Acetylcholinesterase Histochemistry of the habenulo-interpeduncular pathway in the rat and the effects of electrolytic and kainic acid lesions.

The histochemical distribution of acetylcholinesterase (AChE) was studied in the habenulo-interpeduncular pathway of normal rats and after electrolytic and kainic acid lesions of the habenular nuclei. From these combined observations it appears that the AChE-rich projection to the interpeduncular nucleus derives from both the medial and the lateral habenular nuclei. The lateral nucleus of the habenula is the main source of AChE-rich fibres in the fasciculus retroflexus, and a number of stained fibres also derive from the stria medullaris. While total habenular lesion completely deprived the fasciculus retroflexus of AChE-stained fibres, a direct effect on the enzyme distribution in the interpeduncular nucleus was only apparent as its rostral pole. In the remainder of the nucleus the AChE distribution did not undergo obvious changes in comparison with the normal pattern, except for a moderate decrease in overall reaction intensity in cases with subtotal habenular lesion bilaterally. The above results are consistent with the observation derived from experiments involving kainic acid injection into the habenula. The neurotoxic effect of kainic acid was highly selective for specific types of neurons in the lateral habenula, while the neurons of the medial habenula were completely unaffected. The existence of an AChE-rich projection from the lateral habenula to the interpeduncular nucleus was supported by a corresponding decrease in enzyme activity in the lateral habenula and fasciculus retroflexus after kainic acid treatment.