive deficits after SE is not clear. To directly address the cellular concomitants of spatial memory impairment, we recorded the activity of place cells from CA1 in freely moving rats subjected to SE during early development and compared this activity to that in control rats. Place cells discharge rapidly only when the rat's head is in a cell-specific part of the environment called the "firing field." This firing field remains stable over time. Normal place cell function seems to be essential for stable spatial memory for the environment. We, therefore, compared place cell firing patterns with visual-spatial memory in the water maze in SE and control rats. Compared with controls, place cells from the SE rats were less precise and less stable. Concordantly, the water maze performance was also impaired. There was a close relationship between precision and stability of place cells and water maze performance. In contrast, a single, acute, chemically induced seizure produced cessation of place cell activity and spatial memory impairment in water maze performance that reversed within 24 hr. These results strongly bolster the idea that there is a relationship between abnormal place cells and spatial memory. Our findings also suggest that the defects in place cell and spatial memory after SE and acute chemically induced seizures result from different processes.

Telemetry system for reliable recording of action potentials from freely moving rats.

Recording single cells from alert rats currently requires a cable to connect brain electrodes to the acquisition system. If no cable were necessary, a variety of interesting experiments would become possible, and the design of other experiments would be simplified. To eliminate the need for a cable we have developed a one-channel radiotelemetry system that is easily carried by a rat. This system transmits a signal that is reliable, highly accurate and can be detected over distances of > or = 20 m. The mobile part of the system has three components: (1) a headstage with built-in amplifiers that plugs into the connector for the electrode array on the rat's head; the headstage also incorporates a light-emitting diode (LED) used to track the rat's position; (2) a backpack that contains the transmitter and batteries (2 N cells); the backpack also provides additional amplification of the single cell signals; and (3) a short cable that connects the headstage to the backpack; the cable supplies power to the headstage amplifiers and the LED, and carries the physiological signals from the headstage to the backpack. By using a differential amplifier and recording between two brain microelectrodes the system can transmit action potential activity from two nearly independent sources. In a future improvement, two transmitters with different frequencies would be used telemeter signals from four microelectrodes simultaneously.