Rebound Responses Research Articles

Modulation of swimming behavior in the medicinal leech. I. Effects of serotonin on the electrical properties of swim-gating cell 204.

The effects of serotonin on the electrical properties of swim-gating neurons (cell 204) were examined in leech (Hirudo medicinalis) nerve cords. Exposure to serotonin decreased the threshold current required to elicit swim episodes by prolonged depolarization of an individual cell 204 in isolated nerve cords. This effect was correlated with a more rapid depolarization and an increased impulse frequency of cell 204 in the first second of stimulation. In normal leech saline, brief depolarizing current pulses (1 s) injected into cell 204 failed to elicit swim episodes. Following exposure to serotonin, however, identical pulses consistently evoked swim episodes. Thus, serotonin appears to transform cell 204 from a gating to a trigger cell. Serotonin had little effect on the steady-state current-voltage relation of cell 204. However, serotonin altered the membrane potential trajectories in response to injected current pulses and increased the amplitude of rebound responses occurring at the offset of current pulses. These changes suggest that serotonin modulates one or more voltage dependent conductances in cell 204, resulting in a more rapid depolarization and greater firing rate in response to injected currents. Thus, modulation of intrinsic ionic conductances in cell 204 may account in part for the increased probability of swimming behavior induced by serotonin in intact leeches.

Does the release of vasopressin within the supraoptic nucleus of the rat brain depend upon changes in osmolality and Ca 2+/K +?

Microdialysis in conjunction with a highly sensitive radioimmunoassay was used to monitor the in vivo release of arginine vasopressin (AVP) within the supraoptic nucleus (SON) of the rat brain ( n = 70). Thirty-min dialysates were collected in urethane-anesthetized animals before, during and after hypertonic or hypotonic pulses were delivered via the probe. As compared to artificial cerebrospinal fluid (aCSF)-perfused controls, 1M aCSF given over a period of 210 min resulted in an increased intranuclear AVP release which, however, reached its peak only in the post-stimulation period, i.e. after replacement of hypertonic with isotonic aCSF again (rebound phenomenon). Hypertonic (0.5 M, 1 M or 2 M) pulses given 150 min after the first (1 M) pulse resulted in 3 different rebound responses: a marked decrease (to 25.3%, P < 0.001), no change or slight increase (132%, n.s.). As shown by the response to correction of the hypertonicity to normal as well as by perfusion of hypotonic aCSF (0.01 M), release of AVP within the SON appears more responsive to a reduction than to an elevation in the osmolality of aCSF. Osmission of Ca 2+ from and addition of EGTA to the aCSF decreased the osmotically stimulated, but not the basal AVP release. If K +-hypertonic aCSF was used, however, basal AVP levels increased significantly; in contrast, the rebound increase failed to differ from aCSF-perfused controls. It is concluded from these finding that release of AVP from intact neuronal structures in the SON is responsive to changes in osmolality with an apparent greater sensitivity to decrease in osmolality.

Maturational aspects of periodicity coding in cat primary auditory cortex

The click-following responses for single units in the primary auditory cortex of the cat were explored as a function of age. Recordings were obtained in kittens from 9–53 days of age and assembled in four age groups; 10–15 days, 16–21 days, 22–27 days and 30–60 days. Age group means were compared to results obtained in adult cats. The stimulus consisted of one second long click trains presented every three seconds with click rates ranging from 1–32 clicks per second. The response was characterized by entrainment, rate Modulation Transfer Function (rMTF), vector strength (VS) and temporal Modulation Transfer Function (tMTF). Maturational effects on periodicity coding comprised changes in overall responsiveness as well as click-rate dependent changes. The number of spikes elicited by single stimuli increased on average 3-fold between the second post-natal week and adulthood, probably as a result of more efficient synapses in the central auditory pathway and some improvement in thresholds. Adaptation became less pronounced with age; neurons started to respond to the later clicks in the 8 s and 16 s click trains from the third post natal week on. By the end of the first post-natal month the click following responses resembled the adult ones qualitatively, however, increased firing rates and spontaneous rates together with rebound responses continued to produce quantitative differences between the 30–60 day olds and the adults. Limiting rates for the tMTF (50% of the response at sol1 s ) increased from 6 Hz in the 10–15 day old to 12 Hz in adults. The decrease in the duration of the post-activation suppression coupled with the increased response with age to trains with higher click rates suggested that the maturation of inhibitory processes in the cortex play a major role in this rate dependence.

Evidence for a persistent sodium conductance in neurons from the nucleus prepositus hypoglossi

Intracellular recordings were made from 39 neurons in a slice preparation of the prepositus hypoglossi nucleus from guinea pigs. Morphological characteristics were confirmed by dying neurons with Lucifer yellow. The neurons were spontaneously active, firing in the range of 8–50 spikes/s. Spike duration was short (0.32 ms) and the spikes were followed by fast and slow afterhyperpolarizations. The current vs frequency relationship was linear during steady state firing, but showed dual firing ranges corresponding to the first, third and fifth interspike interval. The instantaneous frequency of the first few interspike intervals could reach 500 spikes/s. Depolarizing and hyperpolarizing responses to square pulses displayed initial sag and rebound responses sensitive to extracellular Cs +, pharmacologically classifying the responses as a result of a Q-like current. Substitution of Ca 2+ in the medium with the inorganic calcium blockers Mn 2+ or Co 2+ resulted in oscillatory firing, depolarizing excursions being sensitive to tetrodotoxin (TTX). Mn 2+ or Co 2+ in combination with extracellular Cs + elicited TTX-sensitive plateau potentials, blocked in Na +-free solution. In conclusion, the prepositus neurons displayed spontaneous activity in the slice preparation and active membrane properties above as well as below the threshold of the action potential. In addition, the prepositus neurons possess a persistent sodium conductance that can be uncovered by inorganic calcium blockers. It may be involved in sustaining the spontaneous discharge.

Electrophysiological properties of hypoglossal motoneurons of guinea-pigs studied in vitro

Intracellular recordings were made from the hypoglossal nuclear complex in brain slices from guinea-pigs. Retrograde transport of horseradish peroxidase from the tongue confirmed the identity of the visually identified hypoglossal nucleus. Eighteen neurons were stained by intracellular electrophoresis of Lucifer Yellow through the recording pipette. Two types of neurons were encountered, motoneurons with maximal discharge rates of 90 Hz and another type with maximal discharge rates of 250 Hz. Motoneurons were prevalent in the hypoglossal nucleus and the other type prevailed in the adjoining nucleus prepositus hypoglossi. In both nuclei the two types were mixed. Antidromic spikes elicited from hypoglossal root fibres had initial segment and somatodendritic components. Electrical stimulation of the reticular matter dorsolateral to the hypoglossal nucleus elicited excitatory postsynaptic potentials and strychnine sensitive inhibitory postsynaptic potentials. Motoneurons responded to depolarizing current pulses with a train of spikes. The initial spike interval was much shorter than the rest and fast adaptation occurred over three to four intervals. Slow adaptation was most prominent when the neuron was depolarized and discharged at a high rate. High threshold calcium spikes were evoked by depolarizing pulses when sodium spikes were blocked by tetrodotoxin and the potassium conductance reduced by tetraethylammonium bromide. Motoneurons discharged in a single range, inflections on the frequency-current plot being absent. Spikes and spike trains evoked by depolarizing pulses were followed by afterhyperpolarizations with fast and slow parts. The fast phase was eliminated by tetraethyl-ammonium bromide, possibly because the delayed rectifier was involved. A calcium dependent potassium conductance was probably involved in the slow phase, because it was sensitive to inorganic calcium blockers. The amplitude of the afterhyperpolarization following trains of spikes depended on the frequency of the preceding spikes. At constant frequency, the amplitude depended, in addition, on the strength of stimuli arising from different hyperpolarized potentials. Afterdepolarizing potentials were absent. Lissajous plots of double ramp current stimulation showed anomalous rectification between resting potential and spike threshold. The rectification was sensitive to inorganic calcium blockers. Subthreshold responses showed initial sags and rebound responses in all healthy cells and these were eliminated by caesium. Barium, substituted for calcium, unleashed a depolarizing plateau potential sensitive to tetrodotoxin, indicating the presence of a persistent sodium conductance. The membrane of hypoglossal motoneurons contains voltage dependent conductances in the voltage range around resting potential and spike threshold. These conductances provide for effective switching between the discharging and the non-discharging state of the motoneuron.

Rebound cardiovascular responses following stimulation of canine vagosympathetic complexes or cardiopulmonary nerves.

Electrical stimulation of a canine vagosympathetic complex or a cardiopulmonary nerve can elicit a variety of negative chronotropic and inotropic cardiac responses, with or without alterations in systemic arterial pressure. In the period immediately following cessation of such a stimulation "rebound" tachycardia, increased inotropism above control values in one or more regions of the heart, and (or) elevation in systemic arterial pressure can occur. These "rebound" phenomena are abolished by propranolol or ipsilateral chronic sympathectomy. It is proposed that "vagal" poststimulation "rebound" of the canine cardiovascular system is primarily the result of activation of sympathetic neural elements present in the vagosympathetic complexes or cardiopulmonary nerves.

Stimulation of rhesus monkey GH release: arginine vs plasma volume expansion.

Plasma GH and cortisol responses were examined in unanesthetized adult male rhesus monkeys infused with approximately 30 ml (8.3 ml/kg) of arginine monohydrochloride (Arg 0.5 g/kg), 0.9% saline (NS) or Dextran 75 and following 15 ml infusions of acid-saline (AS), or somatostatin (GHRIH 30 mug/kg). None of the infusions were accompanied by GH elevations. Rebound GH elevations occurred after each 30 ml infusion and after GHRIH but not after 15 ml AS. Although the amplitude of GH responses were not significantly different (p greater than 0.05), responses to Arg and dextran were significantly delayed as compared to responses following NS and GHRIH which did not differ from each other. Both of the latter responses occurred within 15 min of infusion completion. The delay following dextran was significantly longer than that following Arg (p less than 0.05). Since the volume of saline and the dosage of Arg were equivalent to those used in the standard intravenous Arg test, results suggest that effects of infusion volume upon GH secretion and upon GH responses to Arg warrant examination in the human. In addition, it is suggested that GH responses to NS and to dextran are mediated via an intravascular volume control mechanism and that GHRIH may be released upon activation of such a mechanism. GH elevations may represent rebound responses upon termination of GHRIH secretion.

The inhibitory action of type J pulmonary receptor afferents upon the central motor and fusimotor activity and responsiveness in cats.

1. In decerebrate or pentobarbital anesthetized cats the effect of intra-right atrial injection of phenyldiguanide (PDG) upon lumbar fusimotor activity and monosynaptic reflex activity and their responsiveness to pinna stimulation (PS) was investigated. 2. The spontaneous activity of triceps surae muscle spindle afferents or lumbar γ-efferents was in most cases diminished after PDG, sometimes, particularly in decerebrate cats, it was temporarily enhanced. The fusimotor responses to PS were distinctly depressed after injection of PDG, irrespective of the effects upon the spontaneous activity and whether the PS-responses were originally inhibitory (in anesthetized cats), excitatory (in decerebrate cats) or whether rebound responses were present (in anesthetized cats). 3. The monosynaptic gastrocnemius reflex was enhanced after PDG in decerebrate cats, while the facilitatory effects of PS upon the reflex were depressed. 4. The motor effects after intra-right atrial injection of PDG are considered to be reflexogenic, elicited by excitation of juxta pulmonary capillary receptors (type J receptors, Paintal). With respect of the results of other authors it has been assumed that there are three components of central motor actions of the J reflex: a) inhibitory effects mediated by structures rostral to the intercollicular level, b) excitatory effects from brain stem structures below this level, c) depressing effect upon the brain stem responsiveness.

Rebound and Reverberatory Responses by Low-Frequency Neurons in the Inferior Colliculus of the Kangaroo Rat

A small number of low-frequency inferior colliculus neurons show, aside from the usual tone-driven response, late discharges which are a function of the time after the stimulus “off.” These late discharges are always preceded by a silent period of 10 to 50 msec, indicating the presence of inhibition and suggesting that the responses are rebound phenomena following inhibition. Occasionally, the rebound discharges are followed by a second silent period and then additional firing. While rebound and reverberatory responses in the inferior colliculus are not of as long duration as those commonly observed in the medial geniculate, their presence indicates that the processes responsible for such activity are intrinsic to both aggregates. If one ignores the rebound and reverberatory discharges, the response of these neurons to tone bursts is indistinguishable from the response of non-rebound neutrons. The rebound and reverberatory discharges, however, are most sensitive to frequency and intensity, but, unlike the early responses, are relatively insensitive to interaural phase. [Research supported by NINDS Post-doctoral Fellowship and NIH grant to George Moushegian.]