Afferent ingrowth and onset of activity in the rat trigeminal nucleus

A novel in vitro preparation, consisting of the rat brainstem with the trig eminal ganglion attached, has been used to study the anatomical and functio nal development of the trigeminal nucleus from embryonic day (E)13 to postn atal day (P)6. Neurobiotin injections into the trigeminal ganglion showed t hat primary afferents had reached the trigeminal tract by E13 and had grown simple, mainly unbranched, collaterals into all levels of the nucleus by E 15. By E17, these collaterals were extensively branched, with occasional bo utons present. Patches of intense neurobiotin-labelled terminals, correspon ding to whisker-related patterns, were first seen at E20 and became clearer over the next few days. Terminal arbours at this stage were relatively loc alized and densely branched, with many boutons. Responses from the trigemin al nucleus were recorded with suction electrodes, following stimulation of the trigeminal ganglion. Recordings from the main sensory nucleus showed a postsynaptic response was first present at E15. At E16, bath application of AP5 and DNQX showed that the response contained both NMDA and AMPA compone nts, with NMDA predominating (75%). The NMDA : AMPA ratio remained high unt il P1, then gradually declined to 50% by P6. The postsynaptic response was also reduced by bath application of bicuculline, indicating the presence of a GABA(A)-mediated excitatory component. GABAergic excitation was present at all ages but was maximal from E20 to P1, the age at which whisker-relate d patterns are developing. It is hypothesized that both GABAergic excitatio n and NMDA receptor activation play a role in the consolidation of trigemin al connections, and are thus important in the development of whisker-relate d patterns.