Hypothalamic‐evoked effects in cat nucleus tractus solitarius facilitating chemoreceptor reflexes

Abstract

In anaesthetized cats intracellular recordings in the nucleus tractus solitarius (NTS) have been used to identify neurones receiving inputs from the carotid sinus nerve (SN), the aortic nerve and hypothalamic defence area (HDA). In forty-four neurones activated by SN stimulation HDA activation evoked an IPSP in fourteen (32%) and an EPSP in eight (11%). Those neurones receiving an HDA-evoked IPSP, and tested for reflex inputs, responded only to baroreceptor activation. Neurones receiving HDA-evoked EPSPs responded only to chemoreceptor activation. In three neurones unaffected by SN stimulation, short latency inputs from HDA were observed, suggesting direct monosynaptic inputs.

Hypothalamic modulation of laryngeal reflexes in the anaesthetized cat: role of the nucleus tractus solitarii.

“…The bradycardia and hypotension may be elicited by the convergence of SLN inputs onto 'baroreceptor-sensitive' neurones within the NTS (Mifflin, Spyer & Withington-Wray, 1988a). There is evidence that the stimulation of the hypothalamic defence area (HDA), which produces a sustained increase in blood pressure together with tachycardia and tachypnoea, is partly mediated by facilitation of the chemoreceptor reflex within the NTS (Silva-Carvalho, Dawid-Milner, Goldsmith & Spyer, 1993, 1995a and an inhibition of the baroreceptor reflex that is due to hyperpolarization of baroreceptor-sensitive cells within the NTS and mediated by GABA acting at GABAA receptors (Miffin, Spyer & Withington-Wray, 1988b;Jordan, Mifflin & Spyer, 1988). Preliminary reports from this laboratory presented to the Physiological Society (Dawid-Milner, Silva-Carvalho, Goldsmith & Spyer, 1994) indicate that the stimulation of the HDA may counteract the cardiorespiratory responses induced by SLN stimulation, thus indicating an interaction between HDA and SLN inputs.…”

Hypothalamic modulation of the arterial chemoreceptor reflex in the anaesthetized cat: role of the nucleus tractus solitarii.

“…The material presented in this paper shows that, aside from exerting an indirect facilitatory influence on some NTS neurones that are excited on chemoreceptor stimulation as first reported by Silva-Carvalho et al (1993) for example Fig. 3) and as described earlier (Silva-Carvalho et al 1993).…”

“…3) and as described earlier (Silva-Carvalho et al 1993). These patterns of response, and the intranuclear substrate that accounts for them (see SilvaCarvalho et al 1995), can be expected to play a role in the mutual facilitatory interactions that result in enhanced cardiovascular and respiratory responses when these two inputs are activated (and see Dawid-Milner et al 1995 for discussion).…”

“…Chemoreceptor-activated neurones are inhibited by baroreceptor inputs (Mifflin, 1993;Felder & Mifflin, 1994). As mentioned already, the effects of chemoreceptor activation can be facilitated by a conditioning stimulus applied to the HDA (Silva-Carvalho et al 1993), a stimulus that also inhibits the baroreceptor reflex (Mifflin, Spyer & Withington-Wray, 1988;Silva-Carvalho et al 1995). The underlying synaptic mechanisms responsible for this reciprocal control of baroreceptor and chemoreceptor function have not as yet been analysed electrophysiologically.…”

“…This was demonstrated most clearly by Hilton & Joels (1965), who showed that stimulation in the hypothalamic defence area (HDA) of the cat led to a facilitation of the cardiorespiratory effects of activating the chemoreceptors of the carotid body. More recently, a brief report indicated that at least a portion of this effect can be attributed to interactions within the nucleus tractus solitarii (NTS) that were shown to involve a facilitation of sinus nerve-evoked responses when these were preceded by conditioning stimuli applied to the HDA (Silva-Carvalho, Dawid-Milner, Goldsmith & Spyer, 1993). The neuronal interconnections within the NTS that may account for such interactions have been reviewed in an accompanying report .…”