Depotentiation depends on IP3 receptor activation sustained by synaptic inputs after LTP induction

Adam, amigo, brain, and K channel

Voltage-dependent K+ (Kv) channels are diverse, comprising the classical Shab - Kv2, Shaker - Kv1, Shal - Kv4, and Shaw - Kv3 families. The Shaker family alone consists of Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5, Kv1.6, and Kv1.7. Moreover, the Shab family comprises two functional (Kv2.1 and Kv2.2) and several "silent" alpha subunits (Kv2.3, Kv5, Kv6, Kv8, and Kv9), which do not generate K current. However, e.g., Kv8.1, via heteromerization, inhibits outward currents of the same family or even that of Shaw. This property of Kv8.1 is similar to those of designated beta subunits or non-selective auxiliary elements, including ADAM or AMIGO proteins. Kv channels and, in turn, ADAM may modulate the synaptic long-term potentiation (LTP). Prevailingly, Kv1.1 and Kv1.5 are attributed to respective brain and heart pathologies, some of which may occur simultaneously. The aforementioned channel proteins are apparently involved in several brain pathologies, including schizophrenia and seizures.

Synaptic plasticity in hippocampal CA1 neurons of mice lacking inositol-1,4,5-trisphosphate receptor-binding protein released with IP3 (IRBIT)

In hippocampal CA1 neurons of wild-type mice, a short tetanus (15 or 20 pulses at 100 Hz) or a standard tetanus (100 pulses at 100 Hz) to a naive input pathway induces long-term potentiation (LTP) of the responses. Low-frequency stimulation (LFS; 1000 pulses at 1 Hz) 60 min after the standard tetanus reverses LTP (depotentiation [DP]), while LFS applied 60 min prior to the standard tetanus suppresses LTP induction (LTP suppression). We investigated LTP, DP, and LTP suppression of both field excitatory postsynaptic potentials and population spikes in CA1 neurons of mice lacking the inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R)-binding protein released with IP₃ (IRBIT). The mean magnitudes of LTP induced by short and standard tetanus were not different in mutant and wild-type mice. In contrast, DP and LTP suppression were attenuated in mutant mice, whereby the mean magnitude of responses after LFS or tetanus were significantly greater than in wild-type mice. These results suggest that, in hippocampal CA1 neurons, IRBIT is involved in DP and LTP suppression, but is not essential for LTP. The attenuation of DP and LTP suppression in mice lacking IRBIT indicates that this protein, released during or after priming stimulations, determines the direction of LTP expression after the delivery of subsequent stimulations.

Depolarization-initiated endogenous cannabinoid release and underlying retrograde neurotransmission in interneurons of amygdala

The depolarization is also important for the short-term synaptic plasticity, known as depolarization-induced suppression of excitation (DSE). The two major types of neurons and their synapses in the lateral nucleus of amygdala (LA) are prone to plasticity. However, DSE in interneurons has not been reported in amygdala in general and in LA in particular. Therefore, we conducted the patch-clamp experiments with LA interneurons. These neurons were identified by lack of adaptation in firing rate of action potentials. In this study, we show for the first time a transient suppression of neurotransmission at synapses both within the local network and between cortical inputs and interneurons of the LA. The retrograde neurotransmission from GABAergic interneurons were comparable with that of glutamatergic pyramidal cells. That is the axonal terminals of cortical inputs do not posses selectivity toward two neuronal subtypes. However, the DSE of both types of neurons involve an increase in intracellular Ca²⁺ and the release of endogenous cannabinoids (eCB) and activation of presynaptic CB1 receptors. The magnitude of DSE was significantly higher in interneurons compared with pyramidal cells, though developed with some latency.