Ghrelin potentiates cardiac reactivity to stress by modulating sympathetic control and beta-adrenergic response.

Prior evidence indicates that ghrelin is involved in the integration of cardiovascular functions and behavioral responses. Ghrelin actions are mediated by the growth hormone secretagogue receptor subtype 1a (GHS-R1a), which is expressed in peripheral tissues and central areas involved in the control of cardiovascular responses to stress. AIMS: In the present study, we assessed the role of ghrelin - GHS-R1a axis in the cardiovascular reactivity to acute emotional stress in rats. MAIN METHODS AND KEY FINDINGS: Ghrelin potentiated the tachycardia evoked by restraint and air jet stresses, which was reverted by GHS-R1a blockade. Evaluation of the autonomic balance revealed that the sympathetic branch modulates the ghrelin-evoked positive chronotropy. In isolated hearts, the perfusion with ghrelin potentiated the contractile responses caused by stimulation of the beta-adrenergic receptor, without altering the amplitude of the responses evoked by acetylcholine. Experiments in isolated cardiomyocytes revealed that ghrelin amplified the increases in calcium transient changes evoked by isoproterenol. SIGNIFICANCE: Taken together, our results indicate that the Ghrelin-GHS-R1a axis potentiates the magnitude of stress-evoked tachycardia by modulating the autonomic nervous system and peripheral mechanisms, strongly relying on the activation of cardiac calcium transient and beta-adrenergic receptors.

5-HT1A receptors of the rat dorsal raphe lateral wings and dorsomedial subnuclei differentially control anxiety- and panic-related defensive responses.

The dorsal raphe nucleus (DR), the main source of 5-HT projections to brain areas involved in anxiety regulation, is composed by 5 subnuclei that differ morphologically, functionally and neurochemically. Based on immunohistochemical evidence, it has been proposed that whereas 5-HT cells of the dorsomedial (dmDR) and caudal subnuclei are implicated in the pathophysiology of generalized anxiety disorder (GAD), neurons of the lateral wings (lwDR) are associated with panic disorder (PD). We here tested this hypothesis from a behavioral perspective by investigating the consequences of the non-selective stimulation of neurons within the dmDR and lwDR, or the pharmacological manipulation of 5-HT1A receptors located in these nuclei, of male Wistar rats exposed to the elevated T-maze. This test allows the measurement of both a GAD- (i.e. inhibitory avoidance) and a PD- (i.e. escape) related response in the same animal. Intra-dmDR injection of either the excitatory amino acid kainic acid or the 5-HT1A receptor antagonist WAY-100635 facilitated inhibitory avoidance acquisition, suggesting an anxiogenic effect, and inhibited escape expression, a panicolytic-like effect. Microinjection of the 5-HT1A receptor agonist 8-OH-DPAT caused the opposite effect. Administration of the same drugs into the lwDR only altered escape performance. Whereas kainic acid and 8-OH-DPAT facilitated its expression, WAY-100635 inhibited it. At higher doses, kainic acid administration evoked vigorous escape reactions as measured in an open-field. These findings implicate 5-HT neurons of the dmDR in the regulation of both GAD- and PD-related defensive behaviors. They also support a primary role of the lwDR in the mediation of PD-associated responses.

Behavioral consequences of predator stress in the rat elevated T-maze.

Analyses of the behavioral reactions of rodents to predators have greatly contributed to the understanding of defense-related human psychopathologies such as anxiety and panic.We here investigated the behavioral consequences of exposing male Wistar rats to a live cat using the elevated T-maze test of anxiety. This test allows the measurement of two defensive responses: inhibitory avoidance and escape, which in terms of pathology have been associated with generalized anxiety and panic disorders, respectively. For comparative reasons, the effects of exposure to the cat were also assessed in the elevated plus-maze. The results showed that a 5-min exposure to the cat selectively facilitated inhibitory avoidance acquisition, an anxiogenic effect, without affecting escape expression in the elevated T-maze. This was seen immediately but not 30 min after contact with the predator. This short-lived anxiogenic effect was also detected in the elevated plus-maze. Previous administration of the benzodiazepine anxiolytic diazepam (2 mg/kg) decreased the immediate avoidance response to the predator and the neophobic reaction to a dummy cat used as a control stimulus. The drug also impaired inhibitory avoidance acquisition in the elevated T-maze, indicating an anxiolytic effect, without affecting escape performance. The results indicate that the state of anxiety evoked during contact with the predator generalizes to both elevated plus- and T-mazes, impacting on defensive responses associated with generalized anxiety disorder.

The lateral habenula regulates defensive behaviors through changes in 5-HT-mediated neurotransmission in the dorsal periaqueductal gray matter.

Chemical stimulation of the lateral nucleus of the habenula (LHb), an area implicated in the regulation of serotonergic activity in raphe nuclei, affects the acquisition of inhibitory avoidance and escape expression of rats submitted to the elevated T-maze test of anxiety. Here, we investigated whether facilitation of 5-HT-mediated neurotransmission in the dorsal periaqueductal gray (dPAG) accounts for the behavioral consequences in the elevated T-maze induced by chemical stimulation of the LHb. The dPAG in the midbrain, which is innervated by 5-HT fibers originating from the dorsal raphe nucleus (DRN), has been consistently implicated in the genesis/regulation of anxiety- and fear-related defensive responses. The results showed that intra-dPAG injection of WAY-100635 or ketanserin, 5-HT(1A) and 5-HT(2A/2C) receptor antagonists, respectively, counteracted the anti-escape effect caused by bilateral intra-LHb injection of kainic acid (60pmol/0.2microl). Ketanserin, but not WAY-100635, blocked kainic acid's facilitatory effect on inhibitory avoidance acquisition. Overall, the results suggest that the pathway connecting the LHb to the DRN is involved in the control of 5-HT release in the dPAG, and facilitation of 5-HT-mediated neurotransmission in the latter area distinctively impacts upon the expression of anxiety- and fear-related defensive behaviors. While stimulation of 5-HT(1A) receptors selectively affects escape performance, 5-HT(2A/2C) receptors modulate both inhibitory avoidance and escape.

Dorsal raphe nucleus regulation of a panic-like defensive behavior evoked by chemical stimulation of the rat dorsal periaqueductal gray matter.

Electrical or chemical stimulation of the dorsal periaqueductal gray matter (DPAG) evokes escape, a defensive behavior that has been related to panic attacks. Injection of 5-HT(1A) or 5-HT(2A) receptor agonists into this midbrain area inhibits this response. It has been proposed that the impairment of 5-HT mechanisms controlling escape at the level of the DPAG may underlie the susceptibility to panic attacks that characterizes the panic disorder. In this study we evaluated the effects of the pharmacological manipulation of the dorsal raphe nucleus (DRN), which are the main source of 5-HT input to the DPAG, on the escape response evoked in rats by the intra-DPAG injection of the nitric oxide donor SIN-1. The results showed that DRN administration of the 5-HT(1A) receptor agonist 8-OH-DPAT which inhibits the activity of 5-HT neurons favored the expression of escape induced by SIN-1. Intra-DRN injection of the excitatory amino acid kainic acid or the 5-HT(1A) receptor antagonist WAY-100635 did not change escape expression. However, both compounds fully blocked the escape reaction generated by intra-DPAG injection of the excitatory amino acid d,l-homocysteic acid (DLH). Overall, the results indicate that 5-HT neurons in the DRN exert a bidirectional control upon escape behavior generated by the DPAG. Taking into account the effect of WAY-100635 on DLH-induced escape, they also strengthen the view that DRN 5-HT(1A) autoreceptors are under tonic inhibitory influence by 5-HT.

Effects of sibutramine on anxiety-related behaviours in rats.

Sibutramine is an anorexiant drug that inhibits the reuptake of noradrenaline and serotonin, a pharmacological property shared with drugs clinically effective in treating anxiety pathologies. However, the effects of this compound on experimental and clinical anxiety have not been assessed yet. In this study, we evaluated the effects of sibutramine on anxiety-related behaviours which have been related to specific anxiety disorders. Acute injection of sibutramine (5, 10 or 20 mg kg(-1); intraperitoneally) in male Wistar rats impaired inhibitory avoidance in the elevated T-maze (ETM) and in the light/dark transition test, indicative of an anxiolytic effect. The drug also inhibited one-way escape in the ETM. Sibutramine, however, was ineffective in changing rat performance in the elevated plus-maze. Therefore, sibutramine decreased the expression of defensive behaviours that have been associated with generalized anxiety disorder (inhibitory avoidance) and with panic disorder (one-way escape). Yet, in contrast to what has been reported with drugs such as the tricyclic anti-depressants that also inhibit monoamine reuptake, the anxiolytic effects of sibutramine were revealed after a single administration.