Question: 

The mature rat brainstem can be conditioned for SD by lowering the extracellular chloride content and elevating extracellular potassium concentration. Here we describe a form of depolarizations that occurred without triggering only in the brainstem.

Methods: 

In anesthetized and ventilated adult rats (sodium thiopentone, 100 mg/kg, i.p., paralyzed with pancuronium bromide, 1 ml/kg i.v.) we measured DC potentials in the brainstem (in trigeminal nucleus area and in contralateral nucleus tractus solitarii area, NTS) and in the cerebral cortex, and in some experiments we recorded action potentials from neurons in trigeminal nucleus or monitored cardiovascular parameters.

Results: 

After at least 30 min of conditioning in more than 50 % of the rats rapidly recurring depolarizations were observed that occurred initially at intervals of several minutes and subsequently at intervals of 83.3±5.6 s with amplitudes of 9.8±0.6 mV (mean±SEM, n=597 depolarizations). Occasionally even DC amplitudes up to 35 mV were observed. In contrast to classical SD evoked by KCl repetitive depolarizations appeared simultaneously at the recording sites including the NTS area where DC amplitudes of 10.8±0.3 mV (n=154 depolarizations) were monitored. Each depolarization was accompanied by a burst of action potentials in trigeminal nucleus neurons and caused a transient increase in systemic blood pressure and local blood flow as did classical SDs. In no case they evoked SDs in cerebral cortex. As in classical SD the NMDA receptor antagonist MK-801 (3 mg/kg) blocked these repetitive depolarizations which also disappeared after washout of the conditioning superfusate.

Conclusions: 

We conclude therefore that the brainstem has the potential to produce either depolarizations that are slowly spreading, or episodic depolarizations which occur simultaneously throughout the brainstem.