Fos activation and upregulation of nicotinamide adenine dinucleotide phosphate diaphorase in the rat pituitary by acute capsaicin injection

The endocannabinoid, endovanilloid and nitrergic systems could interact in the rat dorsolateral periaqueductal gray matter to control anxiety-like behaviors

Cannabinoid compounds usually produce biphasic effects in the modulation of emotional responses. Low doses of the endocannabinoid anandamide (AEA) injected into the dorsolateral periaqueductal gray matter (dlPAG) induce anxiolytic-like effects via CB1 receptors activation. However, at higher doses the drug loses this effect, in part by activating Transient Receptor Potential Vanilloid Type 1 (TRPV1). Activation of these latter receptors could induce the formation of nitric oxide (NO). Thus, the present study tested the hypothesis that at high doses AEA loses it anxiolytic-like effect by facilitating, probably via TRPV1 receptor activation, the formation of NO. Male Wistar rats received combined injections into the dlPAG of vehicle, the TRPV1 receptor antagonist 6-iodo-nordihydrocapsaicin or the NO scavenger carboxy-PTIO (c-PTIO), followed by vehicle or AEA, and were submitted to the elevated plus maze (EPM) or the Vogel conflict test (VCT). A low dose (5pmol) of AEA produced an anxiolytic-like effect that disappeared at higher doses (50 and 200pmol). The anxiolytic-like effects of these latter doses, however, were restored after pre-treatment with a low and ineffective dose of c-PTIO in both animal models. In addition, the combined administration of ineffective doses of 6-iodo-nordihydrocapsaicin (1nmol) and c-PTIO (0.3nmol) produced an anxiolytic-like response. Therefore, these results support the hypothesis that intra-dlPAG injections of high doses of AEA lose their anxiolytic effects by favoring TRPV1 receptors activity and consequent NO formation, which in turn could facilitate defensive responses.

Cannabinoid modulation of predator fear: involvement of the dorsolateral periaqueductal gray

The present study investigated the effects of systemic or intra-dorsolateral periaqueductal gray (dlPAG) administration of CB1 agonists on behavioural changes induced in rats by predator (a live cat) exposure, a model of panic responses. Since nitric oxide (NO) and cannabinoid neurotransmission are proposed to interact in the dlPAG to modulate defensive responses, we also investigated if NO is involved in the biphasic effects of anandamide (AEA) injected into the dlPAG. The results showed that systemic administration of WIN55,212-2 or intra-dlPAG AEA attenuated the defensive behaviours caused by cat exposure. Both compounds produced biphasic curves. The cannabinoid receptor type 1 (CB1) antagonist AM251 prevented the panicolytic effect of AEA whereas a neuronal NOS inhibitor turned the ineffective high dose of AEA into an effective one. These results suggest that modulation of the cannabinoid system could be a target in the treatment of panic disorders. However, the biphasic effects of these compounds could limit their therapeutic potential.

Activity-dependent heterogeneous populations of nitric oxide synthase neurons in the rat dorsal raphe nucleus

The brainstem dorsal raphe nucleus (DRN) contains an abundant distribution of nitric oxide (NO) synthase (NOS)-containing neuronal profiles in two distinct populations: faint- and intense-immunoreactive cells in midline (ventromedial and dorsomedial) and lateral wing subregions, respectively. This study tested the hypothesis that different functional dynamics underlie the topography of NOS-containing cells in the DRN rostrocaudal and mediolateral neuraxis by using a capsaicin challenge paradigm (50 mg/kg, subcutaneous). Compared with vehicle, capsaicin significantly and preferentially increased nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d, an index of constitutive NOS) reactivity in the rostral midline and caudal lateral wing subregions. Furthermore, capsaicin activated more Fos-positive cells than vehicle within all subregions of the DRN but with a caudal versus rostral predominance in activation pattern. In addition, a high proportion of capsaicin-induced Fos cells in the midline but almost none in lateral wing stained for NADPH-d. These observations suggest the existence of two functionally distinct populations of NOS neurons in the DRN. Furthermore, capsaicin increased galanin immunoreactivity with predominant staining in cell soma and fiber processes in midline and lateral wing subregions of the nucleus, respectively. The total capsaicin-induced galanin immunoreactivity was higher in rostral versus caudal DRN, and a high proportion of galanin-positive cells in the midline also contained NADPH-d and neuronal NOS, thus suggesting a potential NO-galanin interaction in these neurons. The differential pattern of Fos/NADPH-d colocalization across the nucleus suggests that midline and lateral wing NOS neurons of the DRN express their neuromodulatory actions on discrete efferent targets via different intracellular mechanisms.

Capsaicin increases GFAP and glutamine synthetase immunoreactivity in rat arcuate nucleus and median eminence

Antibodies to glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS) were used to determine the effect of s.c. capsaicin (after 75 min) on astroglial cells in the rat arcuate nucleus-median eminence (ARC-ME). Compared to vehicle, capsaicin significantly increased GFAP and GS immunoreactivity in the ARC-ME. Co-localization of GFAP and GS was observed in the ARC-ME complex. Since GS is primarily responsible for glutamate-glutamine metabolism, the increase in total immunostaining for GFAP-and GS- staining suggests a functional adjustment to cope with some of the capsaicin-induced effects. Together with the involvement of nitric oxide synthase in the ARC-ME response to capsaicin, these observations indicate activity-dependent plasticity of the neuron-glia network in response to this stressful/noxious stimulus.