Injury biomechanics, neuropathology, and simplified physics of explosive blast and impact mild traumatic brain injury

Explosive blast shock waves and blunt impact to the head are two types of loading shown to result in mild traumatic brain injury (mTBI). While mTBI from these two causes shares some common features behaviorally, there are distinct differences in the pathophysiology of the underlying injury mechanisms. Various elucidations have been offered in the literature to explain the organic damage associated with mTBI resulting from both types of loading. The current state of understanding in this field is somewhat limited by the degree of appreciation of the physics and biomechanics governing the effects of explosive blast shock waves and blunt impact on the head, which has resulted in the various approaches to the investigation of the operative brain injury "wounding mechanisms". In this chapter we provide a simplified description of terminology associated with forces on the head from explosive blast shock waves and blunt impact, to assist readers in the field in evaluating interpretations of brain injury "wounding" processes. Remarkably, mTBI from either loading is shown generally to result in only a small loss of neurons, with hippocampal neurons appearing to be particularly vulnerable to explosive blast shock waves. Explosive blast studies in large animal models show a unique pattern of periventricular injury, which is different from the classic diffuse axonal injury. Both astrocyte and microglial activation are also seen in explosive blast as well as impact trauma, but this may be a general secondary brain injury response, nonspecific to explosive blast or blunt trauma. Additionally, while moderate to severe impact closed head injuries sometimes result in petechial hemorrhages or hematomas, they do not appear to be associated with explosive blast mTBI even with repeated exposure to blasts.

Serotonin 5-HT1A and 5-HT2/1C receptors in the midbrain periaqueductal gray differentially modulate defensive rage behavior elicited from the medial hypothalamus of the cat

Recent studies have established that the expression of defensive rage behavior in the cat is mediated over a descending pathway from the medial hypothalamus to the dorsolateral quadrant of the midbrain periaqueductal gray matter (PAG). The present study was designed to determine the roles played by 5-HT1A and 5-HT2/1C receptors in this region of PAG in modulating defensive rage behavior elicited from the cat's medial hypothalamus. Monopolar stimulating electrodes were implanted into the medial hypothalamus from which defensive rage behavior could be elicited by electrical stimulation. During the course of the study, the 'hissing' component of the defensive rage response was used as a measure of defensive rage behavior. Cannula-electrodes were implanted into sites within the PAG from which defensive rage could also be elicited by electrical stimulation in order that 5-HT compounds could be microinjected into behaviorally identifiable regions of the PAG at a later time. Microinjections of the selective 5-HT1A agonist, (+)-8-hydroxy-dipropylaminotetralin hydrobromide (8-OHDPAT) (50 pmol, 2.0 and 3.0 nmol), into the PAG suppressed the hissing response in a dose-dependent manner. Administration of the selective 5-HT1A antagonist, 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-benzamide hydrochloride (p-MPPI) (1.5 and 3.0 nmol), blocked the suppressive effects of 8-OHDPAT upon hissing. In contrast, microinjections of the 5-HT2/1C receptor agonist (+)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride ((+)-DOI hydrochloride) (0.01, 1.0 and 1.5 nmol) facilitated the occurrence of hissing elicited from the medial hypothalamus in a dose-dependent manner. Immunohistochemical analysis revealed the presence of 5-HT axons and preterminals throughout the PAG, and in particular, in its dorsolateral aspect which receives major inputs from the medial hypothalamus in association with defensive rage behavior. The overall findings of the study provide evidence that activation of 5-HT1A and 5-HT2/1C receptors within the midbrain PAG differentially modulate the expression of defensive rage behavior elicited from the medial hypothalamus of the cat.

Ascending vocalization pathways in the female ring dove: projections of the nucleus intercollicularis

Nest-coo (courtship) vocalization is an estrogen-dependent component of a sequence of behaviors leading to egg-laying in the ring dove (Streptopelia risoria). Implantation of estrogen in the nucleus intercollicularis of ovariectomized females restores the ability to produce nest-coos. Lesion of n. intercollicularis specifically disrupts nest-cooing and inhibits follicular development. We used anterograde transport of tritiated leucine to identify regions that receive input from n. intercollicularis. As a control, isotope was injected in an auditory nucleus (nucleus mesencephalicus lateralis, pars dorsalis) neighboring the n. intercollicularis. A strong projection to the nucleus was observed, demonstrating a potential interaction between auditory and vocalization systems at the level of the midbrain. Local projections of n. intercollicularis were observed in midbrain and hind brain nuclei including the nucleus tegmenti pedunculo-pontinus, pars compacta and nucleus papillioformis. A conspicuous projection ascended centrally via the mesencephalic central gray. Proceeding rostrally, this pathway was traced to the nucleus periventricularis magnocellularis and tissue surrounding it. At its rostral extent, the pathway was found to project to the nucleus preopticus medialis, the anterior medial hypothalamus, and the lateral hypothalamus. These results are consistent with the possibility of a nucleus intercollicularis projection to regions that regulate reproductive function.

Amphetamine and chick behavior. A role for monoamines in the causation of vocalizations and emotions

The effects of amphetamine (AMP) on the behaviour of domestic chicks were studied by methods of direct observation of behaviour. 7.5 mg/kg d-amphetamine sulphate injected into 5-day-old chicks characteristically facilitated vocalization-a period of peeps followed by short calls (twitters and short peeps), head shakes, forced locomotion and wing drooping; and decreased the duration of eye closure. AMP also increased the responsiveness of chicks to external stimuli. Bilateral lesions in the midbrain inhibited primarily the vocalizations but not the other behavioural changes produced by AMP. Evidence is presented to explain the neurochemical basis of the AMP-induced behaviour: namely, that peeping depends on 5-hydroxytryptamine-dependent mechanisms, which may also influence the postural changes, and head shakes, whereas short calls may be mediated by an interaction between dopaminergic and tryptaminergic mechanisms. It is suggested that the monoamines may also be involved in the attentional changes associated with the vocalizations.