Spreading Depression (SD) Waves in the Brainstem of Immature Rats Propagate Independently of Glutamatergic Synaptic Transmission

Spreading depression (SD) is thought to occur mainly in the cerebral cortex but barely or not at all in other parts of the CNS. Specifically it was believed that the brainstem is not able to generate SDs. Recently, we confirmed that the brainstem of the adult rat does not show SDs but, interestingly, SDs could be elicited in the brainstem of rats younger than 13 days, when excitability was enhanced by replacing extracellular chloride ions with sodium acetate or by transient periods of asphyxia or hypoxia (2min breathing 6% O₂ in N₂). Sustained spreading depressions in the brainstem stopped breathing (Richter et al. J Neurophysiol 2003; 90: 2163–2170). In the present experiments we further studied the mechanism of propagation of SD in the immature brainstem. SDs elicited by KCl were recorded in 11-day-old rat pups (anesthetized with urethane, 1.5g/kg body weight i.p.) with glass microelectrodes in a region close to the trigeminal nucleus at depths of 1600, 1200, and 800µm. We tested whether the elicitation and/or propagation of KCl-induced SD can be inhibited in the immature brainstem either by a blockade of voltage-gated calcium channels (VGCCs) that are involved in glutamate release (P/Q-type by omega-agatoxin IVA,10^–6 M; N-type by omega-conotoxin GVIA,10^–6 M; L- and T-type by flunarizine, 1–2mg/kg body weight) or by a blockade of NMDA receptors by ketamine (doses of 2 to 100mg/kg body weight). Application of agatoxin for 60min to the brainstem surface did not influence SD. Blockade of N-type VGCCs by conotoxin did not influence SD but disturbed the normal breathing rhythm when it was applied for 30–40min. Flunarizine (1–2mg/kg body weight) either injected intraperitoneally or applied onto the brainstem surface did not influence elicitability of SD, SD amplitudes or SD migration velocity in the brainstem within a period of up to 2 hours. SD amplitudes amounted to 12 mV and were accompanied by increases in [K⁺]e up to 40 mM before and after flunarizine. Specific doses of the NMDA receptor blocker ketamine (<10mg/kg body weight) did not influence the SD in the cortex and in the brainstem. Higher doses of ketamine inhibited breathing. AMPA elicited SDs in the cortex, but was barely able to elicit SDs in the brainstem. These findings suggest that SDs in the immature brainstem do not propagate via glutamatergic synaptic transmission. Rather potassium ions seem to mediate both initiation and propagation of SD. Supported by IZKF Jena (IZKF B378–10102).