Anatomical analysis of afferent projections to the medial prefrontal cortex in the rat

An analysis of rat prefrontal cortex in mediating executive function

While it is acknowledged that species specific differences are an implicit condition of comparative studies, rodent models of prefrontal function serve a significant role in the acquisition of converging evidence on prefrontal function across levels of analysis and research techniques. The purpose of the present review is to examine whether the prefrontal cortex (PFC) in rats supports a variety of processes associated with executive function including working memory, temporal processing, planning (prospective coding), flexibility, rule learning, and decision making. Therefore, in this review we examined changes associated with working memory processes for spatial locations, visual objects, odors, tastes, and response domains or attributes, temporal processes including temporal order, sequence learning, prospective coding, behavioral flexibility associated with reversal learning and set shifting, paired associate learning, and decision making based on effort, time discounting, and uncertainty following damage to the PFC in rats. In addition, potential parallel processes of executive function in monkeys and humans based on several theories of subregional differentiation within the PFC will be presented. Specifically, theories based on domain or attribute specificity (Goldman-Rakic, 1996), level of processing (Petrides, 1996), rule learning based on complexity (Wise, Murray, & Gerfen, 1996), executive functions based on connectivity with other brain regions associated with top-down control (Miller & Cohen, 2001), are presented and applied to PFC function in rats with the aim of understanding subregional specificity in the rat PFC. The data suggest that there is subregional specificity within the PFC of rats, monkey and humans and there are parallel cognitive functions of the different subregions of the PFC in rats, monkeys and humans.

Emerging, reemerging, and forgotten brain areas of the reward circuit: Notes from the 2010 Motivational Neural Networks conference

On April 24-27, 2010, the Motivational Neuronal Networks meeting took place in Wrightsville Beach, North Carolina. The conference was devoted to "Emerging, re-emerging, and forgotten brain areas" of the reward circuit. A central feature of the conference was four scholarly discussions of cutting-edge topics related to the conference's theme. These discussions form the basis of the present review, which summarizes areas of consensus and controversy, and serves as a roadmap for the next several years of research.

Prefrontal and monoaminergic contributions to stop-signal task performance in rats

Defining the neural and neurochemical substrates of response inhibition is of crucial importance for the study and treatment of pathologies characterized by impulsivity such as attention-deficit/hyperactivity disorder and addiction. The stop-signal task (SST) is one of the most popular paradigms used to study the speed and efficacy of inhibitory processes in humans and other animals. Here we investigated the effect of temporarily inactivating different prefrontal subregions in the rat by means of muscimol microinfusions on SST performance. We found that dorsomedial prefrontal cortical areas are important for inhibiting an already initiated response. We also investigated the possible neural substrates of the selective noradrenaline reuptake inhibitor atomoxetine via its local microinfusion into different subregions of the rat prefrontal cortex. Our results show that both orbitofrontal and dorsal prelimbic cortices mediate the beneficial effects of atomoxetine on SST performance. To assess the neurochemical specificity of these effects, we infused the α2-adrenergic agonist guanfacine and the D(1)/D(2) antagonist α-flupenthixol in dorsal prelimbic cortex to interfere with noradrenergic and dopaminergic neurotransmission, respectively. Guanfacine, which modulates noradrenergic neurotransmission, selectively impaired stopping, whereas blocking dopaminergic receptors by α-flupenthixol infusion prolonged go reaction time only, confirming the important role of noradrenergic neurotransmission in response inhibition. These results show that, similar to humans, distinct networks play important roles during SST performance in the rat and that they are differentially modulated by noradrenergic and dopaminergic neurotransmission. This study advances our understanding of the neuroanatomical and neurochemical determinants of impulsivity, which are relevant for a range of psychiatric disorders.

Accumbens shell AMPA receptors mediate expression of extinguished reward seeking through interactions with basolateral amygdala

Extinction is the reduction in drug seeking when the contingency between drug seeking behavior and the delivery of drug reward is broken. Here, we investigated a role for the nucleus accumbens shell (AcbSh). Rats were trained to respond for 4% (v/v) alcoholic beer in one context (Context A) followed by extinction in a second context (Context B). Rats were subsequently tested in the training context, A (ABA), or the extinction context, B (ABB). Pre-test injections of the glutamate AMPA receptor antagonist, NBQX (1 µg) into AcbSh had no effect on renewal of alcoholic beer seeking when rats were returned to the training context (ABA). However, NBQX increased responding when rats were tested in the extinction context (ABB). In a second experiment, rats received training, extinction, and test in the same context. Pre-test injections of NBQX (0, 0.3, and 1 µg) into the AcbSh dose-dependently attenuated expression of extinction. We also found that NBQX in the AcbSh had no effect on initial acquisition of extinction or the motivation to respond for reward as measured by break point on a progressive ratio schedule. Finally, we show that pharmacological disconnection of a basolateral amygdala (BLA) → AcbSh pathway via NBQX in AcbSh combined with reversible inactivation of the contralateral BLA attenuates expression of extinction. Together, these results suggest that AcbSh AMPA receptors mediate expression of extinguished reward seeking through glutamatergic inputs from the BLA.

Development of the motivational system during adolescence, and its sensitivity to disruption by nicotine

The brain continues to develop during adolescence, and exposure to exogenous substances such as nicotine can exert long-lasting adaptations during this vulnerable period. In order to fully understand how nicotine affects the adolescent brain it is important to understand normal adolescent brain development. This review summarizes human and animal data on brain development, with emphasis on the prefrontal cortex, for its important function in executive control over behavior. Moreover, we discuss how nicotine exposure during adolescence can disrupt brain development bearing long-term consequences on executive cognitive function in adulthood.

Amygdalar connections in the lesser hedgehog tenrec

The present study analyses the overall extrinsic connectivity of the non-olfactory amygdala (Ay) in the lesser hedgehog tenrec. The data were obtained from tracer injections into the lateral and intermediate portions of the Ay as well as several non-amygdalar brain regions. Both the solitary and the parabrachial nucleus receive descending projections from the central nucleus of the Ay, but only the parabrachial nucleus appears to project to the Ay. There is one prominent region in the ventromedial hypothalamus connected reciprocally with the medial and central Ay. Amygdalar afferents clearly arise from the dorsomedial thalamus, the subparafascicular nuclei and the medial geniculate complex (GM). Similar to other subprimate species, the latter projections originate in the dorsal and most caudal geniculate portions and terminate in the dorsolateral Ay. Unusual is the presence of amygdalo-projecting cells in the marginal geniculate zone and their virtual absence in the medial GM. As in other species, amygdalo-striatal projections mainly originate in the basolateral Ay and terminate predominantly in the ventral striatum. Given the poor differentiation of the tenrec's neocortex, there is a remarkable similarity with regard to the amygdalo-cortical connectivity between tenrec and rat, particularly as to prefrontal, limbic and somatosensorimotor areas as well as the rhinal cortex throughout its length. The tenrec's isocortex dorsomedial to the caudal rhinal cortex, on the other hand, may not be connected with the Ay. An absence of such connections is expected for primary auditory and visual fields, but it is unusual for their secondary fields.

Oscillations and hippocampal-prefrontal synchrony

The hippocampus, a structure required for many types of memory, connects to the medial prefrontal cortex, an area that helps direct neuronal information streams during intentional behaviors. Increasing evidence suggests that oscillations regulate communication between these two regions. Theta rhythms may facilitate hippocampal inputs to the medial prefrontal cortex during mnemonic tasks and may also integrate series of functionally relevant gamma-mediated cell assemblies in the medial prefrontal cortex. During slow-wave sleep, temporal coordination of hippocampal sharp wave-ripples and medial prefrontal cortex spindles may be an important component of the process by which memories become hippocampus-independent. Studies using rodent models indicate that oscillatory phase-locking is disturbed in schizophrenia, emphasizing the need for more studies of oscillatory synchrony in the hippocampal-prefrontal network.

Learning and the motivation to eat: forebrain circuitry

Appetite and eating are not only controlled by energy needs, but also by extrinsic factors that are not directly related to energy balance. Environmental signals that acquire motivational properties through associative learning-learned cues-can override homeostatic signals and stimulate eating in sated states, or inhibit eating in states of hunger. Such influences are important, as environmental factors are believed to contribute to the increased susceptibility to overeating and the rise in obesity in the developed world. Similarly, environmental and social factors contribute to the onset and maintenance of anorexia nervosa and other eating disorders through interactions with the individual genetic background. Nevertheless, how learning enables environmental signals to control feeding, and the underlying brain mechanisms are poorly understood. We developed two rodent models to study how learned cues are integrated with homeostatic signals within functional forebrain networks, and how these networks are modulated by experience. In one model, a cue previously paired with food when an animal was hungry induces eating in sated rats. In the other model, food-deprived rats inhibit feeding when presented with a cue that signals danger, a tone previously paired with footshocks. Here evidence will be reviewed that the forebrain network formed by the amygdala, lateral hypothalamus and medial prefrontal cortex mediates cue-driven feeding, while a parallel amygdalar circuitry mediates suppression of eating by the fear cue. Findings from the animal models may be relevant for understanding aspects of human appetite and eating, and maladaptive mechanisms that could lead to overeating and anorexia.

Excitatory amplification through divergent-convergent circuits: the role of the midline thalamus in limbic seizures

INTRODUCTION

The midline thalamic nuclei are an important component of limbic seizures. Although the anatomic connections and excitatory influences of the midline thalamus are well known, its physiological role in limbic seizures is unclear. We examined the role of the midline thalamus on two circuits that are involved in limbic seizures: (a) the subiculum-prefrontal cortex (SB-PFC), and (b) the piriform cortex-entorhinal cortex (PC-EC).

METHODS

Evoked field potentials for both circuits were obtained in anesthetized rats, and the likely direct monosynaptic and polysynaptic contributions to the responses were identified. Seizures were generated in both circuits by 20 Hz stimulus trains. Once stable seizures and evoked potentials were established, the midline thalamus was inactivated through an injection of the sodium channel blocker tetrodotoxin (TTX), and the effects on the evoked responses and seizures were analyzed.

RESULTS

Inactivation of the midline thalamus suppressed seizures in both circuits. Seizure suppression was associated with a significant reduction in the late thalamic component but no significant change in the early direct monosynaptic component. Injections that did not suppress the seizures did not alter the evoked potentials.

CONCLUSIONS

Suppression of the late thalamic component of the evoked potential at the time of seizure suppression suggests that the thalamus facilitates seizure induction by extending the duration of excitatory drive through a divergent-convergent excitatory amplification system. This work may have broader implications for understanding signaling in the limbic system.

Discrete forebrain neuronal networks supporting noradrenergic regulation of sensorimotor gating

Prepulse inhibition (PPI) refers to the reduction in the startle response when a startling stimulus is preceded by a weak prestimulus, and is an endophenotype of deficient sensorimotor gating in several neuropsychiatric disorders. Emerging evidence suggests that norepinephrine (NE) regulates PPI, however, the circuitry involved is unknown. We found recently that stimulation of the locus coeruleus (LC), the primary source of NE to the forebrain, induces a PPI deficit that is a result of downstream NE release. Hence, this study sought to identify LC-innervated forebrain regions that mediate this effect. Separate groups of male Sprague-Dawley rats received a cocktail solution of the α1-NE receptor agonist phenylephrine plus the β-receptor agonist isoproterenol (equal parts of each; 0, 3, 10, and 30 μg) into subregions of the medial prefrontal cortex (mPFC), nucleus accumbens (NAcc), extended amygdala, mediodorsal thalamus (MD-thalamus), or the dorsal hippocampus (DH) before PPI testing. NE agonist infusion into the posterior mPFC, NAcc shell, bed nucleus of the stria terminalis, basolateral amygdala, and the MD-thalamus disrupted PPI, with particularly strong effects in MD-thalamus. Sites in which NE receptor stimulation did not disrupt PPI (anterior mPFC, NAcc core, central amygdala, and DH) did support PPI disruptions with the dopamine D2 receptor agonist quinpirole (0, 10 μg). This pattern reveals new pathways in the regulation of PPI, and suggests that NE transmission within distinct thalamocortical and ventral forebrain networks may subserve the sensorimotor gating deficits that are seen in disorders such as schizophrenia, Tourette syndrome, and post-traumatic stress disorder.

The dorsomedial hypothalamus and the central pathways involved in the cardiovascular response to emotional stress

Psychological stress elicits increases in sympathetic activity accompanied by a marked cardiovascular response. Revealing the relevant central mechanisms involved in this phenomenon could contribute significantly to our understanding of the pathogenesis of stress-related cardiovascular diseases, and the key to this understanding is the identification of the nuclei, pathways and neurotransmitters involved in the organization of the cardiovascular response to stress. The present review will focus specifically on the dorsomedial hypothalamus, a brain region now known to play a primary role in the synaptic integration underlying the cardiovascular response to emotional stress.

Effects of dorsal or ventral medial prefrontal cortical lesions on five-choice serial reaction time performance in rats

Lesions of the rat medial prefrontal cortex (mPFC) produce behavioral impairments in the 5-choice serial reaction time (5CSRT) task, a widely used measure of sustained and selective visual attention. This experiment compared the effects of "dorsal" (centered on prelimbic and infralimbic cortices) and "ventral" (centered on dorsal peduncular cortex and tenia tecta) mPFC lesions on performance in a variant of the 5CSRT task. Because in some associative learning theories, the predictive validity of events determines the allocation of attention to them, we also examined the effects of cue validity in this task. Operant nosepoke responses to some briefly illuminated ports were consistently (100%) reinforced (CRF) with food, whereas for other ports, responding was reinforced on only 50% of the trials (partial reinforcement, PRF). Different patterns of impairment emerged depending on lesion location within the mPFC. Dorsal- and sham-lesioned rats responded more to CRF than to PRF cues, but ventral-lesioned rats responded similarly to CRF and PRF cues. Additionally, under some conditions of increased attentional demands, dorsal-lesioned rats failed to respond on many trials, whereas the impairment in ventral-lesioned rats was manifested as an increase in response errors. These results demonstrate separable roles for dorsal and ventral mPFC subregions in controlling attention.

Increased GABAergic inhibition in the midline thalamus affects signaling and seizure spread in the hippocampus-prefrontal cortex pathway

PURPOSE

The midline thalamus is an important component of the circuitry in limbic seizures, but it is unclear how synaptic modulation of the thalamus affects that circuitry. In this study, we wished to understand how synaptic modulation of the thalamus can affect interregional signaling and seizure spread in the limbic network.

METHODS

We examined the effect of γ-aminobutyric acid (GABA) modulation of the mediodorsal (MD) region of the thalamus on responses in the prefrontal cortex (PFC) by stimulation of the subiculum (SB). Muscimol, a GABA(A) agonist, was injected into the MD, and the effect on local responses to subiculum stimulation was examined. Evoked potentials were induced in the MD and the PFC by low-frequency stimulation of the SB, and seizures were generated in the subiculum by repeated 20-Hz stimulations. The effect of muscimol in the MD on the evoked potentials and seizures was measured.

KEY FINDINGS

Thalamic responses to stimulation of the subiculum were reduced in the presence of muscimol. Reduction of the amplitudes of evoked potentials in the MD resulted in an attenuation of the late, thalamic components of the responses in the PFC, as well as of seizure durations.

SIGNIFICANCE

Activation of GABA(A) receptors in the midline thalamus not only causes changes within the thalamus, but it has broader effects on the limbic network. This work provides further evidence that synaptic modulation within the midline thalamus alters system excitability more broadly and reduces seizure activity.

Gateways of ventral and dorsal streams in mouse visual cortex

It is widely held that the spatial processing functions underlying rodent navigation are similar to those encoding human episodic memory (Doeller et al., 2010). Spatial and nonspatial information are provided by all senses including vision. It has been suggested that visual inputs are fed to the navigational network in cortex and hippocampus through dorsal and ventral intracortical streams (Whitlock et al., 2008), but this has not been shown directly in rodents. We have used cytoarchitectonic and chemoarchitectonic markers, topographic mapping of receptive fields, and pathway tracing to determine in mouse visual cortex whether the lateromedial field (LM) and the anterolateral field (AL), which are the principal targets of primary visual cortex (V1) (Wang and Burkhalter, 2007) specialized for processing nonspatial and spatial visual information (Gao et al., 2006), are distinct areas with diverse connections. We have found that the LM/AL border coincides with a change in type 2 muscarinic acetylcholine receptor expression in layer 4 and with the representation of the lower visual field periphery. Our quantitative analyses also show that LM strongly projects to temporal cortex as well as the lateral entorhinal cortex, which has weak spatial selectivity (Hargreaves et al., 2005). In contrast, AL has stronger connections with posterior parietal cortex, motor cortex, and the spatially selective medial entorhinal cortex (Haftig et al., 2005). These results support the notion that LM and AL are architecturally, topographically, and connectionally distinct areas of extrastriate visual cortex and that they are gateways for ventral and dorsal streams.

Serotonergic regulation of neuronal excitability in the prefrontal cortex

The cerebral cortex receives a dense serotonergic innervation originating predominantly from the dorsal raphe nucleus. This innervation regulates cortical functioning by activating multiple serotonin receptors that are differentially expressed by pyramidal cells and interneurons. Electrophysiological studies in the prefrontal cortex indicate that receptors of the 5-HT(1A) and 5-HT(2A) subtypes are the main serotonin receptors regulating membrane excitability in pyramidal cells. Most pyramidal cells in layer V coexpress 5-HT(1A) and 5-HT(2A) receptors that together regulate how these neurons encode excitatory input into neuronal firing. In contrast, a subset of large pyramidal cells of deep layer V appears to express exclusively 5-HT(2A) receptors that depolarize and excite these cells. Serotonin also depolarizes and excites at least two classes of GABAergic interneurons by acting on 5-HT(3) and 5-HT(2A) receptors. The differential expression of serotonin receptors in different pyramidal cells and interneurons is consistent with a growing appreciation of the anatomical, molecular and functional heterogeneity of pyramidal cells and interneurons of the cerebral cortex. These findings begin to lay the ground for a cellular-level understanding of the serotonergic regulation of the prefrontal cortex.

The prefrontal cortex communicates with the amygdala to impair learning after acute stress in females but not in males

Acute stress exposure enhances classical eyeblink conditioning in male rats, whereas exposure to the same event dramatically impairs performance in females (Wood and Shors, 1998; Wood et al., 2001). We hypothesized that stress affects learning differently in males and females because different brain regions and circuits are being activated. In the first experiment, we determined that neuronal activity within the medial prefrontal cortex (mPFC) during the stressful event is necessary to disrupt learning in females. In both males and females, the mPFC was bilaterally inactivated with GABA agonist muscimol before the stressor. Inactivation prevented only the impaired performance in females; it had no consequence for performance in males. However, in the second experiment, excitation of the mPFC alone with GABA antagonist picrotoxin was insufficient to elicit the stress effect that was prevented through the inactivation of this region in females. Therefore, we hypothesized that the mPFC communicates with the basolateral amygdala to disrupt learning in females after the stressor. To test this hypothesis, these structures were disconnected from each other with unilateral excitotoxic (NMDA) lesions on either the same or opposite sides of the brain. Females with contralateral lesions, which disrupt the connections on both sides of the brain, were able to learn after the stressful event, whereas those with ipsilateral lesions, which disrupt only one connection, did not learn after the stressor. Together, these data indicate that the mPFC is critically involved in females during stress to impair subsequent learning and does so via communication with the amygdala.

Dissociable roles of prelimbic and infralimbic cortices, ventral hippocampus, and basolateral amygdala in the expression and extinction of conditioned fear

Current models of conditioned fear expression and extinction involve the basolateral amygdala (BLA), ventral medial prefrontal cortex (vmPFC), and the hippocampus (HPC). There is some disagreement with respect to the specific roles of these structures, perhaps due to subregional differences within each area. For example, growing evidence suggests that infralimbic (IL) and prelimbic (PL) subregions of vmPFC have opposite influences on fear expression. Moreover, it is the ventral HPC (vHPC), rather than the dorsal HPC, that projects to vmPFC and BLA. To help determine regional specificity, we used small doses of the GABA(A) agonist muscimol to selectively inactivate IL, PL, BLA, or vHPC in an auditory fear conditioning and extinction paradigm. Infusions were performed prior to extinction training, allowing us to assess the effects on both fear expression and subsequent extinction memory. Inactivation of IL had no effect on fear expression, but impaired the within-session acquisition of extinction as well as extinction memory. In contrast, inactivation of PL impaired fear expression, but had no effect on extinction memory. Inactivation of the BLA or vHPC impaired both fear expression and extinction memory. Post-extinction inactivations had no effect in any structure. We suggest a model in which amygdala-dependent fear expression is modulated by inputs from PL and vHPC, whereas extinction memory requires extinction-induced plasticity in IL, BLA, and/or vHPC.

Projection-specific neuromodulation of medial prefrontal cortex neurons

Mnemonic persistent activity in the prefrontal cortex (PFC) constitutes the neural basis of working memory. To understand how neuromodulators contribute to the generation of persistent activity, it is necessary to identify the intrinsic properties of the layer V pyramidal neurons that transfer this information to downstream networks. Here we show that the somatic dynamic and integrative properties of layer V pyramidal neurons in the rat medial PFC depend on whether they project subcortically to the pons [corticopontine (CPn)] or to the contralateral cortex [commissural (COM)]. CPn neurons display low temporal summation and accelerate in firing frequency when depolarized, whereas COM neurons have high temporal summation and display spike frequency accommodation. In response to dynamic stimuli, COM neurons act as low-pass filters, whereas CPn neurons act as bandpass filters, resonating in the theta frequency range (3-6 Hz). The disparate subthreshold properties of COM and CPn neurons can be accounted for by differences in the hyperpolarization-activated cyclic nucleotide gated cation h-current. Interestingly, neuromodulators hypothesized to enhance mnemonic persistent activity affect COM and CPn neurons distinctly. Adrenergic modulation shifts the dynamic properties of CPn but not COM neurons and increases the excitability of CPn neurons significantly more than COM neurons. In response to cholinergic modulation, CPn neurons were much more likely to display activity-dependent intrinsic persistent firing than COM neurons. Together, these data suggest that the two categories of projection neurons may subserve separate functions in PFC and may be engaged differently during working memory processes.

Different patterns of neuronal activities in the infralimbic and prelimbic cortices and behavioral expression in response to two affective odors, 2,5-dihydro-2,4,5-trimethylthiazoline and a mixture of cis-3-hexenol and trans-2-hexenal, in the freely moving rat

The medial prefrontal cortex (mPFC) is involved in stimulus perception, attentional control, emotional behavior, and the stress response. These functions are thought to be mediated by the infralimbic (IL) and prelimbic (PL) subregions of mPFC; however, few studies have examined the roles of IL and PL cortices in olfactory cognition. In the present study, we investigated the acute effects of two odors, 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) and a mixture of cis-3-hexenol and trans-2-hexenal (green odor: GO), on behavioral responses and IL and PL neuronal activities using extracellular single-unit recordings in a freely moving rat. We found that the total number of spike firings in IL and PL neurons did not change with 10s presentation of odors. TMT presentation induced significant changes in burst firing activity in IL and PL neurons, while GO presentation induced changes in burst firing only in IL neurons. In the temporal profile of the firing activity of IL neurons, TMT exposure induced transient activation and GO exposure induced sustained activation. Those of PL neurons showed sustained activation during TMT exposure and transient activations during GO exposure. GO exposure induced a stretch-attend posture, whereas TMT exposure induced immobility. Furthermore, multiple regression analysis indicated that the property of the odor and neuronal activities of IL and PL regions were correlated with behavioral responses. These findings reveal that olfaction-related neurons exist in IL and PL regions, and that the neurons in these regions might temporarily encode odor information in order to modulate motor outputs by tuning firing properties in the early stage of cognition according to the odor property.

Disrupted thalamic T-type Ca2+ channel expression and function during ethanol exposure and withdrawal

Chronic ethanol exposure produces profound disruptions in both brain rhythms and diurnal behaviors. The thalamus has been identified as a neural pacemaker of both normal and abnormal rhythms with low-threshold, transient (T-type) Ca(2+) channels participating in this activity. We therefore examined T-type channel gene expression and physiology in the thalamus of C57Bl/6 mice during a 4-wk schedule of chronic intermittent ethanol exposures in a vapor chamber. We found that chronic ethanol disrupts the normal daily variations of both thalamic T-type channel mRNA levels and alters thalamic T-type channel gating properties. The changes measured in channel expression and function were associated with an increase in low-threshold bursts of action potentials during acute withdrawal periods. Additionally, the observed molecular and physiological alterations in the channel properties in wild-type mice occurred in parallel with a progressive disruption in the normal daily variations in theta (4-9 Hz) power recorded in the cortical electroencephalogram. Theta rhythms remained disrupted during a subsequent week of withdrawal but were restored with the T-type channel blocker ethosuximide. Our results demonstrate that a key ion channel underlying the generation of thalamic rhythms is altered during chronic ethanol exposure and withdrawal and may be a novel target in the management of abnormal network activity due to chronic alcoholism.

Chronic nicotine exposure produces lateralized, age-dependent dendritic remodeling in the rodent basolateral amygdala

This study investigated the dendritic morphology of neurons located in the right and left basolateral amygdala (BLA) and infralimbic (IL) cortex following chronic nicotine exposure during adolescence or adulthood. Sprague-Dawley rats were administered subcutaneous injections of nicotine (0.5 mg/kg; free base) or saline three times per week for 2 weeks (six total injections). The dose period began on either postnatal day (P) 32 (adolescent) or P61 (adult). Twenty days following the end of dosing, brains were processed for Golgi-Cox staining, and dendrites from principal neurons in the BLA and pyramidal neurons in the IL were digitally reconstructed in three dimensions. Morphometric analysis revealed a contrasting pattern of BLA dendritic morphology between the adolescent and adult pretreatment groups. In the adult control group, basilar dendritic length did not differ with respect to hemisphere. Nicotine induced robust hemispheric asymmetry by increasing dendritic length in the right hemisphere only. In contrast, adolescent nicotine exposure did not produce significant alteration of basilar dendritic morphology. There was, however, an indication that nicotine eliminated a naturally existing hemispheric asymmetry in the younger cohort. At both ages, nicotine produced a reduction in complexity of the apical tree of principal neurons. Chronic nicotine did not affect the dendritic morphology of pyramidal neurons from the IL in either age group, indicating another dimension of anatomical specificity. Collectively, these data implicate the BLA as a target for lasting neuroplasticity associated with chronic nicotine exposure. Further, hemispheric differences in dendritic morphology were uncovered that depended on the age of nicotine exposure, a finding that underscores the importance of considering laterality when investigating neurodevelopmental effects of drug exposure.

Pleasurable behaviors reduce stress via brain reward pathways

Individuals often eat calorically dense, highly palatable "comfort" foods during stress for stress relief. This article demonstrates that palatable food intake (limited intake of sucrose drink) reduces neuroendocrine, cardiovascular, and behavioral responses to stress in rats. Artificially sweetened (saccharin) drink reproduces the stress dampening, whereas oral intragastric gavage of sucrose is without effect. Together, these results suggest that the palatable/rewarding properties of sucrose are necessary and sufficient for stress dampening. In support of this finding, another type of natural reward (sexual activity) similarly reduces stress responses. Ibotenate lesions of the basolateral amygdala (BLA) prevent stress dampening by sucrose, suggesting that neural activity in the BLA is necessary for the effect. Moreover, sucrose intake increases mRNA and protein expression in the BLA for numerous genes linked with functional and/or structural plasticity. Lastly, stress dampening by sucrose is persistent, which is consistent with long-term changes in neural activity after synaptic remodeling. Thus, natural rewards, such as palatable foods, provide a general means of stress reduction, likely via structural and/or functional plasticity in the BLA. These findings provide a clearer understanding of the motivation for consuming palatable foods during times of stress and influence therapeutic strategies for the prevention and/or treatment of obesity and other stress-related disorders.

Forebrain circuits and control of feeding by learned cues

Professor Richard F. Thompson and his highly influential work on the brain substrates of associative learning and memory have critically shaped my research interests and scientific approach. I am tremendously grateful and thank Professor Thompson for the support and influence on my research and career. The focus of my research program is on associative learning and its role in the control of fundamental, motivated behaviors. My long-term research goal is to understand how learning enables environmental cues to control feeding behavior. We use a combination of behavioral studies and neural systems analysis approach in two well-defined rodent models to study how learned cues are integrated with homeostatic signals within functional forebrain networks, and how these networks are modulated by experience. Here, I will provide an overview of the two behavioral models and the critical neural network components mapped thus far, which include areas in the forebrain, the amygdala and prefrontal cortex, critical for associative learning and decision-making, and the lateral hypothalamus, which is an integrator for feeding, reward and motivation.

Reversible Inactivation of Rat Premotor Cortex Impairs Temporal Preparation, but not Inhibitory Control, During Simple Reaction-Time Performance

Previous studies by our lab and others have established a role for medial areas of the prefrontal cortex (mPFC) in the top-down control of action during simple reaction-time (RT) tasks. However, the neural circuits that allow mPFC to influence activity in the motor system have remained unclear. In the present study, we used a combination of tract-tracing and reversible inactivation methods to examine the role of a motor-related area in the rat frontal cortex, called the rostral forelimb area (RFA), in the top-down control of action. Neural tracing studies involved used electrical microstimulation to identify RFA and injections of biotinylated dextran amines (BDA) to map out connections of RFA with other parts of the frontal cortex. Connections were found between RFA and mPFC, the agranular insular cortex, and the primary motor cortex. Reversible inactivations using muscimol infusions into RFA increased response times and eliminated delay-dependent speeding, but did not increase premature responding. These results are markedly different from what is obtained when muscimol is infused into mPFC, which leads to excessive premature responding and a reduction of RTs to stimuli at short delays (Narayanan et al., 2006). We also tested animals during the RT task after inactivating the agranular insular cortex, which contains neurons that projects to and receives from RFA and mPFC, and found no effects on RT performance. Together, these studies suggest that RFA is a premotor region in the rat frontal cortex that competes with mPFC to control action selection. We suggest that RFA controls the threshold that is used to initiate responding and generates prepotent excitation over responding that is crucial for temporal preparation.

Implication of dopaminergic projection from the ventral tegmental area to the anterior cingulate cortex in μ-opioid-induced place preference

Despite the importance of prefrontal cortical dopamine in modulating reward, little is known about the implication of the specific subregion of prefrontal cortex in opioid reward. We investigated the role of neurons projecting from the ventral tegmental area (VTA) to the anterior cingulate cortex (ACG) in opioid reward. Microinjection of the retrograde tracer fluorogold (FG) into the ACG revealed several retrogradely labelled cells in the VTA. The FG-positive reactions were noted in both tyrosine hydroxylase (TH)-positive and -negative VTA neurons. The released levels of dopamine and its major metabolites in the ACG were increased by either the electrical stimulation of VTA neurons or microinjection of a selective μ-opioid receptor (MOR) agonist, (D-Ala²,N-MePhe⁴,Gly-ol⁵) enkephalin (DAMGO), into the VTA. MOR-like immunoreactivity was seen in both TH-positive and -negative VTA neurons projecting to the ACG. The conditioned place preference induced by intra-VTA injection of DAMGO was significantly attenuated by chemical lesion of dopaminergic terminals in the ACG. The depletion of dopamine in the ACG induced early extinction of μ-opioid-induced place preference. The levels of phosphorylated DARPP32 (Thr34) and phosphorylated CREB (Ser133) were increased in the ACG of rats that had maintained the morphine-induced place preference, whereas the increases of these levels induced by morphine were blocked by pre-treatment of a selective dopamine D1 receptor antagonist SCH23390. These findings suggest that VTA-ACG transmission may play a crucial role in the acquisition and maintenance of μ-opioid-induced place preference. The activation of DARPP32 and CREB through dopamine D1 receptors in the ACG could be implicated in the maintenance of μ-opioid-induced place preference.

Cognitive activation by central thalamic stimulation: the yerkes-dodson law revisited

Central thalamus regulates forebrain arousal, influencing activity in distributed neural networks that give rise to organized actions during alert, wakeful states. Central thalamus has been implicated in working memory by the effects of lesions and microinjected drugs in this part of the brain. Lesions and drugs that inhibit neural activity have been found to impair working memory. Drugs that increase activity have been found to enhance and impair memory depending on the dose tested. Electrical deep brain stimulation (DBS) similarly enhances working memory at low stimulating currents and impairs it at higher currents. These effects are time dependent. They were observed when DBS was applied during the memory delay (retention) or choice response (retrieval) but not earlier in trials during the sample (acquisition) phase. The effects of microinjected drugs and DBS are consistent with the Yerkes-Dodson law, which describes an inverted-U relationship between arousal and behavioral performance. Alternatively these results may reflect desensitization associated with higher levels of stimulation, spread of drugs or current to adjacent structures, or activation of less sensitive neurons or receptors at higher DBS currents or drug doses.

The role of nicotinic acetylcholine receptors in the medial prefrontal cortex and hippocampus in trace fear conditioning

Acute nicotine enhances multiple types of learning including trace fear conditioning but the underlying neural substrates of these effects are not well understood. Trace fear conditioning critically involves the medial prefrontal cortex and hippocampus, which both express nicotinic acetylcholine receptors (nAChRs). Therefore, nicotine could act in either or both areas to enhance trace fear conditioning. To identify the underlying neural areas and nAChR subtypes, we examined the effects of infusion of nicotine, or nicotinic antagonists dihydro-beta-erythroidine (DHβE: high-affinity nAChRs) or methyllycaconitine (MLA: low-affinity nAChRs) into the dorsal hippocampus, ventral hippocampus, and medial prefrontal cortex (mPFC) on trace and contextual fear conditioning. We found that the effects of nicotine on trace and contextual fear conditioning vary by brain region and nAChR subtype. The dorsal hippocampus was involved in the effects of nicotine on both trace and contextual fear conditioning but each task was sensitive to different doses of nicotine. Additionally, dorsal hippocampal infusion of the antagonist DHβE produced deficits in trace but not contextual fear conditioning. Nicotine infusion into the ventral hippocampus produced deficits in both trace and contextual fear conditioning. In the mPFC, nicotine enhanced trace but not contextual fear conditioning. Interestingly, infusion of the antagonists MLA or DHβE in the mPFC also enhanced trace fear conditioning. These findings suggest that nicotine acts on different substrates to enhance trace versus contextual fear conditioning, and that nicotine-induced desensitization of nAChRs in the mPFC may contribute to the effects of nicotine on trace fear conditioning.

Regulation of extinction-related plasticity by opioid receptors in the ventrolateral periaqueductal gray matter

Recent work has led to a better understanding of the neural mechanisms underlying the extinction of Pavlovian fear conditioning. Long-term synaptic changes in the medial prefrontal cortex (mPFC) are critical for extinction learning, but very little is currently known about how the mPFC and other brain areas interact during extinction. The current study examined the effect of drugs that impair the extinction of fear conditioning on the activation of the extracellular-related kinase/mitogen-activated protein kinase (ERK/MAPK) in brain regions that likely participate in the consolidation of extinction learning. Inhibitors of opioid and N-methyl-d-aspartic acid (NMDA) receptors were applied to the ventrolateral periaqueductal gray matter (vlPAG) and amygdala shortly before extinction training. Results from these experiments show that blocking opioid receptors in the vlPAG prevented the formation of extinction memory, whereas NMDA receptor blockade had no effect. Conversely, blocking NMDA receptors in the amygdala disrupted the formation of fear extinction memory, but opioid receptor blockade in the same brain area did not. Subsequent experiments tested the effect of these drug treatments on the activation of the ERK/MAPK signaling pathway in various brain regions following extinction training. Only opioid receptor blockade in the vlPAG disrupted ERK phosphorylation in the mPFC and amygdala. These data support the idea that opiodergic signaling derived from the vlPAG affects plasticity across the brain circuit responsible for the formation of extinction memory.

Cross-correlation of instantaneous amplitudes of field potential oscillations: a straightforward method to estimate the directionality and lag between brain areas

Researchers performing multi-site recordings are often interested in identifying the directionality of functional connectivity and estimating lags between sites. Current techniques for determining directionality require spike trains or involve multivariate autoregressive modeling. However, it is often difficult to sample large numbers of spikes from multiple areas simultaneously, and modeling can be sensitive to noise. A simple, model-independent method to estimate directionality and lag using local field potentials (LFPs) would be of general interest. Here we describe such a method using the cross-correlation of the instantaneous amplitudes of filtered LFPs. The method involves four steps. First, LFPs are band-pass filtered; second, the instantaneous amplitude of the filtered signals is calculated; third, these amplitudes are cross-correlated and the lag at which the cross-correlation peak occurs is determined; fourth, the distribution of lags obtained is tested to determine if it differs from zero. This method was applied to LFPs recorded from the ventral hippocampus and the medial prefrontal cortex in awake behaving mice. The results demonstrate that the hippocampus leads the mPFC, in good agreement with the time lag calculated from the phase locking of mPFC spikes to vHPC LFP oscillations in the same dataset. We also compare the amplitude cross-correlation method to partial directed coherence, a commonly used multivariate autoregressive model-dependent method, and find that the former is more robust to the effects of noise. These data suggest that the cross-correlation of instantaneous amplitude of filtered LFPs is a valid method to study the direction of flow of information across brain areas.

Interaction of the rostral basolateral amygdala and prelimbic prefrontal cortex in regulating reinstatement of cocaine-seeking behavior

Previous findings in rats suggest that the rostral basolateral amygdala (rBLA) and prelimbic prefrontal cortex (plPFC) are likely components of cue reinstatement circuitry based on bilateral inactivation of each site alone. In the present investigation, we examined whether the rBLA and plPFC interact to regulate reinstatement of cocaine-seeking behavior elicited by reexposure to a combination of discrete and contextual cocaine-paired cues. After establishing stable baseline responding under a second-order schedule of cocaine reinforcement and cue presentation, rats underwent response-extinction training in which cocaine and cocaine-paired cues were withheld. To test the interaction, rats with asymmetric cannulae placements in the rBLA and plPFC received vehicle or lidocaine infusions prior to reinstatement testing during which cocaine-paired cues were presented, in the absence of cocaine availability, under a second-order schedule. Asymmetric inactivation of the rBLA and plPFC significantly attenuated reinstatement of cocaine-seeking behavior relative to vehicle treatment. As expected, inactivation of the rBLA or plPFC in rats with unilateral cannulae placements did not disrupt reinstatement relative to vehicle treatment. Findings propose critical intrahemispheric interaction between the rBLA and plPFC in regulating reinstatement of cocaine-seeking behavior elicited by reexposure to drug-paired cues.

Induction of fear extinction with hippocampal-infralimbic BDNF

The extinction of conditioned fear memories requires plasticity in the infralimbic medial prefrontal cortex (IL mPFC), but little is known about the molecular mechanisms involved. Brain-derived neurotrophic factor (BDNF) is a key mediator of synaptic plasticity in multiple brain areas. In rats subjected to auditory fear conditioning, BDNF infused into the IL mPFC reduced conditioned fear for up to 48 hours, even in the absence of extinction training, which suggests that BDNF substituted for extinction. Similar to extinction, BDNF-induced reduction in fear required N-methyl-D-aspartate receptors and did not erase the original fear memory. Rats failing to learn extinction showed reduced BDNF in hippocampal inputs to the IL mPFC, and augmenting BDNF in this pathway prevented extinction failure. Hence, boosting BDNF activity in hippocampal-infralimbic circuits may ameliorate disorders of learned fear.

Neurons in the rat anterior cingulate cortex dynamically encode cost-benefit in a spatial decision-making task

Optimal decision-making often requires an assessment of the costs and benefits associated with each available course of action. Previous studies have shown that lesions to the anterior cingulate cortex (ACC) impair cost-benefit decision-making in laboratory animals, but the neural mechanisms underlying the deficit are not well understood. We recorded from ACC neurons in freely moving rats as they performed a spatial decision-making task whereby, in the baseline configuration "2:6B," rats could pursue two or six food pellets, the latter obtained by climbing a barrier [high cost, high reward (HCHR)]. In this configuration, the mean percentage of HCHR choices was 69 +/- 4%, and a substantial portion of ACC neurons (63%) exhibited significantly higher firing for one goal trajectory versus the other; for 94% of these cells, higher firing was associated with the HCHR option. This HCHR bias was not simply attributable to the larger reward, the barrier, or behavioral preference. In intersession and intrasession manipulations involving at least one barrier (2:6B, 2B:6B, and 2:2B), ACC activity rapidly adapted and was consistently biased toward the economically advantageous option relative to the configuration. Interestingly, when only a difference in reward magnitude was presented (2:6, no barrier, HCHR choices of 84 +/- 4%), ACC activity was minimal and nonbiased. One interpretation of our data is that the ACC encodes a relative, integrated cost-benefit representation of available choice options that is biased toward the "better" option in terms of effort/outcome ratio. This representation may be specifically recruited when an assessment of reward and effort is required to optimally perform a task.

Repeated social defeat selectively increases δFosB expression and histone H3 acetylation in the infralimbic medial prefrontal cortex

Exposure to social stress has been linked to the development and maintenance of mood-related psychopathology; however, the underlying neurobiological changes remain uncertain. In this study, we examined numbers of δFosB-immunoreactive cells in the forebrains of rats subjected to 12 episodes of social defeat. This was achieved using the social conflict model whereby animals are introduced into the home cage of older males ("residents") trained to attack and defeat all such "intruders"; importantly, controls were treated identically except that the resident was absent. Our results indicated that the only region in which δFosB-positive cells were found in significantly higher numbers in intruders than in controls was the infralimbic medial prefrontal cortex (mPFC). This same effect was not apparent using another psychological stressor, noise stress. Cells of the infralimbic mPFC also displayed evidence of chromatin remodeling. We found that exposure to repeated episodes of social defeat increased numbers of cells immunoreactive for histone H3 acetylation, but not for histone H3 phosphoacetylation, in the infralimbic mPFC. Collectively, these findings highlight the importance of the infralimbic mPFC in responding to social stress-a finding that provides insight into the possible neurobiological alterations associated with stress-induced psychiatric illness.

Interactive effects of stress and aging on structural plasticity in the prefrontal cortex

Neuronal networks in the prefrontal cortex mediate the highest levels of cognitive processing and decision making, and the capacity to perform these functions is among the cognitive features most vulnerable to aging. Despite much research, the neurobiological basis of age-related compromised prefrontal function remains elusive. Many investigators have hypothesized that exposure to stress may accelerate cognitive aging, though few studies have directly tested this hypothesis and even fewer have investigated a neuronal basis for such effects. It is known that in young animals, stress causes morphological remodeling of prefrontal pyramidal neurons that is reversible. The present studies sought to determine whether age influences the reversibility of stress-induced morphological plasticity in rat prefrontal neurons. We hypothesized that neocortical structural resilience is compromised in normal aging. To directly test this hypothesis we used a well characterized chronic restraint stress paradigm, with an additional group allowed to recover from the stress paradigm, in 3-, 12-, and 20-month-old male rats. In young animals, stress induced reductions of apical dendritic length and branch number, which were reversed with recovery; in contrast, middle-aged and aged rats failed to show reversible morphological remodeling when subjected to the same stress and recovery paradigm. The data presented here provide evidence that aging is accompanied by selective impairments in long-term neocortical morphological plasticity.

Plastic synaptic networks of the amygdala for the acquisition, expression, and extinction of conditioned fear

The last 10 years have witnessed a surge of interest for the mechanisms underlying the acquisition and extinction of classically conditioned fear responses. In part, this results from the realization that abnormalities in fear learning mechanisms likely participate in the development and/or maintenance of human anxiety disorders. The simplicity and robustness of this learning paradigm, coupled with the fact that the underlying circuitry is evolutionarily well conserved, make it an ideal model to study the basic biology of memory and identify genetic factors and neuronal systems that regulate the normal and pathological expressions of learned fear. Critical advances have been made in determining how modified neuronal functions upon fear acquisition become stabilized during fear memory consolidation and how these processes are controlled in the course of fear memory extinction. With these advances came the realization that activity in remote neuronal networks must be coordinated for these events to take place. In this paper, we review these mechanisms of coordinated network activity and the molecular cascades leading to enduring fear memory, and allowing for their extinction. We will focus on Pavlovian fear conditioning as a model and the amygdala as a key component for the acquisition and extinction of fear responses.

Prefrontal control of fear: more than just extinction

Although fear research has largely focused on the amygdala, recent findings highlight cortical control of the amygdala in the service of fear regulation. In rodent models, it is becoming well established that the infralimbic (IL) prefrontal cortex plays a key role in extinction learning, and recent findings are uncovering molecular mechanisms involved in extinction-related plasticity. Furthermore, mounting evidence implicates the prelimbic (PL) prefrontal cortex in the production of fear responses. Both IL and PL integrate inputs from the amygdala, as well as other structures to gate the expression of fear via projections to inhibitory or excitatory circuits within the amygdala. We suggest that dual control of the amygdala by separate prefrontal modules increases the flexibility of an organism's response to danger cues.

Are the dorsal and ventral hippocampus functionally distinct structures?

One literature treats the hippocampus as a purely cognitive structure involved in memory; another treats it as a regulator of emotion whose dysfunction leads to psychopathology. We review behavioral, anatomical, and gene expression studies that together support a functional segmentation into three hippocampal compartments: dorsal, intermediate, and ventral. The dorsal hippocampus, which corresponds to the posterior hippocampus in primates, performs primarily cognitive functions. The ventral (anterior in primates) relates to stress, emotion, and affect. Strikingly, gene expression in the dorsal hippocampus correlates with cortical regions involved in information processing, while genes expressed in the ventral hippocampus correlate with regions involved in emotion and stress (amygdala and hypothalamus).

Synchronized activity between the ventral hippocampus and the medial prefrontal cortex during anxiety

The ventral hippocampus, unlike its dorsal counterpart, is required for anxiety-like behavior. The means by which it acts are unknown. We hypothesized that the hippocampus synchronizes with downstream targets that influence anxiety, such as the medial prefrontal cortex (mPFC). To test this hypothesis, we recorded mPFC and hippocampal activity in mice exposed to two anxiogenic arenas. Theta-frequency activity in the mPFC and ventral, but not dorsal, hippocampus was highly correlated at baseline, and this correlation increased in both anxiogenic environments. Increases in mPFC theta power predicted avoidance of the aversive compartments of each arena and were larger in serotonin 1A receptor knockout mice, a genetic model of increased anxiety-like behavior. These results suggest a role for theta-frequency synchronization between the ventral hippocampus and the mPFC in anxiety. They are consistent with the notion that such synchronization is a general mechanism by which the hippocampus communicates with downstream structures of behavioral relevance.

Serotonin modulates fast-spiking interneuron and synchronous activity in the rat prefrontal cortex through 5-HT1A and 5-HT2A receptors

Alterations of the serotonergic system in the prefrontal cortex (PFC) are implicated in psychiatric disorders such as schizophrenia and depression. Although abnormal synchronous activity is observed in the PFC of these patients, little is known about the role of serotonin (5-HT) in cortical synchrony. We found that 5-HT, released by electrical stimulation of the dorsal raphe nucleus (DRN) in anesthetized rats, regulates the frequency and the amplitude of slow (<2 Hz) waves in the PFC via 5-HT(2A) receptors (5-HT(2A)Rs). 5-HT also modulates prefrontal gamma (30-80 Hz) rhythms through both 5-HT(1A)Rs and 5-HT(2A)Rs, but not 5-HT(2C)Rs, inducing an overall decrease in the amplitude of gamma oscillations. Because fast-spiking interneurons (FSi) are involved in the generation of gamma waves, we examined serotonergic modulation of FSi activity in vivo. Most FSi are inhibited by serotonin through 5-HT(1A)Rs, while a minority is activated by 5-HT(2A)Rs, and not 5-HT(2C)Rs. In situ hybridization histochemistry confirmed that distinct populations of FSi in the PFC express 5-HT(1A)Rs and 5-HT(2A)Rs, and that the number of FSi expressing 5-HT(2C)Rs is negligible. We conclude that 5-HT exerts a potent control on slow and gamma oscillations in the PFC. On the one hand, it shapes the frequency and amplitude of slow waves through 5-HT(2A)Rs. On the other hand, it finely tunes the amplitude of gamma oscillations through 5-HT(2A)R- and 5-HT(1A)R-expressing FSi, although it primarily downregulates gamma waves via the latter population. These results may provide insight into impaired serotonergic control of network activity in psychiatric illnesses such as schizophrenia and depression.

Pathways from the ventral hippocampus and caudal amygdala to forebrain regions that regulate sensorimotor gating in the rat

The neural substrates regulating sensorimotor gating in rodents are studied in order to understand the basis for gating deficits in clinical disorders such as schizophrenia. N-methyl-D-aspartate (NMDA) infusion into the ventral temporal lobe, including caudal parts of the ventral hippocampal region and amygdala, has been shown to disrupt sensorimotor gating in rats, as measured by prepulse inhibition (PPI) of startle. One working model is that reduced PPI after infusion of NMDA into this region is mediated via its efferents to ventral forebrain structures, i.e. medial prefrontal cortex (mPFC) and nucleus accumbens. Yet, PPI-disruptive effects persist after lesions of the precommissural fornix, the principal output pathway of the hippocampal formation. Here, we aimed to characterize non-fornical forebrain projections from this region that might mediate the PPI-disruptive effects of the ventral temporal lobe. Electrolytic lesions of the precommissural fornix in male Sprague-Dawley rats were followed by infusions of fluorogold into the mPFC or by infusions of biotinylated dextan amine into the ventral temporal lobe. Projections from the ventral subiculum and CA1 regions of the ventral hippocampus to the mPFC and accumbens core and shell were interrupted by fornix lesions. Projections to the mPFC and accumbens from other regions of the ventral temporal lobe, particularly the lateral entorhinal cortex and the embedded olfactory and vomeronasal parts of the caudal amygdala, survived fornix lesions. These additional projections coursed rostrally through the amygdala and emerged via the stria terminalis, interstitial nuclei of the posterior limb of the anterior commissure, and the ventral amygdalofugal pathway. PPI-regulatory portions of the ventral temporal lobe innervate the accumbens and mPFC via multiple routes. It remains to be determined which of these non-fornical projections may be responsible for the persistent regulation of PPI after fornix lesions.

Distinct patterns of neuronal inputs and outputs of the juxtaparaventricular and suprafornical regions of the lateral hypothalamic area in the male rat

We have analyzed at high resolution the neuroanatomical connections of the juxtaparaventricular region of the lateral hypothalamic area (LHAjp); as a control and in comparison to this, we also performed a preliminary analysis of a nearby LHA region that is dorsal to the fornix, namely the LHA suprafornical region (LHAs). The connections of these LHA regions were revealed with a coinjection tract-tracing technique involving a retrograde (cholera toxin B subunit) and anterograde (Phaseolus vulgaris leucoagglutinin) tracer. The LHAjp and LHAs together connect with almost every major division of the cerebrum and cerebrospinal trunk, but their connection profiles are markedly different and distinct. In simple terms, the connections of the LHAjp indicate a possible primary role in the modulation of defensive behavior; for the LHAs, a role in the modulation of ingestive behavior is suggested. However, the relation of the LHAjp and LHAs to potential modulation of these behaviors, as indicated by their neuroanatomical connections, appears to be highly integrative as it includes each of the major functional divisions of the nervous system that together determine behavior, i.e., cognitive, state, sensory, and motor. Furthermore, although a primary role is indicated for each region with respect to a particular mode of behavior, intermode modulation of behavior is also indicated. In summary, the extrinsic connections of the LHAjp and LHAs (so far as we have described them) suggest that these regions have a profoundly integrative role in which they may participate in the orchestrated modulation of elaborate behavioral repertoires.

Inactivation or inhibition of neuronal activity in the medial prefrontal cortex largely reduces pup retrieval and grouping in maternal rats

Previous research suggests that the maternal medial prefrontal cortex (mPFC) may play a role in maternal care and that cocaine sensitization before pregnancy can affect neuronal activity within this region. The present work was carried out to test whether the mPFC does actually play a role in the expression of maternal behaviors in the rats and to understand what specific behaviors this cortical area may modulate. In the first experiment, tetrodotoxin (TTX) was used to chemically inactivate the mPFC during tests for maternal behavior latencies. Lactating rats were tested on postpartum days 7-9. The results of this first experiment indicate that there is a large effect of TTX-induced inactivation on retrieval behavior latencies. TTX nearly abolished the expression of maternal retrieval of pups without significantly impairing locomotor activity. In the second experiment, GABA-mediated inhibition was used to test maternal behavior latencies and durations of maternal and other behaviors in postpartum dams. In agreement with experiment 1, it was observed that dams capable of retrieving are rendered incapable by inhibition in the mPFC. GABA-mediated inhibition in the mPFC largely reduced retrieval without altering other indices of maternal care and non-specific behavior such as ambulation time, self-grooming, and inactivity. Moreover, in both experiments, dams were able to establish contact with pups within seconds. The overall results indicate that the mPFC may play an active role in modulating maternal care, particularly retrieval behavior. External factors that affect the function of the frontal cortical site may result in significant impairments in maternal goal-directed behavior as reported in our earlier work.

Neuronal circuits of fear extinction

Fear extinction is a form of inhibitory learning that allows for the adaptive control of conditioned fear responses. Although fear extinction is an active learning process that eventually leads to the formation of a consolidated extinction memory, it is a fragile behavioural state. Fear responses can recover spontaneously or subsequent to environmental influences, such as context changes or stress. Understanding the neuronal substrates of fear extinction is of tremendous clinical relevance, as extinction is the cornerstone of psychological therapy of several anxiety disorders and because the relapse of maladaptative fear and anxiety is a major clinical problem. Recent research has begun to shed light on the molecular and cellular processes underlying fear extinction. In particular, the acquisition, consolidation and expression of extinction memories are thought to be mediated by highly specific neuronal circuits embedded in a large-scale brain network including the amygdala, prefrontal cortex, hippocampus and brain stem. Moreover, recent findings indicate that the neuronal circuitry of extinction is developmentally regulated. Here, we review emerging concepts of the neuronal circuitry of fear extinction, and highlight novel findings suggesting that the fragile phenomenon of extinction can be converted into a permanent erasure of fear memories. Finally, we discuss how research on genetic animal models of impaired extinction can further our understanding of the molecular and genetic bases of human anxiety disorders.

Cortical efferents of the perirhinal, postrhinal, and entorhinal cortices of the rat

We investigated the cortical efferents of the parahippocampal region by placing injections of the anterograde tracers, Phaseolus vulgaris-leuccoagglutinin, and biotinylated dextran amine, throughout the perirhinal (PER), postrhinal (POR), and entorhinal cortices of the rat brain. The resulting density of labeled fibers was evaluated in 25 subregions of the piriform, frontal, insular, temporal, cingulate, parietal, and occipital areas. The locations of labeled terminal fibers differed substantially depending on whether the location of the injection site was in PER area 35, PER area 36, POR, or the lateral or the medial entorhinal (LEA and MEA). The differences were greater for sensory regions. For example, the POR efferents preferentially target visual and spatial regions, whereas the PER efferents target all sensory modalities. The cortical efferents of each region largely reciprocate the cortical afferents, though the degree of reciprocity varied across originating and target regions. The laminar pattern of terminal fibers was consistent with the notion that the efferents are feedback projections. The density and amount of labeled fibers also differed substantially depending on the regional location of injection sites. PER area 36 and POR give rise to a greater number of heavy projections, followed by PER area 35. LEA also gives rise to widespread cortical efferents, arising mainly from a narrow band of cortex adjacent to the PER. In contrast, the remainder of the LEA and the MEA provides only weak efferents to cortical regions. Prior work has shown that nonspatial and spatial information is transmitted to the hippocampus via the PER-LEA and POR-MEA pathways, respectively. Our findings suggest that the return projections follow the same pathways, though perhaps with less segregration.