John I. Hubbard

Responses from osmosensitive neurons of the rat subfornical organ in vitro

Extracellular recordings were made in vitro from 212 single units in the rat subfornical organ (SFO) and 54 single units in the rat medial preoptic area (MPO). Units were exposed to solutions made hyper-osmotic or hypo-osmotic by 1.4-11%. A reversible 30% or greater change in frequency followed the osmotic challenge in tests of 66% of units in the SFO and 46% of units in the MPO. Responses consisted of increases in frequency (excitations) or decreases in frequency (inhibitions) and were either sustained for the whole test period or of a transitory nature. Units responded to either hyperosmotic (SFO, 19%; MPO, 43%) or to hypo-osmotic changes (SFO, 30%; MPO, 28.5%) or to both (SFO, 51%; MPO, 28.5%). The response pattern of the SFO and MPO was significantly different (chi 2 54.0, 3df, P = 0.0001). In both the SFO and MPO the stimulus to which the units responded was a change in tonicity. This was indicated by the findings that similar responses were evoked by hyperosmotic changes made with either mannitol or NaCl and there was no response to solutions containing urea, either as an additive, or as a substitute for NaCl. In the SFO, in the presence of synaptic blockade produced by raising the Mg concentration in the bathing solution to 15 mM, the frequency of 19/27 units fell significantly. Responses of 40% of units to osmotic pressure changes were blocked indicating these responses were synaptically evoked. The responses which survived synaptic blockade when compared with pre-blockade responses were more often transient (P less than 0.02) and more often inhibitions. Post blockade there were also significantly more responses in the SFO to hypo-osmotic than to hyper-osmotic changes (P = 0.01). Our results suggest that while an ability to change their firing rate in response to small changes of osmotic pressure may be a general property of neurons, the neurons of the SFO are specialised for the detection of changes in the extracellular osmotic pressure.

Localization of immunoreactive angiotensin II in the hippocampus and striatum of rat brain

Angiotensin II (ANG II) was estimated by radioimmunoassay in extracts of rat brain. In extracts of whole brain the mean content was 108 +/- 16 fmol/g and estimates of ANG II in kidney and plasma were similar to previous reports. A regional distribution of ANG II was determined. Hippocampus had the highest concentration and cortex the lowest. The concentrations relative to cortex were: hippocampus, 8; striatum, 5; cerebellum, 4; hypothalamus:thalamus:septum:midbrain (HTSM), 3; and medulla, 3.

Angiotensin binding and pressor activity in the rat ventricular system and midbrain

Abstract Specific angiotensin II (AII) binding capacity was investigated in regions of rat brain thought to be involved in the central pressor effects of AII. In the midbrain specific angiotensin II binding capacity was principally localised in the superior colliculi. When the superior colliculi was divided into rostral and caudal portions, the caudal portion had a higher binding capacity than the rostral portion. In the hypothalamus AII binding capacity was only a third of that in the superior colliculi and was localised principally in the lateral hypothalamus. The medial hypothalamus and the preoptic recess each had about a quarter of the binding capacity of the lateral hypothalamus. To investigate whether AII binding capacity could be correlated with pressor activity AII was applied locally (local) to the surface of the superior colliculi (5 μg/5 μl saline) of rats with femoral artery cannulas and anaesthetised with pentobarbitone. These responses were compared with responses to the same dose given intraventricularly (IVT). When the surface of the midbrain was intact there was a pressor response to the local application not significantly different in magnitude and latency from the response to IVT AII. The midbrain was aspirated at the site of AII application and pressor responses to AII evoked during aspiration. As the midbrain lesion was extended the latency of the local pressor response was prolonged (P

Evidence that NMDA receptors contribute to synaptic function in the guinea pig medial vestibular nucleus

Single medial vestibular nucleus neurons were recorded from guinea pig brainstem slices in vitro while superfusing with the selective N-methyl-D-aspartate (NMDA) antagonists, MK801 and CPP. The majority of neurons tested showed a decrease in firing rate in response to these NMDA antagonists, suggesting that NMDA receptors may contribute to the resting activity of MVN neurons.

Neuronal activity in the guinea pig medial vestibular nucleus in vitro following chronic unilateral labyrinthectomy

Unilateral labyrinthectomy (UL) causes ocular motor and postural disorders which disappear over time in a process of recovery known as vestibular compensation. Vestibular compensation is due to CNS plasticity which generates a partial recovery of resting activity in the vestibular nucleus ipsilateral to the UL, however the mechanism of this neural recovery is unknown. It has been suggested that other areas of the CNS may substitute non-vestibular sensory inputs for the missing labyrinthine input, thereby causing vestibular compensation. The present results show that resting activity can be recorded from medial vestibular nucleus (MVN) neurons in vitro, in brainstem slices from guinea pigs which have compensated for an ipsilateral UL. This result suggests that MVN neurons are capable of generating resting activity without inputs from many other CNS areas. Perfusion with high Mg2+ solution did not abolish resting activity in most cases, suggesting that part of the resting activity may be generated spontaneously by the neurons, possibly through changes in the electrical excitability of the cell membrane.

Extracellular recording in rat area postrema in vitro and the effects of cholinergic drugs, serotonin and angiotensin II

Spontaneous extracellular action potentials were recorded from rat area postrema explants in vitro for up to 6 h at 35 degrees C. Their geometric mean frequency was 4.4 +/- 1.7-11 Hz (n = 120) and they were most often recorded caudal to the obex. The frequency of spontaneously discharging units could be increased three-fold by raising the KCl concentration from 5 to 15 X 10(-3) M but a claimed non-specific excitant of neurones, L-glutamic acid at 10(-7)-10(-3) M was without effect. Carbamylcholine at 10(-9)-10(-7) M increased the frequency of spontaneous units (12/13 trials) as did 10(-7) M neostigmine sulphate (14/14 trials). The effects of carbamylcholine and neostigmine were additive and were blocked by atropine sulphate at 10(-6) M (18/18 trials). Atropine also stopped the discharge of spontaneous units while D-tubocurarine did not affect unit discharge frequency. It is suggested that units responding to cholinergic drugs have an afferent input from the dorsal vagus. A number of putative transmitters, serotonin (10(-9)-10(-7) M), angiotensin II (0.5 X 10(-10)-0.5 X 10(-9) M) and dopamine (10(-9)-10(-5) M) which on indirect grounds are thought to affect area postrema neurones, were without effect on unit discharge frequency.

Excitatory effects of atrial natriuretic peptide on rat subfornical organ neurons in vitro

Atrial natriuretic peptide (ANP) was applied in the solution bathing the rat subfornical organ in vitro and the response of 170 neurons recorded extracellularly. A significant change in firing rate was found in 20% of neurons tested at a concentration of 1 X 10(-10) M. At higher concentrations 45% of neurons responded. The response was usually a brief excitation. Tests of a range of concentrations on each of 6 neurons showed the maximal excitation was attained at 5 X 10(-9) M. The excitatory responses were generally (41/51) not affected by synaptic blockade indicating a direct action of ANP. The excitatory effect of ANP upon subfornical neurons can be contrasted with the generally inhibitory effect of changes in osmotic pressure. Neurons excited by ANP also responded to 5% osmotic pressure changes in the proportions (28/43) expected from the known response rate to osmotic pressure changes (66%). The commonest pattern of response (16/28) was excitation by hyper-osmotic solutions and inhibition by hypo-osmotic solutions.

LH-RH in picomole concentrations evokes excitation and inhibition of rat arcuate neurones in vitro

A medial sagittal brain slice was developed, which enabled electrophysiological recording from spontaneously active neurones adjacent to the infundibular recess of the rat arcuate nucleus. Luteinizing hormone-releasing hormone (LH-RH) (10 nM-10 pM) significantly altered the frequency of 21 out of 31 units tested, exciting 14 and inhibiting 7 others. The excitatory responses were often not maintained during the exposure to LH-RH but either returned to pre-exposure frequency or displayed an inhibition of discharge. A neural model incorporating recurrent inhibition of LH-RH-excited neurones is proposed to explain these responses, thought to be endogenously evoked by collateral innervation of the arcuate nucleus by medial preoptic neurones projecting to the median eminence.

Guinea pig medial vestibular nucleus neurons in vitro respond to ACTH4–10 at picomolar concentrations

SummaryThe responses of single guinea pig medial vestibular nucleus (MVN) neurons in vitro to adrenocorticotropic hormone, fragment 4–10 (ACTH4–10) were recorded extracellularly. In coronal slices and in the isolated MVN, neurons were found which responded to ACTH4–10 at picomolar concentrations (10-12M), indicating that ACTH4–10 acts directly on MVN neurons and suggesting the possibility that ACTH4–10 may act as a neurotransmitter in the MVN. In most cases where neurons responded to ACTH4–10 (37/74 neurons), the effect was a decrease in firing. Whether or not the depressive action of ACTH4–10 on the firing rate of MVN neurons in vitro is related to the acceleration of behavioral recovery from unilateral labyrinthectomy (vestibular compensation), which has been reported previously (Flohr and Luneburg 1982; Igarashi et al. 1985), is unclear.

The increased ethanol preference in rats induced by choice, darkness, or drugs is reduced by ritanserin.

We tested the hypothesis that ritanserin, a serotonin S2 antagonist, reduces voluntary and induced forms of ethanol drinking. We gave 10 mg/kg ritanserin IP or SC to groups of rats given either a) a free choice between 3% ethanol and water, or b) kept in the dark for 5 weeks and given a choice between a range of ethanol concentrations (3-25%) and water, or c) implanted with osmotic pumps filled with tetrahydro-beta carboline and given a choice between a range of ethanol concentrations and water. In each case, ritanserin significantly reduced ethanol consumption and ethanol preference for 8-10 days after the last injection.