Long-term morphology of the alumina cream experimental epileptogenic focus

Do glia contribute to behaviour? A neuromodulatory review

1. The links between behavioural state, gross electrophysiology and the activity of neurons and astrocytes are reviewed to stimulate interest in the contributions that glia make to behaviour. 2. Behavioural arousal in which neuronal responsivity ("sensitivity") is elevated is also associated with a sustained (0.5-10 sec) potential shift (SPS). 3. There is powerful and accumulating evidence that the SPS is primarily of glial origin. 4. In epilepsy neurons are hyperactive and there is a massive SPS during seizures. In seizure free periods, epileptic animals frequently have elevated arousal responses and increased neuronal sensitivity, indicating that seizures may be due to elevation of the activity of a normally adaptive sensitizing mechanism. 5. The common finding of an astrocytic pathology in epilepsy and the links between arousal, neuronal sensitization, SPSs and seizures implicates a modulatory role for astrocytes in both health and disease. 6. Glia, especially astrocytes, may modulate neuronal responsiveness by regulation of the microenvironment. 7. At the current state of knowledge, regulation of extracellular ionic K+, Ca2+ and neurotransmitter glutamate and GABA seem to be the most important candidates for modulating neuronal sensitivity in arousal and abnormally for seizure genesis. 8. Both in phylogeny and in ontogeny, glia and neurons have intimate associations. 9. The functional astrocytic syncitium is in a prime position to control the ecology of neuronal populations and thereby their activity. 10. The physiology and biochemistry of glia-neuronal interactions offers exciting new prospects for developments in behavioural neuroscience.

The early involvement of subcortical structures during the development of a cortical seizure focus

Cortical surface electrodes and bipolar depth electrodes were implanted stereotaxically in the ventral posterolateral and paracentral thalamic nuclei, amygdala, hippocampus, and putamen in adult cats to determine the progressive involvement of these structures in the generalization of an experimentally induced seizure disorder. Prior to (45-65 days) and following (50-70 days) subpial injection of 0.04 ml of aluminum hydroxide in the sensorimotor cortex, 30 min EEG records were obtained regularly in each animal. At the time of aluminum hydroxide injection, there was no persistent abnormal EEG activity resulting from electrode implantation. In all animals, intermittent slow waves and epileptiform activity appeared in subcortical or extrafocal structures prior to the development of epileptiform activity in the primary focus. These abnormalities were frequently associated with brief clinical seizures and were clearly independent of abnormal activity in the primary focus. Only later in the development of the seizure disorder was activity in the secondary foci observed to be dependent on frequent epileptiform activity in the primary focus. These results demonstrate that multiple, independent foci develop in subcortical structures before the occurrence of a well developed, fully "mature" cortical primary focus.