Prefrontal cortical inputs to the basal amygdala undergo pruning during late adolescence in the rat

Transformations in affective and social behaviors, many of which involve amygdalar circuits, are hallmarks of adolescence in many mammalian species. In this study, using the rat as a model, we provide the first evidence that afferents of the basal amygdala (BA) undergo significant structural remodeling during adolescence. We used quantitative tract‐tracing and gene expression profiling methods to characterize changes in the medial prefrontal cortical (mPFC) inputs to the BA across ages analogous to the late juvenile period [postnatal day (P) 25], late adolescence (P45), and adulthood (P90) in the rat. As assessed after deposition of Fluorogold into the BA, the number of BA‐projecting neurons in the mPFC remained stable between P25 and P45 but decreased by about 50% between P45 and P90. Anterograde tract tracing with biotin dextran amine deposits centered in the ventral prelimbic cortex revealed that, during this period, the density of mPFC‐derived axon terminals in the BA also decrease significantly, an effect particularly evident in the dorsal basolateral nucleus. Within the BA, there were also highly significant changes in gene expression indicative of neurite or synaptic plasticity, most notably in the Ras/GTPase superfamily, and in pathways that regulate cytoskeletal dynamics and steroid synthesis/lipid metabolism. These data provide convergent evidence that mPFC inputs to the BA are pruned during late adolescence or early adulthood. Moreover, the structural remodeling within these afferents may be accompanied by significant changes in neurite plasticity within the BA. J. Comp. Neurol. 518:2693–2709, 2010. © 2010 Wiley‐Liss, Inc.     

Variations in binding of [H]5-HT to cortical membranes during the female rat estrous cycle

The binding of [³H]5-hydroxytryptamine ([³]-HT) to cortical membranes was examined in female rats during diestrus, proestrus and estrus. Serotonin binding was lowest during the early afternoon of proestrus and highest during the afternoon of estrus with diestrous values in between. The high estrous values were associated with a significant increase in B_max and a decrease in K_d. However, the increase in binding actually took place during the late afternoon of proesttrus, when there was a rapid increase in binding from noon to 18.00 h. Binding remained at this elevated level throughout the next day. There were no changes in the binding of [³H]ketanserin or [³H]spiroperidol to cortical membranes during the estrous cycle, so the differential binding of [³H]5-HT is most likely the result of variations in 5-HT₁ rather than 5-HT₂ receptors. These observations of changes in serotonin binding in a brain area nearly devoid of sex steroid receptors suggest that the hormonal fluctuations accompanying the female estrous cycle influence brain areas other than those classically thought to regulate neuroendocrine function.     

Variations in binding of [3H]5-HT to cortical membranes during the female rat estrous cycle

The binding of [3H]5-hydroxytryptamine ([3H]5-HT) to cortical membranes was examined in female rats during diestrus, proestrus and estrus. Serotonin binding was lowest during the early afternoon of proestrus and highest during the afternoon of estrus with diestrous values in between. The high estrous values were associated with a significant increase in Bmax and a decrease in Kd. However, the increase in binding actually took place during the late afternoon of proestrus, when there was a rapid increase in binding from noon to 18.00 h. Binding remained at this elevated level throughout the next day. There were no changes in the binding of [3H]ketanserin or [3H]spiroperidol to cortical membranes during the estrous cycle, so the differential binding of [3H]5-HT is most likely the result of variations in 5-HT1 rather than 5-HT2 receptors. These observations of changes in serotonin binding in a brain area nearly devoid of sex steroid receptors suggest that the hormonal fluctuations accompanying the female estrous cycle influence brain areas other than those classically thought to regulate neuroendocrine function.     

Delayed onset of late movement-related cortical potentials and abnormal response to lorazepam in catatonia

Catatonia is a psychomotor syndrome with an inability to execute and terminate movements completely, leading consecutively to akinesia and posturing, which both respond almost immediately to benzodiazepines, i.e. gaba-potentiators like lorazepam. However, pathophysiological mechanisms of cortical motor and gaba-ergic dysfunction remain unclear. We therefore investigated movement-related cortical potentials (MRPs) and movement kinematics during a motor task before and after lorazepam.Ten akinetic catatonic patients were compared with 10 psychiatric (similar age, sex, medication, and underlying psychiatric disease but without catatonic syndrome) and 20 healthy controls. MRPs from frontal (F), central (C), and parietal (P) sites were recorded to obtain measures of early and late readiness potential and movement potential. Kinematic measures included parameters for amplitude of movements, peak velocity, average duration of movements, elevation angle, and angle velocity. The motor task consisted in self-initiated extension of the right index finger. All catatonic and psychiatric control patients received intravenous lorazepam (1mg), whereas healthy controls were subjected to a placebo-controlled (10 received lorazepam, 10 received placebo) double-blind study design.Catatonics showed a significantly delayed onset of late readiness and movement potential in central electrodes (Cz, C3) compared with psychiatric and healthy controls. This delayed onset correlated significantly with catatonic motor symptoms and movement duration. Lorazepam led to significantly stronger delays in onset of late readiness potential in left fronto-parietal (F3, C3, P3) electrodes in catatonic patients than in psychiatric and healthy controls.It is concluded that delayed latencies in late MRP components in catatonic patients may reflect their inability to execute and terminate movements completely. Differential and stronger response to lorazepam in catatonia suggests dysfunction in inhibitory control of cortical motor function with increased gaba-ergic sensitivity.     

The cerebral cortical appearance in late paraphrenia

The cortical appearance on CT scan in patients suffering from late paraphrenia was assessed compared to that of age-matched normal controls. There was no difference in overall cortical appearance between the two groups. However, a correlation existed between ventricular size and cortical size and between ventricular size and age in normal controls but this was not present in paraphrenia. The implications of these findings are discussed in relation to a possible structural abnormality in late paraphrenia.     

Hippocampal-prefrontal cortical circuit mediates inhibitory response control in the rat

We investigated the interdependent function of the ventral hippocampus (vHC) and ventral prefrontal cortex (vPFC) in visuospatial attention and inhibitory control using a disconnection lesion approach. Rats were trained, and several aspects of their cognitive performance tested on the 5-choice reaction time task. The animals were prepared with unilateral lesions of both the vPFC and vHC, either in the same hemisphere ("ipsilateral") or in opposite hemispheres ("disconnection"). The disconnection lesion led to both impulsive and compulsive behavior. This deficit is reminiscent of the effects of either bilateral vPFC or bilateral vHC lesions on the same task and is thought to reflect the bihemispheric disruption of the hippocampal-prefrontal circuit. With ipsilateral lesions, behavioral deficits were transient or absent altogether, suggesting that the intact hemisphere was able to exert near normal levels of behavioral control. These behavioral effects were observed in the absence of any changes to visual attention, speed of response, or general motivation. This study provides evidence that optimal inhibitory control of behavior draws upon the functional interaction between the vHC and vPFC.     

Asymmetrical cerebrovascular response to right and left hemisphere cortical suction lesions in the rat

We have previously demonstrated that focal lesions of the right but not left cerebral cortex in the rat produce hyperactivity and depletions of norepinephrine. To assess the cerebrovascular response to suction lesions, postoperative rats were infused intravenously with [¹⁴C]inulin. The brain inulin concentration in the right lesion group was significantly higher than control (no surgery) or left lesion groups in all cortical quadrants, but most prominently in the quadrant posterior and contralateral to the lesion site.     

The thyrotropin response to thyrotropin-releasing hormone in endogenous depression

The maximal response of thyrotropin (TSH) to thyrotropin-releasing hormone (TRH) (Δ max TSH) is reduced in endogenous depression (ED). In the majority of studies Δ max TSH was reduced to the same degree in unipolar and bipolar depression, as well as in manic patients. Decreases present in anorexia nervosa and heroin addiction possibly were secondary to reduced caloric intake and administration of heroin, respectively. When the TRH test was repeated in patients with ED who were clinically cured after short-term antidepressant therapy, the patients could be divided into two groups, one with an increase of Δ max TSH > 2.0 μU/ml, and another with an increase <-2.0 μU/ml. After withdrawal of treatment, patients in the former group did well for at least six months, whereas patients in the latter group relapsed within a median time of two months. Predictions were correct in 60 of 66 patients. However, the change of the TSH response during the course of ED is relatively small, and thus great care should be taken to diminish both the analytical and the normal biological variations. Electroconvulsive therapy (ECT), tricyclic antidepressants, tryptophan and sleep deprivation, respectively, did not influence Δ max TSH, whereas lithium, because of an inhibition of thyroid hormone release and negative feedback, induced an increase of Δ max TSH unrelated to the course of ED. p] Although serum levels of the thyroid hormones and cortisol changed during the course of ED, no relationship could be demonstrated between Δ max TSH on the one hand and the free serum levels of thyroxine and 3,3′,5-triiodothyronine, as expressed by the free indices, or the serum levels of cortisol, on the other hand. Neither did Δ max TSH in the depressed patients correlate significantly with the cerebrospinal fluid (CSF) levels of thyroxine, cortisol or TRH. Thus, CSF levels of TRH were unrelated to the clinical outcome of ED, but 70% of the values exceeded the upper range of neurologic controls. Therefore, it is possible that CSF TRH might be of diagnostic value in patients with ED.     

Variability of cortical evoked potentials in response to tooth pulp stimulation in the freely moving rat

Evoked potentials (EP) due to the stimulation of the upper incisor tooth pulp were recorded from the somatosensory cortex of the freely moving adult rats. Background EEG, motor activity of an animal and respiratory potentials of the olfactory bulb were recorded simultaneously. EP configurations and mean amplitudes of primary complex (P1 + N1) differed significantly during states of sleep, drowsiness, relaxed wakefulness, grooming and exploratory behaviour; primary complex amplitude during intensive motor activities was several times less than during periods without movements. Negative correlation of the EP amplitude and instant respiration rate was found during relaxed wakefulness: it was less pronounced during periods with motor activities. At the same time direct parallelism between changes in EP and respiration rate was absent: the EP depression was maximal during grooming, while the respiration rate was minimal during exploratory behaviour.     

Positive and negative ultrasonic social signals elicit opposing firing patterns in rat amygdala

Rat ultrasonic vocalizations (USVs) are ethologically-essential social signals. Under natural conditions, 22 kHz USVs and 50 kHz USVs are emitted in association with negative and positive emotional states, respectively. Our first experiment examined freezing behavior elicited in naïve Sprague-Dawley rats by a 22 kHz USV, a 50 kHz USV, and frequency-matched tones. None of the stimuli elicited freezing, which is the most commonly-used index of fear. The second experiment examined single-unit responses to these stimuli in the amygdala (AM), which is well-known for its role in innate and acquired fear responses. Among 127 well-discriminated single units, 82% were auditory-responsive. Elicited firing patterns were classified using a multi-dimensional scheme that included transient (phasic) responses to the stimulus onsets and/or offsets as well as sustained (tonic) responses during the stimulus. Tonic responses, which are not ordinarily evaluated in AM, were 4.4-times more common than phasic responses. The 22 kHz stimuli tended to elicit tonic increases in the firing rates, whereas the 50 kHz stimuli more often elicited tonic decreases in firing rates. These opposing tonic responses correspond with the ethological valence of USVs in the two frequency bands. Thus, a relatively-small sample of single-unit responses in AM furnished a more sensitive index of emotional valence than freezing behavior. Latency analysis suggested that stimuli in the two frequency bands are processed through different pathways to AM. One possible interpretation is that phasic responses in AM reflect the detection of a stimulus change, whereas tonic responses indicate the valence of the detected stimulus.     

Gamma-frequency fluctuations of the membrane potential and response selectivity in visual cortical neurons

Fluctuations at frequencies of 25-70 Hz is an inherent property of cortical activity. These rapid, gamma-range fluctuations are apparent in the local field potentials, in spiking of cells and cell groups, and in the membrane potential of neurons. To investigate stimulus dependence of the gamma-frequency fluctuations of the membrane potential, we have recorded intracellularly responses of cells in cat visual cortex to presentation of moving gratings. We found gamma-range fluctuations of the membrane potential in both simple and complex cells. The strength of the gamma-frequency fluctuations correlated with the stimulus optimality. Furthermore, the amplitude of the gamma-frequency fluctuations correlated with the phase of stimulus-imposed slow changes of the membrane potential. The combination of these features makes cortical neurons capable of encoding the slow changes in the visual world in a kind of amplitude modulation of the high frequency fluctuations. This assures reliable transformation of the membrane potential changes into spike responses without compromising the temporal resolution of visual information encoding in the low frequency range.     

Importance of endogenous nitric oxide synthase in the rat hypothalamus and amygdala in mediating the response to capsaicin

Although capsaicin has been shown to activate certain neuronal groups in the hypothalamus and amygdala, the neurotransmitters involved and the exact mechanism of action are not clearly understood at present. The aim of this study was to examine the hypothesis that the effect of capsaicin in the rat hypothalamus and amygdala primarily involves direct activation of the endogenous nitric oxide synthase (NOS) neurons responsible for the synthesis of nitric oxide (NO). Subcutaneous capsaicin injection in male rats, compared with vehicle, caused a significant increase in Fos expression in the paraventricular nucleus (PVN), supraoptic nucleus (SON), and medial and cortical amygdala. The expression of nicotinamide adenine dinucleotide phosphate diaphorase, a histochemical marker for NOS, was also increased in these brain areas in addition to the periventricular and lateral hypothalamic area and central amygdaloid nucleus. Also, capsaicin significantly increased the expression of neuronal NOS messenger RNA and protein in the PVN, SON, and medial amygdala as demonstrated by in situ hybridization and immunohistochemistry, respectively. A higher proportion of the NOS neurons in the PVN, periventricular region, SON and amygdala showed Fos expression in response to capsaicin than vehicle injection. There was little, if any, Fos activation in the NOS-positive neurons in the lateral hypothalamic area. The capsaicin-induced activation of the hypothalamic PVN and SON neurons and the medial amygdaloid nucleus was attenuated in the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) -pretreated animals in comparison with the inactive enantiomer D-NAME. These observations indicate that activation of the endogenous NOS system and production of NO constitute a major pathway through which capsaicin exerts its effect within the hypothalamus and amygdala.     

Neurons in the superior colliculus of echo-locating bats respond to ultrasonic signals

Using conventional electrophysiological techniques, we demonstrate that neurons in the superior colliculus of the big brown bat (Eptesicus fuscus) respond to ultrasonic signals. Most response properties of these neurons are very similar to neurons of the inferior colliculus in the same bat.     

The role of endogenous dopamine in the hypermotility response to intra-accumbens AMPA.

The present study was designed to investigate the role of dopamine in the locomotor stimulant response produced by the bilateral administration of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) into the nucleus accumbens. The hypermotility produced by lower doses of AMPA (up to 0.25 microgram) was inhibited by either SCH23390 or sulpiride, a D1 and D2 receptor antagonist, respectively. The locomotor response to a higher dose of AMPA (0.5 microgram) was greater than the maximum response to intra-accumbal injection of amphetamine and was significantly inhibited only when both the D1 and D2 antagonists were administered together. Alpha-Methyl-p-tyrosine inhibited the locomotor response to AMPA (0.5 microgram), and this inhibition was reversed by the co-injection of AMPA with either SKF38393, a D1 agonist, or quinpirole, a D2 agonist, at doses which were ineffective in the absence of AMPA. AMPA when infused into the nucleus accumbens produced an increase in extracellular dopamine, suggesting that AMPA can enhance dopamine efflux. The injection of AMPA into the nucleus accumbens significantly increased the DOPAC/dopamine ratio, which is different from the decrease in ratio reported for amphetamine. These data suggest that the stimulation of locomotor activity by intra-accumbal AMPA may be the result of an enhancement in dopamine efflux as well as a change in the response to dopaminergic receptor activation.     

Current source-density and neuromagnetic analysis of the direct cortical response in rat cortex

The electrophysiological basis of macropotentials produced by the direct cortical response (DCR) to electrical stimulation was studied using a combination of current source-density (CSD) and neuromagnetic analysis. Current source-density analysis indicated the locations of extracellular sources and sinks in the cortical depth giving rise to each temporal component of the DCR complex. Information about intradendritic currents was obtained from extracranial magnetic field measures. These data indicate that the DCR is composed of a sequential activation of pyramidal cells at different cortical depths. The complex begins with depolarization of cells in the upper and middle layers, followed by depolarization of deeper pyramidal cells with apical dendrites extending near the cortical surface. The complex ends with a positive-negative slow wave sequence indicating possible afterhyperpolarization of surface dendrites and hyperpolarization of cell bodies and basilar dendrites in the depth. These data demonstrate a unique way in which electrical and magnetic measures may be combined to provide complementary information about the spatially and temporally organized cellular currents within local neuronal networks.     

Orbitomedial prefrontal cortical projections to hypothalamus in the rat

A previous study in the rat revealed that distinct orbital and medial prefrontal cortical (OMPFC) areas projected to specific columns of the midbrain periaqueductal gray region (PAG). This study used anterograde tracing techniques to define projections to the hypothalamus arising from the same OMPFC regions. In addition, injections of anterograde and retrograde tracers were made into different PAG columns to examine connections between hypothalamic regions and PAG columns projected upon by the same OMPFC regions. The most extensive patterns of hypothalamic termination were seen after injection of anterograde tracer in prelimbic and infralimbic (PL/IL) and the ventral and medial orbital (VO/MO) cortices. Projections from rostral PL/IL and VO/MO targeted the rostrocaudal extent of the lateral hypothalamus, as well as lateral perifornical, and dorsal and posterior hypothalamic areas. Projections arising from caudal PL/IL terminated within the dorsal hypothalamus, including the dorsomedial nucleus and dorsal and posterior hypothalamic areas. There were also projections to medial perifornical and lateral hypothalamic areas. In contrast, it was found that anterior cingulate (AC), dorsolateral orbital (DLO), and agranular insular (AId) cortices projected to distinct and restricted hypothalamic regions. Projections arising from AC terminated within dorsal and posterior hypothalamic areas, whereas DLO and AId projected to the lateral hypothalamus. The same OMPFC regions also projected indirectly, by means of specific PAG columns, to many of the same hypothalamic fields. In the context of our previous findings, these data indicate that, in both rat and macaque, parallel but distinct circuits interconnect OMPFC areas with specific hypothalamic regions, as well as PAG columns.     

Evoked release of endogenous adenosine from rat cortical slices by K and glutamate

Release of endogenous adenosine from rat cortical slices was determined in response to depolarization by 30 mM K+ and by exposure to glutamate. K+ and glutamate both released adenosine. Glutamate-evoked release was decreased by approximately 50% in the presence of the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid (APV). The adenosine released by glutamate could modulate neurotransmission, and may have a protective effect in pathologic conditions of excess excitation involving glutamate.