Mutsumi Matsukawa

Serotonin and acetylcholine are crucial to maintain hippocampal synapses and memory acquisition in rats

Treatment with serotonin and acetylcholine depletors reduced the number of synapses in the rat hippocampus. Animals that received the drug treatment lost a substantial number of synapses and showed an apparent impairment in memory acquisition. Although the animals were behaviorally impaired following the treatment, spatial memory was nonetheless eventually attained despite the disappearance of long-term potentiation. These data suggest that synapses in the hippocampus that are normally maintained by serotonin and acetylcholine are crucial for normal acquisition of spatial memory. The number of synapses maintained by biogenic amines may be a basic mechanism for neurobehavioral plasticity.

Oxytocinergic innervation to the upper thoracic sympathetic preganglionic neurons in the rat

A combination of retrograde cell body labeling and immunohistochemistry was employed to elucidate how oxytocinergic fibers make contact with sympathetic preganglionic neurons (SPNs) in the rat spinal cord from T1 to T4. SPNs were labeled retrogradely using cholera toxin subunit B (CTb) or horseradish peroxidase-conjugated CTb. Oxytocin-immunoreactive (ir) fibers were found in the intermediate zone, including the sympathetic preganglionic subnuclei. In the central autonomie nucleus and the intercalated nucleus, brown-stained oxytocin-ir varicosities or terminals were frequently observed to stud black-stained dendrites of SPNs. Electron microscopical observations showed that oxytocin-ir terminals form synapses with dendrites or soma of the sympathetic preganglionic neurons. The terminals contained numerous small clear round vesicles and a few large, cored vesicles. These results clearly show that a large proportion of SPNs are innervated by oxytocin-containing fibers. The origin of these fibers is discussed, and it is concluded that they are probably descending fibers from the paraventricular nucleus of the hypothalamus.

Synaptic loss following depletion of noradrenaline and/or serotonin in the rat visual cortex: a quantitative electron microscopic study.

Biogenic amines have a trophic-like role for the formation and the maintenance of synapses in the CNS. We examined the changes in the number of synaptic profiles in the developing and adult rat visual cortex following selective depletion of noradrenaline and/or serotonin. By the drug-induced decreases in levels of noradrenaline or serotonin between 1 and 2 weeks after birth, the number of synaptic profiles was decreased by 29-55% compared with that of control animals. The magnitude of reduction in the number of synaptic profiles was virtually the same following simultaneous depletion of both noradrenaline and serotonin compared with the depletion of noradrenaline or serotonin alone. Later in the developmental period, the function of noradrenaline and serotonin in facilitating synapse formation and maintenance became less prominent than that in younger animals. In the control animals, the number of axosomatic synapses was the highest at around 2 weeks after birth, and decreased with development. The number of axodendritic synapses was the highest between 2 and 7 weeks after birth, and decreased to 50% at 11 weeks after birth. These data demonstrate that synapses in the rat visual cortex are overproduced during the early developmental period. We suggest that both serotonin and noradrenaline are necessary for synapse formation during the early stages of development of the rat visual cortex.

Rose odor can innately counteract predator odor

When animals smell a predator odor such as 2,5-Dihydro-2,4,5-trimethylthiazoline (TMT), even if it is a novel substance, the hypothalamo-pituitary-adrenal (HPA) axis is activated, causing stress-like behaviors. Although the medial part of the bed nucleus of stria terminalis (mBST) is known to be involved in this process, the mechanism remains unclear. Moreover, it is unknown whether there is any odor that can counteract the predator odor, even when the odorants are novel substances for the animals. In this study, we assessed whether rose odor can counteract by counting the number of activated neurons in mice brain following the presentation of rose odor with or without TMT for 30 min. The number of activated cells in the mBST and in the ventrorostral part of the anterior piriform cortex (APC) was significantly reduced by a mixture of TMT and rose odor; however, no significant differences were noted in the dorsal part of the APC and in the olfactory bulb (OB) following TMT presentation with or without rose odor. The results suggest that rose odor may counteract the TMT-induced stress response in the OB and/or APC and suppress the neural circuit to the mBST. It also indicates that there are some odors that can innately counteract predator odor, even when they have not been experienced before.

Species differences in the distribution and coexistence ratio of serotonin and substance P in the monkey, cat, rat and chick spinal cord

Serotoninergic raphe-spinal motor neuron projections exhibit wide species differences in both innervation pattern and coexistence of serotonin and substance P. The coexistence ratios vary widely ranging from more than 80% (rat) to less than 1% (chick). Serotonin and substance P positive fibers are also unevenly distributed in the ventral horn of different species: dense clusters of serotonin and substance P positive fibers were preferentially located in the motor neuron pools of extensor muscles of the hip joint (chick) as well as antigravity muscles of the forelimb (cat and rat).

Stress-related activities induced by predator odor may become indistinguishable by hinokitiol odor.

Predator odors, such as 2,5-dihydro-2,4,5-trimethylthiazoline (TMT), induce a stress-like behavior in some rodents, and there is activation of a complex mix of brain regions including the anterior piriform cortex (APC) and the bed nucleus of stria terminalis (BST). In contrast, rose odor can counteract TMT-induced activation of the ventrorostral part of APC and the medial part of BST. In the present study, two novel odors, woody (hinokitiol) and caraway [S(+)-carvone] odors, were evaluated to determine whether they have an antistress effect. Plasma adrenocorticotropic hormone levels, a marker of stress, and the number of c-Fos-immunopositive cells were determined in APC and BST. Plasma adrenocorticotropic hormone levels were increased by TMT alone and in combination with S(+)-carvone; however, hinokitiol with or without TMT did not have an effect. The number of activated cells in the medial part of BST was increased by TMT alone and in combination with S(+)-carvone or hinokitiol. Although TMT alone activated the medial part of BST, a mixture of TMT and hinokitiol activated both the medial and the lateral part of BST. These data suggest that the selective responses to TMT in the medial part of BST were obscured by activation of more odor-related regions by hinokitiol with TMT. In addition, the ratio of medial to lateral BST activation may be critical in stress-related behavior. In conclusion, hinokitiol can alleviate TMT-induced stress; however, the underlying mechanism appears to be different from that of the rose odor, as found in our previous study.

Identification of adrenoceptor subtype-mediated changes in the density of synapses in the rat visual cortex.

Both serotonin and noradrenaline affect synapse formation and maintenance in the CNS. Although we previously demonstrated that serotonin regulates synaptic density via activation of serotonin(2A) receptor, it was still unclear which receptor subtype mediates the function of noradrenaline. In the present study we tried to identify the noradrenaline receptor (adrenoceptor) subtype, which could regulate the density of synapses in the rat visual cortex. Selective antagonists and/or agonists of adrenoceptor subtypes were administered to six weeks old rats. Changes in the density of axodendritic synapses were quantitatively examined in lamina I, where noradrenaline rather than serotonin is known to regulate the density of synapses. The alpha1 adrenoceptor antagonists (prazosin and 2-{[b-(4-hydroxyphenyl)ethyl]aminomethyl}-1-tetralone) decreased the number of synapses in a dose-dependent manner. In contrast, administrations of the alpha1-agonist (methoxamine) increased the density of synapses. The beta1 adrenoceptor antagonist (atenolol) had no effect on the density of synapses. The alpha2-antagonist (rauwolscine) increased synaptic density, whereas the beta2-antagonist (ICI-118,551) decreased synaptic density. Simultaneous treatments with the alpha1-antagonist and alpha1-agonist caused the alpha1-agonist to competitively block the effect of the alpha1-antagonist and recover the density of synapses to the control values. In addition, the alpha1-antagonist/agonist appeared to show a reverse effect on the changes in synaptic density following alpha2- or beta2-antagonist treatment by acting via the alpha1 receptor. Moreover, decreased synaptic density when a selective noradrenergic neurotoxin (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) was counterbalanced by the alpha1-agonist. These data suggest that noradrenaline regulates the density of synapses in the rat visual cortex primarily via the alpha1 receptor subtype. Both serotonin(2A) and alpha1 receptors are known to couple with phospholipase C, which has been shown to increase intracellular calcium. It may help us to understand the underlying mechanisms for synaptic plasticity in the CNS.

Lamina-selective changes in the density of synapses following perturbation of monoamines and acetylcholine in the rat medial prefrontal cortex

The rat medial prefrontal cortex is known to have diverse brain functions such as learning and memory, attention, and behavioral flexibility. Although these functions are affected by monoamines (dopamine (DA), noradrenaline (NA) and serotonin (5-HT)) and acetylcholine (ACh), the detailed mechanisms remain unclear. These neuromodulators also have effects on synapse formation and maintenance, and regulate plasticity in the central nervous system (CNS). To clarify the effects of these neuromodulators on changes in the density of synapses in the rat medial prefrontal cortex, we separately administered a D1- or D2-antagonist, NA neurotoxin, 5-HT synthetic inhibitor, or muscarinic ACh antagonist for 1 week, and counted the number of synapses on electron microscopic photographs taken from the prelimbic area of the medial prefrontal cortex. The density of synapses in lamina I was regulated by DA via D1-like receptors, and that in laminae II/III was decreased by depletion of NA or ACh. However, 5-HT did not have a regulatory effect on the synaptic density throughout the layers in this brain region. The data in this study and our previous studies indicate that there are appreciable regional differences in the magnitude of biogenic amine-mediated synaptic plasticity in the rat CNS. These neuromodulators may have a trophic-like effect on the selected neuronal circuit to maintain synaptic contacts in the rat CNS. The synaptic density in the medial prefrontal cortex regulated by monoamines and ACh could be important not only for synaptic plasticity in this region but also for pharmacotherapeutic drug treatment.

Retrograde neuronal labelling by E. coli enterotoxin subunit B

The present communication reports that subunit B of Escherichia coli heat-labile enterotoxin (LTB) can be utilized as a powerful tracer in neuroanatomical studies. LTB was injected into the limb muscle of the chick, or into the superior cervical ganglion of the rat. Sections were processed with an immunohistochemical technique using an antibody against LTB. After the LTB injection into the muscle, retrogradely labelled motoneurons were found: the entire extent of dendritic arborizations appeared labeled. After the LTB injection into the ganglion, virtually all of the preganglionic neurons were retrogradely labelled in the rat spinal cord.

Habitat odor can alleviate innate stress responses in mice.

Predatory odors, which can induce innate fear and stress responses in prey species, are frequently used in the development of animal models for several psychiatric diseases including post-traumatic stress disorder (PTSD) following a life-threatening event. We have previously shown that odors can be divided into at least three types; odors that act as (1) innate stressors, (2) as innate relaxants, or (3) have no innate effects on stress responses. Here, we attempted to verify whether an artificial odor, which had no innate effect on predatory odor-induced stress, could alleviate stress if experienced in early life as a habitat odor. In the current study, we demonstrated that the innate responses were changed to counteract stress following a postnatal experience. Moreover, we suggest that inhibitory circuits involved in stress-related neuronal networks and the concentrations of norepinephrine in the hippocampus may be crucial in alleviating stress induced by the predatory odor. Overall, these findings may be important for understanding the mechanisms involved in differential odor responses and also for the development of pharmacotherapeutic interventions that can alleviate stress in illnesses like PTSD.