Antagonism of alpha1-adrenergic and serotonergic receptors in the hypoglossal motor nucleus does not prevent motoneuronal activation elicited from the posterior hypothalamus

Abstract

The perifornical (PF) region of the posterior hypothalamus plays an important role in the regulation of sleep-wake states and motor activity. Disinhibition of PF neurons by the GABA(A) receptor antagonist, bicuculline, has been used to study the mechanisms of wake- and motor activity-promoting effects that emanate from the PF region. Bicuculline activates PF neurons, including the orexin-containing cells that have major excitatory projections to brainstem noradrenergic and serotonergic neurons. Since premotor aminergic neurons are an important source of motoneuronal activation, we hypothesized that they mediate the excitation of motoneurons that results from disinhibition of PF neurons with bicuculline. In urethane-anesthetized, paralyzed and artificially ventilated rats, we found that PF bicuculline injections (1mM, 20 nl) made after combined microinjections into the hypoglossal (XII) nucleus of alpha(1)-adrenergic and serotonergic receptor antagonists (prazosin and methysergide) increased XII nerve activity by 80+/-16% (SE) of the control activity level. Thus, activation of XII motoneurons originating in the hypothalamic PF region was not abolished despite effective elimination by the aminergic antagonists of the endogenous noradrenergic and serotonergic excitatory drives to XII motoneurons and abolition of XII motoneuronal activation by exogenous serotonin or phenylephrine. These results show that a major component of XII motoneuronal activation originating in the posterior hypothalamus is mediated by pathways other than the noradrenergic and serotonergic projections to motoneurons.

Figure 1. Example of the effect of combined prazosin and methysergide injections into the XII nucleus followed by bicuculline injection into the perifornical (PF) hypothalamus on XII nerve activity (A), and control experiments conducted to verify the effectiveness of the aminergic antagonists (B and C)

A: Three injections of the antagonist mix into the XII nucleus depress XII nerve activity, whereas bicuculline injected into the hypothalamic PF region 29 min later increases it, elicits hippocampal theta rhythm and accelerates the central respiratory rate. B: excitatory effect of serotonin (5-HT) injected into the XII nucleus (top trace) is abolished by the antagonist mix (middle trace) and then partially recovers ~3 h later (bottom trace). C: excitatory effect of phenylephrine injected into the XII nucleus (top trace) is also abolished by the antagonist mix (middle trace) and then partially recovers ~3 h later (bottom trace). MA – moving average.

Figure 2. Mean effects of the combined injections of prazosin and methysergide into the XII nucleus and then bicuculline into the perifornical hypothalamus on XII nerve activity and central respiratory rate

A: the antagonist mix injected into the XII nucleus reduced XII nerve activity to 24.9% of its pre-antagonists baseline level. The subsequent hypothalamic injections of bicuculline increased XII nerve activity by 80% of the same baseline level despite antagonism of adrenergic and serotonergic receptors in the XII nucleus. B: the central respiratory rate was not affected by the antagonist injections into the XII nucleus but was increased by the subsequent hypothalamic bicuculline injection.

Figure 3. Medullary and hypothalamic injection sites

A: three antagonist mix injection sites in the XII nucleus shown in a parasagittal plane. B: hypothalamic bicuculline injection site shown in coronal plane. The sites shown in A and B are from the experiment illustrated in Fig. 1A. C: the distribution of all bicuculline injection sites superimposed onto the closest standard cross-sections from a rat brain atlas [25]. Abbreviations: 3V – third ventricle; DMH – dorsomedial hypothalamic nucleus; f – fornix; mt – mammillothalamic tract; VMH – ventromedial hypothalamic nucleus; XII – hypoglossal nucleus.