[Structural basis for the inhibitory function of the parietal cortex efferent systems]

Relative quantitative distribution of all the associative and descending efferent fibers and the ultrastructural organization of the terminals of the parietal cortex areas 5 and 7 in the caudate (NC) and red nucleus (NR) in the cat were analyzed after a local, pointed destruction of the cortex of these areas. The maximal numbers of the associative fibers were found to project to the fundus areas of the motor cortex and to the area of Clare-Bishop; moderate projections were detected to the areas 31, 19 and single degenerating fibers were registered in the areas 1,2, 3a, 3b, 30, and 23. The descending efferents were maximally projecting to NC, NR, reticular nuclei of the thalamus, midbrain, and pons, in all of which, according to the immunocytochemical studies, GABA-ergic terminals are prevalent. On the basis on the electron microscopical studies, it was suggested that the influence of the parietal cortex is mediated by the axo-spinal synapses of the medium shortaxonal spiny cells of the dorsolateral part of NC caput and by the axo-dendritic synapses of Golgi II cells of the parvocellular part of NR. On the basis of the maximal involvement of the fundus areas of the motor cortex, as well as of the inhibitory subcortical (NC) and stem nuclei (NR, reticular nuclei of the thalamus, midbrain, and nuclei pontis), it is suggested that these structures serve as the morphological substrates for the realization of the inhibitory, integrative function of the parietal cortex.

Interactions between orexin-immunoreactive fibers and adrenaline or noradrenaline-expressing neurons of the lower brainstem in rats and mice

Orexins are expressed in neurons of the dorsolateral hypothalamus and their axons widely distribute throughout the central nervous system. The noradrenergic cell groups of the lower brainstem belong to the targets of these orexin projections. Double immunostainings for orexin and phenylethanolamine N-methyltransferase (PNMT), as well as orexin and tyrosine hydroxylase (TH) were applied to demonstrate the orexinergic innervation of catecholamine cell groups in the lower brainstem of the mouse and the rat. In various densities, networks of orexin-positive fibers and terminals were present on neurons of each adrenaline (C1, C2, C3) and noradrenaline (locus coeruleus, A1, A2, A4, A5 and A7) cell groups. The most dense networks of orexin fibers and terminals were detected in the locus coeruleus, the subcoeruleus area, and in the nucleus of the solitary tract. By using confocal microscope to analyze triple immunostainings we could detect that about two-third of the orexin-PNMT or orexin-TH immunopositive close contacts contained synaptophysin (a presynapse-specific protein) in the C1, C2 and C3 adrenaline, or in the A1, A2 noradrenaline cell groups, respectively. Orexin-immunopositive axons in the C1, C2, as well as A1, A2 and A6 cell groups have been examined by an electron microscope. Relatively few asymmetrical (excitatory) synaptic contacts could be demonstrated between PNMT- or TH-positive dendrites and orexin terminals, although the vast majority of orexin-positive axons was located in juxtaposition to PNMT- or TH-positive neurons.

GABAergic structures in the ventral part of the oral pontine reticular nucleus: An ultrastructural immunogold analysis

GABA mediates inhibitory effects in neurons of the ventral part of the oral pontine reticular nucleus (vRPO). Evidence increasingly suggests that GABA plays an important role in the modulation of rapid eye movement (REM) sleep generation in the cat vRPO. Here, we investigate the anatomical substrate of this modulation using GABA immunocytochemistry. Immunoperoxidase labeling revealed a few small GABA-immunoreactive cell bodies scattered throughout the vRPO. The numerical densities of all vRPO synapses and the GABA-immunoreactive synapses were estimated, at the electron microscopical level, by using a combination of the physical disector and the post-embedding immunogold techniques. We estimated that 30% of all vRPO synaptic terminals were immunoreactive to GABA. Our findings support the hypothesis that vRPO neuron activity is significantly controlled by inhibitory GABAergic terminals that directly target somata and the different parts of the dendritic tree, including distal regions. GABAergic input could inhibit vRPO REM sleep-inducing neurons during other states of the sleep-wakefulness cycle such as wakefulness or non-REM sleep.

Branching patterns of parabrachial neurons projecting to the central extended amgydala: single axonal reconstructions

Electrophysiological evidence suggests that the spinoparabrachioamygdaloid pathway carries nociceptive information that may be important for the elaboration of physiological and emotional responses to noxious events. The pontine parabrachial nucleus (pPB) sends a massive projection to the central nucleus of the amygdala (CeA) and lateral bed nucleus of the stria terminalis (BSTL), both regions belonging to a broader macrostructure, the central extended amygdala (EAc). The aim of this study was to examine whether different EAc components are targeted by a same pPB neuron, by reconstructing single axonal branching patterns after anterograde labelling. Small deposits of biotinylated dextran amine in the region of the external lateral pPB result in dense and specific labelling in the whole EAc. Reconstructed axons innervate either the lateral or the capsular part of the CeA with perisomatic or bushy terminals, respectively. A subset of axons enters the stria terminalis rostrally to follow its trajectory caudally toward the CeA. Individual axons targeting the CeA usually send collaterals to other EAc components, especially those projecting to the lateral CeA, which often coinnervate the BSTL. By contrast, only few branches were found outside the EAc. These results suggest that the noxious information travelling from the pPB to the CeA may also be transmitted to other EAc components. This pPB-EAc pathway, which appears distinct from the parabrachiohypothalamic and parabrachiothalamic projections, would be the anatomical basis through which the EAc elaborates the autonomic, endocrine, and emotional components of pain.

Trak1 mutation disrupts GABA(A) receptor homeostasis in hypertonic mice

Hypertonia, which results from motor pathway defects in the central nervous system (CNS), is observed in numerous neurological conditions, including cerebral palsy, stroke, spinal cord injury, stiff-person syndrome, spastic paraplegia, dystonia and Parkinson disease. Mice with mutation in the hypertonic (hyrt) gene exhibit severe hypertonia as their primary symptom. Here we show that hyrt mutant mice have much lower levels of gamma-aminobutyric acid type A (GABA(A)) receptors in their CNS, particularly the lower motor neurons, than do wild-type mice, indicating that the hypertonicity of the mutants is likely to be caused by deficits in GABA-mediated motor neuron inhibition. We cloned the responsible gene, trafficking protein, kinesin binding 1 (Trak1), and showed that its protein product interacts with GABA(A) receptors. Our data implicate Trak1 as a crucial regulator of GABA(A) receptor homeostasis and underscore the importance of hyrt mice as a model for studying the molecular etiology of hypertonia associated with human neurological diseases.

Neurokininergic mechanism within the lateral crescent nucleus of the parabrachial complex participates in the heart-rate response to nociception

We wanted to ascertain whether the lateral parabrachial nucleus was involved in mediating the heart-rate response evoked during stimulation of somatic nociceptors. Reversible inactivation of the lateral parabrachial nucleus, using a GABA(A) agonist, reduced the reflex tachycardia evoked during noxious (mechanical) stimulation of the forelimb by approximately 50%. The same effect was observed after blockade of neurokinin 1 receptors within the lateral parabrachial nucleus, indicating a possible involvement for substance P as a neurotransmitter. Immunocytochemistry revealed a strong expression of substance P-immunoreactive fibers and boutons in all lateral subnuclei, but they were particularly dense in the lateral crescent subnucleus. Histological verification showed that the most effective injection sites for attenuating the noxious-evoked tachycardia were all placed in or near to the lateral crescent nucleus of the lateral parabrachial complex. Many single units recorded from this region were activated by high-intensity brachial nerve stimulation. The brachial nerve evoked firing responses of some of these neurons was reversibly reduced after local delivery of a neurokinin 1 receptor antagonist. However, only a minority of these neurons followed a paired-pulse stimulation protocol applied to the spinal cord, suggesting a predominance of indirect projections from the spinal cord to the parabrachial nucleus. We conclude that the cardiac component of the response to somatic nociception involves indirect spinal pathways that most likely excite neurons located in the lateral crescent nucleus of the parabrachial complex via activation of neurokinin 1 receptors.

Rat trigeminal lamina I neurons that project to thalamic or parabrachial nuclei contain the mu-opioid receptor

Ligands of the mu-opioid receptor are known to inhibit nociceptive transmission in the dorsal horn, yet the cellular site(s) of action for this inhibition remain to be fully elucidated. Neurons located in lamina I of the dorsal horn are involved in distinct aspects of nociceptive transmission. Neurons projecting to the thalamus are thought to be involved in sensory-discriminative aspects of pain perception, while neurons projecting to the parabrachial nucleus are thought to be important for emotional and/or autonomic responses to noxious stimuli. The present study examined these two populations of lamina I projection neurons in the trigeminal dorsal horn to determine if the mu-opioid receptor protein (MOR1) is differentially located in these populations of neurons. Lamina I projection neurons were identified using the retrograde tracer FluoroGold (FGold). FGold was injected into either the contralateral thalamus (ventral posterolateral (VPM)/ventral posterolateral (VPL) thalamic region) or into the ipsilateral parabrachial nuclei. The distribution of MOR1 in these neurons was determined using immunocytochemistry. The distribution of MOR1-ir within these two populations of lamina I projection neurons was examined by both confocal and electron microscopy. We found that both populations of projection neurons contained MOR1. Immunogold analyses revealed the presence of MOR1-ir at membrane sites and within the cytoplasm of these neurons. Cytoplasmic receptor labeling may represent sites of synthesis, recycling or reserve populations of receptors. MOR1 was primarily found in the somata and proximal dendrites of projection neurons. In addition, these neurons rarely received synaptic input from MOR1-containing axon terminals. These results indicate that lamina I neurons in trigeminal dorsal horn that project to the thalamic and parabrachial nuclei contain MOR1 and are likely sites of action for MOR ligands that modulate sensory and/or autonomic aspects of pain transmission in the trigeminal dorsal horn.

Laterodorsal tegmental projections to identified cell populations in the rat ventral tegmental area

Projections from the laterodorsal tegmentum (LDT) to the ventral tegmental area (VTA) contribute to the activity of dopamine (DA) and GABA cells and, hence, to the affective and cognitive functions of this region. LDT afferents arise from neurochemically diverse cell types and mediate multiple functional influences. However, the VTA cell populations that receive LDT afferents are unknown and were investigated here by anterograde and retrograde tract-tracing in combination with immunocytochemistry to distinguish DA and GABA cells. Approximately 50% of the LDT to VTA pathway formed asymmetric, presumably excitatory synapses that innervated DA and GABA cells in rough proportion to their representation within the VTA. This portion of the LDT innervation appeared to selectively target DA but not GABA mesoaccumbens neurons and provide a relatively nonselective input to both DA and GABA mesoprefrontal cells. The remaining LDT axons formed symmetric, presumably inhibitory synapses with a different pattern of cellular targets that included a preferential input to GABA neurons of both mesoaccumbens and mesoprefrontal populations and an apparently selective innervation of mesoprefrontal and not mesoaccumbens DA neurons. These data suggest that the LDT mediates a convergent excitatory and inhibitory influence on both mesoprefrontal DA and GABA cells but a divergent impact on mesoaccumbens neurons that is likely to excite DA cells and inhibit GABA neurons. Combined with our previous description of prefrontal cortical afferents, our data also indicate that mesoaccumbens DA neurons receive putative excitatory drive from the LDT, whereas mesoprefrontal DA cells receive convergent excitation from both cortical and brainstem sources.

Mapping and quantitative analysis of gephyrin cytoplasmic trafficking pathways in motoneurons, using an optimized Transmission Electron Microscopy Color Imaging (TEMCI) procedure

In the present study, an optimized Transmission Electron Microscopy Color Imaging (TEMCI) procedure was used to map and quantify the pathways involved in the trafficking and subcellular targeting of gephyrin in identified abducens motoneurons. Gephyrin is a scaffolding protein, which plays a crucial role in the clustering of the GABA(A) and glycine receptors to the cytoskeleton. TEMCI associated several accurate tools: (i) nanogold immunodetection of gephyrin in motoneurons identified on the basis of their immunoreactivity to Choline Acetyl Transferase, (ii) low magnification color scale coding of gephyrin densities on series of ultrathin sections of motoneurons, which gave a map of the cytoplasmic distribution of the protein, (iii) statistical analysis of the subcellular distribution of the immunolabeling. The color map of gephyrin densities in the cell bodies reflected the distribution of inhibitory synapses over the membrane. The TEMCI analysis of motoneurons with various patterns of synaptic covering made it possible to visualize for the first time the cytoplasmic transport pathway of gephyrin towards its target at synaptic contact. A high magnification quantitative analysis, including the study of 109 inhibitory synapses, showed that most gephyrin-associated immunogold particles (67%) were located in the subsynaptic regions facing the active zones, and the second most densely occupied regions were the perisynaptic regions (19.5% of immunogold particles). A consistent proportion of the gephyrin (11.5%), significantly higher than densities present in the rest of the cytoplasm (2%), was detected in the extrasynaptic submembrane region.

Parabrachial inputs to Fos-immunoreactive neurons in the lateral central nucleus of amygdala activated by hypotension: a light and electron microscopic study in the rat

Morphological features and functional implications of projections of the parabrachial nucleus to the central nucleus of the amygdala were investigated in the rat. The anatomical study was based on injections of the tracers horseradish peroxidase and biotinylated dextran amine. An extremely dense concentration of labeled fibers was found in the lateral and lateral capsular subdivisions of the central nucleus of the amygdala, originating mainly from the external lateral and ventral lateral subnuclei of the parabrachial nucleus. The parabrachial fibers exhibited the morphological characteristic of forming dense pericellular terminal arborizations. The functional implications of this pathway in cardiovascular functions were verified using Fos protein induction in response to hypotension induced by continuous intravenous administration of hydralazine-hydrochloride. In this paradigm, Fos immunoreactivity was found to be confined to the lateral and lateral capsular subdivisions of the central nucleus of the amygdala. Double immunostaining methods were used to visualize, at the electron microscopic level, terminals labeled by biotinylated dextran amine and Fos cell labeling. With this approach, we were able to confirm that Fos-immunoreactive neurons in the central nucleus of the amygdala receive axosomatic terminals from the parabrachial nucleus. The present findings point out that parabrachial inputs to the central nucleus of the amygdala play a relevant role in regulating cardiovascular function.

A novel central pathway links arterial baroreceptors and pontine parasympathetic neurons in cerebrovascular control

1. We tested the hypothesis that arterial baroreceptor reflexes modulate cerebrovascular tone through a pathway that connects the cardiovascular nucleus tractus solitarii with parasympathetic preganglionic neurons in the pons. 2. Anesthetized rats were used in all studies. Laser flowmetry was used to measure cerebral blood flow. We assessed cerebrovascular responses to increases in arterial blood pressure in animals with lesions of baroreceptor nerves, the nucleus tractus solitarii itself, the pontine preganglionic parasympathetic neurons, or the parasympathetic ganglionic nerves to the cerebral vessels. Similar assessments were made in animals after blockade of synthesis of nitric oxide, which is released by the parasympathetic nerves from the pterygopalatine ganglia. Finally the effects on cerebral blood flow of glutamate stimulation of pontine preganglionic parasympathetic neurons were evaluated. 3. We found that lesions at any one of the sites in the putative pathway or interruption of nitric oxide synthesis led to prolongation of autoregulation as mean arterial pressure was increased to levels as high as 200 mmHg. Conversely, stimulation of pontine parasympathetic preganglionic neurons led to cerebral vasodilatation. The second series of studies utilized classic anatomical tracing methods to determine at the light and electron microscopic level whether neurons in the cardiovascular nucleus tractus solitarii, the site of termination of baroreceptor afferents, projected to the pontine preganglionic neurons. Fibers were traced with anterograde tracer from the nucleus tractus solitarii to the pons and with retrograde tracer from the pons to the nucleus tractus solitarii. Using double labeling techniques we further studied synapses made between labeled projections from the nucleus tractus solitarii and preganglionic neurons that were themselves labeled with retrograde tracer placed into the pterygopalatine ganglion. 4. These anatomical studies showed that the nucleus tractus solitarii directly projects to pontine preganglionic neurons and makes asymmetric, seemingly excitatory, synapses with those neurons. These studies provide strong evidence that arterial baroreceptors may modulate cerebral blood flow through direct connections with pontine parasympathetic neurons. Further study is needed to clarify the role this pathway plays in integrative physiology.

Acute moderate hyponatraemia and its rapid correction: effects on striatal and pontine ultrastructure in an animal model of the TURP syndrome

BACKGROUND AND OBJECTIVE

To investigate the effects of moderate hyponatraemia, induced by intravenous application of an electrolyte-free irrigation fluid, as a model of the human transurethral prostate resection syndrome and of its rapid correction by hypertonic saline infusion in rats.

METHODS

Experimental animals received irrigation fluid (Purisole SM) 20 mL kg(-1) body weight, intravenously. In one group, hyponatraemia was subsequently rapidly corrected by infusion of hypertonic saline (NaCl 5.85%), while rats of group two were 'sham-corrected' by infusion of a balanced salt crystalloid solution. Plasma sodium concentrations were analysed during and at the end of the experiments. After 10 days, experimental and untreated control animals were killed humanely, fixed by perfusion and the brains were prepared for electron microscopic investigation of myelin sheets and glial cell numbers in the striatum and pons.

RESULTS

The myelin appearance was unaltered in experimental groups compared to controls, but glial cell numbers were distinctly altered in the pons but not in the striatum. In the pons, oligodendrocytes were significantly reduced in number upon rapid correction of hyponatraemia, while astrocyte numbers were increased in rats with uncorrected hyponatraemia.

CONCLUSIONS

Our electron microscopic data demonstrate that the effects of hyponatraemia and of its rapid correction are multifarious in animals. This may also apply for human patients during transurethral prostate resection.

Prefrontal cortical projections to the rat dorsal raphe nucleus: ultrastructural features and associations with serotonin and gamma-aminobutyric acid neurons

Studies of human brain indicate that both the ventromedial prefrontal cortex (PFC) and the dorsal raphe nucleus (DRN) may be dysfunctional in major depressive illness, making it important to understand the functional interactions between these brain regions. Anatomical studies have shown that the PFC projects to the DRN, although the synaptic targets of this excitatory pathway have not yet been identified. Electrophysiological investigations in the rat DRN report that most serotonin neurons are inhibited by electrical stimulation of the PFC, suggesting that this pathway is more likely to synapse onto neighboring gamma-aminobutyric acid (GABA) neurons than onto serotonin cells. We tested this hypothesis by electron microscopic examination of DRN sections dually labeled for biotin dextran amine anterogradely transported from the PFC and immunogold-silver labeling for tryptophan hydroxylase (TrH) or for GABA. In the DRN, the majority of PFC axons either synapsed onto unlabeled dendrites or failed to form detectable synapses in single sections. Other PFC axons synapsed onto either TrH- or GABA-immunolabeled processes. Considerably more tissue sampling was necessary to detect PFC synapses onto TrH- than onto GABA-labeled dendrites, suggesting that the latter connections are more common. In other cases, PFC terminals and TrH- or GABA-immunoreactive dendrites either were closely apposed, without forming detectable synapses, or were separated by glial processes. These results provide potential anatomical substrates whereby the PFC can both directly and indirectly regulate the activity of serotonin neurons in the DRN and possibly contribute to the pathophysiology of depression.

Cellular interactions between axon terminals containing endogenous opioid peptides or corticotropin-releasing factor in the rat locus coeruleus and surrounding dorsal pontine tegmentum

Recent evidence suggests that certain stressors release both endogenous opioids and corticotropin-releasing factor (CRF) to modulate activity of the locus coeruleus (LC)-norepinephrine (NE) system. In ultrastructural studies, axon terminals containing methionine(5)-enkephalin (ENK) or CRF have been shown to target LC dendrites. These findings suggested the hypothesis that both neuropeptides may coexist in common axon terminals that are positioned to have an impact on the LC. This possibility was examined by using immunofluorescence and immunoelectron microscopic analysis of the rat LC and neighboring dorsal pontine tegmentum. Ultrastructural analysis indicated that CRF- and ENK-containing axon terminals were abundant in similar portions of the neuropil and that approximately 16% of the axon terminals containing ENK were also immunoreactive for CRF. Dually labeled terminals were more frequently encountered in the "core" of the LC vs. its extranuclear dendritic zone, which included the medial parabrachial nucleus (mPB). Triple labeling for ENK, CRF, and tyrosine hydroxylase (TH) showed convergence of opioid and CRF axon terminals with noradrenergic dendrites as well as evidence for inputs to TH-labeled dendrites from dually labeled opioid/CRF axon terminals. One potential source of ENK and CRF in the dorsal pons is the central nucleus of the amygdala (CNA). To determine the relative contribution of ENK and CRF terminals from the CNA, the CNA was electrolytically lesioned. Light-level densitometry revealed robust decreases in CRF immunoreactivity in the LC and mPB on the side ipsilateral to the lesion but little or no change in ENK immunoreactivity, confirming previous studies of the mPB. Degenerating terminals from the CNA in lesioned rats were found to be in direct contact with TH-labeled dendrites. Together, these data indicate that ENK and CRF may be colocalized to a subset of individual axon terminals in the LC "core." The finding that the CNA provides, to dendrites in the area examined, a robust CRF innervation, but little or no opioid innervation, suggests that ENK and CRF axon terminals impacting LC neurons originate from distinct sources and that terminals that colocalize ENK and CRF are not from the CNA.

Grafting of a new target prevents synapse loss in abducens internuclear neurons induced by axotomy

The loss of afferent synaptic boutons is a prominent alteration induced by axotomy on adult central neurons. In this work we attempted to prove whether synapse loss could be reverted by reconnection with a new target. We severed the medial longitudinal fascicle of adult cats and then transplanted embryonic cerebellar primordia at the lesion site immediately after lesion. As previously shown, the transected axons from abducens internuclear neurons penetrate and reinnervate the graft [J Comp Neurol 444 (2002) 324]. By immunocytochemistry and electron microscopy we studied the synaptology of abducens internuclear neurons under three conditions: control, axotomy and transplant (2 months of survival time). Semithin sections of the abducens nucleus were immunostained against calretinin, to identify abducens internuclear neurons, and either synaptophysin (SF), to label synaptic terminals, or glial fibrillary acidic protein (GFAP) to detect the astrocytic reaction. Optical and linear density of SF and GFAP immunostaining were measured. Data revealed a significant decrease in the density of SF-labeled terminals with a parallel increase in GFAP-immunoreactive elements after axotomy. On the contrary, in the transplant group, the density of SF-labeled terminals was found similar to control, and the astrocytic reaction induced by lesion was significantly reduced. At the ultrastructural level, synaptic coverage and linear density of boutons were measured around the somata of abducens internuclear neurons. Whereas a significant reduction in both parameters was found after axotomy, cells of the transplant group received a normal density of synaptic endings. The ratio between F- and S-type boutons was found similar in the three groups. Therefore, these findings indicate that the grafting of a new target can prevent the loss of afferent synaptic boutons produced by the axotomy.

Subcellular localization of the voltage-dependent potassium channel Kv3.1b in postnatal and adult rat medial nucleus of the trapezoid body

A pre-embedding immunocytochemical method was used to study the subcellular distribution of the voltage-dependent potassium channel Kv3.1b in the medial nucleus of the trapezoid body (MNTB) in developing and adult rat. The main finding was the localization of the channel in specific membrane compartments of the calyces of Held and principal globular neurons. Thus, at postnatal day (P) 9 immunoparticles were densely localized in plasma membranes of globular cell bodies and their main dendrites. At P16, a strong Kv3.1b labeling was still observed in these globular cell compartments, but the most remarkable feature was the presence of immunoparticles in synaptic terminal membranes of the calyces of Held. However, the presynaptic and postsynaptic specializations of the calyx of Held-globular cell synapses were virtually devoid of immunoparticles. This same subcellular distribution of Kv3.1b was seen in adult, with membranes of calycine terminals more uniformly labeled. The developmental profile of Kv3.1b expression in MNTB coincides with the functional maturation of the calyx of Held-principal globular neuron synapse. The presence of the channel in this system is crucial for the high-frequency synaptic transmission of auditory signals.

VR1-positive primary afferents contact NK1-positive spinoparabrachial neurons

Neurons in rat superficial dorsal horn that express neurokinin receptor 1 (NK1), a receptor for substance P, play a critical role in the development of hyperalgesia. Thermal hyperalgesia is dramatically reduced after ablation of these neurons, but, paradoxically, not in mice that lack the NK1 receptor (Mantyh et al. [1997] Science 278:275-279). Because primary afferents that express vanilloid receptor 1 (VR1), a receptor for noxious heat, are essential for thermal nociception and hyperalgesia, we reasoned that VR1-positive fibers may terminate onto NK1-expressing dorsal horn neurons. We therefore combined immunofluorescent staining for VR1 and NK1 to show that NK1-positive neurons in lamina I are contacted by VR1-positive fibers. That these contacts represent synapses was verified by staining for the presynaptic marker synaptophysin and by electron microscopy. By combining retrograde tracing with immunocytochemistry, we also found that most NK1-positive cells contacted by VR1-positive fibers project to the lateral parabrachial nucleus. Because quantitative evaluation suggests a preferential targeting of NK1-positive lamina I neurons by fibers containing VR1, these results demonstrate a significant monosynaptic innervation of spinoparabrachial neurons by VR1-positive afferents.

Projections of the basilar pontine nuclei and nucleus reticularis tegmenti pontis to the cerebellar nuclei of the rat

This study showed the precise projection pattern of the basilar pontine nuclei (BPN) and the nucleus reticularis tegmenti pontis (NRTP) to the cerebellar nuclei (CN), as well as the different anatomic features of BPN and NRTP projections. The staining of BPN or NRTP with biotinylated dextran labeled projection fibers to complementary topographic areas in the CN. In fact, BPN principally project to a rostrocaudally oriented column of the nucleus lateralis (NL), which at the midcentral level shifts to the lateroventral part of the nucleus, as well as to the caudolateral part of the nucleus interpositus posterioris. The NRTP projects to a rostrocaudal column of the NL, which at the midcentral level shifts medially, as well as to the nucleus interpositalis and to the caudal part of the nucleus medialis. BPN axons in the CN usually branch into short collaterals of simple morphology that involve small terminal areas, whereas NRTP axons branch into longer collaterals of complex morphology involving terminal areas of different sizes. Each site of injection is at the origin of a set of terminal areas in the CN. The set of projections from different BPN or NRTP areas were partially, but never completely, overlapping. Thus, the set of terminal areas in the CN was specific for each area of both BPN and NRTP. Injection of tetramethyl-rhodamine-dextran-amine into the CN stained cell bodies of BPN and NRTP with different repartition on the two sides. The study showed that CN are innervated by the contralateral BPN and not very much by the ipsilateral BPN, whereas they are innervated by NRTP bilaterally, even if with a contralateral prevalence. In conclusion, this study supports the hypothesis that both BPN and NRTP are concerned in the central program for skilled movements, even if they are probably involved in different functional roles.

Direct projections from the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons: a link to cerebrovascular regulation

Peripheral or central interruption of the baroreflex or the parasympathetic innervation of cerebral vessels leads to similar changes in regulation of cerebral blood flow. Therefore, we sought to test the hypothesis that the cardiovascular nucleus tractus solitarii, the site of termination of arterial baroreceptor nerves, projects to pontine preganglionic neurons whose stimulation elicits cerebral vasodilatation. The current study utilized both light and electron microscopic techniques to analyze anterograde tracing from the cardiovascular nucleus tractus solitarii to preganglionic parasympathetic neurons in the pons. We further used retrograde tracing from that same pontine region to the cardiovascular nucleus tractus solitarii and evaluated the confluence of tracing from the cardiovascular nucleus tractus solitarii to pontine preganglionic neurons labeled retrogradely from the pterygopalatine ganglia. The cardiovascular nucleus tractus solitarii projected to pontine preganglionic parasympathetic neurons, but more rostral and caudal regions of nucleus tractus solitarii did not. In contrast, all three regions of nucleus tractus solitarii projected to the nucleus ambiguus and dorsal motor nucleus of the vagus. Although not projecting to pontine preganglionic parasympathetic neurons, regions lateral, rostral, and caudal to cardiovascular nucleus tractus solitarii sent projections through the pons medial to the preganglionics. The study establishes the presence of a direct monosynaptic pathway from neurons in the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons that project to the pterygopalatine ganglia, the source of nitroxidergic vasodilatory innervation of cerebral blood vessels. It provides evidence that activation of those preganglionic neurons can cause cerebral vasodilatation and increased cerebral blood flow. Finally, it demonstrates differential innervation of medullary and pontine preganglionic parasympathetic neurons by different regions of the nucleus tractus solitarii.

Ultrastructure of endomorphin-1 immunoreactivity in the rat dorsal pontine tegmentum: evidence for preferential targeting of peptidergic neurons in Barrington's nucleus rather than catecholaminergic neurons in the peri-locus coeruleus

Endomorphins are opioid tetrapeptides that have high affinity and selectivity for mu-opioid receptors (muORs). Light microscopic studies have shown that endomorphin-1 (EM-1) -containing fibers are distributed within the brainstem dorsal pontine tegmentum. Here, immunoelectron microscopy was conducted in the rat brainstem to identify potential cellular interactions between EM-1 and tyrosine hydroxylase (TH) -labeled cellular profiles in the locus coeruleus (LC) and peri-LC, an area known to contain extensive noradrenergic dendrites of LC neurons. Furthermore, sections through the rostral dorsal pons, from colchicine-treated rats, were processed for EM-1 and corticotropin releasing factor (CRF), a neuropeptide known to be present in neurons of Barrington's nucleus. EM-1 immunoreactivity was identified in unmyelinated axons, axon terminals, and occasionally in cellular profiles resembling glial processes. Within axon terminals, peroxidase labeling for EM-1 was enriched in large dense core vesicles. In sections processed for EM-1 and TH, approximately 10% of EM-1-containing axon terminals (n=269) targeted dendrites that exhibited immunogold-silver labeling for TH. In contrast, approximately 30% of EM-1-labeled axon terminals analyzed (n = 180) targeted CRF-containing somata and dendrites in Barrington's nucleus. Taken together, these data indicate that the modulation of nociceptive and autonomic function as well as stress and arousal responses attributed to EM-1 in the central nervous system may arise, in part, from direct actions on catecholaminergic neurons in the peri-LC. However, the increased frequency with which EM-1 axon terminals form synapses with CRF-containing profiles in Barrington's nucleus suggests a novel role for EM-1 in the modulation of functions associated with Barrington's nucleus neurons such as micturition control and pelvic visceral function.

A single packet of transmitter does not saturate postsynaptic glutamate receptors

Neurotransmitter is stored in synaptic vesicles and released by exocytosis into the synaptic cleft. One of the fundamental questions in central synaptic transmission is whether a quantal packet of transmitter saturates postsynaptic receptors. To address this question, we loaded the excitatory transmitter L-glutamate via whole-cell recording pipettes into the giant nerve terminal, the calyx of Held, in rat brainstem slices. This caused marked potentiations of both quantal and action potential-evoked EPSCs mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl-D-aspartate (NMDA) receptors. These results directly demonstrate that neither AMPA nor NMDA receptors are saturated by a single packet of transmitter, and indicate that vesicular transmitter content is an important determinant of synaptic efficacy.

GABA(B) receptors in the median raphe nucleus: distribution and role in the serotonergic control of hippocampal activity

Previous studies have shown that serotonergic neurons of the median raphe nucleus have a suppressive effect on theta synchronization in the hippocampus. Median raphe lesion, suppression of 5-HT neuronal activity by administration of GABA(A) receptor antagonist or by glutamate blockade or depletion produced long-lasting non-interrupted hippocampal theta in freely behaving rats independent of behavior and in rats anesthetized with urethane. Serotonergic neurons show a characteristic sleep-wake pattern of activity and there is evidence that GABAergic mechanisms play an important role in their regulation. In this study we analyzed the distribution and subcellular localization of GABA(B) receptors in the midbrain raphe complex using combined 5-HT/GABA(B) receptor immunohistochemistry at the light and electron microscopic levels and studied the effects of their pharmacological manipulation on hippocampal electroencephalographic activity in urethane-anesthetized rats. We found that sustained infusion of the GABA(B) receptor agonist baclofen into the median raphe nucleus, using the microdialysis technique, elicited lasting theta activity in the hippocampus. The effect was antagonized by selective GABA(B) receptor antagonists. The predominant localization of GABA(B) receptors in the median, as well as in dorsal raphe was found on serotonergic neurons which strongly indicates that the increase in theta occurrence after baclofen injection resulted from suppression of the serotonergic output originating from the median raphe. On the electron microscopic level, we found GABA(B) receptors located extrasynaptically indicating that these receptors are preferentially activated by strong inputs, i.e. when GABA released from the synaptic terminals is sufficient to spill over from the synaptic cleft. Such conditions might be satisfied during rapid eye movement sleep when GABAergic neurons in the raphe are firing at their highest rate and in rhythmic synchronized bursts. Our data indicate that midbrain raphe GABA(B) mechanisms play an important role in behavioral state control and in hippocampal activity, in particular.

Quantitative ultrastructural analysis of glycine- and gamma-aminobutyric acid-immunoreactive terminals on trigeminal alpha- and gamma-motoneuron somata in the rat

Detailed knowledge of the inhibitory input to trigeminal motoneurons is needed to understand better the central mechanisms of jaw movements. Here a quantitative analysis of terminals contacting somata of jaw-closing (JC) and jaw-opening (JO) alpha-motoneurons, and of JC gamma-motoneurons, was performed by use of serial sectioning and postembedding immunogold cytochemistry. For each type of motoneuron, the synaptic boutons were classified into four groups, i.e., immunonegative boutons or boutons immunoreactive to glycine only, to gamma-aminobutyric acid (GABA) only, or to both glycine and GABA. The density of immunolabeled boutons was much higher for the alpha- than for the gamma-motoneurons. In the alpha-motoneuron populations, the immunolabeled boutons were subdivided into one large group of boutons containing glycine-like immunoreactivity only, one group of intermediate size harboring both glycine- and GABA-like immunoreactivity, and a small group of boutons containing GABA-like immunoreactivity only. The percentage of immunolabeled boutons was higher for JC than JO alpha-motoneurons, the most pronounced difference being observed for glycine-like immunoreactivity. In contrast, on the somatic membrane of gamma-motoneurons, the three types of immunoreactive bouton occurred at similar frequencies. These results indicate that trigeminal motoneurons are strongly and differentially controlled by premotoneurons containing glycine and/or GABA and suggest that these neurons play an important role for the generation of masticatory patterns.

Synaptic connections between trigemino-parabrachial projection neurons and gamma-aminobutyric acid- and glycine-immunoreactive terminals in the rat

The synaptic connections between gamma-aminobutyric acid (GABA)- and glycine-immunoreactive terminals and neurons projecting to the lateral parabrachial region were examined by a combination of retrograde tracing and immunohistochemical staining in the rat medullary dorsal horn. After injection of horseradish peroxidase (HRP) into the right lateral parabrachial region, HRP retrogradely labeled neurons were observed bilaterally in laminae I, II and III of the medullary dorsal horn with an ipsilateral predominance. GABA- and glycine-like immunoreactive terminals were found in laminae I, II and III. Some of these GABA- and glycine-like immunoreactive terminals were observed chiefly to make symmetric synapses with HRP-labeled neuronal cell bodies and dendritic processes. The present results indicate that neurons in the medullary dorsal horn projecting to the lateral parabrachial region might be modulated by GABAergic and glycinergic inhibitory intrinsic neurons, which might be significantly involved in the regulation of the noxious information transmission.

Group II metabotropic glutamate receptors are differentially expressed in the medial nucleus of the trapezoid body in the developing and adult rat

The existence of a neuronal-glial signalling through the activation of neurotransmitter receptors expressed in glia is well-documented. In excitatory synapses, glutamate released from presynaptic terminals activates not only postsynaptic receptors, but also ionotropic and metabotropic glutamate receptors localized in the glia ensheathing the synapses. The medial nucleus of the trapezoid body of the auditory system is involved in the localization of sounds in the space. In this nucleus, the large excitatory synaptic terminals formed by the calyces of Held on the principal globular cell bodies are wrapped by astrocytic processes. Since these synapses are functional from early postnatal days, glia receiving excitatory synaptic signals from the calyces may participate in modulating the maturation and development of the system. Groups I and II of metabotropic glutamate receptors (mGluRs) have been localized in glial cells in different brain regions. To investigate whether group II mGluRs are present in the medial nucleus of the trapezoid body, we have studied the pattern of expression of mGluR2/3 in the developing and mature nucleus by means of immunocytochemichal methods. The most remarkable finding was the switch in the occurrence of mGluR2/3 from glial to neuronal compartments. Thus, a preferential localization of mGluR2/3 immunoreactivity was observed in astrocytic processes surrounding the calyces of Held during the early postnatal development. In contrast, the main feature in adult rats was the presence of the group II mGluRs in presynaptic calyces of Held and postsynaptic principal globular cells.From these observations we suggest a role for group II mGluRs in neuronal-glial signalling in the calyx of Held-principal globular neuron synapses. Activation of these receptors might be relevant to the maturation and modulation of synaptic transmission in the medial nucleus of the trapezoid body.

Ultrastructural analysis of ventrolateral periaqueductal gray projections to the A7 catecholamine cell group

Stimulation of neurons in the ventrolateral periaqueductal gray produces antinociception that is mediated in part by pontine noradrenergic neurons. Previous light microscopic analysis provided suggestive evidence for a direct projection from neurons in the ventrolateral periaqueductal gray to noradrenergic neurons in the A7 cell group that innervate the spinal cord dorsal horn. Therefore, the present ultrastructural study used anterograde tracing combined with tyrosine hydroxylase immunoreactivity to provide definitive evidence that neurons in the ventrolateral periaqueductal gray form synapses with the somata and dendrites of noradrenergic neurons of the A7 cell group. Injections of the anterograde tracers biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into the ventrolateral periaqueductal gray of Sasco Sprague-Dawley rats yielded a dense innervation in the region of the lateral pons containing the A7 cell group. Electron microscopic analysis of anterogradely labeled terminals (n=401) in the region of the A7 cell group indicated that approximately 10% of these formed plasmalemmal appositions to tyrosine hydroxylase-immunoreactive dendrites with no intervening astrocytic processes. About 23% of these were asymmetric synapses, 10% were symmetric synapses, and 67% did not exhibit clearly differentiated synaptic specializations. The majority of anterogradely labeled terminals (60%) formed plasmalemmal appositions with dendrites and somata that lacked detectable tyrosine hydroxylase immunoreactivity. About 35% of these were symmetric synapses, 9% were asymmetric synapses and 56% did not form synaptic specializations. Approximately 30% of all anterogradely labeled terminals displayed features characteristic of axo-axonic synapses.The present results provide direct ultrastructural evidence to support the hypothesis that the analgesia produced by stimulation of neurons in the ventrolateral periaqueductal gray is mediated, in part, by activation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group.

Ultrastructural and permeability features of microvessels in the hippocampus, cerebellum and pons of senescence-accelerated mice (SAM)

We previously reported that the accumulation of blood-borne radiolabelled serum albumin in brain parenchyma increased with aging, especially in senescence-accelerated mice (SAMP8), which showed age-related deficits in learning and memory. In this study, in order to examine morphological events related to the age-related increase of the brain accumulation of serum albumin, the transvascular passage of blood-borne horseradish peroxidase (HRP) and ultrastructural features of microvessels were examined in the hippocampus, cerebellum and pons of SAMP8 and SAMR1 (control) mice. Ultrastructural examination of the hippocampus showed that the staining for HRP was occasionally spreading throughout the parajunctional cytoplasm of the endothelial cell of aged SAMP8 mice, but not in young SAMP8 mice nor in SAMR1 mice. The number of vessels showing the staining reaction for HRP in the parajunctional cytoplasm of the endothelial cells in aged SAMP8 mice increased significantly compared with that in the others. Electron microscopic morphometry showed that there were no significant differences among the number of HRP-positive vesicles per unit area of the endothelial cell cytoplasm in young and old mice of both strains. The staining reaction for HRP was not seen in the basal lamina of microvessels and the perivascular neuropil in all mice examined. Perivascular lipofuscin-like granules and collagen deposits, swelling of astroglial perivascular endfeet and perivascular cells containing foamy, lipid-like droplets were frequently found in several brain regions of aged SAMP8 mice. The perivascular cells with a few lipid-like droplets and more electron-homogeneous lysosomes were occasionally seen in SAMR1 and young SAMP8, while the other findings were scarcely observed in SAMR1 and young SAMP8 mice. These findings suggest that the blood-brain barrier to HRP was preserved in microvessels in three brain regions of SAM mice but the blood microvessels showed some age-related ultrastructural alterations in SAMP8 brains. Uncontrolled passage of HRP through the parajunctional cytoplasm of the endothelial cells may partly contribute to the age-related increase of accumulation of serum albumin in SAMP8 brains.

Axonal projections and synapses from the supratrigeminal region to hypoglossal motoneurons in the rat

Neural circuits from the supratrigeminal region (Vsup) to the hypoglossal motor nucleus were studied in rats using anterograde and retrograde neuroanatomical tracing methodologies. Iontophoretic injection of 10% biotinylated dextran amine (BDA) unilaterally into the Vsup anterogradely labeled axons and axon terminals bilaterally in the hypoglossal nucleus (XII) as well as other regions of the brainstem. In the ipsilateral XII, the highest density of BDA labeling was found in the dorsal compartment and the ventromedial subcompartment of the ventral compartment, where BDA labeling formed a dense, patchy distribution. Microinjection of 20% horseradish peroxidase (HRP) ipsilaterally or bilaterally into the tongue resulted in retrograde labeling of XII motoneurons confined to the dorsal and ventral compartments of the hypoglossal motor nucleus. Under light microscopical examination, BDA-labeled terminals were observed closely apposing the somata and primary dendrites of HRP-labeled hypoglossal motoneurons. Two hundred and sixty-five of these BDA-labeled terminals were examined at the ultrastructural level. One hundred and twelve BDA-labeled axon terminals were observed synapsing with either the somata (39%, 44/112) or the large or medium-size dendrites (61%, 68/112) of retrogradely labeled hypoglossal motoneurons. Axon terminals containing spherical vesicles (S-type) formed asymmetric synapses with HRP-labeled hypoglossal motoneuron dendrites. In contrast to this, F(F)-type axon terminals, containing flattened vesicles, formed symmetric synapses with both the somata and dendrites of HRP-labeled hypoglossal motoneurons with a preponderance of the contacts on their somata. Axon terminals containing pleomorphic vesicles (F(P)-type) were noted forming both symmetric and asymmetric synapses with HRP-labeled hypoglossal motoneuron somata and dendrites. The present study provides anatomical evidence of neuronal projections and synaptic connections from the supratrigeminal region to hypoglossal motoneurons. These data suggest that the supratrigeminal region, as one of the premotor neuronal pools of the hypoglossal nucleus, may coordinate and modulate the activity of tongue muscles during oral motor behaviors.

AMPA receptor subunit expression in trigeminal neurons during postnatal development

Trigeminal motoneurons (Mo5) and mesencephalic trigeminal neurons (Me5) are important constituents of the neural circuitry responsible for jaw movements. Non-N-methyl-D-aspartate (NMDA) glutamate receptors are a critical component of the brainstem circuitry responsible for reflex and centrally activated jaw movements; however, little is known about the expression of these receptors in neonatal oral-motor circuitry. Receptor immunohistochemistry using affinity-purified polyclonal antibodies directed against GluR1, GluR2/3/4c, and GluR4, respectively, and a monoclonal antibody directed against the GluR2 subunit, were used in rats at postnatal day (P)1, P3, P5, P10, P19-21, P32-35, and P60 to describe the expression of the alpha-amino-d-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor in Mo5 and Me5 neurons. In Mo5, immunoreactivity was noted for all antibodies throughout the time frame sampled. Neurons in caudal portions of Me5 displayed immunoreactivity to each antibody except at P60 when GluR2 immunoreactivity was absent. Neurons located in rostral Me5 displayed GluR2/3/4c and GluR4 immunoreactivity throughout the time frame, GluR1 immunoreactivity emerged at P3 and a transient expression of GluR2 expression was observed between P10 and P32-35. The lack of labeling of some neurons in both regions, coupled with differences in temporal expression, suggests that there are differences in the AMPA receptor phenotype within and between Mo5 and Me5 during postnatal development. Differences in AMPA subunit composition suggest a complex role for AMPA-mediated glutamatergic neurotransmission in brainstem circuits controlling jaw movements.

Ultrastructural synaptic organization of axon terminals in the ventral part of the cat oral pontine reticular nucleus

In an attempt to contribute to the current knowledge of the brainstem reticular formation synaptic organization, the ultrastructure and distribution of synaptic terminal profiles on neurons in the ventral part of the oral pontine reticular nucleus (vRPO), the rapid eye movement (REM) sleep-induction site, were studied quantitatively. Terminals with asymmetric contacts and rounded vesicles were classified according to vesicle density as type I or II (high or low density, respectively). The area, apposed perimeter length, and mitochondrial area of type I terminals, on average, were significantly smaller than those of type II terminals. Type III and IV terminals had symmetric contacts and oval and/or flattened vesicles; type III terminals formed synapses between them and on initial axons. Type V and VI terminals showed characteristics intermediate to those of asymmetric and symmetric synapses. Interestingly, some terminal features were related to both terminal area and postsynaptic dendritic diameter. The percentages of different synapses sampled on somata were as follows: asymmetric synapses (usually formed by type II terminals; mean +/- S.D.), 26.4% +/- 3%; symmetric synapses, 46.7% +/- 5.2%; and intermediate synapses, 26.9% +/- 6.1%. The percentages of different synapses sampled on dendrites were asymmetric synapses, 62.1% +/- 9%; symmetric synapses, 25.6% +/- 8.1%; and intermediate synapses, 12.3% +/- 1.7%. Comparison between large- and small-diameter dendrites revealed that the percentages of symmetric synapses and type II terminals decreased, whereas the percentages of type I terminals increased as postsynaptic dendritic diameters became smaller. Synaptic density was approximately four times lower on somata than on dendrites. The vRPO synaptic organization reflects some patterns that are similar to those found in other regions of the central nervous system as well as specific synaptic patterns that are probably related to its functions: the generation and maintenance of REM sleep and the control of eye movement or limb muscle tone.

Synaptic contacts between serotonergic and cholinergic neurons in the rat dorsal raphe nucleus and laterodorsal tegmental nucleus

We examined synaptic connectivity between cholinergic and serotonergic neurons in the dorsal raphe nucleus and the laterodorsal tegmental nucleus of the rat. To this purpose we employed two variations (the combination of pre-embedding immunogold-silver intensification with avidin-biotin-peroxidase complex technique and the combination of avidin-biotin-peroxidase/3, 3'-diaminobenzidine/silver-gold intensification with avidin-biotin-peroxidase/3,3'-diaminobenzidine reaction) of a double pre-embedding immunoelectron procedure, using primary antibodies against vesicular acetylcholine transporter and serotonin. At the light-microscopic level, serotonin-like immunoreactive neurons in the dorsal raphe nucleus appeared as reddish black and vesicular acetylcholine transporter-like immunoreactive axon terminals were brown colored using a combination of pre-embedding immunogold-silver technique and avidin-biotin-peroxidase complex technique. Serotonin-like immunoreactive fibers projected to the laterodorsal tegmental nucleus. At the electron microscopy level, with both methods we observed in the dorsal raphe nucleus vesicular acetylcholine transporter-immunopositive axon terminals in synaptic contact with serotonin-like immunoreactive dendrites and, to a lesser degree, with serotonin-like immunoreactive cell bodies. These synapses usually were of the symmetrical type. Occasionally we noted, next to vesicular acetylcholine transporter-immunopositive axon terminals, also immunonegative terminals synapsing with the serotonin-like immunoreactive dendrites. In the laterodorsal tegmental nucleus we found serotonin-like immunoreactive axon terminals and immunonegative terminals forming synapses with vesicular acetylcholine transporter-immunoreactive dendrites. Most synapses formed by the serotonin-like immunopositive terminals were of the asymmetrical type. Our results suggest that serotonergic neurons in the dorsal raphe nucleus and cholinergic neurons in the laterodorsal tegmental nucleus may reciprocally influence each other by means of synaptic connectivity. Such connectivity may serve to regulate pain sensation, or be involved in the regulation of the sleeping-waking cycle.

Calbindin-immunoreactive neurons in the reticular formation of the rat brainstem: catecholamine content and spinal projections

Calbindin-D28k (calbindin) is a calcium-binding protein that is distributed widely in the rat brain. The localisation of calbindin immunoreactivity in the medulla oblongata and its colocalisation with adrenaline-synthesising neurons [phenylethanolamine-N-methyltransferase-immunoreactive (PNMT-IR)] was examined (Granata and Chang [1994] Brain Res. 645:265-277). However, detailed information about the distribution of calbindin-IR neurons in the reticular formation of the medulla oblongata in particular is lacking. In this report, the authors address this issue with an emphasis on the quantitation of calbindin-IR neurons, catecholamine neurons [tyrosine hydroxylase (TH)-IR, or PNMT-IR], and spinally projecting neurons in the ventral brainstem. Rats received injections of the retrograde tracing agent cholera toxin B (CTB) into the thoracic spinal cord or into the superior cervical ganglion. Immunocytochemistry was used to reveal calbindin, TH, PNMT, and CTB immunoreactivity. Ten calbindin-IR cell groups were identified within the pontomedullary reticular formation. Seven previously undescribed but distinct clusters of calbindin-IR neurons were found. Within the ventral pons, a population of calbindin-IR neurons occurred dorsal but adjacent to the A5 cell group. These calbindin-IR neurons did not contain either TH or PNMT immunoreactivity, and few if any of these neurons projected to the spinal cord. A distinct group of calbindin-IR neurons was present in the ventral medulla. Seventy-five percent of these calbindin-IR neurons contained TH immunoreactivity, 45% contained PNMT immunoreactivity, and 21% were spinally projecting neurons. Spinally projecting, calbindin-IR neurons were a subpopulation of PNMT-IR cells. In the caudal ventral medulla, no TH-IR or PNMT-IR cells were calbindin-IR. In the intermediolateral cell column, close appositions of calbindin-IR terminals on identified sympathetic preganglionic neurons as well as calbindin-IR synapses indicated that these neurons may affect directly the sympathetic outflow. The results demonstrate for the first time the existence of a new subpopulation of spinally projecting, PNMT-IR neurons in the rostral ventrolateral medulla.

A light and electron microscopic study of GAT-1 positive cells in the monkey brainstem and spinal cord

The distribution of the GABA transporter GAT-1 was studied in the monkey brainstem and spinal cord, using an affinity purified polyclonal antibody against a synthetic peptide corresponding to the C terminus of GAT-1. Very dense staining was observed in the interpeduncular nucleus, the inferior olivary nucleus and the substantia gelatinosa of the spinal cord, whilst dense labelling was observed in the substantia nigra, cochlear nuclei, vestibular nuclei, the spinal nucleus of V, the area postrema and the ventral horn of the spinal cord. Electron microscopy showed that the labelled profiles consisted of axon terminals that formed symmetrical synapses, consistent with GABAergic terminals. Many of the nuclei that were densely labelled for GAT-1 were those that received primary auditory, vestibular, or somatosensory inputs and the high density of GAT-1 in these nuclei suggests that GAT-1 plays an important role in terminating the inhibitory effects of GABA, at these nuclei.

Dendritic and axonal targeting of the vesicular acetylcholine transporter to membranous cytoplasmic organelles in laterodorsal and pedunculopontine tegmental nuclei

Autoregulation of cholinergic neurons in the laterodorsal tegmental (LDT) and pedunculopontine (PPT) nuclei has been implicated in many functions, most importantly in drug reinforcement and in the pathophysiology of schizophrenia. This autoregulation is attributed to the release of acetylcholine, but neither the storage or release sites are known. To determine these sites, we used electron microscopy for the immunocytochemical localization of antipeptide antiserum raised against the vesicular acetylcholine transporter (VAchT) that is responsible for the uptake of acetylcholine into storage vesicles. The cellular and subcellular distribution of VAchT was remarkably similar in the two regions by by using each of two methods, immunogold and immunoperoxidase. In both PPT and LDT nuclei, VAchT labeling was seen mainly on membranous organelles including the trans-Golgi network in many somata. VAchT-immunoreactive tubulovesicles resembling saccules of smooth endoplasmic reticulum were often seen near the plasma membrane in dendrites. The VAchT-containing dendrites comprised almost 50% of the labeled profiles (1027/2129) in PPT and LDT nuclei. The remaining VAchT-immunoreactive profiles were primarily small unmyelinated axons and axon terminals. In axon terminals, VAchT was densely localized to membranes of small synaptic vesicles. The VAchT-immunoreactive axon terminals formed either symmetric or asymmetric synapses. The postsynaptic targets of these axon terminals included dendrites that were with (36/110) or without (74/110) VAchT immunoreactivity. Our results suggest that dendrites, as well as axon terminals, have the potential for storage and release of acetylcholine in the LDT and PPT nuclei. The released acetylcholine is likely to play a major role in autoregulation of mesopontine cholinergic neurons.

The pontine micturition center in rat receives direct lumbosacral input. An ultrastructural study

The act of micturition differs strongly among species. For example, adult cats and humans urinate primarily in a safe environment ('guarded urination'), while rats urinate more reflexively ('reflex urination'). This study in adult rats investigates the existence of direct lumbosacral cord projections to spinally projecting neurons in the pontine micturition center (PMC). Bilateral injections of wheat germ agglutinin horseradish peroxidase in the caudal lumbar and rostral sacral cord resulted in labeled profiles, including retrogradely labeled neurons in the PMC. At the ultrastructural level, anterogradely labeled terminals in the PMC were found, which were filled with many round and some pleiomorphic and flat vesicles. About eleven percent of the terminals contacted retrogradely labeled dendrites. Of the labeled terminals 80% contained asymmetric synaptic clefts, and 20% symmetric synaptic clefts. The results provide evidence that in the rat, unlike the cat, a direct lumbosacral pathway to the PMC exists, which might explain the differences in micturition behavior between rats and cats.

Histological dissociation between intra- and extraparenchymal portion of perforating small arteries after experimental subarachnoid hemorrhage in dogs

This study was conducted to investigate whether the intraparenchymal and extraparenchymal portions of small cerebral arteries show different histopathological changes after subarachnoid hemorrhage (SAH). SAH was produced in dogs by a two-hemorrhage method, and the dogs were perfusion-fixed at 7 and 14 days after SAH. Normal untreated animals were also examined to determine whether or not the histological and morphological changes in the perforating arteries were affected by the perfusion pressure during perfusion-fixation. Control dogs, which received cisternal injection of saline, and untreated normal dogs, which were divided into two groups depending on the perfusion pressure during perfusion-fixation, were also examined. Microvascular corrosion casts produced by arterial injection of polyester resin were examined by scanning electron microscopy (SEM) with morphometric analysis of the caliber of the small perforating arteries. Slices sectioned parallel to the pontine surface were examined by SEM and by light microscopy with morphometric analysis of the luminal diameter and wall thickness of the small perforating arteries. Corrosion casts showed irregular width with folds in the perforating arteries 7 days after SAH. Sectioned slices showed an increased wall thickness of the perforating arteries with intimal corrugation 7 days after SAH. Morphometric analysis revealed that the extraparenchymal portion of the perforating arteries showed no significant differences between any of the groups tested; however, the intraparenchymal portion showed a significant decrease of luminal diameter 7 days after SAH, and a significant increase of wall thickness 7 and 14 days after SAH. The perfusion pressure during perfusion-fixation did not affect the histological changes in the perforating arteries. The results of this study showed that the vasoconstrictive response of the perforating arteries to SAH was different between the extraparenchymal and the intraparenchymal portion, and showed that the intraparenchymal portion of the perforating arteries constrict after SAH, which may affect cerebral ischemia during cerebral vasospasm by increasing total cerebrovascular resistance.

Neuronal vacuolation in raccoons from Oregon

During a 2-year period (1995-1997), vacuoles were detected in neurons of 21/50 (42% prevalence) raccoons (Procyon lotor) in Oregon. Age or sex predisposition was not apparent. Twenty of these raccoons were from within a radius of 40 km of Corvallis in western Oregon. Microscopically, the vacuoles were variable in size, were in the perikarya, and were consistently present in pontine nuclei. Brain tissues were negative for rabies virus antigen by fluorescent antibody test and for the protease-resistant protein prion by immunohistochemistry. Electron microscopic examination of the brain stem of selected animals revealed accumulation of electron-dense material within neuronal perikarya. Light and electron microscopic examination indicated that the accumulated intracellular material had a high lipid content. These lesions suggest a form of neuronal storage condition. Further research is required to identify the composition of the intracellular lipid material, to elucidate the mechanism of neuronal vacuolation in raccoons, and to understand the basis for the apparent geographic restriction of this lesion.

[Experimentally induced actin depolymerization disrupts the adaptive state of a neuron]

The state of filament actin in Mauthner neuron (MN) of gold-fish adapted to long-term natural stimulation was investigated after introduction of actin-depolymerizing cytochalasin D. Cytochalasin was demonstrated to cause almost complete disappearance of long actin bundles that were characteristic for the cytoplasm of adapted fish and were absent from MN of intact fish. Polymeric actin supporting the cytoskeleton integrity was suggested to be involved in mechanisms underlying the increase of MN resistance extreme loads.

Classification of caudal ventrolateral pontine neurons with sympathetic nerve-related activity

This study was designed to answer three questions concerning caudal ventrolateral pontine (CVLP) neurons whose naturally occurring discharges are correlated to sympathetic nerve discharge (SND). 1) What are the proportions of CVLP neurons that have activity correlated to both the cardiac-related and 10-Hz rhythms in SND, to only the 10-Hz rhythm, and to only the cardiac-related rhythm? 2) Do CVLP neurons with activity correlated to the cardiac-related and/or 10-Hz rhythm in SND subserve a sympathoexcitatory or sympathoinhibitory function? 3) Do CVLP neurons with activity correlated to the cardiac-related and/or 10-Hz rhythm in SND project to the thoracic spinal cord? To address these issues we recorded from 476 CVLP neurons in 24 urethan-anesthetized cats. Spike-triggered averaging, arterial pulse-triggered analysis, and coherence analysis revealed that the discharges of 66 of these neurons were correlated to inferior cardiac postganglionic SND. For 39 of these neurons, we were able to determine whether their discharges were correlated to one or both rhythms. The results showed that the CVLP contained a heterogeneous population of neurons with sympathetic nerve-related activity. The discharges of 21 neurons were correlated to both the 10-Hz and cardiac-related rhythms in SND, 9 neurons had activity correlated to only the 10-Hz rhythm, and 9 neurons had activity correlated to only the cardiac-related rhythm. The firing rates of CVLP neurons with activity correlated to both rhythms or to only the 10-Hz rhythm were decreased during the inhibition of SND induced by baroreceptor reflex activation (rapid obstruction of the abdominal aorta). These neurons are presumed to exert sympathoexcitatory actions. The time-controlled collision test verified that 11 of 12 CVLP neurons with activity correlated to both rhythms were antidromically activated by stimulation of the first thoracic segment of the spinal cord. Antidromic mapping at this level showed that the site requiring the least stimulus current to elicit the longest latency response (nearest the terminal) was in the vicinity of the intermediolateral nucleus (IML). In contrast, only 1 of 13 CVLP neurons with activity correlated to only one of the rhythms in SND could be antidromically activated by spinal stimulation. These data demonstrate for the first time that there is a direct pathway from the CVLP to the IML that is comprised almost exclusively of sympathoexcitatory neurons whose discharges are correlated to both the 10-Hz and cardiac-related rhythms in SND.

Convergent prefrontal cortex and mamillary body projections to the medial pontine nuclei: a light and electron microscopic study in the rat

To study the convergence of medial prefrontal cortex and mamillary body projections to the medial pontine nuclei, light and electron microscopic, neuroanatomical, tract-tracing experiments were performed. Injections of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), biotin conjugated to dextran (BD), or rhodamine conjugated to dextran (RD) were made individually or in combinations into the cerebral cortex, hypothalamus, or pons. In addition, injections of WGA-HRP into the medial prefrontal cortex and electrolytic lesions of the mamillary body were made to study the synaptology of afferent projections to the pontine nuclei. In the light microscopic studies, injections of WGA-HRP into the rostromedial pontine nuclei produced dense, retrograde labeling both in the dorsal peduncular area of the medial prefrontal cortex and in the medial mamillary nucleus, pars medialis. Injections of the anterograde tracers BD and RD into the medial prefrontal cortex and the medial mamillary nuclei, respectively, resulted in partially overlapping terminal fields in the rostromedial pontine nuclei. In the electron microscopic studies, injections of WGA-HRP into the dorsal peduncular area and electrolytic lesions of the mamillary body produced anterogradely labeled axon terminals and degenerating axon terminals that synapsed on the same dendrites or neuronal somata in the rostromedial pontine nuclei. The results demonstrate that the medial prefrontal cortex and the medial mamillary nuclei have partially overlapping projections to the rostromedial pontine nuclei and implicate precerebellar relay nuclei in the integration of limbic and/or autonomic functions mediated by convergent projections from the cerebral cortex and the hypothalamus.

Nigral innervation of cholinergic and glutamatergic cells in the rat mesopontine tegmentum: light and electron microscopic anterograde tracing and immunohistochemical studies

The substantia nigra (SN) has long been known as an important source of afferents to the pedunculopontine tegmental nucleus (PPN). However, it has not been established which of the chemospecific cell populations receive this synaptic input. We sought to address this issue by a correlative light and electron microscopic approach that combines anterograde tracing of nigral efferents with pre-embedding choline acetyltransferase (ChAT) and/or glutamate (Glu) immunohistochemistry. Following large bilateral injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in the SN, the labeled nigrotegmental fibers were concentrated in a small area of the mesopontine tegmentum which contained very few ChAT-immunoreactive (ChAT-ir) cell bodies. However, strands of fine varicose fibers penetrated to adjacent regions of the PPN which harbored numerous cholinergic perikarya. The anterogradely labeled boutons were often seen in the proximity of ChAT-ir perikarya and dendrites, but the majority (82-93%) established symmetric synaptic junctions with noncholinergic profiles. In the pars dissipata of the PPN (PPNd), one-third of the labeled terminals synapsed onto noncholinergic perikarya and primary dendrites, while in the pars compacta of the PPN (PPNc) axosomatic synapses were rare. The possibility that the perikarya receiving a rich synaptic input from the SN are glutamatergic was tested in experiments combining anterograde transport of biotinylated tracers biocytin and dextran-amine (BDA) with glutamate immunohistochemistry. In double-labeled sections, Glu-ir perikarya within the terminal plexus of nigrotegmental fibers were surrounded by synaptic terminals. The PPNd also contained retrogradely BDA-labeled neurons which were contacted by anterogradely labeled terminals. These results indicate that although a small subpopulation of cholinergic neurons in the mesopontine tegmentum receive direct synaptic input from the SN, the primary target of nigrotegmental fibers are glutamatergic cells in the PPNd. Our results also provide ultrastructural evidence that some nigrotegmental fibers innervate pedunculonigral neurons.

[Changes in the ratio of granular and agranular endoplasmic reticulum of the Mauthner neurons in fishes as an index of the compensatory processes during sensory stimulation]

Endoplasmic reticulum structure was studied on Mauthner neurons of adult fish and fry. Two-fold decrease of extension of rough endoplasmic reticulum components and similar increase of extension of smooth endoplasmic reticulum components were found to occur following long-term stimulation of Mauthner neurons. The significance of transformation of rough endoplasmic reticulum into smooth one as a result of neuronal compensatory response was discussed.

Direct projections from the lumbosacral spinal cord to Barrington's nucleus in the rat: a special reference to micturition reflex

In the present study, direct projections from the lumbosacral cord to Barrington's nucleus in the rat were investigated by using retrograde and anterograde tracing techniques. After injection of cholera toxin B subunit (CTb) into Barrington's nucleus, a number of moderately CTb-labeled neurons were observed in the lumbosacral cord, with a slight ipsilateral dominance; most were located in the spinal parasympathetic and dorsal commissural nuclei of the lumbosacral cord. In addition, some retrogradely labeled neurons were found in the periaqueductal gray (PAG). These findings were confirmed by an anterograde labeling experiment. After biotinylated dextran amine (BDA) was injected into the lumbosacral cord, dense BDA-labeled axon terminals were found in Barrington's nucleus as well as in the PAG. Injection of BDA into the PAG resulted in many BDA-labeled terminals in Barrington's nucleus. The present results provided clear evidence for a direct projection from the spinal parasympathetic and dorsal commissural nuclei to Barrington's nucleus that could subserve conveying bladder-filling information from the lumbosacral cord to Barrington's nucleus in the micturition reflex of the rat.

The columnar region of the ventral nucleus of the lateral lemniscus in the big brown bat (Eptesicus fuscus): synaptic arrangements and structural correlates of feedforward inhibitory function

Neurons of the columnar region of the ventral nucleus of the lateral lemniscus of Eptesicus fuscus respond with high-precision constant-latency responses to sound onsets and possess remarkably broad tuning. To study the synaptic basis for this specialized monaural auditory processing and to elucidate the excitatory or inhibitory nature of the input and output circuitry, we have used classical transmission electron microscopy, and postembedding immunocytochemistry for gamma aminobutyric acid (GABA) and glycine on serial semithin sections. The dominant putatively excitatory perisomatic input is provided by large calyx-like terminals that possess round synaptic vesicles and asymmetric synaptic contacts. Additionally, calyces contact the dendrites of neighboring neurons. Putatively inhibitory small boutons possess pleomorphic or flattened synaptic vesicles and symmetrical contacts and are sparsely distributed on somata and dendrites. Almost all neurons are glycine-immunoreactive. There is a moderate amount of glycine-immunoreactive puncta; GABA-immunoreactive puncta are rare. This suggests that (1) there is a fast robust excitatory synaptic input via calyx-like perisomatic endings, (2) calyx-like endings distribute frequency-specific excitatory input across isofrequency sheets by virtue of parallel synapses to somata and adjacent dendrites, and thus, dendritic integration may contribute to the broadening of frequency tuning, (3) the columnar region forms an inhibitory glycinergic feedforward relay in the ascending auditory pathway, a relay that is probably involved in creating filters for time-varying signals.

Synaptic excitation in the dorsal nucleus of the lateral lemniscus: whole-cell patch-clamp recordings from rat brain slice

The synaptic events underlying the excitation of neurons in the rat's dorsal nucleus of the lateral lemniscus were studied by whole-cell patch-clamp recordings in a brain slice preparation of the auditory midbrain. Both current-clamp and voltage-clamp data were obtained with the brain slice submerged in artificial cerebrospinal fluid. The rats were between 21 and 35 days of age at the time the recordings were made. Synaptic responses were evoked by a bipolar stimulating electrode placed on the lateral lemniscus just ventral to the dorsal nucleus. To eliminate glycinergic inhibitory responses, all physiological data were gathered with 0.5 microM strychnine added to the saline bath. Under current-clamp conditions, excitatory postsynaptic potentials could be subdivided into early and late components. The early component produced a single, highly reliable, short-latency spike and the later component produced a more variable, long-latency spike or train of spikes. The non-N-methyl-D-aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, completely blocked the early excitatory postsynaptic potential and its associated action potential. The N-methyl-D-aspartate antagonist, D,L-2-amino-5-phosphonovaleric acid, blocked the later excitatory postsynaptic potential and its action potentials. Typically, both early and late excitatory postsynaptic potentials could be recorded from the same cell, but the early excitatory postsynaptic potential was evoked at lower stimulus levels and had a larger amplitude than the later excitatory postsynaptic potential. Under voltage-clamp conditions, dorsal nucleus of the lateral lemniscus neurons responded to stimulation of the lateral lemniscus with excitatory postsynaptic currents. Outward excitatory postsynaptic currents were recorded with holding potentials that depolarized the cell membrane and inward currents were seen when the cell was hyperpolarized. The current-voltage (I-V) relation of the early peak portion of the excitatory postsynaptic current was nearly linear, whereas the I-V relation of the later excitatory postsynaptic current (12 ms after the peak) was non-linear over the range between -50 and - 100 mV. The outward excitatory postsynaptic current consisted of an early current that was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later current that was blocked by D,L-2-amino-5-phosphonovaleric acid. In artificial cerebrospinal fluid with normal concentrations of Mg2+, the inward excitatory postsynaptic current was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, but was not affected by D,L-2-amino-5-phosphonovaleric acid. In Mg2+-free artificial cerebrospinal fluid. however, the early component of the inward excitatory postsynaptic current was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later component was blocked by D,L-2-amino-5-phosphonovaleric acid. The results indicate that both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated synaptic responses are present in dorsal nucleus of the lateral lemniscus neurons of rats at 21-35 days of age. The N-methyl-D-aspartate component had a longer time-course and a higher threshold than the non-N-methyl-D-aspartate component, and was subject to a voltage-dependent Mg2+ block when the cell's membrane was hyperpolarized. The long-duration N-methyl-D-aspartate component is probably responsible for the prolonged inhibitory effect of dorsal nucleus of the lateral lemniscus neurons on physiological responses in the rat's inferior colliculus.

Ultrastructural evidence for a direct pathway from the pontine micturition center to the parasympathetic preganglionic motoneurons of the bladder of the cat

Light microscopy tracing studies have provided evidence that the pontine micturition center (PMC) projects to the area of the intermediolateral cell column of the sacral spinal cord. Although this region contains parasympathetic preganglionic motoneurons of the bladder and colon, it also contains many local interneurons and neurons projecting to supraspinal levels. The present study demonstrates that neurons in the PMC indeed project to preganglionic bladder motoneurons. Wheat germ agglutinin horseradish peroxidase was injected in the PMC and cholera toxin B subunit was injected in the bladder wall. Many anterogradely labeled fibers from the PMC were found to terminate on somata and dendrites of the retrogradely labeled preganglionic bladder motoneurons. The terminals were filled with many round vesicles and possessed an asymmetric synaptic cleft, suggesting an excitatory function.

Input from central nucleus of the amygdala efferents to pericoerulear dendrites, some of which contain tyrosine hydroxylase immunoreactivity

Light microscopic anterograde tracing studies indicate that neurons in the central nucleus of the amygdala (CNA) project to a region of the dorsal pontine tegmentum ventral to the superior cerebellar peduncle which contains noradrenergic dendrites of the nucleus locus coeruleus (LC). However, it has not been established whether the efferent terminals from the CNA target catecholamine-containing dendrites of the LC or dendrites of neurons from neighboring nuclei which may extend into this region. To examine this question, we combined immunoperoxidase labeling of the anterograde tracer biotinylated dextran amine (BDA) from the CNA with immunogold-silver labeling of the catecholamine-synthesizing enzyme tryrosine hydroxylase (TH) in the rostrolateral LC region of adult rats. By light microscopy, BDA-labeled processes were dense in the dorsal pons within the parabrachial nuclei as well as in the pericoerulear region immediately ventral to the superior cerebellar peduncle. Higher magnification revealed that BDA-labeled varicose fibers overlapped TH-labeled processes in this pericoerulear region. By electron microscopy, anterogradely labeled axon terminals contained small, clear as well as some large dense core vesicles and were commonly apposed to astrocytic processes along some portion of their plasmalemma. BDA-labeled terminals mainly formed symmetric type synaptic contacts characteristic of inhibitory transmitters. Of 250 BDA-labeled axon terminals examined where TH immunoreactivity was present in the neuropil, 81% contacted unlabeled and 19% contacted TH-labeled dendrites. Additionally, amygdala efferents were often apposed to unlabeled axon terminals forming asymmetric (excitatory type) synapses. These results demonstrate that amygdaloid efferents may directly alter the activity of catecholaminergic and non-catecholaminergic neurons in this pericoerulear region of the rat brain. Furthermore, our study suggests that CNA efferents may indirectly affect the activity of pericoerulear neurons through modulation of excitatory afferents. Amygdaloid projections to noradrenergic neurons may help integrate behavioral and visceral responses to threatening stimuli by influencing the widespread noradrenergic projections from the LC.

Immunocytochemical evaluation of the blood-brain barrier to endogenous albumin in the olfactory bulb and pons of senescence-accelerated mice (SAM)

The blood-brain barrier (BBB) to endogenous albumin was studied in the olfactory bulb and pons of the senescence-accelerated prone (SAMP8) mouse and senescence-accelerated resistant (SAMR1) mouse strains by using a quantitative immunocytochemical procedure. Ultrathin sections of Lowicryl K4M-embedded samples were exposed to anti-mouse albumin antiserum followed by protein A-gold. Morphometric analysis of the electron micrographs revealed that in the olfactory bulb of both groups of animals, especially in the internal granular layer, some percentage of capillaries and slightly larger microvessels showed leakage of albumin. However, this percentage was larger in SAMP8 than in SAMR1 mice. In the pons, no significant differences in the permeability of blood microvessels were observed in both groups of mice, although a small fraction of capillaries in SAMP8 mice showed limited extravasation of blood plasma albumin. These observations indicate that the BBB in the olfactory bulb of control and SAMP8 mice is not as tight as it is in the pons or in the previously examined cerebral cortex. The labelling density of the neuropil was slightly higher than in the cerebral cortex, suggesting that albumin may have extravasated locally, in addition to having acces to the parenchyma of the olfactory bulb and pons from neighbouring areas supplied with the non-BBB-type of microvasculature. Furthermore, the data obtained suggest that there is limited (segmental), premature age-related impairment of the BBB function in SAMP8 mice.