Spontaneous saccades under different visual conditions in the pigeon.

Spontaneous saccades of both eyes were recorded in head-restrained pigeons placed in 6 different visual conditions (darkness and biocular, uniocular, frontal or lateral viewing). Most saccades (95%) were biocular and directed forward (around the beak axis) and backward (around the horizontal line). In the dark, the proportions of forward and backward saccades were different, they became symmetrical when the visual input involved either the left eye, the lateral fields or both eyes. This spatial reorganization could be mediated by one 'dominant' eye through the lateral and foveal vision. Although some biocular saccades were strictly convergent, divergent or conjugate, most of them showed a higher independence since they had different directions and amplitudes for each eye.     

Eye movements evoked by electrical stimualtion of the brain in anesthetized fishes.

Several eye movements were evoked by electrical stimulation of the brain in anesthetized sunfish and goldfish. Conjugate lateral rolling movements, similar to eye movements observed when an unoperated fish is rotated about its long axis, were evoked from the acoustico-lateral area of the medulla and the eminentia granularis and an adjacent medial portion of the cerebellum. Bilateral and unilateral backward rotations, similar to the eye movements observed when unoperated fish are rotated forward about the interpupillary axis, were evoked from the medial longitudinal fasciculus and areas related to the oculomotor nerve. Bilateral forward rotations, comparable to the eye movements resulting when unoperated fish are rotated backward about the interpupillary axis, were elicited by stimulation near the trochlear nerve roots in the valvula of the cerebellum; unilateral responses resulted from stimulation near the exiting trochlear nerves. Convergence was elicited by stimulation in the midline near the oculomotor complex and the medial longitudinal fasciculus while unilateral vergence responses were triggered by stimulation in the medial longitudinal fasciculus and areas lateral to the oculomotor nucleus. Conjugate eye movements in the horizontal plane were frequently evoked but were not studied in detail.     

Physiological properties and patterns of projection in the cortico-cortical connections from the second somatosensory cortex to the motor cortex, area 4 gamma, in the cat

The physiological properties of neurons in the second somatosensory cortex (SII), and the pattern of projection of these neurons to area 4 gamma of the motor cortex in cat were studied by using single unit recording and collision techniques. Antidromically activated neurons were recorded along the anterior and posterior regions of the lateral bank of the anterior suprasylvian sulcus (ASSS) and from the middle part of the anterior ectosylvian gyrus (AESG) following weak intracortical microstimulation (ICMS) to area 4 gamma. Stimulation of the region around the activated neurons failed to produce muscle contraction or movement with currents of 30 microA or less. The majority of antidromically identified neurons received somatotopically organized afferent inputs from the skin on the contralateral side of the body. A small number of SII neurons received bilateral input. In 91% of the cases receptive field information was available for both the antidromically activated SII neuron and for neurons around the stimulating electrode in area 4 gamma. In 71% of these cases, both cortical sites were activated by sensory input from the same or adjacent peripheral area of the body. Neurons in the rostrocaudal region of the lateral bank of ASSS and the upper part of AESG (forelimb area) projected to the lateral cruciate gyrus of the motor cortex (forelimb area), while neurons in the ventrocaudal region of the medial part of AESG (hindlimb area) projected to the medial part of the postcruciate subregion of the motor cortex (hindlimb area). Antidromically activated SII neurons were typically found in layer III. These results suggest a topographically organized pattern of projection to the motor cortex from SII.     

Movement representation in the dorsal and ventral premotor areas of owl monkeys: A microstimulation study

We used intracortical microstimulation to investigate the lateral premotor cortex and neighboring areas in 14 hemispheres of owl monkeys, focusing on the somatotopic distribution of evoked movements, thresholds for forelimb movements, and the relative representation of proximal and distal forelimb movements. We elicited movements from the dorsal and ventral premotor areas (PMD, PMV), the caudal and rostral divisions of primary motor cortex (Mlc, Mlr), the frontal eye field (FEF), the dorsal oculomotor area (OMD; area 8b), the supplementary motor area (SMA), and somatosensory cortex (areas 3a and 3b). Area PMD was composed of architectonically distinguishable caudal and rostral subdivisions (PMDc, PMDr). Stimulation of PMD elicited movements of the hindlimb, forelimb, neck and upper trunk, face, and eyes. Hindlimb and forelimb movements were represented in the caudalmost part of PMDc. Face, neck, and eye movements were represented in the lateral and rostral parts of PMDc and in PMDr. Stimulation of PMV elicited forelimb and orofacial movements, but not hindlimb movements. Both proximal and distal forelimb movements were elicited from PMDc and PMV, although PMD stimulation elicited mainly shoulder and elbow movements, while PMV stimulation evoked primarily wrist and digit movements. Distal movements were evoked more frequently from PMV than from Mlr or Mlc. Across cases, the median forelimb thresholds for PMDc and PMV were 60 and 36 μA, respectively, values that differ significantly from each other and from the value of 11 μA obtained for Mlr. Our observations indicate that premotor cortex is much more responsive to electrical stimulation than commonly thought, and contains a large territory from which eye movements can be elicited. These results suggest that in humans, much of the electrically excitable cortex located on the precentral gyrus, including cortex sometimes considered part of the frontal eye field, is probably homologous to the premotor cortex of nonhuman primates. © 1996 Wiley-Liss, Inc.     

Abnormalities of Bell''s phenomenon in amyotrophic lateral sclerosis: a clinical and electrophysiological evaluation.

A clinical and electromyographic study of oculomotor function was carried out in a series of 24 patients with amyotrophic lateral sclerosis (ALS). In 15 cases an alteration of Bell's phenomenon was found. In addition, three patients showed some impairment of conjugate ocular motility in the form of upward gaze paly. All cases had preserved oculocephalic reflexes in the vertical and horizontal planes. On clinical and electromyographic grounds, three degrees of altered Bell's phenomenon are suggested: attenuated (short and unsustained upward displacement of the eyeballs after forced closure of the eyelids), abolished (no upward displacement), and inverted (downward instead of upward displacement of the eyes). These oculomotor alterations were not directly related to the type of ALS at onset of the illness, nor with its duration. However they were correlated with the relative degree of the clinical bilateral pyramidal tract signs at the supraspinal level. The common involvement of the corticogeniculate tract in ALS could explain the unexpectedly high incidence of alteration of Bell's phenomenon found in this disease, but is is non-specific and similar lesions from different causes may also produce it.     

Vestibulo-ocular reflex deficits with medial longitudinal fasciculus lesions

The medial longitudinal fasciculus (MLF) is the final common pathway for all conjugate adducting horizontal eye movements, as well as for the vertical–torsional vestibulo-ocular reflex (VOR). MLF lesion causes adduction paresis of ipsilesional (adducting) eye with dissociated nystagmus of contralesional (abducting) eye—the well-known clinical syndrome of internuclear ophthalmoplegia (INO). We measured the VOR stimulation and also any catch-up saccades, from individual semicircular canal (SCC) evoked by the head impulse test (HIT), using head and binocular 3-dimensional scleral search coils in 27 multiple sclerosis (MS) patients, 8 with unilateral, 19 with bilateral INO. In unilateral INO, VOR gain (normal >0.90) from ipsilesional lateral SCC stimulation was 0.48 for the adducting eye and 0.81 for the abducting eye; 0.61 from contralesional anterior SCC stimulation and only 0.29 from contralesional posterior SCC stimulation. In bilateral INO, there were VOR gain deficits from all six SCCs: lateral SCC gains were asymmetrically reduced to 0.45 in the adducting eye and 0.66 in the abducting eye; anterior SCC gain was 0.48 and posterior SCC gain was only 0.19. Horizontal VOR versional dysconjugacy between adducting and abducting eyes at 0.66 was less severe than horizontal catch-up saccade versional dysconjugacy (0.44); normal >0.80. Unexpected partial preservation of horizontal VOR with greater catch-up saccade impairment from the adducting than abducting eye suggests that the ascending tract of Deiters (ATD), an extra-MLF pathway, also mediates the horizontal VOR, but not adducting horizontal saccades. Vertical VOR deficits will produce vertical oscillopsia with any vertical head movement and measurement of the vertical VOR could help with the diagnosis and quantitative evaluation of MLF lesions in suspected MS. Horizontal VOR deficits and catch-up saccade versional dysconjugacy in INO will cause gaze instability and horizontal oscillopsia during active horizontal head movements.     

The role of the posterior medial bed nucleus of the stria terminalis in modulating hypothalamic-pituitary-adrenocortical axis responsiveness to acute and chronic stress

The bed nucleus of the stria terminalis (BST) plays a prominent role in brain integration of acute responses to stressful stimuli. This study tests the hypothesis that the BST plays a complementary role in regulation of physiological changes associated with chronic stress exposure. Male Sprague-Dawley rats received bilateral ibotenate lesions or sham lesions of the posterior medial region of the BST (BSTpm), an area known to be involved in inhibition of HPA axis responses to acute stress. Chronic stress was induced by 14-day exposure to twice daily stressors in an unpredictable sequence (chronic variable stress, CVS). In the morning after the end of CVS, stressed and non-stressed controls were exposed to a novel restraint stress challenge. As previously documented, CVS caused adrenal hypertrophy, thymic involution, and attenuated body weight gain. None of these endpoints were affected by BSTpm lesions. Chronic stress exposure facilitated plasma corticosterone responses to the novel restraint stress and elevated CRH mRNA. Lesions of the BSTpm increased novel stressor-induced plasma ACTH and corticosterone secretion and enhanced c-fos mRNA induction in the paraventricular nucleus of the hypothalamus (PVN). In addition, lesion of the BSTpm resulted in an additive increase in CVS-induced facilitation of corticosterone responses and PVN CRH expression. Collectively these data confirm that the BSTpm markedly inhibits HPA responses to acute stress, but do not strongly support an additional role for this region in limiting HPA axis responses to chronic drive. The data further suggest that acute versus chronic stress integration are subserved by different brain circuitry.     

Effects of chronic rotation and hypergravity on muscle fibers of soleus and plantaris muscles of the rat

Three lines of F₃ Sprague-Dawley rats derived from a single mating were raised either outside the centrifuge at earth gravity (earth control), or under chronic rotation in the center of the centrifuge at 1.03g (rotation control), or in the rim of the centrifuge at 2g (rotation experimental). The rats were killed at 3 months of age, and serial sections of plantaris and soleus muscles were stained for succinic dehydrogenase activity, and for actomyosin ATPase activity following preincubation at pH 10.2. Muscle fibers in a cross-sectional unit area of soleus and both the superficial and deep regions of plantaris were identified as to fiber type according to an enzyme profile and counted. The proportion of each fiber type was calculated, and the diameter of 24 fibers of each type was measured. Analysis of variance was used to determine the effect of experimental treatment and sex on populations and diameters. In soleus, the fiber population shifted from 82% slow oxidative fibers in the controls to 100% slow oxidative fibers in the rotation experimentals in response to the stress of hypergravity. In plantaris, chronic rotation resulted in an increase in fast glycolytic fibers and a corresponding decrease in fast oxidative glycolytic fibers, in both the rotation control and experimental rats. The population changes were similar in both sexes. Muscle fiber diameters were similar in both controls, indicating no response to the stress of chronic rotation. A sexual dichotomy was noted in response to hypergravity, with muscle fiber diameters increasing in females, but decreasing in males.     

GABAergic mechanisms in nucleus tractus solitarius alter blood pressure and vasopressin release

GABAergic agents microinjected into the nucleus tractus solitarius (NTS) influence blood pressure (BP) and plasma vasopressin (VP) levels. The direct-acting GABAergic agonist muscimol (3.75-160 pmol) microinjected bilaterally into the NTS of chloralose-anesthetized, paralyzed, ventilated rats increased BP without significantly altering heart rate. Similar results were obtained using the indirect GABA agonist nipecotic acid (10 nmol), a GABA uptake blocker. In contrast, blocking the action of GABA with bicuculline (5 pmol) elicited a small but consistent decrease in BP. Injections of the vehicle (artificial cerebrospinal fluid) into the NTS or GABAergic agents in an area lateral to the NTS did not alter BP. Unilateral injection of muscimol into the NTS did not elicit immediate pressor responses as did bilateral injections; unilateral muscimol injections following destruction of the contralateral NTS produced a pressor response similar to that seen following bilateral muscimol administration. Stimulation of GABA receptors within the NTS by either muscimol or nipecotic acid resulted in an increase in plasma VP levels. The elevated levels of plasma VP contributed to the pressor response elicited by stimulation of GABAergic receptors as evidenced by the decrease in BP observed following the intravenous administration of a VP pressor antagonist during the pressor response. These studies indicate that tonically active GABAergic mechanisms within the NTS influence BP and VP release, and provide further evidence that VP can be involved in cardiovascular responses elicited from the NTS.     

Laminar organization of spinal dorsal horn neurones activated by C- vs. A-heat nociceptors and their descending control from the periaqueductal grey in the rat

The periaqueductal grey can differentially control A- vs. C-nociceptor-evoked spinal reflexes and deep spinal dorsal horn neuronal responses. However, little is known about the control of A- vs. C-fibre inputs to lamina I and the lateral spinal nucleus, and how this correlates with the control of deeper laminae. To address this, the laminar distributions of neurones expressing Fos-like immunoreactivity were determined following preferential activation of A- or C-heat nociceptors, using fast or slow rates of skin heating, respectively, in the absence or presence of descending control evoked from the periaqueductal grey. In lamina I, numbers of Fos-positive neurones following both fast and slow rates of skin heating were reduced significantly following activation in the ventrolateral and dorsolateral/lateral periaqueductal grey. In contrast, in the deep dorsal horn (laminae III-VI), activation in both the ventrolateral and dorsolateral/lateral periaqueductal grey significantly reduced the numbers of Fos-positive neurones evoked by C- but not A-nociceptor stimulation. C- but not A-heat nociceptor activation evoked Fos bilaterally in the lateral spinal nucleus. Stimulation in the ventrolateral but not the dorsolateral/lateral periaqueductal grey significantly increased the numbers of Fos-positive neurones evoked by A- and C-nociceptor stimulation bilaterally in the lateral spinal nucleus. These data have demonstrated differences in the descending control of the superficial vs. the deep dorsal horn and lateral spinal nucleus with respect to the processing of A- and C-fibre-evoked events. The data are discussed in relation to the roles of A- and C-nociceptors in acute and chronic pain.     

Electroencephalographic studies of chlorpromazine methiodide and somatostatin-induced barrel rotation in rats

It has been reported that intraventricular injection of chlorpromazine methiodide (CPZMI), a quaternary ammonium derivative of chlorpromazine, in rats induces abnormal, twisting postures which may serve as an experimental model of the human movement disorder dystonia. We have shown elsewhere that the behavior induced by intraventricular CPMZI is identical to what has been called "barrel rotation," first observed to follow intraventricular injection of somatostatin (SRIF), which consists of twisting about the long axis, with repetitive lateral rolling. The suitability of barrel rotation, induced by CPZMI or SRIF, as an experimental model for dystonia depends on its physiologic basis. Human dystonia is clinically not a convulsive phenomenon. SRIF-induced barrel rotation has been reported to be associated with epileptiform activity recorded by the electroencephalogram (EEG). The purpose of this study was to investigate EEG activity during CPZMI- and SRIF-induced rotation. We found that CPZMI barrel rotation was not associated with epileptiform activity in cortex, amygdala, or hippocampus, and contrary to prior reports, neither was SRIF rotation. Both CPZMI and SRIF injected in high doses could induce epileptiform activity, but this was associated with clonic motor phenomena and not barrel rotation. We conclude that electroencephalographic criteria do not exclude either CPZMI- or SRIF-induced rotation as models for movement disorders, but their validity as such requires further study.     

Dopaminergic and serotonergic correlates of stimulation-induced circling

Rotation induced by electrical stimulation of the medial forebrain bundle at the level of the lateral hypothalamus was associated with increases in dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striatum ipsilateral to the site of stimulation (i.e. contralateral to direction of turning). The concentrations of DA, DOPAC and HVA within the nucleus accumbens (NAS) were not altered. In the olfactory tubercle (OT), concentrations of DA and both metabolites were, in general, elevated ipsilateral to the electrode. However, relative to non-stimulated controls, HVA concentrations were increased bilaterally in rats exhibiting circling. Stimulation-induced circling also resulted in a bilateral enhancement of striatal serotonin (5-HT) metabolism as indicated by elevated 5-hydroxyindoleacetic acid: 5-HT ratios. No changes in 5-HT metabolism were observed in the NAS. The utilization of 5-HT was elevated in the OT ipsilateral to the electrode in rats that exhibited stimulation-induced rotation. While most subjects that exhibited contraversive rotation in response to the stimulation demonstrated enhanced DA activity, the neurochemical changes were not observed in all subjects. As such, it is concluded that while stimulation of the mesotelencephalic DA system can be associated with stimulation-induced rotation it is not necessary for its elicitation.     

Topography of somatosensory processing: cerebral lateralization and focused attention.

Healthy dextrals underwent fMRI during a task of graphesthesia requiring detection of any number written consecutively from an otherwise random number sequence. Test conditions included (1) focus on unilateral right hand stimuli, (2) focus on unilateral left hand stimuli, (3) focus on right hand only during bilateral hand stimulation, (4) focus on left hand only during bilateral hand stimulation, and (5) rest. Attention to unilateral hand stimulation produced bihemispheric activation with minimal or no activation of ipsilateral primary sensorimotor region. Attention to unilateral left hand stimuli resulted in more activation than attention to unilateral right hand stimuli. Stimulation of the nonattended hand activated the contralateral somatosensory area, but to a lesser spatial extent than attended stimuli. Comparing focused attention to the left versus right side during identical sensory inputs (i.e., bilateral hand stimulation), focused attention to the right hand increased activation in the left somatosensory region, but focused attention to the left hand increased activation in both cerebral hemispheres. Thus, focused attention to unilateral somatosensory stimuli produces bilateral cerebral activation, but the increase in blood flow is greater in the contralateral hemisphere. Unattended stimuli activate the contralateral primary somatosensory area. Left/right asymmetries were demonstrated consistent with cerebral lateralization.     

Clinical study of twenty-one patients with pure midbrain infarction

We investigated the neurological and MRI findings of 21 patients with pure midbrain infarction. The locations of the infarcts were classified into paramedian (PM) area (13 patients), lateral area (6 patients), and PM and lateral areas (1 patient), based on the distribution of each penetrating artery of the midbrain. The infarction of 1 patient showed bilateral multiple lesions. In the PM area group, 12 patients showed eye movement disorders and truncal and gait ataxia. Eye movement disorders consisted of medial longitudinal fasciculus syndrome, oculomotor nerve palsy, and bilateral vertical gaze palsy. The infarcts of the 2 patients with bilateral vertical gaze palsy were located in the PM area on both sides. Eye movement disorders and truncal and gait ataxia were prominent in the PM area group. Ataxic hemiparesis was shown in 2 patients of the PM area group and 4 patients of the lateral area group. We showed that ataxic hemiparesis also occurred in PM area infarction, although ataxic hemiparesis was a prominent feature of lateral area infarction.     

A [C]2-Deoxyglucose analysis of the functional neural pathways of the limbic forebrain in the rat. III. The hippocampal formation

The regions metabolically activated in the rat brain following focal electrical stimulation of various components of the hippocampal formation were identified with the use of [¹⁴C]2-deoxyglucose (2-DG) autoradiography. The results of these experiments, conducted in the rat, showed that in the absence of elicited afterdischarge activity, stimulation of either the CA1 or CA3 field of the dorsal hippocampus resulted in bilateral metabolic activation of only the dorsal hippocampus as well as of a relatively restricted region within the dorsomedial aspect of the lateral septal nucleus, bilaterally. In contrast, stimulation of either the CA1 or CA3 field of the ventral hippocampus resulted in bilateral activation of the ventral hippocampus and no region of the dorsal hippocampus. Following such stimulation, the lateral septal nucleus was also labeled bilaterally, but the activated regions were situated in a position ventrolateral to those resulting from stimulation at dorsal levels. Stimulation of the subicular cortex, in contrast, resulted in only ipsilateral activation of the hippocampal formation and lateral septal nucleus. Further rostral levels of the lateral septal nucleus were noted to be activated following stimulation of subicular cortex as compared to stimulation of the cornu Ammonis.The hypothalamus was directly activated by two pathways, the postcommissural fornix and the medial corticohypothalamic tract. Following stimulation at dorsal and posterior levels of CA1 and the subiculum, the mammillary bodies were demonstrably labeled by input from the postcommissural fornix. Regions of the medial hypothalamus were activated via the medial corticohypothalamic tract following stimulation of the ventral subiculum.The amygdala, stria terminalis and its bed nucleus were also shown to be demonstrably activated following stimulation of the ventral subiculum, ventral CAl field and posterior prosubiculum. This pathway may represent an additional route by which hippocampal modulation may indirectly modulate hypothalamic function.The presence of elicited afterdischarges resulted in more extensive patterns of metabolic labeling within the hippocampal formation and lateral septal nuclei as compared to experiments in which afterdischarges were not elicited. The extent of the demonstrable labeling, both within, and extrinsic to the hippocampal formation appeared to be a function of the duration and severity of the elicited seizure discharge. Additional structures which were demonstrably labeled following the elicitation of seizure activity include the entorhinal cortex-prepyriform area, amygdala, substantia innominata, putamen, substantia nigra, olfactory and prefrontal cortices and medial thalamic nuclei.     

Stabilizing and destabilizing effects of vision and foot position on body sway of healthy young subjects: a posturographic study

A total of 800 posturographic measurements--each characterized by four values (sway path, sway area, anteroposterior sway and lateral sway)--was carried out for this study. Eight different conditions (interfoot distances of 0, 4, 10 and 20 cm, each with eyes open and eyes closed) were compared by statistical means. All sway values were diminished by vision in small interfoot distances. At large interfoot distances, however, the presence of visual control resulted in an increase of lateral sway. Comparing the different foot positions, an increase of anteroposterior sway and a decrease of all other components were found at large interfoot distances no matter whether the eyes were open or closed.     

Subcortical afferents to the interstitial nucleus of Cajal: an anatomical retrograde tracing study in the rabbit.

Organization of brainstem projections to the interstitial nucleus of Cajal (INC) in the rabbit has been studied using the method of retrograde transport of horseradish peroxidase or wheat germ agglutinin-horseradish peroxidase conjugate. Injections of tracers into INC resulted in bilateral labelling in the medial terminal nucleus of the accessory optic tract, medial region of the zona incerta, vestibular nuclei (superior, medial, inferior), rostral portion of the prepositus nucleus and several nuclei of the pontine and medullary reticular formation. Retrograde labelling on the contralateral side was noted in all 4 deep cerebellar nuclei, the lateral vestibular nucleus, group Y, the rostral interstitial nucleus of the medial longitudinal fascicle, INC and mesencephalic reticular formation dorsal and lateral to the red nucleus. Cells of origin for the ipsilateral afferents of INC were found only in the nucleus of the posterior commissure. These data are discussed in relation to other morphological and physiological studies of afferent connectivity of INC in other species.     

Galvanic ocular vestibular evoked myogenic potentials provide new insight into vestibulo-ocular reflexes and unilateral vestibular loss

ObjectiveSynchronous extraocular muscle activity can be recorded from around the eyes at the beginning of a vestibular-evoked eye movement (ocular vestibular evoked myogenic potentials, OVEMPs). As galvanic vestibular stimulation (GVS) evokes the vestibulo-ocular reflex, we wished to investigate GVS-evoked OVEMPs.MethodsWe stimulated 10 normals and 6 patients with unilateral vestibular loss (uVL) with bi/unipolar 4 mA, 2 ms current steps at the mastoid. OVEMPs were recorded from electrodes placed superior and inferior to the eyes.ResultsOVEMPs were present beneath both eyes in all normal subjects: an initial positivity ipsilateral to the cathodal electrode (peak latency 9.9 ms, amplitude 1.3 μV) and an initial negativity contralateral to the cathode (8.8 ms, 2.4 μV). In the patients, stimulation of the affected side produced little or no response. Stimulation of the intact side produced only contralateral responses.ConclusionsThe infra-orbital response is likely produced primarily by the inferior obliques, producing conjugate torsion away from the cathode. The projection to the ipsilateral eye depends upon normal vestibular function on the contralateral side.SignificanceOVEMPs can be evoked by GVS. While bilateral effects are obtained with unilateral stimulation in normals, the primary vestibular pathway to the inferior oblique in humans is crossed.     

Properties of antidromically identified neurons in the enkephalinergic magnocellular dorsal nucleus of the guinea pig hypothalamus

In the guinea pig, immunocytochemical and neuroanatomical studies have demonstrated that enkephalin-containing neurons in the hypothalamic magnocellular dorsal nucleus (MDN) terminate in the lateral septum (LS). In the present investigation, 114 MDN neurons, studied with extracellular recording techniques, were identified by antidromic activation from the LS. Latencies of responses from ipsilateral and contralateral LS were 13.5 and 18.78 ms, respectively, corresponding to an axonal conduction velocity of 0.1 m/s. By using the reciprocal collision test, evidence is presented for bilateral projection of individual MDN neurons to the LS. Fifty-one (44.73%) MDN-LS neurons discharged in a slow irregular pattern. Interspike time histograms were very similar and had a mode of about 280 ms. Peristimulus time histograms were compiled from 15 active MDN-LS neurons. Stimulation which elicited antidromic spikes resulted in a brief silent period in the spontaneous activity which was related to the normal interspike interval pattern of the firing. Prolonged silent periods as well as silent period occurring after subthreshold stimulus and increasing with the stimulus intensity were attributed to inhibitory synaptic effects. On the other hand, some MDN-LS neurons displayed orthodromic excitatory responses following LS stimulation. These observations provide electrophysiological evidence of a direct MDN-LS pathway, in all likelihood of enkephalinergic nature, and indicate that some MDN-LS neurons receive inhibitory and excitatory afferents from the LS.     

Electrochemically stimulated release of luteinizing hormone and ovulation after surgical interruption of lateral hypothalamic connections in the rat

Plasma LH and FSH were studied in adult female rats following bilateral electrochemical stimulation (ECS) of the medial preoptic area (MPOA). By stereotaxic surgery frontal (FC) and frontal-lateral (LFC) retrochiasmatic "deafferenting" cuts were made with a Halász knife (1.5 mm radius). At 3 and 10 weeks after surgery rats were given pentobarbital (32 mg/kg, i.p.) at 13:30h and stimulated at 15:00h with anodal direct current (20 muA for 60 sec) via concentric bipolar steel electrodes placed bilaterally 0.9 mm from the midline. Stimulation at 3 weeks after FC increased plasma LH from a prestimulation level of 95 +/- ng/ml to 227 +/- 51 ng/ml 80 min after ECS, from which it fell to 111 +/- 29 ng/ml at 160 min, with 2 of 7 animals ovulating the next day. LFC females had similar pre-ECS plasma LH levels 3 weeks after surgery (71 +/- 10 ng/ml), but LH concentrations at 80 min (104 +/- 21 ng/ml) and 160 min post-ECS were significantly lower than those of FC rats, and 0 of 5 rats ovulated. Following a similar protocol 10 weeks after surgery, stimulating the MPOA resulted in comparable elevations in plasma LH and 4 of 10 FC animals ovulated; however, the LFC group still retained a significant blocking effect on ovulation (only 1 of 12 ovulated) when compared with controls the next day. Plasma FSH concentrations were not significantly altered by MPOA stimulation at the parameters employed, either before or after deafferentation. However, LFC resulted in reduced ovarian and uterine weights when compared with controls at both 3 and 10 weeks, whereas FC exerted no observable effect on these organs at these intervals of study. The results of these studies suggest that lateral input to the media basal hypothalamus contributes to MPOA mediated release of LH and ovulation as well as to tonic maintenance of ovarian and uterine function.