Studies on the role of NMDA, GABA and adenosine receptors within discrete brain nuclei in the high pressure neurological syndrome in rats

Abstract

We investigated the influence of brain focal injections of 2-amino-7-phosphonoheptanoate (APH), N-methyl-D-aspartate (NMDA), muscimol and 2-chloroadenosine (CAD) on behavioural symptoms of the high pressure neurological syndrome (HPNS) in rats. Each compound was bilaterally injected (in nmole range doses) into the substantia nigra pars reticulata (SNR), entopeduncular nucleus (EP) or ventromedial thalamic nucleus (VLTh) prior to exposure to hyperbaric conditions. Threshold pressure for the occurrence of tremor, myoclonic jerks and convulsions were recorded for drug and vehicle-injected animals. A decrease in threshold pressure for a particular symptom indicates that the reaction to hyperbaric conditions is more pronounced, while an increase in threshold pressure is indicative of protection against HPNS.

Injection of NMDA (an agonist of excitatory NMDA receptor subtype) into the SNR, EP or VLTh produced a decrease in threshold pressure for the occurrence of tremor and its intensity. Injection of APH (a selective antagonist at the NMDA receptor) resulted in protection against tremor in all three structures (the most potent effect was produced by entopeduncular injection). An increase in threshold pressure for convulsions also followed APH injection into each structure (with the most pronounced effect after nigral injection). Injection of muscimol did not change the animals’ response to compression, while injection of CAD produced increased reaction to hyperbaric conditions significantly affecting tremor (nigral and thalamic injections) and convulsions (entopeduncular injection).