STIMULATION-INDUCED RESPONSES OF THE TRIGEMINAL CAUDAL NEURONS IN THEBRAIN-STEM PREPARATION ISOLATED FROM NEWBORN RATS

The brainstem preparation with the trigeminal mandibular nerve attache d was isolated from rats postnatal day 0-6 (P0-P6) to test if the pote ntiation could be induced in neonatal neurons in the trigeminal subnuc leus caudalis by stimulation of the primary afferents. The stimulation -induced potentials in 92 neurons recorded extracellularly, and the sy naptic potentials in 16 neurons recorded by the whole-cell patch clamp technique were examined. The extracellularly recorded neurons respond ed to stimulation (0.5 Hz) with either an increase, a decrease, or lit tle change in spike numbers, and it ere classified as Type 1, Type 2, and Type 3. respectively. Type 1 neurons at P4 and older responded in a low Mg2+ solution with a progressive increase in spike number lastin g for several minutes after the cessation of stimulation, i.e., short- term potentiation, STP. This potentiation was antagonized by 20 mu M o f(+)-MK-801 hydrogen maleate (MK-801) or 25 mu M of 2-amino-5-phosphon ovaleric acid. In contrast, Type 1 at P3 and younger did not exhibit S TP. The age-related distinct response properties were observed between Type 1 neurons at P4-P6 and at P0-P3. The percentage of Type 1 in stu died neurons increased from 24% at P0-P3 to 53% at P4-P6. In the intra cellular experiment, the mean latency of excitatory postsynaptic poten tial (EPSP) of recorded neurons indicated that the conduction velocity of the convergent afferents was 0.37 m/s, in the range of C-fiber. Ne urons were classified as Type E and Type I. Type E responded with EPSP only, or with both EPSP and inhibitory postsynaptic potentials (IPSP) , while Type I responded with IPSP only. In Type E at P4 and older, a single stimulation produced a burst of spike discharges that lasted fo r several seconds. Stimulation at a hyperpolarized membrane potential showed that aggregated slow EPSPs lay under a burst of spike discharge s, and that slow EPSPs, but not a short-latency EPSP, were completely blocked by MK-801. In contrast, Type E at P3 and younger did not evoke a burst of spikes. Morphological examination of recorded neurons show ed that the formation of networks was sparse at P4 and rapidly develop ed up to P4. The results suggest that: (1) short-term potentiation is induced with the development of synaptic network formation in the caud al nucleus at P4 and older, (2) the summation of N-methyl-D-aspartate (NMDA)-mediated slow EPSPs build up a prolonged depolarization: and (3 ) the brainstem preparation is applicable for neurophysiological studi es on the trigeminal pain system. (C) 1998 Elsevier Science B.V.