Abstracts

Rationale:
The locus coeruleus (LC) is the sole source of noradrenaline in the neocortex, hippocampus and large parts of the cerebellum. Noradrenaline, released from the LC, is associated with predominantly anticonvulsant effects, and increased noradrenergic transmission is key to the anticonvulsant mechanisms of vagus nerve stimulation. On the other hand, seizures themselves lead to massive rises in hippocampal noradrenaline concentrations (J Neurochem, 2011; 117(3); 461-9). The temporal dynamics of the LC response resulting in this effect remains unclear, and could be critical to understanding the pathophysiology of seizures since noradrenaline is associated with a wide array of neuromodulatory functions, thus likely affecting the seizure itself. In this study, we provide preliminary data on the temporal dynamics of effects of seizures on LC neuronal activity.
Method:
In isoflurane anesthetized male Sprague-Dawley rats (300 – 400 g, n=4), three hippocampal seizures were evoked with tetanic perforant path stimulation (10 s, 400-600 mA, 250 µs square wave pulses, 20 Hz). Seizures were recorded in the dentate gyrus of the dorsal hippocampus. Neuronal activity in the locus coeruleus was recorded with at 32-channel silicon probe with either 25 or 50 µm contact spacing. Analysis was performed on multiunit activity per contact and on single units isolated using the multichannel spike sorter SpykingCircus (v. 0.9.6) (Elife, 2018, Mar 20;7:e34518). LC activity was identified based on classical parameters: wide spikes ( >0.8 ms), slow firing (typically 0-3 Hz for single units), phasic burst followed by inhibition in response to a mechanical noxious foot pinch and inhibition upon systemic administration of the α2 autoreceptor agonist, clonidine. Spike frequencies were computed based on extracted spike trains and compared before and during the evoked hippocampal seizure. The response of isolated neurons was classified based on the averaged response over the three seizures recorded.
Results:
In four rats, a total of 90 putative LC neurons were isolated, of which 29 (33%) displayed increased, 47(52%) displayed decreased and 14 (15%) showed no change in firing during seizures (Figure 1, for examples). In two of the four rats, visualization of multi-unit activity in relation to seizures revealed a tendency for excited and inhibited putative LC neurons to be organized in anatomically distinct clusters. An example is shown in Figure 2, where spike frequencies have been z-scored relative to baseline, e.g. before seizure induction. In the other two rats, such as spatial pattern could not be demonstrated.
Conclusion:
Hippocampal seizures, evoked by perforant path stimulation, lead to a mixture of activation and inhibition of locus coeruleus neurons, which appeared organized in anatomically distinct clusters. Future research will determine the role of the observed LC response in modulating seizure activity.
Funding:
:LEL is funded by The Research Foundation – Flanders (grant no. 1216520N).