Studies on the role of the NMDA receptor in the substantia nigra pars reticulata and entopeduncular nucleus in the development of the high pressure neurological syndrome in rats

The effect of the focal injection of N-methyl-D-aspartate (NMDA) and 2-amino-7-phosphonoheptanoate (APH) into the substantia nigra pars reticulata (SNR) and entopeduncular nucleus (EP) on behavioural signs of the high pressure neurological syndrome (HPNS) in rats was studied. Doses of 1, 5 and 10 nmoles of NMDA or APH were injected into the SNR or EP, 10-30 min prior to the exposure of animals to a high pressure. Injection of NMDA into either SNR or EP results in a lowering of the threshold pressure for tremor by about 30%. Injection of NMDA into the SNR has no significant effect on clonic seizures whereas its injection into the EP results in a decrease of threshold pressure for clonic seizures. NMDA also facilitates the occurrence of forelimb clonus when injected into the EP. Injection of the NMDA antagonist, APH, into the SNR or EP significantly increases the threshold pressure of tremor (32.8 and 48.2% respectively). Seizure threshold is also increased by the injection of APH into either area, but nigral injections (especially the higher doses) are more protective against seizures than the EP injections. Comparing the two sites blockade of NMDA receptors within the EP is more protective against tremor, whereas in the SNR NMDA blockade is more protective against seizures.