Brainstem projections to the phrenic nucleus: an anterograde and retrograde HRP study in the rabbit.

Brainstem projections to the phrenic nucleus were studied in rabbits using horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) as a retrograde and anterograde neuronal tracer. Injections of 1% WGA-HRP were centered in the phrenic nucleus in the C4-C5 ventral horn in 4 rabbits to identify pontomedullary nuclear groups that contain neurons projecting to the midcervical spinal cord. Regions of the rabbit brainstem that are homologous to the ventral respiratory group (VRG), dorsal respiratory group (DRG), Bötzinger Complex (BötC) and Kölliker-Fuse nucleus in the cat and rat were shown to provide the major pontomedullary projections to the phrenic nucleus. Injections of WGA-HRP into physiologically identified locations within DRG, VRG and BötC anterogradely labelled bulbospinal axons of these groups. These injections produced presumptive terminal labelling in the C4-C5 ventral horn in the region containing the phrenic cell column and the transverse phrenic motoneuron dendrite bundles as defined by WGA-HRP labelling of phrenic motoneurons. These results indicate: 1) The presumptive excitatory (DRG, VRG) and inhibitory (BötC) bulbospinal control of phrenic motoneurons arise from the same medullary respiratory groups in the rabbit as in the cat and rat. 2) The bulbospinal control of phrenic motoneurons is primarily via direct projections to the phrenic motor nucleus, and not through segmental propriospinal interneurons. 3) As in the rat, the bulbospinal contribution of the DRG is less pronounced in the rabbit than in the cat. 4) The rabbit and rat have a slight ipsilateral predominance in their bulbospinal projections to phrenic nucleus; whereas these projections have a contralateral predominance in the cat.

Anatomical and functional connections of neurons of the rostral medullary raphe of the rabbit.

Single cell recordings were made from neurons in the rostral medullary raphe (RMR) of the rabbit. The recording sites were ones that had been shown to yield pressor responses from electrical stimulation and by pressure injections of glutamate. Electrical stimulation of the intermediolateral (IML) region of the spinal cord led to antidromic activation of 12 of the 100 cells studied. Eleven of these cells were located in raphe pallidus or raphe magnus, and one cell was located in raphe obscurus. These findings were consistent with the results of horseradish peroxidase (HRP) histochemistry experiments. Injections of HRP into the IML led to heavy cell body labeling in raphe pallidus and raphe magnus, but sparse labeling in raphe obscurus. Cells in the RMR could be orthodromically activated by electrical stimulation of the putative defense area of the periaqueductal (PAG) but not by stimulation of putative defense areas in the hypothalamus. Most of these cells were located in raphe pallidus or raphe magnus. Similarly, HRP injections into raphe pallidus and raphe magnus led to heavy cell body labeling in the PAG but not the hypothalamus; no cell body labeling was found in the PAG when injections were made into raphe obscurus.

Cardiovascular responses elicited by electrical and chemical stimulation of the rostral medullary raphe of the rabbit.

Electrical stimulation of the rostral medullary raphe (RMR) of the rabbit elicited pressor responses that were accompanied by tachycardia or bradycardia. Stimulation of dorsal sites (the dorsal raphe obscurus) evoked a pressor/tachycardia response and stimulation of ventral sites (the ventral raphe obscurus, raphe magnus and raphe pallidus) produced a pressor/bradycardia response. Electrical stimulation of the RMR after sinoaortic denervation led to an increase in the magnitude of the pressor response elicited from all stimulation sites, a decrease in the magnitude of the bradycardia produced by stimulation at the ventral sites, but had no effect upon the magnitude of the tachycardia observed from stimulation of the dorsal sites. These findings suggest that electrical stimulation of the dorsal sites leads to inhibition of the cardiomotor component of the baroreceptor reflex. The results of vagal blockade experiments demonstrated that baroreceptor attenuation of the pressor responses at ventral sites was mediated primarily by parasympathetic input to the heart. Chemical stimulation of the RMR with L-glutamate also led to a pressor/tachycardia response at the dorsal sites and a pressor/brachycardia response at the ventral sites. This finding provides evidence that neuronal cell bodies, not axon of passage, mediated the responses elicited by electrical stimulation.

The intermediolateral nucleus: an 'open' or 'closed' nucleus?

The sympathetic preganglionic neurons located in the intermediolateral nucleus (IML) that project to the superior cervical ganglion of the rabbit were observed to have two major dendritic orientations after retrograde labeling with horseradish peroxidase. One projection extends longitudinally within IML. The second projection courses medially and presents a triangular shape in horizontal sections. The labeled processes that project medially arise from cells in IML and project through the intercalated nucleus towards the central autonomic area and follow the contour of the central canal. Medially oriented dendrites intruding into other areas of the intermediate grey matter show that IML is an 'open' rather than a 'closed' nucleus as has been recently suggested. The location and distribution of the sympathetic preganglionic neurons projecting to the superior cervical ganglion in the rabbit are compared with those reported for other species.

Nucleus reticularis lateralis involvement in the pressor component of the cerebral ischemic response.

Lesions of the ventromedial portion of the nucleus reticularis lateralis (NRL) of the medulla oblongata in the rabbit significantly and markedly reduced the pressor component of the cerebral ischemic response (CIR) without altering vasomotor tone. Use of horseradish peroxidase as an anterograde and retrograde tracer indicated that the ventromedial NRL projects to the intermediolateral cell column (IML) of the upper thoracic spinal cord. Injections of glutamate into ventromedial NRL elevated blood pressure (BP), indicating that neuronal cell bodies in this area are capable of eliciting BP elevations when stimulated. These findings suggest that cell bodies in the ventromedial NRL are capable of eliciting pressor responses during the CIR via direct projections to sympathetic preganglionic cell bodies.

Daily noise duration influences cardiovascular responses.

In this study, third in a series on non-auditory health effects of noise, we provide data on the previously developed proposition that exposure to intense noise is linked with altered blood pressure regulation through the establishment of a long-term dose----response relationship between the two variables. We also sought to determine the effect of interposed rest sessions on the course of heart rate (HR) and blood pressure (BP) during 97 days of exposure to tape-recorded industrial noise. The stimulus, delivered to 2 female adult macaque monkeys (M. fascicularis) fitted with indwelling cannulas, consisted of pile-driver impacts and other construction noises presented either in 4-hr (Leq24 = 86.6 db) or 8-hr (Leq24 = 89.6 db) daily sessions. When compared to their pre-exposure responses, mean arterial pressure (MAP) for the Ss rose 8.2% during 4-hr and 16.5% during 8-hr exposure sessions, i.e., close to a doubling effect for a doubling of total energy. When compared with control monkeys (N:10) used in earlier studies, however, MAP rose in the present Ss only 44% for the same doubling of energy. In general, HR and BP displayed quite different tendencies during the per-exposure rest sessions and the post-exposure period: HR tended to return to baseline levels upon noise termination whereas MAP, once elevated, tended to remain significantly elevated. Applicability of the "law" of Initial Values was demonstrated for noise-induced BP increases, in the sense that the percentage of change in BP produced by 8-hr exposure sessions in the 2 present Ss was 16.5%, or somewhat less than that previously observed in exposed monkeys with lower baseline levels (i.e., 30.5%), even though absolute levels of per-exposure MAP were similar (i.e., 99 vs 100 mmHg) for the two sets of exposed monkeys.