Stimulation-induced responses of the trigeminal caudal neurons in the brainstem preparation isolated from newborn rats.

ABSTRACT

The brainstem preparation with the trigeminal mandibular nerve attached was isolated from rats postnatal day 0-6 (P0-P6) to test if the potentiation could be induced in neonatal neurons in the trigeminal subnucleus caudalis by stimulation of the primary afferents. The stimulation-induced potentials in 92 neurons recorded extracellularly, and the synaptic potentials in 16 neurons recorded by the whole-cell patch clamp technique were examined. The extracellularly recorded neurons responded to stimulation (0.5 Hz) with either an increase, a decrease, or little change in spike numbers, and were 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 lasting for several minutes after the cessation of stimulation, i.e., short-term potentiation, STP. This potentiation was antagonized by 20 microM of (+)-MK-801 hydrogen maleate (MK-801) or 25 microM of 2-amino-5-phosphonovaleric acid. In contrast, Type 1 at P3 and younger did not exhibit STP. 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 studied neurons increased from 24% at P0-P3 to 53% at P4-P6. In the intracellular experiment, the mean latency of excitatory postsynaptic potential (EPSP) of recorded neurons indicated that the conduction velocity of the convergent afferents was 0.37 m/s, in the range of C-fiber. Neurons 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 for several seconds. Stimulation at a hyperpolarized membrane potential showed that aggregated slow EPSPs lay under a burst of spike discharges, 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 showed that the formation of networks was sparse at P1 and rapidly developed up to P4. The results suggest that (1) short-term potentiation is induced with the development of synaptic network formation in the caudal 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 studies on the trigeminal pain system.