Saccade-related, long-lead burst neurons in the monkey rostral pons

The paramedian pontine reticular formation contains the premotoneuronal cell groups that constitute the saccadic burst generator and control saccadic eye movements. Despite years of study and numerous investigations, the rostral portion of this area has received comparatively little attention, particularly the cell type known as long-lead burst neurons (LLBNs). Several hypotheses about the functional role of LLBNs in saccade generation have been proposed, although there is little information with which to assess them. To address this issue, I mapped and recorded LLBNs in the rostral pons to measure their discharge characteristics and correlate those characteristics with the metrics of the concurrent saccades. On the basis of their discharge and location, I identified three types of LLBNs in the rostral pons: excitatory (eLLBN), dorsal (dLLBN), and nucleus reticularis tegmenti pontis (nrtp) LLBNs. The eLLBNs, encountered throughout the pons, discharge for ipsilateral saccades in proportion to saccade amplitude, velocity, and duration. The dLLBNs, found at the pontomesencephalic junction, discharge maximally for ipsilateral saccades of a particular amplitude, usually <10 degrees , and are not associated with a particular anatomical nucleus. The nrtp LLBNs, previously described as vector LLBNs, discharge for saccades of a particular direction and sometimes a particular amplitude. The discharge of the eLLBNs suggests they drive motor neurons. The anatomical projections of the nrtp LLBNs suggest that their involvement in saccade production is less direct. The discharge of dLLBNs is consistent with a role in providing the "trigger" signal that initiates saccades.     

Distribution and medullary projection of respiratory neurons in the dorsolateral pons of the rat

The dorsolateral pons around the parabrachial nucleus including the Kölliker-Fuse nucleus is closely linked with the medullary respiratory center and plays an important role in respiratory control. We aimed to elucidate the firing properties, detailed distributions, and medullary projections of pontine respiratory neurons in pentobarbitone-anesthetized, paralyzed, and artificially ventilated rats with intact vagi.     

Localization of respiratory rhythm-generating neurons in the medulla of brainstem-spinal cord preparations from newborn rats

We describe the location of Pre-I neurons, which are important to respiratory rhythm generation, in the rostral medulla of brainstem-spinal cord preparations isolated from newborn rats. This neuronal group was delimited in the reticular formation slightly medial to the caudal area of the facial nucleus and near the ventral surface. The effects of electrical stimulation and lesions in that region were also examined with respect to respiratory rhythm generation. Single shock stimulation induced Pre-I neuron firing and reset the phase of the respiratory rhythm. Electrolytic lesions in the Pre-I neuron region reduced the respiratory rate.     

Interactions between rostral pontine and ventral medullary respiratory neurons

Lesioning studies have demonstrated that the respiratory rhythm is generated within the brain stem and that connections between the pons and the medulla must be intact for the generation of eupneic breathing in the decerebrate or anesthetized vagotomized cat. However, the nature of proposed functional connections between pontine and medullary respiratory neurons is not well understood. The possibility of interactions between respiratory neurons of the rostral pons (n. parabrachialis medialis, K?lliker-Fuse nucleus) and the ipsilateral ventral respiratory group (VRG; n. retroambigualis, n. ambiguus, retrofacial nucleus) was investigated because of neuroanatomical and electrophysiological evidence for such connections. Phrenic nerve activity and pontine and medullary single-unit respiratory related activities were recorded extracellularly in 44 decerebrate, vagotomized, paralyzed, and artificially ventilated cats. Cross-correlation analysis was employed to detect and evaluate functional associations of pairs of cells. Eighteen (7%) of the 255 pairs of respiratory neurons analyzed showed evidence of short time scale correlations indicative of a functional interaction. The interpretations of the detected correlations suggest that some cell pairs were correlated due to mono- or paucisynaptic connections, while others were correlated due to the influence of an unobserved shared input. The interpretations for 11 of the 15 cell pairs for which a monosynaptic connection may be postulated involve a projection from a tonically active respiratory neuron. Twelve of the 18 positive correlations involved neurons whose maximum rates of discharge occurred during different parts of the respiratory cycle. The results of this study provide the first evidence of functional connections among pontine and medullary respiratory neurons based on the evaluation of simultaneously recorded spike trains and suggest that the role of the rostral pontine respiratory neurons in the control of the respiratory rhythm may be mediated by various types of interactions. When considered with the results of other studies, our data suggest that monosynaptic interactions between VRG and rostral pontine respiratory neurons play a limited role in the control of the respiratory cycle in the decerebrate vagotomized cat. It is likely that the influence of the pons on ventral medullary neurons (and vice-versa) is also exerted via polysynaptic pathways and/or via brain stem neurons not sampled in this study.     

Rhythmic properties of neurons in the rostral ventrolateral medulla of the rat in vitro: effects of clonidine

The rostral ventrolateral medulla (RVLM) is thought to be the main central site for generation of tonic sympathetic activity. In the rat in vitro slice preparation, we used intracellular recordings to identify different populations of neurons in the RVLM: 43 spontaneously active neurons with regular (R) or irregular (I) patterns of spike firing and 10 silent neurons. The degree of regularity was quantified by the coefficient of variation (CV = SD/mean) of interspike interval durations, as well as by the rhythmic properties of the spike autospectrum and autocorrelation. The distribution of CVs was clustered: R and I neurons were defined as those with CVs 12% (n = 22), respectively. The R-type and I-type neurons resemble the type II and type I neurons, respectively, which were previously characterized in the RVLM in vivo as barosensitive and bulbospinal. Both types may be important in generation of sympathetic tone. Clonidine (1-100 microM) was applied to 10 R-type neurons and 16 I-type neurons. The firing of 21/26 was depressed to the point of silence. However, 18/26 neurons were excited earlier in the perfusion. The later depression of firing occurred in both I and R neurons and in different cases was associated with either hyperpolarization or depolarization.     

The adrenergic modulation of firings of respiratory rhythm-generating neurons in medulla-spinal cord preparation from newborn rat

We analysed the modulation of respiratory neurons by adrenaline or noradrenaline (NA) in a newborn rat brainstem-spinal cord preparation. Adrenaline or NA caused a dose-dependent depression of the respiratory rhythm and induced C4 spinal tonic discharges. The inhibitory effect of adrenaline (ED₅₀=0.5 μM) on the respiratory rhythm was stronger than NA (ED₅₀=5 μM). The adrenaline respiratory rhythm depression was partially blocked by the α₁-antagonist prazosin or by the α₂-antagonist yohimbine. The C4 tonic discharge elicited by adrenaline was blocked by the α₁-antagonist prazosin. The direct effects of adrenaline on pre-inspiratory (Pre-I) neurons were examined in a synaptic blockade solution (low Ca), and fifty-six percent of Pre-I neurons were found to continue firing. In low-Ca solution, Pre-I neurons were excited (n=29 of 39) or depressed (n=5 of 39) by adrenaline, and excited by α₁-agonist phenylephrine or depressed by α₂-agonist clonidine. These results suggest that the respiratory rhythm depression under intact network conditions is mediated by some other inhibitory system. The inhibitory effect of adrenaline on the respiratory rhythm was partially blocked by the GABA_A-antagonists bicuculline or picrotoxin, but not by the GABA_B-antagonist phaclofen. The present results suggest that: (1) respiratory rhythm generation is more sensitive to adrenaline than NA through α-adrenergic action of adrenaline; (2) the activity of Pre-I neurons could be directly regulated by excitation via α₁-receptors and inhibition via α₂-receptors; and (3) the depression of the respiratory rhythm by adrenaline is partly mediated by GABA_Aergic neurons. Received: 8 April 1997 / Accepted: 6 October 1997     

Dopaminergic input to GABAergic neurons in the rostral agranular insular cortex of the rat

Increasing evidence shows that the rostral agranular insular cortex (RAIC) is important in the modulation of nociception in humans and rats and that dopamine and GABA appear to be key neurotransmitters in the function of this cortical region. Here we use immunocytochemistry and path tracing to examine the relationship between dopamine and GABA related elements in the RAIC of the rat. We found that the RAIC has a high density of dopamine fibers that arise principally from the ipsilateral ventral tegmental area/substantia nigra (VTA/SN) and from a different set of neurons than those that project to the medial prefrontal cortex. Within the RAIC, there are close appositions between dopamine fibers and GABAergic interneurons. One target of cortical GABA appears to be a dense band of GABAB receptor-bearing neurons located in lamina 5 of the RAIC. The GABAB receptor-bearing neurons project principally to the amygdala and nucleus accumbens with few or no projections to the medial prefrontal cortex, cingulate gyrus, the mediodorsal thalamic nucleus or contralateral RAIC. The current anatomical data, together with previous behavioral results, suggest that part of the dopaminergic modulation of the RAIC occurs through GABAergic interneurons. GABA is able to exert specific effects through its action on GABAB receptor-bearing projection neurons that target a few subcortical limbic structures. Through these connections, dopamine innervation of the RAIC is likely to affect the motivational and affective dimensions of pain.     

Effects of acetylsalicylic acid and morphine on neurons of the rostral ventromedial medulla in rat

Morphine exerts its analgesic effect via the endogenous pain control system consisting of the periaqueductal grey (PAG) and the rostral ventromedial medulla (RVM). Acetylsalicylic acid (ASA) may also act via this system, but so far this has only been demonstrated for the inhibitory effect on the tail-flick reflex with extremely high doses (200-300 mg/kg). Both drugs show synergistic effects on PAG neurons in vitro. It is unclear whether this mechanism accounts for the well-known analgesic synergism of these drugs in vivo. Thus, the effects of ASA (30 mg/kg) and morphine on off- and on-cells in the RVM and the jaw-opening reflex (JOR) were investigated in anesthetized rats. Under morphine, off-cell activity increased (+34%), on-cell activity decreased (-98%) and the reflex was suppressed (-53%). ASA increased off-cell activity (+20%) and decreased the activity of on-cells (-52%). After preceding ASA administration, the effects of morphine on off- and on-cells and on the reflex did not alter statistically. The experiments document the modulatory effect of a clinically relevant dose of ASA on RVM cells. This effect resembles that of morphine. The results do not support the hypothesis of a mediation of the analgesic synergism of morphine and ASA by the PAG-RVM-network in vivo.     

Evidence for projections from the rostral medullary raphe onto medullary catecholamine neurons in the rat

Following the iontophoretic deposition of Phaseolus vulgaris leucoagglutinin (PHA-L) into the rostral medullary raphe, which included portions of the caudal nucleus raphe magnus, rostral nucleus raphe pallidus, rostral nucleus raphe obscurus and rostral nucleus reticularis paragigantocellularis, two-color immunoperoxidase staining was employed to demonstrate contiguity between PHA-L-immunoreactive (PHA-LI) varicose fibers and boutons and medullary catecholamine (CA) cells. Raphe projections were contiguous with phenylethanolamine N-methyltransferase-immunoreactive (PNMTI) neurons in the C1, C2 and C3 cell groups and with tyrosine hydroxylase-immunoreactive (THI) neurons in the A1 and A2 cell groups. Contiguity between PHA-LI processes and medullary CA cells was observed most frequently in the C1 cell group. Preliminary findings of this study have been presented previously.     

Localization of neurons supplying the temporalis muscle in the rat and monkey.

Following injection of horseradish peroxidase (HRP) solution into the rat and monkey temporalis muscle, a similar pattern of nuclear representation was found in the motor trigeminal nucleus. Labelled neurons were seen throughout the rostro-caudal extent of the ipsilateral motor trigeminal nucleus, in its dorsal, intermediate and ventral portions and its dorsomedial, ventromedial and ventrolateral corners. In addition to the motor trigeminal nucleus labelled neurons were also found in the ipsilateral trigeminal ganglion and mesencephalic trigeminal nucleus. The results were discussed.     

Discharge properties of neurons in the rostral superior colliculus of the monkey during smooth-pursuit eye movements

The intermediate and deep layers of the monkey superior colliculus (SC) comprise a retinotopically organized map for eye movements. The rostral end of this map, corresponding to the representation of the fovea, contains neurons that have been referred to as "fixation cells" because they discharge tonically during active fixation and pause during the generation of most saccades. These neurons also possess movement fields and are most active for targets close to the fixation point. Because the parafoveal locations encoded by these neurons are also important for guiding pursuit eye movements, we studied these neurons in two monkeys as they generated smooth pursuit. We found that fixation cells exhibit the same directional preferences during pursuit as during small saccades-they increase their discharge during movements toward the contralateral side and decrease their discharge during movements toward the ipsilateral side. This pursuit-related activity could be observed during saccade-free pursuit and was not predictive of small saccades that often accompanied pursuit. When we plotted the discharge rate from individual neurons during pursuit as a function of the position error associated with the moving target, we found tuning curves with peaks within a few degrees contralateral of the fovea. We compared these pursuit-related tuning curves from each neuron to the tuning curves for a saccade task from which we separately measured the visual, delay, and peri-saccadic activity. We found the highest and most consistent correlation with the delay activity recorded while the monkey viewed parafoveal stimuli during fixation. The directional preferences exhibited during pursuit can therefore be attributed to the tuning of these neurons for contralateral locations near the fovea. These results support the idea that fixation cells are the rostral extension of the buildup neurons found in the more caudal colliculus and that their activity conveys information about the size of the mismatch between a parafoveal stimulus and the currently foveated location. Because the generation of pursuit requires a break from fixation, the pursuit-related activity indicates that these neurons are not strictly involved with maintaining fixation. Conversely, because activity during the delay period was found for many neurons even when no eye movement was made, these neurons are also not obligatorily related to the generation of a movement. Thus the tonic activity of these rostral neurons provides a potential position-error signal rather than a motor command-a principle that may be applicable to buildup neurons elsewhere in the SC.     

Firing properties of respiratory rhythm generating neurons in the absence of synaptic transmission in rat medulla in vitro

Summary It has previously been demonstrated that Pre-I neurons, localized in the rostral ventrolateral medulla, are important in the generation of the primary respiratory rhythm in brainstemspinal cord preparations from newborn rats. To investigate whether or not Pre-I neurons have endogenous pacemaker properties, we examined Pre-I neuron activity before and after chemical synaptic transmission was blocked by incubation in a low Ca²⁺ (0.2 mM), high Mg²⁺ (5 mM) solution (referred to here as low Ca). After incubation for about 30 min in low Ca, 28 (52%, type-1) out of 54 neurons tested in 27 preparations retained apparent rhythmic (phasic) activity after complete disappearance of C4 inspiratory activity. Sixteen neurons (30%, type-2) fired tonically and 10 (18%, type-3) were silent. We examined the effects of synaptic blockade on 14 inspiratory neurons in the RVL. The firing of all 14 neurons in 9 preparations disappeared concomitantly with the disappearance of C4 activity in low Ca. When the pH of the low Ca solution was lowered with a decrease in NaHCO₃ concentration from 7.4 to 7.1, the firing rate of the Pre-I neurons (type-1) increased from 12 to 18/min. In conclusion, the generator of respiratory rhythm in the newborn rat is probably a neuronal network with chemical synapses that functions mainly through the endogenous Pre-I pacemaker cells. Intrinsic chemoreception in the rhythm generator is probably important in frequency control of respiratory rhythm.     

Respiration-related rhythmic activity in the rostral medulla of newborn rats

There are at least two respiration-related rhythm generators in the medulla: the pre-B?tzinger complex, which produces inspiratory (Insp) neuron bursts, and the parafacial respiratory group (pFRG), which produces predominantly preinspiratory (Pre-I) neuron bursts. The pFRG Pre-I neuron activity has not been correlated with motor neuron activity in slice or block preparations of rostral medulla. In this study, we attempted to detect pFRG Pre-I activity as motor output in the rostral medulla. We recorded respiratory activity of the facial nerve in the brain stem-spinal cord preparation of 0- to 2-day-old rats. Facial nerve activity consisted of preinspiratory, Insp, and postinspiratory activity. Pre- and postinspiratory activity corresponded well with membrane potential trajectories of Pre-I neurons in the rostral ventrolateral medulla. In response to perfusion of 1 microM DAMGO (a mu-opiate agonist), fourth cervical ventral root (C4) Insp activity was depressed and facial nerve activity continued to synchronize with Pre-I neuron bursts. After transverse sectioning between the levels of the pre-B?tzinger complex and the pFRG, C4 Insp activity recovered within 15 min, but facial nerve activity was inhibited. When DAMGO was applied, C4 Insp activity was inhibited, and rhythmic facial nerve activity recovered. Subsequent elevation of K+ concentration reinduced C4 activity, but facial nerve activity was inhibited. Whole cell recordings in the rostral block revealed the presence of putative Pre-I neurons, the activity of which was synchronized with facial nerve activity. These results show that the rostral medulla, not including the pre-B?tzinger complex, produces Pre-I-like rhythmic activity that can be monitored as facial nerve motor output in newborn rat in vitro preparations.     

Activity patterns in respiratory muscles and in respiratory neurones of the rostral medulla of the cat

1. In the rostral medulla the activity patterns of respiratory neurones are of two main types: inspiratory and expiratory (Fig. 1). Some expiratory neurones begin activity early enough to fit Cohen & Wang's (1959) category of ;inspiratory-expiratory'. However, in the rostral medulla, ;inspiratory-expiratory' neurones do not constitute a distinct category within the expiratory group, since all intermediate activity patterns are observed. Similarly, no distinct categories of activity patterns can be distinguished within the inspiratory group.2. Expiratory neurones are rare in the extreme medial and lateral portions of the respiratory regions on each side of the rostral medulla (Fig. 3). Respiratory neurones are not bunched in anatomical clusters within each region, though a medio-dorsal group of neurones on each side is somewhat anatomically separated from the rest.3. At any one time there are at the very least 1000 respiratory neurones on each side of the rostral medulla.4. Many of the patterns of single unit activity recorded in respiratory muscles of the nose, throat and pharynx are different from the patterns of activity typical of respiratory neurones in the medulla near the cranial motor nuclei (Figs. 1 and 4). This suggests that many of the medullary recordings are from cells other than the motor neurones innervating these muscles.     

Cholinergic neurons from the dorsolateral pons project to the medial pons: a WGA-HRP and choline acetyltransferase immunohistochemical study

In this study we determined that cholinergic neurons from the lateral dorsal tegmental (LDT) and peribrachial pontine region (PPG) innervate the medial pontine reticular formation (medial PRF), a region involved in the generation of REM sleep. Wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected into the medial PRF and the brainstem tissue was processed using a combined retrograde transport/immunocytochemical procedure. Results showed that 10-15% of choline acetyltransferase (ChAT) immunoreactive neurons in the LDT and PPG project to the medial PRF. It is hypothesized that these neurons play an important role in the generation of the REM sleep state.     

Parvalbumin in respiratory neurons of the ventrolateral medulla of the adult rat

A column of parvalbumin immunoreactive neurons is closely associated with the location of respiratory neurons in the ventrolateral medulla of the rat. The majority (66%) of bulbospinal neurons in the medullary ventral respiratory column (VRC) that were retrogradely labeled by tracer injections in the phrenic nucleus were also positive for parvalbumin. In contrast, only 18.8% of VRC neurons retrogradely labeled after a tracer injection in the VRC, also expressed parvalbumin. The average cross-sectional area of VRC neurons retrogradely labeled after VRC injections was 193.8 m² ± 6.6 SE. These were significantly smaller than VRC parvalbumin neurons (271.9 m² ± 12.3 SE). Parvalbumin neurons were found in the Bötzinger Complex, the rostral ventral respiratory group (VRG), and the caudal VRG, areas which all contribute to the bulbospinal projection. In contrast, parvalbumin neurons were sparse or absent in the preBötzinger Complex and in the vicinity of the retrotrapezoid nucleus, areas that have few bulbospinal projections. Parvalbumin was rarely colocalized within Neurokinin-1 receptor positive (NK1R) VRC neurons, which are found in the preBötzinger complex and in the anteroventral part of the rostral VRG. Parvalbumin neurons in the Bötzinger Complex and rostral VRG help define the rostrocaudal extent of these regions. The absence of parvalbumin neurons from the intervening preBötzinger complex also helps establish the boundaries of this region. Regional boundaries described in this manner are in good agreement with earlier physiological and anatomical studies. Taken together, the distributions of parvalbumin, NK1R and bulbospinal neurons suggest that the rostral VRG may be subdivided into distinct, anterodorsal, anteroventral, and posterior subdivisions.     

Localization and ontogeny of damage-induced neuronal endopeptidase mRNA-expressing neurons in the rat nervous system

Neuropeptides are crucial mediators in nervous and endocrine systems. Processing and degradation, the major regulatory mechanisms, of enzymes are essential for the control of these peptidergic intercellular signaling systems. Damage-induced neuronal endopeptidase (or endothelin converting enzyme-like1), a member of the neprilysin family, has recently been identified as an M13 zinc metalloprotease. Damage-induced neuronal endopeptidase mRNA expression is strikingly restricted to neurons, and is remarkably induced in response to various types of neuronal injuries, although its function and substrate remain unknown. To clarify the role of damage-induced neuronal endopeptidase, we examined the localization and ontogeny of damage-induced neuronal endopeptidase mRNA expression in the rat nervous system using in situ hybridization. Damage-induced neuronal endopeptidase mRNA was detected at embryonic day 12, and its expression restricted to the ventral region of the neural tube. Subsequently, expression was also apparent in primordia of the striatum, hypothalamus, and cranial motor nuclei during neural development. This specific distribution was relatively maintained in the adult brain, although expression levels became weaker. Expression of damage-induced neuronal endopeptidase was absent in the cerebral cortex, hippocampus, and cerebellum. In addition to prominent expression in CNS, intestinal and sensory ganglia and retina demonstrated transient intense damage-induced neuronal endopeptidase mRNA expression during the embryonic period that then declined, and disappeared after birth. The results indicated that damage-induced neuronal endopeptidase might play an important role in embryonic neural development, in particular in peripheral ganglia derived from the neural crest, and in some neurons originating from the basal plate such as the hypothalamus and cranial motor neurons.     

Coding of locomotor phase in populations of neurons in rostral and caudal segments of the neonatal rat lumbar spinal cord

Several experiments have demonstrated that rostral segments of the vertebrate lumbar spinal cord produce a rhythmic motor output more readily and of better quality than caudal segments. Here we examine how this rostrocaudal gradient of rhythmogenic capability is reflected in the spike activity of neurons in the rostral (L(2)) and caudal (L(5)) lumbar spinal cord of the neonatal rat. The spike activity of interneurons in the ventromedial cord, a region necessary for the production of locomotion, was recorded intracellularly with patch electrodes and extracellularly with tetrodes during pharmacologically induced locomotion. Both L(2) and L(5) neurons tended to be active in phase with their homologous ventral root. L(5) neurons, however, had a wider distribution of their preferred phases of activity throughout the locomotor cycle than L(2) neurons. The strength of modulation of the activity of individual L(2) neurons was also larger than that of L(5) neurons. These differences resulted in a stronger rhythmic signal from the L(2) neuronal population than from the L(5) population. These results demonstrate that the rhythmogenic capability of each spinal segment was reflected in the activity of interneurons located in the same segment. In addition to paralleling the rostrocaudal gradient of rhythmogenic capability, these results further suggest a colocalization of motoneurons and their associated interneurons involved in the production of locomotion.