Akinori Ishikawa

Convergence and Interaction of Hippocampal and Amygdalar Projections within the Prefrontal Cortex in the Rat

The orbital and medial prefrontal cortex (OMPFC) receives inputs from the CA1/subicular (CA1/S) region of the ventral hippocampus and the basolateral nucleus of the amygdala (BLA). Despite many studies about these projections, little is known as to how CA1/S and BLA inputs converge and interact within the OMPFC. Extracellular recordings of single-unit activity in the OMPFC were performed in sodium pentobarbitone-anesthetized rats. OMPFC neurons driven by CA1/S or BLA stimulation were more frequently encountered in the ventral portion of the prelimbic (v-PrL) and infralimbic cortex (IL). OMPFC neurons showing excitatory convergence of both inputs from the CA1/S and BLA were also located predominantly in the v-PrL and IL. The excitatory latencies of these neurons from both the CA1/S and BLA revealed almost identical values. Excitatory responses of OMPFC neurons to CA1/S (or BLA) stimulation were markedly augmented by simultaneous BLA (or CA1/S) stimulation, whereas the inhibitory influence of the BLA (or CA1/S) on CA1/S-induced (or BLA-induced) excitation was apparent when BLA (or CA1/S) stimulation was given 20-40 msec before CA1/S (or BLA) stimulation. Similar results were also observed when reciprocal connections between the CA1/S and BLA were severed to exclude the influences of these connections on one another. From these studies, we concluded that excitatory and inhibitory inputs from the hippocampus and amygdala converge and interact in the v-PrL and IL. Furthermore, the results indicate that simultaneous activation of hippocampal and amygdalar neurons may be important for amplification of OMPFC neuronal activity.

Selective rapid eye movement sleep deprivation impairs the maintenance of long-term potentiation in the rat hippocampus

Rapid eye movement (REM) sleep deprivation (RSD) is known to impair learning and memory. Previous studies have demonstrated that RSD induces an impairment of hippocampal long‐term potentiation (LTP). In most of these studies, RSD was set up prior to LTP induction. In this work, we focused on RSD after LTP induction. We investigated the effect of RSD for 24–48 h after induction of LTP in the dentate gyrus on LTP maintenance and whether a REM rebound after 48 h RSD affected LTP. RSD rats were deprived of REM sleep by stroking their backs using a brush, whereas control rats were allowed to sleep freely. Another control group of rats was awoken during non‐REM sleep (NRS) under the same conditions (NRS group). REM‐deprived rats displayed a faster decay of population spike amplitudes compared with the control and NRS groups over a 24‐h recording time. After 48 h RSD, there was no difference in the population spike amplitudes before or after 4 h of release from RSD. These results suggest that REM sleep after LTP induction in the dentate gyrus plays an essential role in LTP maintenance, whereas a REM rebound does not restore the RSD‐induced impairment of LTP.

Early-life stress induces anxiety-like behaviors and activity imbalances in the medial prefrontal cortex and amygdala in adult rats

Early‐life stress increases the prevalence of psychiatric diseases associated with emotional dysregulation. Emotional regulation requires the inhibitory influence of the medial prefrontal cortex (mPFC) on amygdalar activity, and dysfunction of this system is believed to induce anxiety. Because mPFC and amygdala have dense reciprocal connections and projections between them continue to develop until adolescence, early‐life stress may impair the function of this circuit and cause emotional dysregulation. We examined the effects of stress during circuit development on anxiety‐like behaviors, neural activities in the mPFC and amygdala, and impulse transmission in the mPFC–amygdala circuit in adult rats. Early‐life stress, unpredictable stress twice a day for 12 days following early weaning, increased anxiety‐like behaviors in the open‐field and elevated plus‐maze tests. In the open‐field test, stress altered Fos expression in the mPFC and amygdala. Compared to non‐stressed rats, which were exposed to neither unpredictable stress nor early weaning, stressed rats exhibited decreased Fos expression in the right superficial layers of the infralimbic cortex and increased Fos expression in the right basolateral amygdala and both sides of the central amygdala. Electrophysiological analysis revealed that excitatory latencies of mPFC neurons to amygdalar stimulation in stressed rats were significantly longer than control rats in the right, but not left, hemisphere. Stress had no effect on excitatory latencies of amygdalar neurons to mPFC stimulation in the mPFC–amygdala circuits in the both hemisphere. These data suggest that early‐life stress impairs the mPFC–amygdala circuit development, resulting in imbalanced mPFC and amygdala activities and anxiety‐like behaviors.

Essential role of D1 but not D2 receptors in methamphetamine‐induced impairment of long‐term potentiation in hippocampal–prefrontal cortex pathway

Methamphetamine (MA) abuse induces deficits in cognitive performance that are related to dysfunction of the prefrontal cortex (PFC). The medial portion of the prefrontal cortex (mPFC) in rats that is crucial for cognitive function has been shown to undergo long‐term potentiation (LTP) in the projections from the hippocampus. However, no study has been performed to evaluate the influence of MA on synaptic plasticity in the hippocampal–mPFC pathways. In the present experiments, we investigated the effects of repeated MA administration on hippocampal–mPFC LTP, together with MA‐induced stereotyped behaviors. Repeated MA administration produced behavioral sensitization and LTP impairment in the hippocampal–mPFC pathways. The MA‐induced impairment of hippocampal–mPFC LTP was prevented by the pretreatment of dopamine 1 (D1) but not dopamine 2 (D2) receptor antagonists, while D1 and D2 receptor antagonists attenuated the MA‐induced stereotyped behaviors. These findings suggest that D1 receptors are crucial for the MA‐induced deterioration of synaptic plasticity in the hippocampal–mPFC circuits. Impairment of LTP associated with D1 receptor dysfunction may underlie cognitive deficits in MA‐dependent subjects.

13-cis-retinoic acid alters the cellular morphology of slice-cultured serotonergic neurons in the rat.

Retinoids influence cellular processes such as differentiation, proliferation and apoptosis via retinoic acid receptor (RAR) and retinoid X receptor (RXR), and have therapeutic applications in several cancers and dermatologic diseases. Recent reports indicate that depression occasionally occurs in patients using the acne drug Accutane, the active component of which is 13‐cis‐retinoic acid (13‐cis‐RA). Although impairment of serotonin (5‐HT)‐expressing neurons, including morphologic changes, is thought to be associated with depressive symptoms, the effects of 13‐cis‐RA on 5‐HT neurons have not been examined. The present study demonstrated that 13‐cis‐RA alters the morphology of 5‐HT neurons in cultured rat midbrain slices. The 13‐cis‐RA‐induced changes were partially blocked by RXR and RAR antagonists. Furthermore, cotreatment with RAR and RXR agonists altered the morphology of 5‐HT neurons to a greater extent than the individual application of each agonist. The morphologic changes were completely blocked by RXR antagonist, whereas RAR antagonist partially blocked the effects. These results suggest that 13‐cis‐RA exerts its action on slice‐cultured 5‐HT neurons, at least in part, through specific retinoid receptors. Moreover, RXR has a greater influence on the morphology of 5‐HT neurons than RAR. The receptor‐mediated actions of 13‐cis‐RA presented here may provide a clue for further research on depression associated with the use of 13‐cis‐RA.

Hyperlocomotor activity and stress vulnerability during adulthood induced by social isolation after early weaning are prevented by voluntary running exercise before normal weaning period.

In rodents, the disruption of social-rearing conditions before normal weaning induces emotional behavioral abnormalities, such as anxiety, motor activity dysregulation, and stress vulnerability. The beneficial effects of exercise after normal weaning on emotional regulation have been well documented. However, effects of exercise before normal weaning on emotion have not been reported. We examined whether voluntary wheel running (R) during social isolation after early weaning (early weaning/isolation; EI) from postnatal day (PD) 14-30 could prevent EI-induced emotional behavioral abnormalities in Sprague-Dawley rats. Compared with control rats reared with their dam and siblings until PD30, rats performed R during EI (EI+R) and EI rats demonstrated greater locomotion and lower grooming activity in the open-field test (OFT) during the juvenile period. Juvenile EI ± R rats showed greater learned helplessness (LH) after exposure to inescapable stress (IS; electric foot shock) than IS-exposed control and EI rats. In contrast, EI rats showed increased locomotion in the OFT and LH after exposure to IS compared with control rats during adulthood; this was not observed in EI ± R rats. Both EI and EI ± R rats exhibited greater rearing activity in the OFT than controls during adulthood. EI did not increase anxiety in the OFT and elevated plus-maze. These results suggested that R during EI until normal weaning prevented some of the EI-induced behavioral abnormalities, including hyperlocomotor activity and greater LH, during adulthood but not in the juvenile period.

Cortical spreading depression affects Fos expression in the hypothalamic paraventricular nucleus and the cerebral cortex of both hemispheres

The present experiments were performed to clarify the brain sites whose activity is affected exclusively by cortical spreading depression (CoSD). For this purpose, Fos protein, a product of an immediate early gene, was used as a marker of neuronal activation. Because Fos can be induced by many manipulations such as stress stimuli, we verified CoSD-induced Fos expression by excluding the influence of other factors such as anaesthesia and surgical manipulation. CoSD was induced by applying a KCl solution directly to the dura mater over the cerebral cortex, and Fos expression in the brain was assessed by immunohistochemistry using antibodies against Fos protein. We found that during CoSD, Fos expression was increased specifically in the magnocellular region of the hypothalamic paraventricular nucleus (PVN), as well as in the ipsilateral cortex, whereas reduced Fos expression was observed in both the parvocellular region of the PVN and the whole cortex contralateral to the CoSD site. Consistent with the reduced Fos expression, approximately 40% of neurons in the contralateral cortex revealed a suppression of electrical activity during CoSD. These results suggest that in addition to the ipsilateral cortex, CoSD affects Fos expression exclusively in the PVN and the contralateral cortex.

Neurons in the lateral paragigantocellular nucleus projecting to the infralimbic cortex in rats: electrical activity and response to changes in blood pressure

Neurons in the lateral paragigantocellular nucleus (LPGi) are known to project directly to the infralimbic cortex (IL). However, there have been no physiological studies on LPGi neurons projecting to the IL. In urethane-anesthetized rats, the single-unit activities of LPGi neurons were recorded extracellularly to investigate their electrophysiological properties and responses to changes in blood pressure (BP). The LPGi neurons antidromically driven from the IL were restricted to the ventral part of the LPGi and most spontaneously active neurons revealed suppression of activity following a fall in BP. These results suggest that the LPGi neurons in a very restricted location influence the IL in response to changes in BP.

Juvenile stress alters anxiety behaviors and neural activities in the prefrontal cortex and amygdala in adult rats

The amygdala and serotonergic innervation thereonto are considered to cooperatively regulate emotional states and behaviors. In the present experiments, we investigated how the serotonergic input modulates the excitability of lateral amygdala (LA) neurons by whole cell recordings and voltage-sensitive dye imaging in rat brain slices. Bath-application of serotonin (5-HT) induced a slow afterdepolarization (sADP) in LA neurons. This sADP lasted for more than 5 s. This sADP was also induced by synaptic stimulations. These results suggest that 5-HT enhances the excitability of amygdala neurons by inducing sADP. On the other hand, by using voltage imaging, we observed that bath-application of 5-HT suppressed the excitatory synaptic propagation from LA to endopiriform nucleus, perirhinal cortex, piriform cortex and basolateral amygdala. The suppression by 5-HT was also observed in patch clamp recording from the neurons in endopiriform nucleus and basolateral amygdala. Taken together, it is suggested that 5-HT has two opposite effects, enhancing the excitability of LA and suppressing the synaptic propagation from LA.