Responses Of Individual Neurons Research Articles

Chronic haloperidol administration increases GABA binding and enhances neuronal responsiveness to iontophoresed GABA in rat globus pallidus

Experiments were conducted to assess the effects of chronic haloperidol (CHAL) administration on γ-aminobutyric acid (GABA) receptor binding within the rat globus pallidus (GP) and on the responsiveness of individual pallidal neurons to microiontophoretically applied GABA and glycine. Rats were administered haloperidol in their food for 30 days in increasing concentrations and the experiments were conducted 2 days after termination of the haloperidol treatment. GABA receptor binding and neuronal responsiveness to GABA were significantly increased within the GP following CHAL treatment. The mean EC 50 value for GABA was significantly decreased in the CHAL-treated rats, but there was no change in the EC 50 for glycine. Scatchard analysis of [ 3H]muscimol binding demonstrated a single high-affinity binding site (K d = 5nM) within both control and CHAL-treated rats. The binding capacity (B max) of this high-affinity site was significantly increased in CHAL-treated rats without any change in the dissociation constant (K d) for this site. These results suggest that CHAL administration may lead to a decrease in GABA release by striatopallidal efferents. The results of this study, combined with those of our previous study on SN R neurons, have demonstrated that blockade of striatally mediated dopamine (DA) neurotransmission leads to similar changes in GABAergic mechanisms at the level of the GP and SN R and suggest that DA regulation of the striatopallidal and striatonigral GABAergic pathways need not be differentially organized as has previously been postulated.

Neural encoding of individual words and faces by the human hippocampus and amygdala.

Patients with lesions in the medial temporal lobe (MTL) of the brain, which includes the hippocampus, amygdala and parahippocampal gyrus, are severely impaired in their ability to remember and recognize words or faces which they saw only a short time ago. These lesions also prevent the effect of word repetition on cortical event-related potentials that are associated with these tasks. We have been able to study the response of individual neurons in the human medial temporal lobe to such delayed recognition tasks in epileptic patients undergoing neurosurgery. We found that some MTL neurons preferentially fired on sight of one particular word from a set of ten words used in a memory task, and others fired in response to one particular face. This stimulus-specific firing was maximal during the time that the neocortical event potentials are most sensitive to stimulus repetition, suggesting that the MTL contributes specific information to the cortex during the retrieval of recent memories.

Primate nucleus gracilis neurons: responses to innocuous and noxious stimuli.

1. Recordings were made from 67 neurons in the nucleus gracilis (NG) of anesthetized macaque monkeys. All of the cells were activated antidromically from the ventral posterior lateral (VPL) nucleus of the contralateral thalamus. Stimuli used to activate the cells orthodromically were graded innocuous and noxious mechanical stimuli, including sinusoidal vibration and thermal pulses. 2. The latencies of antidromic action potentials following stimulation in the VPL nucleus were significantly shorter for cells in the caudal compared with the rostral NG. The mean minimum afferent conduction velocity of the afferent conduction velocity of the afferent fibers exciting the NG cells was 52 m/s, as judged from the latencies of the cells to orthodromic volleys evoked by electrical stimulation of peripheral nerves. The overall conduction velocity of the pathway from peripheral nerve to thalamus was approximately 40 m/s. 3. Cutaneous receptive fields on the distal hindlimb usually occupied an area equivalent to much less than a single digit. However, a few cells had receptive fields up to or exceeding the area of the foot. 4. NG cells were classified by their responses to graded mechanical stimulation of the skin as low threshold (LT) or wide dynamic range (WDR). No high-threshold NG cells were found. A special subcategory of pressure-sensitive LT (SA) neurons was recognized. Many of these cells were maximally responsive to maintained indentation of the skin. The sample of NG cells differed from the population of primate spinothalamic and spinocervicothalamic pathways so far examined, in having a larger proportion of LT neurons and a smaller proportion of WDR cells. A few NG cells responded best to manipulation of subcutaneous tissue. 5. Discriminant analysis permitted the NG cells to be assigned to classes determined by a k-means cluster analysis of the responses of a reference set of 318 primate spinothalamic tract (STT) cells. There were four classes of cells based on normalized responses of individual neurons and another four classes based upon responses compared across the population of cells. The NG cells were allocated to the various categories in different proportions than either primate STT cells or spinocervicothalamic neurons, consistent with the view that the functional roles of these somatosensory pathways differ. 6. Some of the pressure-sensitive NG cells were excited when the skin was stretched, suggesting an input from type II slowly adapting (Ruffini) mechanoreceptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of auditory localization cues on neuronal activity in the auditory thalamus of the cat.

1. The response properties of auditory thalamic neurons to the two major localization cues characterizing the azimuth of sound sources in the horizontal plane were investigated in cats. Single-unit responses to auditory stimuli (white noise and tones) presented with interaural phase differences (IPD) or interaural intensity differences (IID) were studied. 2. The proportion of neurons in the medial geniculate body that were sensitive to the localization cues tested was 28% for IPD (n = 253) and 37% for IID (n = 65). Half of the IID-sensitive units were also sensitive to IPD, but when the range of IPDs and IIDs to which each unit responded was converted to the sound-source locations that would generate those ranges they did not always correspond to overlapping azimuth angles. 3. The changes in discharge rate in response to the two localization cues occurred over very broad IPD and IID ranges. If this activity is involved in the representation of acoustic space, then the responses of individual neurons do not provide fine spatial tuning. 4. Contralateral and ipsilateral ear leads were represented in a continuous manner by the maximum discharge rate of IPD-sensitive units. On the other hand, units that were sensitive to IIDs were activated over one of two delimited ranges of IIDs. The first corresponded to IID combinations in which the stimulus was presented at a higher intensity in one ear than in the other (for 15/17 units the contralateral one); these were the lateralized intensity response field units. The second are the centered intensity response field units, whose responses were maximal when the intensity was equal in both ears and decreased when IIDs were introduced.

The effects of contrast on visual orientation and spatial frequency discrimination: a comparison of single cells and behavior.

We have compared the effects of contrast on human psychophysical orientation and spatial frequency discrimination thresholds and on the responses of individual neurons in the cat's striate cortex. Contrast has similar effects on orientation and spatial frequency discrimination: as contrast is increased above detection threshold, orientation and spatial frequency discrimination performance improves but reaches maximum levels at quite low contrasts. Further increases in contrast produce no further improvements in discrimination. We measured the effects of contrast on response amplitude, orientation and spatial frequency selectivity, and response variance of neurons in the cat's striate cortex. Orientation and spatial frequency selectivity vary little with contrast. Also, the ratio of response variance to response mean is unaffected by contrast. Although, in many cells, response amplitude increases approximately linearly with log contrast over most of the visible range, some cells show complete or partial saturation of response amplitude at medium contrasts. Therefore, some cells show a clear increase in slope of the orientation and spatial frequency tuning functions with increasing contrast, whereas in others the slopes reach maximum values at medium contrasts. Using receiver operating characteristic analysis, we estimated the minimum orientation and spatial frequency differences that can be signaled reliably as a response change by an individual cell. This analysis shows that, on average, the discrimination of orientation or spatial frequency improves with contrast at low contrasts more than at higher contrasts. Using the optimal stimulus for each cell, we estimated the contrast threshold of 48 neurons. Most cells had contrast thresholds below 5%. Thresholds were only slightly higher for nonoptimal stimuli. Therefore, increasing the contrast of sinusoidal gratings above approximately 10% will not produce large increases in the number of responding cells. The observed effects of contrast on the response characteristics of nonsaturating cortical cells do not appear consistent with the psychophysical results. Cells that reach their maximum response at low-to-medium contrasts may account for the contrast independence of psychophysical orientation and spatial frequency discrimination thresholds at medium and high contrasts.

Visual orientation and spatial frequency discrimination: a comparison of single neurons and behavior.

Neurons in the visual cortex respond selectively to stimulus orientation and spatial frequency. Changes in response amplitudes of these neurons could be the neurophysiological basis of orientation and spatial frequency discrimination. We have estimated the minimum differences in stimulus orientation and spatial frequency that can produce reliable changes in the responses of individual neurons in cat visual cortex. We compare these values with orientation and spatial frequency discrimination thresholds determined behaviorally. Slopes of the tuning functions and response variability determine the minimum orientation and spatial frequency differences that can elicit a reliable response change. These minimum values were obtained from single cells using receiver operating characteristic (ROC) analysis. The average minimum orientation and spatial frequency differences that could be signaled reliably by cells from our sample were 6.4 degrees (n = 22) and 21.3% (n = 18), respectively. These values are approximately 0.20 of the average full tuning width at one-half height of the cells. Although these average values are well above the behaviorally determined thresholds, the most selective cells signaled orientation and frequency differences of 1.84 degrees and 5.25%, respectively. These values are of the same order of magnitude as the behavioral thresholds. We show that, because of slow fluctuations in a cell's responsivity, ROC analysis overestimates response variability. We estimate that these slow response fluctuations elevated our estimates of single cell "thresholds" by, on average, 30%. Our data point to an approximate correspondence between orientation and spatial frequency discrimination "thresholds" determined behaviorally and those estimated from the most selective single cortical cells. Interpretation of this quantitative correspondence is considered in the discussion.

The response of individual nucleus raphe magnus neurons to microinjections of met-enkephalin at midbrain and at bulbar loci: evidence for midbrain-bulbar convergence on individual raphe neurons.

Local microinjection of met-enkephalin into both the nucleus reticularis paragigantocellularis (Pgc) and into the midbrain periaqueductal gray (PAG) altered the spontaneous discharge of many nociresponsive units in the nucleus raphe magnus (NRM) of the urethane-anesthetized rat. The activity of the majority of units (11 of 14) was suppressed following microinjections into both the PAG and Pgc. Two units showed an increase in spontaneous activity after PAG microinjections and a decrease after Pgc microinjections. One cell showed a decrease in activity after microinjection into Pgc and was unaffected by the PAG microinjection. These findings demonstrate that many nociresponsive units in NRM are affected by microinjections of met-enkephalin into both Pgc and the PAG, indicating a convergent influence of these two regions on single NRM neurons.

Regeneration of primary afferent neurons containing substance P-like immunoreactivity

We compared changes in levels of substance P-like immunoreactivity (SPLI) in L4-6 dorsal root ganglia (DRG), L4-6 dorsal roots, sciatic nerve, tibial nerve and hind foot skin in rats following resection or crush injury of the sciatic nerve. The initial depletion of SPLI, which occurred in all areas sampled, was similar after either type of lesion. In DRG and dorsal roots, recovery to control values occurred in SPLI levels 35–45 days after sciatic crush, but not after resection. In sciatic nerve proximal to the injury, a partial recovery in SPLI content to about 60% of control occurred following crush injury, but not following resection. Distal to the injury, tibial nerve levels recovered rapidly following crush injury, consistent with the previously observed rapid regeneration of SPLI-containing axons. After resection, no recovery was observed until after 35 days, when it appeared that some axons succeeded in crossing the resection zone and regaining the distal nerve stump. Delayed and poor recovery of SPLI levels was observed in foot skin, even after crush injury. This correlated with the poor recovery of the plasma extravasation reaction, a functional index of SP-innervation of skin. In contrast, reinnervation by high-threshold mechanoreceptors was more rapid and complete, in agreement with a previous study. We conclude that although SPLI-containing axons regenerate rapidly, they appear to reinnervate skin less successfully than other afferents. Axon regeneration is associated with a recovery of SPLI levels which fell after axotomy: no recovery occurs if regeneration is prevented. Recovery was almost complete in DRG and roots, but incomplete in sciatic nerve. This peptide transmitter in afferent neurons thus behaves in a similar fashion to previously studied low-molecular weight transmitters and related materials in efferent neurons. Since recovery of SPLI levels begins before there is evidence for target reinnnervation, it seems that axon regeneration is a sufficient condition for reversal of some axotomy-induced changes in these neurons. Further studies on substance P synthesis and on the response of individual DRG neurons to axotomy and regeneration will be required to explain fully the discrepancy between partial recovery of SPLI levels in sciatic nerve and full recovery in DRG and dorsal roots.

Physiological properties of individual cerebral axons studied in vivo for as long as one year.

The long-term stability of conduction velocity and recovery processes were studied in a fast-conducting (corticotectal) and in a more slowly conducting (visual callosal) axonal system. Chronic microelectrode recording methods were used in conjunction with antidromic activation via electrical stimulation at one or more axonal site. These methods enabled 54 axons to be studied for greater than 20 days and seven of these cells to be studied for 101-448 days. The conduction velocities of corticotectal axons were characteristic of myelinated axons and were very stable over time. The conduction velocities of most callosal axons were characteristic of nonmyelinated axons, and 68% of callosal axons had conduction velocities that were stable over long periods of time. Of the remaining callosal axons, approximately one third showed an increase in conduction velocity (8-14%), whereas two thirds showed a progressive and systematic decrease in conduction velocity (6-81%). These changes in conduction velocity were distributed along the callosal axon, rather than limited to a single segment of axon. Although the refractory period of callosal and corticotectal axons showed considerable variability over time, the minimal interval between two conducted impulses was stable. The stability of this property was remarkable because the minimal interspike intervals of different axons with similar conduction velocities often differed greatly. Callosal axons show a supernormal period of increased conduction velocity following the relative refractory period and a subsequent subnormal period of decreased conduction velocity following a burst of prior impulses. In different callosal axons the magnitude of the velocity changes (percent change) differs greatly, even among axons of the same conduction velocity. For a given axon, however, these properties are very stable over time. These results on axonal properties may be useful in studies requiring the examination of extracellular responses of individual neurons over long periods of time. Antidromic latency provides a useful means of identifying a cell, particularly when conduction times are long. The stability of the minimal interspike interval and the supernormal period within individual axons make them suitable as ancillary criteria in identifying individual neurons. These three measures are independent of spike amplitude and waveform, and together they provide a "signature" by which individual cortical neurons can be identified over periods that represent a significant portion of the lifespan of adult mammals.

Primate globus pallidus and subthalamic nucleus: functional organization.

Neuronal relations to active movements of individual body parts and neuronal responses to somatosensory stimulation were studied in the external (GPe) and internal (GPi) segments of the globus pallidus (GP) and the subthalamic nucleus (STN) of awake monkeys. In GPe (n = 249), GPi (n = 151), and STN (n = 153), 47, 29, and 28% of the cells, respectively, discharged in relation to active arm movements, 10, 11, and 15% to leg movements, and 22, 22, and 18% to orofacial movements. Of the neurons whose activity was related to arm movements, 26, 16, and 21% in GPe, GPi, and STN, respectively, discharged in relation to movements of distal parts of the limb. Of cells whose discharge was related to active limb movements, 37, 22, and 20% in GPe, GPi, and STN, respectively, also responded to passive joint rotation, which was usually specific in terms of joint and direction of movement. Only a small percentage of cells responded to muscle or joint palpation, tendon taps, or cutaneous stimulation. Short-latency, direction-specific neuronal responses to load perturbations confirmed the existence of proprioceptive driving. In both GPe and GPi, leg movement-related neurons were centrally located in the rostrocaudal and dorsoventral dimensions. In contrast, arm movement-related cells were found throughout the entire rostrocaudal extent of both segments, although in greater numbers caudally. In the central portions they were situated largely inferior and lateral to leg movement-related neurons. Neurons related to orofacial movements were largely confined to the caudal halves of both segments, where they were located largely ventral to arm movement-related cells. The STN cells whose activity was related to leg movements were observed largely in the central portions of the nucleus in the rostrocaudal and mediolateral dimensions. Cells whose activity was related to arm movements were found throughout the rostrocaudal extent of the nucleus, but were most numerous at the rostral and caudal poles. Neurons related to movements of the facial musculature and to licking and chewing movements were distributed over the entire rostrocaudal extent of the nucleus, where they generally occupied the ventrolateral regions. In all three nuclei, neurons with similar functional properties were sometimes clustered together. Within the arm and leg areas, however, there was no clear evidence for a simple organization of clusters related to different parts of the limb. These studies provide further evidence for a role of the basal ganglia in the control of limb movements.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytophotometric analyses of mesencephalic reticular formation neuronal RNA in soman intoxicated rats

Quantitative azure B cytophotometry was used to monitor ribonucleic acid (RNA) responses of individual neurons within the nucleus cuneiformis (NC) and ventrotegmental nucleus (VTN) of the rat mesencephalic reticular formation following single subcutaneous soman (pinacolyl methylphosphonofluoridate) injections (0.5, 0.9 or 1.5 LD(50)). The sub-lethal (0.5 LD(50)) dosage of soman produced RNA accumulation in NC neurons, but VTN-RNA levels were not significantly altered. In contrast, both reticular nuclei exhibited prominent RNA depletion with higher soman dosages, the severity of which was greater with lethal (1.5 LD(50)) than near-lethal (0.9 LD(50)) dosages. These data indicate that metabolic correlates of enhanced activation of cholinergic reticular nuclei are present only with sub-lethal dosages, and that higher dosages produce characteristics of impaired activation of ascending cholinergic pathways. At present, mechanisms underlying soman-induced metabolic and neurologic deficits remain speculative.

Cytophotometric analyses of thalamic neuronal RNA in soman intoxicated rats

Quantitative azure B-RNA cytophotometry was used to monitor metabolic responses of individual neurons within the ventrobasal nuclear complex (VBC) and nucleus reticularis (NR) of the rat thalamus following administration of soman (0.5, 0.9 or 1.5 LD 50, sc). A dose-dependent depression in brain acetylcholinesterase (AChE) was evidenced. With respect to thalamic RNA responses, a complex pattern of RNA alterations was evidenced, with these two regions generally exhibiting opposite patterns of dose-related RNA changes. With sub-lethal dosages of soman, RNA accumulation was evidenced in the acetylcholine (ACh) mediated excitatory VBC region and RNA depletion in the ACh mediated inhibitory NR neurons. With a lethal dose, an opposite RNA response pattern observed in both thalamic regions. It is postulated that the observed RNA response pattern with sub-lethal dosages of soman is what one would anticipate with cholinergic brainstem reticular formation activation. The absence of such a response with lethal doses strongly suggests some disruption of functional excitatory cholinergic activity and perhaps also an impairment of inhibitory cholinergic synaptic activity.

Cytophotometric analyses of brain neuronal RNA in soman intoxicated rabbits

Quantitative azure B-RNA cytophotometry was used to monitor metabolic responses of individual neurons of the motor cortex (layer V) and caudate nucleus of soman (pinacolyl methylphosphonofluoridate) poisoned rabbits. Time- and dose-dependent RNA responses were related to the extent of acetylcholinesterase (AChE) inactivation of these 2 brain regions and to overt behavioral manifestations of toxication. A complex pattern of RNA responses was evidenced, with RNA depletion occuring at both arousal and convulsive doses of soman. In general, RNA changes paralleled previously reported metabolic responses observed in soman toxicated rats and mice: (1) linear dose-dependent suppression of RNA was evidenced during the depressant phase but not in the acute excitatory phase of toxication; and (2) RNA depletion was more severe following than during the appearance of excitatory symptoms. These data indicate that soman poisoning results in metabolic correlates of impaired rather than excessive CNS activation. It is postulated that metabolic disturbances are related to blockade of central cholinergic excitation, and that disruption of functionally integrated synaptic activity forms a critically important aspect of toxication.

Effect of pentobarbital on neuronal inhibitory response evoked in an isolated slab of cat auditory cortex by intracortical stimulation

Neuronal responses of an acutely isolated slab of auditory cortex (area AI) to intracortical electrical stimulation were studied intracellularly in cats anesthetized with pentobarbital. It was found that 77% of responses were primary IPSPs, and allowing for secondary inhibitory responses, an inhibitory response was observed in 92% of neurons. All types of neuronal responses in the slab were short-latency. The maximal response latency did not exceed 5 msec. Neurons responding to stimulation by IPSPs were found at all depths in the slab, with a maximum in layers II–III. Nearly all primary IPSPswere mono- and disynaptic. Pentobarbital increased the duration of individual neuronal inhibitory responses in the isolated slab of auditory cortex without affecting maximal duration of the IPSP. The mechanisms of the effect of pentobarbital on the amplitude and duration of IPSPs are discussed.

Mechanoafferent neurons innervating tail of Aplysia. I. Response properties and synaptic connections.

Mechanical, chemical, or electrical stimulation of the tail elicits a short-latency (less than 1 s) tail-withdrawal reflex that is graded with the intensity of the stimulus. The tail-withdrawal reflex is not elicited by stimulation of parts of the body outside of the tail region. Mechanoafferent neurons innervating the tail are located in a small subcluster within a large, homogeneous group of medium-size (40-80 micron) cells on the ventrocaudal (VC) surface of each pleural ganglion. The tail sensory neurons within this large VC cluster are activated by tactile pressure or by electrical stimulation of discrete regions of the tail. They adapt slowly to maintained stimulation and sometimes respond to stimulus offset as well. Both mechanical and electrical stimuli produce responses that are graded with the intensity of the stimulus. Cells in the VC cluster appear to be primary mechanoreceptors because they have axons in peripheral nerves (including nerves innervating the tail), they exhibit action potentials lacking prepotentials in response to tactile stimulation, and these action potentials are still produced by cutaneous stimulation when peripheral and central chemical synaptic transmission is blocked. Stimulation of fields all over the body surface evokes synaptically mediated hyperpolarizing responses in individual mechanoafferent neurons that may represent afferent inhibition. Hyperpolarizing responses lasting many seconds can be produced by brief cutaneous stimuli. The mechanoafferent neurons innervating the tail region make strong monosynaptic connections to tail motor neurons in the ipsilateral pedal ganglion, and through these connections this subpopulation of the VC neurons appears to make a substantial contribution to the short-latency tail-withdrawal reflex. In addition, the combined excitatory receptive fields of these mechanoafferents match the excitatory receptive field of the tail-withdrawal reflex. Mechanoafferent neurons in the VC cluster that have receptive fields on other parts of the body (outside the excitatory receptive field of the tail-withdrawal reflex) have not been observed to make monosynaptic connections to the tail motor neurons. The neurons innervating the tail are reliably found in a discrete region within the larger VC cluster. In addition to this gross somatotopic organization, there is evidence of a finer level of somatotopic organization between the position of the excitatory receptive field on the tail and the position of the cell soma in the tail subcluster.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of histamine and serotonin on single unit discharges of suprachiasmatic neurons in rat hypothalamic slices.

Effects of histamine (HA) and serotonin (5-HT) on single neurons of the suprachiasmatic nucleus (SCN) were investigated by iontophoretic application. Forty-nine of 163 neurons were inhibited by HA, while 28 were excited. Thirty-four of 120 neurons were inhibited by 5-HT, while 4 were excited. These substances produced a sustained effect. The inhibitory effect of 5-HT was observed mainly in neurons from the ventrolateral region of SCN where the optic nerve fibers preferentially terminated, whereas that of HA was observed in neurons recorded from both the ventrolateral and dorsomedial regions of SCN. The responses of individual neurons (43) to HA and 5-HT were compared. Six were inhibited by both substances and 12 were inhibited by either of them. Three neurons which were inhibited by HA were excited by 5-HT, while 1 neuron which was excited by HA was inhibited by 5-HT. Pyrilamine slightly (2/7) and Cimetidine strongly (10/13) antagonized the inhibitory effect of HA. The excitatory effect of HA was not antagonized by Cimetidine (0/4), but was perfectly antagonized by pyrilamine (5/5). These results suggest that HA and 5-HT play mainly an inhibitory role in the SCN neuronal activity through their direct effect, and inhibition and excitation induced by HA may be mediated by H2 and H1-receptor, respectively.

Coding of gustatory information in the pontine parabrachial nuclei of the rabbit: Magnitude of neural response

An electrophysiological analysis was carried out on 82 neurons in the parabrachial nuclei to study coding of gustatory stimuli. The magnitude of neural discharge evoked by two concentrations of each of 4 basic taste stimuli were subjected to two types of analyses to evaluate neuronal specificity and across-element spatial patterning in the coding of taste quality. Results were compared and contrasted to those in rodents. Profiles of taste response of individual neurons were analyzed mathematically by Q-technique principal components analysis and cluster analysis to identify basic neuronal patterns of response. Four distinct patterns emerged which, based on the average profiles of response, were characterized as: NaCl-dominant, NaCl HCl, sucrose-dominant, and nonspecific. Analysis of the NaCl- and sucrose-dominant types of neurons indicated that neurons with the most narrowly tuned patterns of response to the taste stimuli could not effectively code specific taste qualities. Comparison of interstimulus correlations in magnitude of response across neurons between qualitatively different tastants and intrastimulus correlations between intensities of given tastants also revealed difficulties with the spatial pattern model of coding. The discrepant findings raise questions about an assumption utilized in the analysis of the model, viz., that all elements contribute homogeneously to the patterning code.

Multiple connections of medial hypothalamic neurons in the rat

The responses of 700 single neurons in the hypothalamus to electrical stimulation of the preoptic area, limbic structures, and midbrain were studied to determine the location of neurons with multiple inputs and to identify by antidromic activation the projection areas of those neurons. Converging excitatory inputs, observed in 134 responsive hypothalamic neurons, were principally derived from the preoptic, limbic, and midbrain areas. Inputs from separate nuclei of the amygdala were noted in the response of individual hypothalamic neurons. Two classes of short latency transsynaptic responses to amygdala stimulation were defined, indicating either separate pathways from the amygdala to the medial hypothalamus or two types of fibers conducting at different velocities. Stimulation of single or multiple sites in the preoptic and limbic areas, as well as in the arcuate nucleus and medial forebrain bundle produced inhibition of hypothalamic neuronal activity. Most antidromically identified medial hypothalamic neurons projected to the preoptic area, median eminence (tuberoinfundibular neurons), or midbrain. Evidence is presented for collateral projections of tuberoinfundibular neurons to the preoptic area and reticular formation. Medial hypothalamic neurons received inputs from the preoptic area, lateral septal nucleus, amygdala, ventral hippocampus (subiculum), and fornix. These findings illustrate a pattern of reciprocal connections between the medial hypothalamus and limbic and midbrain structures. It was concluded that the hypothalamus contains a type of neuron that is equipped to perform complex integrations and to coordinate directly the behavior of neurons in a diversity of anatomical regions.

Graded interactions between identified neurons from the simple eyes of an insect

Intracellular recordings were made of anatomically identified second order neurons of the ocelli of the locust (Orthoptera, Acrididae), and the graded responses of individual neurons to illumination of the 3 ocelli were compared. It was found that two anatomical types of neurons responded with hyperpolarization to illumination of one ocellus, and with depolarization to illumination of another ocellus. The depolarizing responses were abolished with picrotoxin. The 3 anatomical types of large neurons in the median ocellar nerve had responses which were physiologically distinct.

Bilateral coding of sound direction in the CNS of the bushcricketTettigonia viridissima L. (Orthoptera, Tettigoniidae)

1. By means of simultaneous recordings with hook-electrodes the response behavior of a pair of homologous auditory interneurons on either side of the CNS of the bushcricketTettigonia viridissima L. was studied with particular emphasis on bilateral symmetry and directional coding. 2. In about 50% of the preparations the threshold curves of both interneurons were symmetrical, tested within a frequency range from 5 to 40 kHz (Fig. 4a). In the remaining cases significant bilateral asymmetries at frequencies above 20 kHz and/or below 12 kHz could be observed (Figs. 4 and 5). Therefore, in this bushcricket species symmetrical thresholds seem to be guaranteed only within a rather limited frequency band around 20 kHz. This frequency coincides with one (the higher) of the two spectral bands within the conspecific call (Fig. 2). 3. From recordings of unilaterally activated interneurons it can be inferred that the directional sensitivity of the single tympanic organ is as well tuned to this stimulus frequency of about 20 kHz. Maximal intensity differences of 15 to 20 dB between ipsiversus contralateral stimulation could be found (Fig. 3). Above and below this frequency the directionality of the ear was much lower. 4. This frequency dependence of directional hearing could be confirmed on the level of the CNS. Only at a stimulus frequency of about 20 kHz the directional curves of both interneurons were very pronounced (Fig. 10) with precisely encoding the lateralization of the sound source starting at about 10 dB above neuronal thresholds (Figs. 7, 8 and 9). 5. Though in single preparations the intensity-response curves of the two interneurons were notably different (Fig. 6), with respect to right versus left discrimination no auditory ‘handedness’ could be observed. 6. As some insects already orient to a single or a small number of stimuli, individual neuronal responses of the right versus the left interneuron were compared at different sound directions (Figs. 11 and 12). Surprisingly, with frontal stimulation the probability of ‘correct’ (symmetrical) responses was only 17% which abruptly increased as soon as the sound source became lateral. These results are discussed in terms of the zig-zag walk of the localizing insect.