Cooperative glutamatergic and cholinergic mechanisms generate short-term modifications of synaptic effectiveness in prepositus hypoglossi neurons

Abstract

To maintain horizontal eye position on a visual target after a saccade, extraocular motoneurons need a persistent (tonic) neural activity, called "eye-position signal," generated by prepositus hypoglossi (PH) neurons. We have shown previously in vitro and in vivo that this neural activity depends, among others mechanisms, on the interplay of glutamatergic transmission and cholinergic synaptically triggered depolarization. Here, we used rat sagittal brainstem slices, including PH nucleus and paramedian pontine reticular formation (PPRF). We made intracellular recordings of PH neurons and studied their synaptic activation from PPRF neurons. Train stimulation of the PPRF area evoked a cholinergic-sustained depolarization of PH neurons that outlasted the stimulus. EPSPs evoked in PH neurons by single pulses applied to the PPRF presented a short-term potentiation (STP) after train stimulation. APV (an NMDA-receptor blocker) or chelerythrine (a protein kinase-C inhibitor) had no effect on the sustained depolarization, but they did block the evoked STP, whereas pirenzepine (an M1 muscarinic antagonist) blocked both the sustained depolarization and the STP of PH neurons. Thus, electrical stimulation of the PPRF area activates both glutamatergic and cholinergic axons terminating in the PH nucleus, the latter producing a sustained depolarization probably involved in the genesis of the persistent neural activity required for eye fixation. M1-receptor activation seems to evoke a STP of PH neurons via NMDA receptors. Such STP could be needed for the stabilization of the neural network involved in the generation of position signals necessary for eye fixation after a saccade.

Figure 1.

Neural circuits and connectivity. A, A diagram illustrating neural circuits present in the sagittal brainstem slice used here. EBN are located in the PPRF, rostrally to abducens nucleus motoneurons (ABD Mn), and project monosynaptically on PH neurons. Stimuli (St.) applied to the PPRF also activate cholinergic neurons and/or axons. Rec., Recording electrode. B, Location of some (n = 12) biocytin-injected neurons to illustrate the recording area. R, Rostral; D, dorsal; MVn, medial vestibular nucleus. C, Glutamatergic nature of PPRF synaptic contacts on PH neurons. Top, EPSP evoked in a PH neuron by a single subthreshold stimulus (St.; 100 μs, 200 μA) applied to the PPRF. The EPSP was not affected by atropine sulfate (1 μm) or APV (50 μm), but the application of CNQX (10 μm) completely removed the evoked synaptic potential.

Figure 2.

Differential effects of train stimulation of PPRF on PH neurons in presence of glutamatergic and/or cholinergic drugs. A, The top record (Control) illustrates the effect of a PPRF train (100 ms, 200 Hz, 200 μA) on a PH cell. Note the large and sustained depolarization recorded after the end of the train. The middle record shows that the application of APV (50 μm) to the bathing solution did not affect the posttrain activation of the PH neuron. In contrast, this sustained depolarization of the PH neuron was impossible to evoke in the presence of atropine sulfate (1 μm). B, Top, Another example of sustained depolarization, including a lasting burst of action potentials, evoked in a PH neuron by train stimulation (100 ms, 200 Hz, 450 μA) of the PPRF. Superfusion with pirenzepine (bottom; 0.5 μm) completely removed the evoked post-train depolarization. C, The sustained depolarization of PH neurons after train stimulation (100 ms, 200 Hz, 425 μA) of the PPRF was not affected by chelerythrine (2.5 μm). The resting membrane potential for the illustrated neurons is indicated. Calibration in C also applies to B.

Figure 3.

Plots of PH neuron responses after train stimulation. A, Exponential decay of burst firing evoked in PH neurons after train stimulation of PPRF. The illustrated data correspond to a single recording collected from a PH neuron. The burst of action potentials was evoked after a train of stimuli (100 ms, 200 Hz, 400 μA) applied to the PPRF (r = 0.979; p < 0.01). B, Plot of STP duration (in seconds) against sustained cholinergic depolarization duration (in milliseconds). Each point corresponds to the responses of one neuron (n = 10).

Figure 4.

STP of PH neurons after train stimulation of PPRF. A, EPSPs evoked in a PH neuron by single subthreshold stimuli applied to the PPRF 30 s before (Control), and 30 s (Potentiation) and 180 s (Post-Potentiation) after train stimulation (100 ms, 200 Hz, 250 μA; arrow) of PPRF. B, Quantitative analysis of the synaptic potentiation evoked in PH neurons (n = 15) by a train of stimuli (Train Stim.) presented to the ipsilateral PPRF. Potentiation was determined as the percentage increase in the amplitude of the evoked EPSP. Data represent mean percentage ± SD, computed every 30 s. Note that the STP was present at significant values (asterisks) for ∼120 s (p ≤ 0.01). Single pulses applied to the PPRF were presented at a rate of 0.2 Hz. STP was not evoked in the presence of APV (50 μm; n = 8), pirenzepine (0.5 μm; n = 6), or chelerythrine (2.5 μm; n = 5) superfused to the bathing solution.