Chemical Coding of Neurotransmission

National Institute Of Mental Health

Investigators

Linked publications & trials

Abstract

The work of the SMN from October 2024 to September 2025 has been to extend two major projects leveraging IEG signature analysis to link cellular plasticity to functional behavioral outcomes initiated by stress or pain, and continue an additional project, again using IEG signature analysis, to link cellular plasticity to altered pain sensitization in the context of chronic nerve injury. In collaboration with NIMH-IRP’s Unit on Neuromodulation and Synaptic Integration (H. Tejeda, PI), and Tel Aviv University’s Laboratory of Neuroscience (U. Ashery, PI) we have linked RapGEF2-ERK-Egr1 signaling in hippocampus to context-associated fear conditioning, and in basolateral amygdala (BLA) to cue-associated fear conditioning after prior restraint stress (stress-augmented fear learning). The IEG signature for both types of fear conditioning was conditioning-specific Egr1 and Fos induction. Ablation of RapGEF2 expression in CA1 and CA3 of hippocampus, and in BLA, resulted in loss of ERK phosphorylation and Egr1 induction within a subset of CA1 pyramidal neurons in hippocampus (but not in BLA), during context-dependent, but not cue (tone)-dependent learning (freezing to conditioned stimulus), with a parallel loss of context-dependent, but not cue-dependent learning. Augmentation of fear (shock) learning with prior restraint stress resulted in ERK phosphorylation and Egr1 induction in both hippocampus and BLA that was impaired by deletion of RapGEF2, and correlated in this case with parallel impairment in cue-dependent learning. Notably, Fos induction was spared by deletion of RapGEF2 in all cases, indicating that parcellation of induction of the two IEGs Fos and Egr1, first uncovered in chronic psychomotor stimulant administration associated with learned responses to drug administration (Jiang et al., J. Neurosci. 41: 711, 2021), also occurs in the context of fear learning. This project was published in Cellular and Molecular Life Sciences (Jiang et al., Cell Mol. Life Sci. 81: 48, 2024). This work continues with Dr. Hai-ying Zhang's investigations into cAMP-dependent IEG induction after cocaine, in ventral striatum, using novel genetic tools for manipulation of cAMP levels in D1-dopaminoceptive neurons of nucleus accumbens, and effects on cocaine-dependent behaviors. This work by Zhang et al. is now posted in bioRxiv, and is under peer review at Journal of Neurochemistry. A project primarily initiated by Staff Scientist Sunny Jiang has now been completed and published, and is being continued in order to extend findings to other stressors besides restraint. Specific deletion of PACAP from CaMK-expressing glutamatergic neurons of forebrain abolished Fos induction in PVN but not limbic brain areas including extended amygdala, and impaired endocrine but not behavioral effects of restraint stress. Delection of PACAP specifically in prefrontocortical-to-hypothalamic projection neurons phenocopied PACAP knockout from forebrain glutamatergic neurons, establishing the existence of a novel frontocorticohypothalamic projection system, in which PACAP plays a first messenger role in eliciting endocrine responses to restraint stress. Specific deletion of PACAP from parabrachioamygdalar projections, on the other hand, abolished Fos induction in extended amygdala but not PVN, and impaired behavioral but not endocrine effects of restraint stress, while chemogenetic silencing of the targets these projections in CeC resulted in a similar impairment in stress-induced hypophagia. Thus, two separate brain circuits, each requiring PACAP for first-messenger IEG induction, mediate endocrine and behavioral responses to restraint stress (Jiang et al., Biol. Psychiatr. GOS, 3: 673, 2023). Further, induction of the IEG Egr1/Zif268, which also occurs in PVN and CeC following restraint stress, is unaffected by PACAP deletion in either of these two circuits, demonstrating that IEG induction in these stress circuits, as in NAc after psychomotor stimulant administration, is parcellated within distinct intracellular signaling pathways. The role of Egr1/Zif268 in mediating other aspects of the stress response within these circuits awaits elucidation. The project has been extended to analysis of the role of PACAP in supporting PVN-dependent grooming behavior after footshock in mice. This project has been continued into identification of the role of PACAP in a parabrachioamygdalar projection mediating behavioral responses to stress, an aversive stimulus, in collaboration with an NCCIH intramural laboratory which studies this pathway in the context of pain syndromes elicited by nerve injury. The Carrasquillo lab has previously shown that the same PKCdelta-expressing target neuronal population in CeC involved in stress responding undergoes cellular plasticity leading to altered electrophysiological responsiveness that mediates pain sensitization after chronic peripheral nerve injury (see Wilson et al., Cell Reports 29: 332, 2019). Accordingly, pain sensitization after chronic nerve injury was examined in wild-type mice and mice deficient in PACAP expression specifically within the parabrachioamygdalar projection/circuit. A requirement for PACAP expression within these third order neurons, for development of nerve injury-induced allodynia, has been demonstrated (Sudhuman et al., Soc. Neurosci. Abstract, 2024). These observations have led to further investigation of the mechanism whereby PACAP neurotransmission enables intracellular signaling for cellular plasticity within target neurons of the central amygdala causing response to pain sensation. Publications from the SMN and our collaborators in this calendar year, and the last, strengthen the case for specific role(s) of neuropeptide co-transmission in neuronal cellular plasticity underlying stress responding, link this role to Fos induction in neurons engaged in the stress response, and lay a foundation for more detailed examination of the PACAP-dependent post-synaptic events that occur at circuits of stress required to complete their tasks as behavioral and endocrine stress transducers in the brain.