Embryonic spinal cord neurons develop preferential cholinergic projections to sympathetic ganglia explanted from appropriate levels of the neuraxis

To determine whether cholinergic spinal cord neurons can develop preferential projections in vitro within sympathetic ganglia (SGs) of appropriate levels of the neuraxis, organotypic explants of fetal mouse spinal cord (E13) from cervical, thoracic (upper and lower) and lumbar segments were co-cultured with either pairs of neonatal SGs: the rostral superior cervical ganglion (SCG) or a caudally located upper lumbar ganglion (LG). After 3.5-4 weeks of co-culture, levels of the enzyme, choline acetyltransferase (ChAT), were measured in individual spinal cord explants and SCGs or double LGs (to match the target mass of a single SCG). Interaction was assumed to occur primarily between an SCG or LG doublet and the adjacent ipsilateral half of the co-cultured cord segment. An index of cholinergic interaction was defined as the ganglion ChAT activity per unit ChAT activity in half co-cultured cord segment. The index of interaction with the SCG was highest with the T₁/T₂ (1.4) as compared with the T₁₀/T₁₁ (0.79). L₁/L₂ (0.38) and C₂/C₃ (0.11) segments. In contrast, the index of cholinergic interaction with double LGs was highest with the more caudally located T₁₀/T₁₁ (0.62) cord segment as compared with the rostral T₁/T₂ (0.33), cervical C₂/C₃ (0.2) and lumbar L₁/L₂ (0.17) segments. Ganglion compound action potentials evoked in LGs by stimulation of the ipsilateral portion of T₁₀/T₁₁ cord were blocked by the ganglionic antagonist, hexamethonium, as previously observed in co-cultures of SCGs with T₁/T₂ cord. These results indicate that pools of preganglionic neurons in thoracic cord segments can develop in vitro preferential cholinergic projections within SGs of appropriate position. Cervical and lumbar cord segments which contain a preponderance of somatic motoneurons over preganglionic neurons did not interact as effectively with either type of SG. The preferential cholinergic projections from rostral thoracic cord explants within co-cultured SCGs and from caudal thoracic cord explants within LGs may reflect some degree of positional preference intrinsic to embryonic spinal cord neurons and/or their appropriate target SGs, consistent with the positional specificity expressed by preganglionic neurons and SGs in situ.