The parabrachial nucleus (PBN) is important in relaying visceral vagal information throughout the CNS. Previously we have shown an enhanced expression of c-fos in the PBN following stimulation of the cardiac branch of the vagus in the rat (Bulmer et al. 2000) and demonstrated that the PBN receives a direct input from the nucleus tractus solitarii (NTS) (Bulmer et al. 2001). The aim of this study was to determine whether the PBN receives cardiac vagal afferent input via this direct projection from the NTS.

Male Sprague-Dawley rats (300-375 g) were anaesthetised with sodium pentobarbitone (60 mg kg^-1 I.P. supplemented with 18 mg kg^-1 I.V. as required), neuromuscularly blocked with gallamine (12 mg kg^-1 I.V.) and artificially ventilated. Depth of anaesthesia was assessed by the stability of cardiovascular variables during noxious stimuli. Rats were placed in a stereotaxic frame and a concentric bipolar stimulating electrode inserted into the right PBN (bregma -9.10 to 9.20 mm, lateral 2.10 to 2.20 mm, and depth 4.90 to 5.10 mm). Both thoracic vagi were crushed below the level of the heart. Extracellular single-unit activity was recorded using glass capillary microelectrodes (15-32 M¢) filled with 1.5 % biocytin in 0.5 M NaCl. Responses were examined in NTS neurones to cardiac and cervical vagal stimulation (1-15 V, 0.1-1 ms, 1 Hz), and PBN stimulation (100-1000 µA, 0.1-1 ms, 1 Hz). The latency of evoked responses was determined using post-stimulus triggered histograms (50 sweeps, 1 ms bin width). Chemosensitive cardiac and pulmonary afferent endings were stimulated by left ventricular or right atrial injection of phenylbiguanide (PBG; 2-30 µg). NTS recording sites were marked with biocytin using a juxtacellular labelling technique (Pinault, 1996). The PBN stimulating site was marked by an electrolytic lesion (50 µA DC, for 1 min) and animals humanely killed by anaesthetic overdose with sodium pentobarbitone (100 mg kg^-1 I.V.).

A total of 66 NTS neurones were studied that responded with one or more peaks in the post-stimulus histogram at a latency consistent with C-fibre activation (34.7 ± 0.8 ms; mean ± S.E.M.), following stimulation of the cervical vagus. The response to electrical stimulation of the cardiac branch of the vagus was examined in 34 of these NTS neurones that were antidromically activated following PBN stimulation (latency 25.2 ± 0.9 ms). Forty-seven per cent (16/34) were activated synaptically at a latency of 61.3 ± 1.8 ms, of which 81 % (13/16) were also excited in response to left ventricular injection of PBG.

These results indicate that NTS neurones receiving cardiac vagal afferent inputs send projections directly to the PBN.This work was supported by the British Heart Foundation.

Bulmer, D.C.E., Ford, T.W. & Spyer, K.M. (2000). Auton. Neurosci. 82, 54-55.

Bulmer, D.C.E., Ford, T.W. & Spyer, K.M. (2001). J. Physiol. 533, 85P.

Pinault, D. (1996). J. Neurosci. Meth. 65, 113-136.