Newly Identified Aplysia SPTR-Gene Family-Derived Peptides: Localization and Function

When
individual neurons in a circuit contain multiple neuropeptides,
these peptides can target different sets of follower neurons. This
endows the circuit with a certain degree of flexibility. Here we identified
a novel family of peptides, the Aplysia SPTR-Gene
Family-Derived peptides (apSPTR-GF-DPs). We demonstrated apSPTR-GF-DPs,
particularly apSPTR-GF-DP2, are expressed in the Aplysia CNS using immunohistochemistry and MALDI-TOF MS. Furthermore, apSPTR-GF-DP2
is present in single projection neurons, e.g., in the cerebral-buccal
interneuron-12 (CBI-12). Previous studies have demonstrated that CBI-12
contains two other peptides, FCAP/CP2. In addition, CBI-12 and CP2
promote shortening of the protraction phase of motor programs. Here,
we demonstrate that FCAP shortens protraction. Moreover, we show that
apSPTR-GF-DP2 also shortens protraction. Surprisingly, apSPTR-GF-DP2
does not increase the excitability of retraction interneuron B64.
B64 terminates protraction and is modulated by FCAP/CP2 and CBI-12.
Instead, we show that apSPTR-GF-DP2 and CBI-12 increase B20 excitability
and B20 activity can shorten protraction. Taken together, these data
indicate that different CBI-12 peptides target different sets of pattern-generating
interneurons to exert similar modulatory actions. These findings provide
the first definitive evidence for SPTR-GF’s role in modulation
of feeding, and a form of molecular degeneracy by multiple peptide
cotransmitters in single identified neurons.