Gina L. Forster

Laterodorsal tegmental stimulation elicits dopamine efflux in the rat nucleus accumbens by activation of acetylcholine and glutamate receptors in the ventral tegmental area.

Cholinergic and glutamatergic neurons in the laterodorsal tegmentum (LDT) and neighbouring mesopontine nuclei are thought to influence mesolimbic dopaminergic neuronal activity involved in goal‐directed behaviours. We measured the changes in dopamine oxidation current (corresponding with dopamine efflux) in the nucleus accumbens (NAc) in response to electrical stimulation of the LDT using in vivo chronoamperometry in urethane‐anaesthetized rats. LDT stimulation (35 Hz pulse trains for 60 s, 1 s intertrain interval) evoked a three‐component change in dopamine efflux in the NAc: (i) an initial stimulation time‐locked increase in the dopamine signal above baseline, followed by (ii) an immediate decrease below baseline, and thereafter by (iii) a prolonged increase in the dopamine signal above baseline. Intra‐VTA infusion of the nicotinic receptor antagonist mecamylamine (5 μg/0.5 μL) or the ionotropic glutamate receptor antagonist kynurenate (10 μg/μL) attenuated the first LDT‐elicited component. The second suppressive component was abolished by intra‐LDT infusions of either the nonselective or the M2‐selective muscarinic receptor antagonists scopolamine (100 μg/μL) and methoctramine (50 μg/μL), respectively. In contrast, intra‐VTA infusions of scopolamine (200 μg/μL) resulted in a selective attenuation of the third facilitatory component, whereas both second and third components were abolished by systemic injections of scopolamine (5 mg/kg). These results suggest that the initial increase, subsequent decrease, and final prolonged increase in extracellular dopamine levels in the NAc are selectively mediated by LDT‐elicited activation of (i) nicotinic and glutamatergic receptors in the VTA, (ii) muscarinic M2 autoreceptors on LDT cell bodies, and (iii) muscarinic receptors in the VTA, respectively.

Stress coping style predicts aggression and social dominance in rainbow trout

Social stress is frequently used as a model for studying the neuroendocrine mechanisms underlying stress-induced behavioral inhibition, depression, and fear conditioning. It has previously been shown that social subordination may result in increased glucocorticoid release and changes in brain signaling systems. However, it is still an open question which neuroendocrine and behavioral differences are causes, and which are consequences of social status. Using juvenile rainbow trout of similar size and with no apparent differences in social history, we demonstrate that the ability to win fights for social dominance can be predicted from the duration of a behavioral response to stress, in this case appetite inhibition after transfer to a new environment. Moreover, stress responsiveness in terms of confinement-induced changes in plasma cortisol was negatively correlated to aggressive behavior. Fish that exhibited lower cortisol responses to a standardized confinement test were markedly more aggressive when being placed in a dominant social position later in the study. These findings support the view that distinct behavioral-physiological stress coping styles are present in teleost fish, and these coping characteristics influence both social rank and levels of aggression.

Adult rats exposed to early-life social isolation exhibit increased anxiety and conditioned fear behavior, and altered hormonal stress responses.

Social isolation of rodents during development is thought to be a relevant model of early-life chronic stress. We investigated the effects of early-life social isolation on later adult fear and anxiety behavior, and on corticosterone stress responses, in male rats. On postnatal day 21, male rats were either housed in isolation or in groups of 3 for a 3 week period, after which, all rats were group-reared for an additional 2 weeks. After the 5-week treatment, adult rats were examined for conditioned fear, open field anxiety-like behavior, social interaction behavior and corticosterone responses to restraint stress. Isolates exhibited increased anxiety-like behaviors in a brightly-lit open field during the first 10 min of the test period compared to group-reared rats. Isolation-reared rats also showed increased fear behavior and reduced social contact in a social interaction test, and a transient increase in fear behavior to a conditioned stimulus that predicted foot-shock. Isolation-reared rats showed similar restraint-induced increases in plasma corticosterone as group-reared controls, but plasma corticosterone levels 2 h after restraint were significantly lower than pre-stress levels in isolates. Overall, this study shows that isolation restricted to an early part of development increases anxiety-like and fear behaviors in adulthood, and also results in depressed levels of plasma corticosterone following restraint stress.

Pedunculopontine tegmental stimulation evokes striatal dopamine efflux by activation of acetylcholine and glutamate receptors in the midbrain and pons of the rat

The pedunculopontine tegmental nucleus appears to influence striatal dopamine activity via cholinergic and glutamatergic afferents to dopaminergic cells of the substantia nigra pars compacta. We measured changes in striatal dopamine oxidation current (dopamine efflux) in response to electrical stimulation of the pedunculopontine tegmental nucleus using in vivo electrochemistry in urethane‐anaesthetized rats. Pedunculopontine tegmental nucleus stimulation evoked a three‐component change in striatal dopamine efflux, consisting of: (i) an initial rapid increase of 2 min duration; followed by (ii) a decrease below prestimulation levels of 9 min duration; then by (iii) a prolonged increase lasting 35 min. Intra‐nigral infusions of the ionotropic glutamate receptor antagonist kynurenate (10 µg/µL) or the nicotinic cholinergic receptor antagonist mecamylamine (5 µg/0.5 µL) selectively attenuated the rapid first component, while systemic injections of the muscarinic cholinergic antagonist scopolamine (5 mg/kg, i.p.) diminished the second and third components. In addition, intra‐pedunculopontine tegmental nucleus infusions of the M2 muscarinic antagonist methoctramine (50 µg/µL) selectively abolished the inhibitory second component, while intranigral infusions of scopolamine (200 µg/µL) selectively abolished the prolonged third component. Intra‐nigral infusions of the metabotropic glutamate receptor antagonist (+)‐α‐methyl‐4‐carboxyphenylglycine (2 µg/µL) had no effect on pedunculopontine tegmental nucleus‐elicited striatal dopamine efflux. These results suggest that the pedunculopontine tegmental nucleus utilizes nicotinic and ionotropic glutamate receptors in the substantia nigra to mediate rapid activation, M2‐like muscarinic autoreceptors in the pedunculopontine tegmental nucleus to mediate decreased activation, and muscarinic receptors in the substantia nigra (probably of the M5 subtype) to mediate prolonged activation, of the nigrostriatal dopaminergic system.

Adolescent male rats exposed to social defeat exhibit altered anxiety behavior and limbic monoamines as adults.

Social stress in adolescence is correlated with emergence of psychopathologies during early adulthood. In this study, the authors investigated the impact of social defeat stress during mid-adolescence on adult male brain and behavior. Adolescent male Sprague-Dawley rats were exposed to repeated social defeat for 5 days while controls were placed in a novel empty cage. When exposed to defeat-associated cues as adults, previously defeated rats showed increased risk assessment and behavioral inhibition, demonstrating long-term memory for the defeat context. However, previously defeated rats exhibited increased locomotion in both elevated plus-maze and open field tests, suggesting heightened novelty-induced behavior. Adolescent defeat also affected adult monoamine levels in stress-responsive limbic regions, causing decreased medial prefrontal cortex dopamine, increased norepinephrine and serotonin in the ventral dentate gyrus, and decreased norepinephrine in the dorsal raphe. Our results suggest that adolescent social defeat produces both deficits in anxiety responses and altered monoaminergic function in adulthood. This model offers potential for identifying specific mechanisms induced by severe adolescent social stress that may contribute to increased adult male vulnerability to psychopathology.

M5 muscarinic receptors are needed for slow activation of dopamine neurons and for rewarding brain stimulation.

Mesopontine cholinergic neurons (Ch5 and Ch6 cell groups) activate the cerebral cortex via thalamic projections, and activate locomotion and reward via dopamine neurons in the substantia nigra and ventral tegmental area (VTA). Nicotinic receptors in VTA activate dopamine neurons quickly, and are needed for the stimulant and rewarding effects of nicotine in rats. Muscarinic receptors in VTA activate dopamine neurons slowly, and are needed for the rewarding effects of hypothalamic stimulation, but do not increase locomotion. Antisense oligonucleotides targetting M5 mRNA, when infused into the VTA, inhibited M5 receptor binding and rewarding hypothalamic stimulation. Mutant mice with truncated M5 muscarinic receptor genes drank more water than wild-type controls. Spontaneous locomotion and locomotor responses to amphetamine and scopolamine were unchanged. Electrical stimulation near Ch6 induced dopamine release in the nucleus accumbens in two phases, an early phase (0-2 min after stimulation) dependent on nicotinic and gluatamatergic receptors in VTA, and a late phase (8-50 min after stimulation) dependent on muscarinic receptors in VTA. The late phase was lost in M5 mutant mice, while the early phase was unchanged. M5 muscarinic receptors bind slowly to muscarinic ligands, and appear to mediate slow secretions.

Consequences of post-weaning social isolation on anxiety behavior and related neural circuits in rodents

Exposure to adverse experiences in early-life is implicated in the later vulnerability to development of psychiatric disorders, including anxiety and affective disorders in humans. Adverse early-life experiences likely impart their long-term consequences on mental health by disrupting the normal development of neural systems involved in stress responses, emotional behavior and emotional states. Neural systems utilizing the neurotransmitters serotonin, dopamine and the neuropeptide corticotropin-releasing factor (CRF) are implicated in mediating emotive behaviors, and dysfunction of these neurochemical systems is associated with mood/anxiety disorders. These neural systems continue maturing until early or mid-adolescence in humans, thus alterations to their development are likely to contribute to the long-term consequences of adverse early-life experiences. A large body of literature suggests that post-weaning isolation rearing of rodents models the behavioral consequences of adverse early-life experiences in humans. Overall, the majority findings suggest that post-weaning social isolation that encompasses pre-adolescence produces long-lasting alterations to anxiety behavior, while measures of monoaminergic activity in various limbic regions during social isolation suggest alterations to dopamine and serotonin systems. The goal of this review is to evaluate and integrate findings from post-weaning social isolation studies specifically related to altered fear and anxiety behaviors and associated changes in neuroendocrine function and the activity of monoaminergic systems.

Does Serotonin Influence Aggression? Comparing Regional Activity before and during Social Interaction*

Serotonin is widely believed to exert inhibitory control over aggressive behavior and intent. In addition, a number of studies of fish, reptiles, and mammals, including the lizard Anolis carolinensis, have demonstrated that serotonergic activity is stimulated by aggressive social interaction in both dominant and subordinate males. As serotonergic activity does not appear to inhibit agonistic behavior during combative social interaction, we investigated the possibility that the negative correlation between serotonergic activity and aggression exists before aggressive behavior begins. To do this, putatively dominant and more aggressive males were determined by their speed overcoming stress (latency to feeding after capture) and their celerity to court females. Serotonergic activities before aggression are differentiated by social rank in a region‐specific manner. Among aggressive males baseline serotonergic activity is lower in the septum, nucleus accumbens, striatum, medial amygdala, anterior hypothalamus, raphe, and locus ceruleus but not in the hippocampus, lateral amygdala, preoptic area, substantia nigra, or ventral tegmental area. However, in regions such as the nucleus accumbens, where low serotonergic activity may help promote aggression, agonistic behavior also stimulates the greatest rise in serotonergic activity among the most aggressive males, most likely as a result of the stress associated with social interaction.

Corticotropin-releasing factor in the dorsal raphe elicits temporally distinct serotonergic responses in the limbic system in relation to fear behavior

The neurotransmitters serotonin and corticotrophin-releasing factor are thought to play an important role in fear and anxiety behaviors. This study aimed to determine the relationship between corticotrophin-releasing factor-evoked changes in serotonin levels within discrete regions of the limbic system and the expression of fear behavior in rats. The effects of corticotrophin-releasing factor administration to the serotonin cell body regions of the dorsal raphe nucleus on fear behavior, behavioral activity, and extracellular serotonin levels were assessed in freely moving rats with microdialysis probes implanted into the central nucleus of the amygdala and the medial prefrontal cortex. Infusion of corticotrophin-releasing factor (0.5 microg) into the dorsal raphe rapidly induced freezing behavior, which was positively correlated with an immediate increase in serotonin release in the central nucleus of the amygdala. In contrast, cessation of freezing behavior correlated with a delayed and prolonged increase in serotonin release within the medial prefrontal cortex. Our findings suggest that corticotrophin-releasing factor-induced freezing behavior is associated with regionally and temporally distinct serotonergic responses in the limbic system that may reflect differing roles for these regions in the expression of fear/anxiety behavior.

Corticotropin-releasing factor in the dorsal raphe nucleus increases medial prefrontal cortical serotonin via type 2 receptors and median raphe nucleus activity

Interactions between central corticotropin‐releasing factor (CRF) and serotonergic systems are believed to be important for mediating fear and anxiety behaviors. Recently we demonstrated that infusions of CRF into the rat dorsal raphe nucleus result in a delayed increase in serotonin release within the medial prefrontal cortex that coincided with a reduction in fear behavior. The current studies were designed to study the CRF receptor mechanisms and pathways involved in this serotonergic response. Infusions of CRF (0.5 μg/0.5 μL) were made into the dorsal raphe nucleus of urethane‐anesthetized rats following either inactivation of the median raphe nucleus by muscimol (25 ng/0.25 μL) or antagonism of CRF receptor type 1 or CRF receptor type 2 in the dorsal raphe nucleus with antalarmin (25–50 ng/0.5 μL) or antisauvagine‐30 (2 μg/0.5 μL), respectively. Medial prefrontal cortex serotonin levels were measured using in‐vivo microdialysis and high‐performance liquid chromatography with electrochemical detection. Increased medial prefrontal cortex serotonin release elicited by CRF infusion into the dorsal raphe nucleus was abolished by inactivation of the median raphe nucleus. Furthermore, antagonism of CRF receptor type 2 but not CRF receptor type 1 in the dorsal raphe nucleus abolished CRF‐induced increases in medial prefrontal cortex serotonin. Follow‐up studies involved electrical stimulation of the central nucleus of the amygdala, a source of CRF afferents to the dorsal raphe nucleus. Activation of the central nucleus increased medial prefrontal cortex serotonin release. This response was blocked by CRF receptor type 2 antagonism in the dorsal raphe. Overall, these results highlight complex CRF modulation of medial prefrontal cortex serotonergic activity at the level of the raphe nuclei.