Excitatory projections from the anterior hypothalamus to periaqueductal gray neurons that project to the medulla: a functional anatomical study

The present study was designed to investigate the organization of excitatory projections from regions of the anterior hypothalamus that are known to co-ordinate autonomic and sensory functions to medullo-output neurons in the periaqueductal gray. The induction of Fos protein was used to identify neurons in the periaqueductal gray that were activated synaptically by chemical stimulation at sites in the anterior hypothalamus from which either increases or decreases in arterial blood pressure were evoked (pressor sites and depressor sites, respectively). This was combined with retrograde tracing using fluorescent latex microspheres from sites in the medulla. When compared to control animals, neuronal activation at pressor sites in the anterior hypothalamus evoked Fos-like immunoreactivity in significantly more neurons in all but one sub-division of the periaqueductal gray (P at least < 0.05). The majority of Fos-positive neurons following a pressor response were located in the caudal half of the periaqueductal gray where significantly more neurons contained Fos-like immunoreactivity in lateral than in any other sub-division (P < 0.01). In all but two of 14 subdivisions of the periaqueductal gray, the numbers of neurons that expressed Fos-like immunoreactivity following stimulation at depressor sites in the anterior hypothalamus were not significantly different from controls. When neuronal activation at pressor or depressor sites in the anterior hypothalamus was combined with retrograde tracing from the rostral ventrolateral medulla, nucleus raphe magnus and/or nucleus raphe obscurus the majority of double-labelled neurons were located in the caudal half of the periaqueductal gray. Comparisons between the numbers of double-labelled neurons that resulted from different combinations of hypothalamic and medullary injection sites revealed that neuronal activation at pressor sites in the anterior hypothalamus combined with retrograde tracing from the rostral ventrolateral medulla resulted in the greatest numbers of double-labelled neurons. The identification of double-labelled neurons indicates that medullo-output neurons in the periaqueductal gray receive excitatory inputs predominantly from pressor compared to depressor sites in the anterior hypothalamus. These results are discussed in relation to the roles of the different longitudinal columns of the periaqueductal gray, and the organisation of their projections to the medulla, in the co-ordination of autonomic and sensory functions.     

C-nociceptor activation of hypothalamic neurones and the columnar organisation of their projections to the periaqueductal grey in the rat

The induction of Fos protein was used to localise hypothalamic neurones activated by ramps of noxious skin heating delivered at a rate of 2.5 degrees C s(-1) to preferentially activate C-nociceptors. This was combined with retrograde transport of cholera toxin subunit B from identified 'pressor' and 'depressor' sites in the dorsolateral/lateral or the ventrolateral columns of the periaqueductal grey. Fos-positive neurones were found throughout the rostral hypothalamus. Despite this wide distribution, those neurones double labelled retrogradely from the periaqueductal grey were focused in the lateral area of the anterior hypothalamus. More than 20 % of Fos-positive neurones in this region projected to depressor sites in the ventrolateral periaqueductal grey, and 10 % projected to its dorsolateral/lateral sector. These results are discussed in relation to the peripheral inputs to hypothalamic-midbrain pathways and their role in the cardiovascular responses to different components of the pain signal.     

Noxious somatic inputs to hypothalamic-midbrain projection neurones: a comparison of the columnar organisation of somatic and visceral inputs to the periaqueductal grey in the rat

The induction of Fos protein was used to localise hypothalamic neurones activated by noxious somatic stimulation. This was combined with retrograde transport of fluorescent latex microspheres from identified 'pressor' and 'depressor' sites in the dorsolateral/lateral or ventrolateral columns of the periaqueductal grey (PAG). Fos-positive neurones were found throughout the rostral hypothalamus. Of those neurones activated by noxious somatic stimuli that projected to the PAG all but one was retrogradely labelled from sites that included the lateral column. Only one neurone was double labelled following injection of tracer at a depressor site in the ventrolateral PAG. This is in marked contrast to visceroresponsive hypothalamic neurones, a larger proportion of which project to the PAG and which, as reported previously, preferentially target depressor sites in the ventrolateral sector. These results are discussed in relation to the roles of the anterior hypothalamus and the different functional columns of the PAG in co-ordinating autonomic and sensory functions in response to nociceptive inputs originating in different peripheral domains.     

内脏伤害性刺激后Fos在大鼠脑内NOS阳性神经元内的表达

目的：观察一氧化氮合酶(NOS)阳性神经元在内脏伤害性信息传递通路上的分布。方法：给予大鼠内脏伤害性刺激后，采用Fos免疫组织化学(ABC法)和还原型尼克酰胺腺嘌呤二核苷酸脱氢酶(NADPH-d)组织化学双重染色的方法，观察脑内NOS和Fos阳性神经元的分布。结果：脑内Fos／NOS双标阳性神经元主要分布在孤束核，中缝背核，丘脑室旁核，下丘脑室旁核、室周核、背内侧核，中脑导水管周围灰质腹外侧部、背外侧部，臂旁内侧核，内侧缰核，杏仁复合体内侧部等部位。结论：NO是内脏伤害性信息传递和调控通路上的神经递质之一。     

Brain afferents to the lateral caudal ventrolateral medulla: a retrograde and anterograde tracing study in the rat

The ventrolateral medulla (VLM) modulates autonomic functions, motor reactions and pain responses. The lateralmost part of the caudal VLM (VLMlat) was recently shown to be the VLM area responsible for pain modulation. In the present study, the brain sources of VLMlat afferent fibers were determined by tract-tracing techniques. Following injection of cholera toxin subunit B into the VLMlat, retrogradely labeled neurons in the forebrain occurred at the somatosensory, insular, motor, limbic and infralimbic cortices, and at the central amygdaloid nucleus. Retrogradely labeled neurons in diencephalic regions were observed in the lateral hypothalamus, posterior hypothalamus and paraventricular nucleus. In the brainstem, retrograde labeling occurred at the periaqueductal gray, red nucleus, parabrachial area, nucleus raphe magnus, nucleus tractus solitarii, lateral reticular nucleus and dorsal and ventral medullary reticular formation. In the cerebellum, retrogradely labeled neurons occurred at the lateral nucleus. Following injections of the anterograde tracer biotinylated dextran amine (BDA) into the lateral hypothalamus or paraventricular nucleus, anterogradely labeled fibers were mainly observed in the VLMlat. Injections of BDA into the periaqueductal gray, red nucleus or lateral nucleus of the cerebellum resulted in anterograde labeling in the VLMlat and lateral reticular nucleus.The present study gives an account of the brain regions putatively involved in triggering the modulatory actions elicited from the VLMlat. These include areas committed to somatosensory processing, autonomic control, somatic and visceral motor activity and affective reactions. The findings suggest that the VLMlat may play a major homeostatic role in the integration of nociception with other brain functions.     

Collateralized projections from neurons in the rostral medulla to the nucleus locus coeruleus, the nucleus of the solitary tract and the periaqueductal gray.

We have examined collateral projections of locus coeruleus afferent neurons in the rostral medulla to the caudal nucleus of the solitary tract or to the periaqueductal gray using double retrograde labeling techniques in the rat. The present findings confirm previously reported connections to the locus coeruleus, the nucleus of the solitary tract and the lateral periaqueductal gray from the nucleus paragigantocellularis in the rostral ventral medulla. Our results also reveal previously unreported projections from the rostral dorsomedial medulla (in a similar region as locus coeruleus-projecting neurons) to the lateral periaqueductal gray. Following retrograde tracer injections into the nucleus of the solitary tract and the locus coeruleus, doubly labeled neurons were seen in both the nucleus paragigantocellularis and in the rostral dorsomedial medulla. Cell counts revealed that approximately 25% of locus coeruleus-projecting neurons in the nucleus paragigantocellularis, and 12% in the dorsomedial medulla, also innervate the caudal nucleus of the solitary tract. In contrast, no doubly labeled neurons within the rostral ventral medulla were found following injections into the lateral periaqueductal gray and the locus coeruleus, although singly labeled neurons for the two tracers were interdigitated in some regions. Following these injections, numerous neurons were also retrogradely labeled in the dorsomedial medulla in the region of the medial prepositus hypoglossi and the perifascicular reticular formation. A small percentage of locus coeruleus afferents in the dorsal medulla (approximately 10%) also projected to the lateral periaqueductal gray. These results indicate that neurons in both the ventrolateral and dorsomedial rostral medulla frequently send collaterals to both the locus coeruleus and the caudal nucleus of the solitary tract. A small number of neurons in the dorsomedial medulla project to both the locus coeruleus and the lateral periaqueductal gray, but separate populations of neurons project to the locus coeruleus and the lateral periaqueductal gray from the ventrolateral medulla. These results functionally link the locus coeruleus and the nucleus of the solitary tract by virtue of common afferents, and support other studies indicating the importance of central autonomic circuitry in the afferent control of locus coeruleus neurons.     

Patterns of violent aggression-induced brain c-fos expression in male mice selected for aggressiveness

Mice selected for aggressiveness (long and short attack latency mice; LALs and SALs, respectively) constitute a useful tool in studying the neural background of aggressive behavior, especially so as the SAL strain shows violent forms of aggressiveness that appear abnormal in many respects. By using c-Fos staining as a marker of neuronal activation, we show here that agonistic encounters result in different activation patterns in LAL and SAL mice. In LALs, agonistic encounters activated the lateral septum, bed nucleus of stria terminalis, medial amygdala, paraventricular nucleus of the hypothalamus, anterior hypothalamic nucleus and tuber cinereum area (both being analogous with the rat hypothalamic attack area), dorsolateral periaqueductal gray, and locus coeruleus. This pattern is similar with that seen in the territorial aggression of male mice, rats and hamsters, and non-lactating female mice. SALs showed strong fight-induced activations in the central amygdala and lateral/ventrolateral periaqueductal gray. In this strain, no activation was seen in the lateral septum and the dorsolateral periaqueductal gray. This pattern is similar with that seen in other models of violent aggression, e.g., in attacks induced by hypothalamic stimulation in rats, quiet biting in cats, lactating female mice, and hypoarousal-driven abnormal aggression in rats. We suggest here that the excessive activation of the central amygdala and lateral/ventrolateral periaqueductal gray--accompanied by a smaller activation of the septum and dorsolateral periaqueductal gray--underlay the expression of violent attacks under various circumstances.     

Afferents to the periaqueductal gray in the rat. A horseradish peroxidase study

Afferent projections to the periaqueductal gray matter in the rat have been studied by use of the retrograde axonal transport of horseradish peroxidase. Iontophoretic injections of horseradish peroxidase were made in dorsal, lateral and medial areas of the periaqueductal gray, primarily at intercollicular levels. The pattern of projections was similar in all of the injections restricted to the periaqueductal gray. Within the brainstem, numerous reticular formation nuclei were labeled, including nucleus reticularis lateralis, nucleus raphe magnus, pallidus and obscurus, the nucleus reticularis pontis oralis and caudalis, the paralemniscal nucleus and the dorsal and ventral parabrachial nuclei. At diencephalic levels, dense projections were seen from the parafascicular nucleus, dorsal premamillary nucleus, zona incerta, dorsomedial and ventromedial nuclei of the hypothalamus and the retrochiasmatic area, in the ventral portion of the anterior hypothalamus. At forebrain levels, occasional cells were seen in the medial preoptic area, lateral septum and the anterior cingulate cortex. Control injections of horseradish peroxidase into structures adjacent to the periaqueductal gray matter included three well localized deposits in the dorsal raphe. Retrogradely-labeled cells were found in lateral reticular nucleus of the medulla, nucleus raphe magnus, nucleus reticularis pontis caudalis, locus ceruleus, dorsal and ventral parabrachial nuclei, substantia nigra and the lateral hypothalamus. No labeled cells were found in the habenular nuclei. It is suggested that many of the descending hypothalamic and forebrain afferents may be relay centers for descending hippocampal formation efferents. Many of the periaqueductal gray afferent systems receive a direct projection from the hippocampal formation and could therefore coordinate influences from this limbic center with information on homeostatic mechanisms controlled by the hypothalamus. The numerous brainstem afferents to the periaqueductal gray could be involved in relay of ascending sensory information important for initiating any of several behavioral responses known to be controlled by the periaqueductal gray. In addition, certain raphe afferents might play a part in a feedback loop of the pain suppression circuit of which the periaqueductal gray is an important component.     

Excitatory amino acid receptors in the periaqueductal gray mediate the cardiovascular response evoked by activation of dorsomedial hypothalamic neurons

Neurons in the region of dorsomedial hypothalamus are involved in the organization of the physiological responses to emotional stress. We have recently shown that the cardiovascular response evoked by activation of dorsomedial hypothalamus neurons is largely dependent on a synaptic relay with the lateral/dorsolateral periaqueductal gray region. In this study, we aimed to investigate whether excitatory amino acid receptors at the lateral/dorsolateral periaqueductal gray region are involved in mediating the response evoked by activation of dorsomedial hypothalamus neurons. In conscious rats, the cardiovascular effects produced by microinjection of GABA(A) receptor antagonist, bicuculline methiodide into the dorsomedial hypothalamus were evaluated before and after injection of different excitatory amino acid antagonists into lateral/dorsolateral periaqueductal gray region. Pretreatment of lateral/dorsolateral periaqueductal gray region with the non-selective ionotropic excitatory amino acid receptor antagonist kynurenic acid or with the N-methyl-D-aspartate receptor-selective antagonist, MK-801, largely reduced the tachycardic and pressor effects evoked by activation of dorsomedial hypothalamus neurons by bicuculline methiodide microinjection (heart rate 90 and 74%; blood pressure 81 and 84%, respectively). The non-N-methyl-D-aspartate receptor-selective antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, did not alter the cardiovascular response evoked by dorsomedial hypothalamus activation. In an additional series of experiments, microinjection of the N-methyl-D-aspartate receptor agonist, N-methyl-D-aspartate, into the lateral/dorsolateral periaqueductal gray region, evoked an increase in heart rate and a pressor response that was accompanied by an increase in locomotor activity. These effects were not altered by pretreatment of lateral/dorsolateral periaqueductal gray region neurons with 6-cyano-7-nitroquinoxaline-2,3-dione but were completely abolished by MK-801. Altogether, these findings indicate that the cardiovascular response evoked by dorsomedial hypothalamus activation involves a synaptic relay at the lateral/dorsolateral periaqueductal gray region that is mediated at least in large part by excitatory amino acid receptors, possibly N-methyl-D-aspartate receptors.     

大鼠胃肠道伤害性刺激引起的中枢神经系统c-fos表达

本文应用Fos免疫组织化学(ABC法)对大鼠胃肠道伤害性刺激后中枢神经系统内的c-fos表达进行了观察,结果表明:(1)多数核团或部位的c-fos表达于伤害性刺激后的30min开始,2h达高峰,4h后逐渐降低,12h基本恢复正常。(2)Fos免疫反应阳性神经元呈双侧性分布,定位于胸髓(Rexed Ⅰ、Ⅱ、Ⅴ和Ⅹ层)、孤束核、延髓腹外侧区、臂旁外侧核、脑桥室周灰质、中脑导水管周围灰质、楔形核、中缝背核、下丘、丘脑(中线核团、背内侧核、腹后内侧核小细胞部)、外侧缰核、内侧膝状体大细胞部、下丘脑(背内侧核、腹内侧核、室周核等)、中央杏仁核、终纹床核、伏核、外侧隔核、梨状区皮质等。本文对以上部位的c-fos表达规律及其意义进行了初步探讨。     

Attenuation of negative pain affect produced by unilateral spinal nerve injury in the rat following anterior cingulate cortex activation

The affective and the sensory dimensions of pain processing can be differentiated in humans through the use of questionnaires and verbal communication. It is difficult to dissociate these two components of pain processing in rodents, and an understanding of the underlying mechanisms for each component is unclear. The quantification of a novel behavioral response to a repeated noxious cutaneous stimulus together with a measurement of tactile allodynia in nerve-injured rats might be used to differentially explore the sensory and affective components of pain processing in the rat. The present study utilized electrical stimulation of the anterior cingulate cortex, a structure implicated in affective pain processing but not sensory processing, in nerve-injured rats (L5 spinal nerve ligation) and found that the aversive quality of noxious cutaneous hindpaw stimulation was attenuated. There were no effects on sensory processing, because anterior cingulate cortex stimulation did not produce an anti-allodynic effect in L5 spinal nerve ligation animals. Furthermore, anterior cingulate cortex stimulation in animals with bilateral ventrolateral periaqueductal gray area lesions did not affect tactile sensitivity in L5 spinal nerve ligation rats, indicating that an endogenous pain suppression system was not likely activated by anterior cingulate cortex stimulation. However, bilateral ventrolateral periaqueductal gray area lesions in L5 spinal nerve ligation rats blocked the effect produced by anterior cingulate cortex stimulation in the place escape/avoidance paradigm. Specifically, these animals avoided noxious stimulation of the allodynic paw significantly more than anterior cingulate cortex-stimulated, sham or incomplete ventrolateral periaqueductal gray area-lesioned, L5 spinal nerve ligation animals. These findings provide the first quantified report that the activation of the anterior cingulate cortex reduced the aversive quality of repeated noxious tactile stimulation in nerve-injured animals without interfering with normal sensory processing. This effect might require the presence of an intact ventrolateral periaqueductal gray area. It is concluded that the selective manipulation of the anterior cingulate cortex has different effects on pain affect and sensory processing in a rodent model of neuropathic pain.     

Fos activation in hypothalamic neurons during cold or warm exposure: projections to periaqueductal gray matter

The hypothalamus, especially the preoptic area, plays a crucial role in thermoregulation, and our previous studies showed that the periaqueductal gray matter is important for transmitting efferent signals to thermoregulatory effectors in rats. Neurons responsible for skin vasodilation are located in the lateral portion of the rostral periaqueductal gray matter, and neurons that mediate non-shivering thermogenesis are located in the ventrolateral part of the caudal periaqueductal gray matter. We investigated the distribution of neurons in the rat hypothalamus that are activated by exposure to neutral (26 degrees C), warm (33 degrees C), or cold (10 degrees C) ambient temperature and project to the rostral periaqueductal gray matter or caudal periaqueductal gray matter, by using the immunohistochemical analysis of Fos and a retrograde tracer, cholera toxin-b. When cholera toxin-b was injected into the rostral periaqueductal gray matter, many double-labeled cells were observed in the median preoptic nucleus in warm-exposed rats, but few were seen in cold-exposed rats. On the other hand, when cholera toxin-b was injected into the caudal periaqueductal gray matter, many double-labeled cells were seen in a cell group extending from the dorsomedial nucleus through the dorsal hypothalamic area in cold-exposed rats but few were seen in warm-exposed rats. These results suggest that the rostral periaqueductal gray matter receives input from the median preoptic nucleus neurons activated by warm exposure, and the caudal periaqueductal gray matter receives input from neurons in the dorsomedial nucleus/dorsal hypothalamic area region activated by cold exposure. These efferent pathways provide a substrate for thermoregulatory skin vasomotor response and non-shivering thermogenesis, respectively.     

大鼠中脑导水管周围灰质向终纹床核投射的5－羟色胺能神经元对内脏伤害性刺激的FOS表达

本文将ＨＲＰ注入一侧终纹床核，又将Ｆｏｒｍａｌｉｎ导入胃内造成损伤，通过ＨＲＰ呈色、抗ＦＯＳ、抗５－ＨＴ免疫组化染色，在中脑导水管周围灰质发现七种不同标记或免疫反应阳性细胞，即ＨＲＰ逆行标记、５－ＨＴ样阳性和ＦＯＳ样阳性细胞，ＨＲＰ／５－ＨＴ、５－ＨＴ／ＦＯＳ和ＨＲＰ／ＦＯＳ双重标记或免疫反应阳性细胞以及ＨＲＰ／５－ＨＴ／ＦＯＳ三重标记或免疫反应阳性细胞．ＨＲＰ／５－ＨＴ／ＦＯＳ细胞数量很少，出现于中脑导水管周围灰质腹外侧区。     

Fos expression in serotonergic neurons in the rat brainstem following noxious stimuli: an immunohistochemical double-labelling study

In order to detect whether there were different expression patterns of Fos protein induced by somatic or visceral noxious stimulation in the serotonergic neurons in the rat brainstem, an immunohistochemical double-labelling technique for serotonin (5-HT) and Fos was employed after subcutaneous or stomach injection of formalin. The two stimuli were matched in pilot experiments to produce maximum Fos expression. The expression of Fos protein in 5-HT-containing neurons (5-HT/Fos co-localized neurons) could be observed in the ventrolateral subdivision of the midbrain periaqueductal grey, interpeduncular nucleus, paramedian raphe nucleus, all of the brainstem raphe nuclei, the alpha part of the gigantocellular reticular nucleus and the lateral paragigantocellular reticular nucleus. The locations of the 5-HT/Fos co-localized neurons in the brainstem of animals subjected to somatic noxious stimulation were similar to those subjected to visceral noxious stimulation. However, the number and proportion of the 5-HT/Fos co-localized neurons in the median raphe nucleus and nucleus raphe obscurus of the rat subjected to visceral noxious stimulation were statistically greater than those in rats subjected to somatic noxious stimulation. These results suggest that serotonergic neurons in median raphe nucleus and nucleus raphe obscurus have a tendency to higher neuronal activity after visceral noxious stimulation.     

Projections of the ventrolateral pontine vocalization area in the squirrel monkey

In four squirrel monkeys (Saimiri sciureus), the tracer biotin dextranamine (BDA) was injected into the ventrolateral pons at a site at which injection of the glutamate antagonist kynurenic acid blocked vocalization electrically elicited from the periaqueductal gray (PAG). Anterograde projections could be traced into all cranial motor and sensory nuclei involved in phonation, that is, the nucleus ambiguus, facial, hypoglossal and trigeminal motor nuclei, the motorneuron column in the ventral gray substance innervating the extrinsic laryngeal muscles, the nucleus retroambiguus, solitary tract and spinal trigeminal nuclei. Projections were also found into a number of auditory nuclei, namely the nucleus cochlearis-complex, superior olive, ventral and dorsal nuclei of the lateral lemniscus and inferior colliculus. Furthermore, there were projections into the reticular formation of the lateral and dorsocaudal medulla and lateral pons, into nucleus gracilis, inferior and medial vestibular nuclei, lateral reticular nucleus, ventral raphe, pontine gray, superior colliculus, PAG and mediodorsal thalamic nucleus. Injection of the tracer wheat germ agglutinin-conjugated horseradish peroxidase into the ventrolateral pontine vocalization-blocking area in one animal yielded retrograde labeling throughout the PAG. Injection of BDA into a vocalization-eliciting site of the PAG in another animal yielded projections into the ventrolateral pontine vocalization-blocking area. It is concluded that the ventral paralemniscal area in the ventrolateral pons represents a relay station of the descending periaqueductal vocalization-controlling pathway.     

Neurons in the rat medulla oblongata containing neuropeptide Y-, angiotensin II-, or galanin-like immunoreactivity project to the parabrachial nucleus.

Projections from the medulla to the parabrachial complex of the rat were examined for their content of neuropeptide Y-, angiotensin II- or galanin-like immunoreactivity using combined retrograde tracing and immunohistochemical techniques. Rhodamine-labelled latex microspheres were stereotaxically injected into discrete nuclei of the parabrachial complex. After survival of two to five days, colchicine (100 micrograms in 10 microliters saline) was injected into the cisterna magna. One day later, rats were perfused and the brainstems were prepared for visualization of the retrograde tracer and immunoreactivity of one of the three peptides. Retrograde labelling verified that the area postrema, nucleus of the tractus solitarius, caudal spinal nucleus of the trigeminal nerve, parvocellular reticular nucleus, and ventrolateral medulla including the rostral ventrolateral medulla and nucleus paragigantocellularis project to the lateral parabrachial and K?lliker-Fuse nuclei. While most projections were primarily ipsilateral, a small proportion of the projections from the ventrolateral medulla was bilateral. Neurons containing neuropeptide Y-like immunoreactivity were found in the caudal and intermediate nucleus of the tractus solitarius, dorsal to the lateral reticular nucleus and in the nucleus paragigantocellularis. After bilateral microsphere injections into the lateral parabrachial and K?lliker-Fuse nuclei, double-labelled neurons were found dorsal to the lateral reticular nucleus of caudal and intermediate medullary levels, at the ventral surface of the medulla at intermediate levels and in the nucleus paragigantocellularis at rostral levels. Neurons with angiotensin II-like immunoreactivity were observed at the dorsomedial border of the caudal and intermediate nucleus of the tractus solitarius, in the area postrema and in the lateral reticular nucleus and nucleus paragigantocellularis. Of these neurons, small numbers in the nucleus of the tractus solitarius and ventrolateral medulla also projected to the lateral parabrachial and K?lliker-Fuse nuclei. Neurons containing galanin-like immunoreactivity were found in the caudal nucleus of the tractus solitarius, the area postrema, the spinal trigeminal nucleus, the raphe nuclei (pallidus and obscurus), the nucleus paragigantocellularis and dorsal to the lateral reticular nucleus. Of these cells, double-labelled neurons were found in the commissural and medial subdivisions of the caudal nucleus of the tractus solitarius and in the rostral ventrolateral medulla including the ventral surface and the nucleus paragigantocellularis. The results suggest that neuropeptide Y, angiotensin II and galanin may serve as neurochemical messengers in pathways from the medulla to the parabrachial complex. The location of double-labelled neurons suggests that the information relayed by these neurons is related to autonomic activity.     

Afferent projections to the rostral anterior pretectal nucleus of the rat: a possible role in the processing of noxious stimuli

The afferent inputs to the rostral pole of the anterior pretectal nucleus have been examined by utilizing the retrograde axonal transport of a fluorescent dye, Fast Blue. After unilateral injection of the dye into the rostral anterior prectectal nucleus, large numbers of labelled neuronal somata were found in the somatosensory cortex, the ventrolateral geniculate nucleus, the zona incerta, the superior colliculus, the deep mesencephalic nuclei, the pedunculopontine tegmental nucleus and the medial vestibular nucleus. In addition, the contralateral parabigeminal nucleus provided a major input to the rostral part of the anterior pretectal nucleus. Smaller and sparser collections of stained cell bodies could be found in the ventromedial hypothalamus, the posterior pretectal nucleus, the nucleus of the posterior commissure, the peripeduncular nucleus, the periaqueductal central gray, the contralateral anterior pretectal nucleus, and the locus coeruleus. Many of the inputs originated in areas associated with nociceptive pathways. The regional distribution of neurons projecting to the rostral pole of the anterior pretectal nucleus differs substantially from that of the cells innervating the anterior pretectal nucleus proper, i.e. its more caudal parts. It is concluded from this that the rostral pole constitutes a separate nucleus, anatomically distinct from the rest of the anterior pretectal nucleus and other cell groups in the pretectal complex. The demonstration that many of the afferents to the rostral anterior pretectal nucleus arise in regions involved in nociception supports recent electrophysiological and behavioural evidence that this brain area plays a role in the processing of noxious stimuli, rather than as a component in the pretectal control of visual system reflexes.     

The organization of afferent projections to the midbrain periaqueductal gray of the rat

The retrograde transport technique was utilized in the present study to investigate the afferent projections to the periaqueductal gray of the rat. Iontophoretic injections of horseradish peroxidase were made into the periaqueductal gray of 22 experimental animals and into regions adjacent to the periaqueductal gray in 6 control animals. Utilization of the retrograde transport method permitted a quantitative analysis of the afferent projections not only to the entire periaqueductal gray, but also to each of its four intrinsic subdivisions. The largest cortical input to this midbrain region arises from areas 24 and 32 in the medial prefrontal cortex. The basal forebrain provides a significant input to the periaqueductal gray and this arises predominantly from the ipsilateral lateral and medial preoptic areas and from the horizontal limb of the diagonal band of Broca. The hypothalamus was found to provide the largest descending input to the central gray. Numerous labeled cells occurred in the ventromedial hypothalamic nucleus, the lateral hypothalamic area, the posterior hypothalamic area, the anterior hypothalamic area, the perifornical nucleus and the area of the tuber cinereum. The largest mesencephalic input to the periaqueductal gray arises from the nucleus cuneiformis and the substantia nigra. The periaqueductal gray was found to have numerous intrinsic connections and contained a significant number of labeled cells both above and below the injection site in each case. Other structures containing significant label in the midbrain and isthmus region included the nucleus subcuneiformis, the ventral tegmental area, the locus coeruleus and the parabrachial nuclei. The medullary and pontine reticular formation provide the largest input to the periaqueductal gray from the lower brain stem. The midline raphe magnus and superior central nucleus also supply a significant fiber projection to the central gray. Both the trigeminal complex and the spinal cord provide a minor input to this region of the midbrain.The sources of afferent projections to the periaqueductal gray are extensive and allow this midbrain region to be influenced by motor, sensory and limbic structures. In addition, evidence is provided which indicates that the four subdivisions of the central gray receive differential projections from the brain stem as well as from higher brain structures.     

Midbrain periaqueductal gray neurons with substance P- or enkephalin-like immunoreactivity send projection fibers to the nucleus accumbens in the rat.

Employing a combination of retrograde tracing and immunocytochemistry, midbrain periaqueductal gray (PAG) neurons with substance P- or leucine-enkephalin-like immunoreactivity in the rat were found to send projection fibers to the nucleus accumbens bilaterally with an ipsilateral dominance. These neurons were most frequently seen in the ventrolateral and ventral parts of the medial PAG subdivisions at all rostrocaudal levels of PAG.     

The chemical architecture of the rat''s periaqueductal gray based on acetylcholinesterase histochemistry: a quantitative and qualitative study

The chemoarchitecture of the periaqueductal gray has been extensively studied, based on acetylcholinesterase reaction and comparing it to other chemical markers. We have divided the periaqueductal gray into four main longitudinal columns, namely dorsomedial, dorsolateral, lateral and ventrolateral. We also identified the dorsal midline column, the supraoculomotor cap and the juxta-aqueductal ring. The acetylcholinesterase gave rise to a strong reaction in the outer half of the lateral column, the outer half of the dorsomedial column, the supraoculomotor cap and the ventral half of the juxta-aqueductal ring. This labeling was in part complementary to that of the NADPH diaphorase and allowed the lateral column to be differentiated from the ventrolateral column. However, the inner half of both lateral and ventrolateral columns displayed the same chemical properties including acetylcholinesterase, tyrosine hydroxilase and serotonin. Thus, from the chemical view, these inner halves should be considered as one different region. Finally, the juxta-aqueductal ring was composed of two clearly different halves, i.e. dorsal and ventral. The dorsal half did not show any clear differences from the above columns and was negative for acetylcholinesterase, NADPH diaphorase and tyrosine hydroxilase, while the ventral half was clearly different from the lateral and ventrolateral columns and displayed a positive reaction to all those chemical markers. From these results, we strongly suggest the use of acetylcholinesterase histochemistry as a tool for accurate parcellation of the periaqueductal gray.