It has been suggested that noxious information from the lower bowel may reach the medulla oblongata via several spinal pathways terminating in the ventrolateral reticular area, the nuclei of the solitary tract and medullar dorsal column nuclei correspondingly. The contribution of these pathways to visceral pain processing in the medulla is not established. In this study, performed in urethane-anaesthetized rats, we used c-Fos immunohistochemistry to quantitatively characterize and compare neuronal responses in the gracile (Gr) and cuneate (Cu) nuclei, nuclei of the solitary tract (NTS), lateral reticular nucleus (LRt) and ventrolateral reticular area (VLM) of the medulla to a repeated noxious colorectal distension (CRD). When compared with sham distension, after CRD, the most pronounced induction of c-Fos expression was observed in the NTS, while a modest increase was found in the Gr. Simultaneously, the CRD produced a distinct decrease in the c-Fos immunoreactivity in the VLM. Changes in the c-Fos expression observed in Cu and LRt were insignificant. The present results suggest that under anaesthesia the transmission of nociceptive information from the colorectal region to the medulla is mainly accomplished via excitatory spinosolitary afferents and ventrolateral spinal pathway recruiting inhibitory mechanisms in the medullar reticular formation. The excitatory spinal inputs to the dorsal column nuclei seem to be modestly involved.

The orexinergic system interacts with several functional states of emotions, stress, hunger, wakefulness and behavioral arousal through four pathways originating in the lateral hypothalamus (LH). Hundreds of orexinergic efferents have been described by tracing studies and direct immunohistochemistry of orexin in the forebrain, olfactory regions, hippocampus, amygdala, septum, basal ganglia, thalamus, hypothalamus, brain stem and spinal cord. Most of these tracing studies investigated the whole orexinergic projection to all regions of the intracranial part of the CNS. To identify the orexinergic efferents at the subnuclear level of resolution, we focussed on the orexinergic target in the amygdala, which is substantially involved in the LH output and contributes mostly to the functional outcome of the orexinergic system and the basal ganglia. Immunohistochemical identification of axonal orexin A and orexin B in male adult rats has been performed on serial sections. In the extended amygdala many new orexinergic targets were found in the anterior amygdaloid area (dense), anterior cortical nucleus (moderate), amygdalostriatal transition region (moderate), basolateral regions (moderate), basomedial nucleus (moderate), several bed nucleus of the stria terminals regions (few to dense), central amygdaloid subdivisions (dense), posteromedial cortical nucleus (moderate) and medial amygdaloid subnuclei (dense). Furthermore, the entopeduncular nucleus has been newly identified as another target for orexinergic fibers with a high density. These results suggest that subdivisions and subnuclei of the extended amygdala are specific targets of the orexinergic system.

Previous neurophysiological studies have demonstrated that the amygdala has a direct influence upon trigeminal motoneuron activity. The existence of a direct amygdalotrigeminal pathway in rats was proved by anterograde tracing with the neuroanatomical tracer, biotinylated dextran amine (BDA). After ipsilateral BDA application to the central nucleus of the amygdala (AmCe), widespread ipsilateral projections emerging from its medial subnucleus were traced to the trigeminal brainstem nuclear complex, including the principal sensory (Pr5) and mesencephalic trigeminal nucleus (Me5), and their premotoneurons and interneurons, located in the supratrigeminal, intertrigeminal and peritrigeminal nuclei. Sparse BDA-labeled axons and their terminals were also distributed in the contralateral Pr5, interpolar and caudal subnuclei of the spinal trigeminal nucleus. The central lateral amygdaloid nucleus gives rise to a light ipsilateral projection to the pontine part of the Me5. The present data indicate that AmCe sends massive efferents to the trigeminal nuclei in the brainstem, wherein its medial subnucleus sends the major input to them. The medial amygdaloid nucleus sparsely innervates Me5 neurons, specifically those located in its mesencephalic portion, while basomedial and basolateral efferents do not target the trigeminal nuclear complex. These results suggest that the amygdaloid input may modulate the activity of trigeminal sensory and motor neurons and, thus, the amygdala is possibly involved in the control of masticatory behavior.

The recto-anal region is innervated by extrinsic and intrinsic nerves and a number of neuropeptides including substance P (SP) have been suggested to participate in the regulation of intestinal movements. We examined the age-related changes in the distribution of SP-immunoreactive nerve structures in the distal part of the rat large intestine. Using immunohistochemistry, the presence of SP was studied in fresh tissues from Wistar rats at different ages taken at three sampling sites, the distal rectum, anal canal and internal anal sphincter. In the 15-day old rats the myenteric plexus of the distal rectum and anal canal was well outlined by numerous SP-immunoreactive varicose nerve fibres encircling immunonegative perikarya. In the circular muscle layer, nerve fibres and small nerve bundles ran parallel to the muscle cells, while in the longitudinal muscle layer, only occasional nerve fibres were seen. At the level of the internal anal sphincter, no myenteric ganglia were present. Here, thin varicose fibers ran parallel to the smooth muscle cells. In the 3-month old rats, a larger number of intensely staining SP-immunoreactive nerve fibres were found and in the circular muscle layer, thicker nerve strands were observed. In the 26-month old rats, the density and staining intensity of SP-immunopositive nerve fibres in the myenteric plexus was lower than in the 3-month-old rats. Similar changes in the SP-immunostained fibres in the internal anal sphincter were observed. Degenerative alterations in SP-containing fibres during aging appear to play a role in ano-rectal motility and sphincter control.

Age-related changes in the nitric oxide-producing neurons in the rat rectoanal region were studied with reduced nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and morphometry. In 15-day-old rats the myenteric plexus of the distal rectum and anal canal, situated in the space between the longitudinal and circular muscle layers, was exposed by NADPH-d positive perilcarya. In the anal canal the positive myenteric neurons were more intensively stained than in the rectum. The mean optical density (OD) of positive neurons in the rectum was 115.8 +/- 2.4 vs. 89.5 +/- 1.6 in the anal canal. Single nerve fibres and small nerve bundles in the circular muscle layer of the anal canal were frequently seen and were more intensively stained than rectal fibres. In 3-month-old rats a larger number of NADPH-d-positive nerve fibres and an increase in their staining intensity were found. The staining intensity of the perikarya in the anal canal was higher than in the rectum (the OD 70.9 +/- 1.1 vs. 55.5 +/- 1.5). In the circular muscle layer thicker nerve strands were observed. In 26-month-old rats the staining intensity of NADPH-d-positive perikarya in the myenteric plexus was weaker than in the 3-month-old rats. In the rectum their OD was lower than in the anal canal (93.8 +/- 2.05 vs. 78.9 +/- 1.7). The nerve strands were sparse and less intensely stained than in 3-month-old rats. Our results are consistent with the hypothesis that NO is an important neurotransmitter of the recto-anal motility in the ageing gut.

Kamen G. Usunoff1,2, Oliver Schmitt2, Nikolai E. Lazarov1, Dimitar E. Itzev3, Arndt Rolfs4, and Andreas Wree2 1Department of Anatomy and Histology, Medical University, Sofia, Bulgaria, 2Institute of Anatomy, University of Rostock, Rostock, Germany, 3Institute of Neurobiology, Bulgarian Academy of Sciences, Sofia, Bulgaria, and 4Albrecht-Kossel-Institute of Neuroregeneration, Centеr for Mental Health Disease, University of Rostock, Rostock, Germany

Age-related changes in the presence and distribution of substance P (SP)-like immunoreactive nerve structures of the rat recto-anal region were studied by the lightmicroscopical immunohistochemical technique. In 15-day-old rats the myenteric plexus of the distal rectum and anal canal was well outlined by a large number of SP-like immunoreactive varicose nerve fibres, encircling the immunonegative perikarya. In the circular muscle layer scattered fibres and small nerve bundles run parallel to the muscle cells, while in the longitudinal muscle layer single nerve fibres were seen in-between. At the level of the internal anal sphincter no myenteric ganglia were present. Here, only thin varicose nerve fibres running parallel to the smooth muscle cells could be seen. In 3-month-old rats an increasing of the staining intensity and a larger number of SP-like immunoreactive nerve fibres were found. In the circular muscle layer thicker nerve strands were observed. In 26-month-old rats the density and staining intensity of SP-like immunoreactive nerve fibres in the myenteric plexus was lower than in the 3-month-old rats. Similar changes in the SP-like immunoreactive nerve fibres in the internal anal sphincter were encountered.

Since the distal part of the intestine is targeted by a wide range of pathogens, the motility of the recto-anal region has been the object of many experimental and clinical observations. In this study, we investigated descending motor responses in the anal canal as a measure of the activation of autonomic reflex pathways underlying evacuatory recto-anal activity. The partitioned organ bath method was used to register motor responses of the anal canal as induced by balloon distension of the rectum in isolated rat recto-anal preparations. Distension-induced descending responses of the anal canal comprised contractions (with distension at a distance of 15 mm), initial contractions and secondary relaxations (at 10 mm) and short contractions followed by deep relaxations (at 3−5 mm). Decreasing the distance between the distension stimulus and the anal canal resulted in a decreased contraction response and increased relaxation. Tetrodotoxin (0.1 μmol/L) inhibited these responses. Atropine (0.3 μmol/L) decreased contraction and did not change the relaxation response. NG-nitro-L-arginine (0.5 mmol/L) enhanced contraction in both the absence and presence of atropine. L-arginine (0.5 mmol/L) inhibited contraction and extended relaxation in atropine-pretreated preparations. The actions of NG-nitro-L-arginine and L-arginine were more pronounced in the aboral direction. ChAT-positive nerve fibers were observed in myenteric ganglia of the rectum and the anal canal. The density of NADPH-diaphorase-positive neurons was higher in the anal canal region. Our results suggest that locality-dependent activation of the descending reflex neuromuscular communications underlie evacuatory activity in the recto-anal region. This activation response involves long excitatory cholinergic and non-cholinergic pathways along the rectum and short inhibitory nitrergic pathways located predominantly in the anal canal region.

The efferent projections of the anterior and posterodorsal part of the medial nucleus (MePD) in the mouse were studied by means of anterograde axonal tracing using biotinylated dextran amine. The MePD axons ran mainly via the stria terminalis and to a lesser extent via the ventral amygdalofugal pathway. The projections to the forebrain were broadly distributed and varied from very strong to scant. The most significant connections were destined to the bed nucleus of the stria terminalis in which all parts of the medial division were innervated by MePD neurons. Moderate projections reached the limbic striatum (nucleus accumbens), olfactory tubercle and the lateral septal nucleus. The substantia innominata was also innervated by the MePD, and especially the projection to its ventral portion was substantial. The profuse innervation of the medial preoptic nucleus and medial preoptic area indicated significant involvement of the MePD in sexual behavior. Many hypothalamic nuclei were innervated but to a different extent. The very strong innervation of the ventral premammillary nucleus further indicated the involvement of the MePD in the neuronal circuitry for sexual behavior. Substantial projections also reached the anterior hypothalamus and tuber cinereum, while the connections to the lateral hypothalamus were widespread but showed moderate density. MePD strongly innervated the ventrolateral part of the ventromedial hypothalamic nucleus and moderately its remaining parts. The neurosecretory hypothalamic nuclei and the arcuate nucleus contained only a few MePD terminals. The thalamic innervation was very scant and reached the lateral habenular nucleus and the nuclei of the midline. The mesencephalic connections were moderate to sparse and projected to the mesolimbic dopaminergic groups in the ventral tegmental area, the pars lateralis and the dorsal tier of the substantia nigra pars compacta, the periaqueductal gray and the dorsal raphe nucleus. The present results principally resembled data known in other rodent species; however, the efferents of the MePD often differed in extent and/or topical distribution.

The implications of cholinergic and nitrergic transmissions in ascending and descending reflex motor pathways of recto-anal region in rat model were evaluated using: (i) electrical stimulation; (ii) triple organ bath; and (iii) morphological techniques. Electrical stimulation to anal canal induced simultaneous ascending contractile responses of longitudinal and circular muscles of proximal rectum, local contraction of anal canal or contraction followed by relaxation of internal anal sphincter when external sphincter was dissected off. The stimulation of proximal rectum elicited local contractions of both rectal layers and descending contractions of internal sphincter or anal canal. Tetrodotoxin (0.1μM) prevented the electrically elicited events. The ascending excitatory responses and the local and ascending contractions of longitudinal muscle were more pronounced than those of circular muscle suggesting dominant role of ascending reflex pathways and of longitudinal muscle in rectal motor activity. Choline acetyltransferase (ChAT)-containing fibres and nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase-positive neurons were observed in myenteric ganglia of rectum and anal canal. NG-nitro-l-arginine (0.5mM) increased the contractile ascending and descending responses. During atropine (0.3μM) treatment the ascending and descending contractions were suppressed but not abolished and a relaxation revealed in ascending response of circular muscle and in descending responses of internal anal sphincter and anal canal. The relaxation was decreased by NG-nitro-l-arginine and increased by l-arginine (0.5mM). The results suggest that cholinergic excitatory ascending and descending pathways and nitric oxide-dependent inhibitory ascending neurotransmission(s) to rectal circular muscle and inhibitory descending to internal anal sphincter and anal canal are involved in reflex circuitry controlling motor activity of recto-anal region.

Histochemical technique for nicotinamide adenine dinucleotide phosphoate-diaphorase (NADPH-d) and immunohistochemical technique were used to examine the distribution of NADPH-d-positive, substance P- (SP-), choline acetyltransferase- (ChAT-) and tyrosine hydroxylase-(TH-) immunoreactive neuronal elements in the myenteric ganglia and muscle coat of the distal rectum, anal canal and internal anal sphincter of the rat. Numerous NADPH-d-positive nerve cell bodies in myenteric ganglia of both investigated regions were found. Single nerve cell bodies immunoreactive for SP and ChAT were found in, the myenteric ganglia of the rectum and anal canal. High number of NADPH-d-positive nerve fibres in the myenteric ganglia and in the internodal strands were demonstrated. Large number of nerve fibres, demonstrating immunoreactivity for all the three peptides in the myenteric ganglia, their internodal strands, and in the muscle coat between the smooth muscle cells were observed. In the internal and sphincter NADPH-d-, SP-, ChAT-, TH-positive varicose and folded nerve fibres were found.

The subcortical efferent projections of the claustrum. in the rat were investigated by means of retrograde tracing using Fluoro-Gold as an effective retrograde fluorescent marker. The tracer was stereotaxically injected unilaterally in the dorsal ("extrapyramidal") striatum, in the nucleus accumbens (ventral, "limbic" striatum), in the substantia nigra, in the ventral tegmental area, and in the parabrachial nuclear complex in the dorsolateral. pons. ln all cases with successful infiltration of these structures, the ipsilateral claustrum. contained retrogradely labelled neuronal perikarya. The projection towards the dorsal striatum. arises from loosely distributed neurons, intermingled with claustrocortical cells, over the entire territory of the claustrum. On the other hand, the axons to the nucleus accumbens arise from neurons concentrated towards the medial border of the claustrum, close to the external medullary lamina. The projections to the extrapyramidal structures, e.g. dorsal striatum and substantia nigra arise only from the claustrum. proper, whilst the tracts to structures, affiliated with the limbic system (nucleus accumbens, ventral tegmental area) are emitted also by the endopiriform. nucleus (the ventral continua * tion of the claustrum). Unexpectedly strong connection from both the claustrum. and endopiriform. nucleus to the parabrachial nuclear complex is presently reported. The functional significance of this newly described projection remains to be elucidated but it is plausible to speculate that it might be concerned with the pain processing.

The amygdala (Am) is a relatively voluminous gray substance, located in the depth of the ventromedial temporal lobe. The Am has diverse afferent and efferent connections throughout the neuraxis, and is involved in the modulation of neuroendocrine functions, visceral effector mechanisms, and in complex patterns of behavior: learning and memory, aggression and defense, pain modulation, reproduction, food intake, etc. A recently revealed important function of the Am is that it acts as the brain 'lighthouse' which constantly monitors the environment for stimuli which signal a threat to the organism. The data from patients with extensive lesions of the striate cortex indicate that unseen fearful and fear-conditioned faces elicit increased Am responses. Thus, also extrageniculostriate pathways are involved. A multisynaptic pathway from the retina to the Am via the superior colliculus and several thalamic nuclei was recently suggested. We here present data based on retrograde neuronal labeling that the parabigeminal nucleus emits a substantial bilateral projection to the Am. This small cholinergic nucleus (Ch8 group) in the midbrain tegmentum is a subcortical relay visual center that is reciprocally connected with the superior colliculus. We suggest the existence of a second extrageniculostriate multisynaptic connection to Am: retina - superior colliculus - parabigeminal nucleus - Am. This pathway might be very effective since all tracts listed above are bilateral. The function of the Am by the rapid response to the sources of threat before conscious detection is significantly altered by various neuropsychiatric diseases. Biomedical Reviews 2008; 19: 1-16.

We modeled the common clinical conditions of human stroke in fully conscious rats through an occlusion of the middle cerebral artery (MCAO) by means of unilateral microinjection of Endothelin-1 (ET1) in the vicinity of the artery (EMCAO model). Since the role of serotonin (5-HT) system in the regulation of the cerebral blood flow has been known for long time and no data are available at present for the effects of 5-HT antagonists in focal ischemia models, we further tested whether a blockade of the serotonin-2A (5-HT2A) receptors by ketanserin (20 min post-ET1) would diminish the late EMCAO-induced functional and morphological changes. The long-term neurological (postural reflex) and electroencephalogram (EEG) changes in the somatosensory cortical region (S1FL) were used to assess the effects of ketanserin on the post-ischemic changes. The study was supplemented by a histopathological examination of S1FL area and striatum of both hemispheres. The EMCAO/ ketanserin-treated rats showed much smaller neurological deficits than the EMCAO rats treated with vehicle. This effect was observed on day 3 and lasted until the end of experiments-14 days after EMCAO. The depression of alpha and beta EEG frequencies found after EMCAO was significantly and earlier restored following ketanserin. Notably, there was not augmentation of the pathological slow EEG waves at day 3 post-ET1 in the EMCAO ketanserin-treated rats compared with that observed in the EMCAO vehicle-treated rats. Although there were mild morphological changes in the penumbral S1FL cortical region after EMCAO, ketanserin reduced the histopathological difference between the ipsilateral and contralateral cortical S1FL regions, but did not change the difference between striatum of both sides. Ketanserin reduced the infarct size in ipsilateral hemisphere (mainly cortex). In conclusion, the results showed that treatment with ketanserin at the early stage of stroke may reduce the consequences of ischemia by improvement of functional and morphological recovery at later stages. Ketanserin appears to be a promising candidate for mitigating the consequences of stroke.

The nitric oxide-producing neurons in rat amygdala (Am) were studied by means of immunohistochemical technique for detection of the specific synthetic enzyme nitric oxide synthase (NOS). Almost all nuclei of the Am contained NOS-positive neurons and fibres, but their density, somatodendritic morphology and the intensity of staining of different subpopulations varied. The strongly stained cells displayed labelling of the dendritic tree with Golgi impregnation-like quality, whilst the processes of the lightly stained neurons were less successfully followed. Numerous strongly stained neurons were present in the external capsule and in the intraamygdaloid fibre bundles. A large number of small, strongly stained cells was present in the amygdalostriatal transition area. Condensation of deeply stained cells occurred in the lateral Am nucleus. In the basolateral nucleus, the strongly NOS-positive neurons were few, and were located along the lateral border of the nucleus. These cells clearly differed from the large efferent, pyramidal neurons of the basolateral nucleus. The basomedial nucleus contained numerous positive cells, but most of them were only lightly labelled. A moderate number of strongly stained neurons appeared in the medial division of the central Am nucleus, and a larger accumulation of positive cells was present in the lateral and the capsular divisions. The medial Am nucleus contained numerous moderately stained small neurons and displayed the strongest diffuse neuropil staining. The anterior Am area contained numerous NOS-positive neurons; in its dorsal part the cells were moderately stained, whereas in the ventral part the neurons were strongly positive. The intercalated Am nucleus lacked NOS-positive neurons, but an appreciable plexus of fine tortuous axons was present.

The striatum is divided into two parts: dorsal striatum, implicated in the motor neuronal circuitry of the basal ganglia, and ventral striatum (nucleus accumbens), affiliated to the limbic system. We investigated the differential afferent connections to both striatal divisions from the dopaminergic, noradrenergic, serotonergic and cholinergic neuronal groups of the rostral brain stem by means of retrograde axonal transport of the fluorescent tracer Fluoro-Gold. The dopaminergic neurons of the ventral midbrain tegmentum emit a mighty projection to either of the striatum parts. The dorsal striatum is innervated by numerous neurons in the ventral tier of substantia nigra, pars compacta and pars lateralis and by only few scattered neurons in the ventral tegmental area. Conversely, numerous neurons in the ventral tegmental area send axons to the nucleus accumbens, whilst few cells in the dorsal tier of pars compacta innervate the ventral striatum. Only occasional noradrenergic neurons in locus ceruleus project to the dorsal striatum, whereas the ceruleal projection to the ventral striatum is substantial. The serotonergic dorsal raphe nucleus innervates the entire striatum. The dorsal striatum receives axons from cells concentrated near the midline in the rostral sector of the dorsal raphe nucleus. The projection to the ventral striatum is stronger and is emitted by numerous neurons throughout the dorsal raphe nucleus. In contrast to the profuse innervation of substantia nigra, subthalamic nucleus and globus pallidus by the cholinergic reticular formation of the mesopontine tegmentum, the projection to the striatum is moderate. The cells of origin are mainly located in pars dissipata of the pedunculopontine tegmental nucleus that innervates the dorsal striatum, and in the laterodorsal tegmental nucleus that sends axons to the nucleus accumbens.

Nicotinamide adenosine dinucleotide phosphate-diaphorase (NADPH-d) staining after muscarinic receptor activation by (4-Hydroxy-2-butynyl)-1-trimethylamnionium-m-chlorocarbanilate chloride (McN-A-343) in the myenteric neurons of guinea-pig gastric fundus was investigated.The staining intensity of myenteric neurons was evaluated by their optical density (01)) in arbitrary units (AU). After muscarinic activation by McN-A-343 (10 mu M), the OD of NADPH-d neurons in the control group was 81.7 +/- 1.3 AU. Muscarinic activation after blockade of muscarinic M-1 subtype receptors by telenzepine (0.1 mu M) significantly increased the OD of the neurons to 59.0 +/- 1.0 AU. After blockade Of M-4 receptors by tropicamide (0.1 mu M), however, the OD does not change (82.0 +/- 1.1 AU) toward control.These results indicate that the blockade of M, muscarinic receptors potentiates the release of nitric oxide, while M-4 are not involved in modulation of nitrergic neurotransmission.

The direct excitatory pathway from the basal nucleus of the amygdala (BNA) to deep layers of the prelimbic (PL) and infralimbic (IL) cortical areas is reliably demonstrated. Nevertheless, the precise neurochemical mechanisms mediating BNA-PL/IL transmission are still unclear. It was shown that a number of neurons in the rat cerebral cortex is NO-synthesizing. We made ail attempt to find out whether amygdaloid inputs can modulate nitrergic activity of PL and IL cells. The aim of this study was to examine I lie distribution of nicotinamide adenine dinucleotide phosphate-diphorase- (NADPH-d) positive neurons within deep PL and IL layers in control and following electrical stimulation of BNA. Stimulation of BNA was achieved by monopolar tungsten electrode with three 10 s series of rectangular pulses of 3-5 V with duration of 0.5 ms and frequency of 50 Hz presented in 20 s intervals. Immediately after stimulation the animals were perfused with 4% paraformaldehyde. Thirty pm of thick coronal section were made and processed for NADPH-d reaction. The mean number of the positive neurons per section and their mean optical density (OD) was separately calculated for layers V and VI of the PL or IL cortical areas. Our results show that electrical stimulation of BNA provoked significant increase in number and OD of NADPH-d- positive cells in layer V of PL. The number of NO-synthesizing cells in PL layer VI and in V-VI layers of IL cortical area did not change, but their OD also significantly increased. The obtained results suggest that the BNA influence on the PL and IL cortical areas is partially mediated by local NO-depending mechanisms.

The parabigeminal nucleus (Pbg) is a subcortical visual center that besides reciprocal connections with the superior colliculus (SC), also projects to the amygdala (Am). The Pbg-Am connection is part of a multineuronal pathway that conveys extrageniculostriate inputs of the retina to the Am, and it rapidly responds to the sources of threat before conscious detection. The present study demonstrates that Pbg projects bilaterally to Am and SC. The ipsilateral projections arise from separate cell populations, whilst the contralaterally projecting Pbg neurons emit branching axons that simultaneously innervate Am and SC.

Immunohistochemistry and computer assisted image analysis were used to examine the age-related changes in tyrosine hydroxylase- (TH-) immunoreactivity in substantia nigra (SN), ventral tegmental area (VTA), locus ceruleus (LC) and dopamine-betahyroxylase- (DBH-) immunoreactivity in LC and subceruleus nuclei of the rat. The findings in 3-month-old rats were compared with 28-month-old rats. In SN TH-positive neurons were concentrated in pars compacta and to a lesser extent - in pars lateralis. In VTA the TH-positive neurons were present over the entire area. In LC the immunoreactive perikarya were densely arranged and superimposed, but in subceruleus nuclei they were less numerous and individual cells were clearly discernible. The DBHimmunoreaction distinctly demonstrated the noradrenergic LC and subceruleus neurons. The results indicate of only subtle signs of cell loss in the dopaminergic neuronal population of SN and VTA, whilst the cell loss of the noradrenergic neurons in LC and subceruleus nuclei is evident. On the other hand, considerable age-related dendritic alterations were observed in all catecholaminergic nuclei. Cross-sectional area and optical density (OD) of the TH-immunoreactive neurons in SN, VTA and LC, and of the DBH-immunoreactive neurons in LC and subceruleus nuclei were investigated. In aging the cross-sectional area decreased statistically and OD of the neurons in SN decreased with 13%. In VTA the cross-sectional area did not change its dimensions, while the OD increased with 19%. In LC and subceruleus nuclei the cross-sectional area decreased with 36% and the OD of the neurons decreased with 16%. In conclusion, the most resistant to age-related changes catecholaminergic region in the rat is the VTA, followed by the pars compacta of SN. Rodent LC is a very vulnerable region. Biomedical Reviews 2007; 18: 45-58.