KCa2 channels transiently downregulated during spatial learning and memory in rats.

Small-conductance calcium-activated potassium channels (K(Ca)2) are essential components involved in the modulation of neuronal excitability, underlying learning and memory. Recent evidence suggests that K(Ca)2 channel activity reduces synaptic transmission in a postsynaptic NMDA receptor-dependent manner and is modulated by long-term potentiation. We used radioactive in situ hybridization and apamin binding to investigate the amount of K(Ca)2 subunit mRNA and K(Ca)2 proteins in brain structures involved in learning and memory at different stages of a radial-arm maze task in naive, pseudoconditioned, and conditioned rats. We observed significant differences in K(Ca)2.2 and K(Ca)2.3, but not K(Ca)2.1 mRNA levels, between conditioned and pseudoconditioned rats. K(Ca)2.2 levels were transiently reduced in the dorsal CA fields of the hippocampus, whereas K(Ca)2.3 mRNA levels were reduced in the dorsal and ventral CA fields of the hippocampus, entorhinal cortex, and basolateral amygdaloid nucleus in conditioned rats, during early stages of learning. Levels of apamin-binding sites displayed a similar pattern to K(Ca)2 mRNA levels during learning. Spatial learning performance was positively correlated with levels of apamin-binding sites and K(Ca)2.3 mRNA in the dorsal CA1 field and negatively correlated in the dorsal CA3 field. These findings suggest that K(Ca)2 channels are transiently downregulated in the early stages of learning and that regulation of K(Ca)2 channel levels is involved in the modification of neuronal substrates underlying new information acquisition.

Small-conductance Ca(2+)-activated K(+) channels: Heterogeneous affinity in rat brain structures and cognitive modulation by specific blockers.

Small-conductance calcium-activated potassium channels (K(Ca)2) generating the medium afterhyperpolarization seen after an action potential modulate the neuronal integration signal. The effects of two K(Ca)2 channel blockers, apamin, specific to K(Ca)2.2 and K(Ca)2.3 channels, and lei-Dab7, which binds to K(Ca)2.2 channels only, were compared to evaluate the involvement of K(Ca)2 channel subunits in behavior, spatial learning and memory in rats. Intracerebroventricular (9-5 ng) injections of lei-dab7 decreased locomotor activity, food intake and body weight in rats deprived of food. A dose of 3 ng lei-Dab7 had no effect on these types of behavior. We therefore used this dose for attention and memory tasks. No modification to attention or memory was observed in a spatial radial-arm maze task with rats given 3 ng lei-Dab7, whereas apamin (0.3 ng) improved reference memory and accelerated changes of strategy from egocentric to allocentric. These findings suggest that K(Ca)2.3 blockade improves memory in rats. Lei-Dab7 entirely outcompeted the binding of iodinated apamin to 64 brain structures (mean IC(50): 34.5 nM), although IC(50) values were highly variable. By contrast, overall IC(50) values for apamin were close to mean values (11.3 pM). The very low affinity of the hippocampus and neocortex for lei-Dab7 may account for the absence of a behavioral effect of this compound. The variability of IC(50) values suggests that K(Ca)2 channel composition varies considerably as a function of the brain structure considered.

Differential effects of two blockers of small conductance Ca2+-activated K+ channels, apamin and lei-Dab7, on learning and memory in rats.

SK channels are responsible for long-lasting hyperpolarization following action potential and contribute to the neuronal integration signal. This study evaluates the involvement of SK channels on learning and memory in rats, by comparing the effects of two SK channel blockers, i.e., apamin which recognizes SK2 and SK3 channels, and lei-Dab7 which binds SK2 channels only. lei-Dab7 totally competes and contests apamin binding on whole brain sections (IC(50): 11.4 nM). Using an olfactory associative task, intracerebroventricular blocker injections were tested on reference memory. Once the task was mastered with one odor pair, it was then tested with a new odor pair. Apamin (0.3 ng), injected before or after the acquisition session, improved new odor pair learning in a retention session 24 hours later, whereas lei-Dab7 (3 ng) did not significantly affect the mnesic processes. These results indicated that the blockage of SK channels by apamin facilitates consolidation on new odor associations; lei-Dab7, containing only SK2 subunits, remains without effect suggesting an involvement of SK3 channels in the modulation of the mnesic processes.

Correspondences between the binding characteristics of a non-natural peptide, Lei-Dab7, and the distribution of SK subunits in the rat central nervous system.

Small-conductance calcium-activated potassium channels (SK1-SK3 channels) are responsible for long-lasting hyperpolarization following action potential and contribute to the neuronal firing and integration signal. Two peptide toxins: apamin and Leiurotoxin 1, block this SK channels with high affinities. We generated a modified Leiurotoxin 1 (Lei-Dab7) that inhibits SK2 channels with a high selectivity. Competitive binding of radio-iodinated apamin to different rat brain structures, in the presence of native apamin and Lei-Dab7, has shown that dissociation constants differ by a factor of 1000 and thus demonstrated that ligand affinity is as important as ligand selectivity for a specific receptor. However, the lack of ligands discriminating between SK channel subunits is impeding the understanding of the role of each heteromeric SK channel type in different tissues. Our study aims to better understand the molecular combinations of SK channels and their association with specific functional implications. On this purpose, a clustering technique allows us to identify five groups of brain structures reflecting singular profiles of affinity and selectivity of Lei-Dab7 in comparison with apamin. The analysis of correspondences between Lei-Dab7 binding and distribution of SK subunits in these groups of brain structures suggests that functional heteromeric SK channels are involved in specific information processes.