PACAP Enhances Axon Outgrowth in Cultured Hippocampal Neurons to a Comparable Extent as BDNF

<div><p>Pituitary adenylate cyclase-activating polypeptide (PACAP) exerts neurotrophic activities including modulation of synaptic plasticity and memory, hippocampal neurogenesis, and neuroprotection, most of which are shared with brain-derived neurotrophic factor (BDNF). Therefore, the aim of this study was to compare morphological effects of PACAP and BDNF on primary cultured hippocampal neurons. At days <i>in vitro</i> (DIV) 3, PACAP increased neurite length and number to similar levels by BDNF, but vasoactive intestinal polypeptide showed much lower effects. In addition, PACAP increased axon, but not dendrite, length, and soma size at DIV 3 similarly to BDNF. The PACAP antagonist PACAP6–38 completely blocked the PACAP-induced increase in axon, but not dendrite, length. Interestingly, the BDNF-induced increase in axon length was also inhibited by PACAP6–38, suggesting a mechanism involving PACAP signaling. K252a, a TrkB receptor inhibitor, inhibited axon outgrowth induced by PACAP and BDNF without affecting dendrite length. These results indicate that in primary cultured hippocampal neurons, PACAP shows morphological actions via its cognate receptor PAC₁, stimulating neurite length and number, and soma size to a comparable extent as BDNF, and that the increase in total neurite length is ascribed to axon outgrowth.</p></div

PACAP and BDNF comparably increase axon, but not dendrite, length.

<p>Primary hippocampal neurons were cultured with 10 nM PACAP or 2 nM BDNF for 1 to 3 DIV and double-immunostained for pNF and MAP2. (A) Representative pNF- (red) and MAP2- (green) immunostained images of neurons. (B, C) Time-dependent effects of PACAP (closed circles), BDNF (closed triangles), and vehicle (open circles) on axon (B) and dendrite (C) length. Values represent mean ± SEM of 60 neurons from three independent experiments. **<i>P</i> < 0.01, two-way ANOVA followed by Tukey-Kramer test. Scale bar, 20 μm.</p

The effect of TrkB receptor inhibitor K252a on neurite outgrowth.

<p>Primary hippocampal neurons were cultured with 10 nM PACAP or 2 nM BDNF in the presence or absence of 200 nM K252a for 3 DIV and double-immunostained for pNF and MAP2. Representative pNF- (red) and MAP2- (green) immunostained images of neurons (A), axon length (B), and dendrite length (C) were shown. Values represent mean ± SEM of 60 neurons from three independent experiments. **<i>P</i> < 0.01 vs. control, ^##<i>P</i> < 0.01 vs. without K252a, two-way ANOVA followed by Tukey-Kramer test. Scale bar, 20 μm.</p

Comparable effects of PACAP and BDNF treatment on total neurite length in cultured hippocampal neurons at DIV 3.

<p>Primary hippocampal neurons were cultured with PACAP or BDNF for 2 to 3 DIV and immunostained for MAP2. (A) Representative MAP2-immunostained images of neurons at DIV 3. (B) Total neurite length of cultured hippocampal neurons treated with 10 nM PACAP and/or 2 nM BDNF. (C-E) Dose-dependent effects of PACAP (circles) and VIP (triangles) on total neurite length (C), and number of total (D) and primary (E) neurites. Values represent mean ± SEM of 69–75 neurons from three independent experiments. ^$$<i>P</i> < 0.01 vs. control, one-way ANOVA followed by Tukey-Kramer test; **<i>P</i> < 0.01 vs. control, ^##<i>P</i> < 0.01 vs. identical VIP dose, two-way ANOVA followed by Tukey-Kramer test. Scale bar, 20 μm.</p