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Electrophysiological identification of neurons in the parabrachial nucleus projecting directly to the hypothalamus in the rat
Brain Research, 322 (1984) 388-392 Elsevier
388 BRE 20489
Electrophysiological identification of neurons in the parabrachial nucleus projecting directly to the hypothalamus in the rat JOHN CIRIELLO2, DUNCAN LAWRENCE l and QUENTIN J. PITTMAN I
t Department of Pharmacology and Therapeutics, Faculty of Medicine, University of Calgary, Calgary, Alberta T2N IN4 and 2Department of Physiology, Health Sciences Centre, University of Western Ontario, London, Ontario N6A 5C1 (Canada) (Accepted July 10th, 1984)
Key words: parabrachial nucleus - - paraventricular nucleus of the hypothalamus - - supraoptic nucleus - supraoptic commissure - - ascending visceral reflex pathways
Experiments were done in urethane anesthetized rats to identify single units in the region of the parabrachial nucleus (PBN) projecting directly to 'cardiovascular' responsive sites in either the paraventricular nucleus of the hypothalamus (PVH) or the supraoptic commissure and nucleus (SOC-SON) region. Fifty-five single units were antidromically activated in the ipsilateral PBN by electrical stimulation of either the PVH (n = 27) or SOC-SON region (n = 28) with latencies corresponding to conduction velocities of 0.3-5.1 m/s. The axons of PBN units projecting to the PVH conducted at significantlyslower velocities (0.5 + 0.04 m/s) than those projecting to the SOC-SON region (1.6 + 0.25 m/s). These data suggest that ascending fibers from the PBN to the PVH are unmyelinated, whereas those to the SOC-SON region are primarily a little myelinated. In addition, since the PBN is known to receive cardiovascular and visceral afferent inputs, it is suggested that these neurons likely function in relaying this afferent information to hypothalamic areas involved in autonomic regulation. Recent electrophysioiogical and anatomical evidence suggests that the parabrachial nucleus (PBN) plays an important role in cardiovascular regulation. This suggestion is based on the following observations. First, focal electrical stimulation of the PBN has been shown to elicit increases in arterial pressure and heart rate 10. Second, neurons in the PBN have
the PBN region which send efferent axons directly to 'cardiovascular' responsive sites in the hypothalamus are reported. Experiments were done in adult male Wistar rats (233-290 g) anesthetized with urethane (1.5 g/kg, i.p.). A polyethylene catheter inserted into the femo-
been shown to alter their rate of discharge during electrical stimulation of the carotid sinus3 and aortic depressor3.7 nerves. Third, the PBN has been dem-
ral artery and connected to a transducer was used to record arterial pressureS. Rectal temperature was continuously monitored and m a i n t a i n e d at 37-38 eC using radiant heat. The head of the animal was fixed
onstrated to receive direct projections from the nucleus of the solitary tract 9,12, the primary site of termination of cardiovascular afferent fibers 6. Finally, the PBN has been shown to project to forebrain areas known to be involved in autonomic function 4,15. Taken together this evidence suggests that PBN neurons function in the integration and relay of autonomic afferent information. However, little is known about the functional properties of PBN neurons which project to forebrain areas. In the present study some of the electrophysiological characteristics of neurons in
in a Kopf stereotaxic instrument and access to the paraventricular nucleus of the hypothalamus (PVH), supraoptic commissure and nucleus ( S O C - S O N ) region, and PBN was obtained by a unilateral parietal and occipital craniotomy. Exposed nervous tissue was covered with warm mineral oil to prevent drying. Bipolar stainless steel electrodes insulated with Insl-X (0.25 mm tip diameters; 5 0 - 7 0 Kf2 initial impedance at 1 kHz in saline) were stereotaxically placed in the regions of the P V H (n = 7) and S O C SON (n = 6) at 'cardiovascular' responsive sites. The
Correspondence: J. Ciriello, Department of Physiology, Health Sciences Centre, University of Western Ontario, London, Ontario N6A 5C1 Canada. 0006-8993/84/$03.00© 1984 Elsevier Science Publishers B.V.
389
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.
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Fig. 1. Antidromic responses of single units in different regions of the PBN to electrical stimulation of the PVH (A) and SOCSON (B) region. Each record is 5 superimposed sweeps and the stimuli were delivered at the arrows. Note the constant latency and high following frequency of the evoked antidromic spikes. Calibration in A, 2 ms and 200/~V and in B, 5 ms and 200/tV. PBL, lateral PBN; PBM, medial PBN.
PBL
stimulus applied to either the P V H or S O C - S O N regions was a rectangular pulse of 0.2 ms duration at 0.5 Hz and at a m a x i m u m current intensity of 0.5 m A . This intensity was chosen as it r e p r e s e n t e d 5 times the current required to elicit an increase in arterial pressure of a p p r o x i m a t e l y 25 m m Hg when a
PBM
10 s train of pulses (0.2 ms) at 80 Hz was appliedL Procedures for stimulation of hypothalamic regions, for recording of electrical activity from single units, and for histological identification of stimulation and recording sites have been described in detail elsewhere 2,5,13. Single units in the PBN were activated antidromically by electrical stimulation of either the P V H or S O C - S O N region according to established
A
1 15-
I PVH n=27 E=O.S S.E. -- 0 . 0 4
10
r l SOC -SON n=28 E= 1.6 S.E. = 0 . 2 5
Number of Units
5
II 0.6
1.2
1.8 2.4 . .3.0. Conduction Velocity (m/s)
.
.
. 3.6 .
4.8
"
"
5:4
Fig. 2. Histogram of conduction velocities of single units in the PBN antidromically activated by stimulation of the PVH (hatched arca) and SOC-SON region. The mean conduction velocities for the two groups were significantly different (P < 0.001).
390
A
B 1.6
0.8
,' - - ,
..
n,-
.'V:.
,
':
'- " -I~';" t//--',1 -~
,
..,
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,
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Fig. 3. Transverse sections of the hypothalamus extending from 0.4 to 1.6 mm anterior to bregma showing the location of stimulation sites (dots) in the SOC-SON region (A) and PVH (B) which evoked antidromic single unit responses in the PBN. AH, anterior hypothalamic nucleus; POA, preoptic area; PVH, paraventricular nucleus; OC, optic chiasma; OT, optic tract; SOC, supraoptic commissure; SON, supraoptic nucleus; VMH, ventromedial hypothalamicnucleus. criteria (for a review see ref. 8): (1) constant latency .of the evoked spike; (2) high following frequency of the evoked spike; (3) occurrence of a single evoked spike at threshold and suprathreshold stimulus intensities; and (4) collision of evoked and spontaneous spikes. Not all single units antidromicaUy activated were evaluated for all criteria as only 13% of the units were spontaneously active and were tested by the collision method. All spontaneously active units showed collision of the evoked and spontaneous spikes. Once a unit was assessed as being antidromically activated, the minimum current required to evoke the antidromic spike was determined. Fifty-five histologically verified single units in the region of the ipsilateral PBN were antidromically activated by electrical stimulation of the PVH and S O C - S O N region. Of these units, 7 were discharging spontaneously (mean discharge rate, 2.8 + 0.8
spikes/s) and 48 were silent. Antidromically activated units responded with a latency which was constant for any one unit and followed rates of stimulation of 83-500 Hz (Fig. 1). The mean threshold currents to evoke antidromic spikes in the PBN during stimulation of the PVH and S O C - S O N region were 0.32 + 0.18 mA and 0.23 + 0.17 mA, respectively. All units responded with a single spike (mean duration, 1.2 + 0.05 ms) at threshold and suprathreshold stimulus intensities. The latencies of the antidromic responses to stimulation of the PVH (mean latency, 18.2 + 1.2 ms) corresponded toconduction velocities of these ascending fibers of 0.5 + 0.04 m/s (range, 0.3-1.3 m/s), using measured conduction distances of 7.4-7.9 mm. On the other hand, the latencies of the antidromic responses to stimulation of the region of the S O C - S O N (mean latency, 9.2 + 1.1 ms) corresponded to conduction velocities of 1.6 + 0.25 m/s
391 -6.8
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Fig. 4. Transverse sections through the region of the PBN (6.87.4 mm posterior to bregma) showing the location of single units antidromically activated by stimulation of the PVH (O) and SOC-SON region (A). A, aqueduct of Sylvius; BC, brachium conjunctivum; KF, K611iker-Fuse nucleus; PBL, lateral parabrachial nucleus; PBM, medial parabrachial nucleus. (range, 0.5-5.1 m/s), using measured conduction distances of 8.2-8.7 mm. Although the ranges of conduction velocities of PBN units responding to stimulation of either the PVH or S O C - S O N region overlap (Fig. 2), the mean conduction velocities for the two groups were significantly different (P < 0.001). The anatomical distribution of stimulation sites in the region of the PVH and S O C - S O N is shown in Fig. 3 and the distribution of single units in the PBN region antidromically activated by stimulation of either the PVH or S O C - S O N region is shown in Fig. 4. Single units antidromically activated by stimulation of the PVH were found to be approximately equally distributed between the lateral and medial PBN. On the other hand, units activated antidromically by stimulation of the S O C - S O N region were found primarily in the medial PBN and in the K611iker-Fuse nucleus, the area immediately ventral to the medial PBN. These data provide electrophysiological evidence
of direct pathways from neurons in the region of the PBN to 'cardiovascular' responsive sites in the PVH and S O C - S O N region. The finding of neurons in the PBN projecting to the PVH is supported by the neuroanatomical demonstration of direct connections between the PBN and PVH using the autoradiographic method 15 and the techniques of retrograde transport of fluorochromes t6 and horseradish peroxidase TM. In addition, the demonstration in the present study of antidromically activated units in the PBN and K611iker-Fuse nucleus to stimulation of the S O C - S O N region is consistent with the finding in the rat of labelled fibers coursing through the S O C - S O N region following the injections of tritiated amino acids into the region of the PBN zS. However, it was suggested in this latter study that the fibers coursing through the S O C - S O N region did not terminate in the SON, but rather continued on to terminate primarily in the amygdala 15. This suggests that the single units recorded in the present study in the PBN to stimulation of the S O C - S O N region were likely activated as a result of stimulating fibers projecting to the amygdala. This is a likely possibility as the histologically verified stimulation sites in the S O C - S O N region were found primarily along the dorsal border of the SON (refer to Fig. 3A). However, the possibility cannot be totally excluded that some of the unit responses in the PBN resulted from the activation of a small number of fibers terminating in the region of the SON, which may not have been observed in the previous autoradiographic study 15. The latencies of the antidromic units corresponded to conduction velocities which ranged between 0.3 and 5.1 m/s. This suggests that the axons of these ascending PBN neurons are unmyelinated and small myelinated. An unexpected finding was that axons of PBN neurons projecting to or through the PVH conducted at significantly slower velocities than those projecting to the S O C - S O N region (refer to Fig. 2). These data suggest that there are at least two distinct populations of single units in the PBN projecting to .forebrain structures identified on the basis of their conduction velocities. Furthermore, it is suggested that the ascending pathway to the PVH consists of unmyelinated fibers whereas that projecting to or through the S O C - S O N region is composed primarily of small myelinated fibers. Finally, this suggestion of two different types of neurons in the PBN is support-
392 ed by the topographic representation within the PBN of single units projecting to either the PVH or S O C -
mus, it is likely that they are involved in mediating cardiovascular information to these sites. Single units
SON region; those projecting to the PVH were all confined to the PBN in both the lateral and medial di-
in the PVH, SON, amygdala and PBN have previously been shown to alter their firing frequency during
vision, whereas those projecting to the S O C - S O N region were located primarily in the medial PBN and
stimulation of baroreceptor and chemoreceptor afferent fibers 2-4. On t h e other hand, these neurons
the K611iker-Fuse nucleus, the area immediately ventral to the medial PBN. The functional significance of these two different types of neurons in the PBN is u n k n o w n , but may be related to the chemical
may also participate in neural mechanisms associated with visceral, neuroendocrine and behavioral functionsl,Xt.
transmitters associated with these neurons or to the modality of the afferent information relayed. On the basis of the present data, the function of these ascending pathways from the PBN to the forebrain remains speculative. However, since PBN neu-
This work was supported by the Ontario Heart F o u n d a t i o n and the Medical Research Council of Canada. Dr. J. Ciriello is a Canadian Heart F o u n d a t i o n Scholar, Dr. Q. J. Pittman is a Medical Research Council of Canada Scholar and Dr. D. Lawrence is
rons were antidromically activated by stimulation of
an Alberta Heritage F o u n d a t i o n Medical Research Fellow.
'cardiovascular' responsive sites in the hypothala-
1 Berntson, G. G., Attack, grooming, and threat elicited by stimulation of the pontine tegmentum in cats, Physiol. Behay., 11 (1973) 81-87. 2 Calaresu, F. R. and Ciriello, J., Projections to the hypothalamus from buffer nerves and nucleus tractus solitarius in the cat, Arner. J. Physiol., 239 (1980) R130-R136. 3 Cechetto, D. F. and Calaresu, F. R., Parabrachial units responding to stimulation of buffer nerves and forebrain in the cat, Amer. J. Physiol., 245 (1983) R811-R819. 4 Cechetto, D. F. and Calaresu, F. R., Response of single units in the amygdala to stimulation of buffer nerves in the cat, Amer. J. Physiol., 244 (1983)R646-R651. 5 Ciriello, J. and Calaresu, F. R., Role of paraventricular and supraoptic nuclei in central cardiovascular regulation in the cat, Amer. J. Physiol., 239 (1980) R137-R142. 6 Ciriello, J. and Calaresu, F. R., Projections from buffer nerves to the nucleus of the solitary tract: an anatomical and electrophysiological study in the cat, J. autonom. Nerv. Syst., 3 (1981) 299-310. 7 Hamilton, R. B., Ellenberg, H., Loskowsky, D. and Schneiderman, N., Parabrachial area as mediator of bradycardia in rabbits, J. autonom. Nerv. Syst., 4 (1981) 261-281. 8 Lipski, J., Antidromic activation of neurones as an analytic tool in the study of the central nervous system, J. Neurosci. Meth., 4 (1981) 1-32. 9 Loewy, A. D. and Burton, H., Nuclei of the solitary tract:
efferent projections to the lower brain stem and spinal cord of the cat, J. comp. Neurol., 181 (1978)421-450. l0 Mraovitch, S., Kumada, M. and Reis, D. J., Role of the nucleus parabrachialis in cardiovascular regulation in cat, Brain Research, 232 (1982) 57-75. 11 Norgren, R., Taste pathways to hypothalamus and amygdala, J. comp. Neurol., 166 (1976) 17-33. 12 Norgren, R., Projections from the nucleus of the solitary tract in the rat, Neuroscience, 3 (1978) 207-218. 13 Pinman, Q. J., Increases in antidromic latency of neurohypophyseal neurons during sustained activation, Neurosci. Lett., 37 (1983) 239-243. 14 Sakumato, T., Tohyama, M., Satoh, K., Kimato, Y., Kinugasa, T., Tanizawa, O., Kurachi, K. and Shimizu, N., Afferent fiber connections from the lower brainstem to hypothalamus studied by horseradish peroxidase method with special reference to noradrenaline innervation, Exp. Brain Res., 31 (1978) 81-94. 15 Saper, C. B. and Loewy, A. D., Efferent connections of the parabrachial nucleus in the rat, Brain Research, 197 (1980) 219-317. 16 Sawchenko, P. E., Swanson, L. W., Steinbusch, H. W. M. and Verhofstad, A. A. J., The distribution and cells of origin of serotonergic inputs to the paraventricular and supraoptic nuclei of the rat, Brain Research, 277 (1983) 355-360.
388 BRE 20489
Electrophysiological identification of neurons in the parabrachial nucleus projecting directly to the hypothalamus in the rat JOHN CIRIELLO2, DUNCAN LAWRENCE l and QUENTIN J. PITTMAN I
t Department of Pharmacology and Therapeutics, Faculty of Medicine, University of Calgary, Calgary, Alberta T2N IN4 and 2Department of Physiology, Health Sciences Centre, University of Western Ontario, London, Ontario N6A 5C1 (Canada) (Accepted July 10th, 1984)
Key words: parabrachial nucleus - - paraventricular nucleus of the hypothalamus - - supraoptic nucleus - supraoptic commissure - - ascending visceral reflex pathways
Experiments were done in urethane anesthetized rats to identify single units in the region of the parabrachial nucleus (PBN) projecting directly to 'cardiovascular' responsive sites in either the paraventricular nucleus of the hypothalamus (PVH) or the supraoptic commissure and nucleus (SOC-SON) region. Fifty-five single units were antidromically activated in the ipsilateral PBN by electrical stimulation of either the PVH (n = 27) or SOC-SON region (n = 28) with latencies corresponding to conduction velocities of 0.3-5.1 m/s. The axons of PBN units projecting to the PVH conducted at significantlyslower velocities (0.5 + 0.04 m/s) than those projecting to the SOC-SON region (1.6 + 0.25 m/s). These data suggest that ascending fibers from the PBN to the PVH are unmyelinated, whereas those to the SOC-SON region are primarily a little myelinated. In addition, since the PBN is known to receive cardiovascular and visceral afferent inputs, it is suggested that these neurons likely function in relaying this afferent information to hypothalamic areas involved in autonomic regulation. Recent electrophysioiogical and anatomical evidence suggests that the parabrachial nucleus (PBN) plays an important role in cardiovascular regulation. This suggestion is based on the following observations. First, focal electrical stimulation of the PBN has been shown to elicit increases in arterial pressure and heart rate 10. Second, neurons in the PBN have
the PBN region which send efferent axons directly to 'cardiovascular' responsive sites in the hypothalamus are reported. Experiments were done in adult male Wistar rats (233-290 g) anesthetized with urethane (1.5 g/kg, i.p.). A polyethylene catheter inserted into the femo-
been shown to alter their rate of discharge during electrical stimulation of the carotid sinus3 and aortic depressor3.7 nerves. Third, the PBN has been dem-
ral artery and connected to a transducer was used to record arterial pressureS. Rectal temperature was continuously monitored and m a i n t a i n e d at 37-38 eC using radiant heat. The head of the animal was fixed
onstrated to receive direct projections from the nucleus of the solitary tract 9,12, the primary site of termination of cardiovascular afferent fibers 6. Finally, the PBN has been shown to project to forebrain areas known to be involved in autonomic function 4,15. Taken together this evidence suggests that PBN neurons function in the integration and relay of autonomic afferent information. However, little is known about the functional properties of PBN neurons which project to forebrain areas. In the present study some of the electrophysiological characteristics of neurons in
in a Kopf stereotaxic instrument and access to the paraventricular nucleus of the hypothalamus (PVH), supraoptic commissure and nucleus ( S O C - S O N ) region, and PBN was obtained by a unilateral parietal and occipital craniotomy. Exposed nervous tissue was covered with warm mineral oil to prevent drying. Bipolar stainless steel electrodes insulated with Insl-X (0.25 mm tip diameters; 5 0 - 7 0 Kf2 initial impedance at 1 kHz in saline) were stereotaxically placed in the regions of the P V H (n = 7) and S O C SON (n = 6) at 'cardiovascular' responsive sites. The
Correspondence: J. Ciriello, Department of Physiology, Health Sciences Centre, University of Western Ontario, London, Ontario N6A 5C1 Canada. 0006-8993/84/$03.00© 1984 Elsevier Science Publishers B.V.
389
A
.
I!
.
A l/f, - VVV
B
I
i ,
I I In I lZ
1
Fig. 1. Antidromic responses of single units in different regions of the PBN to electrical stimulation of the PVH (A) and SOCSON (B) region. Each record is 5 superimposed sweeps and the stimuli were delivered at the arrows. Note the constant latency and high following frequency of the evoked antidromic spikes. Calibration in A, 2 ms and 200/~V and in B, 5 ms and 200/tV. PBL, lateral PBN; PBM, medial PBN.
PBL
stimulus applied to either the P V H or S O C - S O N regions was a rectangular pulse of 0.2 ms duration at 0.5 Hz and at a m a x i m u m current intensity of 0.5 m A . This intensity was chosen as it r e p r e s e n t e d 5 times the current required to elicit an increase in arterial pressure of a p p r o x i m a t e l y 25 m m Hg when a
PBM
10 s train of pulses (0.2 ms) at 80 Hz was appliedL Procedures for stimulation of hypothalamic regions, for recording of electrical activity from single units, and for histological identification of stimulation and recording sites have been described in detail elsewhere 2,5,13. Single units in the PBN were activated antidromically by electrical stimulation of either the P V H or S O C - S O N region according to established
A
1 15-
I PVH n=27 E=O.S S.E. -- 0 . 0 4
10
r l SOC -SON n=28 E= 1.6 S.E. = 0 . 2 5
Number of Units
5
II 0.6
1.2
1.8 2.4 . .3.0. Conduction Velocity (m/s)
.
.
. 3.6 .
4.8
"
"
5:4
Fig. 2. Histogram of conduction velocities of single units in the PBN antidromically activated by stimulation of the PVH (hatched arca) and SOC-SON region. The mean conduction velocities for the two groups were significantly different (P < 0.001).
390
A
B 1.6
0.8
,' - - ,
..
n,-
.'V:.
,
':
'- " -I~';" t//--',1 -~
,
..,
V H'll
,
', ",
Z.-..
Fig. 3. Transverse sections of the hypothalamus extending from 0.4 to 1.6 mm anterior to bregma showing the location of stimulation sites (dots) in the SOC-SON region (A) and PVH (B) which evoked antidromic single unit responses in the PBN. AH, anterior hypothalamic nucleus; POA, preoptic area; PVH, paraventricular nucleus; OC, optic chiasma; OT, optic tract; SOC, supraoptic commissure; SON, supraoptic nucleus; VMH, ventromedial hypothalamicnucleus. criteria (for a review see ref. 8): (1) constant latency .of the evoked spike; (2) high following frequency of the evoked spike; (3) occurrence of a single evoked spike at threshold and suprathreshold stimulus intensities; and (4) collision of evoked and spontaneous spikes. Not all single units antidromicaUy activated were evaluated for all criteria as only 13% of the units were spontaneously active and were tested by the collision method. All spontaneously active units showed collision of the evoked and spontaneous spikes. Once a unit was assessed as being antidromically activated, the minimum current required to evoke the antidromic spike was determined. Fifty-five histologically verified single units in the region of the ipsilateral PBN were antidromically activated by electrical stimulation of the PVH and S O C - S O N region. Of these units, 7 were discharging spontaneously (mean discharge rate, 2.8 + 0.8
spikes/s) and 48 were silent. Antidromically activated units responded with a latency which was constant for any one unit and followed rates of stimulation of 83-500 Hz (Fig. 1). The mean threshold currents to evoke antidromic spikes in the PBN during stimulation of the PVH and S O C - S O N region were 0.32 + 0.18 mA and 0.23 + 0.17 mA, respectively. All units responded with a single spike (mean duration, 1.2 + 0.05 ms) at threshold and suprathreshold stimulus intensities. The latencies of the antidromic responses to stimulation of the PVH (mean latency, 18.2 + 1.2 ms) corresponded toconduction velocities of these ascending fibers of 0.5 + 0.04 m/s (range, 0.3-1.3 m/s), using measured conduction distances of 7.4-7.9 mm. On the other hand, the latencies of the antidromic responses to stimulation of the region of the S O C - S O N (mean latency, 9.2 + 1.1 ms) corresponded to conduction velocities of 1.6 + 0.25 m/s
391 -6.8
,-', ', .'
o,..V
B,-"3'.to.
...'0
',.:C'--,,.',
,",1
.'1
V-) - - ' " -7.0
---'; ,
,, ,,,
;;
.... .
|
I
-7.2
- 7.4
/
1ram
Fig. 4. Transverse sections through the region of the PBN (6.87.4 mm posterior to bregma) showing the location of single units antidromically activated by stimulation of the PVH (O) and SOC-SON region (A). A, aqueduct of Sylvius; BC, brachium conjunctivum; KF, K611iker-Fuse nucleus; PBL, lateral parabrachial nucleus; PBM, medial parabrachial nucleus. (range, 0.5-5.1 m/s), using measured conduction distances of 8.2-8.7 mm. Although the ranges of conduction velocities of PBN units responding to stimulation of either the PVH or S O C - S O N region overlap (Fig. 2), the mean conduction velocities for the two groups were significantly different (P < 0.001). The anatomical distribution of stimulation sites in the region of the PVH and S O C - S O N is shown in Fig. 3 and the distribution of single units in the PBN region antidromically activated by stimulation of either the PVH or S O C - S O N region is shown in Fig. 4. Single units antidromically activated by stimulation of the PVH were found to be approximately equally distributed between the lateral and medial PBN. On the other hand, units activated antidromically by stimulation of the S O C - S O N region were found primarily in the medial PBN and in the K611iker-Fuse nucleus, the area immediately ventral to the medial PBN. These data provide electrophysiological evidence
of direct pathways from neurons in the region of the PBN to 'cardiovascular' responsive sites in the PVH and S O C - S O N region. The finding of neurons in the PBN projecting to the PVH is supported by the neuroanatomical demonstration of direct connections between the PBN and PVH using the autoradiographic method 15 and the techniques of retrograde transport of fluorochromes t6 and horseradish peroxidase TM. In addition, the demonstration in the present study of antidromically activated units in the PBN and K611iker-Fuse nucleus to stimulation of the S O C - S O N region is consistent with the finding in the rat of labelled fibers coursing through the S O C - S O N region following the injections of tritiated amino acids into the region of the PBN zS. However, it was suggested in this latter study that the fibers coursing through the S O C - S O N region did not terminate in the SON, but rather continued on to terminate primarily in the amygdala 15. This suggests that the single units recorded in the present study in the PBN to stimulation of the S O C - S O N region were likely activated as a result of stimulating fibers projecting to the amygdala. This is a likely possibility as the histologically verified stimulation sites in the S O C - S O N region were found primarily along the dorsal border of the SON (refer to Fig. 3A). However, the possibility cannot be totally excluded that some of the unit responses in the PBN resulted from the activation of a small number of fibers terminating in the region of the SON, which may not have been observed in the previous autoradiographic study 15. The latencies of the antidromic units corresponded to conduction velocities which ranged between 0.3 and 5.1 m/s. This suggests that the axons of these ascending PBN neurons are unmyelinated and small myelinated. An unexpected finding was that axons of PBN neurons projecting to or through the PVH conducted at significantly slower velocities than those projecting to the S O C - S O N region (refer to Fig. 2). These data suggest that there are at least two distinct populations of single units in the PBN projecting to .forebrain structures identified on the basis of their conduction velocities. Furthermore, it is suggested that the ascending pathway to the PVH consists of unmyelinated fibers whereas that projecting to or through the S O C - S O N region is composed primarily of small myelinated fibers. Finally, this suggestion of two different types of neurons in the PBN is support-
392 ed by the topographic representation within the PBN of single units projecting to either the PVH or S O C -
mus, it is likely that they are involved in mediating cardiovascular information to these sites. Single units
SON region; those projecting to the PVH were all confined to the PBN in both the lateral and medial di-
in the PVH, SON, amygdala and PBN have previously been shown to alter their firing frequency during
vision, whereas those projecting to the S O C - S O N region were located primarily in the medial PBN and
stimulation of baroreceptor and chemoreceptor afferent fibers 2-4. On t h e other hand, these neurons
the K611iker-Fuse nucleus, the area immediately ventral to the medial PBN. The functional significance of these two different types of neurons in the PBN is u n k n o w n , but may be related to the chemical
may also participate in neural mechanisms associated with visceral, neuroendocrine and behavioral functionsl,Xt.
transmitters associated with these neurons or to the modality of the afferent information relayed. On the basis of the present data, the function of these ascending pathways from the PBN to the forebrain remains speculative. However, since PBN neu-
This work was supported by the Ontario Heart F o u n d a t i o n and the Medical Research Council of Canada. Dr. J. Ciriello is a Canadian Heart F o u n d a t i o n Scholar, Dr. Q. J. Pittman is a Medical Research Council of Canada Scholar and Dr. D. Lawrence is
rons were antidromically activated by stimulation of
an Alberta Heritage F o u n d a t i o n Medical Research Fellow.
'cardiovascular' responsive sites in the hypothala-
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