The prepositus hypoglossi nucleus (PHN) is involved in the maintenance of eye position following horizontal eye movements. Previous studies show that neurons belonging to this nucleus are able to transform eye velocity signals coming from paragigantocellular formation (RDPG) to a position signal, in order to send this information to the external ocular motor nucleus. This transformation could be mediated by acetylcholine. In the present work we try to determine if the above-mentioned neurotransmitter could be a modulator in the signal transformation process.
For this purpose, we made a current-clamp intracellular recording of neurons from brain stem slices obtained from Wistar rats (60-80 g) (Yajeya et al. 2000). Rats were deeply anesthetized with halothane and humanely killed. All the experiments strictly conformed to National guidelines.
The recorded neurons (n = 70) were classified according to their type of spike as type A (80 %) and type B (20 %). External stimuli in RDPG were used to study postsynaptic potentials, resulting in EPSPs in all cases, which were totally blocked with perfusion of CNQX (n = 10). After TTX blockage of evoked synaptic activity, the administration of carbachol (Cch, 10 µM) produced a significant depolarization (10.22 ± 1.2 mV, P < 0.01, Student’s paired t test), suggesting a postsynaptic effect (n = 10). On the other hand, variations in the amplitude of postsynaptic potentials were also studied under the action of Cch, showing a 56.6 % reduction (n = 10). Paired-pulse stimulation was used to determine the possible site of Cch action and the differences observed in paired-pulse depression indicate a presynaptic effect (n = 5) (S2/S1, control 60%, problem 95%).
Our results show that afferents fibres to NPH arriving from RDPG are glutamatergic and muscarinic agonist Cch acts as modulator at pre- and postsynaptic sites.
This work was supported by grants PBI 2000-11900 (DGICYT), 01/0385 (FIS), PM1998-OO11 and a fellowship by Ministerio de Ciencia y Tecnología de España FP99 Program.