Distribution and activation of nitrergic neurons in response to aversive stimulus exposure in the Tropidurus hispidus lizard: Involvement of glutamatergic circuitry.

Introduction: Fear is a response to real aversive stimuli. Studies on phylogenetically distant species like reptiles can offer valuable insights into the neural mechanisms of fear.

Objective: To investigate the activation and distribution of nitrergic neurons in the telencephalon of Tropidurus hispidus lizards and evaluate the role of glutamatergic modulation via NMDA receptors following exposure to an aversive stimulus.

Methods: Lizards were exposed to a live cat, and 24 h later, NADPH-diaphorase histochemistry was performed in the telencephalon to quantify neuron number and optical density. In the second stage, animals received i.c.v. injections of the NMDA receptor antagonist AP5 (1.25, 2.5, or 5.0 μg/μl), followed by cat exposure and histochemical analysis.

Results: Exposure to the cat led to increased freezing time in lizards. These animals also showed an increased number of nitrergic neurons in the dorsal cortex, anterior dorsal ventricular ridge (ADVR), and dorsolateral amygdala, as well as elevated integrated optical density (IOD) in the striatum, ADVR, dorsolateral amygdala and lateral amygdaloid nucleus. The AP5 1.25 and 5.0 μg/μl groups exhibited some head or limb movements even in the presence of the cat. The 1.25 μg/μl group showed reduced neuron counts and IOD in the dorsolateral amygdala; the 2.5 μg/μl and 5.0 μg/μl groups showed reduced IOD and neuron counts in the striatum.

Conclusion: Tropidurus hispidus lizards show fear-like behavior and decreased exploration after aversive stimuli, with nitric oxide in the telencephalon - particularly in the striatum and dorsolateral amygdala - modulating this response via NMDA receptor activation.