'Homing' of Lucifer Yellow liposomes into hypothalamic neurons: a combined neuroanatomical Golgi and tracing technique

Liposome delivery to the brain with rapid short-pulses of focused ultrasound and microbubbles

Liposomes are clinically used drug carriers designed to improve the delivery of drugs to specific tissues while minimising systemic distribution. However, liposomes are unable to cross the blood-brain barrier (BBB) and enter the brain, mostly due to their large size (ca. 100 nm). A noninvasive and localised method of delivering liposomes across the BBB is to intravenously inject microbubbles and apply long pulses of ultrasound (pulse length: >1 ms) to a targeted brain region. Recently, we have shown that applying rapid short pulses (RaSP) (pulse length: 5 μs) can deliver drugs with an improved efficacy and safety profile. However, this was tested with a relatively smaller 3-kDa molecule (dextran). In this study, we examine whether RaSP can deliver liposomes to the murine brain in vivo. Fluorescent DiD-PEGylated liposomes were synthesized and injected intravenously alongside microbubbles. The left hippocampus of mice was then sonicated with either a RaSP sequence (5 μs at 1.25 kHz in groups of 10 ms at 0.5 Hz) or a long pulse sequence (10 ms at 0.5 Hz), with each pulse having a 1-MHz centre frequency (0.35 and 0.53 MPa). The delivery and distribution of the fluorescently-labelled liposomes were assessed by fluorescence imaging of the brain sections. The safety profile of the sonicated brains was assessed by histological staining. RaSP was shown to locally deliver liposomes across the BBB at 0.53 MPa with a more diffused and safer profile compared to the long pulse ultrasound sequence. Cellular uptake of liposomes was observed in neurons and microglia, while no uptake within astrocytes was observed in both RaSP and long pulse-treated brains. This study shows that RaSP allows a targeted and safe delivery of liposomal drugs into the murine brain with potential to deliver drugs into neuronal and glial targets.

Neural connections of hypothalamic neuroendocrine nuclei in the rat

The secretion of many hormones, including oxytocin, vasopressin and growth hormone, is not constant but shows a day-night rhythm. The suprachiasmatic nucleus (SCN) is thought to generate most mammalian biological rhythms and previous studies have reported suprachiasmatic efferents to the paraventricular nucleus (PVN) and the supraoptic nucleus (SON). We used in vivo extracellular electrophysiological techniques to show that the SCN also sends direct and indirect neural projections to the arcuate nucleus (ARC). This projection consisted of both excitatory and inhibitory components and may contribute to the entrainment of the rhythm in growth hormone secretion to the day-night cycle. Some SCN neurones appear to project to both the SON and the ARC. The SCN in turn receives excitatory and inhibitory inputs from the ARC and the peri-nuclear zone of the SON (peri-SON), which may provide feedback information, as well as allowing nonphotic entrainment of the SCN, for example, in response to feeding. Our data thus suggest extensive two-way connections between the SCN and its target nuclei which may contribute to the generation of day-night neuroendocrine rhythms. They also suggest the existence of indirect retinal projections to the ARC and PVN. We further investigated the retinal projection to the SCN. We were unable to demonstrate a significant difference in retinal input to those suprachiasmatic cells which had efferent projections to particular hypothalamic targets (SON and/or ARC), and those which did not.

Spontaneous and stimulated firing in cultured rat suprachiasmatic neurons

Neurons from the suprachiasmatic nucleus (SCN) of the hypothalamus, the site of a circadian pacemaker in mammals, were isolated from embryonic rat. After mechanical dissociation neurons were brought into culture for 1-2 weeks, using a chemically defined medium. Recordings were made from 74 bipolar neurons using two different configurations of the patch-clamp technique. During cell attached patch recordings, 45% of neurons fired spontaneously. The mean firing rate was 0.7 +/- 0.6 Hz and the firing pattern was irregular. In whole cell recordings 73% of the investigated neurons showed spontaneous activity with an irregular firing pattern. The mean spontaneous firing rate with an intracellular Cl- concentration of 145 mM was 1.0 +/- 0.6 Hz. The resting membrane potential of the bipolar neurons was estimated to be -62 +/- 24 mV. An intracellular Cl- concentration of 145 mM depolarised the membrane potential. It also increased the probability of spontaneous firing. A depolarising current stimulus produced an action potential with a threshold voltage of -46 +/- 9 mV. Suprathreshold stimuli resulted in repetitive firing with a mean frequency of 12 +/- 4 Hz. The minimum interspike interval was 52 +/- 14 ms. All action potentials either occurring spontaneously or elicited by current stimuli were abolished by the Na(+)-channel blocker TTX. These results indicate that our cultured neurons have some electrophysiological properties in common with SCN neurons in brain slices and in vivo.

The microwave Rio-Hortega technique: a 24 hour method

Application of microwaves in histochemistry and cytochemistry generally speeds up the technique. Microwaves stimulate diffusion into the tissue and influence the proteins and membrane of the cell. Silver impregnation techniques for the brain, such as the fast Rio-Hortega or the slow Golgi-Cox technique, normally require a minimum time period of 7 days and 20 days respectively. Using microwaves, the Rio-Hortega technique can be completed within 24 h. In sections prepared from mature brains, good silver impregnation of cell bodies, of axons and their terminals, and of dendrites and their spines are obtained. An explanation is given as to why the method cannot be further reduced in time. To our knowledge, this is the first report of the application of microwave irradiation for colouring pieces of tissue.

Specific neuronal staining by in vitro uptake of lucifer yellow

Neurons and glial cells can be stained by Lucifer Yellow CH in vitro to produce a Golgi-like fluorescent or electron-dense stain. This technique has been applied successfully in the retinas of several species, rat brain slices and embryonic chick spinal cord. The relative proportion of stained neurons residing in different retinal layers can be modified by manipulating extracellular concentrations of calcium, magnesium and cobalt. In many cases this technique can be a useful adjunct to traditional neuroanatomical techniques.