Topographical organization of the rat suprachiasmatic-paraventricular projection

Indirect projections from the suprachiasmatic nucleus to the ventrolateral preoptic nucleus: a dual tract-tracing study in rat

The suprachiasmatic nucleus (SCN) contains a master clock for most circadian rhythms in mammals, including daily sleep-wake cycles. The ventrolateral preoptic nucleus (VLPO) plays a key role in sleep generation and, as such, might be an important target of the SCN circadian signal. However, direct SCN projections to the VLPO are limited, suggesting that most of the SCN output to the VLPO might be conveyed indirectly. We examined this possibility by microinjecting selected known major targets of SCN efferents with biotinylated dextran-amine and/or cholera toxin B subunit, followed by analyses of retrograde labelling in the SCN and anterograde labelling in the VLPO. Retrograde labelling results confirmed that the medial preoptic area, subparaventricular zone, dorsomedial hypothalamic nucleus and posterior hypothalamic area all received projections from the SCN; these projections arose predominantly from the shell, as opposed to the core, of the SCN. Anterograde labelling results indicated that these same nuclei also projected to the VLPO, mainly its medial and ventral aspects. Comparison of the results of injections of similar sizes across different target groups indicated that the rostral part of the medial preoptic area and the caudal part of the dorsomedial hypothalamic nucleus were particularly noteworthy for the abundance of both SCN source neurons and efferent fibres and terminals in the VLPO. These results suggest that the SCN might provide indirect input to the VLPO via the medial preoptic area and the dorsomedial hypothalamic nucleus, and that these indirect neuronal pathways might play a major role in circadian control of sleep-wake cycles.

A subpopulation of efferent neurons in the mouse suprachiasmatic nucleus is also light responsive

The suprachiasmatic nucleus (SCN) is the site of a circadian clock with input (afferent) pathways for photic entrainment and output (efferent) pathways for expression of overt, measurable rhythms. To determine whether there are individual neurons in the mouse SCN that might be part of both pathways, we performed double-label immunohistochemistry for light-induced c-Fos and the retrograde tracer cholera toxin subunit B (CtB), 2 weeks after CtB was iontophoresed into the subparaventricular area (subPVA). A minority of neurons was found that were both efferent to the subPVA and responsive to light. This cellular subset may function as a direct channel through the SCN for photic inputs to influence neural outputs, and its existence highlights the topographical heterogeneity of SCN tissue.

Local circuit regulation of paraventricular nucleus stress integration: glutamate-GABA connections

Limbic neurocircuits play a central role in regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Limbic influences on adrenocortical hormone secretion are mediated by transynaptic activation or inhibition of hypophysiotrophic neurons in the medial parvocellular paraventricular nucleus (PVN). Projections from the ventral subiculum, prefrontal cortex, medial amygdala, lateral septum, paraventricular thalamus and suprachiasmatic nucleus (SN) terminate in the immediate surround of the PVN, an area heavily populated by GABAergic interneurons. As such, these regions are positioned to modulate paraventricular output via excitation or inhibition of interneuronal projections into the PVN. In addition, the same limbic and diencephalic regions have projections to local PVN-projecting hypothalamic and basal telencephalic nuclei, including the dorsomedial and medial preoptic nuclei and the bed nucleus of the stria terminalis. These regions are involved in both inhibitory and excitatory regulation of the stress axis, indicating that they contain heterogeneous neuronal populations whose relative impact on the PVN is determined by the nature of afferent stimuli. Thus, limbic modulation of the pituitary-adrenocortical system appears to be a multisynaptic process integrated at the level of local PVN-projecting neurocircuits. Local circuits are likely the primary integrators of anticipatory stress responses, and may indeed be the focus of HPA dysfunction seen with aging or affective disease.

Light-induced c-Fos expression in suprachiasmatic nuclei neurons targeting the paraventricular nucleus of the hamster hypothalamus: phase dependence and immunochemical identification

The suprachiasmatic nuclei (SCN) contain a master clock driving the majority of circadian rhythms in mammals. It is believed that the SCN confers circadian rhythmicity as well as light responsiveness to pineal melatonin secretion via a direct projection to the paraventricular nucleus of the hypothalamus (PVN). Neurons in the SCN respond to light during subjective night with an expression of the immediate early gene c-fos. The number and distribution of c-Fos protein-containing neurons depend on the zeitgeber time (ZT) at which the light stimulus is presented. To investigate whether this phase-dependent activity is present in the SCN output neurons targeting the PVN, we combined retrograde cholera toxin subunit B (ChB) tracing from the PVN with c-Fos immunohistochemistry. Male golden hamsters were injected iontophoretically with ChB into the PVN area and 7 days later given a 1.5-hr light stimulus at either ZT 14 or ZT 19 followed by vascular fixation. Light stimulation at ZT 19 gave rise to more c-Fos containing neurons in the SCN than light presented at ZT 14. Double immunostaining for ChB and c-Fos revealed that light stimulation at ZT 14 induced c-Fos expression in 26.6% +/- 2.8% of the retrogradely filled perikarya, whereas light-stimulation at ZT 19 increased this fraction to 40.7% +/- 1.9%. This demonstrates the presence of a phase-dependent c-Fos induction in the suprachiasmatic-paraventricular projection system. Triple immunohistochemistry showed that light-activated output neurons contained both gastrin-releasing peptide and vasoactive intestinal polypeptide and to a lesser extent vasopressin. The present findings provide functional evidence of light activation of central pathways involved in the regulation of circadian output rhythms.

How does the circadian clock send timing information to the brain?

This paper discusses circadian output in terms of the signaling mechanisms used by circadian pacemaker neurons. In mammals, the suprachiasmatic nucleus houses a clock controlling several rhythmic events. This nucleus contains one or more pacemaker circuits, and exhibits diversity in transmitter content and in axonal projections. In Drosophila, a comparable circadian clock is located among period -expressing neurons, a sub-set of which (called LN-vs) express the neuropeptide PDF. Genetic experiments indicate LN-vs are the primary pacemakers neurons controlling daily locomotion and that PDF is the principal circadian transmitter. Further definition of pacemaker properties in several model systems will provide a useful basis with which to describe circadian output mechanisms.

Contrasting effects of ibotenate lesions of the paraventricular nucleus and subparaventricular zone on sleep-wake cycle and temperature regulation

The suprachiasmatic nucleus (SCN), the circadian pacemaker for the brain, provides a massive projection to the subparaventricular zone (SPZ), but the role of the SPZ in circadian processes has received little attention. We examined the effects on circadian rhythms of sleep, body temperature, and activity in rats of restricted ibotenic acid lesions of the ventral or dorsal SPZ that spared the immediately adjacent paraventricular hypothalamic nucleus (PVH) and the SCN. Ventral SPZ lesions caused profound reduction of measures of circadian index of sleep (by 90%) and locomotor activity (75% reduction) but had less effect on body temperature (50% reduction); dorsal SPZ lesions caused greater reduction of circadian index of body temperature (by 70%) but had less effect on circadian index of locomotor activity (45% reduction) or sleep (<5% reduction). The loss of circadian regulation of body temperature or sleep was replaced by a strong ultradian rhythm (period approximately 3 hr). Lesions of the PVH, immediately dorsal to the SPZ, had no significant effect on any circadian rhythms that we measured, nor did the lesions affect the baseline body temperature. However, the fever response after intravenous injection of lipopolysaccharide (5 microg/kg) was markedly decreased in the rats with PVH lesions (66.6%) but not dorsal SPZ lesions. These results indicate that circadian rhythms of sleep and body temperatures are regulated by separate neuronal populations in the SPZ, and different aspects of thermoregulation (circadian rhythm and fever response) are controlled by distinct anatomical substrates.

Topographic organization of suprachiasmatic nucleus projection neurons

The mammalian circadian pacemaker, the hypothalamic suprachiasmatic nucleus (SCN), has two subdivisions. The core is located above the optic chiasm, receives primary and secondary visual afferents, and contains neurons producing vasoactive intestinal polypeptide and gastrin-releasing peptide. The shell largely surrounds the core, receives input from nonvisual sources and contains neurons producing arginine vasopressin and calretinin. In this study, we tested the hypothesis that SCN efferent projections are topographically organized with respect to the subdivision of origin. Injections of retrograde tracers were placed in major sites of efferent termination, described from prior studies that used anterograde tracers (Watts and Swanson, [1987] J. Comp. Neurol. 258:230-252; Watts et al. [1987] J. Comp. Neurol. 258:204-229). After retrograde tracer injections in the medial preoptic area, dorsomedial and paraventricular hypothalamic nuclei, bed nucleus of stria terminalis, paraventricular thalamic nucleus, zona incerta, and medial subparaventricular zone, retrogradely labeled SCN cells are clustered in the shell with few labeled neurons in the core. After injections centered in the lateral subparaventricular zone, peri-suprachiasmatic region, lateral septum, or ventral tuberal area, the majority of neuronal label is in the core with moderate to sparse neuronal label in the shell. Both subdivisions are labeled after injections in the paratenial thalamic nucleus. The same pattern of retrograde labeling is found with four tracers, cholera toxin-beta subunit, Fluoro-Gold, the Bartha strain of pseudorabies virus, and biotinylated dextran amine. These data extend our understanding of the significance of the division of the SCN into shell and core by demonstrating that the subdivisions differ in the pattern of projections. Together with prior observations that the subdivisions differ with respect to afferents, local connections, and neuroactive substances, the present study provides an anatomic basis for discrete control of circadian function by the SCN core and shell. In this novel view, the nature of the signal conveyed to areas receiving core or shell projections varies as a function of the subdivision from which innervation is derived.

Pseudorabies virus and the functional architecture of the circadian timing system

Transneuronal tracing of neuronal circuitry with neurotropic viruses has provided valuable insights in the way in which the nervous system imposes temporal organization on physiological processes and behavior. The swine alpha herpes virus known as pseudorabies virus, or PRV, has been particularly useful in this regard. Early studies identified attenuated mutants with selective tropism for visual circuitry involved in circadian regulation, and subsequent experiments employing this virus have provided considerable insight into the polysynaptic organization of the suprachiasmatic nuclei and associated circuitry. This literature, which has emerged during the past decade, is the subject of this mini review.

Neural connections of hypothalamic neuroendocrine nuclei in the rat

The secretion of many hormones, including oxytocin, vasopressin and growth hormone, is not constant but shows a day-night rhythm. The suprachiasmatic nucleus (SCN) is thought to generate most mammalian biological rhythms and previous studies have reported suprachiasmatic efferents to the paraventricular nucleus (PVN) and the supraoptic nucleus (SON). We used in vivo extracellular electrophysiological techniques to show that the SCN also sends direct and indirect neural projections to the arcuate nucleus (ARC). This projection consisted of both excitatory and inhibitory components and may contribute to the entrainment of the rhythm in growth hormone secretion to the day-night cycle. Some SCN neurones appear to project to both the SON and the ARC. The SCN in turn receives excitatory and inhibitory inputs from the ARC and the peri-nuclear zone of the SON (peri-SON), which may provide feedback information, as well as allowing nonphotic entrainment of the SCN, for example, in response to feeding. Our data thus suggest extensive two-way connections between the SCN and its target nuclei which may contribute to the generation of day-night neuroendocrine rhythms. They also suggest the existence of indirect retinal projections to the ARC and PVN. We further investigated the retinal projection to the SCN. We were unable to demonstrate a significant difference in retinal input to those suprachiasmatic cells which had efferent projections to particular hypothalamic targets (SON and/or ARC), and those which did not.

GABA(B) presynaptically modulates suprachiasmatic input to hypothalamic paraventricular magnocellular neurons

This study used whole cell patch clamp recordings in rat hypothalamic slice preparations to evaluate the effects of GABA(B) receptor activation on GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) in paraventricular nucleus magnocellular neurons evoked by electrical stimulation in the suprachiasmatic nucleus (SCN). Baclofen induced a dose-dependent (1-10 microM) and reversible reduction in SCN-evoked IPSC amplitude (11/11 cells), blockable with 2-hydroxysaclofen (300 microM; 3/3 cells). IPSCs displayed paired-pulse depression (PPD), attenuated by both baclofen and 2-hydroxysaclofen, but neither altered resting membrane conductances or IPSC time constants of decay. Baclofen induced a significant dose-dependent (1-100 microM) reduction in frequency, but not amplitude, of spontaneous IPSCs and miniature IPSCs, reversible with 2-hydroxysaclofen pretreatment. Baclofen effects and PPD persisted in slices pretreated with pertussis toxin (PTX) and N-ethylmaleimide, implying that these GABA(B) receptors are coupled to PTX-insensitive G proteins. Responses were unaltered by barium (2 mM) or nimodipine, ruling out involvement of K(+) channels and L-type Ca(2+) channels. Thus pre- and postsynaptic GABA(B) and GABA(A) receptors participate in SCN entrainment of paraventricular neurosecretory neurons.

Topographical organization of projections from the subiculum to the hypothalamus in the rat

The projections from the subiculum to the hypothalamus were comprehensively examined in the rat by using the anterograde Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde cholera toxin B subunit (CTb) methods. Tracing of efferents with PHA-L indicated that the medial preoptic region received projection fibers from the temporal two-thirds of the subiculum, whereas the anterior, tuberal, and mammillary regions received those from the full longitudinal extent of the subiculum. The subicular projections to the anterior and tuberal hypothalamic regions were also found to be organized in a topographical manner such that the temporal-to-septal axis of origin in the subiculum determined a ventromedial-to-dorsolateral axis of termination in the medial zone of the hypothalamus: Massive labeled fibers from the temporalmost part of the subiculum terminated in the subparaventricular zone and its caudal continuum around the dorsal and medial aspects of the ventromedial nucleus, and those from progressively more septal parts terminated in progressively more dorsolateral parts of the medial zone. In addition, the temporal-to-septal axis of origin in the subiculum tended to determine a medial-to-lateral axis of termination in the preoptic region as well as a ventral-to-dorsal axis of termination in the mammillary region. Furthermore, the temporal-to-septal axis of origin in the septal two-thirds of the subiculum corresponded to a ventrolateral-to-dorsomedial axis of termination in the medial mammillary nucleus. The topographical projections from the subiculum to the medial zone of the hypothalamus were confirmed by CTb experiments, representatively in the subicular projections to the anterior hypothalamic region. These results suggest that different populations of neurons existing along the longitudinal axis of the subiculum may exert their influences on the execution of different hypothalamic functions.

Tracing autonomic innervation of the rat pineal gland using viral transneuronal tracing

We have used the neurotropic Bartha strain of pseudorabies virus (PRV) to characterise the pathway linking the endogenous circadian pacemaker of the suprachiasmatic nucleus (SCN) to the pineal gland. This low virulent strain of virus replicates within synaptically linked neurones and is ideally suited to visualise the multisynaptic pathways through which the SCN modulates the activity of the rat pineal gland. Using specific antibodies against PRV, we could follow the immunohistochemical pattern of the spatiotemporal passage of virus through the sympathetic trunk and the neuraxis. The time course of virus infection indicated that the most prominent pathway from the SCN to the pineal gland is via a final sympathetic innervation from the superior cervical ganglion (SCG). The pathway arises in the dorsomedial portion of the SCN from where neurones project to the dorsal parvicellular subdivision of the hypothalamic paraventricular nucleus (PVN) to form synaptic contact with neurones descending to the intermediolateral nucleus (IML) of the upper thoracic spinal cord. The neurones of the IML constitute the presynaptic sympathetic input synaptically connected to postsynaptic sympathetic neurones in the SCG which constitute the final input to the pineal gland. Removal of the superior cervical ganglion (SCGX) prior to viral infection completely abolished infection of neurones in this circuit. However, an additional parasympathetic projection from the superior salivatory nucleus via the sphenopalatine ganglion to the pineal gland was observed in SCGX animals.

GABA release from suprachiasmatic nucleus terminals is necessary for the light-induced inhibition of nocturnal melatonin release in the rat

The daily rhythm of melatonin production in the mammalian pineal is driven by the endogenous circadian pacemaker in the suprachiasmatic nuclei. The major release period of melatonin is closely linked to the dark phase of the 24-h day/night cycle. Environmental light will affect melatonin release in two ways: (i) it entrains the rhythm of the circadian oscillator; and (ii) it causes an acute suppression of nocturnal melatonin release. These two effects of light are both mediated by the suprachiasmatic nucleus and enable the pineal gland to convey information about day length to the reproductive system through changes in melatonin levels. Glutamate is currently believed to be the major transmitter in the retinal ganglion cell fibers reaching the suprachiasmatic nucleus. At present no information is available, however, about the transmitter(s) implicated in the further propagation, i.e. from the suprachiasmatic nucleus onwards, of the light information. In the present study we provide evidence that the endogenous release of GABA from suprachiasmatic nucleus terminals is implicated in the further transmission of light information to the pineal gland. Bilateral administration of the GABA-antagonist bicuculline to hypothalamic target areas of the suprachiasmatic nucleus completely prevents the inhibitory effect of nocturnal light on melatonin secretion and the present study thus documents that retina-mediated photic activation of suprachiasmatic nucleus neurons induces the release of GABA from efferent suprachiasmatic nucleus nerve terminals, resulting in an inhibition of melatonin release by the pineal gland. Together with our previous (electro)physiological data these results identify GABA as an important mediator of rapid synaptic transmission of suprachiasmatic nucleus output to its target areas.

Interleukin-1beta and calcitonin, but not corticotropin-releasing factor, alter sleep cycles when injected into the rat hypothalamic lateral paraventricular area

Interleukin-1beta (II-1beta) is a cytokine known to have somnogenic properties. We have previously shown that II-1beta decreases food intake when injected into the lateral part of the paraventricular nucleus of the hypothalamus (PVH), and, because food intake and sleep are closely related behaviors, we tested the hypothesis that II-1beta could alter sleep when injected into the lateral PVH area. We compared the effects of II-1beta with those of two other peptides involved in feeding behavior and known to act in the PVH area, the corticotropin-releasing factor (CRF) and salmon calcitonin (sCT). The EEG of rats was recorded for 48 h after the injection. The results showed that CRF had no effects, II-1beta reduced significantly sleep duration during the first 5 h following the injection, and sCT profoundly affected sleep cycles, producing an almost 30-h long insomnia, with a major reduction of slow wave sleep and a long period of alternation of REM sleep and wakening. It is concluded that (i) the area between the lateral part of the PVH and the fornix is a brain site involved in sleep regulation, (ii) II-1beta, a peptide generally considered as somnogenic, decreases sleep when administered in this area, and (iii) sCT is an extremely potent suppressor of slow wave sleep.

The suprachiasmatic nucleus-paraventricular nucleus interactions: a bridge to the neuroendocrine and autonomic nervous system

Vasopressin (VP) is one of the principal neurotransmitters of the suprachiasmatic nucleus (SCN). By means of anatomical, physiological and electrophysiological techniques we have demonstrated that VP containing pathways from the SCN serve to affect neuroendocrine and 'autonomic' neurons in the paraventricular nucleus. By direct and indirect connections VP serves to inhibit corticosterone secretion, not only by affecting ACTH secretion but also by controlling the adrenal cortex via a neuronal route. Apart from controlling the pineal and adrenal, we also observed that the SCN is able to influence the heart. Subjecting rats or humans to light affects heart rate in a dose-dependent manner. These results suggest an important role for the SCN and VP in the SCN in the regulation of neuroendocrine and autonomic functions.

Suprachiasmatic pacemaker organization analyzed by viral transynaptic transport

The suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circadian pacemaker, is a paired structure with two subdivisions, a ventral core receiving photic input and a dorsal shell receiving non-photic input. Rhythmicity is thought to be generated by individual SCN neurons which are coupled to achieve synchrony [D.K. Welsh, D.E. Logothetis, M. Meister, S.M. Reppert, Individual neurons dissociated from rat suprachiasmatic nucleus express independently phased circadian firing patterns, Neuron, 14 (1995) 697-706]. Normally, the core and shell, and the nuclei on each side, act in unison to transmit rhythmicity to effector systems. It is not known how coupling between neurons in the two subdivisions, and between the two SCNs, takes place. In the present study, we analyze the intrinsic, commissural, and efferent projections of the SCN using the swine herpesvirus (pseudorabies virus, PRV) as a tool for transynaptic analysis of circuits and small iontophoretic injections of the conventional tracer horseradish peroxidase (HRP) conjugated to fluorescein. We find that the core and shell each project through commissural efferents to homologous contralateral areas. The core projects densely to shell but we find little reciprocal innervation. The two subdivisions project to different hypothalamic areas, with the core projecting to the lateral subparaventricular zone and shell to the dorsomedial hypothalamic nucleus and medial subparaventricular zone. These data are the first demonstration that connections within the SCN, and from the SCN to effector regions, are topographically organized and lend insight into the flow of information through and out of the pacemaker.

Postmortem anterograde tracing of intrahypothalamic projections of the human dorsomedial nucleus of the hypothalamus

Together with the paraventricular nucleus (PVN), the dorsomedial nucleus of the hypothalamus (DMH) acts as one of the hypothalamic centers that integrate autonomic and central information. The DMH in the rat brain has extensive intrahypothalamic connections and is implicated in a wide variety of functions. Up until now, no knowledge has been available to indicate that the human DMH might have functions similar to those of the rat DMH. In the present study, intrahypothalamic efferent projections of the human DMH were revealed by a recently developed in vitro postmortem tracing method. It was found that the most densely innervated areas are the PVN, the ventromedial nucleus of the hypothalamus, and the area below the PVN. Other significant terminal fields include the periventricular nucleus, the lateral hypothalamic area, and the medial part of the anteroventral hypothalamic area. Scarce fibers project to the suprachiasmatic nucleus, infundibular nucleus, posterior hypothalamic nucleus, and posterior part of the bed nucleus of the stria terminals. The projections of the ventral and dorsal part of the DMH show some differences. The dorsal part of the DMH has denser projections to the dorsal part of the PVN than to the ventral part of the PVN. In contrast, the ventral part of the DMH has denser projections to the ventral part of the PVN. Labeled fibers in the PVN from ventral and dorsal DMH appear to run near many vasopressin and oxytocin neurons of different sizes, and also near some corticotropin- releasing hormone neurons, suggesting that the DMH neurons may directly affect the functioning of these PVN neurons. In many aspects, the observed projections of the human DMH resemble those of the rat, indicating that the organization of DMH intrahypothalamic projections of human is similar to that of rat. The functional significance of DMH intrahypothalamic connections is discussed.

Dorsocaudal SCN microknife-cuts do not block short day responses in Siberian hamsters given melatonin infusions

Siberian hamsters (Phodopus sungorus sungorus) undergo photoperiod-induced physiological and behavioral adaptations. These adaptations, including changes in reproductive and metabolic status, are triggered by the pineal gland through the nocturnal secretion of its principal hormone, melatonin. The possible CNS sites of melatonin action determined through radiolabeled melatonin binding include the paraventricular and reuniens nuclei of the thalamus and the suprachiasmatic nucleus (SCN). However, we do not know the mechanisms and circuitry involved in the transmission of melatonin signals. Bilateral electrolytic lesions of the SCN (SCNx) block the responses to short day-like (long duration) melatonin signals delivered daily via the timed infusion paradigm, suggesting that the SCN receives and transmits short-day melatonin signals. The purpose of the present experiment was to answer the following question: are short-day melatonin signals transmitted to other brain structures from the SCN through its dorsomedial/dorsocaudal fiber projections? Pinealectomized adult male hamsters given horizontal knife cuts (kc) just dorsocaudal to the SCN (SCN-kc), sham-kc, or SCNx were given daily subcutaneous short day-like melatonin infusions via the timed infusion paradigm for 6 weeks. Only the hamsters given SCNx exhibited long day-like gonadal, epididymal fat pad, and body masses. Therefore, short day melatonin signals received by the SCN were not transmitted to other areas of the central nervous system through SCN efferents projecting dorsomedially or dorsocaudally.

Nicotine and nicotinic receptors in the circadian system

Considerable data support a role for cholinergic influences on the circadian system. The extent to which these influences are mediated by nicotinic acetylcholine receptors (nAChRs) has been controversial, as have the specific actions of nicotine and acetylcholine in the suprachiasmatic nucleus (SCN) of the hypothalamus. In this article we review the existing literature and present new data supporting an important role for nAChRs in both the developing and adult SCN. Specifically, we present data showing that nicotine is capable of causing phase shifts in the circadian rhythms of rats. Like light and carbachol, nicotine appears to cause phase delays in the early subjective night and phase advances in the late subjective night. In the isolated SCN slice, however, only phase advances are seen, and, surprisingly, nicotine appears to cause the inhibition rather than the excitation of neurons. Among nAChR subunit mRNAs, alpha 7 appears to be the most abundant subunit in the adult SCN, whereas in the perinatal period, the more typical nAChRs with higher affinity for nicotine predominate in the SCN. This developmental change in subunit expression may explain the dramatic sensitivity of the perinatal SCN to nicotine that we have previously observed. The effects of nicotine on the SCN may contribute to alterations caused by nicotine in other physiological systems. These effects might also contribute to the dependence properties of nicotine through influences on arousal.

Direct link from the suprachiasmatic nucleus to hypothalamic neurons projecting to the spinal cord: a combined tracing study using cholera toxin subunit B and Phaseolus vulgaris-leucoagglutinin

By combining retrograde and anterograde tracing, evidence for a bineuronal connection from the suprachiasmatic nucleus (SCN) to the intermediolateral cell column in the spinal cord (IML) was obtained. The retrograde tracer cholera toxin subunit B (ChB) was pressure-injected into the spinal cord and the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was iontophoretically injected into the SCN. The two tracers were visualized simultaneously by a double immunohistochemical procedure. In the hypothalamus, ChB injections gave rise to retrogradely labeled cell bodies in the paraventricular nucleus, retrochiasmatic area, perifornical region, lateral hypothalamic area, and the posterior hypothalamic area. The SCN were found to project to all of these areas. Furthermore, spinal-projecting neurons were found in the brain stem, but no efferents from the SCN were observed to innervate these areas. In the most sparsely innervated areas, the lateral hypothalamic area and the perifornical region, only occasionally a PHA-L fiber in close apposition to a ChB-ir cell body was observed. This was also the case in the retrochiasmatic area and posterior hypothalamic area, although these areas received a moderate number-immunoreactive (ir) PHA-L-ir fibers. The highest number of closely apposed PHA-L-ir fibers and ChB-ir cell bodies was observed in the dorsal parvicellular and in the ventral division of the medial parvicellular paraventricular nucleus, which were also the areas receiving the densest input from the SCN. By anterograde tracing from the paraventricular nucleus of the hypothalamus, the exact topography of the terminal field formed by descending paraventricular neurons was established. Thus, it was confirmed that the paraventricular nucleus of the hypothalamus predominantly innervates the IML. The present study suggests the existence of a bineuronal link between the SCN and the IML, possibly involved in transmission of circadian signals from the endogenous clock to the pineal gland and other organs receiving sympathetic afferents.

Characterization of the multisynaptic neuronal control of the rat pineal gland using viral transneuronal tracing

Knowledge of the polysynaptic pathway conveying photic information to the pineal gland is based upon studies employing lesions, knife cuts and classical tracers. In the present investigation we used viral transneuronal tracing to re-examine the organization of this circuitry. This was accomplished by injecting a neurotropic alpha herpesvirus (pseudorabies virus) into the gland and localizing viral antigen in infected neurones at various postinoculation intervals. This approach is based upon the demonstrated ability of this virus to invade axon terminals, replicate in neurones and pass retrogradely through a multisynaptic circuit. Immunohistochemical localization of viral antigen revealed the progressive appearance of infected neurones in the superior cervical ganglion (SCG), intermediolateral nucleus of the upper thoracic spinal cord (IML), parvicellular subdivisions of the hypothalamic paraventricular nucleus (PVN), and the suprachiasmatic nucleus (SCN). Other infected cell groups known to project to the IML also became infected. Infection of the PVN reproducibly involved neurones in the dorsal, medial and lateral parvicellular subdivisions and preceded the appearance of infected neurones in the SCN and other regions of hypothalamus. Topographic analysis of virus infected neurones within the SCN revealed differential infection of SCN subdivisions that suggested topography in the projection of the SCN to the PVN. Removal of the SCG eliminated infection within the aforementioned circuitry and revealed a parasympathetic innervation from the sphenopalatine ganglion. The data provide further detail on the cellular identity and synaptology of neural circuitry controlling the rhythmic secretion of melatonin by the rat pineal gland.

The structural organization of connections between hypothalamus and cerebral cortex

Motivated behavior requires coordinated somatic, autonomic, and endocrine responses, and may be divided into initiation, procurement, and consummatory phases (Swanson, L.W. and Mogenson, G.J., Neural mechanisms for the functional coupling of autonomic, endocrine and somatomotor responses in adaptative behavior, Brain Res. Rev., 3 (1981) 1-34). Obviously, such behavior may involve the entire central nervous system, although it is important to identify circuitry or systems that mediate the behavior directed toward specific goal objects. This problem has recently been clarified by the identification of hypothalamic subsystems important for the execution of instinctive behaviors related to ingestion, reproduction, and defense. These subsystems are modulated by sensory (reflex), central control (e.g., circadian), and voluntary (cortical) inputs. The latter are dominated by inputs from the ventral temporal lobe and medial prefrontal region, which are both direct and via associated parts of the basal nuclei (ganglia). Hypothalamic output is characterized by descending projections to brainstem and spinal motor systems, and by projections back to the cerebral cortex, which are both direct and via a continuous rostromedial part of the dorsal thalamus. This thalamic region includes the anterior, medial, and midline groups, which in turn innervate a continuous ring of cortex that includes the hippocampal formation and the cingulate, prefrontal, and insular regions. Parts of this thalamic region also innervate the ventral striatum, which receives a massive input from the cortical rings as well.

Innervation of the sheep pineal gland by nonsympathetic nerve fibers containing NADPH-diaphorase activity

We used the NADPH-diaphorase histochemical method as a potential marker for nitric oxide synthase (NOS)-containing nerve fibers innervating the pineal gland of the sheep. Nerve fibers containing NADPH-diaphorase activity provide dense innervation of the sheep pineal gland. The nerve fibers were located in the pineal capsule, in the connective tissue septae separating the lobull of the gland, and penetrating between the pinealocytes. The nerve fibers were either smooth or endowed with boutons en passant. After bilateral removal of the superior cervical ganglion, the dense network of NADPH-diaphorase-positive fibers was still present in the gland. Ganglionectomy affected neither the distribution nor the appearance of the NADPH-diaphorase-positive fibers. Most of the NADPH-diaphorase-positive fibers also contained peptide histidine isoleucine and vasoactive intestinal polypeptide, and a comparatively smaller fraction contained neuropeptide Y. Pinealocytes never exhibited NADPH-diaphorase activity. These results demonstrate a major neural input to the sheep pineal gland with NADPH-diaphorase-positive nerve fibers of nonsympathetic origin.

Distribution of vasopressin and vasoactive intestinal polypeptide (VIP) fibers in the human hypothalamus with special emphasis on suprachiasmatic nucleus efferent projections

The human suprachiasmatic nucleus (SCN) is located in the basal part of the anterior hypothalamus and is considered as the biological clock that generates circadian rhythms and synchronizes the daily activity pattern with the environmental light-dark cycle. However, the mechanisms and pathways by which the SCN transmits its information to the other brain areas are unknown. Therefore, in the present study, we investigated the efferent projections of the SCN by the immunocytochemical staining of two major peptidergic SCN neurotransmitters: vasopressin (VP) and vasoactive intestinal polypeptide (VIP). It confirmed that these peptides are present in different subdivisions of the SCN. The results of this investigation show that VP and VIP fibers arising from the SCN were detected to branch extensively and hence seem to innervate the SCN itself and the central and medial part of the anteroventral hypothalamic area (AVH), the area below the paraventricular nucleus (sub-PVN), the ventral part of the paraventricular nucleus (PVN), and the dorsomedial nucleus of the hypothalamus (DMH). There appeared to be substantial congruity between the presumptive human SCN projections and those as observed by tracing in rat or hamster. Regarding the anatomical organization of the human SCN projections, the main projection areas appeared to be the AVH, the sub-PVN, the ventral part of the PVN, and the DMH. The observation that VIP and in particular VP fibers pass between the SCN and the PVN suggests that the human SCN and the PVN may have a direct anatomical connection. In addition, VP and VIP fibers were detected in several other hypothalamic areas that are not known to have clear direct connections to the SCN. The possible origin of these VP and VIP fibers is discussed.

Neurones in the supraoptic nucleus of the rat are regulated by a projection from the suprachiasmatic nucleus

1. In the rat, projections from the suprachiasmatic nucleus (SCN) to the supraoptic nucleus (SON) of the hypothalamus were characterized in vivo using extracellular recordings and in slice preparations using both extracellular and whole-cell patch clamp recording. 2. Of 117 magnocellular neurones recorded in the SON in vivo, fifteen (13%) displayed a short latency excitation, sixty-eight (58%) a short latency inhibition, six (5%) were unresponsive and twenty-eight (24%) gave long latency responses following SCN stimulation. 3. The responses of putative vasopressin cells in the SON to SCN stimulation in vivo (4 out of 61 cells, 7% excited; 49 out of 61 cells, 80% inhibited) were significantly different from those of putative oxytocin cells (10 out of 50 cells, 20% excited and 16 out of 50 cells, 32% inhibited; P < 0.02, test for differences between proportions). 4. Recordings in vitro using patch technology in whole-cell mode showed both inward and outward currents in SON cells at holding potentials near resting membrane potential following stimulation of the SCN region. The outward currents could be blocked by bicuculline (10 microM; n = 7) and the inward currents were blocked by the non-NMDA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (5 microM; n = 4). 5. We conclude that there is a strong projection from the SCN to the SON with both inhibitory (GABAergic) and excitatory (glutamatergic) components which may regulate the daily changes in neurohypophysial hormone secretion.

Synaptic contacts between gonadotropin-releasing hormone-containing fibers and neurons in the suprachiasmatic nucleus and perichiasmatic area: an anatomical substrate for feedback regulation?

The suprachiasmatic nucleus (SCN) is critically involved in the generation and entrainment of circadian rhythms in mammalian species. Both the occurrence and the timing of the luteinizing hormone surge on the afternoon of proestrus in the female rodent are critically dependent on the integrity of the SCN. Recently, we demonstrated the presence of a monosynaptic pathway from the SCN to the gonadotropin releasing hormone (GnRH) neurons in the preoptic area. In addition, we found that interaction between the SCN and the GnRH system may be found close to the SCN, since we observed apposition of SCN efferents and GnRH fibers at the ultrastructural level in that region. The aim of the present study was to investigate the presence of synaptic contacts between GnRH fibers and structures in the SCN and surrounding perichiasmatic area (periSCN). At the light microscopical level, the immunoreactivity for GnRH showed a considerable overlap with the immunoreactivity for vasopressin and vasoactive intestinal peptide, two neuropeptides synthesized by SCN neurons. At the ultrastructural level, we demonstrated synaptic input of GnRH-containing axons on immunocytochemically unidentified structures in the SCN/peri-SCN region. The present results clearly demonstrate that the SCN and periSCN are postsynaptic targets of GnRH fibers. It is hypothesized that the GnRH input in the SCN region represents an anatomical substrate for feedback-control between these systems.

Physiological mapping of local inhibitory inputs to the hypothalamic paraventricular nucleus

Local inhibitory synaptic inputs to neurons of the rat hypothalamic paraventricular nucleus (PVN) were studied by using glutamate microstimulation and conventional intracellular and whole-cell patch-clamp recording in coronal, horizontal, and parasagittal slices of rat hypothalamus. PVN cells were classified as magnocellular or parvocellular neurons on the basis of electrophysiological and post hoc immunohistochemical analyses; GABA-producing neurons were localized with in situ hybridization. Glutamate microstimulation of different sites around the PVN evoked volleys of postsynaptic potentials in 43% of the PVN neurons tested. Some responses to stimulation at each site were blocked by bicuculline, suggesting that they were mediated by the activation of presynaptic GABA neurons. In the coronal plane, presynaptic inhibitory sites were located lateral to the PVN and ventral to the fornix, corresponding to the lateral hypothalamic area and the posterior bed nucleus of the stria terminalis (BNST). In the horizontal plane, presynaptic inhibitory sites were found rostral, lateral, and caudal to the nucleus, corresponding to parts of the anterior hypothalamic area, the posterior BNST, the medial preoptic area, and the dorsomedial hypothalamus. In the parasagittal plane, presynaptic inhibitory neurons were revealed at sites rostral and caudal to the nucleus, corresponding to the medial preoptic area and the dorsomedial hypothalamus, and in a site dorsal to the optic chiasm that included the suprachiasmatic nucleus. These presynaptic sites each contained GABA-producing neurons based on in situ hybridization with a glutamic acid decarboxylase riboprobe and together formed a three-dimensional ring around the PVN. Unexpectedly, both magnocellular and parvocellular neurons received inhibitory synaptic inputs from common sites.

Persistence of nonphotic phase shifts in hamsters after serotonin depletion in the suprachiasmatic nucleus

Serotonin-containing fibres (5-HT) project from the raphe complex to the suprachiasmatic nucleus (SCN). Previous studies have suggested that this pathway may be involved in nonphotic resetting of the circadian clock. For example, 5-HT agonists are capable of phase shifting the biological clock both in vivo and in vitro, producing phase response curves (PRCs) similar in shape to those of other nonphotic stimuli. Therefore we studied the role of the serotonergic projection to the SCN in nonphotic phase shifts by bilateral injection of the selective 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) onto the SCN of hamsters. About 50 days after the administration of the neurotoxin, the 5-HT and 5-HIAA (5-hydroxyindole acetic acid) levels were severely depleted in the SCN, as revealed by high performance liquid chromatography (HPLC), and immunocytochemistry (ICC). The average level of 5-HT depletion was 88% in Experiment 1 and 95% in Experiment 2. This treatment had no effect on the magnitude of phase shifts produced by 3 h of novelty-induced wheel-running starting at circadian time (CT) 4, the peak of the advance region of the PRC to this stimulus. The effect of 5-HT depletion on shifts produced by running at CT 22 were inconclusive because of changes in the behavior of control animals. No changes in the phase angle of entrainment of animals in a 14:10 light:dark (LD) cycle were detected in depleted animals. The results suggest that the 5-HT projection from the raphe to the SCN is not essential for activity-induced phase shifts in hamsters.

Anterograde, transneuronal transport of herpes simplex virus type 1 strain H129 in the murine visual system

Herpes simplex virus (HSV) undergoes retrograde and anterograde axonal transport as it establishes latency and later intermittently reactivates. Most strains of HSV show preferential retrograde transport within the central nervous system (CNS), however. Previous experiments suggest that an exception to this is HSV type 1 (HSV-1) strain H129, since this virus appears to spread primarily in the CNS via anterograde, transneuronal movement. The objective of the present study was to test how specifically this virus spreads in the visual system, a system with well-described neuronal connections. In the present study, the pattern of viral spread was examined following inoculation into the murine vitreous body. Virus was initially detected in the retina and optic tract. Virus then appeared in all known primary targets of the retina, including those in the thalamus (e.g., lateral geniculate complex), hypothalamus (suprachiasmatic nucleus), and superior colliculus (superficial layers). In previous studies, many strains of HSV were shown to infect these structures, even though they spread predominantly in a retrograde direction. However, the H129 strain was unique in then spreading, via anterograde transport, to the primary visual cortex (layer 4 of area 17) via thalamocortical connections. At later times after infection, specific labeling was also detected in other cortical and subcortical areas known to receive projections from the visual cortex. No labeling was ever detected in the contralateral retina, which is consistent with a lack of retrograde spread of HSV-1 strain H129. These results demonstrate the specific anterograde movement of this virus from the retina to subcortical and cortical regions, with no clear evidence for retrograde spread. HSV-1 strain H129 should be generally useful for tracing sensory pathways and may provide the basis for designing a virus vector capable of delivering genetic material via anterograde pathways within the CNS.

State-dependent cellular activity patterns of the cat paraventricular hypothalamus measured by reflectance imaging

Activity within the cat paraventricular hypothalamus (PVH) during sleep and waking states was measured by quantifying intrinsic tissue reflectivity. A fiber optic probe consisting of a 1.0 mm coherent image conduit, surrounded by plastic fibers which conducted 660 nm source light, was attached to a charge-coupled device camera, and positioned over the PVH in five cats. Electrodes for assessing state variables, including electroencephalographic activity, eye movement, and somatic muscle tone were also placed. After surgical recovery, reflected light intensity was measured continuously at 2.5 Hz during spontaneously varying sleep/waking states. Sequential state transitions from active waking to quiet waking, quiet sleep and active sleep were accompanied by progressively increased levels of PVH activity. Overall activity was highest during active sleep, and decreased markedly upon awakening. Moment-to-moment activity oscillated in the 0-0.1 Hz range, especially during active sleep and active waking; this oscillation diminished during quiet sleep. Distinct sub-regions of enhanced or diminished activity emerged within the imaged area in a state-dependent manner. We conclude that PVH activity changes with behavioral state in a regionally specific manner, and that overall activity increases during quiet sleep, and is even more enhanced in active sleep. PVH activation could be expected to stimulate pituitary release of adrenocorticotropic hormone (ACTH) and affect input to autonomic regulatory sites. Since ACTH and corticotropin releasing factor elicit arousal, and since the PVH projects to other brain areas which modulate state, we speculate that the PVH plays a role in shaping characteristics of sleep/waking states.

Dexamethasone rapidly increases hypothalamic neuropeptide Y secretion in adrenalectomized ob/ob mice

A single intracerebroventricular injection of dexamethasone (DEX) rapidly (within 30 min) suppresses brown adipose tissue thermogenesis and increases plasma insulin concentrations in adrenal-ectomized (ADX) ob/ob mice but not in ADX lean mice. Intracerebroventricular neuropeptide Y (NPY) administered intracerebroventricularly causes these same metabolic changes within 30 min in both ob/ob and lean ADX mice. We therefore hypothesized that DEX exerts these rapid-onset metabolic actions in ob/ob mice via a phenotype-specific enhancement of NPY secretion within the central nervous system. In support of this hypothesis, DEX (a type II glucocorticoid receptor agonist) administered intracerebroventricularly selectively lowered NPY concentrations in the whole hypothalamus of ADX ob/ob mice by 35% and in the arcuate nucleus region by approximately 70% within 30 min but not in the brain stem or hippocampus or in any of these regions of lean mice. DEX also functioned in vitro to enhance depolarization-dependent release of NPY from hypothalamic blocks of ADX ob/ob mice but not of ADX lean mice. Thus DEX acts in the hypothalamus of ob/ob mice in a phenotype-specific manner to evoke rapid transport of NPY from cell bodies within the arcuate nucleus to terminal regions including the dorsomedial and ventromedial hypothalamic regions for release.

Ultraviolet light entrains rodent suprachiasmatic nucleus pacemaker

It has long been assumed that, in contrast to other vertebrates, mammals are ultraviolet blind. Recent evidence indicates, however, that the spectral sensitivity of the retina in rodents extends into the ultraviolet range. This finding, combined with reports that ultraviolet light can suppress nocturnal melatonin release and reverse the effect of short photoperiod on the gonads, invites speculation about the role of ultraviolet light in photoperiodic control of physiological and behavioral functions. One idea is that ultraviolet light participates in retinally mediated processes underlying photic entrainment of a pacemaker located in the hypothalamic suprachasmatic nucleus that generates circadian rhythms. Consistent with such a function, we now show that ultraviolet light is capable of inducing phase shifts in circadian rhythms in the rat and of inducing in the hypothalamic suprachiasmatic nucleus expression of the transcription factor Fos, a known cellular correlate of light-induced phase shifts of the hypothalamic suprachiasmatic nucleus pacemaker.

Direct projection from the suprachiasmatic nucleus to hypophysiotrophic corticotropin-releasing factor immunoreactive cells in the paraventricular nucleus of the hypothalamus demonstrated by means of Phaseolus vulgaris-leucoagglutinin tract tracing

The diurnal rhythm of the activity of the hypothalamo-pituitary-adrenal axis is generated by the circadian pacemaker located in the suprachiasmatic nuclei (SCN). However, the neuronal circuit connecting the SCN with the neurosecretory corticotropin-releasing factor (CRF) neurons in the paraventricular nucleus of the hypothalamus is not clear. To investigate the existence of a direct link between the SCN and the CRF neurons in the PVN we combined microiontopheretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) into the SCN with immunohistochemical detection of CRF in adrenalectomized male rats. The majority of the PHA-L-ir axons originating from the SCN terminated in the subparaventricular area. A minor contingent of fibers continued into the PVN proper, involving the medial and dorsal parvicellular subnuclei of the PVN. All PHA-L injections involving the entire SCN gave rise to PHA-L positive fibers endowed with boutons en passage and terminal boutons contacting CRF positive cell bodies in the PVN. Notably, varicosities on the PHA-L labelled fibers were present in close proximity to cell bodies and proximal dendrites of a subportion of the CRF neurons located in the periphery of the CRF cell cluster. The present study provides the first evidence to suggest a direct connection between the SCN and the CRF producing neurons of the hypothalamo-pituitary-adrenocortical axis in the PVN. Considering the sparse number of PHA-L-ir varicosities in close proximity to the CRF-ir cells, it seems likely that this direct pathway constitutes but a part of a projection system from the SCN, possibly involving multisynaptic pathways, influencing the hypothalamo-pituitary-adrenocortical axis.