Dorsal horn neurons presynaptic to lamina I spinoparabrachial neurons revealed by transynaptic labeling.

Sensory input to supraspinally projecting lamina I (LI) neurons arises both directly from primary afferents and via neurons intrinsic to the spinal dorsal horn. The types of neurons presynaptic to those projection neurons remain poorly known. To address this question we used retrogradely transported adenoviral vectors encoding green fluorescent protein (GFP) and a GFP-TTC (fragment C of the tetanus toxin) fusion protein, labeling respectively spinoparabrachial projection neurons and neurons presynaptic to them. The expression of GFP by infected neurons labeled the entire dendritic tree, enabling a more complete and quantitative morphological description of spinoparabrachial neurons than previous methods. These neurons were located in spinal LI, with dendritic arbors oriented extensively in the rostrocaudal axis (1,089.8 +/- 91.5 microm) and displaying low spine density. In contrast, their dendrites did not extend significantly ventrally (29.2 +/- 3.5 microm). The use of transynaptic tracer GFP-TTC revealed a population of local circuit LI neurons presynaptic to LI projection neurons. These local circuit LI neurons had distinct morphological properties, in particular significantly longer ventrally oriented dendrites (80.1 +/- 10.1 microm). The transynaptic tracer also revealed a population of stalked cells, some being highly spiny, directly in contact with spinal projection neurons. However, stalked cells were not the only lamina II cells in direct contact with projection neurons. Intracellular injections with Lucifer yellow in parasagittal slices of fixed tissue confirmed the above observations. Overall, these experiments demonstrated that neurons projecting to the parabrachial nucleus had their dendritic branching almost exclusively in LI and had sparse dendritic spines, in contrast with local circuit neurons that often extended ventrally and could be very spiny.