Fear is induced by innate and learned mechanisms involving separate pathways. Here, we used an olfactory-mediated innate-fear versus learned-fear paradigm to investigate how these pathways are integrated. Notably, prior presentation of innate-fear stimuli inhibited learned-freezing response, but not vice versa. Whole-brain mapping and pharmacological screening indicated that serotonin-2A receptor (Htr2a)-expressing cells in the central amygdala (CeA) control both innate and learned freezing, but in opposing directions. In vivo fiber photometry analyses in freely moving mice indicated that innate but not learned-fear stimuli suppressed the activity of Htr2a-expressing CeA cells. Artificial inactivation of these cells upregulated innate-freezing response and downregulated learned-freezing response. Thus, Htr2a-expressing CeA cells serve as a hierarchy generator, prioritizing innate fear over learned fear.

Fyn-tyrosine-kinase-deficient mice exhibit defects in the Morris water maze test and long-term potentiation (LTP) induction in the hippocampus, and given that LTP has been postulated as the neural basis for memory formation, Fyn may be required for hippocampus-dependent memory formation. However, how Fyn is involved in the process of memory formation is unclear. To investigate the role of Fyn in hippocampal memory formation, we first tested the behavior of Fyn-deficient mice by contextual fear conditioning. A mouse was placed in a context and a foot shock was delivered, so that the mouse associated the context with the shock. We found that the freezing response of Fyn-deficient mice to the context was impaired at 24 h after conditioning. We then measured freezing at 1 h after conditioning, and found that their short-term contextual fear memory was also impaired. We used Western blotting to examine the mode of Fyn activation in dorsal hippocampal tissue following contextual fear conditioning. Fyn activation peaked as early as 5-10 min after contextual fear conditioning and persisted for at least 40 min. Concomitant increases in tyrosine phosphorylation of several proteins, including NR2B, were also observed, but no increases in tyrosine phosphorylation were observed in Fyn-deficient mice. Thus, both short-term and long-term (24-h) contextual fear memory were impaired in Fyn-deficient mice, and Fyn activation in the dorsal hippocampus transiently increased after contextual fear conditioning. These findings strongly suggest that activation of the Fyn signaling pathway is involved in hippocampus-dependent formation of contextual fear memory.

Fyn-mediated tyrosine phosphorylation of N-methyl-D-aspartate (NMDA) receptor subunits has been implicated in various brain functions, including ethanol tolerance, learning, and seizure susceptibility. In this study, we explored the role of Fyn in haloperidolinduced catalepsy, an animal model of the extrapyramidal side effects of antipsychotics. Haloperidol induced catalepsy and muscle rigidity in the control mice, but these responses were significantly reduced in Fyn-deficient mice. Expression of the striatal dopamine D 2 receptor, the main site of haloperidol action, did not differ between the two genotypes. Fyn activation and enhanced tyrosine phosphorylation of the NMDA receptor NR2B subunit, as measured by Western blotting, were induced after haloperidol injection of the control mice, but both responses were significantly reduced in Fyndeficient mice. Dopamine D 2 receptor blockade was shown to increase both NR2B phosphorylation and the NMDA-induced calcium responses in control cultured striatal neurons but not in Fyndeficient neurons. Based on these findings, we proposed a new molecular mechanism underlying haloperidol-induced catalepsy, in which the dopamine D 2 receptor antagonist induces striatal Fyn activation and the subsequent tyrosine phosphorylation of NR2B alters striatal neuronal activity, thereby inducing the behavioral changes that are manifested as a cataleptic response.Typical antipsychotic agents, such as haloperidol and chlorpromazine, have extrapyramidal side effects (EPS) 2 that resemble Parkinson disease. Drug-induced catalepsy, the impairment of movement initiation, in rodents is an animal model of EPS and is mainly caused by blockade of the dopamine D 2 receptor (D 2 -R) (1, 2).Haloperidol-induced responses are also dependent on N-methyl-Daspartate receptor (NMDA-R) activity, because prior administration of the NMDA-R antagonist MK-801 attenuates haloperidol-induced catalepsy (3, 4). D 2 -R and NMDA-R are co-expressed in close proximity along the dendrites of medium spiny neurons in the striatum, and they are functionally coupled in terms of controlling extrapyramidal functions (5).The NMDA-Rs are hetero-oligomeric ligand-gated ion channels composed of a single NR1 subunit and one type of NR2 (A-D) subunit (6). The most abundant receptor subunits in the striatum are NR1, NR2A, and NR2B (7, 8). These three subunits are involved in extrapyramidal functions (5), and we have found that an NR2B-selective antagonist attenuates haloperidol-induced catalepsy (9).Phosphorylation of tyrosine residues on the NMDA-R has been reported to modulate its channel characteristics (10, 11). Depriving the striatum of dopaminergic input increases the tyrosine phosphorylation of the striatal NMDA-R and the motor response (12, 13), but infusing the striatum with a tyrosine kinase inhibitor, genistein, attenuates both the tyrosine phosphorylation and the motor response induced by dopaminergic deprivation (13).Fyn is a member of the Src family kinases (SFKs) and is associated with the NMDA-R at postsynaptic d...