The physiological and morphological characteristics of interneurons caudal to the trigeminal motor nucleus in rats

Electrophysiological and morphological properties of rat supratrigeminal premotor neurons targeting the trigeminal motor nucleus

The electrophysiological and morphological characteristics of premotor neurons in the supratrigeminal region (SupV) targeting the trigeminal motor nucleus (MoV) were examined in neonatal rat brain stem slice preparations with Ca(2+) imaging, whole cell recordings, and intracellular biocytin labeling. First, we screened SupV neurons that showed a rapid rise in intracellular free Ca(2+) concentration ([Ca(2+)]i) after single-pulse electrical stimulation of the ipsilateral MoV. Subsequent whole cell recordings were generated from the screened SupV neurons, and their antidromic responses to MoV stimulation were confirmed. We divided the antidromically activated premotor neurons into two groups according to their discharge patterns during the steady state in response to 1-s depolarizing current pulses: those firing at a frequency higher (HF neurons, n = 19) or lower (LF neurons, n = 17) than 33 Hz. In addition, HF neurons had a narrower action potential and a larger afterhyperpolarization than LF neurons. Intracellular labeling revealed that the axons of all HF neurons (6/6) and half of the LF neurons (4/9) entered the MoV from its dorsomedial aspect, whereas the axons of the remaining LF neurons (5/9) entered the MoV from its dorsolateral aspect. Furthermore, the dendrites of three HF neurons penetrated into the principal sensory trigeminal nucleus (Vp), whereas the dendrites of all LF neurons were confined within the SupV. These results suggest that the types of SupV premotor neurons targeting the MoV with different firing properties have different dendritic and axonal morphologies, and these SupV neuron classes may play unique roles in diverse oral motor behaviors, such as suckling and mastication.

The pronociceptive dorsal reticular nucleus contains mostly tonic neurons and shows a high prevalence of spontaneous activity in block preparation

Despite the importance and significant clinical impact of understanding information processing in the nociceptive system, the functional properties of neurons in many parts of this system are still unknown. In this work we performed whole cell patch-clamp recording in rat brain stem blocks to characterize the electrophysiological properties of neurons in the dorsal reticular nucleus (DRt), a region known to be involved in pronociceptive modulation. We also compared properties of DRt neurons with those in the adjacent parvicellular reticular nucleus and in neighboring regions outside the reticular formation. We found that neurons in the DRt and parvicellular reticular nucleus had similar electrophysiological properties and exhibited mostly toniclike firing patterns, whereas neurons outside the reticular formation showed a larger diversity of firing patterns. Interestingly, more than one-half of the neurons also showed spontaneous activity. While the general view of the reticular formation, being a loosely associated mesh of groups of neurons with diverse function, and earlier reports suggests more electrophysiological heterogeneity, we showed that this is indeed not the case. Our results indicate that functional difference of neurons in the reticular formation may mostly be determined by their connectivity profiles and not by their intrinsic electrophysiological properties. The dominance of tonic neurons in the DRt supports previous conclusions that these neurons encode stimulus intensity through their firing frequency, while the high prevalence of spontaneous activity most likely shapes nociceptive modulation by this brain stem region.

Response properties of temporomandibular joint mechanosensitive neurons in the trigeminal sensory complex of the rabbit

The neurophysiological properties of neurons sensitive to TMJ movement (TMJ neurons) in the trigeminal sensory complex (Vcomp) during passive movement of the isolated condyle were examined in 46 rabbits. Discharges of TMJ neurons from the rostral part of the Vcomp were recorded with a microelectrode when the isolated condyle was moved manually and with a computer-regulated mechanostimulator. A total of 443 neurons responding to mechanical stimulation of the face and oral cavity were recorded from the brainstem. Twenty-one TMJ neurons were detected rostrocaudally from the dorsal part of the trigeminal principal sensory nucleus (NVsnpr), subnucleus oralis of the trigeminal spinal nucleus, and reticular formation surrounding the trigeminal motor nucleus. Most of the TMJ neurons were located in the dorso-rostral part of the NVsnpr. Of the TMJ units recorded, 90 % were slowly adapting and 26 % had an accompanying resting discharge. The majority (86 %) of the TMJ units responded to the movement of the isolated condyle in the anterior and/or ventral directions, and half were sensitive to the condyle movement in a single direction. The discharge frequencies of TMJ units increased as the condyle displacement and constant velocity (5 mm/s) increased within a 5-mm anterior displacement of the isolated condyle. Based on these results, we conclude that sensory information is processed by TMJ neurons encoding at least joint position and displacement in the physiological range of mandibular displacement.

Dopamine triggers skeletal muscle tone by activating D1-like receptors on somatic motoneurons

The dopamine system plays an integral role in motor physiology. Dopamine controls movement by modulation of higher-order motor centers (e.g., basal ganglia) but may also regulate movement by directly controlling motoneuron function. Even though dopamine cells synapse onto motoneurons, which themselves express dopamine receptors, it is unknown whether dopamine modulates skeletal muscle activity. Therefore, we aimed to determine whether changes in dopaminergic neurotransmission at a somatic motor pool affect motor outflow to skeletal muscles. We used microinjection, neuropharmacology, electrophysiology, and histology to determine whether manipulation of D(1)- and D(2)-like receptors on trigeminal motoneurons affects masseter and/or tensor palatini muscle tone in anesthetized rats. We found that apomorphine (a dopamine analog) activated trigeminal motoneurons and triggered a potent increase in both masseter and tensor palatini tone. This excitatory effect is mediated by D(1)-like receptors because specific D(1)-like receptor activation strengthened muscle tone and blockade of these receptors prevented dopamine-driven activation of motoneurons. Blockade of D(1)-like receptors alone had no detectable effect on basal masseter/tensor palatini tone, indicating the absence of a functional dopamine drive onto trigeminal motoneurons, at least during isoflurane anesthesia. Finally, we showed that D(2)-like receptors do not affect either trigeminal motoneuron function or masseter/tensor palatini muscle tone. Our results provide the first demonstration that dopamine can directly control movement by manipulating somatic motoneuron behavior and skeletal muscle tone.

GABAB receptor-mediated tonic inhibition of noradrenergic A7 neurons in the rat

Noradrenergic (NAergic) A7 neurons that project axonal terminals to the dorsal horn of the spinal cord to modulate nociceptive signaling are suggested to receive tonic inhibition from local GABAergic interneurons, which are under the regulation of descending analgesic pathways. In support of this argument, we presently report GABA(B) receptor (GABA(B)R)-mediated tonic inhibition of NAergic A7 neurons. Bath application of baclofen induced an outward current (I(Bac)) in NAergic A7 neurons that was blocked by CGP 54626, a GABA(B)R blocker. The I(Bac) was reversed at about -99 mV, displayed inward rectification, and was blocked by Ba(2+) or Tertipian-Q, showing it was mediated by G protein-activated inward-rectifying K(+) (GIRK) channels. Single-cell RT-PCR results suggested that GIRK1/3 heterotetramers might dominate functional GIRK channels in NAergic A7 neurons. Under conditions in which GABA(A) and glycine receptors were blocked, bath application of GABA inhibited the spontaneous firing of NAergic A7 neurons in a dose-dependent manner. Interestingly, CGP 54626 application not only blocked the effect of GABA but also increased the firing rate to 126.9% of the control level, showing that GABA(B)Rs were constitutively active at an ambient GABA concentration of 2.8 μM and inhibited NAergic A7 neurons. GABA(B)Rs were also found at presynaptic excitatory and inhibitory axonal terminals in the A7 area. Pharmacological activation of these GABA(B)Rs inhibited the release of neurotransmitters. No physiological role was found for GABA(B)Rs on excitatory terminals, whereas those on the inhibitory terminals were found to exert autoregulatory control of GABA release.

Roles of A-type potassium currents in tuning spike frequency and integrating synaptic transmission in noradrenergic neurons of the A7 catecholamine cell group in rats

We investigated voltage-dependent K(+) currents (I(K)) in noradrenergic (NAergic) A7 neurons. The I(K) evoked consisted of A-type I(K) (I(A)), which had the characteristics of a low threshold for activation (approximately -50 mV), fast activation/inactivation, and rapid recovery from inactivation. Since the I(A) were blocked by heteropodatoxin-2 (Hptx-2), a specific Kv4 channel blocker, and the NAergic A7 neurons were shown to be reactive with antibodies against Kv4.1/Kv4.3 channel proteins, we conclude that the I(A) evoked in NAergic neurons are mediated by Kv4.1/Kv4.3 channels. I(A) were also evoked using voltage commands of a single action potential (AP), a subthreshold voltage change between two consecutive APs, or excitatory postsynaptic potential (EPSP) activity recorded in current-clamp mode (CCM). Blockade of the I(A) by 4-AP, a broad spectrum I(A) blocker, or by Hptx-2 increased the half-width and spontaneous firing of APs and reduced the amount of synaptic drive needed to elicit APs in CCM, showing that the I(A) play important roles in regulating the shape and firing frequency of APs and in synaptic integration in NAergic A7 neurons. Since these neurons are the principal projection neurons to the dorsal horn of the spinal cord, these results also suggest roles for Kv4.1/4.3 channels in descending NAergic pain regulation.

Intrinsic membrane properties of pre-oromotor neurons in the intermediate zone of the medullary reticular formation

Neurons in the lower brainstem that control consummatory behavior are widely distributed in the reticular formation (RF) of the pons and medulla. The intrinsic membrane properties of neurons within this distributed system shape complex excitatory and inhibitory inputs from both orosensory and central structures implicated in homeostatic control to produce coordinated oromotor patterns. The current study explored the intrinsic membrane properties of neurons in the intermediate subdivision of the medullary reticular formation (IRt). Neurons in the IRt receive input from the overlying (gustatory) nucleus of the solitary tract and project to the oromotor nuclei. Recent behavioral pharmacology studies as well as computational modeling suggest that inhibition in the IRt plays an important role in the transition from a taste-initiated oromotor pattern of ingestion to one of rejection. The present study explored the impact of hyperpolarization on membrane properties. In response to depolarization, neurons responded with either a tonic discharge, an irregular/burst pattern or were spike-adaptive. A hyperpolarizing pre-pulse modulated the excitability of most (82%) IRt neurons to subsequent depolarization. Instances of both increased (30%) and decreased (52%) excitability were observed. Currents induced by the hyperpolarization included an outward 4-aminopyridine (4-AP) sensitive K+ current that suppressed excitability and an inward cation current that increased excitability. These currents are also present in other subpopulations of RF neurons that influence the oromotor nuclei and we discuss how these currents could alter firing characteristics to impact pattern generation.

Neurokinin 1 receptor activates transient receptor potential-like currents in noradrenergic A7 neurons in rats

Noradrenergic (NAergic) A7 neurons are involved in modulating nociception by releasing noradrenaline in the dorsal spinal cord. Since NAergic A7 neurons receive dense Substance P (Sub-P) releasing terminals from ventromedial medulla, here we tested the effect of Sub-P on them. Bath application of Sub-P induced an inward current (I(Sub-P)) in NAergic neurons, which was significantly blocked by Neurokinin 1 (NK1) receptor antagonist. The I(Sub-P) was reversed at approximately -20 mV, blocked by several TRP channel blockers, enhanced by OAG and negatively regulated by PKC. Immunohistochemistry staining showed that NAergic A7 neurons express high level of TRPC6 channel proteins, which is consistent with pharmacological properties of I(Sub-P) shown above, as TRPC6 channel is shown to be augmented by OAG and inhibited by PKC. In conclusion, the above results provide mechanism underlying postsynaptic action of Sub-P on NAergic A7 neurons and a role for TRPC6 channel in NAergic pain modulation.

Physiological and morphological properties of, and effect of substance P on, neurons in the A7 catecholamine cell group in rats

The A7 catecholamine cell group consists of noradrenergic (NAergic) neurons that project to the dorsal horn of the spinal cord. Here, we characterized their morphology and physiology properties and tested the effect of substance P (Sub-P) on them, since the results of many morphological studies suggest that A7 neurons are densely innervated by Sub-P-releasing terminals from nuclei involved in the descending inhibitory system, such as the lateral hypothalamus and periaqueductal gray area. Whole cell recordings were made from neurons located approximately 200 microm rostral to the trigeminal motor nucleus (the presumed A7 area) in sagittal brainstem slices from rats aged 7-10 days. After recording, the neurons were injected with biocytin and immunostained with antibody against dopamine-beta-hydroxylase (DBH). DBH-immunoreactive (ir) cells were presumed to be NAergic neurons. They had a large somata diameter ( approximately 20 microm) and relatively simple dendritic branching patterns. They fired action potentials (AP) spontaneously with or without blockade of synaptic inputs, and had similar properties to those of NAergic neurons in other areas, including the existence of calcium channel-mediated APs and a voltage-dependent delay in initiation of the AP (an indicator of the existence of A-type potassium currents) and an ability to be hyperpolarized by norepinephrine. Furthermore, in all DBH-ir neurons tested, Sub-P caused depolarization of the membrane potential and an increase in neuronal firing rate by acting on neurokinin-1 receptors. Non-DBH-ir neurons with a smaller somata size were also found in the A7 area. These showed great diversity in firing patterns and about half were depolarized by Sub-P. Morphological examination suggested that the non-DBH-ir neurons form contacts with DBH-ir neurons. These results provide the first description of the intrinsic regulation of membrane properties of, and the excitatory effect of Sub-P on, A7 area neurons, which play an important role in pain regulation.

Intrinsic membrane properties and morphological characteristics of interneurons in the rat supratrigeminal region

The membrane properties and morphological features of interneurons in the supratrigeminal area (SupV) were studied in rat brain slices using whole-cell patch clamp recording techniques. We classified three morphological types of neurons as fusiform, pyramidal, and multipolar and four physiological types of neurons according to their discharge pattern in response to a 1-sec depolarizing current pulse from -80 mV. Single-spike neurons responded with a single spike, phasic neurons showed an initial burst of spikes and were silent during the remainder of the stimulus, delayed-firing (DF) neurons exhibited a slow depolarization and delay to initial spike onset, and tonic (T) neurons showed maintained a discharge throughout the stimulus pulse. In a subpopulation of neurons (10%), membrane depolarization to around -44 mV produced a rhythmic burst discharge (RB) that was associated with voltage-dependent subthreshold membrane oscillations. Both these phenomena were blocked by the sodium channel blocker riluzole at a concentration that did not affect the fast transient spike. Low doses of 4-AP, which blocks low-threshold K+ currents, transformed bursting into low-frequency tonic discharge. In contrast, bursting occurred with exposure to cadium, a calcium-channel blocker. This suggests that persistent sodium currents and low-threshold K+ currents have a role in intrinsic burst generation. Importantly, RB cells were most often associated with multipolar neurons that exhibited either a DF or a T discharge. Thus, the SupV contains a variety of physiological cell types with unique morphologies and discharge characteristics. Intrinsic bursting neurons form a unique group in this region. .

Postnatal development of noradrenergic terminals in the rat trigeminal motor nucleus: A light and electron microscopic immunocytochemical analysis

The noradrenergic (NA) innervation in the trigeminal motor nucleus (Vmot) of postnatal and adult rats was examined by light and electron microscopic immunocytochemistry using antibodies against dopamine-beta-hydroxylase or tyrosine hydroxylase. NA fibers were identified in the Vmot as early as the day of birth (postnatal day 0; P0). A continuous increase in the density of labeled fibers was observed during development up to P20, with a slight decrease at P30 and in the adult. Electron microscopic analysis of serial ultrathin sections revealed that, at P5, nearly half (46%) of the examined NA terminals made synaptic contact with other neuronal elements with membrane specializations. The percentage of examined NA varicosities engaged in synaptic contacts increased at P15 (74%), then decreased in the adult (64%). At all developmental ages, the majority of contacts made by these boutons were symmetrical, the postsynaptic elements being mainly dendrites and occasionally somata. Interestingly, some of the NA terminals made axo-axon contacts with other unidentified boutons. These results show that, although the density of NA fibers increases during postnatal development, functional NA boutons are present in the Vmot at early postnatal ages. Some of these fibers might exert their effects via nonsynaptic release of noradrenaline, the so-called volume transmission, but, in the main, they form conventional synaptic contacts with dendrites, somata, and other axonal terminals in the Vmot. These results are consistent with previous electrophysiological studies that propose an important role for the NA system in modulating mastication.

Input-output organization of jaw movement-related areas in monkey frontal cortex

The brain mechanisms underlying mastication are not fully understood. To address this issue, we analyzed the distribution patterns of cortico-striatal and cortico-brainstem axon terminals and the origin of thalamocortical and intracortical fibers by injecting anterograde/retrograde tracers into physiologically and morphologically defined jaw movement-related cortical areas. Four areas were identified in the macaque monkey: the primary and supplementary orofacial motor areas (MIoro and SMAoro) and the principal and deep parts of the cortical masticatory area (CMaAp and CMaAd), where intracortical microstimulation produced single twitch-like or rhythmic jaw movements, respectively. Tracer injections into these areas labeled terminals in the ipsilateral putamen in a topographic fashion (MIoro vs. SMAoro and CMaAp vs. CMaAd), in the lateral reticular formation and trigeminal sensory nuclei contralaterally (MIoro and CMaAp) or bilaterally (SMAoro) in a complex manner of segregation vs. overlap, and in the medial parabranchial and Kölliker-Fuse nuclei contralaterally (CMaAd). The MIoro and CMaAp received thalamic projections from the ventrolateral and ventroposterolateral nuclei, the SMAoro from the ventroanterior and ventrolateral nuclei, and the CMaAd from the ventroposteromedial nucleus. The MIoro, SMAoro, CMaAp, and CMaAd received intracortical projections from the ventral premotor cortex and primary somatosensory cortex, the ventral premotor cortex and rostral cingulate motor area, the ventral premotor cortex and area 7b, and various sensory areas. In addition, the MIoro and CMaAp received projections from the three other jaw movement-related areas. Our results suggest that the four jaw movement-related cortical areas may play important roles in the formation of distinctive masticatory patterns.

Identification of c-Fos immunoreactive brainstem neurons activated during fictive mastication in the rabbit

In the present study we used the expression of the c-Fos-like protein as a "functional marker" to map populations of brainstem neurons involved in the generation of mastication. Experiments were conducted on urethane-anesthetized and paralyzed rabbits. In five animals (experimental group), rhythmical bouts of fictive masticatory-like motoneuron activity (cumulative duration 60-130 min) were induced by electrical stimulation of the left cortical "masticatory area" and recorded from the right digastric motoneuron pool. A control group of five animals (non-masticatory) were treated in the same way as the experimental animals with regard to surgical procedures, anesthesia, paralysis, and survival time. To detect the c-Fos-like protein, the animals were perfused, and the brainstems were cryosectioned and processed immunocytochemically. In the experimental group, the number of c-Fos-like immunoreactive neurons increased significantly in several brainstem areas. In rostral and lateral areas, increments occurred bilaterally in the borderzones surrounding the trigeminal motor nucleus (Regio h); the rostrodorsomedial half of the trigeminal main sensory nucleus; subnucleus oralis-gamma of the spinal trigeminal tract; nuclei reticularis parvocellularis pars alpha and nucleus reticularis pontis caudalis (RPc) pars alpha. Further caudally-enhanced labeling occurred bilaterally in nucleus reticularis parvocellularis and nucleus reticularis gigantocellularis (Rgc) including its pars-alpha. Our results provide a detailed anatomical record of neuronal populations that are correlated with the generation of the masticatory motor behavior.