Endocannabinoid neuromodulation in the neostriatum decreases the GABAergic striato-nigral disinhibitory function and increases the nigro-collicular inhibitory pathway activity.

We previously reported the involvement of neostriato-nigral projections in the organisation of innate fear and panic attack-like responses organised by dorsal midbrain neurons, such as the periaqueductal grey matter and the deep layers of the superior colliculus (dlSC). In addition, several lines of evidence have demonstrated that cannabinoid receptor type 1 is found in the neostriatum (caudate nucleus and putamen; CPu). In the present study, we investigated the role of endocannabinoid neuromodulation in CPu in the expression of unconditioned fear-related behavioural responses elicited by microinjections of the γ-aminobutyric acid (GABA)A receptor selective antagonist bicuculline (BIC) in the dlSC. Wistar rats received injection of vehicle or anandamide (AEA) at 0.5, 5, 50, 100 pmol in CPu, followed by injections of BIC in a dose of 40 ng in the dlSC. The treatment of the CPu with AEA in a dose of 5 and 50 pmol attenuated the unconditioned fear-related behaviour, such as defensive alertness, defensive immobility and escape, induced by GABAA receptor blockade in dlSC. These findings suggest that endogenous cannabinoids acting on CPu neurons exert an indirect modulatory influence on the activity of superior colliculus neurons, possibly through an inhibitory activity on neostriato-nigral disinhibitory connections that modulate the nigro-collicular inhibitory GABAergic pathways.