Relationship between the ventromedullary clonidine-sensitive area and the posterior hypothalamus

Hypothalamic, midbrain and bulbar areas involved in the defense reaction in rabbits

The present study mapped neuroanatomical sites in the hypothalamus and periaqueductal gray (PAG) of the rabbit which, when stimulated electrically, evoked the cardiorespiratory components of the defense reaction (CRDR). This included increases in heart rate, blood pressure, hindlimb blood flow and respiration rate. All of the components of the CRDR were elicited by electrical stimulation of the posterior hypothalamus, at sites dorsal and medial to the fornix. Although there were regions throughout the PAG in which electrical stimulation elicited concomitant increases in blood pressure, hindlimb blood flow and respiration rate, only stimulation of the dorsal PAG evoked tachycardia. Injection of horseradish peroxidase into the rostral ventrolateral medulla (RVLM) led to heavy retrograde and anterograde labeling in the region of the hypothalamus that yielded the CRDR when stimulated electrically. Heavy labeling was also observed in the dorsal and ventral PAG. The results of this study provide evidence that the posterior hypothalamus and the dorsal PAG are nodal structures in the mediation of the CRDR and that cells in posterior hypothalamus, dorsal PAG and ventral PAG make monosynaptic connections with the RVLM.

The alpha-adrenergic receptor: radiohistochemical analysis of functional characteristics and biochemical differences

The partial agonist [3H]para-aminoclonidine was used to label alpha 2-adrenergic binding sites in intact sections of the rat central nervous system using in vitro labeling receptor autoradiographic techniques. The distribution of alpha 2-agonist binding sites closely parallels the reported distribution of noradrenergic and adrenergic cell groups and their terminal fields, particularly the projections of the medullary catecholamine neurons. This distribution of alpha 2 binding sites confirms physiological studies which indicate that the anti-hypertensive actions of alpha 2-agonist compounds are mediated centrally in medullary and spinal centers involved in the control of parasympathetic and sympathetic outflow. Further, the high concentrations of alpha 2 binding sites in pontine and limbic areas such as the locus coeruleus, parabrachial nucleus, dorsal raphe, hypothalamus, amygdala, bed nucleus of the stria terminalis, septum and entorhinal cortex offer an anatomical basis for understanding the anxiolytic and antidepressant actions of drugs like clonidine. The antagonists [3H]prazosin and [3H]WB4101 were used to study the distribution of alpha 1-adrenergic binding sites in the rat forebrain and biochemical studies were performed to analyze the marked differences that were initially seen in the distribution of [3H]prazosin and [3H]WB4101 binding sites. Several pieces of evidence derived from both biochemical and autoradiographic studies suggest that [3H]prazosin and [3H]WB4101 act at distinctly different binding sites. However, both sites may represent components of an alpha 1-adrenergic receptor-effector complex since a high degree of overlap was seen in the binding site distribution of these two ligands and since kinetic interactions could be demonstrated in at least one region of the brain, the hippocampus. Differences noted in the relative displacements of [3H]prazosin and [3H]WB4101 binding in various forebrain regions could reflect differences in the coupling efficiency of the [3H]prazosin and [3H]WB4101 component of the hypothesized complex. Further, in some regions, [3H]WB4101 labeled a binding site that is different from the alpha 1-receptor. Thus, [3H]prazosin and [3H]WB4101 binding sites seen in forebrain regions such as lamina V of the cortex, thalamic nuclei and dorsal raphe probably represent alpha 1-adrenergic receptors and confirm electrophysiological and biochemical studies which demonstrate that adrenergic transmission in these regions can be mediated through an alpha 1-receptor.

Distribution of alpha 2 agonist binding sites in the rat and human central nervous system: analysis of some functional, anatomic correlates of the pharmacologic effects of clonidine and related adrenergic agents

Using [3H]para-aminoclonidine, alpha 2 adrenergic binding sites have been mapped in the rat and human CNS using in vitro labeling autoradiographic techniques. In both the rat and human thoracic spinal cord, high densities of alpha 2 binding sites were associated with the substantia gelatinosa and the intermediolateral cell column. In the rat medulla, high binding site density was observed in the medial nucleus of the solitary tract, dorsal motor nucleus of the vagus, raphe pallidus and the substantia gelatinosa of the trigeminal nucleus, while lower levels of specific binding were found in the lateral and ventrolateral medulla. In the human, a similar distribution was observed. However, significantly lower levels of specific binding were seen in the medial nts as opposed to the dmv. In the rat, high levels of specific binding were seen at pontine and midbrain levels in the locus coeruleus, parabrachial nucleus and periaqueductal gray. In the forebrain, several hypothalmic and limbic regions, including the paraventricular and arcuate nuclei of the hypothalamus, the central, medial and basal nuclei of the amygdala, lateral septum and bed nucleus of the stria terminalis and pyriform, entorhinal and insular cortex were labeled. Each of these regions are involved in either modulating autonomic functions directly or integrating somatosensory and/or affective function with autonomic mechanisms. Further, these regions are interrelated by reciprocal connections, and neurons that utilize noradrenaline or adrenaline as their neurotransmitter form a vital part of these connections. Thus, these functional, anatomical and neurochemical correlates of the alpha 2 binding site distribution establish a neurological basis for the complex pharmacological effects of centrally acting alpha 2 agonists.

The nucleus reticularis lateralis: a region highly sensitive to clonidine

Slow bilateral microinjections of a low dose of clonidine (75 ng/kg) in the cat's nucleus reticularis lateralis (NRL) lead to significant hypotension and bradycardia. This finding confirms the existence of a ventromedullary highly sensitive site of action of clonidine. It is suggested that clonidine inhibits some vasopressive and cardioacceleratory structures within the NRL region.

Effects on blood pressure of noradrenaline and isoprenaline administered into the third ventricle of the brain of anaesthetized and conscious cats

1 Noradrenaline administered into the third ventricle of the brain (IIIv) of both conscious and anaesthetized cats induced increases in blood pressure accompanied by small and variable heart rate effects. The pressor responses were reduced after autonomic ganglion blockade indicating their likely central origin. 2 Noradrenaline when administered either into a lateral cerebral ventricle (i.c.v.) in conscious and anaesthetized cats or into the cisterna magna (i.c.) in anaesthetized cats induced falls in blood pressure accompanied by bradycardia. 3 Pressor responses to IIIv noradrenaline in conscious cats were partly blocked by either propranolol or thymoxamine indicating a possible involvement in the responses of excitatory adrenoreceptors of both alpha and beta types. 4 Pressor responses induced by IIIv isoprenaline in conscious cats were blocked by propranolol but not by thymoxamine suggesting the effect is mediated solely via excitatory beta-adrenoreceptors. 5 In anaesthetized cats prior i.c. administration of noradrenaline reduced the pressor responses induced by IIIv noradrenaline. 6 In conscious cats i.v. clonidine reduced pressor responses to IIIv noradrenaline without depressing peripheral vascular noradrenaline sensitivity. 7 The results suggest the involvement of excitatory and inhibitory alpha-adrenoreceptors and of excitatory beta-adrenoreceptors in central blood pressure control. It is also concluded that activation of inhibitory alpha-adrenoreceptors in the hind brain region can suppress the cardiovascular effects of stimulating excitatory alpha- and beta-adrenoreceptors located in the region of the third ventricle.