Mechanisms mediating the brain stem control of somatosensory transmission in the dorsal horn of the cat's spinal cord: an intracellular analysis

Summary The effect of brainstem stimulation was studied on neurones recorded intracellularly in the superficial and deeper laminae of the lumbosacral dorsal horn of the spinal cord in anaesthetised cats. Stimulation in the nucleus locus coeruleus (LC) produced a hyperpolarisation in 4/13 multireceptive neurones and produced a biphasic action consisting of a hyperpolarisation which was followed by a depolarisation in 3/13 neurones. These actions were produced irrespective of whether the multireceptive neurone was located in the superficial or deeper laminae of the dorsal horn. Stimulation failed to produce postsynaptic potentials in the remaining 6/13 multireceptive neurones. The amplitude of hyperpolarisation was increased by the passage of depolarising pulses through the recording microelectrode and decreased by hyperpolarising pulses. Stimulation in other brainstem areas such as, the lateral (FTL), paralemniscal (FTP) and central (FTC) divisions of the tegmental field and the nuclei raphe magnus (NRM) and reticularis magnocellularis (RMc) also hyperpolarised neurones in the dorsal horn. The polarity of hyperpolarisation evoked from some brainstem areas (FTP, FTC, RMc) could be reversed to depolarisation by the passive diffusion of ions from the recording microelectrode containing 3M-KCl. Brainstem (LC, NRM, FTP, FTL) stimulation generated long lasting (700 ms) hyperpolarisation on 4/4 selectively nocireceptive neurones of lamina I. There was, however, no effect on the activity of 5/5 neurones recorded in laminae I/II which in addition to receiving excitatory cutaneous inputs were inhibited by heat stimuli. Stimulation in LC also produced dorsal root potentials (DRPs) and reduced the amplitude of simultaneously recorded excitatory postsynaptic potentials (EPSPs) generated by the activation of primary afferent fibres in 3 multireceptive neurones. It is concluded that inhibition of nociceptive transmission in the spinal cord from LC and other brainstem areas may involve both pre- and postsynaptic mechanisms.     

大鼠前脑和脑干向中央上核的纤维投射

用辣根过氧化物酶(HRP)微电泳技术研究大鼠前脑和脑干向中央上核(CS)的纤维投射。将HRP输入CS后,观察到缰核、脚间核和被盖背核区有较多的标记细胞,斜角带核、下丘脑核和乳头体核有少量标记细胞。在外侧缰核、脚间核和被盖背核区可见丰富的顺行标记终末。结果表明,前脑的缰核和脑干的脚间核和被盖背核区是CS的主要传入来源,它们之间存在往返的纤维联系。     

Serotonergic projections from the midbrain periaqueductal gray and nucleus raphe dorsalis to the nucleus parafasccicularis of the thalamus

By a double-labeling method combining the retrograde tracing of horseradish peroxidase and the immunocytochemical technique, serotonin-like immunoreactive neurons in the midbrain periaqueductal gray (PAG) and nucleus raphe dorsalis (DR) of the rat were observed to send projection fibers to the nucleus parafascicularis of the thalamus bilaterally with an ipsilateral dominance. These serotonin-containing projecting neurons were observed mainly at the middle-caudal levels of the ventrolateral subdivision of the PAG and less at the middle-rostral levels of the DR.     

中缝大核内催产素在大鼠痛行为调制中的作用

目的：探讨中缝大核（NRM）内催产素（OT）在痛行为调制中的作用，及其和内源性5－HT在痛行为调制中的关系。方法：以钾离子透入法引起大鼠甩尾反应的电流强度（mA)为痛行为反应指标，测定动物痛阈，观察NRM内催产素对大鼠痛行为反应的影响。结果：NMR内注入催产素后，大鼠的痛阈明显增加；注入抗催产素血清后，大鼠的痛阈则明显降低。NRM内注入噻庚啶后，可阻断OT镇痛的效应。结论：NRM内的催产素在痛行为调制的复杂过程中发挥着一定作用，且其效应可能是通过内源性5－HT系统中介的。     

Overactive bladder in Parkinson's disease: alteration of brainstem raphe detected by transcranial sonography

Urinary dysfunction is very common in idiopathic Parkinson's disease (PD) and manifests primarily with symptoms of overactive bladder (OAB). Affection of central serotonergic systems has been suggested to play a role in OAB. The objective of this study was to evaluate whether in PD patients with OAB symptoms a specific alteration of the brainstem raphe (BR), which contains serotonergic neurons, can be detected with transcranial sonography (TCS). Of 116 PD patients enrolled, 19 had PD-related OAB symptoms (OAB+) unlike remaining 97 patients (OAB−). Patients were examined by a sonographer blinded to the clinical data. Reduced echogenicity of BR was found in 12 (63%) OAB+ patients but only in 18 (19%) of 93 assessable OAB− patients (Mann–Whitney U -test, P  < 0.001). In OAB+ patients, lower raphe echogenicity score was associated with longer duration of OAB symptoms ( anova , P  = 0.033). Other TCS findings such as echogenicity of substantia nigra, thalami, lenticular and caudate nuclei, and widths of third and lateral ventricles did not differ between OAB+ and OAB− patients. TCS findings suggest a pathogenetic role of BR in OAB related to PD. Alterations may reflect disturbance of its central serotonergic system.     

Effects of a selective 5-HT reuptake blocker,citalopram, on the sensitivity of 5-HT autoreceptors: Electrophysiological studies in the rat brain

Summary Citalopram (CIT), is a selective serotonin (5-HT) reuptake blocker and a clinically effective antidepressant. The present electrophysiological studies were undertaken to investigate in vivo the acute and long-term effects of CIT administration on 5-HT neurotransmission. In a first series of experiments, a single dose of CIT (0.05–0.5 mg/kg) was administered intravenously to naive rats while recording the activity of a 5-HT-containing neuron in the nucleus raphe dorsalis. A dose-response relationship of the inhibitory effect of CIT on the firing activity of 5-HT neurons was obtained with an ED₅₀ of 0.23±0.03 mg/kg. In a second series of experiments, rats were treated with CIT (20 mg/kg/day, i.p.) for 2, 7 and 14 days. In rats treated for 2 days, there was a marked reduction in the firing activity of 5-HT neurons in the nucleus raphe dorsalis; there was a partial recovery after 7 days and a complete recovery after 14 days of treatment. The response of 5-HT neurons to intravenously administered LSD was decreased in rats treated for 14 days with CIT, indicating a desensitization of the somatodendritic 5-HT autoreceptor. In a third series of experiments, carried out in rats treated with CIT (20 mg/kg/day, i.p.) for 14 days, the suppression of firing activity of CA₃ hippocampal pyramidal neurons produced by microiontophoretically-applied 5-HT and by the electrical activation of the ascending 5-HT pathway was measured. Long-term treatment with CIT did not modify the responsiveness of these neurons to microiontophoretically-applied 5-HT; however, the effect of the electrical activation of the ascending 5-HT pathway on these same neurons was enhanced. To determine if 5-HT reuptake blockade could be responsible for this enhancement, CIT (1 mg/kg) was injected intravenously in naive rats while stimulating the ascending 5-HT pathway; it failed to modify the effectiveness of the stimulation. To assess the involvement of the 5-HT terminal autoreceptor, methiothepin, a 5-HT autoreceptor antagonist, was injected intravenously (1 mg/kg) in naive rats and in rats treated for 14 days with CIT while stimulating the ascending 5-HT pathway. Methiothepin enhanced the effect of the stimulation in naive rats but failed to do so in the CIT-treated rats. It is concluded that long-term CIT treatment enhances 5-HT neurotransmission by desensitizing both the somatodendritic and terminal 5-HT autoreceptors.     

Morphology and distribution of serotoninergic and oculomotor internuclear neurons in the cat midbrain

Serotoninergic fibers have been reported in both the abducens and facial nuclei of the cat. Furthermore, serotoninergic dorsal raphe and oculomotor internuclear neurons occupy similar locations in the periaqueductal gray overlying the oculomotor and trochlear motor nuclei. To resolve the issue of whether these two populations of neurons overlap, serotoninergic fibers were assayed in the abducens and facial nucleus; then the morphologies and distributions of identified serotoninergic neurons and oculomotor internuclear neurons were determined. Both the abducens and facial nuclei contained varicosities labelled with antibody to serotonin, but a much higher density of immunoreactive fibers was present in the latter, especially in its medial aspect. Distinct synaptic profiles labelled with antibodies to serotonin were observed in both nuclei. In both cases, terminal profiles contained numerous small, predominantly spheroidal, synaptic vesicles as well as a few, large, dense-core vesicles. These profiles made synaptic contacts onto dendritic and, in the facial nucleus, somatic profiles that occasionally displayed asymmetric, postsynaptic, membrane densifications. Following injection of horseradish peroxidase into either the abducens or facial nuclei, double-label immunohistochemical techniques demonstrated that the serotoninergic and oculomotor internuclear neurons form two distinct cell populations. The immunoreactive serotoninergic cells were distributed within the dorsal raphe nucleus, predominantly caudal to the retrogradely labelled oculomotor internuclear neurons. The latter were located in the oculomotor nucleus along its dorsal border and in the adjacent supraoculomotor area. Intracellular injection of horseradish peroxidase revealed that oculomotor internuclear neurons have multipolar somata with up to ten long, tapering dendrites that bifurcate approximately five times. Their dendritic fields were generally contained within the nucleus and adjacent supraoculomotor area. In contrast, putative serotoninergic neurons were often spindle-shaped and exhibited far fewer primary dendrites. Many of these long, narrow, sparsely branched dendrites crossed the midline and extended to the surface of the cerebral aqueduct. In the vicinity of the aqueduct they branched repeatedly to form a dendritic thicket. The axons of the intracellularly stained serotoninergic neurons emerged either from the somata or the end of a process with dendritic morphology, and in some cases they produced axon collaterals within the periaqueductal gray. Thus the oculomotor internuclear and serotoninergic populations differ in both distribution and morphology.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of electric and chemical stimulation of the raphe obscurus on phrenic nerve activity in the cat

The effect of electrical and chemical (l-glutamate) stimulation of the raphe obscurus on phrenic nerve activity was examined in the cat. Phrenic nerve activity was recorded from a C5 nerve root in anesthetized, paralyzed and artificially ventilated cats. Neural discharge was quantitated by integrating the phrenic nerve activity. The respiratory frequency was determined from the integrated nerve signal. Focal electrical stimulation (18–144 μA; 5–40 Hz; 100 μs pulse duration) resulted in significant (P < 0.05) increases in both integrated phrenic nerve (IPN) amplitude and respiratory frequency. These changes were dependent upon current intensity and frequency of stimulation. The largest increases in IPN amplitude and respiratory frequency were47 ± 17%and146 ± 8%, respectively. To insure that the changes in integrated phrenic nerve activity (IPNA) were the result of stimulation of cell bodies and not axons of passage,l-glutamate (100, 200 nmol) was microinjected (100 nl) into the raphe obscurus. Significant (P < 0.05) dose-related changes occurred in integrated phrenic nerve amplitude with an increase of44 ± 13% at 100 nmol and80 ± 13% at 200 nmoll-glutamate. No significant increase in respiratory frequency was observed withl-glutamate microinjection. The results suggest that the raphe obscurus may be involved in respiratory control.     

Aminergic and cholinergic afferents to REM sleep induction regions of the pontine reticular formation in the rat

Microinjection of cholinergic agonists in a dorsolateral part of the mesopontine tegmentum has been shown to induce a rapid eye movement (REM) sleep-like state. Physiological evidence indicates that not only acetylcholine but also various amine transmitters, including those implicated in behavioral state regulation, affect neuronal activity in this region of the pontine reticular formation. In the present study, sources of select aminergic and cholinergic inputs to this REM sleep induction zone were identified and quantitatively analyzed by using fluorescence retrograde tracing combined with immunofluorescence in the rat. In addition to previously demonstrated cholinergic projections from the pedunculopontine and laterodorsal tegmental nuclei, the REM sleep induction zone received various aminergic inputs that originated in widely distributed regions of the brainstem and hypothalamus. Serotoninergic afferents represented a mean of 44% of all aminergic/cholinergic source neurons projecting to the REM sleep induction zone, which was comparable to the mean percentage of 39% represented by cholinergic afferent neurons. The serotoninergic afferents originated from the raphe nuclei at all brainstem levels, with heavier projections from the pontine than from the medullary raphe nuclei. Unexpectedly, an additional major serotoninergic input was provided by serotoninergic neurons in the nucleus prosupralemniscus (B9). Noradrenergic afferent neurons represented a mean of 14% of all aminergic/cholinergic source neurons, which was only about one-third of the mean percentage of either cholinergic or serotoninergic source neurons. These noradrenergic projection neurons were located not only in the locus ceruleus (8%) but also in the lateral tegmentum, including the A5 (4%) and A7 (2%) cell groups. Histaminergic neurons in the tuberomammillary hypothalamic nucleus represented a minor group of afferent neurons (3%), and a still smaller input came from adrenegic C1 neurons. The pattern of these transmitter-specific afferent connections appeared to be similar regardless of the longitudinal level within the REM sleep induction zone. The present results are consistent with previous behavioral and physiological evidence for a role of the pontine REM sleep induction zone in triggering REM sleep. The regulation of REM sleep induction would be best understood in terms of a state-dependent interplay of cholinergic, serotoninergic, and other inputs all acting convergently upon neurons in the REM sleep-inducing region of the pontine reticular formation.     

Dye coupling between dorsal raphe neurones

Neurones in the dorsal raphe nucleus (DRN) were impaled and filled with biocytin in coronal slices of midbrain taken from young adult rats. The electrophysiological properties and gross morphology of the cells were similar to those reported previously for serotonergic neurones in the DRN. Of 27 cases in which filled neurones were recovered in histological material, almost half (48%) showed labelling of two or three cells, although only one cell had been recorded from. Coupled cells were identified as close or distantly coupled, depending on the distance from the soma of the presumed impaled cell (23.5±15 m, n=7 and 150±26.5 m, n=10 respectively). Whereas close-coupled cells may have been artefactually coupled by the penetrating electrode, coupling between distant cells is most likely to be a result of transfer of biocytin through gap junctions. Camera lucida reconstructions of pairs of labelled cells revealed extensive overlap of dendritic fields and numerous crossings between dendrites. When examined at high magnification under a light microscope, many of the crossing dendrites were found to travel in different focal planes. Nevertheless, for each pair of cells, at least one point of close apposition was observed between dendrites or between the axon and a dendrite of the presumed impaled and coupled cell. The incidence of dye coupling between neurones in the DRN may reflect a relatively high level of electrotonic coupling between the neurones. This form of coupling may be important in determining the synchronous nature of firing of neurones in the DRN.