Immunohistochemical evidence for the adrenergic medullary longitudinal bundle as a major ascending pathway to the hypothalamus

Distribution and morphology of the catecholaminergic neural elements in the human hypothalamus

Previous studies have demonstrated that catecholaminergic, tyrosine hydroxylase (TH)-immunoreactive (IR) perikarya and fibers are widely distributed in the human hypothalamus. Since TH is the key and rate-limiting enzyme for catecholaminergic synthesis, these IR neurons may represent dopaminergic, noradrenergic or adrenergic neural elements. However, the distribution and morphology of these neurotransmitter systems in the human hypothalamus is not entirely known. Since the different catecholaminergic systems can be detected by identifying the neurons containing the specific key enzymes of catecholaminergic synthesis, in the present study we mapped the catecholaminergic elements in the human hypothalamus using immunohistochemistry against the catecholaminergic enzymes, TH, dopamine beta-hydroxylase (DBH) and phenylethanolamine-N-methyltransferase (PNMT). Only a few, PNMT-IR, adrenergic neuronal elements were found mainly in the infundibulum and the periventricular zone. DBH-IR structures were more widely distributed in the human hypothalamus occupying chiefly the infundibulum/infundibular nucleus, periventricular area, supraoptic and paraventricular nuclei. Dopaminergic elements were detected by utilizing double label immunohistochemistry. First, the DBH-IR elements were visualized; then the TH-IR structures, that lack DBH, were detected with a different chromogen. In our study, we conclude that all of the catecholaminergic perikarya and the majority of the catecholaminergic fibers represent dopaminergic neurons in the human hypothalamus. Due to the extremely small number of PNMT-IR, adrenergic structures in the human hypothalamus, the DBH-IR fibers represent almost exclusively noradrenergic neuronal processes. These findings suggest that the juxtapositions between the TH-IR and numerous peptidergic systems revealed by previous reports indicate mostly dopaminergic synapses.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Direct and indirect inhibition by catecholamines of hypocretin/orexin neurons

Hypothalamic hypocretin enhances arousal, similar to the actions of norepinephrine (NE). The physiological actions of NE were examined in hypocretin neurons identified by selective green fluorescent protein expression in transgenic mouse hypothalamic slices using whole-cell recording. NE induced an outward current, inhibited spike frequency, and hyperpolarized hypocretin neurons dose dependently. Similar actions were evoked by the selective alpha2 adrenergic agonist clonidine. The alpha2 antagonist idazoxan increased spike frequency, suggesting tonic NE-mediated inhibition. The NE-induced current was inwardly rectified, and the reversal potential was dependent on external potassium concentration; it was blocked by barium in the bath and by GTP-gamma-S in the pipette, suggesting activation of a G-protein inward rectifying K+ (GIRK) current. NE and clonidine decreased calcium currents evoked by depolarizing voltage steps. The selective alpha1 adrenergic agonist phenylephrine had no effect on membrane potential but did increase IPSC frequency; miniature IPSC frequency was also increased, in some cells without any effect on amplitude, suggesting a facilitative presynaptic action at alpha1 receptors on GABAergic axons that innervate hypocretin neurons. NE therefore inhibits hypocretin neurons directly through two mechanisms: activation of a GIRK current, depression of calcium currents, and indirectly through increased inhibitory GABA input. Similar to NE, dopamine and epinephrine reduced or blocked spikes and, in the presence of TTX, showed direct hyperpolarizing actions. The action of dopamine was blocked by the D2 receptor antagonist eticlopride, whereas a D1/5 antagonist had no effect. These data suggest that catecholamines evoke strong inhibitory actions on hypocretin neurons and suggest negative feedback from catecholamine cells that may be excited by hypocretin.

Overlapping distributions of orexin/hypocretin- and dopamine-beta-hydroxylase immunoreactive fibers in rat brain regions mediating arousal, motivation, and stress

A double-label immunohistochemical study was carried out to investigate overlap between dopamine-beta-hydroxylase (DbetaH) -immunopositive projections and the projections of hypothalamic neurons containing the arousal- and feeding-related peptide, orexin/hypocretin (HCRT), in rat brain. Numerous intermingled HCRT-immunopositive and DbetaH-immunopositive fibers were seen in a ventrally situated corridor extending from the hypothalamus to deep layers of the infralimbic cortex. Both fiber types avoided the nucleus accumbens core, caudate putamen, and the globus pallidus. In the diencephalon, overlap was observed in several hypothalamic areas, including the perifornical, dorsomedial, and paraventricular nuclei, as well as in the paraventricular thalamic nucleus. Intermingled HCRT-containing and DbetaH-containing fibers extended from the hypothalamus into areas within the medial and central amygdala, terminating at the medial border of the lateral subdivision of the central nucleus of the amygdala. Dense overlap between the two fiber types was also observed in the periaqueductal gray, particularly in the vicinity of the dorsal raphe, as well as (to a lesser extent) in the ventral tegmental area, the retrorubral field, and the pedunculopontine tegmental nucleus. Hypocretin-containing cell bodies, located in the perifornical and lateral hypothalamus, were embedded within a dense plexus of DbetaH-immunopositive fibers and boutons, with numerous cases of apparent contacts of DbetaH-containing boutons onto HCRT-immunopositive soma and dendrites. HCRT-containing fibers were observed amid the noradrenergic cells of the locus coeruleus, and in the vicinity of the A1, A2, and A5 cell groups. Hence, the projections of these two arousal-related systems, originating in distinctly different parts of the brain, jointly target several forebrain regions and brainstem monoaminergic nuclei involved in regulating core motivational processes.

Expression of estrogen receptor-alpha and c-Fos in adrenergic neurons of the female rat during the steroid-induced LH surge

Epinephrine is an important neurotransmitter that is synthesized in relatively few neurons of the medullary regions C1-C3. Epinephrine is involved, among others in the control of most neuroendocrine systems, such as corticotropin releasing hormone-, gonadotropin releasing hormone- and oxytocin/vasopressin-containing neurons as part of complex feedback loop systems that often include interactions with the gonadal or adrenal steroid hormones. In order to determine if the interactions between gonadal steroid hormones with the adrenergic neurons are direct or involve steroid-receptive interneurons that in turn innervate the adrenergic neurons, dual immunohistochemistry was applied to identify if estrogen receptor-alpha (ERalpha) protein was expressed by adrenergic, phenylethanolamine-N-methyl transferase (PNMT)-positive neurons and if estradiol can activate these neurons as determined by the transient expression of the transcription factor c-Fos. The results show that an average of 22% of all PNMT neurons in the C1 region, 38% in C2 and 42% in the C3 region express estrogen receptor-alpha protein with the highest numbers of dual labeled neurons in the central levels of the C1-C3 regions. Overall, the percentages of dual labeled PNMT/ERalpha neurons did not change during the steroid-induced LH surge. In contrast, the percentage of c-Fos expressing PNMT neurons changed significantly during the LH surge. Thus, c-Fos immunoreactivity was highest in all three regions at 1200 h with 69% of the PNMT neurons in C1, 60% in C2 and 79% in C3 co-expressing c-Fos. C-Fos expression was lowest before and after the surge with 39% of the PNMT neurons in the C2 region containing c-Fos at 0800 h, 52% c-Fos-positive PNMT neurons in C1 and 54% in area C3. The results show that many adrenergic neurons are direct targets for estradiol and that most PNMT neurons in the brainstem are activated during the initiation of the steroid-induced LH surge which suggests that epinephrine is one of the triggers that stimulates GnRH release during the surge.

Adrenergic projections from the lower brainstem to the hypothalamic paraventricular nucleus, the lateral hypothalamic area and the central nucleus of the amygdala in rats

Fine networks of phenylethanolamine N-methyltransferase (PNMT)-immunoreactive fibers are found in the hypothalamic paraventricular nucleus--mainly in the anterior, dorsal and dorso-medial parvicellular subdivisions, the lateral hypothalamus (dorsal, lateral and ventral to the fornix) and in the central amygdaloid nucleus. Coronal hemisections of the brainstem through the rostral level of the medulla oblongata show that most hypothalamic and amygdaloid PNMT fibers arise from the medullary adrenergic cell groups. Fourteen, but not 10 days after total hemisections, PNMT fibers disappeared almost completely from the hypothalamus and amygdala, ipsilateral to the knife cuts. A small decrease was also observed in the ventral, lateral hypothalamus on the contralateral side. Partial depletion of PNMT-immunoreactivity in the hypothalamus and the amygdala after medial or lateral brainstem hemisections indicates that ascending PNMT-immunoreactive fibers pass through mainly the lateral portion of the medulla, but some fibers also in its medial portion. Midsagittal transection of the diencephalon slightly reduced PNMT immunostaining in the paraventricular nucleus and the lateral hypothalamus bilaterally. The results show that the ascending PNMT system essentially is ipsilateral, but probably with a small crossing-over component, both at the diencephalic and lower brainstem level.

Posterior hypothalamic receptors involved in the cardiovascular changes elicited by electrical stimulation of the rostral ventrolateral medulla

The posterior hypothalamic receptors involved in the cardiovascular responses to electrical stimulation of the rostral ventrolateral medulla were investigated in urethane-anaesthetized rats. Electrical stimulation of the rostral ventrolateral medulla produced a significant increase in systolic blood pressure. This response was significantly attenuated by the prior administration of d,l-propranolol (20 micrograms), clonidine (8 micrograms), atropine (8 micrograms) or methysergide (10 micrograms) into the posterior hypothalamus, but not by cimetidine (11 micrograms), chlorpheniramine (12 micrograms), naloxone (10 micrograms) or a vasopressin V1 antagonist (100 ng). The effect of clonidine (8 micrograms) on the pressor response to stimulation of the rostral ventrolateral medulla was antagonized by idazoxan (66 micrograms). These results confirm that the cardiovascular changes elicited by stimulation of the rostral ventrolateral medulla area are, in part, centrally modulated by alpha 2 and beta-adrenoceptors in the posterior hypothalamus which exert respectively, inhibitory and stimulatory effect. Furthermore the results indicate the involvement of posterior hypothalamic cholinergic and serotonergic receptors in the pressor response produced by stimulation of the rostral ventrolateral medulla.

Electrophysiological characterization of putative C1 adrenergic neurons in the rat

Recent studies in the rat have demonstrated that at least two populations of sympathoexcitatory reticulospinal neurons reside in the nucleus reticularis rostroventrolateralis. It appears that only one of these populations consists of C1 adrenergic neurons. The present study used both double-labeling (one retrograde tracer and immunohistochemistry) and triple-labeling (two retrograde tracers and immunohistochemistry) to determine if C1 adrenergic neurons, which are immunoreactive for phenylethanolamine N-methyltransferase, exhibit a projection pattern that is sufficiently unique to permit the electrophysiological discrimination between C1 adrenergic and non-adrenergic neurons in the nucleus reticularis rostroventrolateralis. Double-labeling experiments indicated that 71% (range: 53-80) of phenylethanolamine-N-methyltransferase-immunoreactive neurons in the nucleus reticularis rostroventrolateralis could be retrogradely labeled from the thoracic cord, as were 76% (range: 67-94) following tracer injection in the central tegmental tract at pontine levels. Triple-labeling experiments indicated that 88% (range: 82-93) of nucleus reticularis rostroventrolateralis neurons with projections to both spinal cord and central tegmental tract were phenylethanolamine-N-methyltransferase-immunoreactive. Single-unit recording, in nucleus reticularis rostroventrolateralis, was used to identify antidromic potentials elicted from stimulation sites in the spinal cord and/or central tegmental tract. Since clonidine is known to reduce central adrenaline turnover, sensitivity to this drug was used to identify putative adrenergic neurons. Twenty-six nucleus reticularis rostroventrolateralis neurons with axonal projections to both the ipsilateral spinal cord and the central tegmental tract were recorded in halothane-anesthetized rats. All these cells were barosensitive, pulse-modulated, and 16 of the 16 cells tested exhibited a 66 +/- 8% reduction in activity upon the intravenous administration of clonidine (20 micrograms/kg). Most (13 out of 16) exhibited a strong respiratory modulation. The conduction velocity of their spinal collateral was generally low (0.9 +/- 0.1 m/s) and their firing rate moderate (7.4 +/- 1.2 spikes/s). Forty-three nucleus reticularis rostroventrolateralis cells with axonal projections exclusively to the thoracic cord were studied for comparison. These cells were strongly barosensitive and pulse-synchronous, had a high discharge rate (25 +/- 3 spikes/s) and a moderate conduction velocity (3.4 +/- 0.3 m/s). Only one of the 15 cells tested was inhibited by clonidine and only two to these 15 cells exhibited a detectable respiratory modulation. Thus barosensitive nucleus reticularis rostroventrolateralis neurons with axonal projections to both the spinal cord and the central tegmental tract likely belong to the C1 adrenergic cell group. It is concluded that this subgroup of adrenergic neurons probably subserves a vasomotor function.