Properties and mechanisms of spontaneous activity in the embryonic chick hindbrain

Abstract

Spontaneous activity regulates many aspects of central nervous system development. We demonstrate that in the embryonic chick hindbrain, spontaneous activity is expressed between embryonic days (E) 6-9. Over this period the frequency of activity decreases significantly, although the events maintain a consistent rhythm on the timescale of minutes. At E6, the activity is pharmacologically dependent on serotonin, nACh, GABA(A), and glycine input, but not on muscarinic, glutamatergic, or GABA(B) receptor activation. It also depends on gap junctions, t-type calcium channels and TTX-sensitive ion channels. In intact spinal cord-hindbrain preparations, E6 spontaneous events originate in the spinal cord and propagate into lateral hindbrain tissue; midline activity follows the appearance of lateral activity. However, the spinal cord is not required for hindbrain activity. There are two invariant points of origin of activity along the midline, both within the caudal group of serotonin-expressing cell bodies; one point is caudal to the nV exit point while the other is caudal to the nVII exit point. Additional caudal midline points of origin are seen in a minority of cases. Using immunohistochemistry, we show robust differentiation of the serotonergic raphe near the midline at E6, and extensive fiber tracts expressing GAD65/67 and the nAChR in lateral areas; this suggests that the medial activity is dependent on serotonergic neuron activation, while lateral activity requires other transmitters. Although there are differences between species, this activity is highly conserved between mouse and chick, suggesting that developmental event(s) within the hindbrain are dependent on expression of this spontaneous activity.