Influence of nucleus tractus solitarius stimulation and baroreceptor activation on rat parabrachial neurons.

The parabrachial nucleus (PBN) within the dorsolateral pons is a major recipient of autonomic-related inputs from more caudal levels of the brainstem and, in particular, the nucleus of the solitary tract (NTS). Although the anatomical projections from the NTS to the PBN are well characterized, less is known concerning the influence of activating NTS efferents on PBN neurons and the response of the latter to cardiovascular-related inputs. The present study examined the response of PBN neurons to electrical stimulation of the depressor area within the NTS in urethane anesthetized rats, and subsequently, the influence of arterial baroreceptor activation and systemic angiotensin II (ANG II) on these cells. Extracellular single-unit PBN recordings indicated that 92 of 227 (40.5%) cells were orthodromically excited and 35 of 227 (15.4%) inhibited consequent to NTS stimulation. Ten (4.5%) PBN cells displayed antidromic activation from the NTS. Of 41 of 119 neurons responding to both NTS stimulation and baroreceptor activation, 29.3% revealed a excitatory and 31.7% an inhibitory response to the two stimuli. Fifteen PBN cells responded to NTS stimulation, baroreceptor activation, and the administration of systemic ANG II, with six cells displaying either an excitatory or inhibitory response to all three stimuli. These observations provide electrophysiological support for reciprocal connections between the NTS and PBN and indicate the presence of both excitatory and inhibitory projections to the pontine nucleus. A population of neurons influenced by activation of NTS efferents also reveal a similarity of responses to inputs originating from peripheral arterial baroreceptors and systemic ANG II.     

Responses of rat lateral hypothalamic neurons to periaqueductal gray stimulation and nociceptive stimuli

The effects of dorsal periaqueductal gray (D-PAG) stimulation and noxious stimuli on neural activity in the lateral hypothalamic area (LHA) were investigated in 56 adult male anesthetized rats. Strong tail pinch was used as noxious stimulation. We examined 234 extracellular and 75 intracellular recordings of LHA responses to electrical stimulation of D-PAG. To determine neurotransmitter candidates, the effects of the opioid agonist, morphine, and its antagonist, naloxone were investigated by systemic administration and microelectrophoresis. Of 234 spontaneously firing LHA neurons, 70 (30%) were inhibited by D-PAG stimulation. Of these 70 neurons, 26/40 tested (65%) were glucose-sensitive, 16/19 (84%) were inhibited by morphine and 12/18 (67%) were inhibited by tail pinch. Glucose-sensitive neurons were selectively inhibited by morphine and tail pinch. Naloxone attenuated inhibitory responses to D-PAG stimulation, tail pinch and electrophoretic morphine. From intracellular recordings these polysynaptic inhibitory responses to D-PAG stimulation were considered to be inhibitory postsynaptic potentials (IPSPs) with 6.1 +/- 3.2 ms (mean +/- S.D.) latency and reversal membrane potential of about -78 mV. Since LHA glucose-sensitive neurons receive, selectively, both inhibitory opioid inputs from the D-PAG and inhibitory inputs through noxious stimulation, we suggest that D-PAG might be an intermediate site for transmission of noxious stimuli to the LHA.     

Sensory input to single neurons in the amygdala of the cat

We determined functionally the afferent projections from exteroceptors and enteroceptors (visceral receptors) to single cells of the amygdala. Recordings were made in chloralosed cats from 249 cells in four subnuclei of the amygdala. Forty-six percent of cells tested responded to carotid sinus nerve stimulation and more than one-half of them responded to selective baroreceptor or chemoreceptor activation or to electrical stimulation of the locus ceruleus. Of 11 cells responding similarly (inhibition) to carotid sinus nerve stimulation and to selective baroreceptor activation, all were inhibited by locus ceruleus stimulation, also. Approximately 17% of cells tested responded to at least one exteroceptive stimulus (acoustic, optic, or tactile) whereas 14% responded to more than one exteroceptive stimulus in a similar manner. Amygdalar cells were also identified that responded to visceral (renal nerve) and somatic (iliac nerve) stimulation. Many cells received input both from enteroreceptors and from exteroceptors. These results suggest that baroreceptor input to single neurons in the amygdala is often convergent with input from the locus ceruleus. In addition, the convergence of both internal and external sensory inputs to single neurons is further evidence that the amygdala is a site for the integration of responses to arousal.     

Responses of rostroventrolateral medulla spinal vasomotor neurones to chemoreceptor stimulation in rats

The activity of spinal projecting 'vasomotor' neurones of the rostroventrolateral medulla (RVL) has been recorded in anaesthetised and paralysed rats. They responded to carotid body chemoreceptor stimulation (0.208 M NaH2PO4) with an increase in discharge that preceded an evoked increase in arterial blood pressure. This initial excitatory response was followed by a fall in discharge correlated to the rise in blood pressure. RVL neurones, which had equivalent sensitivity to baroreceptor inputs but no spinal projecting axons, failed to exhibit the initial excitatory response on chemoreceptor stimulation. These observations suggest that the pressor response to chemoreceptor stimulation is mediated, at least partially, by excitation of RVL-spinal 'vasomotor' neurones.     

Responses of rat dorsal horn neurons to natural stimulation and to iontophoretically applied norepinephrine

Extracellular recordings were obtained of 177 neurons throughout the lumbar spinal dorsal horn of urethane- or halothane-anesthetized rats. These neurons all responded to iontophoretically applied L-glutamate and their responses to natural stimulation of the ipsilateral hindlimb were characterized. Iontophoretically applied norepinephrine was tested on 94 of these neurons. Fifty-one neurons were inhibited and 22 were excited. Norepinephrine produced a biphasic inhibitory/excitatory effect on nine neurons. Norepinephrine was exclusively inhibitory on superficial dorsal horn neurons that responded only to innocuous brush and touch and on neurons in the nucleus proprius that responded to brush, touch, and noxious skin pinch. Norepinephrine excited some superficial brush/touch/pinch neurons and produced short inhibitions that were followed by prolonged excitations of some nucleus proprius neurons that responded only to noxious skin pinch. Neurons in the base of the dorsal horn that responded to low-threshold proprioceptive stimulation were excited by norepinephrine. Both the inhibitory and excitatory effects of norepinephrine were stereoselective, but they were not blocked by receptor subtype-selective antagonists. Desensitization to norepinephrine occurred for 30% of the neurons. This study demonstrates that the inhibitory effects of norepinephrine on rat dorsal horn neurons are not restricted to neurons that are responsive to noxious stimuli and that some of these neurons are primarily excited by norepinephrine. The excitatory effects of norepinephrine on low-threshold proprioceptive neurons may contribute to norepinephrine's known enhancement of spinal flexor reflex activity.     

GABA-mediated corticofugal inhibition of taste-responsive neurons in the nucleus of the solitary tract

The nucleus of the solitary tract (NST) receives descending connections from several forebrain targets of the gustatory system, including the insular cortex. Many taste-responsive cells in the NST are inhibited by gamma-aminobutyric acid (GABA). In the present study, we investigated the effects of cortical stimulation on the activity of gustatory neurons in the NST. Multibarrel glass micropipettes were used to record the activity of NST neurons extracellularly and to apply the GABA(A) antagonist bicuculline methiodide (BICM) into the vicinity of the cell. Taste stimuli were 0.032 M sucrose (S), 0.032 M NaCl (N), 0.00032 M citric acid (H), and 0.032 M quinine hydrochloride (Q), presented to the anterior tongue. Each of 50 NST cells was classified as S-, N-, H-, or Q-best on the basis of its response to chemical stimulation of the tongue. The ipsilateral insular cortex was stimulated both electrically (0.5 mA, 100 Hz, 0.2 ms) and chemically (10 mM DL-homocysteic acid, DLH), while the spontaneous activity of each NST cell was recorded. The baseline activity of 34% of the cells (n=17) was modulated by cortical stimulation: eight cells were inhibited and nine were excited. BICM microinjected into the NST blocked the cortical-induced inhibition but had no effect on the excitatory response. Although the excitatory effects were distributed across S-, N-, and H-best neurons, the inhibitory effects of cortical stimulation were significantly more common in N-best cells. These data suggest that corticofugal input to the NST may differentially inhibit gustatory afferent activity through GABAergic mechanisms.     

Electrophysiological analysis of corticofugal influences on neurons of the preoptic area]

Evoked potentials (EPs) and neuronal responses of the medial (MPO) and lateral (LPO) preoptic region (RPO) and adjacent areas of the hypothalamus to stimulation of the prefrontal (area 8), cingulum (area 24), periamigdaloideus (RPA) cortex and hippocampus (area CA3) have been studied on cats anesthetized with ketamine. Cortical areas were selected according to their evolutionary status. It is determined that the most pronounced EPs are generated in the RPO to stimulation of the periamigdaloideus and cingulum cortex. A close correlation between responses of single units and EPs waveforms is shown. The majority of neurons responsible for cortical stimuli are located mainly in the basal part of the LPO. It is this area of the RPO where the prevalence of initial excitatory reactions is observed. The MPO contains much less neurons responding to cortical stimulation. In this area of the RPO mainly initial inhibitory reactions are registered. For the LPO a ratio between the inhibitory and excitatory reactions is 0.6:1, for the MPO the same ratio is 5.8:1. In the area of the RPO adjoining the bed nucleus stria terminalis (BST) the initial inhibitory reactions prevail over the initial excitatory ones too (ratio 4.9:1). In the area of the RPO immediately adjoining the nucleus supraopticus the initial excitatory reactions are registered and the ratio between the initial inhibitory reactions and initial excitatory ones is 0.7:1. In the RPO a pronounced convergence of the stimuli from the different cortical areas to the definite cells is found as well. In the RPO which is a zone of wide convergence 3/4 of neurons responded to stimulation of two and more cortical inputs. The combinations of inputs from the prefrontal and cingulum cortex as well as from prefrontal and periamigdaloideus cortex are the most frequent types of convergence of different cortical stimuli to the preoptic neurons.     

The effect of morphine on responses of ventrolateral orbital cortex (VLO) neurons to colorectal distension in the rat

In 49 halothane-anesthetized rats, we characterized the responses of single neurons in the ventrolateral orbital cortex (VLO) to a noxious visceral stimulus (colorectal balloon distension, CRD), and studied the effects of intravenous morphine on these responses using standard extracellular microelectrode recording techniques. One hundred and four neurons were isolated on the basis of spontaneous activity. Fifty-seven (55%) responded to CRD, of which 32% had excitatory and 68% had inhibitory responses. Neurons showed tendencies toward graded responses to graded CRD pressures (20–100 mmHg), with maximum excitation or inhibition occurring at 80 or 100 mmHg, respectively. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 80 of the VLO neurons isolated. Thirty-three (41%) of these neurons (21 CRD-responsive and 12 CRD-nonresponsive) had cutaneous receptive fields, of which 79% were large and bilateral, 18% were small and bilateral, 3% were small and ipsilateral. Ninety-four percent of these neurons responded only to noxious cutaneous stimulation, 6% responded to both noxious and innocuous stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.0625, 0.125 and 0.25 mg/kg i.v.) produced statistically significant dose-dependent attenuation of neuronal responses to CRD. Naloxone (0.4 mg/kg i.v.) reversed the effects of morphine. Morphine and naloxone had no significant effects on spontaneous activity. These data support the involvement of VLO neurons in visceral nociception.     

Cortical responses to electrical and gustatory stimuli in the rabbit.

The distribution of surface positive cortical potentials evoked by electrical stimulation of the chorda tympani, glossopharyngeal and lingual nerves which innervate the tongue was mapped in rabbits. All projections were bilateral. Judging from the extent of the cortical response area and the amplitude and latency of the responses, the major projection of the chorda tympani was ipsilateral, whereas that of the lingual and the glossopharyngeal nerves was contralateral. Both the chorda tympani and the glossopharyngeal nerve project to a confined area in the insular cortex and the lingual nerve projects to the appropriate part of the somatotopic pattern of somatic sensory area I. Further, a single unit study was undertaken to characterize the response of units in the cerebral cortex which was induced by gustatory stimulation of the anterior tongue, Twenty-four gustatory units were found in the insular cortex and the claustrum. The gustatory units were divided into an early response type (21 units) and a late response type (3 units) based on latency measurements. Gustatory units were also classified according to discharge patterns into excitation type (21 units) and inhibition type (4 units). Eleven units responded to 1 or 2 kinds of conventional taste stimuli, and 13 units responded to more than 3 different taste stimuli. Sensitivities of cortical units to the 4 conventional taste stimuli were found to be mutually independent and randomly distributed among cortical units. The frequency of discharges increased in the excitation type units and decreased in the inhibition type units monotonically with the excitation type units and decreased in the inhibition type units monotonically with an increse of NaCl concentration exfept at the highest concentrations.     

Characterization, distribution and lateralization of baroreceptor-related neurons in the rat insular cortex

The insular cortex contains a site of cardiovascular representation. Stimulation experiments suggest a discrete localization within the rostral posterior insula. In 34 urethane-anesthetized male Sprague-Dawley rats, we investigated whether cells responsive to baroreceptor stimulation with phenylephrine and sodium nitroprusside were selectively clustered within the insula compared with the surrounding frontoparietal cortex, the extent of distribution of these responsive cells within the insula, and whether there was any lateralization of response. In addition, we characterized the cells as SE (sympathoexcitatory), SI (sympathoinhibitory) or null cells using the criteria of Barman. Of the 128 insular cells investigated with extracellular recording techniques, 70% responded to baroreceptor manipulations compared to 32% of the 57 cells investigated outside the insula (P<0.0001). The majority of the responsive cells were SE units and were distributed widely throughout the insular cortex including anterior areas not previously thought to be involved in cardiovascular control. Within the rostral posterior insula from which cardiovascular effects are mainly obtained in stimulation experiments, lateralization was identified, with significantly more cells responding to blood pressure changes being found within the right posterior insula than the left (P<0.003). These data confirm the importance of the right posterior insula in the rat as a site of cardiovascular representation; identify a more extensive distribution of cells responsive to blood pressure changes within the insula than previous studies and imply more widespread convergence of visceral afferent information within the insula.     

Insular cortex and neuropsychiatric disorders: a review of recent literature.

The insular cortex is located in the centre of the cerebral hemisphere, having connections with the primary and secondary somatosensory areas, anterior cingulate cortex, amygdaloid body, prefrontal cortex, superior temporal gyrus, temporal pole, orbitofrontal cortex, frontal and parietal opercula, primary and association auditory cortices, visual association cortex, olfactory bulb, hippocampus, entorhinal cortex, and motor cortex. Accordingly, dense connections exist among insular cortex neurons. The insular cortex is involved in the processing of visceral sensory, visceral motor, vestibular, attention, pain, emotion, verbal, motor information, inputs related to music and eating, in addition to gustatory, olfactory, visual, auditory, and tactile data. In this article, the literature on the relationship between the insular cortex and neuropsychiatric disorders was summarized following a computer search of the Pub-Med database. Recent neuroimaging data, including voxel based morphometry, PET and fMRI, revealed that the insular cortex was involved in various neuropsychiatric diseases such as mood disorders, panic disorders, PTSD, obsessive-compulsive disorders, eating disorders, and schizophrenia. Investigations of functions and connections of the insular cortex suggest that sensory information including gustatory, olfactory, visual, auditory, and tactile inputs converge on the insular cortex, and that these multimodal sensory information may be integrated there.     

Convergence of afferent inputs from the chorda tympani, lingual-tonsillar and pharyngeal branches of the glossopharyngeal nerve, and superior laryngeal nerve on the neurons in the insular cortex in rats

The responses of single neurons in the insular cortex to electrical stimulation of the chorda tympani (CT), lingual-tonsillar branch of the glossopharyngeal (LT-IXth) nerve, pharyngeal branch of the glossopharyngeal (PH-IXth) nerve, and superior laryngeal (SL) nerve were recorded in anaesthetized and paralyzed rats. Ninety-four neurons responding to stimulation of at least one of the four nerves were identified from the insular cortex. Most of the neurons were located in the posterior portion of the insular cortex; the mean location was 0.8 mm anterior to the anterior edge of the joining of the anterior commissure (AC) and was 1.4 mm dorsal to the rhinal fissure (RF). Of the 94 neurons, 84 (89%) received convergent inputs from two or more nerves, and the remaining 10 (11%) received inputs from one nerve. The neurons responding to the CT stimulation were distributed more anteriorly than those responding to other three nerves in the anterior-posterior dimension. Our results indicate that the neurons recorded mainly from the posterior portion of the insular cortex receive convergent inputs from the oropharyngolaryngeal regions.     

Electrical stimulation of thalamic Nucleus Submedius inhibits responses of spinal dorsal horn neurons to colorectal distension in the rat

In 78 halothane-anesthetized rats, we characterized the responses of single neurons in the dorsal horn of L(6)-S(1) spinal segments to a noxious visceral stimulus (colorectal balloon distension, CRD), and studied the effects of focal electrical stimulation of Nucleus Submedius (Sm) on these responses using standard extracellular microelectrode recording techniques. A total of 102 neurons were isolated on the basis of spontaneous activity. Eighty (78%) responded to CRD, of which 70% had excitatory and 30% had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 100 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 73 of the spinal dorsal horn neurons isolated. Fifty-seven (78%) of these neurons (46 CRD-responsive and 11 CRD-nonresponsive) had cutaneous receptive fields, of which 35 (61%) were small and ipsilateral, 14 (25%) were large and ipsilateral, 7 (12%) were large or small and bilateral, and 1 (2%) was small and contralateral. Sixty-one percent of these neurons responded to both noxious and innocuous cutaneous stimulation, 35% responded only to noxious stimulation, and 4% responded only to innocuous stimulation. Electrical stimulation (50-300 microA) of the contralateral Sm produced intensity-dependent attenuation of the CRD-evoked activities of most neurons (18/28 of CRD-excited and 7/12 of CRD-inhibited) tested. Sm stimulation produced facilitation of CRD responses of only one neuron (CRD-inhibited). Sm stimulation had no effects on spontaneous activity. These data indicate that Sm may be involved in the descending inhibitory modulation of visceral nociception at the spinal level.     

Responses of neurons in the nucleus basalis of Meynert to various afferent stimuli in rats

The effects of innocuous and noxious mechanical stimulation of skin, and of baroreceptor and chemoreceptor stimulation, on the activity of single neurons in the nucleus basalis of Meynert (NBM), whose axons project to the cortex, were examined in urethane-anesthetized adult rats. Most of the neurons were not significantly influenced by innocuous mechanical cutaneous stimulation or baroreceptor stimulation, while they were excited by noxious mechanical cutaneous stimulation and chemoreceptor stimulation. The NBM neurons were excited more intensely and frequently by nociceptive mechanical stimulation to a fore- or hindpaw than by that to the back or face. The function of these NBM neurons is discussed.     

The effect of morphine on responses of nucleus ventroposterolateralis neurons to colorectal distension in the rat

In 71 halothane-anesthetized rats, we characterized the responses of single neurons in the nucleus ventroposterolateralis (VPL) of the thalamus to a noxious visceral stimulus (colorectal balloon distension; CRD) and studied the effects of intravenous morphine on these responses using standard extracellular microelectrode recording techniques. One hundred nine neurons were isolated on the basis of spontaneous activity. Sixty-four (59%) responded to CRD, of which 52 (81 %) had excitatory and 12 (19%) had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 80 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 95 of the VPL neurons isolated. Eighty-three of these neurons (48 CRD responsive and 35 CRD nonresponsive) (87%) had cutaneous receptive fields, of which 96% were small and contralateral and 4% were large and contralateral or bilateral. Ninety-four percent of these neurons responded to both noxious and innocuous cutaneous stimulation, and 6% responded to only noxious stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.125, 0.25, 0.5, 1, and 2 mg/kg, i.v) produced statistically significant dose-dependent attenuation of neuronal responses to CRD. Naloxone (0.4 mg/ kg, i.v.) reversed the effects of morphine. Morphine and naloxone had no significant effects on spontaneous activity. These data support the involvement of VPL neurons in visceral nociception and are consistent with a role of VPL in sensory-discriminative aspects of nociception.     

Reactions of neurons of the preoptic area in the presence of serial cortical stimuli]

Evoked potentials (EPs) and neuronal responses of the medial (MPO) and lateral (LPO) preoptic region (RPO) and adjacent areas of the anterior hypothalamus to serial (6-300 s) stimulation of the prefrontal (area 8), cingulum (area 24), periamygdaloideus (RPA) cortex and hippocampus (field CA3) were studied on experimental cats anesthetized by ketamine. Cortical areas were selected according to their evolutionary status. Responses of preoptic neurons to serial stimulation were grouped into four main types: excitatory, inhibitory, excitatory on-off-effect and inhibitory on-off-effect. No pronounced changes in the patterns of response reactions were found when frequency of the stimuli increased, only intensity of the reactions changed. Neurons responding to serial cortical stimulation were located mainly in the central part of the MPO and in the basal part of the LPO. These areas of the RPO contained neurons with the most pronounced convergence of different cortical stimuli to the same cell. In serial cortical stimulation inhibitory reactions developed more frequently than excitatory ones (ratio 3.4:1). A gradient of inhibitory reactions to stimulation of different cortical areas was observed while comparing them in terms of evolution from new to old structures. Prevalence of inhibitory responses over excitatory ones was minimal (ratio 1.7:1), when the neocortex (area 8) was stimulated and it was more pronounced (ratio 1.9:1) during stimulation of the evolutionary intermediate cortex (area 24). That prevalence was still more significant (ratio 4.5:1) in case of paleocortical (RPA) stimulation and it was the most pronounced (ratio 10.2:1) during stimulation of the archicortex (hippocampus).     

Nociceptive projection from tooth pulp to the lateral hypothalamus in rats

The response of the rat lateral hypothalamic (LHA) neurons to tooth pulp electrical stimulation and the sensory projection pathway from the incisor pulp to the LHA were studied by electrophysiology and histology. 1) LHA neurons that responded to contralateral lower incisor pulp stimulation were found in the lateral part of the LHA. These neurons also responded to intensive tail pinch, but not to innocuous stimuli nor to applied glucose. 2) Histological study after injection of WGA-HRP into the lateral part of the LHA revealed many retrogradely labeled neurons in the ventral part of the periventricular gray (PVG) in the mesencephalon. 3) The PVG neurons responded antidromically to LHA stimulation and to contralateral lower incisor pulp stimulation. 4) After injection of WGA-HRP into the ventral portion of the PVG, many labeled cells were found in the contralateral subnucleus caudalis in the spinal tract of the trigeminal nucleus (NTST) where termination of the pulpal afferent was previously reported. It is thus suggested that the PVG is the most likely site of transmission relay of nociceptive inputs from incisors to the LHA.     

Somatic nociception activates NK1 receptors in the nucleus tractus solitarii to attenuate the baroreceptor cardiac reflex

There is limited information regarding the integration of visceral and somatic afferents within the nucleus of the solitary tract (NTS). We studied the interaction of nociceptive and baroreceptive inputs in this nucleus in an in situ arterially perfused, un-anaesthetized decerebrate preparation of rat. At the systemic level, the gain of the cardiac component of the baroreceptor reflex was attenuated significantly by noxious mechanical stimulation of a forepaw. This baroreceptor reflex depression was mimicked by NTS microinjection of substance P and antagonized by microinjection of either bicuculline (a GABAA receptor antagonist) or a neurokinin type 1 (NK1) receptor antagonist (CP-99994). The substance P effect was also blocked by a bilateral microinjection of bicuculline, at a dose that was without effect on basal baroreceptor reflex gain. Baroreceptive NTS neurons were defined by their excitatory response following increases in pressure within the ipsilateral carotid sinus. In 27 of 34 neurons the number of evoked spikes from baroreceptor stimulation was reduced significantly by concomitant electrical stimulation of the brachial nerve (P < 0.01). Furthermore, the attenuation of baroreceptor inputs to NTS neurons by brachial nerve stimulation was prevented by pressure-ejection of bicuculline from a multi-barrelled microelectrode (n = 8). In a separate population of 17 of 45 cells tested, brachial nerve stimulation evoked an excitatory response that was antagonized by blockade of NK1 receptors. We conclude that nociceptive afferents activate NK1 receptors, which in turn excite GABAergic interneurons impinging on cells mediating the cardiac component of the baroreceptor reflex.     

Responses of neurons in the caudal intralaminar thalamic complex of the rat to stimulation of the uterus, vagina, cervix, colon and skin

This study examined responses of 35 neurons in the caudal intralaminar (IL) thalamic nuclei in 12 adult female virgin rats to mechanical stimulation of the skin (brush, pressure, pinch) and to 4 different visceral stimuli (noxious distension of the uterine horns and vaginal canal; gentle distension of the colon and probing the cervix). As in male rats and other species, many IL neurons () responded to frankly noxious somatic stimuli applied to several bodily regions. Some of these () also responded to one or more of the visceral stimuli (mainly the noxious ones), while responded only to a visceral stimulus. Thus, unlike neurons in lateral thalamus studied under identical conditions, IL neurons appear to be signalling information primarily when intense somatic and visceral stimuli are frankly above the noxious threshold.     

Visceral cortex: integration of the mucosal senses with limbic information in the rat agranular insular cortex

The organization of the subcortical and cortical connections of the rat agranular insular cortex was examined. Retrogradely transported dyes were used to map the agranular insular cortex efferents to brainstem visceral nuclei (the nucleus of the solitary tract and the parabrachial nucleus), to gustatory-visceral and limbic thalamic nuclei (medial ventrobasal and mediodorsal thalamus, respectively), and to association cortex (medial prefrontal and contralateral agranular insular cortex). The results revealed that a specific area within the ipsilateral agranular insular cortex projected to all of the subcortical and cortical areas listed above. This area of overlap in the agranular insular cortex stretched from the level of the genu of the corpus callosum rostrally to the crossing of the anterior commissure caudally. Anterograde projections from the medial ventrobasal and mediodorsal thalamus and from the olfactory bulb to the agranular insular cortex were mapped with wheat germ agglutinin conjugated to horseradish peroxidase. The terminal cortical projections from these areas were generally separate, except in an area where they overlap immediately medial to the rhinal fissure in the agranular insular cortex. This overlap area matched the area in the agranular insular cortex where there was an overlap of cortical efferent cells projecting to the brainstem, thalamus, and association cortex, as revealed in the retrograde tracing studies. We refer to this region of convergence in the agranular insular cortex as the visceral cortex, and suggest its involvement in the efficient integration of specific visceral sensory stimuli with correlated limbic or motivational consequences. The visceral cortex may help regulate the organism's visceral response to stress.