NMDARs in granule cells contribute to parallel fiber–Purkinje cell synaptic plasticity and motor learning

Long-term synaptic plasticity is believed to be the cellular substrate of learning and memory. Synaptic plasticity rules are defined by the specific complement of receptors at the synapse and the associated downstream signaling mechanisms. In young rodents, at the cerebellar synapse between granule cells (GC) and Purkinje cells (PC), bidirectional plasticity is shaped by the balance between transcellular nitric oxide (NO) driven by presynaptic N-methyl-D-aspartate receptor (NMDAR) activation and postsynaptic calcium dynamics. However, the role and the location of NMDAR activation in these pathways is still debated in mature animals. Here, we show in adult rodents that NMDARs are present and functional in presynaptic terminals where their activation triggers NO signaling. In addition, we find that selective genetic deletion of presynaptic, but not postsynaptic, NMDARs prevents synaptic plasticity at parallel fiber-PC (PF-PC) synapses. Consistent with this finding, the selective deletion of GC NMDARs affects adaptation of the vestibulo-ocular reflex. Thus, NMDARs presynaptic to PCs are required for bidirectional synaptic plasticity and cerebellar motor learning.

Excitatory postsynaptic calcium transients at Aplysia sensory–motor neuron synapses allow for quantal examination of synaptic strength over multiple days in culture

A more thorough description of the changes in synaptic strength underlying synaptic plasticity may be achieved with quantal resolution measurements at individual synaptic sites. Here, we demonstrate that by using a membrane targeted genetic calcium sensor, we can measure quantal synaptic events at the individual synaptic sites of Aplysia sensory neuron to motor neuron synaptic connections. These results show that synaptic strength is not evenly distributed between all contacts in these cultures, but dominated by multiquantal sites of synaptic contact, likely clusters of individual synaptic sites. Surprisingly, most synaptic contacts were not found opposite presynaptic varicosities, but instead at areas of pre- and postsynaptic contact with no visible thickening of membranes. The release probability, quantal size, and quantal content can be measured over days at individual synaptic contacts using this technique. Homosynaptic depression was accompanied by a reduction in release site probability, with no evidence of individual synaptic site silencing over the course of depression. This technique shows promise in being able to address outstanding questions in this system, including determining the synaptic changes that maintain long-term alterations in synaptic strength that underlie memory.

α2δ-2 is Required for Functional Postsynaptic Calcium Channel Nanodomain Signaling

α2δ proteins (CACNA2D1-4) are required for normal neurological function, although the mechanisms whereby α2δ proteins control neuronal output remain unclear. Using whole-cell recordings of mouse cerebellar Purkinje cells, we show that α2δ-2 is required for coupling postsynaptic voltage-dependent calcium entry to effector mechanisms controlling depolarization-induced suppression of excitation as well as action potential afterhyperpolarization. Our findings indicate that α2δ-2 is necessary for the function of postsynaptic calcium signaling nanodomains.

Excitatory postsynaptic calcium transients at Aplysia sensory-motor neuron synapses allow for quantal examination of synaptic strength over multiple days in culture

The ability to monitor changes in strength at individual synaptic contacts is required to test the hypothesis that specialized synapses maintain changes in synaptic strength that underlie memory. Measuring excitatory post-synaptic calcium transients through calcium permeable AMPA receptors is one way to monitor synaptic strength at individual synaptic contacts. Using a membrane targeted genetic calcium sensor, we demonstrate that one can measure synaptic events at individual synaptic contacts in Aplysia sensory-motor neuron synapses. These results show that synaptic strength is not evenly distributed between all contacts in these cultures, but dominated by multiquantal sites of synaptic contact. The probability, quantal size and quantal content can be measured over days at individual synaptic contacts using this technique. Surprisingly, most synaptic contacts were not found opposite presynaptic varicosities, but instead at areas of pre- and post-synaptic contact with no visible thickening of membranes. This technique shows promise in being able to address whether specialized synapses maintain synaptic strength underlying memory.

Depression of substantia nigra dopamine transmission is driven by retrograde neurotensin release and is enhanced by methamphetamine self-administration

Midbrain dopamine neurons play central roles in reward learning and motivated behavior, and inhibition at somatodendritic dopamine D2 receptor (D2R) synapses blunts psychostimulant reinforcement. Release of the neuropeptide neurotensin in the midbrain increases following methamphetamine exposure and induces long-term depression of D2R synaptic currents (LTDDA), however the source of neurotensin that drives LTDDA is not known. Here we show that LTDDA is driven by neurotensin released by dopamine neurons. Optogenetic stimulation of dopamine neurons was sufficient to induce LTDDA in the substantia nigra, but not the ventral tegmental area, and was dependent on neurotensin receptors, postsynaptic calcium, and vacuolar-type H+-ATPase activity in the postsynaptic cell. Further, LTDDA was enhanced in mice that had self-administered methamphetamine. These findings reveal a novel form of signaling between dopamine neurons involving release of the peptide neurotensin, which may act as a feed forward mechanism to increase dopamine neuron excitability and methamphetamine self-administration.