A study of hypothalmic neurosecretory cells of bullfrogs in vitro

Patterns of spontaneous unit preoptic neurosecretory cell discharges in the rainbow trout, Salmo gairdneri

Extracellular antidromic potentials recorded from the neurosecretory cell body were characterized by the following criteria: constant latency, the ability to follow a high frequency rate of stimulation and the collision test. The latency of the antidromic potentials ranged from 12 to 24 ms (17.46 +/- 3.10 SD) which gave a mean conduction velocity of 0.19 m/s, typical of unmyelinated nerve fibers. Two components could be clearly distinguished in the antidromic potential. A small "A" spike which showed constant latency and a large "B" spike with a variable latency and amplitude. A delay of 6.5 ms between the two spikes could occur and sometimes the "B" spike was blocked leaving only the "A" spike. Four patterns of spontaneous activity seem to emerge: Type I (26% of units, M +/- SD = 0.77 +/- 0.32 sp/s) corresponds to a slow and irregular pattern of activity; Type II (28% of units, M = 1.58 +/- 0.47 sp/s) is hard to classify and may be related to an irregular bursting pattern of activity; Type III (28% of units, M = 2.59 +/- 1.19 sp/s) corresponds to a continuous pattern of activity; Type IV (18% of units) represents a rhythmic pattern of activity with an active phase of about 3 min (M = 2.42 +/- 0.90 min), a silent phase of about 4 min (M = 3.89 +/- 3.02 min) and a maximal frequency of unit discharge in the range 2-18 sp/s. No statistical differences exist for the mean dorsal aortic pressure (DAP) between the four types of neurosecretory cell activity.

Possible projections from regions of paraventricular and supraoptic nuclei to the spinal cord: electrophysiological studies

The existence of monosynaptic connections between neurons in the paraventricular nucleus (PVN) of the hypothalamus and the intermediolateral cell column (ILC) of the spinal cord was studied by electrophysiological techniques in chloralose-anesthetized cats. Sympathetic preganglionic discharges (recorded from the 2nd or 3rd thoracic white ramus) were evoked by microstimulation of certain regions in or near the PVN with short train of pulses and below 50 microA current. By recording responses of 'identified' and 'non-identified' neurosecretory cells in the PVN and supraoptic nucleus (SON) to stimulation of the ILC of the thoracic cord, it was possible to identify antidromically evoked action potentials in 9 out of 297 neurons tested. Among them, 2 neurons were also antidromically excited by the pituitary stalk stimulation, 5 were orthodromically excited by the same stimulus and the remaining 2 were not excited by the stalk stimulation. Our results indicate that some PVN neurons, though small in number, send axons directly to the ILC of the cord, and that a very few neurons among these also send their axons to the pituitary gland.

Effect of membrane polarization and synaptic activity on the timing of antidromic invasion

The latency from the stimulus (S) to the IS and SD components of the antidromic spike was measured in motoneurones and spinocerebellar tract cells following displacement of the membrane potential either by current pulses or by synaptic potentials. Changes in the latency to the SD spike (S-SD delay) were mainly caused by changes in the IS-SD delay and varied from 10 to 100 musec per mV change in membrane potential, depending on the initial value of the IS-SD delay. Changes in the S-IS delay were also observed and these changes could, especially in spinocerebellar cells, give a significant contribution to the change in the total delay. EPSPs shortened the S-SD delay as efficiently as current-evoked depolarizations of similar magnitude while IPSPs were often more effective in prolonging the delay than current-evoked hyperpolarizations. This difference was related to the larger conductance increase during IPSPs than during IPSPs and to the longer IS-SD delays at hyperpolarized potentials. The presented data contribute to the understanding of the method which uses extracellular recording of antidromic latency changes as an indirect measure of intracellular membrane potential changes. Our results show that the recording of antidromic latency changes is a particularly sensitive method for detecting inhibition of neurones.

Rhythmic patterns of discharge in hypothalamic neurosecretory neurons of cats and dogs

Recent studies in rats suggest that vasopressin- and oxytocin-secreting neurons in supraoptic nuclei and paraventricular nuclei of the hypothalamus show two different patterns of activity: one a "bursting" or rhythmic pattern and the other, irregular continuous discharges. This possibility was investigated in cats and dogs anesthetized with chloralose or Nembutal by recording electrical activity of single supraoptic and paraventricular neurons. Only some of the "identified" neurosecretory cells showed rhythmic, intermittent discharges ("bursting" cells in rats); the majority showed an irregular continuous firing pattern. Furthermore, the pattern of discharge sometimes changed from one ot the other during long periods of observation. This occurred without apparent stimulus in certain instances; in others, the rhythmic firing was associated with fluctuation in blood pressure and heart rate and was likely to be caused by changes in baroreceptor activity. Possible origins of rhythmic discharges and the physiological importance of such patterns in terms of hormone secretion are discussed.

Some electrophysiological properties of neurones of rat locus coeruleus

1. Electrical activity of neurones of the locus coeruleus (LC) was studied in rats anaesthetized with urethane. By stimulating the dorsal pathway (DP) of axons of LC neurones in the mid=brain and observing field responses in the dorsolateral tegmentum of the pons, micro-electrodes were oriented to record unit discharges of LC neurones. They were evoked by DP stimulation mainly during the negative wave of the field response. 2. In the extraceullar records of spike discharges of LC meurones A and B spikes were distinguished. Very often the third component (C spike) was observed to ride on the descending stroke of the B spike. When present in the evoked discharge, it was also seen in the spontaneous discharge. 3. The DP-elicited unit discharges of LC neurones were classified into three types. The type 1 response had a fixed latency and a distinct A-B step. In the type 2 response the A spike occurred with a fixed latency, but the B spike followed it with variable delays, sometimes exceeding 5 msec. Being supported by the data of the collision test with spontaneous discharges, the type 1 and 2 responses were assumed to be due to antidromic excitation. The type 3 response whose characteristic was a wide variation of the latency from stimulation to stimulation was categorized as orthodromic excitation. Among seventy-four responses, forty-four were type 1, eight type 2 and twenty-two type 3. The conduction velocities of axons of LC neurones, determined from the latencies of the A spike of the type 1 and 2 responses, ranged from 0-3 to 1-4 m/sec with a mean of 0-69 m/sec. 4. Delay of the B spike in antidromic excitation was observed as a unique property of LC neurones. It was seen in the response to a single shock of DP (type 2 response) or in the response to the second shock of DP following the first one shortly (type 1 response). Since delay of the B spike in the type 2 response could not be ascribed to refractoriness, it was suggested that DP stimulation produced an inhibitory effect upon LC neurones. 5. LC neurones were invaded antidromically from the frontal or visual cortex, hippocampus, cerebellum or from varied combinations of them. About 70% of LC neurones were activated antidromically from the frontal cortex. The antidromic latencies ranged from 15 to 90 msec. 6. Some LC neurones were activated trans-synaptically by stimulation of those forebrain sites which received axonal projections from LC. All LC neurones examined were excited trans-synaptically by eletrical stimulation of the skin and the optic nerve. The sensory inputs arising from a vast area of the skin or those from the skin and the optic nerve were proved to converge on to the same LC neurones.

Inhibition and facilitation of antidromically identified tubero-infundibular neurones following stimulation of the median eminence in the rat

1. Stimulation of the median eminence of female rats inhibited the spontaneous firing of antidromically identified tubero-infundibular units. Some units could be inhibited by stimuli subthreshold for the antidromic spike. A conditioning stimulus of subthreshold intensity for the antidromic spike also delayed or abolished the invasion of the soma and dendrites of the same unit by antidromic action potentials evoked by a suprathreshold stimulus given 2-10 msec later. 2. Although strychnine (0-2 mg/kg, i.v.) did not significantly alter the inhibition evoked by stimulation of the median eminence, it was abolished by picrotoxin (2-6 mg/kg, iv.). 3. In seven of the fifty-seven identified units examined stimulation of the median eminence facilitated the spontaneous firing. However, after an i.v. injection of picrotoxin the facilitory response was observed in thirty-seven of the forty-six units tested. Post-stimulus time histograms obtained from the same unit before and after an injection of picrotoxin demonstrated that the latency and duration of the facilitation did not always coincide with that of the inhibition. 4. After an injection of picrotoxin the field potential evoked by antidromic stimulation of the median eminence consisted of a small positive wave followed by a negative wave. Frequently the negative wave of the field potential was accompanied by a convulsive discharge. The latency of the negative wave appears to be identical to that of the facilitation seen in nearby single units. 5. The facilitation evoked by antidromic stimulation in the presence of picrotoxin was blocked by an i.v. injection of alpha-methyl-p-tyrosine (250 or 375 mg/kg). None of the nine units sampled from rats pre-treated with alpha-methyl-p-tyrosine injected twice I.P. (250 mg/kg for each) were facilitated by stimulation of the median eminence following I.V. picrotoxin, while eight of the eleven units sampled from control rats pre-treated with L-tyrosine could be facilitated by antidromic stimulation. 6. These results suggest that tuber-infundibular neurosecretory neurones are inhibited and facilitated by neural pathways which could involve the axon collaterals of the neruosecretory neurones which project to the external layer of the median eminence. It is also suggested that GABA-releasing neurones mediate the inhibition and catecholaminergic neurones are involved in the facilitory pathways. Presumably the facilitation is normally masked by the activity of the GABA-releasing neurones.