The axotomy-induced neuropeptides galanin and pituitary adenylate cyclase-activating peptide promote axonal sprouting of primary afferent and cranial motor neurones

Regulation of neuronal pituitary adenylate cyclase-activating polypeptide expression during culture of guinea-pig cardiac ganglia

The trophic neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) increases in many different neuron types following injury; a response postulated to support cell survival and regeneration. In acutely isolated cardiac ganglia, approximately 1% of the cardiac neurons exhibited PACAP immunoreactivity whereas after 72 h in culture, approximately 25% of the neurons were PACAP immunoreactive. In contrast, there was no increase in vasoactive intestinal polypeptide (VIP)-immunoreactive (IR) cells. Using a combination of immunocytochemical and molecular techniques, we have quantified PACAP expression, during explant culture of guinea-pig cardiac ganglia. Using real time polymerase chain reaction, PACAP transcript levels increased progressively up to 48 h in culture with no further increase after 72 h. PACAP transcript levels were reduced by neurturin at 48 h in culture but not after 24 or 72 h in culture. In addition, neurturin partially suppressed the percentage of PACAP-IR neurons after 72 h in culture, an effect mediated by activation of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase signaling pathways. The addition of different known regulatory molecules, including ciliary neurotrophic factor (CNTF), interleukin-1 beta (Il-1beta), tumor necrosis factor-alpha (TNFalpha), fibroblast growth factor basic (bFGF), transforming growth factor-beta (TGF-beta) and nerve growth factor (NGF) did not increase the percentage of PACAP-IR neurons after 24 h in culture; a result indicating that the generation and secretion of these factors did not stimulate PACAP expression. The presence of 20 nM PACAP or 10 muM forskolin increased the percentage of PACAP-IR cardiac neurons in 24 h cultures, but not in 72 h cultures. Neither treatment enhanced the number of VIP-IR neurons. The addition of the PACAP selective receptor (PAC(1)) receptor antagonist, M65 (100 nM) suppressed the 20 nM PACAP-induced increase in percentage of PACAP-IR cells in 24 h cultures indicating the effect of PACAP was mediated through the PAC(1) receptor. However, 100 nM M65 had no effect on the percentage of PACAP-IR cells in either 24 or 48 h cultures not treated with exogenous PACAP, suggesting that endogenous release of PACAP likely did not contribute to the enhanced peptide expression. We postulate that the enhanced PACAP expression, which occurs in response to injury is facilitated in the explant cultured cardiac ganglia by the loss of a target-derived inhibitory factor, very likely neurturin. In intact tissues the presence of neurturin would normally suppress PACAP expression. Lastly, our results indicate that many common trophic factors do not enhance PACAP expression in the cultured cardiac neurons. However, the stimulatory role of an, as yet, unidentified factor cannot be excluded.

Peripheral nerve injury induces reorganization of galanin-containing afferents in the superficial dorsal horn of monkey spinal cord

Peripheral nerve injury-induced structural and chemical modifications of the sensory circuits in the dorsal horn of the spinal cord contribute to the mechanism of neuropathic pain. In contrast to the topographic projection of primary afferents in laminae I-IV in the rat spinal cord, the primary afferents of Macaca mulatta monkeys almost exclusively project into laminae I-II of the spinal cord. After peripheral nerve injury, up-regulation of galanin has been found in sensory neurons in both monkey and rat dorsal root ganglia. However, the nerve injury-induced ultrastructural modification of galanin-containing afferents in the monkey spinal cord remains unknown. Using immunoelectron microscopy, we found that 3 weeks after unilateral sciatic nerve transection, the number of galanin-containing afferents was increased in ipsilateral lamina II of monkey spinal cord. Branching of these galanin-positive afferents was often observed. The afferent terminals contained a large number of synaptic vesicles, peptidergic vesicles and mitochondria, whereas the number of synapses was markedly reduced. Some of the afferents-enriched microtubules were often packed into bundles. Moreover, galanin-labeling could be associated with endosomal structures in many dendrites and axonal terminals of dorsal horn neurons. These results suggest that peripheral nerve injury induces an expansion of the central projection of galanin-containing afferents in lamina II of the monkey spinal cord, not only by increasing galanin levels in primary afferents but also by triggering afferent branching.