Aim: 

Protohormones like PACAP-38 are known to be involved in the learning processes in both Invertebrates and Vertebrates. We have analysed the effects of PACAP-38 in the onset of non associative learning in the leech H. medicinalis. Our study has been carried out at both cellular and behavioural level.

Methods: 

Segmental ganglia dissected out from adult leeches were used to perform intracellular recordings from T sensory neurons. These cells show an AHP during the firing whose amplitude can be modulated to induce events of synaptic plasticity. Leeches with the first segmental ganglion disconnected from the cephalic ganglion respond to weak electrical stimuli (activating T cells) with a swim cycle. We have measured the time between the stimulus application and the onset of the swim (latency) as an index of the changes in the swim induction.

Results: 

Five minutes 5·10^-3 nM PACAP-38 extracellular perfusion reduced significantly the AHP amplitude without affecting the input resistance. This effect was still present also after the perfusion with 1 nM apamine, a blocker of the K⁺/Ca²⁺-operated channels which partially determine the AHP. This suggests that PACAP-38 affected the Na⁺ /K⁺-ATPase activity which is the main determinant of the AHP in T cells. Moreover, PKA activity seems to be unnecessary in the PACAP action.

Behavioural observations showed that low concentrations of PACAP-38 (0.01 nM-1nM) induced sensitisation of swim induction in a dose-dependent manner.

Conclusion: 

PACAP-38 seems to modulate cellular mechanisms involved in learning processes in the leech as well in other invertebrates and vertebrates models.