Effects of anxiolytic drugs on escape behavior induced by dorsal central gray stimulation in rats

Effects of Withania somnifera on oral ethanol self-administration in rats

Recent evidence has shown that Withania somnifera Dunal (Ashwagandha or Indian ginseng), a herbal remedy used in traditional medicine, impairs morphine-elicited place conditioning. Here, we investigated the effect of W. somnifera roots extract (WSE) on motivation for drinking ethanol using operant self-administration paradigms. Wistar rats were trained to self-administer ethanol (10%) by nose-poking. The effects of WSE (25-75 mg/kg) were evaluated on acquisition and maintenance, on ethanol breakpoint under a progressive-ratio schedule of reinforcement and on the deprivation effect and reinstatement of seeking behaviours. Moreover, on the basis of the recent suggestion of an involvement of GABAB receptors in WSE central effects, we studied the interaction between WSE and GABAB ligands. The effect of WSE on saccharin (0.05%) oral self-administration was also tested. The results show that WSE reduced the acquisition, maintenance and breakpoint of ethanol self-administration. WSE also reduced the deprivation effect, reinstatement of ethanol-seeking behaviours and saccharin reinforcement. Furthermore, the GABAB receptor antagonist, phaclofen, counteracted the ability of WSE to impair the maintenance of ethanol self-administration. These findings show that WSE, by an action that may involve GABAB receptors, impairs motivation for drinking ethanol and suggest that further investigations should be performed to determine whether W. somnifera may represent a new approach for the management of alcohol abuse.

Examining face and construct validity of a noninvasive model of panic disorder in Lister-hooded rats

RATIONALE

Increasing evidence suggests that defensive escape behavior in Lister-hooded (LH) rats induced by ultrasound application may be an animal model of panic disorder.

OBJECTIVE

The objectives of this study were to further explore the face and construct validity of ultrasound-induced escape behavior by characterizing the autonomic and neuroendocrine response to ultrasound, and to examine the underlying neuronal structures by comparing the effects of the anxiolytic with panicolytic properties, diazepam, with a preclinical anxiolytic without panicolytic-like activity, the NOP agonist Ro 64-6198.

MATERIALS AND METHODS

LH rats were implanted with telemetry transmitters to monitor heart rate and core body temperature before, during, and after ultrasound application. Blood samples were taken after ultrasound application for corticosterone analysis. Ultrasound-induced c-Fos expression was measured in different periaqueductal gray (PAG) and amygdala subregions after treatment with diazepam or Ro 64-6198.

RESULTS

Ultrasound application increased heart rate and body temperature, but did not alter plasma corticosterone levels. Ultrasound application increased c-Fos expression in the dorsal and dorsolateral PAG (dPAG, dlPAG) and amygdaloid subregions. Diazepam, but not Ro 64-6198, reduced c-Fos expression in the dPAG/dlPAG, while Ro 64-6198, but not diazepam, reduced c-Fos expression in the central amygdala.

CONCLUSIONS

Similar to human panic attacks, ultrasound application to LH rats activated the autonomic, but not the neuroendocrine, stress system. Also, like in humans, the current data confirm and extend that the dPAG/dlPAG plays a key role in ultrasound-induced escape behavior. These observations suggest that ultrasound-induced escape behaviors in LH rats have face and construct validity for panic disorders.

Evidence that NMDA-dependent limbic neural plasticity in the right hemisphere mediates pharmacological stressor (FG-7142)-induced lasting increases in anxiety-like behavior: study 3--the effects on amygdala efferent physiology of block of NMDA receptors prior to injection of FG-7142 and its relationship to behavioral change

The findings of this study support the hypothesis that N-methyl-D-aspartate (NMDA) receptors mediate the initiation of long-term potentiation (LTP) and behavioral changes induced by the anxiogenic beta-carboline, FG-7142. Unlike previous work, this study examined the effects of FG-7142 on LTP of amygdala efferents in both hemispheres. 7-amino-phosphono-heptanoic acid (AP7), a competitive NMDA receptor blocker, given prior to administration of FG-7142, prevented LTP in amygdala efferent transmission to the medial hypothalamus and periacqueductal gray (PAG). When given FG-7142 alone, cats showed lasting behavioral changes accompanied by LTP in all pathways studied. Duration of LTP, and its relationship to behavioral change, depended on the pathway and the hemisphere of the pathway. Correlation and covariance analyses indicate that LTP in the left amygdalo-ventromedial hypothalamic pathway mediates initiation, but not maintenance, of increased defensiveness. This finding replicates previous work. A new finding is that increased local excitability in the right basal amygdala (reduced threshold for evoked response), and LTP in the right amygdalo-PAG pathway, may be important for maintenance of increases in defensive behavior. Furthermore, the effects of flumazenil, a benzodiazepine receptor antagonist, on behavior and physiology single out the importance of right amygdalo-PAG LTP as a critical mediator of increased defensiveness. Flumazenil reversed the increase in defensiveness produced by FG-7142 in a drug-dependent manner as described in Adamec (1998a). Moreover, flumazenil reversed LTP only in the right amygdalo-PAG pathway. The findings of the present study suggest that response to FG-7142 may be a useful model of the effects of traumatic stressors on limbic system function in anxiety, especially in view of the recent data in humans implicating right hemispheric function in persisting negative affective states.

Neurochemical mechanisms of the defensive behavior in the dorsal midbrain

Some regions in the mesencephalon, such as dorsal periaqueductal gray, inferior colliculus and deep layers of superior colliculus have been grouped together as a continuous strip of midbrain structures involved in the integration of the different components of aversive states in the brain. In fact, escape behavior and defensive, or fear-like behavior often result when these sites are electrically or chemically stimulated. Moreover, the behavioral responses induced by stimulation of these structures are, in general, accompanied by increases in mean arterial blood pressure, heart rate and respiration, and by analgesia. Both the behavioral and autonomic consequences of electrical stimulation of the mesencephalic tectum was shown to be attenuated by minor tranquilizers, probably through enhancement of GABAergic neurotransmission. Besides GABAergic interneurons which exert a tonic inhibitory control on neural circuits responsible for the behavioral correlates of the aversion in the above-mentioned structures, several other mechanisms such as opioid, neuropeptides, serotonergic and excitatory amino acids have also been implicated in the regulation of these processes. As to the analgesia that accompanies these aversive states it is mediated by non-opioid mechanisms, particularly by serotonergic ones through 5-HT2 receptors. Now, efforts have been made to characterize the mode of action of these neurotransmitters on their multiple receptors and how they interact with each other to produce or regulate the neural substrates of aversion in the midbrain.

Functional characteristics of the midbrain periaqueductal gray

The major functions of the midbrain periaqueductal gray (PAG), including pain and analgesia, fear and anxiety, vocalization, lordosis and cardiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferents from nociceptive neurons in the spinal cord and sends projections to thalamic nuclei that process nociception. The PAG is also a major component of a descending pain inhibitory system. Activation of this system inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety produced by stimulation of amygdala. Stimulation of PAG produces vocalization and its lesion produces mutism. The firing of many cells within the PAG correlates with vocalization. The PAG is a major site for lordosis and this role of PAG is mediated by a pathway connecting the medial preoptic with the PAG. The cardiovascular controlling network within the PAG are organized in columns. The dorsal column is involved in pressor and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the current information about the anatomy, physiology, and pharmacology of the PAG, a model is proposed to account for the interactions between these different functional components.

Neural substrate of defensive behavior in the midbrain tectum

It has been shown that the gradual increase in the intensity of electrical stimulation of the dorsal periaqueductal gray (DPAG), deep layers of the superior colliculus (DLSC) and inferior colliculus of rats induces, in a progressive manner, characteristic aversive responses such as arousal, freezing, and escape behavior. The DPAG-DLSC together with the periventricular gray substance of the diencephalon, amygdala and the inferior colliculus, constitute the neural substrate of aversion in the brain. In general, the behavioral responses induced by midbrain tectum stimulation are accompanied by increases in the mean arterial blood pressure, heart rate, and respiration. Both the behavioral and autonomic consequences of electrical stimulation of the mesencephalic tectum have been shown to be attenuated by minor tranquilizers, probably through enhancement of GABAergic neurotransmission. Besides GABAergic mechanisms several lines of evidence have clearly implicated opioid, serotonergic, and excitatory amino acids-mediated mechanisms in the control of the neural substrates commanding defensive behavior in the brain aversive system.

Mechanisms of defense in the inferior colliculus

The inferior colliculus (IC) is a well known relay station for auditory pathways in the brainstem. In the present review we are suggesting that aversive states are also generated and elaborated in the inferior colliculus and that this structure may be part of a brain system commanding defensive behavior. The evidences presented in this review have been obtained from experiments carried out with the combined use of intracerebral microinjections and of electrical stimulation of the inferior colliculus. This electrical stimulation caused a behavioral activation together with autonomic reactions usually observed as part of the defense reaction. NMDA--an excitatory amino acid--, or bicuculline--a GABAA antagonist--injected into the IC mimicked the effects of its electrical stimulation. The IC electrical stimulation showed clear aversive properties as rats submitted to a switch-off paradigm quickly learned to interrupt it. Systemic administration as well as IC microinjections of the anxiolytic compound midazolam caused dose-dependent increases in the latency and reductions in the frequency of switch-off responses to the inferior colliculus electrical stimulation. Similar results were obtained following microinjections into this brainstem structure of the GABAA agonist muscimol. These results suggest that neural substrates responsible for defensive behavior in the inferior colliculus may be depressed by benzodiazepines as part of the anxiolytic action of these compounds. This anti-aversive action may be produced by the enhancement of GABAA mechanisms. Serotonergic mechanisms seem also to be involved in the modulation of these aversive states as IC microinjections of zimelidine, a 5-HT uptake blocker, caused a significant inhibition of the switch-off responses in the shuttle-box.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiaversive action of benzodiazepines on escape behavior induced by electrical stimulation of the inferior colliculus

In the present work, evidence is presented for the involvement of inferior colliculus in the generation and elaboration of aversive responses which suggests that this structure may be part of a brain system that commands aversive states. Electrical stimulation of the inferior colliculus of rats placed inside an open field allowed the determination of thresholds for the escape response. Afterward these rats were placed inside a shuttle box and submitted to a switch-off paradigm. Electrical stimulation of the inferior colliculus was applied at a current intensity 5% below the escape threshold. This electrical stimulation showed clear aversive properties: the rats quickly learned to interrupt it. Systemic administration (3 and 5.6 mg/kg) as well as inferior colliculus microinjections (10 and 20 nmol) of the anxiolytic compound midazolam caused dose-dependent increases in the latency and reductions in the frequency of switch-off responses to the inferior colliculus electrical stimulation. Similar results were obtained following microinjections into this brainstem structure of the GABA-A agonist muscimol (0.1 and 0.5 nmol). These results suggest that neural substrates commanding defensive behavior in the inferior colliculus may be depressed by benzodiazepines as part of the anxiolytic action of these compounds. This antiaversive action may be produced by the enhancement of GABA-A mechanisms.