Previous studies have implicated Cdk5 in the regulation of neuronal excitability 
. Cdk5 has been suggested to both increase 
and decrease 
neurotransmitter release and modulate striatal neuron excitability 
. Cdk5 has specifically been implicated in exocytosis and endocytosis via phosphorylation of numerous substrates including synapsin, amphiphysin I, dynamin, and others 
. We previously demonstrated that conditional loss of Cdk5 led to enhancements in plasticity and learning via modulation of NMDAR degradation 
. Furthermore, transient overexpression of Cdk5-activating cofactor, p25, increases NMDAR-mediated plasticity and synaptic transmission 
Here, we further characterize the enhancement of hippocampal plasticity in Cdk5 KO mice by studying fEPSPs during theta bursts and tetanic stimuli. We first show that within the SC/CA1 pathway, Cdk5 KO mice displayed altered LTP-inducing TBS topography. During the detailed LTP analyses, we find that loss of Cdk5 also leads to an NMDAR-independent enhancement in PTP, a presynaptic form of short-term plasticity. Although Cdk5 KO had no effect on paired pulse facilitation (PPF) with interstimulus intervals between 25 and 800 
, Cdk5 KO did lead to depression with a shorter 10 ms inter-stimulus interval. Cdk5 may theoretically confer these changes by increasing the number of presynaptic vesicles, increasing presynaptic Ca2+
-influx, or increasing sensitivity to presynaptic Ca2+
. Changes in probability of presynaptic vesicle release are usually accompanied by broad impairments in PPF. However, since loss of Cdk5 only conferred a PPF deficit at a very short interstimulus interval, additional information is necessary to better elucidate Cdk5′s role in the presynapse. Although several studies implicate Cdk5 in vesicle release and recycling 
, the exact nature of Cdk5′s role in the presynaptic compartment is still unclear. Future studies examining Cdk5′s role in presynaptic terminal would be beneficial.
Conditional loss of Cdk5 initially leads to enhanced learning, plasticity and increased NMDAR-mediated currents 
. In the present study, electrophysiological extracellular hippocampal recordings in vitro
reveal that conditional loss of Cdk5 also leads to elevations in fEPSPs with a predominant NMDA component and reduced threshold for population-spike activity. It is known that the NR2B NMDAR subunit directly modulates neuronal excitability and contributes to seizures 
, so it is possible that a similar mechanism is involved following loss of Cdk5. Other mechanisms may also contribute to the enhanced hippocampal excitability. For example, loss of Cdk5 led to a subtle impairment in fEPSP repolarization in the hippocampal SC/CA1 pathway. Nonetheless, the data suggests that Cdk5 functions to attenuate hippocampal neuronal excitability via several mechanisms including the modulation of NMDARs.
Over time, Cdk5 KO mice displayed an increase in seizure susceptibility, suggesting a progressive increase in excitability. Chronic loss of Cdk5 increased the propensity for pharmacologically- and audiogenically-induced seizures. The reduced threshold for behavioral seizure activity correlated with EEG/EMG evidence of spontaneous seizures. Given the complexity, impact, and significance of spontaneous seizure activity, additional EEG recordings over an extended period of are warranted in future studies. In addition, conditional Cdk5 KO mice also displayed an increase in behavioral startle reactivity. We also found an association between the seizure phenotype and the lethality in Cdk5 KO mice: after 10 weeks of Cdk5 KO, the majority of Cdk5 KO mice displayed signs of increased behavioral excitability and propensity towards handling-induced seizures. Up to 40% of affected mice died soon after displaying seizure activity.
The electrophysiological and behavioral experiments reveal similar trends. Initially, Cdk5 KO mice display increased NMDAR-mediated currents, enhanced plasticity, and impaired neuronal repolarization. Later, mice exhibit increased startle reactivity and reduced threshold for in vitro population spikes. Then, mice exhibit spontaneous electrographic seizures, handling-induced seizures, audiogenic seizures and lower threshold for pharmacologically-induced seizures. Although the experiments were performed at varying time-points after Cdk5 KO, together, the results suggest that loss of Cdk5 leads to a progressive increase in neuronal and behavioral excitability, ultimately leading to seizures.
Abnormal expression and dysregulation of Cdk5 and its cofactors have been demonstrated in tissues from human cortical dysplasia 
and hippocampal sclerosis 
. KO of p35 leads to cortical lamination defects, seizures, and lethality 
. Mice lacking p35 also display abnormal morphological and functional organization of the hippocampus, dysplastic hippocampi, heterotopic pyramidal cells, and granule cell dispersion and may serve as a model for cortical dysplasia 
. In this study we show that chemically-induced status epilepticus and electroconvulsive shock in healthy animals induced acute generation of a Cdk5-activating cofactor, p25. Chronic loss of Cdk5 is associated with both seizures and reduced levels of p25. These published and new findings indicate that Cdk5 and its cofactors may play key regulatory roles in neuronal excitability.
Recent data have demonstrated dual roles for both Cdk5 and its activating cofactor, p25, in learning and plasticity 
. A dichotomy may also exist for Cdk5/p25′s role in pathological neuronal excitability associated with seizures. Loss of Cdk5 increases excitability and leads to seizures, which corresponds with decreased levels of p25. Acute seizures in healthy animals and chronic seizures in human epileptics produces elevated levels p25 
. During periods of increased Ca2+
-influx, neurons produce p25 following calpain activation 
. Initially, Cdk5/p25 may serve as a homeostatic molecule to dampen excitatory transmission and inhibit seizure activity. However, over-excitation could result in excessive Ca2+
-influx and p25 generation, aberrant Cdk5 activity and neurotoxicity 
. Thus, Cdk5 may serve to inhibit abnormal epileptiform activity when a neuron is in a normal state or promote cell death following excess and non-physiological Ca2+
influx. We previously showed that Cdk5 facilitates calpain-mediated degradation of the NR2B NMDAR subunit 
. Cdk5 KO may impair calpain-mediated p25 generation and thereby disrupt normal homeostatic mechanisms that prevent seizures. Future studies on Cdk5, calpain, p25, and NR2B may better our understanding of the divergent roles for Cdk5 in neuronal physiology and disease.
Current anticonvulsant therapeutics increase inhibitory neurotransmission, suppress high frequency neuronal firing by reducing voltage-gated Na+
channels availability, or inhibit voltage-gated T-type Ca2+
-channels. Such therapeutic options produce unwanted side effects, are only efficacious in 70% of adults suffering from recurrent seizures 
, and generally alleviate the symptoms rather than curing or modifying the underlying etiology 
. Aberrations in neuronal excitability can produce abnormal electrical discharges in the brain leading to seizure activity. Thus, a better understanding of the cellular mechanisms underlying neuronal excitability will aid in the development of novel therapeutics for seizures.
In future studies, it will be worthwhile to directly assess whether the increase in neuronal excitability following Cdk5 KO has any affects on presynaptic transmission, neuronal fiber sprouting, cell count, and cell survival. Furthermore, it would be interesting to study how long-term loss of Cdk5 affects hippocampal plasticity and learning. Although short-term loss of Cdk5 produced enhancements in plasticity and learning 
, it is possible that chronic loss of Cdk5 and the associated epileptiform activity leads to neurodegeneration and impaired synaptogenesis, learning, and structural plasticity