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1. In order to assess the importance of the parabrachial nuclei in modulating cardiorespiratory activity, electric current or microinjections of glutamate were used to stimulate discrete regions of the parabrachial nuclei in anaesthetized rats. 2. Stimulation of cell bodies in the medial region of the parabrachial nuclei and in the Kölliker-Fuse nuclei, caused an expiratory facilitatory response. This consisted mainly of a decrease in respiratory rate as measured by observing phrenic nerve activity. 3. Stimulation of cell bodies in the lateral region of the parabrachial nuclei caused an inspiratory facilitatory response. This consisted mainly of an increase in respiratory rate. 4. At the majority of sites (16 out of 20) where changes in respiratory rate were elicited by glutamate injection or electrical stimulation an increase in blood pressure was observed. The coexistence of increases in blood pressure and heart rate indicates the presence of inhibition of the heart rate component of the baroreflex and/or an increase in cardiac sympathetic drive. 5. The expiratory facilitatory response was not evoked reflexly by the rise in blood pressure since it was still present after administration of guanethidine, which abolished the rise in blood pressure. 6. The interactions between the parabrachial nuclei and the medullary respiratory complex in eliciting these changes are discussed.

The origin of cardiac vagal preganglionic motoneurones in the rat is still controversial and knowledge of the chemistry of synaptic inputs onto these neurones is limited. In this investigation vagal preganglionic motoneurones innervating the heart were identified by the retrograde transport of cholera toxin conjugated to horseradish peroxidase (CT-HRP) combined with the immunocytochemical localization of 5-hydroxytryptamine. Injection of CT-HRP into the myocardium resulted in the retrograde labelling of neurones primarily in the ventral regions of the nucleus ambiguus (75.1%). Labelled neurones were also distributed in a narrow band through the reticular formation extending between the dorsal motor nucleus of the vagus nerve and the nucleus ambiguus (17.3%) as well as in the dorsal motor nucleus itself (7.6%). A combination of retrograde labelling with immunocytochemistry for 5-hydroxytryptamine revealed that the neuronal perikarya and the dendrites of cardiac vagal motoneurones in the nucleus ambiguus were often ensheathed in 5-hydroxytryptamine-immunoreactive axonal boutons. Electron microscopic examination of this material confirmed that there were synaptic specializations between these boutons and the cardiac vagal motoneurones. The identification of 5-hydroxytryptamine-containing synaptic inputs to this population of vagal motoneurones provides further detail towards the understanding of the regulation of heart rate by the parasympathetic nervous system.

Since the inhibitory neurotransmitter γ-aminobutyric acid (GABA) has been implicated in virtually all neuronal systems, it would be surprising if it were not also involved in the pathways responsible for maintaining and controlling blood pressure and heart rate. However, the exact role that it plays in cardiovascular control is still under intensive investigation and GABAergic inhibitory connections between cell groups are still being mapped anatomically and functionally. In addition, in recent years it has become apparent that GABA can act on at least two types of receptor (GABAA and GABAB), which are both capable of mediating pre- and postsynaptic inhibition but function via different ionic mechanisms. Agonists acting at the GABAA-receptor subtype cause inhibition by opening of Cl− channels whereas agonists at the GABAB-receptor subtype cause postsynaptic inhibition via a K+-conductance (Newberry and Nicoll, 1985) and presynaptic inhibition of transmitter release probably through a reduction in Ca2+-conduc-tance (see Huston et al., 1990).

There is both anatomical and neurophysiological evidence to indicate that the primary synapses in the arterial chemoreceptor reflex are located within the nucleus tractus solitarius (NTS). Indeed, it appears that these afferents make connections with neurones in a relatively restricted portion of the NTS, particularly the commissural nucleus and the medial subnucleus in close proximity to the obex (for review see Donoghue et al., 1984; Housley et al., 1987; Jordan and Spyer, 1987). In contrast, there is little detailed evidence to support the notion that the respiratory neurones of the ventrolateral NTS are contacted directly by these afferents. The role of intrinsic circuits within the NTS in mediating the chemoreceptor control of respiratory activity, and the attended influence of this control on cardiovascular activity, have thus become a focus of interest in our laboratory. This report will summarize the results of neurophysiological, neuroanatomical, and immunocytochemical studies that have been designed to address these questions.